. Chem. 69, 1929Chem. 69, (1991. ' The structures of five stabilized phosphonium ylides 1-5 were studied by 'H, 13C, and 3 '~ NMR. Various trapping experiments were followed by investigation of the relative reactivities of these ylides vis-8-vis benzaldehyde. A novel effect of some significance, the influence of acidic contaminants (conjugate acids, HzO, etc.) on the structure and reactivity of these compounds, is discussed.

The reactions of stabilized phosphoranes with cyclic anhydrides give enol-lactones as final products. The initial condensation, however, leads to the formation of acyclic adducts that are observable by NMR, can be easily trapped, and, in some cases, can be isolated. A study of the mechanism of these condensations by NMR spectroscopic methods and by various trapping experiments is described and the reaction pathway is proposed.

A study by NMR spectroscopic methods and trapping experiments of the mechanism of Wittig reactions between stabilized phosphoranes and unsymmetrically substituted cyclic anhydrides suggests that two reactions are involved: ( I ) a low-energy, reversible formation of acyclic adducts; and (2) a higher energy "Wittig olefination" reaction leading to enollactones. The latter, more selective, transformation requires a more highly organized transition state in which T-stacking and stabilizing complexations are important factors.

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Synthesis of Fused-Ring Cyclobutenones via a Tandem (2 + 2) Cycloadditionβ-Elimination Sequence.-Photoreaction of the enol acetates (I) of 1,3-cyclohexanediones with the cycloalkenes (II) gives the fused cyclobutanes (III). These are converted into the corresponding cyclobutenes (IV) by base-induced . beta.-elimination of acetic acid. -(GALATSIS, P.; ASH-BOURNE, K. J.; MANWELL, J. J.; WENDLING, P.; DUFAULT, R.; HATT, K. L.; FERGUSON, G.; GALLAGHER, J. F.; J.