Base-Catalyzed 1,6-Conjugate Addition/ipso-Cyclization of Ynone-Tethered Pyrroles with para-Quinone Methides: Access to Spiro-Fused Cyclohexadienone/Pyrrolizine Frameworks.

A convenient synthesis of spiro-fused cyclohexadienones/pyrrolizines from pyrrolyl ynones and p-quinone methides through a base-catalyzed 1,6-conjugate addition/ipso-cyclization tandem reaction is described. The protocols feature good substrate tolerance, mild reaction conditions, and high yields of target products. The problem of reaction stereoselectivity raised during the scope study is easily solved by employing a one-pot, two-step approach that allows directing the reaction sequence to obtain the desired products exclusively with the (Z)-configuration of the enone moiety. A tentative reaction mechanism is proposed with the support of experimental studies.