Enantioselective Synthesis of Cyclobutanes via Sequential Rh-catalyzed Bicyclobutanation/Cu-catalyzed Homoconjugate Addition

Enantiomerically
enriched cyclobutanes are constructed by a three-component process
in which t-butyl (E)-2-diazo-5-arylpent-4-enoates
are treated with Rh2(S-NTTL)4 to provide enantiomerically enriched bicyclobutanes, which can subsequently
engage in homoconjugate addition/enolate trapping sequence to give
densely functionalized cyclobutanes with high diastereoselectivity.
This three-component, two-catalyst procedure can be carried out in
a single flask. Rh2(S-NTTL)4-catalyzed reaction of t-butyl (Z)-2-diazo-5-phenylpent-4-enoate gives the Büchner cyclization
product in excellent enantioselectivity.