ChemInform Abstract: Asymmetric Diels-Alder Reactions with Chiral Acetylenic Carbene Complexes as Dienophiles.

Abstract

A new synthetic method is developed for the asymmetric Diels-Alder reactions of acetylenic dienophiles. Acetylenic Fischer carbene complexes with chiral pyrrolidines as the heteroatom stabilizing substituent were anticipated to block three of the four possible approaches of a diene and lead to selective asymmetric cycloadditions. A series of complexes of the type (CO)5MvC((C4H7N)CR2OMe)- CuCH (MaCr, W; RaH, Me, Ph) were prepared by aminolysis of the corresponding methoxy complexes that have the terminal acetylene protected as a silane. Michael addition is completely suppressed if the alkyne is protected as a triisopropylsilyl group and after aminolylsis by 1,2-addition of a pyrrolidine the terminal alkynyl carbene complexes can be obtained predominately as E-isomers upon protodesilylation. The cycloadditions of the E-isomers (but not Z-isomers) of these complexes occur with significant asymmetric induction with 2-tri- isopropylsiloxy-1,3-pentadiene (66-73% de) but not with cyclopentadiene and a-triisopropylsiloxyvinyl cyclohexene. A model is presented to account for the observed stereoselectivities. q 2000 Published by Elsevier Science Ltd.