Ethyl α-Nitrocinnamates in the Synthesis of Highly Functionalized Isoxazoles

Primary nitro compounds: progress in the synthesis of isoxazoles by condensation with aldehydes or activated ketones

Among the known strategies directed towards the synthesis of isoxazole derivatives, the reactions of aldehydes with primary nitro compounds deserve a comprehensive treatment, including the historical development as well as the more recent applications. The reactions of aldehydes with primary nitro compounds in a 1 : 2 molar ratio have been shown to lead to isoxazoline-N-oxides or isoxazole derivatives, via β-dinitro derivatives. Several modifications of the process allowed the formation of products bearing substituents at various positions of the heterocyclic ring with control of regioselectivity. Ketones are reported to react with primary nitro compounds, only if activated (β-diketones, α-nitroketones, or strained ketones), to give isoxazole derivatives. Symmetric 2,4-dinitroglutarates formed from aromatic aldehydes and nitroacetate undergo ring closure to form isoxazole derivatives or, according to reaction conditions, 5-hydroxy-6-oxo-4-aryl-6H-1,2-oxazine-3-carboxylates ("oxazinones"), by loss of alcohol instead of water. Isoxazole-4-carbaldehydes are obtained by the reaction of 3-oxetanone with primary nitro compounds.

Efficient synthesis of tetra- and penta-substituted benzenes via a domino annulation reaction of a pyridinium ylide and chalcone o-enolate

A very simple and highly efficient protocol for synthesizing tetra- and penta-substituted benzene derivatives has been developed.

Nitroacetic Esters in the Regioselective Synthesis of Isoxazole-3,5-dicarboxylic Acid Derivatives

An efficient and high-yielding strategy to prepare "unsymmetrical" 4-aryl-isoxazol-3,5-dicarboxylic acid derivatives from nitroacetic esters and aromatic aldehydes has been developed. The strategy is based on the isolation and usage of the previously missed intermediate of the Dornow reaction-5-hydroxy-6-oxo-4-aryl-6H-1,2-oxazine-3-carboxylates. In addition, the mechanism of the Dornow reaction was partially revised.

Copper-mediated oxidative [3 + 2]-annulation of nitroalkenes and pyridinium ylides: general access to functionalized indolizines and efficient synthesis of 1-fluoroindolizines

A general method for the synthesis of substituted indolizines by copper(ii) acetate-promoted oxidative [3 + 2]-annulation of α-fluoronitroalkenes with in situ generated pyridinium ylides was developed. Application of the copper(ii) acetate-2,6-lutidine system provides efficient access to various 1-fluoroindolizines in up to 81% yield. Both electron-rich and electron-deficient nitroalkenes as well as different pyridinium and isoquinolinium salts can be involved in the reaction. Moreover, it was found that copper-mediated annulation is applicable for other α-substituted (alkyl, chloro, and ester) nitroalkenes giving rise to the corresponding indolizines. First synthesis of monofluorinated [3,2,2]cyclazines was demonstrated via oxidative annulation of 3-unsubstituted fluoroindolizines with diethyl acetylene dicarboxylate.

Employment of α-nitroketones in organic synthesis

This review summarizes the recent developments of the reactions of α-nitroketones and the consequent formation of a variety of structural frameworks is discussed.

Convenient construction of dibenzo[b,d]furanes and 2,6-diaryl-4-(2-hydroxyphenyl)pyridines via domino reaction of pyridinium ylides with 2-aryl-3-nitrochromenes

The DBU promoted domino annulation reaction of 2-aryl-3-nitrochromenes with two molecular 4-(N,N-dimethylamino)-1-phenacylpyridinium bromides in DMF resulted in unprecedented 1-benzoyl-2,4-diaryldibenzo[b,d]furane derivatives in good yields.

Highly stereoselective synthesis of 2,3-dihydrofurans via a cascade Michael addition-alkylation process: a nitro group as the leaving group

The Michael addition-alkylation process between gem-benzoyl-nitrostyrenes and 1,3-dicarbonyl compounds proceeded smoothly in the presence of a bifunctional squaramide, exclusively providing 2,3-dihydrofurans as trans-diastereomers in 33-92% isolated yields and excellent enantioselectivities (29->99% ee).

Stepwise cycloaddition reaction of N-phenacylbenzothiazolium bromides and nitroalkenes for tetrahydro-, dihydro- and benzo[d]pyrrolo[2,1-b]thiazoles

The triethylamine promoted stepwise 1,3-dipolar cycloaddition reaction of N-phenacylbenzothiazolium bromides with nitroalkenes in ethanol resulted in a mixture of two isomeric tetrahydrobenzo[d]pyrrolo[2,1-b]thiazoles with cis/trans/cis- and all-trans-configurations. More importantly, the corresponding dihydrobenzo[d]pyrrolo[2,1-b]thiazoles can be selectively prepared in refluxing ethanol and the benzo[d]pyrrolo[2,1-b]thiazoles can be obtained in satisfactory yields by sequential dehydrogenation with DDQ as oxidizer. On the other hand, the similar cycloaddition reaction of N-phenacylbenzothiazolium bromides with 1-methy-1-nitroalkenes in refluxing ethanol afforded benzo[d]pyrrolo[2,1-b]thiazoles with splitting out of nitro group. The stereochemistry of the spiro compounds was clearly elucidated on the basis of NMR spectra and sixteen single crystal structures.

Diastereoselective synthesis of benzo[d]chromeno[3′,4′:3,4]pyrrolo[2,1-b]thiazoles via cycloaddition reaction of benzothiazolium salts with 3-nitrochromenes

Diastereospecific synthesis of benzo[d]chromeno[3′,4′:3,4]pyrrolo[2,1-b]thiazoles via cycloaddition reaction of benzothiazolium salts with 3-nitrochromenes.

Cobalt salt-catalyzed carbocyclization reactions of α-bromo-N-phenylacetamide derivatives

A Co(ii)-catalyzed carbocyclization reaction for the synthesis of 4-substituted quinolin-2-(1H)-ones is described.