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 la and ethyl acr ylate, employing phase-transfer catalysis (PTC) conditions in the presence of substoichiometric amounts of Lewis acids and inorganic bases, has been s tudied for the first time. Effects were analysed in the order: solvent, pha se transfer agent, inorganic base, metal salt (Lewis acid), quantity of met al 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 mo l-%), 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 w ere compared to those obtained under thermal 1,3-dipolar cycloaddition reac tion conditions and to those previously reported using metallodipoles in th e presence of organic bases.