Unexpected Substituent Effects in Spiro-Compound Formation: Steering -Aryl Propynamides and DMSO toward Site-Specific Sulfination in Quinolin-2-ones or Spiro[4,5]trienones.

A highly substituent-dependent rearrangement allows for the novel and SOCl-induced divergent synthesis of 3-methylthioquinolin-2-ones and 3-methylthiospiro[4.5]trienones through intramolecular electrophilic cyclization of -aryl propyamides. DMSO acts as both solvent and sulfur source, and use of DMSO-/ enables the incorporation of SCH or SCD moieties to the 3-position of the heterocyclic framework. Different -substituents trigger divergent reaction pathways leading to the formation of quinolin-2-ones for mild substituents and spiro[4,5]trienones for both electron-withdrawing and -donating substituents, respectively. On the basis of both computational and experimental results, a new mechanism has been put forward that accounts for the exclusive spirolization/defluorination process and the surprising substituent effects.