Chiral synthesis of alcohols via asymmetric epoxidation Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/k0698b87r

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  • Enantioselective epoxidation of crotyl alcohol, followed by a cuprate mediated methylation, gave a 1:1 mixture of (2S) -3- methyl -1,2- butanediol and (2R,3S)-2-methy1-1,3- butanediol. The latter was separated as its 2,4,6-trimethylbenzenesulfonate derivative and converted via the iodide to the corresponding phosphonium salt. The separated butanediol derivatives were independently transformed to (S)-3-methyl -2-butanol, thereby proving configuration and establishing optical purity. The primary trityl ether of the chiral butane-1,3- diol was converted to (2S,3R)-2-methy1-3-tert-butyldimethylsilyloxybutanal. The reaction of this aldehyde with methylmagnesium bromide and with methyllithium gave, after removal of the silyl group, meso and (2S,4S)-3-methy1-2,4- pentanediols. A study of this reaction at various temperatures led to the conclusion that stereoselectivity can be predicted by the Cram model when methyllithium is the reactant but not when the Grignard reagent is employed. In the latter case, a coordination model in which magnesium complexes in bidentate fashion to the carbonyl and 3-oxygen atoms successfully predicts the stereochemical outcome.
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