Acid-catalysed rearrangements of steroid alkenes. Part 1. Rearrangement of 5α-cholest-7-ene

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Abstract
In the presence of boron trifluoride–diethyl ether or anhydrous toluene-p-sulphonic acid–acetic acid, 5α-cholest-7-ene (1) is transformed into 5α-cholest-8(14)-ene (2a) and 5α-cholest-14-ene (3a) and then viaC-ring contraction into the ring C/D-rearranged 12(13→14)abeo-5α-cholest-13(17)-ene (4a). Isomerisation at C-20 in the cholestene (4a) occurs giving compound (4b). Both can then undergo a reversal of the C-ring contraction, regenerating the cholestenes (2a) and (3a) and also forming (20S)-5α-cholest-8(14)-ene (2b) and -14-ene (3b). Two other rearrangement products have been identified as 14β-methyl-18-nor-5α,13β-cholest-17(20)-enes (6a,b). They may be formed via 14β-methyl-18-nor-5α-cholest-12-enes and -13(17)-enes (5a,b). Experiments with anhydrous toluene-p-sulphonic acid–O-deuteriated acetic acid indicate that the rearrangement occurs by a predominantly stepwise process.