CHAPTER–IIIshodhganga.inflibnet.ac.in/bitstream/10603/44333/11/11_chapter_3.pdf · aldehyde 7 by...

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CCHHAAPPTTEERR––IIIIII

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Chapter III

-55-

INTRODUCTION

Myxomycetes, also known as true slime molds are distinctive organisms as they are

not animals, plants or fungi. Conventionally myxomycetes are classified as primitive

animals in zoology due to their property to roam around to grow and eat, while in

mycology they are considered as fungi because of fungi like spore producing reproductive

stage. Modern biological classification in general place myxomycetes in separate kingdom

of Protozoa in the company of other primitive unicellular organisms like amoeba.

NH

NH

O

HN

OH

O

O

O

OH

O

MeO

HN

HN

O

OMe

OH

O

HN O

OH

O

MeO

OH

Melleumin A (1) Melleumin B (2)

23 4

6

81011

1

12

34

6

810

11

Melleumin A (1), and its seco acid methyl ester, melleumin B (2) were isolated

from the cultured plasmodium of the myxomycete Physarum melleum.1 Myxomycetes P.

melleum were collected at Tokorozawa, Saitama Prefecture, Japan in 2001. The crude

extract of the cultured plasmodium of P. melleum exhibited antimicrobial activity against

Bacillus subtillis. The plasmodiums of this myxomycete was mass-cultured in the

laboratory on agar plates in the presence of Escherichia coli using known methods.2 Thus

harvested plasmodial cells were extracted with 90% MeOH and 90% acetone, and the

combined extract was partitioned between ethyl acetate and water. The ethyl acetate-

soluble layer was purified by silica gel column chromatography, and the fraction eluting

with 5–9% MeOH in chloroform, containing UV (254 nm)-positive spots on TLC were

Chapter III

-56-

isolated and further separated by HPLC on ODS column eluting with 50% MeOH to give

two UV (254 nm)-positive compounds, named melleumin A (1) and B (2), in 0.007% and

0.02% yield, respectively. These novel peptide lactone and its seco acid methyl esters were

fully characterized by NMR, HRMS, UV spectroscopic methods in conjunction with

chromatographic separation and chemical degradation methods of analysis. Initially

unknown stereochemistry at C-4 was confirmed later as 4S by total synthesis.3 Melleumin

A and B consist of four residues, p-methoxybenzoic acid, L-threonine, glycine, and an

unusual amino acid, a tyrosine attached acetic acid.

Wnt proteins are the signalling molecules secreted to regulate cell to cell

interactions during embryogenesis. Mutation of Wnt genes, Wnt pathways and proteins

cause developmental defects. Among humans various diseases including cancer are caused

by abnormal Wnt signalling. Unusual Wnt signalling is known to drive formation of

various human cancers. So molecules which inhibit these Wnt signalling pathways will

eventually block the growth of particular cancer cell lines. 10-epi-Melleumin B has shown

Wnt signal inhibition. This finding suggests that members of this melleumin structural

family can be possible candidates as small-molecule Wnt signal Inhibitors. Small molecule

Wnt signal inhibitors are sought after potential therapeutics for various cancers and other

serious diseases.

Contemporary approaches for the synthesis of Melleumin A and B.

As evident from the above report, melleumin A (1) and B (2) were isolated only

0.007% and 0.02% yields respectively. These quantities were significantly insufficient for

the detailed biological evaluations. To address this problem few strategies have been

developed for the total syntheses of melleumin A and B, which are briefly summarized in

this section.

Chapter III

-57-

Scheme-1

Ishibashi’s approach to melleumin B:

The Ishibashi group, who isolated melleumin A, B were first to report the total

synthesis of melleumin B. They also determined previously unknown stereochemistry at

C-4 position3. They disconnected melleumin B at N-8–C-9 amide linkage to give simpler

acid (3) and amine (4) precursors. While acid 3 was sourced from known compound 5,

amine 4 was derived from 6 which was in turn synthesized from L-tyrosine derived

aldehyde 7 by reacting it with lithium enolate of ethyl acetate. The stereochemistry at

newly generated chiral center in 6 was determined by the modified Mosher’s method and

chiral HPLC methods (Scheme-1).

Huang’s Approach to Melleumin A:

Huang et. al reported the first total synthesis of melleumin A4. Their strategy

includes diphenyl phosphoryl azide (DPPA) mediated macrolactamization at N-5, C-6

junction. While advanced precursor 8 was derived from intermediate alcohol 9 and acid 10

Chapter III

-58-

via Yamaguchi esterification protocol in good yields, alcohol 9 was conveniently made

from L-threonine using routine functional group transfer reactions. They synthesized key

acid intermediate 10 using Jouin and Castro’s method with minor modification. (Scheme-

2)

Scheme-2: Ishibashi’s approach to melleumin A

Ishibashi and coworker’s used macrolactamization strategy to construct N-8–C-9

amide bond in advanced dipeptide ester precursor 14, which was sourced using

dicyclohexyl carbodimide (DCC) mediated esterification between alcohol 15 and acid 16.

Alcohol 15 was derived from L-threonine, while acid 16 was sourced using amide bond

formation between Boc-NH-glycine and β-hydroxy ester intermediate 17 derived amine.

The β-hydroxy ester intermediate 17 was obtained by stereoselectively reducing L-tyrosine

derived β-keto ester intermediate 18 using K-selectride (Scheme-3).

Chapter III

-59-

(Scheme-3)

Chapter III

-60-

PRESENT WORK:

In view of the possible Wnt signal inhibiting properties of melleumin A, B and

their structural derivatives and scarcity of these compounds from natural sources, we

decided to undertake synthesis of melleumin A, B. Melleumin A and its seco acid methyl

ester melleumin B consist of four residues viz. p-methoxybenzoic acid, L-threonine,

glycine, and an unusual amino acid, a tyrosine-attached acetic acid.

Retrosynthetic strategy:

Our retro synthesis is illustrated in Scheme 4. Taking the advantage of close

structural relationship between cyclic lactone melleumin A and it open chain seco acid

methyl ester melleumin B, we planned a common strategy for both the targets. Upon

examination of melleumin A structure, an obvious disconnection at ester linkage gives

intermediate 19, which can also be converted to melleumin B by simple functional group

transformation reactions. This crucial intermediate 19 was envisioned to be sourced from

union of p-methoxy benzoyl threonine-glycine methyl ester 20 and β-hydroxy ester

intermediate 17. The methyl ester 20 was clearly accessible from simple union of Boc-

threonine (22) with glycine methyl ester (23) followed by coupling with p-methoxybenzoic

acid (21). While known compound β-hydroxy ester intermediate 17 was planned to be

synthesized from tyrosine derived aldehyde (7), utilizing Reformatsky reaction

(Scheme-4).

Chapter III

-61-

Scheme 4: Retrosynthetic analysis.

Chapter III

-62-

Result and Discussion

Thus as illustrated in Scheme 5, Boc-Threonine (24) was subjected to isobutyl

chloroformate mediated activation followed by treatment with glycine methyl ester in

presence of DIEPA forming new peptide bond, to give BocNH-Thr-Gly-OMe dimer (25)

in good yields. 1H NMR spectrum of 24 showed singlet peak at δ 3.75 ppm, doublet peak

at δ 5.68 ppm & mass peak at 313 [M+Na]+ in ESI-MS spectrum to confirm the

conversion. Here we sought extension of threonine-NH terminus with p-methoxybenzoic

acid (21). So accordingly, the dimer 25 was subjected to Boc-deprotection using usual 50%

TFA in dichloromethane condition, but to our surprise, there was no progress in reaction

even after 12 h run time. Next we tried this deprotection using neat TFA for 12 h & 24 h

run times, which too failed to remove the t-butoxycarbonyl (Boc) protecting group of 25.

Then the report of using thiophenol in TFA for Boc-deprotection6 rescued us. Thus 25 on

treatment with TFA in presence of 1.0 equivalent of thiophenol gave free amine TFA salt

26 which was subjected to EDCI –HOBt mediated amide coupling with p-methoxybenzoic

acid (21) to give advanced intermediate 20 in very good yields. 1H NMR spectrum of 20

showed disappearance of peaks corresponding to Boc-group at δ 1.45 ppm. Additional

singlet at δ 3.84 ppm in 3-4 ppm region, two doublets in aromatic region and mass peak at

347 [M+Na]+ in ESI-MS spectrum confirmed the validity of the structure. Now hydrolysis

of the methyl ester of advanced intermediate 20 was the next task. Accordingly it was

treated with LiOH to give free acid 27 which was then used for further extension of chain

towards our target molecule (Scheme 5).

COOHBocHN

HO

BocHN

HO

NH

O

OMe

O

HN

HO

NH

O

OMe

O

O

MeO

isobutyl chloroformate,

NMM, 23

DIPEA, CH2Cl2, 0oC to rt

12 h, 68%

TFA, thiophenol (1eq)

0 oC to rt, 2 h

TFA.H2N

HO

NH

O

OMe

O

21, HOBt, EDCI, CH2Cl2

DIPEA, 0 oC to rt, 4 h, 89%

LiOH, THF:MeOH:H2O

0 oC to rt, 2 h, 96%

20

HN

HO

NH

O

OH

O

O

MeO

27

2425 26

Scheme 5: Synthesis of fragment 20.

Chapter III

-63-

For the synthesis of tyrosine β-hydroxy ester intermediate 17, we started from

tyrosine derived known alcohol7 28, which on SO3-pyridine mediated oxidation gave

corresponding aldehyde8 7, which was directly taken for the next reaction without further

purification. Here we planned Reformatsky addition9 of ethyl bromoacetate to aldehyde 7.

Thus activated Zinc was taken in dry THF under nitrogen atmosphere. To it ethyl

bromoacetate was added slowly drop-wise at room temperature and stirred for 30 min.

Then this mixture was heated to 65 oC and aldehyde was added slowly. The reaction was

stirred at the same temp for 4 hours. Usual work up gave the products that were mixture of

diastereomeric β-hydroxy esters 17 and 29. This diastereomeric mixture was separated by

silica gel column chromatography and analytical data of 17 was matched with the reported

values3. Our next objective was to remove t-butoxycarbonyl-(Boc-) protection of β-

hydroxy ester 17. Accordingly it was treated with 50% TFA in DCM solution to give TFA

salt of the free amine, 30 which was taken to next step without further characterization.

Now free acid 27 and amine 30 were ready with us to carry out the crucial peptide

coupling reaction. Treatment of the acid 27 with EDCI & HOBt in presence of DIPEA,

followed by amine 30 gave the advanced intermediate 19 in good yields (Scheme 6).

Chapter III

-64-

NHBoc

OBn

OH

SO3-Pyridine, TEA

DCM:DMSO

Zn, BrCH2COOEtTHF, rt, 30 min.

then aldehyde, 65 °C,4 h, 66%

NHBoc

OBn

OH

EtOOC

NHBoc

OBn

OH

EtOOC

+

TFA:CH2Cl2 1:1, 0 °C to rt

2 h

28

17

29

NHBoc

OBn

O

7

H

TFA.NH2

OBn

OH

EtOOC

30

HOBt, EDCI,15 min,then 30 in DCM, DIPEA,0 °C to rt, 4 h 70%

HO

HN

O

NH

O

MeO

NH

O

OBn

EtOOC

OH

19

HN

HO

NH

O

OH

O

O

MeO

27

Sche

me 6: Synthesis of advanced intermediate 19

After synthesizing the advanced intermediated 19, our first target was to achieve

the total synthesis of melleumin B. Accordingly advanced intermediate (19) was subjected

to LiOH mediated ester hydrolysis. The acid, thus obtained, was cleanly converted to its

methyl ester using diazomethane in DCM. The crude product upon DCM evaporation was

subjected to Pd-C catalyzed hydrogenation reaction using a hydrogen filled balloon to give

melleumin B in 67% yields. Spectroscopic data viz.1H NMR,

13C NMR and mass spectral

data of our synthetic melleumin B (2) were in conformity with that of the isolated product

melleumin B1 (Scheme 7).

