Organocatalytic asymmetric [2+2] cycloaddition of alkynes with quinones

A chiral phosphoric acid catalyzed enantioselective [2 + 2] cycloaddition of alkynylindols or alkynylnaphthols with quinones is disclosed. A class of functionalized cyclobutenes with excellent yields, diastereo- and enantioselectivities were prepared under mild reaction conditions (70 examples, up to 99% yield, 99% ee, all > 50:1 dr). Mechanistic studies revealed that a dearomatization of indole or naphthol occurred to initiate the cycloaddition, followed by an intramolecular Michael addition with in situ generated allene-iminium or vinylidene-quinone methide intermediate. The competitive [2 + 3] cycloaddition was prevented in this catalytic system. An interesting central to axial chirality conversion via a rearrangement process was realized during transformation of the product.