Experimental and Theoretical Study on the Regioselective Radical Cyclization Reactions of 1-(-Alkenylaryl)-2-amido-1-ketones for the Construction of Indeno[2,1-][1,3]oxazin-9-ones.

A simple and efficient two-step method for the construction of novel 2,4,9a-trisubstituted-4a,9a-dihydroindeno[2,1-][1,3]oxazin-9-ones has been developed. The NHC-catalyzed aza-benzoin reaction of -alkenyl benzaldehydes with -acylarylimines afforded 1-(-alkenylaryl)-2-amido-2-aryl-1-ethanones, which underwent regioselective 5-exo-trig radical cyclization to furnish the three-ring-fused heterocyclic products, generally in good yields. The synthetic method displayed good tolerance toward the nature of substituents, substitution pattern, and steric hindrance of -alkenyl benzaldehydes. Based on this method, the synthesis of unprecedented dihydrobenzo[6,7]indeno[2,1-][1,3]oxazin-7-ones and dihydropyrido[2',3':3,4]cyclopenta[1,2-][1,3]oxazin-9-ones has been achieved by employing -alkenylnaphthaldehyde and -alkenylnicotinaldehyde as substrates. The regioselectivity between 5- and 6- radical cyclization reactions of different 1-(-alkenylaryl)-2-amido-2-aryl-1-ethanones were elucidated with DFT calculations.