Easy and stereoselective one-step incorporation of two 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 obtained, 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 transformed 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-epoxyamides was determined by comparing the IR, NMR, and polarimetric data with 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 reaction medium that took into account, at first, the principal electrical interactions between the carbonyl group and those unprotected hydroxyl groups with partial hydroxylate character and, secondarily, the preferred equatorial approach (exo) of the nucleophile. Finally, we studied the cyclization of the reaction products by an initial Payne transposition of the gamma-hydroxy-alpha,beta-epoxide to alpha-hydroxy-beta,gamma-epoxide, followed by its cyclization to a C-glycofuranoside.