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A chiral N-heterocyclic carbene (NHC)-catalyzed Diels–Alder reaction of 2-oxoindolin-3-ylidenes and α-chloroaldehydes was developed for the synthesis of fused pyrano2,3-bindoles in good to excellent yields (up to 99%) with high cis-diastereoselectivities (>99:1 dr) and enantioselectivities (up to >99% ee).
Yang et al. (Fri,) studied this question.