SYNTHESIS OF 6-AMINO-6-DEOXY-2,3,4,5- TETRA-O-METHYL-D-GALACTONIC ACID,A KEY PRECURSOR OF A STEREOREGULAR POLYAMIDE[1]

The title compound (17) was synthesized by two alternative sequences, starting from D-galactose diacetonide (1) and from methyl 6-O-tosyl-α-D-galactopyranoside (9). Compound 1 was converted into the 6-bromo-6-deoxy derivative 2 or mesylated to 3. Nucleophilic substitution of the leaving group in 2 and 3 by sodium azide led to the 6-azido-6-deoxy derivative 4, which on treatment with methanol under acidic conditions afforded a mixture of the corresponding methyl β-furanoside (5) and α-pyranoside (6). Methylation of the free hydroxyl groups of 5 and 6 gave the respective per-O-methyl derivatives 7 and 8. In order to maintain the size of the sugar ring during the sequence, compound 8 was alternatively prepared from 9, by acetylation, substitution by azide and per-O-methylation. Hydrolysis of the glycoside followed by oxidation and further 5-O-methylation afforded the 6-azido-6-deoxy carboxylic acid 16 which was converted into 17 (38% overall yield from 9) by hydrogenolysis of the azide function.