Formation of Carbocycles by Intramolecular Conjugate Displacement: Scope and Mechanistic Insights
2009-04-29T00:00:00Z (GMT) by
A detailed study has been made of a method of ring closure categorized as an all-carbon intramolecular conjugate displacement (ICD). This reaction involves intramolecular addition of a carbanion, which is stabilized by at least one electron-withdrawing group, to a Michael acceptor which has a leaving group in an allylic position. The process formally resembles a combination of Michael addition and S<sub>N</sub>2′ displacement. The overall result is formation of a ring with loss of the allylic leaving group and shift of the original double bond to a new location spanning the positions of the electron-withdrawing substituent of the Michael acceptor subunit and the original allylic leaving group. The starting materials are easily prepared by a selenium-based version of the Morita−Baylis−Hillman reaction. The cyclizations are transition metal free and occur under mild conditions, using DBU or Cs<sub>2</sub>CO<sub>3</sub> in MeCN or THF. Acetate is a suitable leaving group and the electron-withdrawing substituent of the Michael acceptor unit can be CO<sub>2</sub>R, SO<sub>2</sub>Ph, or CN. Six- and seven-membered rings are formed efficiently, and complex structures, such as those resembling the core of CP-225,917, are easily assembled. The products of these ICD reactions are themselves classical Michael acceptors. A range of mechanisms probably operates, depending on the structure of the starting material and the reaction conditions, but conclusive evidence for a stepwise mechanism was obtained in a suitably biased case, while other observations are compatible with a concerted process or a stepwise path involving a short-lived carbanion that evades capture by a proton source.