Photo-bromination of carbohydrate derivatives. Part 2. Penta-O-acetyl-β-D-glucopyranose; the 5-bromo-derivative and products of further bromination

Abstract

Photo -bromination of penta-O-acetyl-β-D-glucopyranose with N-bromosuccinimide in carbon tetrachloride causes specific replacement of H-5, and the 5-bromo-derivative can be isolated crystalline in high yield. By-products of the reaction contain monobromoacetyl and dibromoacetyl groups. When bromine was used as reagent the 5-substituted compound was again formed as initial product, but, ultimately, tetra-O-acetyl-α-D-glucopyranosyl bromide and dibromides with halogen substituents at C-1 and C-5, and with both bromine atoms at C-1, were also produced. A two-step route from glucose penta-acetate to 1,2,3,4-tetra-O-acetyl-6-deoxy-β-D-xylo-hex-5-enose is defined. Since the C-5 epimer of the penta-acetate affords the same products, it is proposed that the initial bromination proceeds by way of a discrete radical at the tertiary site.