SYNTHESES OF THE ANTI-AIDS DRUG 2',3'-DIDEOXYCYTIDINE FROM CYTIDINE

Two efficient syntheses of the anti-AIDS drug 2',3'-dideoxycytidine (3) from N4-acetylcytidine (4) are described. In one, silylation of the C-5' hydroxyl group of 4 with tert-butyldimethylsilyl chloride followed by treatment with 1',1'-(thiocarbonyl)diimidazole gave the cyclic thionocarbonate 7, which on reaction with 1,3-dimethyl-2-phenyl-1,3-diazaphospholidine gave the crystalline alkene 8. Hydrogenation of 8, followed by desilylation with tetrabutylammonium fluoride and hydrolysis, gave 3 in 27% overall yield from 4. In the other synthesis, 4 was converted into a regioisomeric mixture of bromo acetates 11 with 2-acetoxy-2-methylpropanoyl bromide. Reductive elimination of 11 with zinc-copper couple in acetic acid or electrochemically gave the crystalline alkene 15, whose stereostructure was established by a single-crystal X-ray analysis. Hydrogenation of 15, followed by hydrolysis, gave ddC (3). In a through process, which is suitable for large-scale work, this second synthesis gave 3 in over 40% overall yield from 4. The use of (S)-(-)-2-acetoxypropanoyl bromide, of 2-acetoxybenzoyl bromide, and of hydrogen bromide/acetic acid in the bromoacetylation of 4 is also described.