Studies on ketoses .11. Pyranoid exo- and endo-glycals from D-fructose and L-sorbose: Practical routes for their acquisition and some ensuing reactions

The reductive elimination of the benzoylated beta-D-fructopyranosyl and alpha-L-sorbopyranosyl bromides 10 and 23 has been examined in order to determine suitable conditions for effecting the elimination from the exo- as well as the endo-positions. Exposure of 10 and 23 to Zn/HOAc or Zn/N-methylimidazole gave the tribenzoates of exo-D-fructal (1) and exo-L-sorbal (25), in yields of 79 and 80%, respectively; minor products (ca. 10% each) were the corresponding endo-D-fructal (6) and endo-L-sorbal esters (26). For the reductive elimination to occur primarily from the endo-position, enhancement of the leaving group capacity of the respective ring acyloxy group was required, and materialized via the 3-O-mesyl analogues of fructosyl bromide 10, which smoothly reacted with Zn/HOAc to give the endo-D-fructal esters 20 and 21; their yields, over the five steps from diacetone-fructose, were in the 50% range. Accordingly, the methodology for generating ketose-derived glycal esters in exo- or endo-version, is now available. Some ensuing reactions of the exoglycals have been exploited, that is, their ozonolysis to the respective pyranoid pentonolactones, and their hydrogenation to 1,5-anhydro-6-deoxy-hexitols.