Recent developments in the heterocyclic ketene aminal-based synthesis of heterocycles

Synthetic Access to α-Oxoketene Aminals by the Nucleophilic Addition of Enol Silane-Derived Palladium(II) Enolates to Carbodiimides

Synthetically important α-oxoketene aminal intermediates can now be accessed from readily available and inexpensive carbodiimides as starting materials via the nucleophilic addition of palladium enolates derived from enol silane precursors. This operationally simple method features mild reaction conditions, including open air atmosphere, ligand-free metal catalysis, broad substrate scope, and multi-gram scalability. Select synthetic applications that take advantage of the enamine character of α-oxoketene aminals and involve C-nucleophilic additions to electrophilic systems, including an α,β-unsaturated ester, an azo dicarboxylate, an aralkyl halide, and an aldehyde, are demonstrated.

Rapid and catalyst free synthesis of new bis(benzo[g]chromene) and bis(pyrano[3,2-c]chromene) derivatives and optimization of reaction conditions using response surface methodology

4,4'-(1,4-phenylene)bis(2-(alkylamino)-3-nitro-4H-benzo[g]chromene-5,10-dione) and 4,4'-(1,4-phenylene)bis(2-(alkylamino)-3-nitropyrano[3,2-c]chromen-5(4H)-one) derivatives are synthesized by a one-pot, multi-component reaction of N-alkyl-1-(methylthio)-2-nitroethenamine (derived from the reaction of various amines and 1,1-bis(methylthio)-2-nitroethene) with terephthalaldehyde or isophthalaldehyde, and 2-hydroxy-1,4-naphthoquinone or 4-hydroxycoumarin in EtOH/H2O (85 : 15) as the solvent at 89 °C. Response surface methodology (RSM) is used to investigate the effect of reaction temperature and water content of aqueous ethanol on the product yields and reaction time. The notable features of this work are the optimization of reaction conditions with minimal experiments, absence of catalyst, good yields, simple work-up and the non-chromatographic purification of products.

Efficient synthesis of new functionalized 2-(alkylamino)-3-nitro-4-(aryl)-4H-benzo[g]chromene-5,10-dione

Structurally diverse benzo[g]chromenes were conveniently synthesized by one-pot, multi-component reaction of N-alkyl-1-(methylthio)-2-nitroethenamine (derived from the reaction of nitroketene dithioacetal with various amines), aromatic aldehydes and 2-hydroxy-1,4-naphthoquinone in CH₃CN at room temperature within 10-25 min, without any catalyst. The notable advantages of the protocol are mild reaction conditions, absence of catalyst, very short reaction time, simple purification process involving no chromatographic process, compatibility with various functional groups and excellent product yields.

An efficient synthesis of new imidazo[1,2-a]pyridine-6-carbohydrazide and pyrido[1,2-a]pyrimidine-7-carbohydrazide derivatives via a five-component cascade reaction

A highly efficient and straightforward synthesis of N-fused heterocyclic compounds including N'-(1-(4-nitrophenyl)ethylidene)imidazo[1,2-a]pyridine-6-carbohydrazide and N'-(1-(4-nitrophenyl)ethylidene)pyrido[1,2-a]pyrimidine-7-carbohydrazide derivatives is successfully achieved via a five-component cascade reaction utilizing cyanoacetohydrazide, 4-nitroacetophenone, 1,1-bis(methylthio)-2-nitroethylene and various diamines in a mixture of water and ethanol. The new efficient domino protocol involving a sequence of N,N-acetal formation, Knoevenagel condensation, Michael reaction, imine-enamine tautomerization and N-cyclization as key steps. The merit of this catalyst free approach is highlighted by its easily available starting materials, operational simplicity, clean reaction profile, the use of environmentally benign solvents and tolerance of a wide variety of functional groups.

Simple synthesis of new imidazopyridinone, pyridopyrimidinone, and thiazolopyridinone derivatives and optimization of reaction parameters using response surface methodology

The reaction between ketene animal/ketene N,S-acetals (derived from diamines/cysteamine hydrochloride and 1,1-bis(methylthio)-2-nitroethene) with aromatic aldehydes, and Meldrum's acid led to the title compounds. The reaction conditions were optimized using response surface methodology (RSM). The two independent variables (temperature and water content of aqueous ethanol), and the responses (yield of product and reaction time) were studied. The range of each parameter selected was: T = 25–100 °C and water = 0–100%. The optimal values were: T = 72 °C and water = 33%. This work offers significant advantages including use of a green solvent, experimental simplicity, absence of catalyst, a simple work-up and purification process, moderate to good yields, and preparation of potentially bioactive compounds.

The one-pot reaction of various diamines with cysteamine hydrochloride or 1,1-bis(methylthio)-2-nitroethene, aromatic aldehydes, and Meldrum's acid led to pyridone compounds in good yields. The reaction conditions were optimized using response surface method.

Synthesis of 5-amino-N′-(9H-fluoren-9-ylidene)-8-nitro-7-aryl-1,2,3,7-tetrahydroimidazo[1,2-a]pyridine-6-carbohydrazide derivatives based on heterocyclic ketene aminals

A new class of tetrahydroimidazo[1,2-a]pyridine derivatives has been successfully prepared via a five-component domino reaction using cyanoacetohydrazide, 9-fluorenone, aromatic aldehydes, 1,1-bis(methylthio)-2-nitroethene and ethylenediamine in ethanol at reflux. The new efficient cascade approach involves a sequence of N,N-acetal formation, Knoevenagel condensation, Michael addition, imine–enamine tautomerization and N-cyclization as key steps. The merit of this protocol is highlighted by its available and economical starting compounds, operational simplicity, clean reaction profile and tolerance of a wide diversity of functional groups.

A new class of tetrahydroimidazo[1,2-a]pyridine derivatives has been successfully prepared via a five-component domino reaction using cyanoacetohydrazide, 9-fluorenone, aromatic aldehydes, 1,1-bis(methylthio)-2-nitroethene and ethylenediamine in ethanol at reflux.