Enantiospecific total synthesis of L-2′,3′-dideoxyisonucleosides via regioselective opening of optically active C2-symmetric 1,4-pentadiene bis-epoxide

A new method for the synthesis of L-2',3'-dideoxyisonucleosides is described. The readily available, optically active C-2-symmetric bis-epoxide (2S,4S)-1,2:4,5-diepoxypentane (5) was prepared by a short route from readily available starting materials. The key step of the new synthesis is the opening of 5 with nucleophiles, which proceeds highly regioselectively; e.g., reaction with sodium sulfide affords a 5:1 mixture of the tetrahydrothiophenediol 9a and the tetrahydrothiopyrandiol 14, and reaction with sodium hydroxide gives exclusively the tetrahydrofurandiol 9b via a preferred 5-exo cyclization. These five-membered diols 9a,b can be converted in only four steps into the modified dideoxyuridine and adenosine isonucleosides 4a-c, one of which (4c) has shown good antiviral activity. In addition, we have examined the opening of the analogous six carbon bis-epoxide, (2S,5S)-1,2:5,6-diepoxyhexane (23), which affords a 3:1 mixture of the hexahydrothiepinediol 24 and the tetrahydrothiopyrandiol 25 with sodium sulfide via a preferred 7-endo cyclization. An alternate route to these two optically active bis-epoxides 5 and 23 was also examined, namely the asymmetric dihydroxylation of 1,4-pentadiene and 1,5-hexadiene followed by selective sulfonylation and epoxide : formation. The asymmetric reaction produces a nearly 1:1 mixture of optically active and meso tetrols, e.g., 28-9 and 32-3. Unfortunately, the tetrols, their simple derivatives, and the final sulfonates and epoxides could not be readily separated by Rnv simple means.