Enantioselective phosphine-catalyzed [6 + 1] annulations of α-allyl allenoates with 1,1-bisnucleophiles

Organocatalytic annulations between allenes and bisnucleophiles represent one of the most convenient routes to various carbocycles and heterocycles. However, most examples are limited to the formation of five- and six-membered rings, probably owing to relatively easy handling of short-chained biselectrophiles. Here we report long-chained α-allyl allenoate-derived 1,6-biselectrophiles for the first time, enabling a phosphine catalyzed [6 + 1] annulation with readily available 1,1-bisnucleophilic reagents. The reaction proceeds via a tandem γ-umpolung addition and δ′-addition process, smoothly constructing both seven-membered N-heterocycles and carbocycles with a broad scope of substrates, high atom economy and excellent enantioselectivity (up to 99% yield and up to 96% ee). Mechanistic experiments revealed a conversion of the 1,6-dipole into a 1,6-biselectrophilic intermediate through proton abstraction. A phosphine catalyzed [6 + 1] annulation of α-allyl allenoates is achieved. Both seven-membered N-heterocycles and carbocycles were synthesized. Mechanistic experiments have verified the 1,6-biselectrophilic intermediate from 1,6-dipole.