An Efficient Synthesis of 2-Phenyl-4H-1-benzothiopyran-4-ones from Thiosalicylic Acid

Abstract

2 such as antimalarial, antimicrobial, and antifungal activity and are useful as potent inhibitors of steroid sulfatase. 3 In general, 2-phenyl-4H-1-benzothiopyran-4-ones are synthesized by the condensation of thiophenols with ethyl benzoylacetates in polyphosphoric acid at 90 o C, but the yields are low to mode- rate. 4 The cyclization of ethyl β-(arylthio)cinnamates, derived from Michael addition of thiophenols to ethyl phenylpro- piolates, with stannic chloride or phosphorus pentoxide/met- hanesulfonic acid gives 2-phenyl-4H-1-benzothiopyran-4-ones. 5 However, this method could not be applied for the synthesis of methoxy substituted thioflavones because competitive cyclization into the cinnamyl aromatic ring, rather than the sulfur-bearing ring, occurs when cinnamyl ring is activated by methoxy substituents. The reaction of S-aroyl derivatives of thiosalicylic acid with N-phenyl(triphenylphosphoranylidene) ethemine 6 or (trimethylsilyl)methylenetriphenylphosphorane 7