Enantioselective Phosphine‐Catalyzed Formal 4+4 Annulation of α,β‐Unsaturated Imines and Allene Ketones: Construction of Eight‐Membered Rings

The first highly enantioselective phosphine-catalyzed formal 4+4 annulation has been developed. In the presence of amino-acid-derived phosphines, the unprecedented 4+4 annulations between benzofuran/indole-derived α,β-unsaturated imines and allene ketones proceeded smoothly, thus affording azocines, bearing either a benzofuran or an indole moiety, in excellent yields and with nearly perfect enantioselectivities (≥98 % ee in most cases). This work marks the first efficient asymmetric construction of optically enriched eight-membered rings by phosphine catalysis.