The use of nonactivated iminodienophiles in the stereoselective aza-Diels-Alder reaction

Abstract This paper describes the preparation of nitrogen-containing bicycles by the aza-Diels–Alder reaction of nonactivated iminodienophiles and cyclopentadiene. Readily available starting materials such as ( S )-(−)-lactate and l -amino acids were used for the preparation of chiral aldehydes with high enantiomeric excess. The improved oxidation procedure by Dess–Martin periodinane was employed for the synthesis of l -alanine derived phthalimide protected aldehyde 14 , which was difficult to obtain in high enantiomeric excess by other methods. The influence of different Lewis acids on the stereoselectivity of the aza-Diels–Alder reaction was also investigated: It was found that the use of a combination of BF 3 ·Et 2 O and TFA in the cycloaddition leads to complete racemization of the imine prepared from 14 whereas the use of TiCl 4 gives the cycloaddition products 17a and 17b with high enantioselectivity (90%).