Studies on Conformation and Reactivity. III. The Polyphosphoric Acid-catalyzed Ring Opening of 4, 5-Epoxy-3-oxo Steroids. 3. The Synthesis of 16α, 17-Epoxy-4-ethyl-thiopregn-4-ene-3, 20-dione and its Analogs

An example of the mild and selective catalytic action of PPA for the ring opening reaction of α, β-epoxy ketone systems under different steric environment in the steroid nucleus was described. A remarkably inert character of the 16α, 17-epoxy-20-oxo system in the pregnane series for the ring opening under PPA catalysis with such nucleophile as alkylmercaptans was observed. The effcient catalytic action of PPA for normal ring opening at C-4 of 4β, 5 : 16α, 17-diepoxy-5β-pregnane-3, 20-dione (IIb) was reported. Ethanethiol reacted with the diepoxide (IIb) in PPA-dioxane affording 16α, 17-epoxy-4-ethylthiopregn-4-ene-3, 20-dione (IV) and a further product, 3, 4-bis (ethylthio)-16α, 17-epoxypregna-3, 5-dien-20-one (V). Ethanedithiol and 2-mercaptoethanol reacted with IIb, as expected, affording 16α, 17-epoxy-20-oxopregna-3, 5-dieno[3, 4-b]dithiane (VI) and its oxathiane derivative (VII) respectively.