Phosphoric Acid Catalyzed Route to α-Amino Esters

Significance

The phosphoric acid catalyzed transfer hydrogenation of imines is established for a variety of substrates. Acyclic α-imino esters, however, have not been employed, even though the products arising from this transformation are interesting amino acid precursors. The authors now report the use of bisphenol-derived phosphoric acids as catalysts for a highly enantioselective reaction, leading to PMP-protected α-amino esters in excellent yields. Bisphenol-derived phosphoric acids previously employed for the amidation of Boc-imines by the same group (J. Am. Chem. Soc. 2005, 127, 15696-15697) proved superior to the most commonly employed class of BINOL-derived phosphoric ­acids in this reaction. The method can be applied to preformed imines as well as to a one-pot protocol with in situ generation of the imines, albeit at the cost of somewhat reduced yields in the latter case.