Intramolecular -Cycloadditions of Photochemically Generated Oxyallyl Zwitterions: A Route to Functionalized Cycloctanoid Skeletons

Article abstract of DOI:10.1021/jo00076a047

<4 + 3>-Cycloaddition of photochemically generated, 4-pyrone derived oxyallyl zwitterions and furans was examined as a potential approach to keto-bridged cyclooctenes.In one case, intermolecular cycloaddition proceeded to give adduct 3 with complete diastereoselectivity and in moderate yield, but the reaction was not general.Substrates 5a-i were then prepared in one to three steps from either 3-methyl-4H-pyran-4-ones or 3-hydroxy-2-methyl-4H-pyran-4-one, and their viability in intramolecular <4 + 3>-cycloadditions was examined.Substrates 5a-e, in which the tether wasattached to the incipient zwitterion via a carbon, underwent cycloaddition to give <4 + 3>-cycloadducts 7 and 8 in varying yields, along with small amounts of cage compound 9.In contrast, 5f-i, in which the tether was attached to the incipient zwitterion via an oxygen, gave only solvent-trapping products 10.In one case (5e), isomeric pyran-2-ones 11e/12e were produced in addition to <4 + 3>-cycloadduct 8e and cage compound 9e.Moreover, it was found that 11e and 12e both underwent subsequent photochemical crossed <4 + 4>-cycloadditions to give lactone-bridged cyclooctadienes 13e and 14e.The structures of 11e/12e and their <4 + 4>-cycoadducts were confirmed by synthesis of each of the 2-pyrones via alternative routes.