Asymmetric Formal [4 + 2] Annulation of o-Quinone Methides with β-Keto Acylpyrazoles: A General Approach to Optically Active trans-3,4-Dihydrocoumarins.

An asymmetric cascade reaction between β-keto acylpyrazoles and o-quinone methides in a formal [4 + 2] fashion to access potentially pharmacological active trans-3,4-dihydrocoumarins has been achieved efficiently by using a quinine-based chiral squaramide as the catalyst. The desired products were obtained in high yields with excellent diastereo- and enantioselectivities (up to 96% yield, >19/1 dr and 96% ee) under mild reaction conditions.