Elucidation of Intramolecular Reactions of Acyl Radicals

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In this work, several new aspects of intramolecular behaviors of acyl radicals have been studied. Intramolecular homolytic substitution of an acyl radical at sulfur was found to be an efficient process. By using t-butyl group as a leaving radical, a variety of gamma-thiolactones were prepared based on this intramolecular S_H2 reaction. The rate of cyclization is approximately 7.5 x 10^3s^<-1> at 25ﾟC.Ab initio MO calculations suggest that the reaction occurs via T-shaped transition state where the incoming acyl radical and the leaving alkyl radical have a linear structural relationship. The system can be successfully extended to the synthesis of gamma-selenolactones. Acyl radical cyclization onto O-C double bond was also examined. Using a slow mediator, the formation of gamma-lactone ring was observed. This formal radical 5-endo cyclization product was turned out to be formed via iodine atom transfer to acyl radical followed by ionic 5-endo cyclization. Ab initio MO studies were carried out to know the mechanism of intramolecular cyclization behavior of 5-hexenoyl radical. It was suggested that the formation of six-membered redical in this system is due to the isomerization of the initially formed five-membered radical.Acyl radical cyclizations onto N-C double bonds were investigated. Unlike the corresponding alkyl radical cyclizations, the cyclizations took place in complete nitrogen-philic manner to give lactam rings.The isomerization behavior of alpha, beta-unsaturated acyl radicals was studied in terms of theory and synthesis. Ab initio MO calculations suggest that E-form and Z-form can interconvert very easily via a ketennyl radical. This is in good accord with several experimental results which include that 5-exo cyclization of alpha, beta-unsaturated acyl radicals gave uniform cyclized product irrespective of the starting geometries of the radical.