Use of a New Spirophosphine To Achieve Catalytic Enantioselective [4 + 1] Annulations of Amines with Allenes To Generate Dihydropyrroles

Due
in part to the common occurrence of five-membered nitrogen heterocycles
in bioactive molecules, the discovery of methods for the enantioselective
synthesis of such structures is a useful endeavor. Building on a single
example by Tong of a phosphine-catalyzed [4 + 1] annulation of an
amine with an allene that furnished an achiral dihydropyrrole in 22%
yield, we have developed, with the aid of a new chiral spirophosphine
catalyst, a method with increased utility, specifically, improved
yield, enhanced scope (the use of γ-substituted allenes), and
good ee. The enantioenriched dihydropyrrole products can be transformed
into other interesting families of compounds with very good stereoselectivity.