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.