Enantioselective Construction of Chiral Cyclopropa[c]coumarins via Lewis Base-Catalyzed Cyclopropanation

The first highly enantioselective construction of chiral cyclopropa­[c]­coumarins was described. Using commercially available (bis)­cinchona alkaloid (DHQ)₂PYR as the chiral Lewis base catalyst, together with Cs₂CO₃ as the achiral base, the reaction of a series of coumarin-3-carboxylate and 3-benzoyl coumarins with tert-butyl 2-bromoacetate could give rise to the corresponding cyclopropa­[c]­coumarins bearing three continuous chiral stereocenters in 83–93% ee and 90–97% ee, respectively. The reaction is proposed to proceed via an in situ generated ammonium ylide intermediate.