Abstract
An intermolecular-intramolecular Michael reaction sequence was developed for the synthesis of cyclopentanes and cyclohexanes. Silyl ketene acetals and silyl enol ethers were used as the nucleophilic initiators of the ring closure of bis-enones and bis-enoates. Using experimental conditions developed for the Mukaiyama-Michael and group transfer polymerization reactions, three procedures were developed to accomplish these cyclizations. In the first procedure, anionic catalysts effected the silyl ketene acetal initiated cyclization of bis-enoates or bis-enones. The reactions were clean and high yielding. The formation of cyclopentanes was completely stereoselective, while cyclohexanes were formed with moderate stereoselectivity. Four chiral centers could be formed with complete control of relative configuration in the conversion of unsymmetrical bis-enoate 31 to cyclopentane 33. Model studies directed toward the synthesis of polycyclics from polyenoates were not promising. In the second procedure, Znl2 was an effective Lewis acid catalyst for the cyclization of bis-enoates initiated by silyl ketene acetals. These reactions were also clean and high yielding. Cyclopentanes again were formed with higher stereoselectivity than cyclohexanes, although in both cases the degree of selectivity was lower, and the sense of selectivity was different, than for the corresponding anion-catalyzed reactions. The Znl2 procedure allowed the use of r-butyldimethylsilyl ketene acetals, which were ineffective initiators for the anion-catalyzed protocol. In the third procedure, the combination SnCl2/trityl chloride successfully mediated the silyl enol ether or silyl ketene acetal initiated cyclization of bisenones. The reactions again were high yielding, with completely stereoselective formation of cyclopentanes and moderately stereoselective formation of cyclohexanes. The sense of stereoselection was the same as in the anioncatalyzed procedure...
Klimko, Peter Gerard (1992). Silyl group transfer cyclizations. Texas A&M University. Texas A&M University. Libraries. Available electronically from
https : / /hdl .handle .net /1969 .1 /DISSERTATIONS -1307082.