Asymmetric Diels-Alder reactions employing modified camphor-derived oxazolidin-2-one chiral auxiliaries

✍ Scribed by Malcolm R. Banks; Alexander J. Blake; J.I.G. Cadogan; Allan A. Doyle; Ian Gosney; Philip K.G. Hodgson; Paul Thorburn

• Publisher: Elsevier Science
• Year: 1996
• Tongue: French
• Weight: 882 KB
• Volume: 52
• Category: Article
• ISSN: 0040-4020
• DOI: 10.1016/s0040-4020(96)00070-1

✦ Synopsis

Transposition of the dormant methyl group from C-1 in chiral oxazolidin-2-one 2 to C-7 in a six step synthetic sequence from (IR)-camphor 5 creates a novel transfigomer 4 with sufficient n-topological bias to induce excellent levels of asymmetric induction in Lewis-acid catalysed Diels-Alder reactions of its a, 13-unsaturated carboximide derivatives with cyclopentadiene. Further modification of 4 by replacement of the C-7 methyl group with an ethyl substituent raises the level of diastereoselectivity for the acrylate derivative Ila from 81 to >95% de.. 1 2 of asymmetric induction in alkylation, acylation and aldol reactions, but Dieis-Alder cycloadditions were much less successful due to the lack of sufficient n-topological bias imparted by the auxiliary when bearing Nacryloyl substituents, e.g. 3. The origin of the poor stereoselection arises from the free rotation of the carbonyl-C~ bond in the N-acryloyl moiety of the dienophile. This allows attack by the diene on both enantiofaces, i.e. the dienophile can react in either the S-trans or S-cis conformation despite the adjacent enantiopure bornyl substituent and the presence of a Lewis-acid catalyst [Et2AIC! or TiCI2(OPr~2)], which brings about bidentate chelation and freezes the N-carbonyl rotor. In order to render the faces diastereotopic and bring about