High-Pressure Cycloaddition of Tropones to Ethoxyethene. An Occurrence of [4+2] and [8+2] Cycloadditions

Although thermal cycloaddition of tropone to ethoxyethene under ordinary pressure conditions is synthetically impractical, the reaction was greatly improved under 10000 bar to give six products with 95% conversion (1 bar=105 Pa): the major products obtained in 66% yield were 9α- and 9β-ethoxybicyclo[3.2.2]nona-3,6-dien-2-ones, Diels–Alder adducts, which were accompanied by other two 8-ethoxy derivatives and two [8+2] cycloadducts. 2-Methoxy- and 2-chlorotropones similarly gave better results under high-pressure conditions. The 13CNMR spectra of these Diels–Alder adducts provided useful information regarding structure elucidations.