Adventures in Synthesis, or, Formic Acid is my Friend
David E. Lewis
Department of Chemistry
University of Wisconsin - Eau Claire
Gustavus Adolphus College, April 8, 2005
Lewis Research Group Members contributing to this talk
Graduates
2004
The Big Picture
What got us started?
Flowers that are beautiful
• Aconitum spp. — monkshoods (wolfsbane)– Ornamental species; late-flowering.– Beautiful blue or purple flowers resembling the hood of a monk’s
habit (hence the name)
• Delphinium spp. — larkspurs– Native to western USA– Beautiful blue flowers with a long “spur” resembling the foot of a lark
(hence the name)– Many ornamental cultivars available
…but deadly
•Aconitum spp.– A. napellus one of the most toxic plants known
•Delphinium spp.– Responsible for cattle losses worth tens of millions of dollars annually in western states of United States
•All parts of both species are toxic– Occasionally responsible for poisoning by ingestion of honey (even the nectar and pollen contain the toxic compounds!)
An evil reputation since antiquity…
...., lurida terribiles miscent aconita novercae,
Ovid, Metamorphoses, Book I, 144-150.
huius in exitium miscet Medea, quod olimattulerat secum Scythicis aconiton ab orbis......quae quia nascuntur dura vivacia caute,
agrestes aconita vocant. .....
Ovid, Metamorphoses, Book VII, 406-420.
..., nec miseros fallunt aconita legentis,
Virgil, Georgics, Book II, 152.
....constat omnium venenorumocissimum esse aconitum ....
Pliny, Natural History, Book XXVII, II.
That from the Echydnaean monster's jaws
Deriv'd its origin, and this the cause.
Thro' a dark cave a craggy passage lies,
To ours, ascending from the nether skies;
Thro' which, by strength of hand, Alcides drew
Chain'd Cerberus, who lagg'd, and restive grew,
With his blear'd eyes our brighter day to view.
Thrice he repeated his enormous yell,
With which he scares the ghosts, and startles Hell;
At last outragious (tho' compell'd to yield)
He sheds his foam in fury on the field,-
Which, with its own, and rankness of the ground,
Produc'd a weed, by sorcerers renown'd,
The strongest constitution to confound;
Call'd Aconite, because it can unlock
All bars, and force its passage thro' a rock.
— John Dryden’s translation of Ovid, VII, 404-420
The aconitane skeletonThe carbon skeleton has six rings: A, B, C, D, E and F
aconitine: the defining toxin
NR
AN
R
B
NR
C
NR
D
NR
E NR
F
The synthetic challenge: Bridged rings
• The hexacyclic carbon skeleton of these molecules has:– 2 bridged-ring carbocyclic systems based on 5- and 6-membered
rings– 2 bridged-ring heterocyclic systems based on 5- and 6-membered
rings
NR
NR
NR
NR
• The hexacyclic carbon skeleton of these molecules has:– 3 fused-ring carbocyclic systems based on 5- and 6-membered rings– 1 fused-ring heterocyclic system ( a cis-perhydroquinoline)– 1 spirocyclic ring system
The synthetic challenge: Fused rings
NR
NR
NR
NR
NR
The synthetic challenge• These molecules are densely
functionalized– In aconitine, only 8 of the 19
skeletal carbon atoms do not carry a functional group; in cardopetaline (the simplest member of this class), 5 of 19 skeletal carbons still carry a functional group
• These molecules are densely populated with stereocenters– In cardiopetaline, only 7 of 19
skeletal carbons are not chiral centers (fortunately, all are not independent; in fact 9 chiral centers are fixed by the carbon skeleton)
EtN
OH
OMe
MeO
EtN
HO
OH
OHMeO
OMe
OMe
OMe
OCOPh
OAc
EtN
OH
OMe
MeO
Our focusthe “southern hemisphere”
The spirocycic A/F ring systemThe heterocyclic ring
RN
RN
Retrosynthetic analysis of target
E'N
O
E R
E'NHR
O
E
E'NHR
O
E
E'NHR
O
X
E
E'NHR
O
X
E
E'NHR
OH
E
Ar
OH
X
(–)
(–)
(–)
N
O
R
OR
The first key intermediate
The aldehyde group must be present in protected form capable of surviving a number of different reactions, but capable of being revealed at a later stage of the synthesis. An aryl group is the most logical precursor in light of our preliminary results.
