Intramolecular [4 + 3]-cycloadditions of photochemically generated oxyallyl zwitterions: a route to functionalized cyclooctanoid skeletons
作者:F. G. West、Claudia Hartke-Karger、Daniel J. Koch、Cynthia E. Kuehn、A. M. Arif
DOI:10.1021/jo00076a047
日期:1993.11
[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 was attached 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]-cycloadducts were confirmed by synthesis of each of the 2-pyrones via alternative routes.