Enantioselective

Further Studies on the [1,2]-Wittig Rearrangement of 2-(2-Benzyloxy)aryloxazolines

The behaviour of 14 ortho-functionalised 2-aryloxazolines (11 of them prepared and characterised for the first time) with butyllithium has been examined. Significant limitations to the Wittig rearrangement of such systems are revealed. In terms of asymmetric Wittig rearrangement, good diastereoselectivity is obtained with a valine-derived 4-isopropyl oxazoline, but this is compromised by racemisation upon hydrolysis. More encouraging selectivity is achieved in the Wittig rearrangement of an acyclic phenylalanine-derived ortho-benzyloxy benzamide.

Access to Diarylmethanols by Wittig Rearrangement of ortho-, meta-, and para-Benzyloxy-N-Butylbenzamides

The N-butyl amide group, CONHBu, has been found to be an effective promoter of the [1,2]-Wittig rearrangement of aryl benzyl ethers and thus allow the two-step synthesis of isomerically pure substituted diarylmethanols starting from simple hydroxybenzoic acid derivatives. The method is compatible with a wide range of functional groups including methyl, methoxy, and fluoro, although not with nitro and, unexpectedly, is applicable to meta as well as ortho and para isomeric series.

alpha-Substituted acylsilanes via a highly selective [1,4]-Wittig rearrangement of alpha-benzyloxyallylsilane

alpha-Benzyloxyallylsilane undergoes efficient [1,4]-Wittig rearrangement to generate an enolate intermediate that can be trapped with various electrophiles, thereby providing a new synthetic approach to substituted acylsilanes.

A Class II Aldolase Mimic

[structures: see text] A class II aldolase-mimicking synthetic polymer was prepared by the molecular imprinting of a complex of cobalt (II) ion and either (1S,3S,4S)-3-benzoyl-1,7,7-trimethylbicyclo[2.2.1]heptan-2-one (4a) or (1R,3R,4R)-3-benzoyl-1,7,7-trimethylbicyclo[2.2.1]heptan-2-one (4b) in a 4-vinylpyridine-styrene-divinylbenzene copolymer. Evidence for the formation of interactions between the functional monomer and the template was obtained from NMR and VIS titration studies. The polymers imprinted with the template demonstrated enantioselective recognition of the corresponding template structure, and induced a 55-fold enhancement of the rate of reaction of camphor (1) with benzaldehyde (2), relative to the solution reactions, and were also compared to reactions with a series of reference polymers. Substrate chirality was observed to influence reaction rate, and the reaction could be competitively inhibited by dibenzoylmethane (6). Collectively, the results presented provide the first example of the use of enantioselective molecularly imprinted polymers for the catalysis of carbon-carbon bond formation.

Planar Chiral Cyclic Ether: Asymmetric Resolution and Chirality Transformation

Remarkably stable planar chirality was found in a nine-membered diallylic cyclic ether. Enantiomerically enriched ether was prepared by a kinetic resolution of a racemic one, which is valuable as a novel type of chiral building block.

Aqueous aldol catalysis by a nicotine metabolite

Nornicotine, an endogenous tobacco alkaloid and minor nicotine metabolite, can catalyze aldol reactions at physiological pH. Catalysis appears to be due to a covalent enamine mechanism, an unprecedented reaction with small organic molecule catalysts in aqueous buffer. Kinetic parameters for nornicotine as well as other related alkaloids were measured and demonstrate that both the pyrrolidine and pyridine rings are critical for optimal catalysis. Substrate compatibility of this catalyst and its implications in vivo are discussed.

External chiral ligand-induced enantioselective versions of the [2,3]-Wittig sigmatropic rearrangement

The external chiral ligand-induced enantioselective [2,3]-Wittig rearrangements of crotyl benzyl ethers and crotyl propargylic ethers are described. The most notable is that treatment of (E)-crotyl propargylic ethers with a t-butyllithium/(S;S)-bis(oxazoline) complex provides a relatively high enantioselectivity (up to 89% ee), together with a high threo-diastereoselectivity. Furthermore, examples of the "asymmetric catalytic version" of the rearrangement of crotyl benzyl ethers are presented.