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Cyclic ethyl 2-diazo-3-hydroxy carboxylates were prepared by treating ethyl diazoacetate with LDA followed by reaction with a series of cyclic ketones. Further treatment of these R-diazo--hydroxy esters with boron trifluoride etherate in various solvents affords an unusual array of products. Product types and ratios were found to be strongly dependent on ring size and the solvent used. The reaction proceeds by Lewis acid complexation of the alcohol functionality of the diazo hydroxy ester with BF 3 etherate followed by neighboring-group participation of the diazo moiety to generate a cycloalkylidene diazonium salt. Loss of nitrogen produces a highly reactive, destabilized, linear vinyl cation. Ring expansion Via a 1,2-methylene shift leads to the formation of a more stable, bent cycloalkenyl vinyl cation. A subsequent 1,2-methylene shift results in ring contraction ultimately leading to a stable allylic cation. This cation is either trapped by the solvent or else undergoes cyclization with the adjacent ester group to give a lactone. Computational studies at the 6-31G* level were performed to determine the geometry of the optimized vinyl cations. Relative energies suggest a moderate energy gain for isomerization of the initial vinyl cation V 1 to the rearranged vinyl cation V 2 followed by a large stabilization in energy for subsequent conversion to the allyl cation A 1 . Compared with isolated product distributions, the energy profiles suggest kinetically-controlled V 1 f V 2 f A 1 migrations. Finally, the calculations suggest that in diethyl ether the carbocations may be coordinated to a molecule of solvent resulting in "protected" cationic intermediates with nonlinear geometries.

A variety of substituted 2-cyclopentenones are obtained in good yields (5342%) via intramolecular 1,5carbon-hydrogen insertion reactions of [b-(p-toluenesulfonyl)alkylidene]carbenes derived from Michael addition of sodium p-toluenesulfinate to fl-ketoethynyl(phenyl)iodonium triflates. An extension of the methodology using 8-amidoethynyl(pheny1)iodonium triflates provides a facile synthesis of y-lactams including fused bicyclic systems.The isomerization of a substituted cyclopentenone on silica gel is also reported.The development of new synthetic methods for the preparation of substituted cyclopentenones continues to be an area of intense interest' as a result of the ubiquity of the cyclopentenone nucleus in nature. Prostaglandins,z ambrosin,3 dicranenones? jasmonoids,5 and aromatin6 are but a few of the natural products incorporating this structural unit. Among the current, extensively employed synthetic methods for the construction of cyclopentenones are the Nazarov' and related cationic cyclizations and the Pau~on-Khand~.~ Co2(CO)~-mediated cyclizations of alkynes with olefins. However, each of these methods has limitation~.~-9Intramolecular carbon-hydrogen insertion reactions of carbenes have been widely used for the construction of a large variety of five-membered ring systems."J For example, gas-phase ther-Dedicated to Professor Jerome A. Berson on the occasion of his 70th birthday. Nantermet, P. G.; Trotter, B. W.; Vallin, I. M.; Watson, P. S.; McKerlie, L. A.; Reinhold, T. L.; Cheung, A. W.-H.; Stetson, K. A,; Dedopoulou, D.; Gray, J. L. A., tom Dieck, H., Eds.; Springer: Berlin, 1988; p 233. Krafft, M. E.; Scott, I. L.; Romero, R. H.; Feibelmann, S.; Van Pelt, C. E.

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