Alkoxide-catalyzed ring expansion of 1,3-cyclobutanediones with aldehydes

Catalytic asymmetric synthesis of spirocyclobutyl oxindoles and beyond via [2+2] cycloaddition and sequential transformations

“… 16 In continuation with our interest in chiral N , N ′-dioxide/metal Lewis acid catalysis 17 and inspired by elegant studies 5,13,14 on the N -allenamide chemistry, herein, we demonstrated that catalysts could promote the direct asymmetric [2+2] cycloaddition of ( E )-alkenyloxindoles with N -allenamides for straightforward access to optically active spirocyclobutyl oxindole derivatives ( Scheme 1b ). Moreover, further diversified transformation 18 of the internal C C bond-involved product catalyzed by a copper salt enabled the one-pot synthesis of tetradropyranoindole derivatives, which seem to be the formal [4+2] cycloaddition adducts of the terminal C C bond of N -allenamides. Further transformation in the presence of acid and water gave 4′-methylenedihydrofuranone-substituted oxindole derivatives.…”