EASY AND STEREOSELECTIVE ONE-STEP INCORPORATION OF 2 ASYMMETRIC CARBONS IN PYRANOSE DERIVATIVES TO ACYCLIC EPOXYAMIDES - NEW, POTENTIALLY USEFUL ACYCLIC CHIRAL TEMPLATES

We reacted N,N-diethyl-2-(dimethylsulfuranylidene)acetamide with 4,6-O -alkylidene-glycopyranoses under several experimental conditions and o btained, stereoselectively, derivatives of acyclic 3-(polyhydroxyalkyl )-alpha,beta-epoxyamides. In this way, and in one stage, we introduced , highly stereoselectively, two new chiral carbons with a substituted asymmetric epoxide group that could then be regioselectively transform ed and, in addition, obtained highly functionalized acyclic structures starting from easily obtained cyclohemiacetalic monosaccharides. The configuration of the new chiral carbons of the resulting trans-epoxyam ides was determined by comparing the IR, NMR, and polarimetric data wi th another epoxyamide of known configuration. We attempted to explain the stereochemistry of the major products by proposing a preferential conformation for the different starting aldehyde sugars in the basic r eaction medium that took into account, at first, the principal electri cal interactions between the carbonyl group and those unprotected hydr oxyl groups with partial hydroxylate character and, secondarily, the p referred equatorial approach (exo) of the nucleophile. Finally, we stu died the cyclization of the reaction products by an initial Payne tran sposition of the gamma-hydroxy-alpha,beta-epoxide to alpha-hydroxy-bet a,gamma-epoxide, followed by its cyclization to a C-glycofuranoside.