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

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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.