β-Elimination in aldonolactones. Synthesis of 3,6-dideoxy L-arabino-hexose (ascarylose)

Benzoylation of L-rhamnono-1,5-lactone (1) with an excess of benzoyl chloride and pyridine for 16 h afforded 2,4-O-benzoyl-3,6-dideoxy-L-erythro-hex-2-enono-1,5-lactone (2). Catalytic hydrogenation of 2 was stereoselective and gave crystalline 2,4-di-O-benzoyl-3,6-dideoxy-L-arabino-hexono-1,5-lactone (3). Reduction of the lactone 3 with disiamylborane afforded 2,4-di-O-benzoyl-3,6-dideoxy-L-arabino-hexopyranose (4) which, on debenzoylation, gave 3,6-dideoxy-L-arabino-hexose (ascarylose) (7) in good overall yield. The sugar was identified as the corresponding alditol (ascarylitol) and by convertion into methyl 3,6-dideoxy-α-L-arabino-hexopyranoside (methyl ascaryloside, 6).