Structure Elucidation of a Cryptic Condensation Product from Diacetyl and Arylamine – Then and Now

Abstract

Some 70 years ago, the reaction of diacetyl (1) and arylamines (2) in hot phosphoric acid was reported to give a new type of condensation products, but no structure was assigned to them. The case is presented to recapitulate the methods and rationales used in classical structure elucidation of organic molecules through reaction networks, before spectroscopic or crystallographic methods were generally available. The difficulties and limits of the classical approach are exemplified through this real-life problem, which could not be solved by the methodology of its time. A representative condensation product 4a has been resynthesized from 1 and p-toluidine (2a), and its structure elucidation by means of 2D NMR techniques is outlined.