THE DIASTEREOSPECIFIC SYNTHESIS OF NEW 2',3'-CIS-ALPHA-FUSED CARBOCYCLIC NUCLEOSIDES

The first diastereospecific synthesis of [3.3.0]- and [3.4.0]-alpha-ci s-fused-carbocyclic nucleosides 10, 12 and 20, starting directly from 2'-O-(TBDMS) or 3'-O-(TBDMS) derivatives of 5'-O-MMTr-2',3'-seco-ribo- thymidines, 1 and 13 (ref. 4), have been reported. The key steps invol ve the unsymmetrical modification of the 2'- and 3'-hydroxyls in seco- nucleosides 1 and 13 and their diastereospecific recyclisalion to the furanose-fused carbocyclic rings using either radical cyclization %)-- >4(90%)-->5(83%)-->6-->7(77%)-->8(61%)-->(41%) and 11(42%);9-->10 (87% ) and 11-->12(84%)] or Diels-Alder reaction 1%)-->16(80%)-->17-->18(36 %)-->19(69%)-->20(84%)].