Diels-Alder reaction of 4-bromo-6-spiroepoxycyclohexa-2,4-dienone with electron-rich and neutral dienophiles

Article abstract of DOI:10.1016/S0040-4020(98)01041-2

Spirodienone 1, prepared by Adler-Becker oxidation of 4-bromo-2- hydroxymethylphenol, undergoes [4+2] cycloaddition with various dienophiles (enol ethers, enol esters, styrenes, N-methylvinylacetamide) under thermal conditions (20-160°C). Three sets of experiments have been carried out, either with CH₂Cl₂ as solvent or neat with 1, or under tandem oxidation- cycloaddition conditions with phase-transfer catalysis. Complete regio- and syn diastereofacial selectivities were obtained but a switch in endo/exo selectivity has been observed between enol ethers and styrenes (endo addition), enol esters (low selectivity) and an enamide (exo addition). The FMO analysis confirms that theses reactions are under LUMO(diene) control and that the observed regioselectivity is in agreement with orbital coefficients. Except for vinyl acetate, the formation of the major isomer is qualitatively confirmed at the AM1 level.