Unanticipated stereoselectivity in the reaction of primaquine alpha-aminoamides with substituted benzaldehydes

Abstract

Imidazolidin-4-ones are commonly employed as skeletal modifications in bioactive oligopeptides, either as proline surrogates or for protection of the N-terminal amino acid against aminopeptidase- and endopeptidase-catalyzed hydrolysis. Imidazolidin-4-one synthesis usually involves the reaction of an alpha-aminoamide moiety with a ketone or an aldehyde to yield an imine, followed by intramolecular cyclization. We have unexpectedly found that imidazolidin-4-one formation is stereoselective when benzaldehydes containing o-carboxyl or o-methoxycarbonyl substituents are reacted with alpha-aminoamide derivatives of the antimalarial drug primaquine. A systematic computational and experimental study on the stereoselectivity of imidazolidin-4-one formation from primaquine alpha-aminoamides and various substituted benzaldehydes has been carried out, and they have allowed us to conclude that intramolecular hydrogen-bonds involving the CO oxygen of the o-substituent play a crucial role.