Unsaturated carbohydrates. Part 22. Alkenes from 5-bromohexopyranose derivatives

Abstract

Base-catalysed elimination of hydrogen bromide from 5-bromo-β-D-glucopyranose penta-acetate and penta-benzoate gives predominantly the endocyclic products, whereas treatment with zinc–acetic acid affords a means of obtaining the exocyclic 6-deoxy-5-enose esters. All eight alkenes obtainable from these two bromo-compounds are reported together with their ¹H and ¹³C n.m.r. spectra. Unsaturated compounds derived from penta-O-benzoyl-5-bromo-α-D-glucopyranose and acetylated 5-bromouronates are also reported. The conformations of all the alkenes are discussed and, in one case, the influences of the anomeric and allylic effects can be compared.