The levorotatory diol 7 has been sequentially monosilylated, dehydrated, and oxidized at the allylic methylene group to provide (+)-12. The enantiomeric dextrorotatory diol 7 has been directed down a different sequence of steps involving monosilylation, dehydration, hydroboration, Swern oxidation, and regioselective introduction of a conjugated double bond to generate (-)-33. The novel feature of these transformations is that two key deoxycarbospironucleoside intermediates of the proper absolute configuration have been made available from enantiomerically related precursors. Also reported is a highly practical and reliable means for the formation of novel 2'-deoxyribonucleosides of novel structural type from these spirocyclic cyclopentenones.
[reaction: see text] The enantiomers of spiro[4.4]nonane-1,6-diol have been transformed by different reaction pathways into the two possible carbaspironucleoside epimers with natural C1' absolute stereochemistry.
The stereochemical course of the epoxidation of the silyl enol ethers of 2-tert-butyldimethylsilyloxycycloheptanone and -cyclooctanone has been investigated and shown to proceed exclusively anti to the existing alpha-substituent. 2-(Benzyloxy)cyclooctanone behaves similarly, and the presence of a transannular double bond does not alter the outcome. Alpha-ketol rearrangements are seen to operate during ensuing fluoride ion-induced removal of the silyl protecting groups in select examples. The preferred means for generating the cis isomers of the alpha,alpha'-dihydroxy cycloalkanones involves methylenation of the monoprotected trans-dihydroxy ketones with the Nysted reagent, followed by oxidation and subsequent reduction with sodium borohydride. Ozonolysis and fluoride ion-induced desilylation complete the route.
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