Pummerer-Type Reactions in the (2-Methylsulfanyl-2-phosphonyl) Thiopyran 1-Oxide Series.

Abstract Two (3,6-dihydro-2-methylsulfanyl-2 H -thiapyran-2-yl)phosphonate derivatives have been chemoselectively oxidized on the thiopyran sulfur. The obtained allylic six-membered cyclic sulfoxides 2a and 2b were reacted under Pummerer reaction conditions leading to new thiopyran derivatives ( 4a and 6b , respectively). In both studied cases, the nucleophilic attack of β,γ-unsaturated thionium ion intermediate took place regioselectively at the γ-position (even when occupied by a methyl substituent like in 2b ). An unexpected second product 7b was however obtained from substrate 2b (having the dimethyl-substituted double bond). Dephosphorylation of 7b under basic conditions led to an original conjugated tri-unsaturated trifluromethylcarbonyl thiopyran product ( 8b ). These results represent new original examples of the Pummerer reaction.