Presentation on theme: "Adventures in Synthesis, or, Formic Acid is my Friend David E. Lewis Department of Chemistry University of Wisconsin - Eau Claire Gustavus Adolphus College,"— Presentation transcript:
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
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 olim attulerat 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 venenorum ocissimum 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 skeleton The 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 rings
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 focus the “southern hemisphere” The spirocycic A/F ring system The heterocyclic ring
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.
Preparing the starting compounds: the Fossé reaction Reaction succeeds when X = OH, OMe, NR 2 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.
But… Reaction fails when X = H –benzhydrol returns only starting material
A Partial Solution: Benzhydrylation of Active Methylene Compounds R 1 = R 2 = Me73% R 1 = Me; R 2 = OEt61% R 1 = R 2 = OEttrace R 1, R 2 = OCMe 2 O29%* Gullickson, G.C.; Lewis, D.E. Aust. J. Chem. 2003, 56, 385-388. [Bowie Festschrift]
Aust. J. Chem. 2003, 56, 385-388. There are limitations… triphenylmethanol always returns triphenylmethane
…and unexpected results This Ritter reaction is highly reproducible, and highly chemoselective Aust. J. Chem. 2003, 56, 385-388.
Ritter reactions by solvolysis of benzhydrol in formic acid Gullickson, G.C.; Lewis, D.E. Synthesis, 2003, 681-684.
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
Mechanism… 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. 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 propionanilide a RIsolated Yield 4-MeOC 6 H 4 31 (40) 2-ClC 6 H 4 15 (16) 2-C 4 H 3 O b 26 (33) C 6 H 5 CH=CH31 (37) CHMe 2 19 (26) 2-C 10 H 7 b 24 (29) 1-C 10 H 7 52 (64) Me b 25 (27) a Isolated yields of purified anti isomer. Values in parentheses are isolated yields of crystalline material prior to recrystallization. b Structures of these compounds have been determined by single crystal X-ray structure analysis.
What’s happening here? product ratio is largely insensitive to: –temperature –solvent –length of reaction effects of other metal ions –zinc 2 eq. ZnCl 2 reverses stereochemical preference to approximately 70:30 syn. 1 eq. ZnCl 2 yields exclusively anti, but only in low yield. –magnesium reaction fails when MgCl 2 is added
Rationalization of reaction stereochemistry –rapid equilibrium between aldols –slower equilibrium involving acetal dianion Z-azaenolateZ-enolateO-nucleophile
Cyclization of -hydroxyanilides permits early incorporation of heterocyclic ring reaction tolerates even sensitive functional groups –cinnamyl group does not lead to polymerization R = C 6 H 5 10% * R = p-MeOC 6 H 5 33% R = -C 10 H 7 69% *single run; not optimized R = C 6 H 5 CH=CH52% R = p-MeOC 6 H 4 79% R = -C 10 H 7 75 % R = -C 10 H 7 40% E:Z ≈ 3:2 Glen C. Gullickson, Jessica A. Walters
The reaction has its limitations 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
Why the observed regiochemistry of cyclopropane cleavage? H° f = – 63.3 kcal/mole H° 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
Attempted Grignard synthesis of allyl carbinols
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 -C 10 H 7 —CHO -C 10 H 7 —CH(OH)—CH 2 CH=CH 2 81 (CH 3 ) 2 CH—CHO(CH 3 ) 2 CH—CH(OH)—CH 2 CH=CH 2 70 cyclohexanoneH 2 C=CHCH 2 —CH(OH)(CH 2 ) 5 76 C 6 H 5 COCH 3 C 6 H 5 C(CH 3 )(OH)CH 2 CH=CH 2 82 CH 3 —CO 2 C 2 H 5 CH 3 —C(OH)(CH 2 CH=CH 2 ) 2 77 C 6 H 5 —CO 2 CH 3 C 6 H 5 —C(OH)(CH 2 CH=CH 2 ) 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-Bu57%R = n-C 6 H 13 74% R = n-C 8 H 18 66% Deprez, N.R.; McNitt, K.A.; Petersen, M.E.; Brown, R.G.; Lewis, D.E. Tetrahedron Lett. 2005, 46, 2149-2153.
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 mitochondrial probe n = 6 InstantMito LMT-1 n = 4 InstantMito LMT-2
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