Polycyclic isophosphindole derivatives by intramolecular trapping of 2-phospha-1,3-diyls or isomeric bis(methylene)phosphoranes

5-Alkylidene-5,7a-dihydro-3H-[1,2,4]diazaphospholo[4,3-c][l,2,3]diazaphosph oles 10a-c were obtained from (1-diazo-2-oxoalkyl)silane 8 and 2-acetyl-2H- 1,2,3-diazaphospholes 9a-c. Thermally induced elimination of molecular nitr ogen from 10 generated polycyclic isophosphindole (benzo[c]phosphole) deriv atives 13 by means of intramolecular [3+2] cycloaddition of an intermediate 2-phospha-1,3-diradical or an isomeric bis(methylene)phosphorane onto a be nzene ring.