Development of Efficient Methods for the Synthesis of Medium- and Large-Sized Carbocycles with the Aid of Samarium

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Medium- as well as large-sized carbocyclic compounds whose ring skekitorls are composed entirely of sp^3 carbons have been prepared in high yields under mild conditions by utilizing Sml_2-promoted intramolecular Reformatsky reaction. The reaction seems to proceed through samarium enorate intermediate as the result of successive two-ekx-tron reduction of a-bromo esters. The distinct property of samarium such as large ionic radius, flexible coordination, and high oxophilicity should play an important role for the cyclizadion : The samarium-linked large-sized chelate, from which the carbon-carbon bond formation would take place like a ring contraction, and the six-membered chelate after cyclization might be involved in the process.Conjugated esters were instantaneously hydrodimerized at room temperature by use of the reduction system, Sml_2-THF-HMPA, in the presence of proton sources, and perfect stereoselection was realized in the reaction of N, N-dibenzyl crotonamide. However, the method could riot be successfully applied to macrocyclization. In connection with the present study, a very efficient method for the selective reduction of alpha, beta-unsaturated esters and amkles via Sml_2-promoted electron transfer process was also developed.Pinacol coupling reaction with lanthanide triflate-samarium (II) iodide system afforded medium-sized carbocycle in moderate yield in the preliminary experiment. This approach seems to be promising and is being further studied in this laboratory.All other efforts to develop useful cyclization methods to botain medium-sized carbocycles ended in failure : The attempted methods are (i) intramolecular Barbier-type reaction of w-oxo allylic halides, (ii) acyloin condensation, and (iii) ketone-alkene or ketone-alkyne reductive coupling reaction. But, the last method was found to be useful as carboncarbon bond-forming reaction via radical process.