Synthesis, properties, and reactions of alpha- and beta-D-glucopyranosyl esters of some tripeptides.

The 2,3,4,6-tetra-O-benzyl-1-O-(N-benzyloxycarbonyltripeptidyl)-D-glucopyranoses ), 8, and 13 were synthesised from 2,3,4,6-tetra-O-benzyl-alpha-D-glucopyranose and the active esters of the appropriate N-protected tripeptides (Gly-Cly-Gly-, L-Phe-Gly-Gly-, and Gly-Gly-L-Phe-) in the presence of imidazole; the anomeric mixtures were resolved and the alpha and beta anomers characterised. The beta anomer of 13, containing the L and D enantiomers (ratio approximately 3:1) of Gly-Gly-Phe- as the aglycon, could be resolved by column chromatography into the pure isomeric forms. Catalytic hydrogenolysis of the beta anomers, in the presence and absence of a strong acid, yielded the free 1-esters 2 beta, 9 beta, and 14 beta, which were characterised as the monoxalate or trifluoroacetate salts and as free bases. Similarly, the alpha anomers afforded 2 alpha, 9 alpha, and 14 alpha, whereas omission of the strong acid led to accompanying 1 leads to 2 acyl migration, to give the 2-O-acyl derivatives. All of the compounds prepared were converted into the N-acetyl and/or peracetylated derivatives. The 1-esters 2 beta and 9 beta, both in the charged and uncharged form, and the trifluoroacetate salt of 14 beta, are susceptible to cleavage by beta-D-glucosidase; the enzyme had no effect on the uncharged form of 14 beta. This difference between 14 beta and its salt is discussed in conformational terms.