Epoxidation of the 5-ylidene-l,3-dioxane-4-ones 3 with dimethyldioxirane (4) affords enantiomerically pure oxiranes 5 in satisfactory yields. These products 5 are novel hydroxyethyloxiranecarboxylic acid derivatives and can be reduced to enantiomericaily pure 5 -(ot-hydroxyalkyl)-1,3 -dioxane-4-ones 6.
Homochiral 5-ylidene-l,3-dioxane-4-ones 3 are easily available from naturally occurring (R)-poly-3-hydroxybutanoic acid via the 1,3-dioxan-4-one 1.1 Aldol reaction of 1 with aldehydes gives hydroxyalkyl derivatives 2 that are easily dehyrated to the 5-ylidene-l,3-dioxane-4-ones 3. These or,B-unsaturated ester derivatives 3 have been found to undergo highly stereoselective addition reactions to the C-C double bond, e. g. with organocuprates l,Z or alkyl radicals 3. Recently we found an effective access to enantiomerically pure hydroxyethylcyclopropane carboxylic acid derivatives by cycloaddition of diazomethane to 5-ylidene-l,3dioxane-4-ones 3 and subsequent elimination of N2 .4 In this paper we report on the epoxidation of 5-ylidene-1,3-dioxane-4-ones 3, making use of 2,2-dimethyldioxirane (4), which already served as an effective epoxidizing reagent of other a,13-unsaturated carbonyl compounds. 5.6 Interaction of reactants 3 and 4 in acetone at room temperature clearly formed hydroxyethyloxirane-carboxylic acid derivatives 6. All products 67 were enantiomerically pure according to NMR spectroscopy. The configuration of the oxiranes 5 was proved by X-ray crystal analysis of compound 5c (see Fig. 1), revealing an attack of the dioxirane from the re-face of 2. Surprisingly the epoxidation occurs from the opposite side from the dipolar cycloaddition of diazomethane to 5-ylidene-l,3-dioxanones 3. 4 To obtain further evidence for the stereo-chemistry found by AMBERG and SEEBACH I in the aidol reaction of the dioxanone 1 with aldehydes affording 2 as the major stereoisomer (e. g. 7 : 1 diastereomeric mixture in case of R = Et) the oxirane 5b was reduced with SmI 2 .8
The hydroxyethyldioxanone 6b was obtained as the only stereoisomer. Its structure was confirmed by X-ray