Studies on conformation and reactivity—I : The polyphosphoric acid-catalyzed ring opening of 4,5-epoxy-3-oxo steroids—the synthesis of 4-ethylthiocholest-4-EN-3-one and its analogs

Abstract The efficient catalytic action of polyphosphoric acid, when used in the presence of suitable nucleophiles, of both normal and abnormal ring opening of 4β,5-epoxy-5β-cholestan-3-one (I) is reported. In acetic acid, I affords 2α-acetoxycholest-4-en-3-one (XIIa; abnormal product) whilst, in marked contrast, ethanethiol reacts with I in dioxane affording 4-thylthiocholest-4-en-3-one (XV; normal product) and a further product, 3,4-bis-(ethylthio)cholesta-3,5-diene (XVI). Ethanedithiol and β-mercaptoethanol react with I, as expected, affording cholesta-3,5-dieno[3,4-b]dithiane (XVII) and its oxathiane derivative (XIX) respectively. The nature of the reactions is briefly discussed and presence of some unique conjugation in the SC 3 S and OC 3 C 4 S systems in the dithiane (XVII) and oxathiane (XIX) is suggested on the basis of their UV absorption data.