Steroidal sulphur compounds. Part XI. 4-Thia-5α- and 5β-cholestane and their oxides and dioxides

Abstract

Cholesterol was converted in nine steps into 4-thia-5β-cholestane in 36% overall yield. The final step involved the intramolecular addition of a thio-radical, generated photocatalytically from A-nor-3,5-secocholest-5-ene-3-thiol, to the double bond to give 4-thia-5β-cholestane stereospecifically. Another approach involved the intramolecular addition of a sulphenic acid, generated by pyrolysis of 3-t-butylsulphinyl-A-nor-3,5-secocholest-5-ene, to the double bond to give 4-thia-5β-cholestane 4α-oxide stereospecifically. Base-catalysed isomerisation of 4-thia-5β-cholestane 4,4-dioxide and 4-thia-5β-cholestane 4α-oxide gave 4-thia-5α-cholestane 4,4-dioxide and 4-thia-5α-cholestane 4α-oxide quantitatively, whereas 4-thia-5β-cholestane 4β-oxide gave a ca. 56 : 44 mixture of starting material and 4-thia-5α-cholestane 4β-oxide. Reduction of 4-thia-5α-cholestane 4β-oxide gave 4-thia-5α-cholestane. The oxides were configurationally stable at 190°, but underwent photocatalysed stereomutation at sulphur without concomitant isomerisation at C-5.