Organocatalytic asymmetric synthesis of both cis- and trans-configured pyrano[2,3-b]chromenes via different dehydration pathways

Abstract

The [3 + 3]-cycloaddition reaction of chroman-2-ols with β,γ-unsaturated α-ketoesters via an enamine-catalyzed sequential process is reported. The protocol provides efficient access to polycyclic hemiketal-containing compounds, which could be converted into both enantiomers of cis- and trans-fused pyrano[2,3-b]chromene derivatives with excellent stereoselectivity by different dehydration reaction pathways. The catalytic system could be recycled on a larger scale eight times without significantly affecting the yield and stereoselectivity. The possibilities for further structural diversification are demonstrated by various transformations.