Unusual dihydrazone formation in the fischer‐indole cyclization. The synthesis of novel nonclassical annulated 2,4‐Diamino‐pyrrolo[2,3‐ d ]pyrimidine antifolates

Abstract The synthesis of seven novel tetracyclic 2,4‐diaminopyrrolo[2,3‐ d ]pyrimidines as conformationally restricted nonclassical antifolates was achieved via an unusual Fischer‐indole cyclization of dihydrazones. An attempted synthesis of 2,4‐diamino‐6‐hydrazinopyrimidine afforded 2‐amino‐4,6‐dihydrazinopyrimidine which when reacted with appropriate ketones gave the dihydrazones. The dihydrazones in turn under Fischer‐indole cyclization conditions afforded target conformationally restricted tetracyclic products.