Synthesis of the Carbohydrate Moiety of Bleomycin. 2,3,4,6-Tetra-O-Substituted D-Mannose Derivatives

As part of the synthesis of the carbohydrate moiety of bleomycin [2-O-(3-O-carbamoyl-α-D-manno-pyranosyl)-L-gulopyranose], two different approaches were employed for the synthesis of appropriate 3-O-car-bamoylmannose precursors. One approach, which required introduction of the carbamoyl group following coupling of the L-gulose and D-mannose moieties, involved the preparation of the strained tricyclic ortho ester 4-O-benzyl-l,2,3-O-orthoacetyl-6-O-trityl-ß-D-mannoyranose (6b). The preparation of this compound and a study of its chemistry is reported. An alternate approach, which provided mannose derivatives of utility for the synthesis of bleomycin, involved the preparation of four compounds that contained the 3-0-carbamoyl group prior to condensation with L-gulose. These key intermediates were 2,4,6-tri-O-acetyl-3-O-(N-acetylcarbamoyl)-α-D-mannopyranosyl bromide (20), 4,6-di-O-acetyl-3-O-(N-acetylcarbamoyl)-l,2-O-(l-ethoxyethylidene)-ß-D-mannopyranose (21), 4,6-di-O-acetyl-3-O-carbamoyl-l,2-O-(l-ethoxyethylidene)-ß-D-mannopyranose (22), and 2,4,6-tri-O-acetyl-3-O-carbamoyl-α-D-mannoyranosyl chloride (23).