N,N-Diisopropyl-N-phosphonyl imines lead to efficient asymmetric synthesis of aziridine-2-carboxylic esters

Abstract

Highly diastereoselective asymmetric synthesis of chiral aziridine-2-carboxylic esters is reported for 20 examples with good yields (51–87%) and excellent diastereoselectivities (>99 : 1 dr for most cases). The modified N-phosphonyl imines have proven to be superior to previous imine auxiliaries for the aza Darzens reaction by using a secondary isopropyl group to replace the primary benzyl group for N,N-diamino protection. In the meanwhile, a special operation by slowly adding the pre-cooled imine solution at −78 °C into the preformed β-bromo lithium enolate mixture at this temperature in the presence of 4 Å molecular sieves was found to be crucial in terms of yields and diastereoselectivity. The present method can provide an easy and general access to β-hydroxy α-amino acids and other important amino building blocks.