ENOLIZATION OF ALDOSULOSE DERIVATIVES . 4-0-ACETYL-1,6-ANHYDRO-2,3-0-ISOPROPYLIDENE-BETA-D-THREO-HEX-3-ENOPYRANOSE 3,4-ENEDIOL ACETATE OF A FUSED-RING KETO SUGAR AND FORMATION OF A BRANCHED-CHAIN SUGAR DERIVATIVE

1,6-Anhydro-2,3-O-isopropylidene-β-D-lyxo-hexopyranos-4-ulose (1), a keto sugar having a [3.2.1] bicyclic ring-system, is converted by acetic anhydride-triethylamine at room temperature into a crystalline dimer (5), a branched-chain sugar derivative apparently formed by attack of C-3 of the 3,4-enediolate anion of a molecule of 1 on the carbonyl group of a second molecule of 1. Under more vigorous conditions, the reagent converts the dimer 5 into the crystalline 3,4-enediol acetate (2) of the ketone 1; no 4,5-enediol acetate is formed because of steric reasons (Bredt rule). The enediol acetate 2 is hydrolyzed readily to the parent ketone 1, and is converted by ethanolic base into the crystalline, hydrated ketone 3. Reduction of 2 with borohydride gives 1,6-anhydro-2,3-O-isopropylidene-β-D-talopyranose (6). The n.m.r. spectrum of the enediol acetate 2, analyzed with the use of spin decoupling, shows a long-range coupling between H-2 and the exo proton at H-6. © 1969.