A NOVEL ROUTE TO ISOANNULATED HETEROAROMATIC-COMPOUNDS .2. THE DIPOLE ROUTE TO FURO/THIENO PYRROLES AND ALPHA-PYRIDONES, AND THEIR BENZO DERIVATIVES

A new methodology involving l, 7-dipolar cyclizations of conjugated dipoles followed by a multistep rearrangement affording ring-contracted products has been employed for the synthesis of various examples of the title compounds. Under different thermolysis conditions the annulated alkynyl nitrones 15-19, 45, and 46 are converted into isoannulated pyrroles and a-pyridones, respectively, with yields up to 90% (general reaction A --> B + C). The product distribution is strongly influenced by the nature of the terminal substituent R, hence supporting a mechanism with oxo carbenes as central intermediates which undergo either 6-pi cyclization to the pyrrole systems or Wolff rearrangement leading to conjugated ketenes, the precursors of the observed a-pyridones (see Scheme 1). Diels-Alder reactions were performed with benzofuropyridone 40a, napthopyridone 62 as well as with the corresponding dihydronaphtho compounds 64a, b. Whereas the tert-butyl derivatives 22b, 23b and 41b are rather unreactive against dienophiles, they are easily transformed into the cyclic imides 31, 32 and 52, respectively, under the influence of oxygen and Light suggesting the formation and subsequent [4 + 2] cycloaddition of O-1(2) as important steps.