STUDIES ON KETOSES .11. PYRANOID EXO-GLYCALS 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 an d alpha-L-sorbopyranosyl bromides 10 and 23 has been examined in order to determine suitable conditions for effecting the elimination from t he exo- as well as the endo-positions. Exposure of 10 and 23 to Zn/HOA c or Zn/N-methylimidazole gave the tribenzoates of exo-D-fructal (1) a nd exo-L-sorbal (25), in yields of 79 and 80%, respectively; minor pro ducts (ca. 10% each) were the corresponding endo-D-fructal (6) and end o-L-sorbal esters (26). For the reductive elimination to occur primari ly from the endo-position, enhancement of the leaving group capacity o f the respective ring acyloxy group was required, and materialized via the 3-O-mesyl analogues of fructosyl bromide 10, which smoothly react ed with Zn/HOAc to give the endo-D-fructal esters 20 and 21; their yie lds, over the five steps from diacetone-fructose, were in the 50% rang e. Accordingly, the methodology for generating ketose-derived glycal e sters in exo- or endo-version, is now available. Some ensuing reaction s 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.