Boron trifluoride-catalysed rearrangements of some tetrasubstituted A-norcholestane epoxides. Evidence for the formation of an oxetan intermediate in a novel fragmentation reaction

Abstract

3′,3′-Dimethyl-(3R)-spiro[A-norcholestane-3,2′-oxiran](1) and its 3S-isomer (2) give, on treatment with boron trifluoride–ether, acetone and A-norcholest-3(5)-ene (11). The oxetan (10). formed together with the allylic alcohol (9) during the chromatographic separation over alumina of (1) and (2), shows a similar behaviour, when submitted to the action of the same reagent. Similar treatment of 3-(1-deuterio-1-methylethyl)-3,5β-epoxy-A norcholestane (15) leads to acetone and 3-deuterio-A-norcholest-3(5)-ene (12). These results suggest a mechanism for all fragmentation reactions involving an oxetan intermediate such as (10).