CAN THE PROGRESS OF FISCHERS INDOLE SYNTHESIS BE STOPPED

The cycloadducts of isoquinoline N-phenylimide and ethylenic dipolarop hiles are a new class of ene-phenylhydrazines. Their hydrate rearrange ment corresponds to the key step of Fischer's indole synthesis, but th e reaction stops at the 2-aminoindoline stage, e.g., 16, because too m uch strain would build up in the 8-membered ring on indole formation. The model 23, lacking the medium-sized ring, smoothly undergoes indoli zation. The structures of type 16a aminals - besides the all-H parent mainly the diastereoisomeric 12,13-dicarboxylic esters and 13-carbonit riles - were clarified by Xray and NMR analyses as well as by conversi ons. The pentacyclic aminals form a rigid bowl; the boat vs. chair con formation of the hydropyrimidine ring C is discussed. (C) 1998 Publish ed by Elsevier Science Ltd. All rights reserved.