Heterocyclic β‐Enamino esters. 28. The reaction of heterocyclic β‐Enamino esters and nitriles with cyclic amidines. A simple route to azolopyrimidines

Whereas 2‐amino‐3‐ethoxycarbonyl‐4,5‐dihydrofurans Ia‐c condense with 5‐membered amidine derivatives, via elimination of ethanol to afford the azolopyrimidines IIIa,b, XI, and XIVa,b, the 2‐amino‐3‐cyano‐4,5‐dihydrofurans Id,e give with the same reagents, under elimination of ammonia, the novel ring systems of furo‐azolopyrimidines XVIII and XXa,b. 2‐Amino‐3‐ethoxycarbonyl‐5,6‐dihydro‐4H‐thiopyrane (XXI) reacts with 5‐amino‐1,2,4‐triazole (II) to yield the triazolo[1,5‐a]pyrimidine XXII, and with 2‐aminobenzimidazole to XXIII. The mechanism of these reactions is discussed. XIVb and VIIb are cyclized in a secondary step to give the novel furo[2,3‐d]benzimidazo[1,2‐a]pyrimidine XXVI, and furo[2,3‐d]‐1,2,4‐triazolo[1,5‐a]pyrimidine XXVIII respectively, besides the acetoxy derivatives XVII and XXIX.