Herein we report two processes facilitated by diisopropylethylamine (DIPEA) for the synthesis of novel bridged polycyclic molecule analogues to natural products. The use of 4-bromoisochroman-3-one initiated an autoxidation reaction, followed by a Diels-Alder cycloaddition in the presence of electron-deficient dienophiles. Mechanistic studies revealed isochromane-3,4-dione as a key intermediate, which undergoes in situ dienolization/dearomatization followed by a [4 + 2] cycloaddition. Subsequently, the synthesis and direct application of isochromane-3,4-diones in the Diels-Alder reaction enabled the development of an alternative method with an enhanced efficiency and improved atom economy. In addition to chalcones, other enones and common electron-poor alkenes, bearing ester, nitro and cyano electron-withdrawing groups (including both terminal, acyclic and cyclic alkenes), were successfully reacted. The mechanism was also investigated, and a subsequent reductive ring opening was successfully carried out.

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http://dx.doi.org/10.1021/acs.joc.4c02522DOI Listing

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