Iminium Catalytic [2+2] Cycloaddition

Significance

An organocatalytic enantioselective [2+2] cycloadditon of unactivated alkenes 2 with 3 to give optically active 1-acyloxycyclobutanecarbaldehydes is reported. A further ring expansion yields optically active 2-hydroxycyclopentanone derivatives 7. Several acyloxy groups were tested and electronic or steric effects did not have a great influence on chemical yields and stereo­selectivities. The alkene scope was screened. Cyclic and acyclic trialkylethenes give moderate to good yields with high enantiomeric excess (up to 89% yield, 95% ee, dr up to 5:95). However, 1,1- and 1,2-dialkyl­ethenes showed no reactivity.