Wittig–Horner mediated synthesis of 4-vinyl sulfide derivatives of pyrazoles

Abstract

The synthesis of a series of 4-vinyl sulfide derivatives of 1,3-diarylpyrazoles, including their corresponding sulfoxides and sulfones, is reported. Access to the target vinyl sulfides was stereoselectively achieved, in moderate to good yields, by the n-BuLi-mediated Wittig–Horner reaction of 4-formylpyrazoles with arylthiophosphonates and α-chloroarylthiophosphonates in dimethoxyethane. Their oxidation with H2O2in AcOH and mCPBA in CH2Cl2afforded satisfactory yields of the expected vinyl sulfoxides and vinyl sulfones, respectively. Enrichment in the more stable isomers during both oxidation processes was detected and a plausible general mechanistic explanation was given to these observations.