Conformationally locked nucleosides. Synthesis of 3(R,S)-(adenin-9-yl)-1- and 3(R,S)-(cytosin-1-yl)-1-hydroxymethylbicyclo[2,1,1]hexanes

3,3-Di(isopropoxycarbonyl)-1,1-dimethoxycyclobutane (1) was converted, in multi-steps, to 3-(1,3-dithiacyclohex-2-yl)-1,1-di(p-tosyloxymethyl)cyclobutane (7), which was subjected to a butyllithium-mediated cyclization to give the bicyclo[2,1,1]hexane ring system 8. Further transformations afforded 1-(t-butyldimethylsilyloxymethyl)-3(R,S)-hydroxybicyclo[2,1,1]hexanes (11), which were condensed with 6-chloropurine and 4-acetylcytosine via Mitsunobu reactions to give, after further treatment, 3(R,S)-(adenin-9-yl)- and 3(R,S)-(cytosin-1-yl)-1-hydroxymethylbicyclo[2,1,1]hexanes (14 and 17), respectively.