Asymmetric Diels-Alder reactions with chiral acetylenic carbene complexes as dienophiles

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)(5)M=C((C4H7N)CR2OMe)C=CH (M=Cr, W; R=H, 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-triisopropylsiloxy-1,3-pentadiene (66-73% de) but not with cyclopentadiene and alpha-triisopropylsiloxyvinyl cyclohexene. A model is presented to account for the observed stereoselectivities. (C) 2000 Published by Elsevier Science Ltd.