ENANTIOSELECTIVE SYNTHESIS OF 2-METHYL-2-HYDROXY-GAMMA-BUTYROLACTONE AND ITS APPLICATION IN THE ASYMMETRIC-SYNTHESIS OF FRONTALIN AND MEVALONOLACTONE

The asymmetric hydroxylation of the enolates of fully substituted acyclic ester 8 and lactone 10 with (camphorylsulfonyl)oxaziridines 1a-c was studied. The stereoselectivities of the tertiary alpha-hydroxy carbonyl products were highly dependent on the enolate structure, the oxidizing reagents, and the reaction conditions. While high diastereoselectivity (up to 94%) was obtained for enolates of fully substituted menthol ester 8 with substoichiometric amounts of oxaziridine 1a, the yields were unsatisfactory. On the other hand, the enantioselective alpha-hydroxylation of the sodium enolate of 2-methyl-gamma-butyrolactone (10) with [(8,8-dimethoxycamphoryl)sulfonyl]oxaziridine (1c) afforded alpha-hydroxy lactone 11a in 70% yield and 84% ee. The enantiomeric excess was improved to > 93% ee by crystallization of the corresponding benzoyl ester 11c. The utility of both enantiomers of 11c were demonstrated in the formal asymmetric syntheses of the pheromone, (1S,5R)-(-)-frontalin (13) and in the asymmetric synthesis of (R)-(-)-mevalonolactone (20).