Divergent pathways in the intramolecular Diels-Alder reaction of 2(1H)-pyrazinones substituted at the 3-position with a phenylalkyne containing side chain.

2(1H)-Pyrazinones bearing an X-(o-C6H4)-C=C-R moiety (X=O or MI; R=H, Ph or TMS) at position 3 were subjected to intramolecular Diels-Alder reaction. For the ether compounds (X=O) cycloaddition-elimination occurred readily to produce either benzofuro[2,3-c]pyridin-1(2-H)-ones or benzofuro[2,3-b]pyridines. For the aniline derivatives (X=NH, R=H or TMS) thermolysis in acetic anhydride resulted in a similar product distribution of beta-carbolinones and alpha-carbolines which. however, differed from that obtained previously in refluxing tetrahydronaphthalene. This result is explained by the cycloaddition proceeding from the aniline NH-acetylated precursor. However, the aniline derivatives with Ph as the acetylenic end group (X=NH, R=Ph) reacted via a divergent pathway to produce N-(2-oxopyrazin-3-yl)-2-Ph-substituted indoles. (C) 1999 Elsevier Science Ltd. All rights reserved.