Adventures in Synthesis, or, Formic Acid is my Friend

transcript

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

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

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

• 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

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)

Our focusthe “southern hemisphere”

The spirocycic A/F ring systemThe heterocyclic ring

Retrosynthetic analysis of target

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.

EEtO2C

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.

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]

HCO2H/Δ

R1 R 2

O O R2

Aust. J. Chem. 2003, 56, 385-388.

There are limitations…

CO2EtNC

PhPhPh

NC CO2Et

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.

CO2EtNCHN

NC CO2Et

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 +

Mechanism…

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.

Ph2) H2O

1) RCHOPh

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.

Stereochemistry assigned by single crystal X-ray structure analysis

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.

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

OSiMe3

Me3Si Me

OSiMe3

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 OHCO2H/Δ

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

O OH(ZnO)

HCO2H/Δ

The reaction has its limitationsHN

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

CO2EtNC

EtO2 C

CNEtO2C

Na/NH3

NH4OAc/AcOH

PhMe/Δ

≈30% (crude)

60-65%

What’s happening? A putative answer…

CO2EtNC

NH3 (SN2)

How might we overcome the problem? …the plan…

CNEtO2C

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.

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

CO2H•H2OO

Br MeOH

CO2MeO

Br CO2Me

TsOH/Δ 75-80%

LDA/Et2O/0°C

≈50%

Joel D. Lischefski

Possible reaction pathways with base

Base-Promoted Fragmentation

Homoconjugate Addition

What actually happened?

Joel D. Lischefski

CO2MeO

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.

Side benefit: a “green” synthesis sequence experiment for the

organic laboratory

Lischefski, J.D.; Lewis, D.E. J. Chem. Educ. accepted for publication

CO2HCO2H

CO2H•H2OO

H2OCO2H

H2O/Δ+

detergent>95%60-70%

NaOCl/NaBr/H2O50-80%

NaHCO3/H2O/Δ

50-60%≈100%O

Attempted Grignard synthesis of allyl carbinols

Br MgBr

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

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.

H2CONN

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

H2NNH2

Δ, 71%

C6H13NH2 (1 eq)

PhMe/Δ, 89%

A new stain for cholesterol-rich microdomains

InstantLipo Sep-1

1) NaOMe/DMF

2) Br(CH2)7CH3

And for Golgi apparatus…

TsCl (2 eq.)/CH2Cl216 h, 60%

InstantGolgi McN-1

A new mitochondrial probe

n = 6 InstantMito LMT-1n = 4 InstantMito LMT-2

N/EtOH/ΔMe2N

(CH2)n N NMe2

(CH2)n 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

Adventures in Synthesis, or, Formic Acid is my Friend

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