No abstract

A new multicomponent catalytic approach to important fused imidazole bicyclic acetic esters, whose core is present in many biologically active principles, is presented. It is based on the sequential cyclization‐alkoxycarbonylation‐isomerization of readily available N‐heterocyclic propargylamine derivatives, carried out under oxidative conditions using a simple catalytic system consisting of PdI2 (1 mol%) in conjunction with KI (1 equiv.), in the presence of AcONa as additive (1 equiv.) at 100 °C under 20 bar of a 4 : 1 mixture CO‐air. Under the optimized conditions, several N‐(prop‐2‐yn‐1‐yl)pyridin‐2‐amines were smoothly converted into alkyl 2‐(imidazo[1,2‐a]pyridin‐3‐yl)acetates in fair yields (51–77 %). The method was also applied to the conversion of N‐(prop‐2‐yn‐1‐yl)pyrimidin‐2‐amine into 2‐(imidazo[1,2‐a]pyrimidin‐3‐yl)acetate and of N‐(prop‐2‐yn‐1‐yl)pyrazin‐2‐amine into 2‐(imidazo[1,2‐a]pyrazin‐3‐yl)acetate. Some of the newly synthesized bicyclic derivatives have shown promising luminescence properties.

Dedicated to Professor Cesare Gennari on the occasion of his 70th birthdayWe report a direct, multicomponent approach to an important class of fused-ring heterocycles, namely, imidazo[2,1-b]thiazole derivatives starting from simple and readily available building blocks (N-Boc-N-propargylthiazol-2-amines, carbon monoxide, a secondary amine, and oxygen). The process takes place under the catalysis of the simple PdI 2 /KI system under relatively mild conditions ( 100°C under 20 atm of a 4 : 1 mixture of CO-air), and leads to 2-(imidazo[2,1-b]thiazol-5-yl)-N,N-dialkylacetamides in yields up to 87 %. The mechanistic pathway leading to carbonylated imidazo[2,1-b]thiazole occurs through an ordered sequence of steps, involving amine-induced N-deprotection, oxidative aminocarbonylation of the triple bond (with CspÀ H activation and formation of a 2-ynamide intermediate), dearomative cyclization (by intramolecular conjugate addition of thioazolic nitrogen to the 2-ynamide moiety) and aromatization by proton-shift isomerization. When starting from N-Boc-N-(propargyl)benzo[d]thiazol-2-amine, tricyclic benzimidazothiazoles were formed in moderate yields.