Chapter III

-65-

Scheme 7: Total Synthesis of Melleumin B.

As per our retro-synthetic analysis we planned to construct the lactone ester linkage

in melleumin A using Yamaguchi macrolactonisation reaction10

. With this intent, the

intermediate 19 was treated with LiOH to give free acid 31 which is nothing but

Yamaguchi macrolactonisation substrate with free acid and alcohol functionalities. Upon

treating the free acid 31 with 2,4,6-trichlorobenzoyl chloride in presence of base under

various modifications (as illustrated in Table 1) failed to yield the desired cyclized product

32. With this failure we thought of removing benzyl protection of free acid 31 and then

trying Yamaguchi macrolactonisation. Accordingly free acid 31 was de-benzylated using

Pd-C and hydrogen filled balloons, and the resulting product was subjected to identical

Yamaguchi conditions (as illustrated in Table 1). To our dejection this time too, the

macrolactonization reaction failed to yield any desired product and the starting material

was seen to be decomposed upon TLC and mass analysis (Scheme 8).

Table 1

S.No Reagents and conditions used for Yamaguchi macrolactonisation Result

1 2,4,6-trichlorobenzoyl chloride, Et3N, DMAP, DCM, reflux, 12 h Decomposition

2 2,4,6-trichlorobenzoyl chloride, Et3N, DMAP, THF, reflux, 12 h Decomposition

3 2,4,6-trichlorobenzoyl chloride, Et3N, DMAP, tolune, reflux,12 h Decomposition

Chapter III

-66-

Scheme 8: Synthetic trials for Melleumin A.

This failure of lactonization forced us to device an alternate macrolactonization

strategy and we thought that oxidative cleavage and subsequent lactonzation of alkenols

under mild reaction conditions11, 12

as illustrated in Scheme 9 can be used in the synthesis

of melleumin A.

Scheme 9

Building upon the possibility of forming lactone core of melleumin A using this

methodology, we constructed alkenol precursor 38. Here we started with aldehyde 7,

which was treated with allylmagnesium bromide13

and the product mixture was then

subjected to NaH and benzyl bromide mediated protection to give fully protected terminal

Chapter III

-67-

alkene intermediates 34, 35. Now Boc-protection of 34, 35 were removed using usual TFA

in DCM methodology to give free amine•TFA salts 36, 37 respectively. These amine

intermediates 36, 37 were then subjected to amide coupling reaction using EDCI and

HOBt to give our required alkenol precursor 38 and its epimer 39 respectively (Scheme

10).

1. (Allyl)MgBr THF,

0 °C to rt, 2 h, 66%

NHBoc

OBn

OBn

NHBoc

OBn

OBn

+

TFA:CH2Cl2 1:1,

0 °C to rt, 2 h

34 35

NHBoc

OBn

O

7

H

NH•TFA

OBn

OBn

36

TFA:CH2Cl2 1:1,

0 °C to rt, 2 h

NH•TFA

OBn

OBn

37

2. NaH, BnBr, THF,

0 °C to rt, 12 h, 69%

HO

HN

O

NH

O

MeO

NH

O

OBn

OBn

38

HN

HO

NH

O

OH

O

O

MeO

27

HOBt, EDCI,15 min,

then 36 in DCM, DIPEA,

0 °C to rt, 4 h 58%

HOBt, EDCI,15 min,then 37 in DCM, DIPEA,0 °C to rt, 4 h 54%

HO

HN

O

NH

O

MeO

NH

O

OBn

OBn

39

27

Scheme 10

Chapter III

-68-

Now alkenol 38 was ready to try this OsO4-Oxone mediated oxidative lactonization

strategy. Accordingly 0.1M solution of alkenol 38 was treated with 4.0 equivalents of

oxone and 1.0 mole % OsO4 simultaneously and progress of reaction was monitored by

TLC analysis and MS analysis, but to our disappointment no desired product 40 was seen

is MS analysis despite complete consumption of SM 38. The reaction was repeated with

various solvents at room temperature and moderate heating conditions (Table-3) but no

desire product was formed, while SM was decomposed. Same reactions were tried with

epimeric Alkenol 39, this too did not yield any cyclized lactone product (Scheme-11).

Scheme 11

With these results we stopped working any further on this Scheme.

Chapter III

-69-

Table 3

S.No Starting

material Solvent & temp conditions

Expected

product Result

1 38 DMF, a) rt; b) 60 °C 40 Decomposition

2 38 Acetonitrile, a) rt; b) 50 °C 40 Decomposition

3 38 Acetone/water, a) rt; b) 50 °C 40 Decomposition

4 38 Methanol, a) rt; b) 50 °C 40 Decomposition

5 38 HMPA, a) rt; b) 60 °C 40 Decomposition

6 38 Dioxane, a) rt; b) 60 °C 40 Decomposition

7 38 Xylene, a) rt; b) 60 °C 40 Decomposition

8 38 DMF, a) rt; b) 60 °C 40 Decomposition

All above mention conditions were also examined for Alkenol 39, but no desired

product was formed.

To summarize we have successfully synthesised melleumin B, and perused total

synthesis of melleumin A. The synthetic strategy developed here is simple and amenable

for modifications for synthesis of melleumin B, melleumin A analogues for Wnt signal

inhibition activity evaluations.

Chapter III

-70-

EXPERIMENTAL SECTION

Methyl 2-((2S, 3R)-2-(tert-butoxycarbonylamino)-3-hydroxybutanamido)acetate (25):

To the solution of Boc-Threonine (24) (5.0 g, 22.8 mmoles) in dry CH2Cl2 (60 mL) at 0 ºC,

N-methylmorpholine (3.51 mL, 31.92 mmol) was added slowly, in a drop-wise manner,

with stirring under nitrogen atmosphere. After 10 min stirring, isobutyl chloroformate

(3.57 mL, 27.36 mmol) was added to the reaction mixture slowly and stirred further 30

min. at the same temperature. Then solution of glycine methyl ester (23) (5.17 g, 27.36

mmol) in dry CH2Cl2 (60 mL), pre-mixed with DIEPA (11.91 mL, 68.4 mmol), was added

slowly at same temperature. The reaction mixture was allowed to warm to rt slowly and

stirred for 12 h. Then the reaction mixture was quenched with saturated aqueous NH4Cl

solution and extracted with EtOAc. The combined organic layers were washed with water,

brine, dried (Na2SO4) and concentrated in vacuo. Purification of the crude by column

chromatography (SiO2, 50% EtOAc in petroleum ether) afforded Boc-Thr-Gly-OMe 25 as

white solid (4.50 g, 68%).

Rf = 0.4 (SiO2, 80% EtOAc in PE); 1H NMR (300 MHz, CDCl3): δ 7.37-7.27 (m, 1H), 5.68

(d, J = 7.55 Hz, 1H), 4.40-4.32 (m, 1H), 4.18 (d, J = 7.5 Hz, 1H), 4.11-3.99 (m, 2H), 3.75

(s, 3H), 3.71 (bs, 1H), 1.45 (s, 9H) 1.20 (d, J = 6.7 Hz, 3H); 13

C NMR (75 MHz, CDCl3):

δ 171.7, 170.2, 156.3, 80.2, 67.0, 58.5, 52.3, 41.1, 28.2, 18.2; MS: m/z (%): 313 [M+ Na]+;

HRMS calculated for C12H22N2O6Na [M+Na]+: 313.1375; Found: 313.1385.

Methyl 2-((2S, 3R)-3-hydroxy-2-(4-methoxybenzamido)butanamido)acetate (20):

HN

HO

NH

O

OMe

O

O

MeO

20

Chapter III

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To a solution of Boc-Thr-Gly-OMe 25 (3.0 g, 10.33 mmol) in dry CH2Cl2 (12 mL) was

added trifluoroacetic acid (12 mL) at 0 °C followed by slow addition of thiophenol (1.08

mL, 10.33 mmol). The reaction mixture was then allowed to warm to rt and stirred for 2 h.

The reaction mixture was then concentrated in vacuo to give TFA•Thr-Gly-OMe 26.

The p-methoxybenzoic acid 21 (1.65 g, 10.84 mmol) was dissolved in CH2Cl2 (20 mL) and

cooled to 0 °C. Then it was sequentially treated with HOBt (2.49 g, 16.26 mmol) and

EDCI (3.12 g, 16.26 mmol). After 15 min, TFA•Thr-Gly-OMe 26, prepared above and

dissolved in CH2Cl2 (10 mL), was added to the reaction mixture followed by DIPEA (5.66

mL, 32.52 mmol). After stirring for 8 h at room temperature, the reaction mixture was

diluted with EtOAc, washed with saturated NH4Cl solution, water, brine, dried (Na2SO4),

filtered and concentrated in vacuo. Purification by column chromatography (SiO2, 60-63%

EtOAc in petroleum ether eluant) afforded the advanced intermediate 20 (2.98 g, 89%).

Rf = 0.4 (SiO2, 80% EtOAc in PE); [α]24

D = –57.6 (c 1.41, CHCl3); IR: νmax 3330, 3083,

2947, 1743, 1640, 1499, 1253, 1025, 766, 693 cm-1

; 1H NMR (300 MHz, CDCl3): δ 7.83-

7.78 (m, 2H), 7.65-7.61 (m, 1H), 7.30-7.27 (m, 1H), 6.94-6.89 (m, 2H), 4.71 (dd, J = 7.5,

3.0 Hz, 1H), 4.53-4.46 (m, 1H), 4.31 (bs, 1H), 4.04 (dd, J = 6.0, 7.8 Hz, 2H), 3.84 (s, 3H),

3.72 (s, 3H), 1.25 (d, J = 6.7 Hz, 3H); 13

C NMR (75 MHz, CDCl3): δ 171.1, 170.0, 167.6,

162.6, 129.0, 125.4, 113.7, 66.7, 57.4, 55.3, 52.3, 41.1, 18.0; MS: m/z (%): 347 [M+ Na]+;

HRMS calculated for C15H20N2O6Na [M+Na]+: 347.1219; Found: 347.1210.

(3S, 4S)-ethyl 5-(4-(benzyloxy)phenyl)-4-(tert-butoxycarbonylamino)-3-

hydroxypentanoate (17):

To a solution of tyrosine derived known alcohol 28 (2.0 g, 5.59 mmol) in dry CH2Cl2 (10

mL) was added with stirring, DMSO (8 mL), Et3N (3.90 mL, 27.97 mmol) and SO3-Py

complex (2.22 g, 13.97 mmol) portion wise at 0 °C under nitrogen atmosphere. After 60

min of stirring at 0 °C, the reaction was quenched with saturated aqueous NH4Cl (15 mL)

and extracted with EtOAc (2 x 25 mL). The combined organic extracts were washed with

Chapter III

-72-

saturated NH4Cl solution, water, brine, dried (Na2SO4), filtered and concentrated in vacuo.

The aldehyde 7 (Rf = 0.5, 30% EtOAc in petroleum ether) thus obtained by flash

chromatography (1.95 g, crude) was directly used for the next reaction.

