Thermal Electrocyclic Reactions of 2-Aza-1,3-butadiene Derivatives. A New N-Heterocyclic Annelation

Article abstract of DOI:10.1021/jo00174a036

A general, three-step annelation sequence, which ultimately gives 3,4-dihydro-2-quinolines and related derivatives (3), is described.The cyclization step is accomplished by pyrolysis of a 1-arenyl-2-aza-1,3-butadiene analogue (2) that apparently undergoes successive six-?-electron electrocyclization and 1,5-hydrogen migration reactions to yield the product.The conjugated azadienes, 2, are prepared by the base-catalyzed isomerization of the unconjugated isomers, 1.Compounds 1 are prepared by condensing arenyl ketones or aldehydes with 2-propenyl-1-amine.Steric effects of substituents on the azadiene chain and steric and electronic effects of the arenyl group on the cyclization step were studied.The following general conclusions were drawn: alkyl substituents R on the C=N terminus of 2 hinder a competing degradative process (commencing with a four-?-electron electrocyclization) and improve the yield of products 3; electron-withdrawing substituents on Ar of 2 or electron-withdrawing Ar groups enhance the yield of cyclized products, but they impart little regioselectivity to the reaction; regioselectivity may be imparted by ? bond fixation in Ar; electrocyclization also proceeds well with ?-electron excessive Ar groups on 2.The preferred conformation of the heterocyclic product 3 can be readily deduced by 1H NMR spectroscopy.