Silica-Gel Catalyzed Stereoselective Conversion of Stabilized Phosphorus Ylides to Dialkyl ( Z )-2-(2-Methoxycarbonyl-phenoxy)-2-butenedioates in Solvent-Free Conditions

Protonation of the highly reactive 1:1 intermediates, produced in the reaction between triphenylphosphine and dialkyl acetylenedicarboxylates, by methyl salisylate leads to vinyltriphenylphosphonium salts, which undergo a Michael addition reaction with a conjugate base to produce dialkyl 2-(2-methoxycarbonyl-phenoxy)-3-(triphenylphosphoranylidene)butanedioates. Silica-gel powder was found to catalyze the stereoselective conversion of dialkyl 2-(2-methoxycarbonyl-phenoxy)-3-(triphenylphosphoranylidene)butanedioates to dialkyl (Z)−2−(2−methoxycarbonyl−phenoxy)−2−butenedioates in solvent−free conditions under microwave (0.5 KW, 3 min) and thermal (90°C, 90 min) conditions.