Photochemical Cyclization of Tertiary Buta-2,3-dienamides to β-Lactams Upon Triplet Energy Transfer

A series of N,N-disubstituted buta-2,3-dienamides was prepared from 3-butynoic acid and probed as substrates in a light-induced photocyclization. It was found that xanthen-9-one (10 mol%) promotes the desired reaction to 3-vinyl-substituted 2-azetidinones (β-lactams) when performed at λ = 350 nm in acetonitrile as the solvent. Evidence was collected by transient absorption spectroscopy that the catalyst promotes excitation of the allene amide to its triplet state by Dexter energy transfer. Upon intramolecular hydrogen atom transfer from one of the nitrogen substituents, the ensuing 1,4-diradical undergoes C─C bond formation to the lactam product. If the substituent at the nitrogen atom is a primary benzyl group, the product displays a stereogenic center in 4-position and is formed exclusively as the trans-product (eleven examples, 18%-73% yield). If the substituent is secondary, 4,4-disubstituted products are formed. If the buta-2,3-dienamide is substituted at the terminal carbon atom, the substituent at C3 in the 2-azetidinone is an (E)-configured alkenyl group. Two alternative reaction pathways were observed, i.e. an intramolecular para photocycloaddition for N-phenyl substituted substrates and an elimination from the 1,4-diradical intermediate. The vinyl group at C3 can serve as useful handle for consecutive transformations.