INTRAMOLECULAR [4-CYCLOADDITIONS OF PHOTOCHEMICALLY GENERATEDOXYALLYLZ WITTERIONS - A ROUTE TO FUNCTIONALIZED CYCLOOCTANOID SKELETONS(3])

[4 + 3]-Cycloaddition of photochemically generated, 4-pyrone derived o xyallyl 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 m oderate 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-on es or 3-hydroxy-2-methyl-4H-pyran-4-one, and their viability in intram olecular [4 + 3]-cycloadditions was examined. Substrates 5a-e, in whic h the tether was attached to the incipient zwitterion via a carbon, un derwent 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), isom eric pyran-2-ones 11e/12e were produced in addition to [4+3]-cycloaddu ct 8e and cage compound 9e. Moreover, it was found that 11e and 12e bo th underwent subsequent photochemical crossed [4 + 4]-cycloadditions t o give lactone-bridged cyclooctadienes 13e and 14e. The structures of 11e/12e and their [4 + 4]-cycloadducts were confirmed by synthesis of each of the 2-pyrones via alternative routes.