Simple and Efficient Stereoselective Synthesis of (Z)- and (E)-Alkylidene 2,3-Dimethoxybutenolides from L-Ascorbic Acid and D-Isoascorbic Acid

Reactions of 2,3-di-O-methyl-5,6-di-O-toxyl-L-ascorbic acid (5b) and 2,3-di-O-methyl-5,6-di-O-toxyl-D-isoascorbic acid (11) with excess of carbon, oxygen and nitrogen nucleophiles produced γ-(Z)- and (E)-alkylidene-2,3-dimethoxybutenolides 6a-e and 12a-d, respectively, in good yields. The formation of the reaction products 6a-e and 12a-d has been rationalised through the involvement of two reaction processes. The ditosylates undergo a stereoselective antiperiplanar E2 reaction first with the nucleophiles to yield exocyclic allylic tosylates of the type 6 or 12 which in a subsequent S_N2 reaction give rise to the observed products 6a-e and 12a-d.

alkylidine butenolides - tosylation - ascorbic acid