Heterocyclic compounds with bridgehead nitrogen atoms. Part 11. Formation of azocino[2,1,8-cd]pyrrolizines in the reactions of indolizines with dimethyl acetylenedicarboxylate

Abstract

5-Methyl-1-phenylindolizine and its 1 -acetyl derivative reacted with dimethyl acetylenedicarboxylate to give both 1 : 1 -and 1 : 2-adducts. UV and ¹H NMR spectroscopy showed that these types of adducts are dihydro derivatives of pyrrolo[2,1,5-cd]indolizines and of azocino[2,1,8-cd]pyrrolizines, respectively. Contrary to previous claims, the 1 : 2-adducts similarly obtained from 6- and 7-methyl-2-phenylindolizines are also derivatives of the latter ring system; X-ray crystallography (of one such adduct) revealed stereochemical differences relative to the 1 : 2-adduct from 5-methyl-2-phenylindolizine, thus accounting for substantial associated differences in spectroscopic properties. Dehydrogenation products, previously claimed to be [2.3.4]cyclazines, are fully unsaturated azocino[2,1,8-cd] pyrrolizines. X-Ray crystallography showed that this hitherto unrecognised N-bridged [12] annulene ring system is markedly non-planar.