Intramolecular Oxyallyl–Carbonyl (3 + 2) Cycloadditions

Cycloadditions involving
oxyallyl intermediates typically require
an electron-rich diene or alkene, but we have discovered the first
examples of the cycloaddition of heteroatom-stabilized oxyallyls onto
carbonyl groups. An oxazolidinone-substituted oxyallyl undergoes chemoselective
(3 + 2) cycloaddition onto the carbonyl group of a tethered dienone
in preference to formation of the expected (4 + 3) cycloadduct. Density
functional theory calculations indicated that the (3 + 2) cycloaddition
takes place through a concerted, highly asynchronous mechanism. The
transition state features simultaneous interactions of the oxyallyl
LUMO with the carbonyl π and lone-pair orbitals, making this
reaction “hemipseudopericyclic” (halfway between purely
pericyclic and purely pseudopericyclic). Further (3 + 2) cycloadditions
involving tethered phenyl ketones and a tethered enone were predicted
theoretically and verified experimentally.