Enantioselective Functionalization of Prochiral Cyclobutanones and Cyclobutenones

Enantioselective synthesis of cyclobutane derivatives is still a challenging topic in asymmetric synthesis. [2+2]-Cycloaddition and skeleton rearrangement are two primary strategies to this end. Recently, functionalization of cyclobutanones and cyclobutenones, which are readily available via [2+2]-cycloadditions as prochiral substrates, has emerged as a powerful tool to access versatile four-membered ring compounds. Herein, we summarize some recent advances in these areas from our and other groups.

Cage-confined photocatalysis for wide-scope unusually selective [2 + 2] cycloaddition through visible-light triplet sensitization

Light-induced [2 + 2] cycloaddition is the most straightforward way to generate cyclobutanes, which are core structures of many natural products, drugs and bioactive compounds. Despite continuous advances in selective [2 + 2] cycloaddition research, general method for intermolecular photocatalysis of acyclic olefins with specific regio- and diastereoselectivity, for example, syn-head-to-head (syn-HH) cyclobutane derivatives, is still lack of development but highly desired. Herein, we report a cage-confined photocatalytic protocol to enable unusual intermolecular [2 + 2] cycloaddition for α,β-unsaturated carbonyl compounds. The syn-HH diastereomers are readily generated with diastereoselectivity up to 99%. The cage-catalyst is highly efficient and robust, covering a diverse substrate range with excellent substituent tolerance. The mimic-enzyme catalysis is proposed through a host-guest mediated procedure expedited by aqueous phase transition of reactant and product, where the supramolecular cage effect plays an important role to facilitate substrates inclusion and pre-orientation, offering a promising avenue for general and eco-friendly cycloaddition photocatalysis with special diastereoselectivity.

Catalytic asymmetric [2+2] cycloaddition between quinones and fulvenes and a subsequent stereoselective isomerization to 2,3-dihydrobenzofurans

A chiral Lewis acid catalyzed enantioselective [2+2] cycloaddition between quinones and fulvenes was reported for the first time. The method afforded a series of [6,4,5]-tricyclic cyclobutane derivatives in good yields with excellent regio- and stereoselectivities. Furthermore, the [2+2] adducts could be easily converted into formal [3+2] adducts efficiently and stereoselectively.

Synthesis, structure, and stereospecific cross-[2+2] photocycloaddition of pseudodimeric complexes based on ammonioalkyl derivatives of styryl dyes

Two different styryl dyes form pseudodimeric complexes via hydrogen bonding and stacking interactions; irradiation of these complexes gives rctt-cyclobutane derivatives.