137117-07-8Relevant academic research and scientific papers
Studies on the Fischer Indole Synthesis: Rearrangements of Five-, Six- and Seven-membered Cyclic Hydrazones of Pyrazoline, Tetrahydropyridazine and Tetrahydro-1,2-diazepine Series in Polyphosphoric Acid
Benincori, Tiziana,Brenna, Elisabetta,Sannicolo, Franco
, p. 2139 - 2145 (2007/10/02)
The rearrangements of a few cyclic phenylhydrazones structurally related to 1-phenyl-Δ2-pyrazoline, 1-phenyl-1,4,5,6-tetrahydropyridazine, and 1-phenyl-4,5,6,7-tetrahydro-1,2-diazepine in hot polyphosphoric acid (PPA) are described.The five-membered-ring substrates (the pyrazolines) did not undergo the sigmatropic rearrangement typical of the Fischer indolization, the main reaction course being homolytic N-N bond cleavage, leading to benzidine and its 4-(2-benzoylethyl)-4'-(3-phenyl-Δ2-pyrazolin-1-yl) derivative.The six-membered heterocycles underwent two different reactions, both involving the tautomeric enehydrazine form: the first one is the sigmatropic rearrangement, expected for open-chain hydrazones, affording 4-acyl-1,2,3,4-tetrahydroquinoline derivatives; the second one is a retro-Diels-Alder reaction producing methyleneaniline and α,β-unsaturated carbonyl compounds.The seven-membered-ring substrate gave a 5-acyltetrahydrobenzazepine, resulting from the rearrangement, together with both a pyrrolidine and a pyrazine by-product; their formation involves homolytic N-N bond cleavage of the ring, a δ hydrogen abstraction, followed by intramolecular or intermolecular ring closure.Chemical proofs are given for the new structures.
