Studies on Digitalis Glycosides. XXIV. Ring-opening Reaction of 14, 15-Epoxide with Thiocyanic Acid, and Preparation of 3β-Hydroxy-14β, 15β-epithio-5β-card-20(22)-enolide

Abstract

Ring-opening reaction of 3β-acetoxy-14α, 15α-epoxy-5β-card-20(22)-enolide (Ib) with thiocyanic acid proceeded smoothyl to afford 3β-acetoxy-14β-thiocyanato-15α-hydroxy-5β-card-20(22)-enolide (II) and 3β-acetoxy-14β-isothiocyanato-15α-hydroxy-5β-card-20(22)-enolide (III). On the contrary, the reaction with 3β-acetoxy-14β, 15β-epoxy-5β-card-20(22)-enolide (VIII) did not proceed so smoothly as a result of steric hindrance, giving 3β-acetoxy-14β-hydroxy-15α-thiocyanato-5β-card-20(22)-enolide (IX) together with 3β-acetoxy-15-oxo-5β, 14α-card-20(22)-enolide (X). Ring closure of the mesylate of II with a weak alkali resulted in the formation of 3β-acetoxy-14β, 15β-epithio-5β-card-20(22)-enolide (VIIb) and 3β-acetoxy-14α, 15α-epoxy-5β-card-20(22)-enolide (Ib).