To a stirred suspension of Zn dust (1.7 g, 16.77 mmol), activated according to

published procedure, in dry THF (25 mL) under argon was added drop-wise ethyl

bromoacetate (1.85 mL, 16.77 mmol) at rt. After additional stirring for 0.5 h, the reaction

mixture was warmed to 60 °C, and a solution of the aldehyde, prepared above and

dissolved in dry THF (25 mL), was added slowly. The mixture was stirred for 0.5 h at the

same temperature, quenched by the addition of aqueous 1 N HCl. After THF was

evaporated under reduced pressure, the residue was extracted with EtOAc. The extract was

washed successively with aqueous 0.5 N HCl and brine, and dried over anhydrous MgSO4.

The filtrate was concentrated in vacuo to give an oily residue.

(3S,4R)-ethyl 5-(4-(benzyloxy)phenyl)-3-hydroxy-4-(2-((2R,3S)-3-hydroxy-2-(4-

methoxybenzamido)butanamido)acetamido)pentanoate (19):

To a solution of the advanced intermediate 20 (250 mg, 0.771 mmol) in THF:MeOH:H2O

(3:1:1, 3 mL) at 0 °C, LiOH.H2O (97 mg, 2.31 mmol) was added and stirred at the same

temperature for 3 h. The reaction mixture was then acidified to pH 2 with 1N HCl. It was

diluted with EtOAc, washed with brine, dried (Na2SO4), filtered and concentrated in vacuo

to get the acid 27 (220 mg, crude) which was directly used in the peptide coupling

reaction.

Chapter III

-73-

To a solution of β-hydroxy ester 17 ( 326 mg, 0.734 mmol) in dry CH2Cl2 (2 mL) was

added trifluoroacetic acid (2 mL) at 0 °C. The reaction mixture was then allowed to warm

to rt and stirred for 2 h. The reaction mixture was then concentrated in vacuo to give the

TFA salt of the free amine 30 that was directly used for the next reaction.

Acid 27 (220 mg, crude) prepared above was dissolved in CH2Cl2 (3 mL) and cooled to 0

°C. Then it was sequentially treated with HOBt (178 mg, 1.16 mmol) and EDCI (222 mg,

1.16 mmol). After 15 min., TFA salt of the free amine 30, prepared above and dissolved in

CH2Cl2 (3 mL), was added to the reaction mixture followed by DIPEA (0.40 mL, 2.31

mmol). After stirring for 12 h at room temperature, the reaction mixture was diluted with

EtOAc, washed with saturated NH4Cl solution, water, brine, dried (Na2SO4), filtered and

concentrated in vacuo. Purification by column chromatography (SiO2, 80% EtOAc in

petroleum ether eluant) afforded the advanced intermediate 19 (326 mg, 70%). Rf = 0.3

(silica gel, 100% EtOAc).

Melleumin B:

Chapter III

-74-

To a solution advanced intermediate 19 (200 mg, 0.31 mmol) in THF:MeOH:H2O (3:1:1, 3

mL) at 0 °C, LiOH.H2O (33 mg, 0.77 mmol) was added and stirred at the same



to get the acid (170 mg, crude) which was dissolved in CH2Cl2 (3 mL) and cooled to 0 °C

and treated with excess of diazomethane (freshly prepared ethereal solution) and stirred for

1 h. Reaction mixture was then concentrated in vacuo to get ester (180 mg, crude) which

was taken to final step without further purification. To a stirred solution of above crude

ester in EtOAc (4 mL), catalytic Pd-C (10%) was added and the mixture was hydrogenated

for 4 h using a H2-filled balloon. It was then filtered through a short pad of Celite and the

filter cake was washed with EtOAc. The filtrate and washings were combined and

concentrated in vacuo. Purification by column chromatography (SiO2, 8% MeOH in

CH2Cl2) afforded melleumin B as a white solid (110 mg, 67%).

Rf = 0.3 (SiO2, 10% MeOH in chloroform); [α]24

D = 0.2 (c 1.0, MeOH); IR: νmax 3410,

2925, 1735, 1655, 1640, 1630, 1610, 1495,1453, 1250 cm-1

; 1H NMR (DMSO, 500 MHz):

δ 9.08 (brs, 1H), 8.25 (br t, J = 5.7 Hz, 1H), 7.95 (d, J = 7.9 Hz, 1H), 7.90 (d, J = 8.9 Hz,

2H), 7.54 (d, J = 9.0 Hz, 1H), 7.02 (d, J = 8.9 Hz, 2H), 6.95 (d, J = 8.1 Hz, 2H), 6.60 (d, J

= 8.1 Hz, 2H), 5.12 (d, J = 5.7 Hz, 1H), 5.08 (d, J = 6.0 Hz, 1H), 4.31 (dd, J = 7.3, 4.9 Hz,

1H), 4.08 (m, 1H) 3.92 (m, 1H), 3.87 (m, 1H), 3.82 (s, 3H), 3.75 (dd, J = 6.5, 17.1 Hz,

1H), 3.57 (dd, J = 4.9, 15.5 Hz, 1H), 3.55 (s, 3H), 2.73 (dd, J = 13.7, 5.7 Hz, 1H), 2.47 (dd,

J = 13.7, 8.6 Hz, 1H), 2.38 (dd, J = 15.5, 3.3 Hz, 1H), 2.29 (dd, J = 15.5, 9.7 Hz, 1H), 1.12

(d, J = 6.5 Hz, 3H); 13

C NMR (CDCl3, 75 MHz): δ 171.8, 170.7, 168.5, 166.3, 161.8,

155.4, 129.9, 129.4, 129.1, 126.1, 114.9, 113.5, 67.5, 66.6, 60.1, 55.4, 54.7, 51.1, 42.1,

38.3, 35.0, 20.0; MS: m/z (%): 532 (100) [M+H]+; HRMS calculated for C26H34O9 N3

[M+H]+: 532.2295; Found: 295.2294.

Chapter III

-75-

tert-butyl (2S,3S)-3-(benzyloxy)-1-(4-(benzyloxy)phenyl)hex-5-en-2-ylcarbamate (34)

&

tert-butyl (2S,3R)-3-(benzyloxy)-1-(4-(benzyloxy)phenyl)hex-5-en-2-ylcarbamate (35)

:

To a solution of tyrosine derived known alcohol 28 (2.0 g, 5.59 mmol) in dry CH2Cl2 (10

mL) was added with stirring, DMSO (8 mL), Et3N (3.90 mL, 27.97 mmol) and SO3-Py

complex (2.22 g, 13.97 mmol) portion wise at 0 °C under nitrogen atmosphere. After 60

min of stirring at 0 °C, the reaction was quenched with saturated aqueous NH4Cl (15 mL)

and extracted with EtOAc. The combined organic extracts were washed with saturated

NH4Cl solution, water, brine, dried (Na2SO4), filtered and concentrated in vacuo. The

aldehyde 7 (Rf = 0.5, 30% EtOAc in petroleum ether) thus obtained by flash

chromatography (1.95 g, crude) was directly used for the next reaction.

This aldehyde 7 was dissolved in dry THF (15 mL) under nitrogen and cooled to –

78 °C, to this allyl magnesium bromide solution (2.0 M in THF; 8.5 mL, 16.77 mmole)

was added slowly drop-wise, RM was then brought to 0 °C and stirred for 2 hrs, quenched

with 1N aq.HCl extracted with ethyl acetate, washed with sat. aq. NaHCO3 solution, dried,

passed through small bed of silica gel before next reaction.

The crude mixture obtained above was dissolved in dry THF and cooled to 0 °C.

To it NaH (60% in mineral oil, 0.336 g, 8.38 mmole) was added slowly portion-wise,

followed by drop-wise addition of benzyl bromide (0.73 mL, 6.15 mmole). Reaction

mixture was then slowly warmed to rt and stirred overnight, then quenched by addition of

sat. aq NH4Cl, diluted with ethyl acetate, washed with water, dried, and concentrated in

vacuo. Purification by column chromatography (SiO2, 20%-30% EtOAc in PE) afforded

compounds 34 (620 mg, 23%) and 35 (570 mg, 21%) as white solids.

Chapter III

-76-

Analytical data for 34


D = –6.96 (c 1.25, CHCl3); IR: νmax 3364, 2929,

1685, 1499, 1517, 1242, 736, cm-1

; 1H NMR (300 MHz, CDCl3): δ 7.44-7.28 (m, 10H),

7.03 (d, J = 8.4 Hz, 2H), 6.86 (d, J = 8.6 Hz, 2H), 5.78-5.63 (m, 1H), 5.07 (d, J = 7.4 Hz,

1H), 5.02 (s, 3H), 4.92 (d, J = 9.6 Hz, 1H), 4.66 (d, J = 11.1 Hz, 1H), 4.40 (d, J = 11.3 Hz,

1H) 3.96-3.85 (m, 1H), 3.40-3.33 (m, 1H), 2.87-2.69 (m, 2H), 2.47-2.37 (m, 1H), 2.31-

2.20 (m, 1H), 1.40 (s, 9H); 13

C NMR (75 MHz, CDCl3): δ 157.2, 155.5, 138.3, 137.1,

134.0, 130.8, 130.1, 128.5, 128.4, 127.8, 127.7, 127.4, 117.8, 114.7, 78.9, 77.7, 71.9, 69.9,

53.8, 38.1, 35.4, 28.3; MS: m/z (%): 347 [M+ Na]+; HRMS calculated for C15H20N2O6Na

[M+Na]+: 347.1219; Found: 347.1210.

Analytical data for 35


D = –37.9 (c 1.00, CHCl3); IR: νmax 3330, 2974,

2935, 1680, 1510, 1169, 1253, 646 cm-1

; 1H NMR (300 MHz, CDCl3): δ 7.44-7.27 (m,

10H), 7.08 (d, J = 8.5 Hz, 2H), 6.88 (d, J = 8.5 Hz, 2H), 5.94-5.80 (m, 1H), 5.19-5.04 (m,

2H), 5.02 (s, 2H), 4.65 (d, J = 11.5 Hz, 1H), 4.59-4.48 (m, 3H), 4.03-3.91 (m, 1H), 3.64-

3.56 (m, 1H), 2.92 (dd, J = 14.2, 4.3 Hz, 1H), 2.71-2.60 (m, 1H), 2.52-2.28 (m, 2H), 1.32

(s, 9H); 13

C NMR (75 MHz, CDCl3): δ 157.3, 155.3, 137.2, 134.4, 130.8, 130.3, 128.6,

128.4, 127.9, 127.8, 127.7, 127.4, 117.6, 114.7, 80.2, 72.0, 70.0, 53.6, 35.4, 34.6, 28.3;

MS: m/z (%): 347 [M+ Na]+; HRMS calculated for C15H20N2O6Na [M+Na]

+: 347.1219;

Found: 347.1210.

N-((2R,3S)-1-(2-((2R,3S)-3-(benzyloxy)-1-(4-(benzyloxy)phenyl)hex-5-en-2-ylamino)-

2-oxoethylamino)-3-hydroxy-1-oxobutan-2-yl)-4-methoxybenzamide (38):

Chapter III

-77-

To a solution advanced intermediate 20 (250 mg, 0.771 mmol) in THF:MeOH:H2O (3:1:1,

3 mL) at 0 °C, LiOH.H2O (97 mg, 2.31 mmol) was added and stirred at the same




reaction.

