Pilot Plant Preparation of an α_vβ₃ Integrin Antagonist. Part 1. Process Research and Development of a (S)-β-Amino Acid Ester Intermediate: Synthesis via a Scalable, Diastereoselective Imino-Reformatsky Reaction

Described are four process research investigations directed toward discerning a scalable, enantioselective method for preparing (S)-β-amino acid ester 3, a key intermediate to the α_vβ₃ integrin antagonist 1. Reported are an asymmetric Michael reaction approach, attempts to enantioselectively hydrogenate an enamine, resolution of (±)-3 via diastereomeric salt formation, and a synthetic route employing a novel, diastereoselective imino-Reformatsky reaction. This last research initiative proved successful and was employed as the enabling route to initial API supply. Process development of this enabling chemistry is reported. The technical issues researched and optimized were (1) the necessity of employing MEM-protection for high yield and diastereoselectivity in the imino-Reformatsky reaction, (2) the reaction kinetics of MEM chloride hydrolysis and the application of these data to an on-scale quench procedure, (3) the efficient formation of the (S)-phenylglycinol imine 15 in NMP and a dehydration of this product solution on-scale, employing molecular sieves, (4) a calorimetric study of the Reformatsky reaction and the application of these data, (5) the replacement of Pb(OAc)₄ with NaIO₄ and the use of methylamine to sequester competing oxazolidine formation, and (6) further development of the isolation and purification protocol for the ethyl ester, p-TsOH salt of (S)-3. The results and challenges associated with two campaigns in which the potential commercial process was practiced are discussed.