Access to Tri- and Tetracyclic Structures by Thermally Promoted and High-Pressure-Promoted [4+2] Cycloadditions of 2-, 3- or 4-Vinyl-Substituted Binuclear Heterocycles

A set of 3‐(2‐alkoxyvinyl)benzofurans, ‐furopyridines and‐indoles 5–8 have been tested in [4+2] cycloaddition reactions. The results indicate that they all behave as good dienes, even with only moderately activated dienophiles such as acrylates or MVK. The endo/exo selectivity observed in the adducts depends on the activation conditions: in general, thermal conditions tend to favour the exo isomers, while high pressures preferentially provide the endo isomers. Only the (E) isomers of these dienes proved reactive, the (Z) isomers generally being recovered unaltered. The [4.4.0] binuclear heterocycles such as the isochromene compound 13 and the isoquinolinones 14–16 tested here proved to be significantly less reactive. Finally, the resulting tricyclic adducts 19 were engaged in ene reactions with NPM. Additions involving solely the endo isomers of these adducts took place, stereoselectively affording the succinimide derivatives 29, although in low yields. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)