Synthesis of Isochroman Derivatives via Oxa-Pictet–Spengler Reaction of Vinylogous Esters

Trimethylsilyl triflate (TMSOTf) can be efficiently used for the synthesis of isochromans via Pictet-Spengler type reaction of vinylogous esters in good yields. The methodology is applied for the formal synthesis of key intermediate alcohols for the synthesis of D-4 antagonist sonepiprazole (+/-)-U-101387 and the isochroman (+/-)-U-54537.