Asymmetric synthesis of pipecolic acid derivatives using the aza-Diels-Alder reaction
摘要:
Imines of the type R-N=CHCO2Et can be coerced into undergoing a (4 + 2) cycloaddition with substituted dienes if the reaction is carried out in DMF in the presence of both water and acid; these reactions show extremely high regio- and diastereoselectivity. Use of the 1-phenylethyl group as a chiral auxiliary leads to moderate asymmetric induction (typical d.e. ca. 70%); moreover, the diastereoisomers are surprisingly easy to separate, giving a short general route to optically pure substituted pipecolic acid derivatives.