Asymmetric hetero Diels–Alder reaction using chiral cationic metallosalen complexes as catalysts

Chiral cationic (R,S)- or (R,R)-(salen)–manganese(III) and –chromium(III) complexes served as the catalysts for asymmetric hetero Diels–Alder reaction of Danishefskyʹs diene with aldehydes, achieving high enantioselectivity (up to 97% ee at 0°C). The reactions of aldehydes bearing no precoordinating functional group were well effected by using (R,R)-complexes as catalysts, while those of aldehydes bearing a precoordinating functionality were better effected by using (R,S)-complexes.