Synthesis of 2',3'-dideoxy-2',3'-alpha-methanocytidine.

A six-stage synthesis of 2',3'-dideoxy-2',3'-alpha-methanocytidine (3) from (5S)-5-benzoyloxymethyl-(5H)-furan-2-one (5) is described. The key step involved the stereoselective formation of (1R,4S,5S)-4-benzoyloxymethyl-3-oxabicyclo[3.1.0]hexan-2-one (7) via 1,3-dipolar cycloaddition of diazomethane to 5 followed by photoinduced elimination of nitrogen. Reduction of 7 to the corresponding lactol followed by acetylation yielded primarily 1-O-acetyl-5-O-benzoyl-2,3-dideoxy-beta-D-ribofuranose (8). Reaction of 8 with 2,4-bis(trimethylsilyl)cytosine and EtAlCl2 followed by deprotection and chromatography gave 3, which exhibited only weak activity against the human immunodeficiency virus (HIV).