E
OH
E'NHR
E'NHR
OH
EEtO2C
OH
CHO
NHR
EtO2C
OH
Ph
CN
Preparing the starting compounds:the Fossé reaction
• Reaction succeeds when X = OH, OMe, NR2
Fossé, R. Compt. rend. 1907, 145, 1290-1293; 1908, 146, 1039-1042, 1277-1280; Bull. Soc. Chim. France 1909, 3, 1075; Ann. chim. phys. 1910, 18, 400-432, 503-530, 531-569.
OH
X X
CO2H
X X
CH2 (CO2H)2/Δ
But…
• Reaction fails when X = H– benzhydrol returns only starting material
OHCO2H
CH2 (CO2H)2/Δ
A Partial Solution: Benzhydrylation of Active Methylene Compounds
R1 = R2 = Me 73%
R1 = Me; R2 = OEt 61%
R1 = R2 = OEt trace
R1, R2 = OCMe2O 29%*
Gullickson, G.C.; Lewis, D.E. Aust. J. Chem. 2003, 56, 385-388. [Bowie Festschrift]
OH R1
O
HCO2H/Δ
R1 R 2
O O R2
O
Aust. J. Chem. 2003, 56, 385-388.
There are limitations…
Ph Ph
CO2EtNC
Ph
Ph Ph
O O
Ph
H
PhPhPh
H
PhPhPh
OH
PhPhPh
OH
PhPhPh
O O
NC CO2Et
HCO2H/Δ
HCO2H/Δ
triphenylmethanol always returns triphenylmethane
…and unexpected results
This Ritter reaction is highly reproducible, and highly chemoselective
Aust. J. Chem. 2003, 56, 385-388.
Ph Ph
CO2EtNCHN
PhPh
O
OH
PhPh
NC CO2Et
HN
PhPh
CO2Et
O
HN
PhPh
CO2H
O
HCO2H/Δ(80%)
Ritter reactions by solvolysis of benzhydrol in formic acid
Gullickson, G.C.; Lewis, D.E. Synthesis, 2003, 681-684.
Nitrile Yield Nitrile YieldCH2(CN)2 60% CH3CH2CN 86%
EtO2CH2CN 80% CH2=CHCN 65%C6H5CN 80% CH2=C(Me)CN 73%
p-MeC6H4CN 76%
which may lead to a useful application…
• only bornyl compounds are obtained: no isobornyl compounds are isolated– bornylamides are racemic
• bornyl formate is a liquid, b. ≈50°C near 1 mm Hg• the bornylamides are solids, m. >100°C.• the products are easily separated by vacuum distillation
Glen C. Gullickson, Joel D. Lischefski, Paul J. Erdman
(ZnO)OCHO NHCOR
HCO2H/RCN/Δ +
OH
Mechanism…
OH
(ZnO)
OCHO
OCHO NHCOR
HCO2HRCN/Δ +
By using a chiral nitrile, we may be able to obtain both amides optically pure
An alternative approach to the target
Use an aldol addition to build the system…
anti isomer obtained stereochemically pure from crude reaction mixture by direct crystallization.
Typical recovery of unreacted propionanilide: 10-30%.
Gullickson, G.C.; Khan, M.A.; Baughman, R.G.; Walters, J.A. Lewis, D.E. Synthesis, accepted for publication.
HN R
O OH
Ph2) H2O
1) RCHOPh
N
OLi
Li
HN R
O OH
Ph
HN
O
Ph
Aldol additions using amide dianions
Baughman, R.G.; Gullickson, G.C.; Khan, M.A.; Lewis, D.E. Acta Crystallogr. C, submitted for publication; manuscript under revision.
OH
O
HN
OH
O
HN
O
HN
OHO
Stereochemistry assigned by single crystal X-ray structure analysis
HN
OH
O
anti Aldols from dilithiated propionanilidea
R Isolated Yield
4-MeOC6H4 31 (40)
2-ClC6H4 15 (16)
2-C4H3Ob 26 (33)
C6H5CH=CH 31 (37)
CHMe2 19 (26)
2-C10H7b 24 (29)
1-C10H7 52 (64)
Meb 25 (27)aIsolated yields of purified anti isomer. Values in parentheses are isolated yields of crystalline material prior to recrystallization. bStructures of these compounds have been determined by single crystal X-ray structure analysis.
HN R
O OH
Ph
HN
O
Ph
1) 2 BuLi/THF/0‚C2) RCHO/0°C3) H2O
What’s happening here?• product ratio is largely insensitive to:
– temperature– solvent– length of reaction
• effects of other metal ions– zinc
• 2 eq. ZnCl2 reverses stereochemical preference to approximately 70:30 syn.
