A novel synthesis of 2-deoxy-alpha-glycosides

The key step of the synthesis involves the reaction of glycals [3,4,6-tri-O-acetyl-D-glucal (1), the new glycal derivative 4-O-acetyl-1,5-anhydro-2,6-dideoxy-3-C-methyl-3-O-methyl-L-ribo-hex-1-enitol (2), and 3-acetamido-4,6-di-O-acetyl-1,5-anhydro-2,3-dideoxy-D-arabino-hex-1-enitol (3)] with 1.5 molar equivalents of several alcohols in the presence of N-bromosuccinimide in acetonitrile to give mainly the corresponding 2-bromo-2-deoxy-alpha-glycopyranosides (4--21). The glycopyranosides (4-8 and 16-21) from 1 and 3 have the alpha-D-manno configuration and those (10--15) from 2 have the alpha-L-altro configuration. The yields are high from 1, virtually quantitative from 2, and moderate from 3. Debromination of the 2-bromo-2-deoxy compounds with tributylstannane and a radical initiator gives the corresponding 2-deoxy-alpha-glycopyranosides (22-38) in quantitative yields. In particular, the branched-chain glycal 2 reacts with alcohols to give exclusively the corresponding alpha-glycopyranosides (27--32) of cladinose in strikingly high overall yields. The stereoselectivity and regiospecificity of the bromination reaction are described. 1,3-Dibromo-5,5-dimethylhydantoin and N-bromoacetamide are also found to be useful for the reaction.