Enantiospecific cyclization of methyl N-(tert-butoxycarbonyl)-N-(3-chloropropyl)-D-alaninate to 2-methylproline derivative via ‘memory of chirality’ in flow

We report for the very first time a continuous-flow route to perform the intramolecular cyclization of haloalkyl-substituted α-amino esters via memory of chirality (MoC), using lithium bis(trimethylsilyl)amine as a base and methyl N-(tert-butoxycarbonyl)-N-(3-chloropropyl)-D-alaninate as a model reactant. The various reaction parameters, such as temperature, residence time, reactant stoichiometry, or type and concentration of the base were optimized to maximize the yield of the cyclized product and its enantiomeric excess. At the conditions identified, the reaction was eventually scaled up, reaching a productivity of 11 g h−1. Compared to the standard batch protocols available in the literature, the use of a microreactor enables a better control of the exothermicity associated with the addition of the organolithium reagent to the reaction mixture, resulting in operations at more practical temperatures, with high enantiospecificity and full conversion of the reactive amino ester within a few seconds of residence time.