C-nucleoside studies. Part 13. A new synthesis of 2,3,5-tri-O-benzyl- α(and β)-D-ribofuranosylethyne involving benzyloxy participation, and a synthesis of α-showdomycin

2,3,4,6-Tetra-O-benzyl-D-glucitol (5) reacts with toluene-p-sulphonyl chloride in pyridine at 60°C to form mainly the furanoid products 2,3,6-tri-O-benzyl-1,4-anhydro-D-glucitol (10) and its 5-toluene-p-sulphonate (11) with loss of the 4-O-benzyl group. The pyranoid product tetra-O-benzyl-1,5-anhydro-D-glucitol preponderates when the intermediate 2,3,4,6-tetra-O-toluene-p-sulphonyl-D-glucitol (6) is converted into its 0-5 oxyanion. Benzyloxy participation has been exploited in a new synthesis of 2,3,5-tri-O-benzyl-a (and ß)-D-ribofuranosylethyne, (20) and (4), from 2,3,4,5-tetra-O-benzyl-aldehydo-D-ribose. A synthesis of 2-a-D-ribofuranosylmaleimide, the a-isomer of showdomycin, from (20) is described.