Heterocyclic β‐Enamino Esters, 52. — New Approaches to Condensed Pyrimidines via Iminophosphoranes
The heterocyclic and carbocyclic 2‐(triphenylphosphoranyl‐ideneamino)‐3‐carboxylates 1a, b, 4a–d, and 8a, b, d react with isocyanates in a novel type of pyrimidine annulation to give heterocondensed 2‐alkoxy‐4‐pyrimidinones 3a–d, 5a–h, furthermore 14‐17 (revised constitutions) as well as 4(3H)‐quinazolinones 9a–g, 10a, b. The constitution of model compound 3a has been established by X‐ray analysis. — Crossover experiments reveal that partially saturated enamino esters, such as 1a, b, 4a–d, undergo strictly an intramolecular 1,3‐shift of the ester alkoxide; a mechanism accounting for this annulation reaction is discussed. Under these conditions, the anthranilates 8 afford a stabilized carbodiimide to which alcohols or amines add before condensation occurs to 4(3H)‐quinazolinones 9a–c, 10a, b; however, with the phenyl esters 8b, d the intramolecular 1,3‐migration of the phenoxide is clearly favoured to afford 9f,g. — Reaction of the iminophosphoranes 19a,b of the 1,4‐dihydropyridines 18a,b with acetylenedicarboxylates results in 1‐addition and Aza‐Wittig annulation to give the pyrido[1,2‐a]pyrimidines 21a–c.