To a solution of allyl alcohol 34 ( 375 mg, 0.771 mmol) in dry CH2Cl2 (2 mL) was added

trifluoroacetic acid (2 mL) at 0 °C. Reaction mixture was then allowed to warm to rt and

stirred for 2 h. The reaction mixture was then concentrated in vacuo to give the TFA salt of

the free amine 36 that was directly used for the next reaction.

Acid 27 (202 mg, crude), prepared above was dissolved in CH2Cl2 (3 mL) and cooled to 0






concentrated in vacuo. Purification by column chromatography (SiO2, 80% EtOAc in




D = –35.3 (c 0.85, CHCl3); IR: νmax 3307, 2927, 1643,

1503, 1251, 742 cm-1

; 1H NMR (300 MHz, CDCl3): δ 7.76 (d, J = 8.7 Hz, 2H), 7.44-7.27 (m, 11H),

Chapter III

-78-

7.06 (d, J = 7.4 Hz, 1H), 7.00 (d, J = 8.5 Hz, 2H), 6.92-6.82 (m, 4H), 6.54 (d, J = 9.2 Hz, 1H),

5.74-5.59 (m, 1H), 5.06-4.97 (m, 4H), 4.65-4.54 (m, 2H), 4.45-4.32 (m, 2H), 4.30-4.20 (m, 1H),

3.93-3.87 (m, 2H), 3.83 (s, 3H), 3.43-3.37 (m, 1H), 2.85-2.69 (m, 2H), 2.40-2.27 (m, 1H), 2.22-

2.10 (m, 1H), 1.91 (bs, 1H), 1.12 (d, J = 6.4 Hz, 3H); 13

C NMR (75 MHz, CDCl3): δ 171.7, 168.3,

167.4, 162.6, 157.4, 137.9, 137.0, 133.6, 130.2, 129.0, 128.6, 128.5, 128.0, 127.9, 127.5, 125.4,

118.1, 114.8, 113.8, 77.7, 72.2, 69.9, 66.9, 57.6, 55.4, 52.7, 43.3, 37.5, 35.5, 18.5; MS: m/z (%):

702 [M+ Na]+; HRMS calculated for C40H45N3O7Na [M+Na]

+: 702.3155; Found: 702.3134.

N-((2R,3S)-1-(2-((2R,3R)-3-(benzyloxy)-1-(4-(benzyloxy)phenyl)hex-5-en-2-ylamino)-

2-oxoethylamino)-3-hydroxy-1-oxobutan-2-yl)-4-methoxybenzamide (39):

To a solution of advanced intermediate 20 (250 mg, 0.771 mmol) in THF:MeOH:H2O

(3:1:1, 3 mL) at 0 °C, LiOH.H2O (97 mg, 2.31 mmol) was added and stirred at the same




reaction.

Chapter III

-79-

To a solution of allyl alcohol 35 ( 375 mg, 0.771 mmol) in dry CH2Cl2 (2 mL) was added

trifluoroacetic acid (2 mL) at 0 °C. Reaction mixture was then allowed to warm to rt and

stirred for 2 h. The reaction mixture was then concentrated in vacuo to give the TFA salt of

the free amine 37 that was directly used for the next reaction.

Acid 27 (202 mg, crude) prepared above was dissolved in CH2Cl2 (3 mL) and cooled to 0






concentrated in vacuo. Purification by column chromatography (SiO2, 80-85% EtOAc in




D = –32.5 (c 1.79, CHCl3); IR: νmax 3307, 2927, 2935,

1641, 1503, 1251, 1026, 740 cm-1

; 1H NMR (300 MHz, CDCl3): δ 7.75 (d, J = 8.8 Hz, 2H), 7.42-

7.20 (m, 11H), 7.09 (d, J = 6.8 Hz, 1H), 7.01 (d, J = 7.8 Hz, 2H), 6.88 (d, J = 8.8 Hz, 2H), 6.82

(d, J = 8.8 Hz, 2H), 6.30 (d, J = 8.8 Hz, 1H), 5.87-5.78 (m, 1H), 5.15-5.07 (m, 2H), 4.97 (s, 2H),

4.60 (d, J = 11.7 Hz, 1H), 4.50-4.44 (m, 2H), 4.40-4.29 (m, 2H), 3.82 (s, 3H), 3.71 (d, J = 5.8 Hz,

2H), 3.57-3.53 (m, 1H), 2.91 (dd, J = 14.6, 4.9 Hz, 1H), 2.70-2.64 (m, 1H), 2.48-2.40 (m, 1H),

2.37-2.30 (m, 1H), 1.16 (d, J = 5.8 Hz, 3H); 13

C NMR (75 MHz, CDCl3): δ 171.7, 168.2, 167.6,

162.6, 157.3, 138.3, 137.0, 134.2, 130.3, 130.0, 129.1, 128.5, 128.4, 127.9, 127.7, 127.5, 125.3,

117.6, 114.7, 113.8, 79.6, 71.9, 69.9, 66.8, 58.1, 55.4, 52.4, 43.0, 35.2, 34.2, 29.6, 18.7; MS: m/z

(%):702 [M+ Na]+; HRMS calculated for C40H45N3O7Na [M+Na]

+: 702.3155; Found: 702.3150.

Chapter III

-80-

REFERENCES:

1. Nakatani, S.; Kamata, K.; Sato, M.; Onuki, H.; Hirota, H.; Matsumoto, J.;

Ishibashi, M. Tetrahedron Lett. 2005, 46, 267.

2. a) Ishibashi, M. In Studies in Natural Products Chemistry; Atta-ur-Rahman, Ed.;

Elsevier: Amsterdam, 2003; Vol. 29, pp 223–262. b) Ishibashi, M.; Iwasaki, T.;

Imai, S.; Sakamoto, S.; Yamaguchi, K.; Ito, A. J. Nat. Prod. 2001, 64, 108–110. c)

Misono, Y.; Ishibashi, M.; Ito, A. Chem. Pharm. Bull. 2003, 51, 612–613.

3. Hanazawa, S.; Arai, M. A.; Li, X.; Ishibashi, M. Bioorg. Med. Chem. Lett. 2008,

18, 95–98.

4. Luo, J.-M.; Dai, C.-F.; Lin, S.-Y.; Huang, P.-Q. Chem. Asian J. 2009, 4, 328 – 335.

5. Arai, M. A.; Hanazawa, S.; Uchino, Y.; Li, X.; Ishibashi, M. Org. Biomol. Chem.,

2010, 8, 5285–5293.

6. Lundt, B. F.; Johansen, N. L.; Volund, A.; Markussen, J. Int. J. Peptide Protein

Res. 1978, 12, 258-268.

7. Green, R.; Taylor, P. J. M.; Bull, S. D.; James T. D.; Mahon, M. F; Merritt, A. T.

Tetrahedron Asymm. 2003, 14, 2619.

8. Parikh, J. R.; Doering W. V. E. J. Am. Chem. Soc.1967, 89, 5505.

9. (a) Picotin, G.; Miginiac, P. J. Org. Chem. 1987, 52, 4796. (b) Beignet, J.; Jervis, P.

J.; Cox, L. R. J. Org. Chem. 2008, 73, 5462.

10. Inanaga, J.; Hirata, K.; Saeki, H.; Katsuki, T.; Yamaguchi, M. Bull. Chem. Soc.

Jpn. 1979, 52, 1989.

11. a) Schlecht, M. F.; Kim, H. Tetrahedron Lett. 1985, 26, 127. b) Baskaran, S.;

Islam, I.; Raghavan, M.; Chandrasekaran, S. Chem. Lett. 1987, 1175. c)

Chakraborty, T. K.; Chandrasekaran, S. Tetrahedron Lett. 1980, 21, 1583. d)

Rathore, R.; Vankar, P. S.; Chandrasekaran, S. Tetrahedron Lett. 1986, 27, 4079. e)

Baskaran, S.; Islam, I.; Vankar, P. S.; Chandrasekaran, S. J. Chem. Soc., Chem.

Commun. 1990, 1670.

Chapter III

-81-

12. Schomaker, J. M.; Travis, B. R.; Borhan B. Org. Lett. 2003, 5, 3089.

13. Harrison, B. A.; Gierasch, T. M.; Nielan C.; Pasternak, G. W.; Verdine G. L. J. Am.

Chem. Soc.2002, 124, 13352.

. 001122334455667788991010

1H NMR SPECTRUM OF COMPOUND 25 (CDCl3, 300MHz

BocHN

HO

NH

O

OMe

O

25

00252550507575100100125125150150175175200200

13C NMR SPECTRUM OF COMPOUND 25 (CDCl3, 75MHz)

BocHN

HO

NH

O

OMe

O

25

001122334455667788991010

1H NMR SPECTRUM OF COMPOUND 20 (CDCl3, 300MHz)

HN

HO

NH

O

OMe

O

O

MeO

20

00252550507575100100125125150150175175200200


HN

HO

NH

O

OMe

O

O

MeO

20

0.00.00.50.51.01.01.51.52.02.02.52.53.03.03.53.54.04.04.54.55.05.05.55.56.06.06.56.57.07.07.57.58.08.08.58.5


NHBoc

OBn

OBn

34

252550507575100100125125150150175175200200


NHBoc

OBn

OBn

34

0.00.00.50.51.01.01.51.52.02.02.52.53.03.03.53.54.04.04.54.55.05.05.55.56.06.06.56.57.07.07.57.58.08.0


NHBoc

OBn

OBn

35

252550507575100100125125150150175175200200


NHBoc

OBn

OBn

35

0.00.00.50.51.01.01.51.52.02.02.52.53.03.03.53.54.04.04.54.55.05.05.55.56.06.06.56.57.07.07.57.58.08.0


HO

HN

O

NH

O

MeO

NH

O

OBn

OBn

38

00252550507575100100125125150150175175200200


HO

HN

O

NH

O

MeO

NH

O

OBn

OBn

38

0.00.00.50.51.01.01.51.52.02.02.52.53.03.03.53.54.04.04.54.55.05.05.55.56.06.06.56.57.07.07.57.58.08.08.58.5


HO

HN

O

NH

O

MeO

NH

O

OBn

OBn

39

00252550507575100100125125150150175175200200


HO

HN

O

NH

O

MeO

NH

O

OBn

OBn

39

HO

HN

O

NH

O

MeO

NH

O

OH

MeOOC

OH

Melleumin B

0011223344556677889910101111

1H NMR SPECTRUM OF Melleumin B (DMSO, 500MHz)

HO

HN

O

NH

O

MeO

NH

O

OH

MeOOC

OH

Melleumin B

252550507575100100125125150150175175200200

13C NMR SPECTRUM OF Melleumin B (DMSO, 100MHz)

LIST OF PUBLICATIONS

1. “Total Synthesis of (+)-Mupirocin H from D-Glucose.”

Sandip P Udawant, Tushar Kanti Chakraborty*

J. Org. Chem. 2011, 76, 6331–6337

2. “Synthesis and characterization of Boc-protected 4-amino- and 5-amino-pyrrole-2-

carboxylic acid methyl esters.”

Tushar Kanti Chakraborty,* Sandip P. Udawant et. al

Tetrahedron letters 2006, 47, 4631-4634.

3. “Synthetic studies on Melleumin A, B.”

