Transformations of cellobiose derivatives into analogues of lactose

The preparation of benzyl beta-cellobioside 2,3,6,2',3'-pentabenzoate (2b) via a 4',6'-benzylidene acetal in 27% overall yield from cellobiose is described. This pentabenzoate has been utilised for conventional syntheses of the 6'-amino-, 4'-amino-, and 4',6'-diamino-analogues of lactose, via the 4',6'-dimesylate. In addition, reaction of the pentabenzoate 2b with sulphuryl chloride initially afforded the 6'-chloro derivative, which was then slowly transformed into a mixture of the 4',6'-dichloro-lactoside and an isomer in which the non-reducing ring of the disaccharide had been transformed into a 3',6'-dideoxy-3',6'-dichloro-gulopyranoside. The latter probably arose by neighbouring-group participation by the 3'-benzoloxy group prior to the introduction of the secondary 3'-chloro substituent. The former dichlorocompound was subsequently converted into 4',6'-dideoxy-4',6'-dichloro-lactose. It was found that the rate of nucleophilic displacement at C-4' of a cellobioside was much lower than that for the corresponding alpha-linked disaccharide, maltose. Furthermore, nucleophilic displacement at C-4' of a lactoside was accompanied by substantial elimination, to give the 4'-ene, together with some cleavage of the interglycosidic bond. tthe origins of these effects are discussed.