Abstract
Pyridinium ylides are well recognized as dipoles for cycloaddition reactions. In its turn, the microwave-assisted interaction of N-(cyanomethyl)-2-alkylpyridinium salts with enaminones unexpectedly proceeds as a domino sequence of cycloisomerization and cyclocondensation reactions, instead of a 1,3-dipolar cycloaddition. The reaction takes place in the presence of sodium acetate as base and employs benign solvents. The optical properties of the resulting pyrido[2,3-b]indolizines were studied, showing green light emission with high fluorescence quantum yields.
Highlights
Indolizines, and in particular the annulated ones, are frequently found to exhibit useful biological [1,2,3,4,5,6] and optical properties [7,8,9,10,11,12,13,14]
In this work we discovered that the reactions of 2-alkyl-N-(cyanomethyl)pyridinium salts 1 with enaminones 2 proceed unexpectedly as a two-component domino sequence of cycloisomerization and cyclocondensation reactions, while cycloaddition processes were not observed (Scheme 1)
With the optimized conditions in hand, we went on to investigate the reaction scope. It turned out the optimized conditions in hand, we went on to investigate the reactionwas scope. It turned that theWith reaction of N-cyanomethyl-2,3-dimethylpyridinium salt with enaminone more effective, out that the reaction of
Summary
Indolizines, and in particular the annulated ones, are frequently found to exhibit useful biological [1,2,3,4,5,6] and optical properties [7,8,9,10,11,12,13,14]. Interaction of ylides with alkenes or alkynes leads to a different outcome [24,25,26]. Another approach towards the indolizine scaffold is based on intramolecular cyclization of 2-alkylpyridinium ylides. Following our interest in the chemistry of cyanomethylpyridinium salts [28,29,30], we recently showed that the interaction of N-(cyanomethyl)pyridinium chlorides with enaminones under basic conditions proceeds as a pseudo-three-component reaction, resulting in the formation of pyridoindolizine-10-carbonitriles [31]. In this work we discovered that the reactions of 2-alkyl-N-(cyanomethyl)pyridinium salts 1 with enaminones 2 proceed unexpectedly as a two-component domino sequence of cycloisomerization and cyclocondensation reactions, while cycloaddition processes were not observed (Scheme 1).
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