Can the progress of Fischer's indole synthesis be stopped?

Abstract

The cycloadducts of isoquinoline N-phenylimide and ethylenic dipolarophiles are a new class of ene-phenylhydrazines. Their hydrazo rearrangement corresponds to the key step of Fischer's indole synthesis, but the reaction stops at the 2-aminoindoline stage, e.g., 16, because too much strain would build up in the 8-membered ring on indole formation. The model 23, lacking the medium-sized ring, smoothly undergoes indolization. The structures of type 16a aminals - besides the all- H parent mainly the diastereoisomeric 12,13-dicarboxylic esters and 13-carbonitriles - were clarified by X-ray and NMR analyses as well as by conversions. The pentacyclic aminals form a rigid bowl; the boat vs. chair conformation of the hydropyrimidine ring C is discussed.