Oxathiaborolium-catalyzed enantioselective [2 + 2] cycloadditions.

The generation of sulfur-stabilized borenium cations by SnCl₄, a halophilic Lewis acid, leads to the formation of oxathiaborolium pentachlorostannate, which is an excellent Lewis acid and is successfully used to catalyze the [2 + 2] cycloadditions of N-substituted maleimides and silyl enol ethers to afford cycloadducts with excellent enantioselectivities (17 examples, up to 96% ee). This efficient synthetic methodology provides a new viable and significantly efficient route to ring-fused cyclobutanes bearing quaternary stereogenic centers by combining pot, atom, and step economy to synthesize complex organic molecules.