• 1 eq. ZnCl2 yields exclusively anti, but only in low yield.– magnesium
• reaction fails when MgCl2 is added
Enolate stereochemistry
Me3SiCl
Ph
HN
O
BuLi
PhN
Me
OSiMe3
SiMe3
PhN
Me
OLi
Li
PhN
OSiMe3
Me3Si Me
PhN
OLi
Li Me
PhN
OSiMe3
Me
SiMe3
+
21 1 1.7
+ +
: :
Rationalization of reaction stereochemistry
– rapid equilibrium between aldols– slower equilibrium involving acetal dianion
Z-azaenolate Z-enolate O-nucleophile
Cyclization of -hydroxyanilides
• permits early incorporation of heterocyclic ring• reaction tolerates even sensitive functional groups
– cinnamyl group does not lead to polymerization
HN R
O OH
HN OHCO2H/Δ
R
R = C6H5
10%*R = p-MeOC6H5 33%
R = -C10H7 69%
*single run; not optimized
R = C6H5CH=CH 52%
R = p-MeOC6H4 79%
R = -C10H7 75 %
R = -C10H7 40%
E:Z ≈ 3:2
Glen C. Gullickson, Jessica A. Walters
HN R
O OH(ZnO)
HN O
R
HCO2H/Δ
The reaction has its limitationsHN
O
NMe2
HN
O
NMe2
HN
O
NMe2
HN
O OH
NMe2
HN
O
NMe2
HN
O OH
NMe2
HN
O
NMe2
HCO2H/Δ
Similarly…
Glen C. Gullickson, Jessica A. Walters
Attempts at an alternative approach to closing the A-ring
Attempted spirocyclization by reductive alkylation
Joseph M. Schaefer, Paul J. Erdman
OHC
OH
O
CO2EtNC
EtO2C
O
CN
HN
OH
NCO
EtO2 C
OH
CN
CNEtO2C
Br Br
Na/NH3
NH4OAc/AcOH
PhMe/Δ
≈30% (crude)
60-65%
What’s happening? A putative answer…
EtO2C
O
CN
HN
OH
NCO
O
CO2EtNC
EtO2C
O
NC
Br
EtO2C
O
NC
NH2
Br Br
NH3 (SN2)
NH3 (SN2)
NH3 (SN2)
SN2
How might we overcome the problem? …the plan…
OH
NN
CNO
NN
O
O
CN
OH
CNEtO2C
OH
CNEtO2C
H2NHN
OH
HN
HN
CNO
Br Br
Δ
Br Br/base
NH2NH2
EtOH/Δ
…the reality…
Erdman, P.J.; Gosse, J.L.; Jacobson, J.A.; Lewis, D.E. Synth. Commun. 2004, 34, 1141-1149.
NN
HO
OH
OH
HN
HN
CNO
OH
CNEtO2C
OH
CNEtO2C
H2NHNNH2NH2
EtOH/Δ
Have we tried anything about the “northern hemisphere”?
Retrosynthetic analysis of the “northern hemisphere”
Synthesis of the nortricyclane synthon: the norbornane pathway
CO2Me
OO
MeO2C
O
O
CO2H•H2OO
O
Br MeOH
CO2MeO
O
Br CO2Me
OO
TsOH/Δ 75-80%
LDA/Et2O/0°C
≈50%
Joel D. Lischefski
Possible reaction pathways with base
Base-Promoted Fragmentation
Homoconjugate Addition
OO
OOMe
H
H
OO
OOMe
H
OMeO
OO
base?
OO
OOMe
OO
OOMe
Nu
OO
NuOMe
O
What actually happened?
Joel D. Lischefski
OO
CO2Me
CO2MeO
O
Br
NaNH2
NaNH2
NaOH
Me2SO
OO
OO
CO2Me
OO
HO
HNO
OO
+
Why the observed regiochemistry of cyclopropane
cleavage?
ΔH°f = – 63.3 kcal/mole ΔH°f = – 75.1 kcal/mole
Calculations at the AM1 level predict an overwhelming preference for the endo bridged norbornane ring system.
