Synthesis of 3-amino-2,3,6-trideoxy-D-ribo-hexose hydrochloride

The title sugar, the 5-epimer of daunosamine, was prepared in a sequence of high-yielding steps from methyl alpha-D-mannopyranoside (1). Conversion of 1 into methyl 3-acetamido-4-O-benzoyl-6-bromo-2,3,6-trideoxy-alpha-D-ribo-hexopyranoside (2), followed by reduction with hydrogen and Raney nickel, gave the 4-benzoate (3) of methyl 3-acetamido-2,3,6-trideoxy-alpha-D-ribo-hexopyranoside (4). Saponification of 3 gave 4 as an oil that gave a crystalline 4-acetate (8). N-Deacetylation of 4 was effected with barium hydroxide, and the resultant glycoside was hydrolyzed to give 3-amino-2,3,6-trideoxy-D-ribo-hexose hydrochloride (7). The 3-benzamido analogue (5) of 4 was prepared from 4 by N-deacetylation and subsequent benzoylation, and hydrolysis of 5 gave crystalline 3-benzamido-2,3,6-trideoxy-D-ribo-hexose (6). The crystalline 3-acetamido analogue (9) of 6 was obtained by acid hydrolysis of the glycoside 4.