Chiral Amine‐Catalyzed Stereoselective [4+2] Annulations of Alkenyl Thiazolones and Aliphatic Aldehydes via a Step‐Wise Mechanism.

An efficient diastereo‐ and enantioselective formal oxo‐Diels‐Alder reaction of enolizable aliphatic aldehydes and 5‐alkenyl thiazolones was developed through chiral amine catalysis. With a robust enamine activation strategy, both simple aliphatic aldehydes and a challenging aqueous acetaldehyde could participate well as competent dienophiles in the reaction. A variety of thiazole‐fused dihydropyran derivatives were facilely produced in up to 99% yield with up to >99:1 e.r. under mild conditions. Interestingly, a high dosage of the inexpensive aqueous acetaldehyde in a similar catalytic system could lead to a densely functionalized thiazolone product through a three‐component cascade reaction, suggesting a step‐wise mechanism for the established [4+2] annulation process. [ABSTRACT FROM AUTHOR]