Tushar Kanti Chakraborty*, Sandip P. Udawant

(Manuscript under preparation)

Published: June 27, 2011

r 2011 American Chemical Society 6331 dx.doi.org/10.1021/jo200396q | J. Org. Chem. 2011, 76, 6331–6337

NOTE

pubs.acs.org/joc

Total Synthesis of (+)-Mupirocin H from D-Glucose†

Sandip P. Udawant‡ and Tushar Kanti Chakraborty*,§

‡CSIR-Indian Institute of Chemical Technology, Hyderabad 500607, India§CSIR-Central Drug Research Institute, Lucknow 226001, India

bS Supporting Information

Mupirocin, a naturally occurring metabolite isolated fromPseudomonas fluorescens (a soil isolate, NCIB 10586)

proved highly efficient in treatment of skin infections and oneof the most successful topical antibiotics for eradication of nasalStaphylococcus aureus including methicillin-resistant Staphylococ-cus aureus (MRSA).1 Mupirocin is a mixture of pseudomonicacids2 with principal constituent pseudomonic acid A accountingfor 90%, and the other components present are pseudomonicacids B, C, and D shown in Figure 1. Structural complexityand important biological activities of these pseudomonic acidsattracted synthetic organic chemists resulting in many totalsyntheses of these molecules.3 Studies on biosynthesis of pseu-domonic acids from Pseudomonas fluorescens and its biosyntheticgene cluster resulted in isolation of novel pseudomonic acidanalogues mupirocin W4 (1) and mupirocin H5 (2), amongwhich 1 has shownmoderate bioactivity similar to those of earlierreported pseudomonic acids. Despite excellent antibiotic proper-ties of mupirocin, it is associated with poor metabolic stabilityand poor bioavailability restricting it to be used as topicalantibiotic. Moreover emerging resistance to mupirocin is also amatter of serious concern.6 In this scenario, development ofbetter alternatives to mupirocin is significant that led us toperform total synthesis of mupirocin H. The versatility of thepresent route lies in the fact that three out of its six chiral centersare derived from D-glucose followed by late-stage introduction ofits trans double bond making this pathway useful for totalsynthesis of many other structurally similar molecules likepseudomonic acids and analogues, mupirocin W, antibioticthiomarinols,7 and their analogues for biological evaluations.The common core for all these molecules, shown in red inFigure 1, can be derived using the present strategy.

Our retrosynthetic analysis is illustrated in Scheme 1. Weplanned to generate the γ-lactone functionality in the final stepof our synthesis by acid-catalyzed oxidative lactonization of4-hydroxynitrile moiety 3, whose E-olefinic linkage suggested

Figure 1. Chemical structures of pseudomonic acids, selected thiomar-inols, and mupirocin W, H having a common core, shown in red, whichcould be derived using the present strategy.

Received: March 4, 2011

ABSTRACT: Enantioselective total synthesis of mupirocin H is accomplishedstarting from D-glucose featuring strategic application of D-glucose derivedchirality, diastereoselective Still�Barrish hydroboration, and further elaborationof carbon chain to furnish a phenyltetrazolyl sulfone intermediate, which oncoupling with (2S,3S)-2-methyl-3-(triisopropylsilyloxy)butanal under Julia�Kocienski olefination conditions gave an advanced E-olefinic intermediateselectively. The E-olefin was transformed to the 4-hydroxynitrile, a prefinalsubstrate, which on acid-catalyzed oxidative lactonization furnished the targetmolecule mupirocin H in 19 steps from known compound 6 (longest linearsequence) with an overall yield of 4.96%.

6332 dx.doi.org/10.1021/jo200396q |J. Org. Chem. 2011, 76, 6331–6337

The Journal of Organic Chemistry NOTE

disconnection of double bond to Julia�Kocienski olefinationprecursors, sulfone 4, and aldehyde 7. Further simplification of 4via Still�Barrish hydroboration, one-carbon homologation,Mitsunobu reaction, and peroxide oxidation resulted in terminalalkene precursor 5, which could easily be resourced from D-glucose-derived known alcohol 6. On the other hand, aldehyde 7was planned to be derived from another known alcohol 8.

To synthesize sulfone 4, as depicted in Scheme 2, we beganwith D-glucose which was transformed into alcohol 6 using aknown procedure.8 This alcohol 6 was subjected to Swernoxidiation9 conditions to give the corresponding keto intermedi-ate which under one-carbon Wittig olefination10 conditionsafforded the olefin 9 in 72% yield. Treatment of 9 with 50%aqueous TFA followed by LiBH4 reduction afforded triol 10,which was subjected to selective TBDPS protection to give 11 in75% yield over three steps. Compound 11 on treatment with 2,2-dimethoxypropane in presence of CSA (cat.) gave the terminalalkene 5, the Still�Barrish hydroboration precursor, in 95%yield. Thus, 5 on treatment with 9-borabicyclononane (9-BBN)in dry THF at 0 �C followed by treatment with H2O2 and NaOHgave 12 as a single diastereomer in 85% yield.11

In the course of the above hydroboration reaction, stericbulkiness of the boron ligand in 9-BBN oriented the reagent tosuch a face of the alkene 5, which ensured minimum interactionswith allylic hydroxyl (here protected as its acetonide) group ofalkene 5, thereby imparting maximum stability to that transitionstate (TS). The above facial bias can be explained using theHouk12 rationale for two possible TSs as depicted in Figure 2.13

As seen in TS-2, the allylic acetonide protected hydroxyl group ofalkene and the boron ligands are in eclipsed form generatingsteric repulsive strain which is destabilizing this TS making itnonfavored, whereas in TS-1 the allylic acetonide-protectedhydroxyl group of alkene and the boron ligands is in noneclipsedform, making this TS more stable and favored, ultimately result-ing in the formation of single diastereomer 12 in this reaction.The diastereoselection observed in the Still�Barrish hydrobora-tion is widely documented, and many examples of such hydro-boration are reported.14

The alcohol 12 was tosylated with TsCl and triethylamine togive a tosylate, which on heating with an excess of NaCN andcatalytic amount of NaI in DMSO produced 13 in 80% yield.Intermediate 13 on reaction with DIBAL-H at �78 �C formedintermediate aldehyde,15 which on reduction with NaBH4 gavethe alcohol 14 in 89% yield. Mitsunobu reaction of 14 and1-phenyl-1-1H-tetrazole-5-thiol (PT-SH) formed sulfide 15 in86% yield, which on oxidation with H2O2 and ammoniummolybdate afforded the sulfone 4 in 82% yield.

Synthesis of the aldehyde 7, depicted in Scheme 3, was startedfrom the known compound 8, which was prepared from(E)-4-(benzyloxy)but-2-en-1-ol by employing the Sharpless asym-metric epoxidation reaction, followed by dimethylcopper lithium

Scheme 1. Retrosynthetic Analysis of Mupirocin H (2) Scheme 2. Synthesis of Sulfone 4

Figure 2. Transition states for hydroboration of alkene 5.



mediated regioselective epoxide opening.16 The diol 8 on selec-tive protection of a primary hydroxyl group as TBDMS etherfollowed by protection of secondary hydroxyl group as a TIPSether gave the fully protected compound 17 in 90% overall yieldin two steps. Compound 17 on hydrogenation in presence ofPd�C in ethyl acetate gave 18 in 91% yield. Here we sought theremoval of the free hydroxyl group of 18. Hence, it was convertedto tosyl derivative, which on treatment with LiEt3BH (Super-hydride) in dry THF gave 19 in 86% yield.17 Compound 19 ontreatment with HF�pyridine complex in THF gave the alcohol20, which on Dess�Martin periodinane oxidation18 furnishedaldehyde 7.

With sulfone 4 and aldehyde 7 in hand, the stage was set forthe crucial Julia�Kocienski olefination.19 Accordingly, 4 wastreated with KHMDS in dry THF at �78 �C, followed by slowaddition of 7 over 10 min period, and the same temperature wasmaintained for 1.5 h. It was then warmed to rt and further stirredfor 12 h to furnish the E-olefinic compound 21 in 76% yield (withtrace amounts of Z-isomer which was separated by simplecolumn chromatography). Compound 21 was then treated with3 N KOH in MeOH solution under refluxing conditions to givethe TBDPS-deprotected compound20 22 in 84% yield, which onLi�liquid ammonia mediated benzyl deprotection21 affordeddiol 23 in 72% yield. The diol on selective monotosylation22 gaveprimary O-tosylate, which was heated with NaCN and catalyticamount of NaI in DMSO to give our prefinal substrate 3 in 77%yield. This compound with terminal 4-hydroxy-1-nitrile moietywas proven to be ideal for our end game. Thus, it was heated with5% concd HCl in MeOH solution23 generating the required γ-lactone functionality with concomitant removal of diacetonideand triisopropylsilyl (TIPS) protecting groups, all in one pot, togive our target molecule mupirocin H in 64% yield (Scheme 4).The 1H and 13C NMR spectra and specific rotation of syntheticmupirocin H (2) were found to be in good agreement with thereported values.5

In summary, we have achieved the total synthesis of mupirocinH using D-glucose as chiral pool material and Still�Barrishhydroboration and Julia�Kocienski olefination as key steps.Three chiral centers out of six were obtained from D-glucose.The convergent strategy developed here can be utilized in thetotal synthesis of other natural products of the same family likepseudomonic acids, mupirocin W, and thiomarinols. Currentlywe are working in this direction.

’EXPERIMENTAL SECTION

(3aR,5R,6R,6aR)-6-(Benzyloxy)-2,2-dimethyl-5-(prop-1-en-2-yl)tetrahydrofuro[2,3-d][1,3]dioxole (9). (R)-1-((3aR,5R,6R,6aR)-6-(Benzyloxy)-2,2-dimethyltetrahydrofuro[2,3-d][1,3]dioxol-5-yl)ethanol(6) was prepared fromD-glucose using a knownprocedure.8 Alcohol6 (8.0 g,27.37 mmol) was subjected to Swern oxidation reaction using same proce-dure as described in our previous work24 to give the corresponding ketone.The ketone (Rf = 0.60, 30% EtOAc in PE), thus obtained, was directly usedafter flash chromatography for the next reaction.

To a suspension of methyltriphenylphophonium iodide (22.13 g,54.74 mmol) in dry THF (150 mL) at 0 �C was added n-BuLi (1.6 M inhexane, 34.21 mL, 54.74 mmol) slowly. After being stirred at the sametemparature for 10 min, the mixture was warmed to rt rapidly, stirred for25 min, and then cooled to �50 �C, and to this was added slowly theabove ketone dissolved in dry THF (25 mL). The mixture was stirred atthe same temperature for 1 h, then gradually warmed to rt, and stirredfor 24 h. Water was added to the mixture, which was extracted withEtOAc, washed with brine, dried (Na2SO4), and concentrated in vacuo.Purification by column chromatography (SiO2, 8% EtOAc in PE)yielded compound 9 as a colorless oil (5.72 g, 72%): Rf = 0.6 (SiO2,30% EtOAc in PE); [R]24D = +60.5 (c 2.62, CHCl3); IR νmax 2924, 2857,1726, 1513, 1454 cm�1; 1H NMR (500 MHz, CDCl3) δ 7.31�7.26(m, 5H), 5.70 (d, J = 3.9 Hz, 1H), 5.12 (s, 1H), 4.94 (s, 1H), 4.70 (d, J =11.6 Hz, 1H), 4.54 (d, J = 11.6 Hz, 1H), 4.52 (dd, J = 3.9, 3.8 Hz, 1H),4.39 (d, J = 8.7 Hz, 1H), 3.52 (dd, J = 8.7, 3.9 Hz, 1H), 1.68 (s, 3H), 1.60(s, 3H), 1.35 (s, 3H); 13CNMR (75MHz, CDCl3)δ 141.2, 137.4, 128.2,127.7, 114.2, 112.6, 103.5, 81.1, 79.7, 77.4, 71.8, 26.6, 26.4, 17.4;MSm/z308 [M + NH4]

+; HRMS calcd for C17H22O4Na [M + Na]+ 313.1416,found 313.1422.(2S,3R,4R)-3-(Benzyloxy)-1-(tert-butyldiphenylsilyloxy)-5-

methylhex-5-ene-2,4-diol (11). Aqueous TFA (50%, 18.25 mL,1 mL/mmol) was added to compound 9 (5.30 g, 18.25 mmol) at 0 �C,and the resulting mixture was allowed to warm to rt and stirred for 3 h.Then TFA was removed in vacuo, and the residue was dissolved inDCM, washed with saturated aq NaHCO3 solution and brine, dried(Na2SO4), and concentrated in vacuo. The lactol (Rf = 0.60, 50% EtOAcin PE), thus obtained, was directly used after flash chromatography fornext reaction.