OO
OO
Side benefit: a “green” synthesis sequence experiment for the
organic laboratory
Lischefski, J.D.; Lewis, D.E. J. Chem. Educ. accepted for publication
CO2HO
O
Br
O
O
O
Δ
CO2HCO2H
CO2H•H2OO
O
Br
H2OCO2H
CO2H
H2O/Δ+
detergent>95%60-70%
NaOCl/NaBr/H2O50-80%
NaHCO3/H2O/Δ
50-60%≈100%O
O
OO
Attempted Grignard synthesis of allyl carbinols
Br MgBr
HO
CHO
Mg/Et2O
Mg/Et2O
This problem is not new…
• formation of “biallyl” observed as a major problem by 1920’s
• Gilman developed the method for preparing allylmagnesium bromide in the 1940’s– 3-fold excess of magnesium– slow addition of allyl bromide to magnesium– temperature control: temperature kept below 15°C
throughout addition.
Our solution…• return to original method: the Barbier-
Grignard addition– involves adding a solution containing both
halide and carbonyl compound to magnesium in ether
– traditionally, equimolar amounts of halide and carbonyl compound are used.
– our modification: use 1 eq. excess of magnesium and allyl bromide
Barbier-Grignard addition of allylmagnesium bromide
carbonyl compound
product(s) isolated yield
-C10H7—CHO -C10H7—CH(OH)—CH2CH=CH2 81
(CH3)2CH—CHO (CH3)2CH—CH(OH)—CH2CH=CH2 70
cyclohexanone H2C=CHCH2—CH(OH)(CH2)5 76
C6H5COCH3 C6H5C(CH3)(OH)CH2CH=CH2 82
CH3—CO2C2H5 CH3—C(OH)(CH2CH=CH2)2 77
C6H5—CO2CH3 C6H5—C(OH)(CH2CH=CH2)2 43
Br
Mg
OR
R' R
R'HO
Sormunen, G.J.; Lewis, D.E. Synth. Commun. 2004, 34, 3473-3480.
Some observations
• a full equivalent excess of allyl bromide and magnesium is not needed– the aqueous quench after the addition
gives copious quantities of gas
• the minimum amount of allyl bromide and magnesium is under active investigation
And now for something completely different…
An organic chemist’s adventures with fluorescence
…with apologies to Monty Python
Fluorescent Tröger’s bases
R = n-Bu 57% R = n-C6H13 74%R = n-C8H18 66%
Deprez, N.R.; McNitt, K.A.; Petersen, M.E.; Brown, R.G.; Lewis, D.E. Tetrahedron Lett. 2005, 46, 2149-2153.
NN
NO O
R
OH
N
O
O
R
H2CO
NH2
NO O
R
H2CONN
N
N
O O
R
R
O O
HCl/EtOH/ΔHCl/EtOH
Fluorescence Microscopy
• high selectivity for the target molecule or organelle.• resistant enough to photochemical degradation under normal
illumination conditions to permit the target cell feature to be visualized conveniently.
• preferably sufficiently non-toxic to allow live cells to be used for the experiment.
• highly fluorescent (i.e. it should have a high quantum yield for fluorescence), so that only small amounts of the dye are needed to visualize the cell target of interest.
• large Stokes shift to minimize problems from light scattering by the cell
• preferably easy to make from readily available, inexpensive starting materials, and chemically stable to permit long-term storage.
A new lysosomal stain
InstantLyso LLT-1
NO O
HN
NH2
H2NNH2
Δ, 71%
NO O
Cl
C6H13NH2 (1 eq)
PhMe/Δ, 89%
OO O
Cl
A new stain for cholesterol-rich microdomains
InstantLipo Sep-1
NH
O O
NH2
N
C8H17
O O
NH2
1) NaOMe/DMF
2) Br(CH2)7CH3
80%
And for Golgi apparatus…
NO O
HN
NH2
NO O
HN
NH
SO
O
Me
TsCl (2 eq.)/CH2Cl216 h, 60%
InstantGolgi McN-1
A new mitochondrial probe
n = 6 InstantMito LMT-1n = 4 InstantMito LMT-2
H2NNH
O
O
N/EtOH/ΔMe2N
H2NN
O
O
(CH2)n N NMe2
H2NN
O
O
(CH2)n Br
Br
1) NaOMe/MeOH/DMF
2) Br(CH2)nBr/DMF
n=4, 76%; n=6, 91%
n=4, 56%n=6, 30%
Where to now?
• Synthesis of new fluorescent dyes using formic acid cyclization as an important reaction step
• Elucidating reason for responsiveness of Tröger’s base fluorescence spectra to medium
• Application of new fluorescent dyes to fluorescence microscopy
Financial Support
• UW-Eau Claire Office of Research and Sponsored Programs
• Research Corporation• Cottrell College Science Award
• National Institutes of Health• AREA Grant