Arylazo-glycenosides. Part I. Synthesis and reactions of some 2- and 3-arylazo-derivatives of methyl 4,6-O-benzylidene-2,3-dideoxy-D-hex-2-enopyranosides

Abstract

Preparations of the α- and β-anomers of methyl 2-O-benzoyl-4,6-O-benzylidene-D-ribo-hexopyranosid-3-ulose [(2) and (1)] and of methyl 3-O-benzoyl-4,6-O-benzylidene-D-ribo-hexopyranosidulose [(6) and (5)] are described. The phenylhydrazones [(12) and (28)] derivable from the α- and β-anomers of the 2-benzoate [(2) and (1)] have been converted by a base-catalysed elimination of benzoic acid into, respectively, the α- and β-anomers of methyl 4,6-O-benzylidene-2,3-dideoxy-3-phenylazo-D-erythro-hex-2-enopyranoside [(14) and (29)]. The phenylhydrazone (37) of the α-anomer of the corresponding 3-benzoate has likewise afforded the α-D-2-phenylazo-derivative (39).The 3-phenylazo-derivatives (14) and (29) have been shown to be useful synthetic intermediates: they undergo 1,4-addition reactions in a highly stereoselective fashion with a wide range of nucleophiles. For example, meth-oxide and azide ions, amines, acetic acid, and metal hydrides have each been added to the β-anomer (29), and 2-methoxyethoxide, methylmagnesium iodide, and the enolate derived from acetylacetone, as well as the aforementioned reagents, have been added to the α-form (14). Both these anomers gave 3-phenylhydrazones with the D-arabino- and D-ribo-structures: with the α-anomer a majority of the reagents afforded compounds with the D-arabino-configuration, whereas additions to the β-anomer usually afforded a product with the D-ribo-structure.The α-D-2-phenylazo-derivative (39) underwent addition reactions less readily: sodium borohydride was the only reagent found to add smoothly, to afford the phenylhydrazone (42) of a 3-deoxyhexoside in high yield. On the other hand use of sodium azide gave a poor yield of a 3-azido-product with the D-arabino-configuration. Sodium methoxide, however, did not add to the unsaturated system in compound (39): instead it induced rearrangement to yield methyl 4.6-O-benzylidene-3,4-dideoxy-α-D-glycero-hex-3-enopyranosid-2-ulose phenylhydrazone (41).Explanation of the differences in reactivity of these compounds and a rationalisation of the stereochemistry of the addition reactions are attempted.