The lactol obtained above was dissolved in THF/MeOH (1:1,60 mL) and cooled to 0 �C, and LiBH4 (0.795 g, 36.50 mmol) wasslowly added. The reaction mixture was then allowed to warm to rt,

Scheme 3. Synthesis of Aldehyde 7 Scheme 4. Julia�Kocienski Coupling between Sulfone 4 andAldehyde 7 and Completion of Synthesis



stirred for 3 h, quenched slowly using saturated NH4Cl solution,extracted with DCM, washed with brine, dried (Na2SO4), and concen-trated in vacuo. The (2S,3R,4R)-3-(benzyloxy)-5-methylhex-5-ene-1,2,4-triol (10) (Rf = 0.30, 60% EtOAc in PE), thus obtained, wasdirectly used after flash chromatography for next reaction.

Triol 10 was subjected to selective mono-TBDPS protection usingthe same procedure as described in our previous work.25 Purificationby column chromatography (SiO2, 10% EtOAc in PE) afforded 11 as acolorless oil (6.72 g, 75% over three steps):Rf= 0.4 (SiO2, 20% EtOAc inPE); [R]24D = �6.44 (c 6.1, CHCl3); IR νmax 3565, 3421(br), 2929,1461 cm�1; 1H NMR (300 MHz, CDCl3) δ 7.67�7.64 (m, 5H),7.46�7.33, (m, 6H), 7.28�7.23 (m, 2H), 7.17�7.14 (m, 2H), 5.10(s, 1H), 4.97 (s, 1H), 4.60 (d, J = 11.3 Hz, 1H), 4.47 (d, J = 10.6 Hz, 1H),4.31 (d, J = 6.0 Hz, 1H), 3.93�3.88 (m, 2H), 3.79 (dd, J = 11.3, 6.8 Hz,1H), 3.63 (dd, J = 11.3, 5.3 Hz, 1H), 2.85 (bs, 1H), 1.80 (s, 3H), 1.08(s, 9H); 13C NMR (75 MHz, CDCl3) δ 144.3, 137.8, 135.5, 133.0,132.8, 129.8, 128.3, 127.9, 127.8, 113.1, 79.7, 76.2, 73.6, 73.1, 64.7, 26.8,19.2, 19.0; MS m/z 513 [M + Na]+; HRMS calcd for C30H38O4SiNa[M + Na]+ 513.2437, found 513.2426.(((4S,5R,6R)-5-(Benzyloxy)-2,2-dimethyl-6-(prop-1-en-

2-yl)-1,3-dioxan-4-yl)methoxy)(tert-butyl)diphenylsilane (5).Diol 11 (6.50 g, 13.25 mmol) was protected to give 5 using the sameprocedure as described in our previous work.24 Purification by columnchromatography (SiO2, 2 to 5% EtOAc in PE) afforded compound 5 as acolorless oil (6.68 g, 95%): Rf = 0.4 (SiO2, 10% EtOAc in PE);[R]24D = �41.0 (c 3.63, CHCl3); IR νmax 2932, 2859, 1462, 1381 cm

�1;1H NMR (300 MHz, CDCl3) δ 7.80�7.70 (m, 5H), 7.44�7.33 (m, 6H),7.31�7.20 (m, 4H), 5.24 (s, 1H), 5.08 (d, J = 1.5 Hz, 1H), 4.59(d, J = 10.6 Hz, 1H), 4.50 (d, J = 9.8 Hz, 1H), 4.24 (d, J = 9.1 Hz, 1H),3.98 (dd, J = 11.3, 3.8 Hz, 1H), 3.84 (dd, J = 11.3, 1.5 Hz, 1H), 3.78(m, 1H), 3.69 (dd, J = 9.1, 9.1 Hz, 1H), 1.89 (s, 3H), 1.48 (s, 3H), 1.47(s, 3H), 1.08 (s, 9H); 13CNMR (75MHz, CDCl3)δ 143.0, 137.9, 135.9,135.6, 134.0, 133.4, 129.5, 128.3, 128.0, 127.8, 127.6, 127.4, 115.7, 98.2,77.4, 74.0, 73.6, 71.4, 63.5, 29.3, 26.8, 19.4, 17.7; MS m/z 548 [M +NH4]

+; HRMS calcd for C33H42O4SiNa [M + Na]+ 553.2750, found553.2740.(R)-2-((4R,5R,6S)-5-(Benzyloxy)-6-((tert-butyldiphenylsily-

loxy)methyl)-2,2-dimethyl-1,3-dioxan-4-yl)propan-1-ol (12). Asolution of 9-BBN (0.5 M in THF, 67.8 mL, 33.9 mmol) was slowly addeddropwise at 0 �C to a solution of alkene (5) (6.0 g, 11.3 mmol) in dry THF(30 mL). After the addition, the mixture was allowed to warm to rt, stirredfor 12 h, and then treated with EtOH (21.1 mL), aqueous NaOH (3 N,14 mL), and 30% aqueous H2O2 (14 mL) and stirred at rt for 2 h. Themixture was saturated with solid K2CO3 and extracted with Et2O. Com-bined organic layers were washed with brine, dried (Na2SO4), filtered, andconcentrated in vacuo. Purification by column chromatography (SiO2, 10%EtOAc in PE) afforded compound 12 as colorless oil (5.27 g, 85%):Rf= 0.5(SiO2, 20% EtOAc in PE); [R]24D =�4.4 (c 3.96, CHCl3); IR νmax 3530,2931, 2865, 1427 cm�1; 1H NMR (300 MHz, CDCl3) δ 7.80�7.70(m, 5H), 7.45�7.22 (m, 10H), 4.72 (d, J = 11.3 Hz, 1H), 4.59 (d, J = 10.6Hz, 1H), 3.98 (dt, J = 11.3, 3.0 Hz 1H) 3.91�3.77 (m, 4H) 3.72 (m, 1H),3.55 (dd, J = 10.6, 3.8 Hz, 1H), 2.64 (bs, 1H), 2.04 (m, 1H) 1.43 (s, 3H),1.42 (s, 3H), 1.14 (d, J = 7.5 Hz, 3H), 1.10 (s, 9H); 13C NMR (75 Hz,CDCl3) δ 137.6, 135.9, 135.6, 133.8, 133.2, 129.5, 128.5, 127.9, 127.6,127.4, 98.5, 77.6, 74.4, 74.3, 71.5, 64.2, 63.4, 34.5, 29.3, 26.8, 19.4, 18.9, 14.8;MS m/z 566 [M + NH4]

+; HRMS calcd for C33H44O5SiNa [M + Na]+:571.2855, found 571.2850.(R)-3-((4R,5R,6S)-5-(Benzyloxy)-6-((tert-butyldiphenylsily-

loxy)methyl)-2,2-dimethyl-1,3-dioxan-4-yl)butanenitrile (13).To a solution of 12 (5.0 g, 9.11mmol) inDCM(30mL)was added Et3N(3.81 mL, 27.33mmol) followed by TsCl (2.084 g, 10.93 mmol) at 0 �C.After being stirred at the same temperature for 15 min, DMAP (0.111 g,0.91 mmol) was added, and the reaction mixture was allowed to warm tort, then stirred for 12 h followed by quenching with saturated aqueous

solution of NH4Cl and extracted with EtOAc. The combined organiclayers were washed with water and brine, dried (Na2SO4), and con-centrated in vacuo. The tosylate (Rf = 0.5, 10% EtOAc in PE), thusobtained, was directly used after flash chromatography for the nextreaction.

To a solution of above tosylate in dry DMSO (30 mL) were addedsodium cyanide (3.57 g, 72.88 mmol) followed by sodium iodide(0.136 g, 0.911 mmol), and the reaction mixture was stirred at 90 �C for 2 h.After being cooled to rt, the mixture was diluted with EtOAc, washedwith water and brine, dried (Na2SO4), and concentrated in vacuo.Purification by column chromatography (SiO2, 5% EtOAc in PE) affordedcompound 13 as colorless oil (4.06 g, 80% over two steps): Rf = 0.6(SiO2, 10% EtOAc in PE); [R]24D = +13.34 (c 3.45, CHCl3); IR νmax

2953, 2864, 2362, 2324, 1695, 1516 cm�1; 1H NMR δ 7.79�7.70 (m,5H), 7.45�7.24 (m, 10H), 4.76 (d, J = 11.3 Hz, 1H), 4.60 (d, J = 10.6,Hz, 1H), 4.00 (dd, J = 11.3, 3.0 Hz, 1H), 3.86 (dd, J = 11.3, 1.5 Hz, 1H),3.71�3.56 (m, 3H), 2.34�2.20 (m, 3H), 1.40 (s, 6H), 1.18 (d, J = 6.8Hz, 3H), 1.10 (s, 9H); 13C NMR (75 MHz, CDCl3) δ 137.4, 135.9,135.6, 134.7, 133.7, 133.1, 129.6, 128.6 128.2, 128.1, 127.6, 127.4, 119.7,98.3, 74.6, 74.1, 70.7, 63.3, 31.1, 29.0, 26.8, 19.4, 19.1, 18.2, 17.4;MSm/z575 [M + NH4]

