The Addition of N-Bromoacetamide to 2,3-Unsaturated Nitro Sugars, a New Approach to 2,3-Diamino Sugars. Synthesis of Derivatives of 2,3-Diamino-2,3-dideoxy-D-mannose and -D-talose
The methyl 4,6-O-benzylidene-2,3-dideoxy-3-nitrohex-2-enopyranosides having the α-D-erythro (1), α-D-threo (10), β-D-erythro (19), and β-D-threo (24) configurations react smoothly with N-bromoacetamide (NBA) in the presence of sodium acetate in acetone or aqueous acetone. The main products are 2-acetamido-3-bromo-2,3-dideoxy-3-nitrohexose derivatives (2, 11, 20, 25, and 26). Depending on the amount of sodium acetate provided, varying proportions of 2-O-acetyl-3-bromo-3-deoxy-3-nitrohexose derivatives (3, 12, and 21) are formed also. The reactions are highly stereoselective in that the acetamido or acetoxy substituent appears to enter exclusively in the position trans to the anomeric methoxyl group. Generally the nitro group in the addition products appears to be oriented equatorially. An exceptional case is the reaction of 24, which gives two C-3 epimers (25 and 26). Borohydride reduction of 3, 12, and 21 results in loss of the bromo and acetoxy substituents leading to known, saturated 2,3-dideoxy-3-nitro derivatives (4, 13, and 23). Compounds 2, 11, 20, 25, and 26 are debrominated by sodium borohydride to give vicinal acetamidonitro derivatives having the following configurations: α-D-manno (5, from 2), β-D-talo (14, from 11), β-D-gluco (22, from 20), and β-D-galacto (27, from 25 and 26). Compound 5 was converted by standard reaction sequences into methyl 2,3-diacetamido-2,3-dideoxy-α-D-mannopyranoside (8) and its 4,6-diacetate 9. Compound 14 was similarly converted into the corresponding derivatives (16 and 17) of hitherto unknown 2,3-diamino-2,3-dideoxy-D-talose.