Catalytic generation of ortho -quinone dimethides via donor/donor rhodium carbenes.

Substrates engineered to undergo a 1,4-C-H insertion to yield benzocyclobutenes resulted in a novel elimination reaction to yield ortho -quinone dimethide ( o -QDM) intermediates that undergo Diels-Alder or hetero-Diels-Alder cycloadditions. The analogous benzylic acetals or ethers avoid the C-H insertion pathway completely and, after hydride transfer, undergo a de-aromatizing elimination reaction to o -QDM at ambient temperature. The resulting dienes undergo a variety of cycloaddition reactions with high diastereo- and regio-selectivity. This is one of the few examples of catalytic generation of o -QDM without the intermediacy of a benzocyclobutene and represents one of the mildest, ambient temperature processes to access these useful intermediates. This proposed mechanism is supported by DFT calculations. Moreover, the methodology was applied to the synthesis of (±)-isolariciresinol in 41% overall yield.