Synthetic steroids. Part V. A transannular hydrogen transfer encountered in the formation of 1α,5-cyclo-5α-cholest-2-ene

Abstract

The reaction of toluene-p-sulphonyl chloride in pyridine with 5α-cholest-1-en-3β-ol (I; R = C₈H₁₇) has been reported to yield directly 1α,5-cyclo-5α-cholest-2-ene (II). This rearrangement is now shown to be initiated by a C-5α to C-3α transannular migration of hydrogen, followed by movement of the π-electrons of the Δ¹-bond to give a 1α,5α-bridge. The reaction is then completed by proton extrusion from C-3 to give the cyclosteroid (II). This mechanism has been verified by considering the solvolysis products of 5α-deuteriocholest-1-en-3β-ol and 5α-fluorocholest-1-en-3β-ol.

Application of this solvolysis to 5α-androst-1-en-3β-ol and its 17β-methoxy-derivative also gives cyclosteroids but these are accompanied by the corresponding 1,3-dienes. The ratio of cyclosteroid to diene is found to vary with the size of the 17-substituent.