Syntheses of 2H-1-Benzothiopyran-2-ones (Thiocoumarins) and Related Compounds from Benzenethiols and Diketene

Abstract The products formed by the reaction of benzenethiols with diketene in the presence of H2SO4 are (E)-β-(arylthio)crotonic acids (2) and/or isomeric (Z)-β-(arylthio)crotonic acids and not, as has been reported, S-phenyl 3-oxobutanethioates (1). Compounds 1a–k, as the precursor of thiocoumarins, were prepared from benzenethiols and diketene in the presence of triethylamine. The reaction of 1 with various condensing agents has been examined to prepare 2H-1-benzothiopyran-2-ones (thiocoumarins). It is found that 4-methyl(thiocoumarins) were conveniently prepared by the reaction of 1 with anhydrous aluminium chloride in yields of 16–48%. When 1 was treated with PPA, isomeric 2-methyl(thiochromones) were preferentially obtained in yields of 5–66%, and compound 2 was isolated as an intermediate. The spectral characteristics of 4-methyl(thiocoumarins) have also been described.