Synthesis of the Disaccharide Moiety of Bleomycin. 2-O-(3-O-Carbamoyl-α-D-mannopyranosyl)-L-gulopyranose Derivatives

The synthesis of the carbohydrate moiety of bleomycin [2-O-(3-O-carbamoyl-α-D-mannopyranosyl)-L-gulo-pyranose] is described. A key parameter in defining a successful strategy was the lability of the carbamoyl group. Several approaches were investigated; the most successful involved the coupling of 1,6-di-O-acetyl-3,4-di-O-benzyl-β-L-gulopyranose (19) and 2,4,6-tri-O-acetyl-3-O-carbamoyl-α-D-mannopyranosyl chloride (17) via the agency of silver trifluoromethanesulfonate and tetramethylurea. Also reported is the synthesis of l,6-di-O-acetyl-3,4-di-O-benzyl-2-O-(2,3,4,6-tetra-O-acetyl-α-D-mannopyranosyl)-L-gulopyranose (16), a dissacharide useful for the synthetic elaboration of decarbamoyl bleomycin.