Catalytic cis-selective [2 + 2] cycloaddition reaction of neutral alkenes with α,β-unsaturated aldehydes/ketones

Generally, cyclobutanes obtained from the intermolecular [2 + 2] cycloaddition reactions of acyclic alkenes suffer from poor diastereoselectivities (dr). Herein, a chiral oxazaborolidinium ion (COBI)-catalyzed [2 + 2] cycloaddition reaction of acyclic neutral alkenes with α,β-unsaturated aldehydes/ketones is presented. The cis-formed cyclobutanes are produced in excellent diastereo- and enantioselectivities (ee). The cis-selectivity depends on the steric hindrance of the substituent attached to boron in COBI. The resulting cyclobutanes have two adjacent larger substituents cis-oriented and are thus structurally strained. Note that additional transformations involving the cleavage of the carbon–carbon bond can be seen in enantioselective ring enlargement reactions with the aldehydes. Moreover, this new synthetic method leads to a short synthesis of a junionone variant.