+; HRMS calcd for C34H43NO4SiNa [M + Na]+

580.2984, found 580.2980.(R)-3-((4R,5R,6S)-5-(Benzyloxy)-6-((tert-butyldiphenylsily-

loxy)methyl)-2,2-dimethyl-1,3-dioxan-4-yl)butan-1-ol (14).Nitrile 13 (3.90 g, 6.99 mmol) was converted to alcohol 14 using atwo-step protocol viz. DIBAL-H mediated conversion of nitrile toaldehyde, followed by NaBH4 reduction using the same procedure asdescribed in our previous work.24 Purification by column chromatogra-phy (SiO2, 18% EtOAc in PE) afforded compound 14 as a colorless oil(3.5 g, 89%): Rf = 0.4 (SiO2, 20% EtOAc in PE); [R]24D =�3.4 (c 0.35,CHCl3); IR νmax 3618, 2933, 2856, 1694, 1462 cm�1; 1H NMR (300MHz, CDCl3) δ 7.80�7.71 (m, 5H), 7.44�7.22 (m, 10H), 4.70 and4.64 (ABq, J = 10.6 Hz, 2H), 3.98 (dd, J = 11.3, 3.0 Hz, 1H), 3.86 (dd, J =11.3, 1.5 Hz, 1H), 3.80�3.68 (m, 4H), 3.58 (m, 1H), 2.43 (bs, 1H), 2.19(m, 1H), 1.81�1.62 (m, 2H), 1.43 (s, 6H), 1.10 (s, 9H), 1.04 (d, J =6.8 Hz, 3H); 13C NMR (75 MHz, CDCl3) δ 137.9, 135.9, 135.6, 133.8,133.3, 129.5, 128.4, 127.9, 127.8, 127.5, 127.4, 98.3, 76.5, 74.3, 74.2, 71.0,63.5, 59.8, 32.7, 29.9, 29.0, 26.8, 19.3, 19.2, 16.6; MS m/z 585 [M +Na]+; HRMS calcd for C34H46O5SiNa [M + Na]+ 585.3012, found585.3000.5-((R)-3-((4R,5R,6S)-5-(Benzyloxy)-6-((tert-butyldiphenyl-

silyloxy)methyl)-2,2-dimethyl-1,3-dioxan-4-yl)butylthio)-1-phenyl-1H-tetrazole (15).Triphenylphosphine (3.84 g, 14.65mmol),1-phenyl-1-1H-tetrazole-5-thiol (1.57 g, 8.79 mmol), and the alcohol 14(3.30 g 5.86 mmol) were dissolved in dry THF (18 mL). This solutionwas cooled to 0 �C, and diisopropyl azodicarboxylate (DIAD, 2.88 mL,14.65 mmol) was slowly added and then the mixture allowed to warm to rt.After 12 h of stirring, the reactionmixture was directly concentrated in vacuo.Purification by column chromatography (SiO2, 12% EtOAc in PE) affordedcompound 15 as a colorless oil (3.64 g, 86%): Rf = 0.5 (SiO2, 20% EtOAc inPE); [R]24D=+31.0 (c 0.73,CHCl3); IRνmax 2930, 2858, 1795, 1499 cm�1;1H NMR (300 MHz, CDCl3) δ 7.71�7.61 (m, 5H), 7.44�7.38 (m, 5H),7.35�7.23 (m, 5H), 7.22�7.11 (m, 5H), 4.58 and 4.52 (ABq, J = 10.9 Hz,2H), 3.88 (dd, J = 11.5, 2.8 Hz, 1H), 3.76 (d, J = 11.5 Hz, 1H), 3.66�3.58(m, 3H), 3.49 (m, 1H), 3.19 (m, 1H), 2.07�1.94 (m, 2H), 1.74 (m, 1H),1.33 (s, 3H), 1.32 (s, 3H), 1.01 (d, J = 6.8 Hz, 3H), 0.98 (s, 9H); 13C NMR(75 MHz, CDCl3) δ 154.3, 137.8, 135.9, 135.6, 133.8, 133.6, 133.3, 129.9,129.6, 129.5, 128.4, 127.8, 127.6, 127.4, 123.7, 98.1, 76.4, 74.2, 70.8, 63.6, 32.2,31.7, 29.4, 29.2, 26.8, 19.3, 19.2, 16.5; MS ESIMS (m/z)C41H50N4O4SSiNa[M +Na]+ 745; HRMS calcd for C41H50N4O4SSiNa [M +Na]+ 745.3219,found 745.3197.5-((R)-3-((4R,5R,6S)-5-(Benzyloxy)-6-((tert-butyldiphenyl-

silyloxy)methyl)-2,2-dimethyl-1,3-dioxan-4-yl)butylsulfonyl)-1-phenyl-1H-tetrazole (4). To a solution of compound 15 (3.40 g,



4.70 mmol) in absolute EtOH (15 mL) at 0 �C was added a premixedsolution of (NH4)6Mo7O24 3 4H2O (1.89 g, 7.05mmol) inH2O2 (30% aq,9.6 mL, 94 mmol) using a glass pipet. The resulting yellow solution wasthen removed from the ice bath and allowed to warm to rt. After 12 h,saturated aqueous NaHCO3 solution was added to the reaction mixtureand the aqueous layer was extracted with Et2O. The combined organiclayers were washed with brine, dried (Na2SO4), and concentrated invacuo. Purification by column chromatography (SiO2, 15% EtOAc in PE)afforded compound 4 as colorless oil (2.91 g, 82%): Rf = 0.4 (SiO2, 20%EtOAc in PE); [R]24D = +20.3 (c 1.75, CHCl3); IR νmax 2991, 2933, 1462,1384 cm�1 ; 1H NMR (300 MHz, CDCl3) δ 7.71�7.57 (m, 6H),7.56�7.46 (m, 4H), 7.36�7.16 (m, 10H), 4.64 (d, J = 10.9 Hz, 1H), 4.54(d, J = 10.9 Hz, 1H), 3.89 (dd, J = 11.3, 3.0 Hz, 1H), 3.83�3.73 (m, 2H),3.66�3.56 (m, 3H), 3.48 (m, 1H), 2.07�1.95 (m, 2H), 1.88 (m, 1H),1.33 (s, 6H), 0.99�1.01 (m, 12H); 13CNMR (75MHz, CDCl3) δ 153.4,137.8, 135.9,135.6, 133.8, 133.2, 131.3, 129.6, 129.5, 128.5, 128.0, 127.6,127.4, 125.0, 98.3, 76.1, 74.3, 71.1, 63.5, 54.2, 31.6, 29.2, 26.8, 22.6, 19.4,19.1, 16.5; MS m/z 777 [M + Na]+; HRMS calcd for C41H54N5O6SiS[M + NH4]

+ 772.3564, found 772.3548.(2R,3R)-1-(Benzyloxy)-4-(tert-butyldimethylsilyloxy)-3-

methylbutan-2-ol (16). Compound 8 (5.50 g, 26.16 mmol), pre-pared using a known procedure,16 was selectively protected as primaryTBDMS ether using the same procedure as described in our previouswork.26 Purification by column chromatography (SiO2, 10% EtOAc inPE) afforded pure compound 16 as colorless oil (7.96 g, 94%): Rf = 0.6(SiO2, 10% EtOAc in PE); [R]24D =�5.0 (c 2.28, CHCl3); IR νmax 3480(br), 2929, 1531 cm�1; 1H NMR (300 MHz, CDCl3) δ 7.28�7.17 (m,5H), 4.52 and 4.46 (ABq, J = 12.1 Hz, 2H), 3.68�3.62 (m, 2H),3.56�3.39 (m, 4H), 1.80 (m, 1H), 0.80�0.83 (m, 12H),�0.02 (s, 6H);13C NMR (75 MHz, CDCl3) δ 138.1, 128.3, 127.6, 127.5, 74.2, 73.3,72.7, 66.7, 37.2, 25.7, 18.0, 13.3, �5.7; MS m/z 325 [M + H]+; HRMScalcd for C18H33O3Si [M + H]+ 325.2198, found 325.2206.(6R,7R)-7-(Benzyloxymethyl)-9,9-diisopropyl-2,2,3,3,6,-

10-hexamethyl-4,8-dioxa-3,9-disilaundecane (17). Compound16 (7.0 g, 21.57 mmol) was converted to TIPS ether 17 using thesame procedure as described in a previously published work27 withTIPS-OTf and 2,6-lutidine. Purification by column chromatogra-phy (SiO2, 1% EtOAc in PE) afforded compound 17 as a colorless liquid(9.95 g, 96%): Rf = 0.5 (SiO2, in PE); [R]24D =�3.7 (c 0.9, CHCl3); IRνmax 2932, 2862, 1725, 1464 cm�1; 1H NMR (300 MHz, CDCl3) δ7.53�7.18 (m, 5H), 4.42 (s, 2H), 4.28 (m, 1H), 3.59 (m, 1H), 3.54�3.36(m, 3H), 1.92 (m, 1H), 1.00�1.01 (m, 21H,), 0.81�0.86 (m, 12H),0.03 to�0.06 (m, 6H); 13CNMR(75MHz, CDCl3) δ 138.5, 128.1, 127.6,127.3, 73.1, 73.0, 72.8, 64.9, 40.7, 25.9, 18.1, 12.7, 12.4,�5.5;MSm/z 481[M +H]+; HRMS calcd for C27H52O3Si2Na [M + Na]+ 503.3353, found503.3365.(2R,3R)-4-(tert-Butyldimethylsilyloxy)-3-methyl-2-(triiso-

propylsilyloxy)butan-1-ol (18). To a stirred solution of 17 (8.0 g,16.64 mmol) in EtOAc (50 mL) was added catalytic Pd�C (10%), andthe mixture was hydrogenated overnight using a H2-filled balloon. It wasthen filtered through a short pad of Celite, and the filter cake was washedwith EtOAc. The filtrate and washings were combined and concentratedin vacuo. Purification by column chromatography (SiO2, 6% EtOAc inPE) afforded compound 18 as a colorless liquid (5.91 g, 91%): Rf = 0.3(SiO2, 10% EtOAc in PE); [R]24D = +16.4 (c 2.77, CHCl3); IR νmax

3496 (br), 2935, 2864, 1465 cm�1; 1HNMR (300MHz, CDCl3) δ 3.98(m, 1H), 3.68�3.53 (m, 4H), 3.01 (bs, 1H), 2.02, (m, 1H), 1.09�1.03(m, 21H), 0.93 (d, J = 6.8 Hz, 3H), 0.89 (s, 9H), 0.06 (s, 6H); 13CNMR(75 MHz, CDCl3) δ 74.3, 64.8, 63.5, 40.0, 25.8, 18.1, 12.6, 11.9,�5.58,�5.62; MS m/z 391 [M + H]+; HRMS calcd for C20H46O3Si2Na[M + Na]+ 413.2883, found 413.2900.(6R,7S)-9,9-Diisopropyl-2,2,3,3,6,7,10-heptamethyl-4,8-

dioxa-3,9-disilaundecane (19). Compound 18 (5.50 g, 14.07 mmol)was subjected to tosylation following the same procedure as per the

synthesis of compound 13. The tosylate (Rf = 0.6, 10% EtOAc in PE), thusobtained, was directly used after flash chromatography for next reaction.

A solution of the above tosylate in dry THF (75 mL) at 0 �C wastreated with lithium triethylborohydride (1.0 M in THF, 70.35 mL,70.35 mmol) slowly. After being stirred for 5 h at the same temperature,the reaction mixture was quenched with methanol (24 mL) and 1 NNaOH (24 mL). The mixture was treated with 30% aqueous H2O2

(8 mL), stirred for 16 h at rt, and then concentrated in vacuo. Theresidue was taken up in saturated aqueous NaHCO3 solution andextracted with Et2O. The combined organic extracts were washed withbrine, dried (Na2SO4), filtered, and concentrated in vacuo. Purificationby column chromatography (SiO2, 1% EtOAc in PE) afforded pure 19(4.54 g, 86%) as a colorless oil: Rf = 0.6 (silica gel, 10% EtOAc in PE);[R]24D = +3.0 (c 1.79, CHCl3); IR νmax 2993, 2862, 1464 cm�1; 1HNMR (300 MHz, CDCl3) δ 4.08 (m, 1H), 4.52�4.42 (m, 2H), 1.86(m, 1H), 1.07�1.06 (m, 24H), 0.88 (s, 9H), 0.84 (d, J = 6.8 Hz, 3H),0.04�0.02 (m, 6H); 13C NMR (75 MHz, CDCl3) δ 68.8, 65.6, 43.1,25.9, 18.5, 18.2, 12.6, 10.8,�5.5,�5.4; MS m/z 375 [M + H]+; HRMScalcd for C20H47O2Si2 [M + H]+ 375.3114, found 375.3125.(2R,3S)-2-Methyl-3-(triisopropylsilyloxy)butan-1-ol (20).

