Synthesis of 6-deoxy-6-halolaminarans and conversion of 6-chloro-6-deoxylaminaran into the 6-amino-6-deoxy derivative

Abstract

The reaction of laminaran, a linear polyglucan with β-(1 → 3)-linkages 1, with trityl chloride followed by conventional acetylation gave a product that contained predominantly 6- O-trityl-2,4-di- O-acetate ( 3). Detritylation using hot aqueous acetic acid afforded a product containing 2,6-di- O-acetate ( 4), 2,4-di- O-acetate ( 5), and 2,4,6-tri- O-acetate ( 6) glucosyl residues. Similarly, the 2,4-di- O-benzoate ( 8) was prepared and then converted to the 6-bromo-6-deoxy derivative ( 9), using triphenylphosphine and carbon tetrabromide. The benzoyl groups in 9 could not be removed completely under conventional conditions. Direct halogenation of laminaran ( 1) with triphenylphosphine and carbon tetrabromide was unsuccessful. The replacement of the primary hydroxyl groups in 1 by halogens was achieved using methanesulfonyl halide-DMF complex. Syntheses of the 6-bromo-6-deoxy-2,4-di- O-formate ( 10) and the 6-chloro-6-deoxy-2,4-di- O-formate ( 12) were thus achieved. The formyl groups in 10 and 12 were removed under mild basic conditions to afford 11 and 13. Treatment of the 6-chloro-6-deoxy-2,4-di- O-formate derivative 12 with sodium azide in DMF afforded the free 6-azido-6-deoxy compound 14. However, when 6-chloro-6-deoxylaminaran ( 13) was treated with sodium azide it gave a compound which contained both the azide and chloro groups at C-6 positions in the polymeric chain. Catalytic hydrogenation of the water soluble 6-azido-6-deoxy-2,4-di- O-succinyl derivative 15 to give the desired 6-amino-6-deoxy compound 16 was unsuccessful. Chemical hydrogenation using triphenylphosphine in DMF gave the intermediate 6-triphenylphosphinimine 17, which on subsequent treatment with aq 30% ammonia afforded a compound containing both the 6-phosphinimino and the 6-amino groups. The structures of all the compounds were confirmed by 13C NMR and by FTIR spectroscopy.