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.