The Effect of Phase-Transfer Catalysis in the 1,3-Dipolar Cycloaddition Reactions of Azomethine Ylides − Synthesis of Substituted Prolines Using AgOAc and Inorganic Base in Substoichiometric Amounts

The 1,3-dipolar cycloaddition reaction between imino ester 1a and ethyl acrylate, employing phase-transfer catalysis (PTC) conditions in the presence of substoichiometric amounts of Lewis acids and inorganic bases, has been studied for the first time. Effects were analysed in the order: solvent, phase transfer agent, inorganic base, metal salt (Lewis acid), quantity of metal salt and the substituents on the 1,3-dipole. The two best methods found in this study [method A: THF, TBAC (10 mol-%), KOH (10 mol-%), AgOAc (10 mol-%), room temp.; method B: toluene, KOH (10 mol-%), AgOAc (10 mol-%), room temp.] were then applied to the cycloaddition reaction of the imino esters 1a and 5a (derived from alanine and glycine) with alkenes. These results were compared to those obtained under thermal 1,3-dipolar cycloaddition reaction conditions and to those previously reported using metallodipoles in the presence of organic bases.