Hantzsch pyridine synthesis using microwave

In a 1998 publication, the concept of microwave-assisted parallel synthesis in plate format was introduced for the first time. Using the three-component Hantzsch pyridine synthesis as a model reaction, libraries of substituted pyridines were pre-... 

The rapid synthesis of heteroaromatic Hantzsch pyridines can be achieved by aromatization of the corresponding 1,4-DHP derivative under microwave-assisted conditions [51]. However, the domino synthesis of these derivatives has been reported in a domestic microwave oven [58,59] using bentonite clay and ammoniiun nitrate, the latter serving as both the source of ammonia and the oxidant, hi spite of some contradictory findings [51,58,59], this approach has been employed in the automated high-throughput parallel synthesis of pyridine libraries in a 96-well plate [59]. In each well, a mixture of an aldehyde, ethyl acetoacetate and a second 1,3-dicarbonyl compound was irradiated for 5 min in the presence of bentonite/ammonium nitrate. For some reactions, depending upon the specific 1,3-dicarbonyl compound used. 

Suarez et al. [114] also reported Hantzsch synthesis under microwave irradiations. It was observed that in the conventional method the reaction mixture in the presence of ethanol was refluxed for 8 h to give 55 % yield of the product. The same reactants heated at 120°C in the presence of alumina and DMF for 6 min gave 5% yield and when heated for 1 h gave 30% yield. The reaction mixture when taken in dry media with alumina and heated at 80-85 °C for 6 min, gave 40 % yield. The same reactants under microwave irradiation in the presence of alumina and DMF at 120 °C gave 95 % of the product within 6 min of irradiation (Scheme 11.56). In this case, a small amount of polar DMF allows the temperature to reach up to 120 °C. This technique was also used for the synthesis of various substituted pyridines [21]. 

The Hantzsch dihydropyridine synthesis is still one of the most important routes to pyridine ring synthesis. The use of microwaves reduces the heating times (10-15 min) and also increases the yield (51-92%) where as under conventional conditions, it requires a long period of heating (about 12 hr) and poor to moderate yield were obtained (15-61%) (Ohberg and Westmen, 2001). 

A variety of conditions (solution, dry media, solvent-free) has been used for microwave-assisted synthesis of Hantzsch 1,4-DHP only procedures involving solvent-free conditions under the action of irradiation led to the aromatized pyridine derivatives. 

A survey of microwave activation in the chemistry of Hantzsch 1,4-dihydropyridines (1,4-DHP) was reported in 2003 [195]. The experimental method proposed more than a century ago remains the most widely used for synthesis of these heterocycles. Since 1992 the process has been adapted to microwave irradiation under a variety of conditions to reduce the reaction time and enhance the yield. Among these experiments, Zhang reported a solvent-free process starting from 3-aminocrotonate (20 mmol), methyl acetoacetate (20 mmol), and aromatic aldehydes (20 mmol) in a domestic oven [196]. Yields from 59 to 77% were reported for 10-min reaction. A variety of conditions (solution, dry media, solvent-free) has been used for microwave-assisted synthesis of Hantzsch 1,4-DHP. Only procedures involving solvent-free conditions under the action of microwave irradiation led to the aromatized pyridine derivatives. 

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