Compound 20 was synthesized from compound 19 (4.20 g, 11.21mmol), following the same procedure as described in our previouswork.24 Purification by column chromatography (SiO2, 4% EtOAc inPE) afforded the pure alcohol 20 (2.48 g, 85%) as a colorless oil.Analytical data of 20 was matched with literature values.28

(((4S,5R,6R)-5-(Benzyloxy)-2,2-dimethyl-6-((2R,6R,7S,E)-6-methyl-7-(triisopropylsilyloxy)oct-4-en-2-yl)-1,3-dioxan-4-yl)-methoxy)(tert-butyl)diphenylsilane (21). Alcohol 20 (1.12 g,4.30 mmol) was subjected to Dess�Martin oxidation using the sameprocedure as described in our previous work.24 The (2S,3S)-2-methyl-3-(triisopropylsilyloxy)butanal (7) (Rf = 0.6, 10% EtOAc in PE), thusobtained, was directly used after flash chromatography for nextreaction.

A solution of sulfone 4 (2.70 g, 3.58 mmol) in dry THF (50 mL)at �78 �C was treated with dropwise addition of potassium bis-(trimethylsilyl)amide (KHMDS, 0.5M in toluene, 14.32mL, 7.16mmol)under nitrogen. The resulting yellow solution was stirred at�78 �C for1 h, and then a solution of aldehyde 7 in dry THF (20 mL) was slowlyadded via syringe over 10 min. The reaction mixture was stirredat �78 �C for 1.5 h and then at rt for overnight. The cloudy whitereaction mixture was quenched with water, and extracted with EtOAc,and the organic layer was washed with brine, dried (Na2SO4), filtered,and concentrated in vacuo. Purification by column chromatography(SiO2, 2% EtOAc in PE) afforded compound 21 as a colorless liquid(2.14 g, 76%): Rf = 0.3 (SiO2, 5% EtOAc in PE); [R]24D = +9.43 (c 1.24,CHCl3); IR νmax 2935, 2865, 1712, 1462 cm�1; 1H NMR (300 MHz,CDCl3) δ 7.73�6.63 (m, 5H), 7.36�7.16 (m, 10H), 5.36�5.24(m, 2H), 4.62 (d, J = 10.6 Hz, 1H), 4.54 (d, J = 11.3 Hz, 1H), 3.91(dd, J = 11.3, 3.0 Hz, 1H), 3.87�3.75 (m, 2H), 3.71�3.57 (m, 3H),2.27�2.17 (m, 2H), 1.93�1.80 (m, 2H), 1.34 (s, 6H), 1.01�0.96 (m,33H), 0.93�0.90 (m, 6H); 13CNMR (75MHz, CDCl3) δ 138.0, 136.0,135.6, 134.0, 133.3, 129.5, 129.2, 128.4, 127.8, 127.7, 127.6, 127.4, 98.1,76.2, 74.3, 71.9, 71.3, 63.6, 44.1, 33.7, 33.4, 29.3, 26.8, 19.4, 19.3, 19.1,18.1, 16.8, 14.1, 12.5; MS m/z 810 [M + Na]+; HRMS calcd forC48H78NO5Si2 [M + NH4]

+ 804.5418, found 804.5445.((4S,5R,6R)-5-(Benzyloxy)-2,2-dimethyl-6-((2R,6R,7S,E)-6-

methyl-7-(triisopropylsilyloxy)oct-4-en-2-yl)-1,3-dioxan-4-yl)methanol (22). Compound 21 (500 mg, 0.63 mmol) was treatedwith 3 NKOH solution inMeOH (3mL), and the resulting mixture wasrefluxed at 70 �C for 12 h. The reaction mixture was then dilutedwith EtOAc, washed with water and brine, dried (Na2SO4), filtered,and concentrated in vacuo. Purification by column chromatography(SiO2, 10% EtOAc in PE) afforded compound 22 as a colorless liquid(289.3 mg, 84%): Rf = 0.3 (SiO2, 10% EtOAc in PE); [R]24D = +20.2



(c 0.96, CHCl3); IR νmax 3619, 2931, 2866, 1741, 1461 cm�1; 1H NMR

(300 MHz, CDCl3) δ 7.37�7.27 (m, 5H), 5.42�5.28 (m, 2H), 4.61(s, 2H), 3.90 (m, 1H), 3.83�3.65 (m, 4H), 3.48 (dd, J = 9.4, 9.3Hz, 1H),2.33�2.19 (m, 2H), 2.04 (m, 1H), 1.96�1.80 (m, 2H), 1.44 (s, 3H),1.38 (s, 3H), 1.07�1.03 (m, 24H), 1.01�0.96 (m, 6H); 13C NMR (75MHz, CDCl3) δ 137.7, 134.1, 129.0, 128.5, 128.0, 98.4, 76.4, 74.5, 73.7,71.8, 71.3, 62.3, 44.1, 33.4, 33.3, 29.3, 19.4, 19.1, 18.2, 16.9, 14.2, 12.5;MS m/z 571 [M + Na]+; HRMS calcd for C32H56O5Si Na [M + Na]+

571.3794, found 571.3774.(4S,5R,6R)-4-(Hydroxymethyl)-2,2-dimethyl-6-((2R,6R,7S,

E)-6-methyl-7-(triisopropylsilyloxy)oct-4-en-2-yl)-1,3-diox-an-5-ol (23). A solution of lithium in liquid ammonia was prepared bystirring lithium (50 mg, 7.2 mmol, cleaned by successive dipping inhexane and ether, and cut into small pieces) and liquid ammonia(∼20 mL) at�78 �C for 0.5 h. To this dark blue Li�NH3 solution wasadded compound 22 (200 mg, 0.36 mmol) in dry THF (3 mL) viacannula under nitrogen atmosphere. After being stirred at �78 �C for0.5 h, the reaction was quenched by addition of excess solid NH4Cl anddiluted with ether, and ammonia was evaporated at rt. More ether wasadded, and the mixture was filtered and concentrated in vacuo.Purification by column chromatography (SiO2, 20% EtOAc in PE)afforded compound 23 as a colorless liquid (120.3 mg, 72%): Rf = 0.3(SiO2, 20% EtOAc in PE); [R]24D = +8.94 (c 2.13, CHCl3); IR νmax

3620, 3390, 2928, 2863, 1836, 1708, 1462 cm�1; 1H NMR (500 MHz,CDCl3) δ 5.43�5.33 (m, 2H), 3.90 (m, 1H), 3.78 (d, J = 3.5 Hz, 2H),3.66 (m, 1H), 3.60�3.54 (m, 2H), 2.35�2.21 (m, 3H), 2.03 (m, 1H),1.93�1.86 (m, 2H), 1.45 (s, 3H), 1.37 (s, 3H), 1.05�1.03 (m, 24H),0.99�0.96 (m, 6H); 13C NMR (75 MHz, CDCl3) δ 134.2, 129.0, 98.5,76.4, 73.5, 71.9, 64.9, 63.1, 44.1, 34.0, 33.8, 29.3, 19.4, 19.2, 18.2, 18.1,16.4, 14.2, 12.5; MSm/z 481 [M + Na]+; HRMS calcd for C25H50O5SiNa [M + Na]+ 481.3325, found 481.3348.2-((4S,5S,6R)-5-Hydroxy-2,2-dimethyl-6-((2R,6R,7S,E)-6-

methyl-7-(triisopropylsilyloxy)oct-4-en-2-yl)-1,3-dioxan-4-yl)acetonitrile (3). Compound 23 (100 mg, 0.22 mmol) was sub-jected to the tosylation following the same procedure as per thesynthesis of compound 13, except for the initial addition of di-n-butyltin oxide (5 mg, 0.02 mmol). The tosylate (Rf = 0.6, 20% EtOAcin PE), thus obtained, was directly used after flash chromatography forthe next reaction.

The above tosylate was converted to nitrile 3 following the sameprocedure as per the synthesis of compound 13. Purification by columnchromatography (SiO2, 12% EtOAc in PE) afforded compound 3as a colorless oil (79.5 mg, 77%): Rf = 0.5 (SiO2, 20% EtOAc in PE);[R]24D = +14.55 (c 1.45, CHCl3); IR νmax 3476 (br), 2935, 2866, 2263,1491 cm�1; 1H NMR (300 MHz, CDCl3) δ 5.46�5.33 (m, 2H), 3.90,(m, 1H), 3.81 (m, 1H), 3.55 (m, 1H), 3.41 (m, 1H), 2.77 (dd, J = 16.8,3.8 Hz, 1H), 2.65 (dd, J = 16.8, 6.2 Hz, 1H), 2.30�2.22 (m, 2H),1.97�1.87 (m, 2H), 1.65 (bs, 1H), 1.46 (s, 3H), 1.40 (s, 3H), 1.10�1.04(m, 24 H), 1.01�0.97 (m, 6H); 13C NMR (75 MHz, CDCl3) δ 134.4,128.8, 117.3, 99.1, 76.6, 71.9, 70.1, 67.5, 44.1, 33.9, 33.7, 29.1, 21.6, 19.4,19.2, 18.2, 16.6, 14.4, 12.6; MS m/z 490 [M + Na]+; HRMS calcd forC26H49NO4Si Na [M + Na]+ 490.3328, found 490.3323.Mupirocin H (2). To the nitrile 3 (50 mg, 0.11 mmol) in methanol

(5 mL) was added concd HCl (0.25 mL). The resulting solution wasstirred at 65 �C for 3 h. After the reactionmixture was cooled to rt, 15mLeach of DCM and half-saturated aqueous NaCl solution were added.The organic layer was separated, and the aqueous layer was extractedwith DCM (3 � 15 mL). The combined organic layers were dried(Na2SO4) and concentrated in vacuo. Purification by column chroma-tography (SiO2, 12% MeOH in chloroform) afforded mupirocin H (2)as a colorless oil (18.8 mg, 64%): Rf = 0.3 (SiO2, 10% MeOH inchloroform); [R]24D = +25.8 (c 0.64, CHCl3 (lit.

5 [R]20D = +30.5, c 1.3,CHCl3)); IR νmax 3375 (br), 2965, 2924, 1759, 1457 cm

�1; 1H NMR(CDCl3, 300 MHz) δ 5.59 (ddd, J = 15.1, 8.3, 6.8 Hz, 1H), 5.37

(dd, J = 15.1, 8.3 Hz, 1H), 4.58 (m, 1H), 4.44 (dd, J = 5.3, 3.0 Hz, 1H),3.57 (dd, J = 6.8, 6.0 Hz, 1H), 3.49 (m, 1H), 2.93 (dd, J = 18.1, 7.6 Hz,1H), 2.50 (dd, J = 18.1, 3.8 Hz, 1H), 2.36�2.21 (m, 2H), 2.06 (m, 1H),1.89 (m, 1H), 1.18 (d, J = 6.8 Hz, 3H), 1.04 (d, J = 6.8 Hz, 3H), 0.98(d, J = 6.8 Hz, 3H); 13C NMR (CDCl3, 75 MHz) δ 176.2, 134.7, 129.6,88.0, 75.1, 71.6, 68.4, 45.3, 38.3, 35.3, 34.6, 20.6, 17.0, 16.0; MSm/z 290(100) [M + NH4]

+; HRMS calcd for C14H24O5 Na [M + Na]+

295.1521, found 295.1522.

’ASSOCIATED CONTENT

bS Supporting Information. Copies of 1H and 13C NMRfor all the new compounds. This material is available free ofcharge via the Internet at http://pubs.acs.org.

’AUTHOR INFORMATION

Corresponding Author*E-mail: [email protected].

Notes†CDRI Communication no. 8089.

’ACKNOWLEDGMENT

S.P.U. thanks CSIR, India, for a research fellowship.

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