Metal-Free [5 + 2] Cyclization of Photogenerated Siloxyketenes and Vinyl Aziridines for the Synthesis of Azepanones

Abstract

Ketenes constitute a versatile and highly reactive class of intermediates with widespread utility in modern organic synthesis. Among them, siloxyketenes, a class of oxygen-substituted variants, generated in situ from α-ketoacylsilanes through a photoinduced [1,3]-silyl migration, have garnered considerable attention due to their distinctive structural features. However, their synthetic utility has been confined to the construction of four- to six-membered rings. The development of methods that exploit these intermediates for the assembly of medium-sized rings remains exceedingly limited and is therefore highly desirable. Herein, we developed a metal-free, visible-light-driven [5 + 2] cyclization reaction for the synthesis of azepanone derivatives. Under irradiation of blue LEDs at 20 °C, α-ketoacylsilanes and vinyl aziridine were used as readily available substrates to realize this cyclization. The reaction features mild reaction conditions, operational simplicity, a broad substrate scope, and it generates diverse azepanones bearing a tertiary alcohol motif in good to excellent yields (up to 96%). The reaction proceeds via a photochemically induced [1,3]-silyl migration to form reactive siloxyketenes, followed by nucleophilic addition and a ring-closing process. This work expands the application of α-ketoacylsilanes as siloxyketene precursors and provides a concise and efficient approach to constructing medium-sized rings under mild conditions.