The photochemistry of carbohydrate derivatives. Part IV. Photochemical rearrangement of 4,6-O-o-nitrobenzylideneglycopyranosides

Abstract

Methanolic solutions of methyl 4,6-O-o-nitrobenzylideneglycopyranosides rearranged upon irradiation with u.v. light into mixtures of the corresponding amorphous glycopyranoside 4- and 6-o-nitrosobenzoates. Trifluoroperacetic acid converts these photoproducts into the corresponding 4- and 6-o-nitrobenzoates, which have been separated. The conditions used for this oxidation have been shown not to cause migration of the nitrobenzoyl groups.

The 2,3-di-O-acetyl derivatives [(12) and (13)] of methyl α- and β-D-galactoside gave the corresponding galactoside 4- and 6-nitrobenzoates [(14) and (15), and (16) and (17)], both in ca. 7 : 3 ratios whereas the diacetates of methyl α- and β-D-glucoside [(1) and (2)] and α-D-mannoside (18) gave 4- and 6-nitrobenzoates [(6) and (7), (8) and (9), and (19) and (20)], all in ca. 3 : 7 ratios. Similar rearrangements were observed with methyl 4,6-O-o-nitrobenzylidene-α-D-glucosides in which the 2- and 3-positions were either unprotected (4), tosylated (5), or methylated (3), and also with the corresponding 2,3-anhydro-α-D-alloside derivative (26). Methyl 2,3-di-O-acetyl-4,6-O-(2,4-dinitrobenzylidene)-α-D-glucoside (24) gave upon similar treatment the corresponding 4- and 6-dinitrobenzoates also in a ca. 3 : 7 ratio. Methyl 2,3 : 4,6-bis-(O-o-nitrobenzylidene)-α-D-mannoside (29) gave methyl mannopyranoside 2,6-bis-(o-nitrobenzoate)(31) and 2,4-bis-(o-nitrobenzoate)(30) as the major products.