Thermische und photochemische Umwandlungen von Hetero-1,3,5-hexatrienen in fünfgliedrige Ringe mögliche pericyclische Reaktionen

Synthesis of Pyrroles from 1-Dialkylamino-3-phosphoryl(or phosphanyl)allenes through 1,5-Cyclization of Conjugated Azomethine Ylide Intermediates

1-Dialkylamino-1,3-diaryl-3-diphenylphosphanylallenes 3a-e are thermally converted into a-annulated 3,5-diarylpyrroles 6a-f and [a]-annulated benzo[c]azepines 7a,b,d. These transformations are likely to include conjugated azomethine ylide intermediates that can undergo either a 1,5- or a 1,7-electrocyclization. The periselectivity is markedly shifted toward 1,5-cyclization when the diphenylphosphanyl substituent is replaced by the diphenylphosphoryl group. Thus, 1-dialkylamino-3-(diphenylphosphoryl)allenes 4a-f yield pyrroles 6 exclusively and with improved yields, unless the 3-aryl substituent in the allene is too electron-rich (e.g., benzodioxol-5-yl, 4f --> 7f). The preparation and thermal transformation of aminoallenes 4 over three or four steps can be conducted as a one-pot procedure, thus providing a convenient synthesis of [a]-annulated 3,5-diarylpyrroles from enaminoketones.

Theoretical Evidence for Oxygenated Intermediates in the Reductive Cyclization of Nitrobenzenes

Deoxygenation of nitroaromatics is a classic synthetic method for the construction of nitrogen heterocycles. The generally accepted mechanism involves exhaustive deoxygenation to a singlet nitrene. We present theoretical evidence for an alternative, 6pi-electron 5-atom electrocyclization of nitroso-styrenes, -stilbenes, and -biphenyls to nitronates. A downstream 1,5-H shift and tautomerization leads to N-hydroxy heterocycles. [reaction: see text]