Fully Substituted Carbon Centers by Diastereoselective Spirocyclization: Stereoselective Synthesis of (+)-Lepadiformine C
作者:Matthew A. Perry、Matthew D. Morin、Brian W. Slafer、Scott A. Wolckenhauer、Scott D. Rychnovsky
DOI:10.1021/ja104250b
日期:2010.7.21
Reductive lithiation of N-Boc alpha-amino nitriles generated alpha-amino alkyllithium reagents with unexpected selectivity. The intermediate radical prefers to align with the nitrogen lone pair, and this interaction leads to an A(1,3)-strain effect that biases the conformation of the radical. In cyclohexane rings with alpha-substituents the net effect is an inversion of configuration on reductive lithiation. In the presence of a tethered electrophile the alkyllithium cyclizes to produce a spiro compound, again with inversion of configuration. The overall result is retention of configuration in the cyclization reaction. The same overall selectivity is found with alpha-oxygen alkyllithium cyclizations, but in this case both steps proceed with retention. The difference can be explained by careful consideration of the intermediate geometries. The alpha-amino spirocyclization was utilized in a concise and stereoselective synthesis of lepadiformine C.
Synthesis of cyclic and acyclic enol ethers (vinyl ethers)
作者:Paul G. Gassman、Stephen J. Burns、Keith B. Pfister
DOI:10.1021/jo00058a027
日期:1993.3
A general method has been developed for the conversion of both cyclic and acyclic acetals of cyclic ketones, acyclic ketones, and aldehydes into enol ethers through treatment of the acetal with trimethylsilyl triflate in the presence of N,N-diisopropylethylamine. The range of isolated yields of enol ethers from the various classes of acetals was as follows: cyclic acetals of cyclic ketones, 83-98%; acyclic acetals of ketones, 72-94%; acyclic and cyclic acetals of aldehydes, 65-90%.