Synthesis and photoreactivity of some 5-alkylidene- and 5-alkylidenamine-2,5-dihydroisoxazoles

5-Alkylidene-2,5-dihydroisoxazoles and 5-alkylidenamine-2,5-dihydroisoxazoles were easily prepared from the corresponding 5-chloro-2-methylisoxazolium triflate and an enolizable compound or alkylamine. Their photochemical reactivity leads to photoisomers that in some cases constitute new heterocycles systems. The photorearrangement involves either triplet or singlet state depending on substituents and experimental conditions.     

Anthocyanidins and related compounds—XVII : Reactions of flavylium salts and 2-hydroxychalcones with hydroxylamine and hydrazine

Flavylium salts and 2-hydroxychalcones (1-phenyl-3-(2-hydroxyphenyl)-2-propen-1-ones) react with hydroxylamine in pyridine to form 2,5-dihydroisoxazoles. These undergo thermal and acid-catalyzed rearrangements to isomeric 4,5-dihydroisoxazoles and chalcone oximes, respectively. With hydrazine, flavylium salts yield phenolic 4,5-dihydro-3,5-diphenyl-1H-pyrazoles. Since these readily condense with acetone to form cyclic acetonides, the hydrazination reaction involves initial nucleophilic attack at position 2 of the flavylium nucleus.     

Photoreaction of some 5-alkylidene-2,5-dihydroisoxazoles: facile construction of novel unclassical β-lactam containing heterocycles

The photochemical behaviour of some 5-alkylidene-2,5-dihydroisoxazoles was investigated. This reaction produced cis-4,5-dihydrofuroazetidinones in high yields.     

Tetraacetylethylene and nitrile oxides: Synthesis of spirofuranisoxazoles

trans and cis-3-Hexen-2,5-dione, 2 , reacted with nitrile oxides to give 4,5-dihydroisoxazoles 3a-c with the trans configuration. On the contrary the reaction between 3,4-diacetyl-3-hexen-2,5-dione, 1 , with nitrile oxides yielded 3-aryl-8,9-diacetyl-7-hydroxy-7-methyl-1,6-dioxa-2-azaspiro[4.4]nona-3,8-dienes 9a-e. The reaction is completely regiospecific. The cycloadducts show ring-open chain tautomerism.     

On the viability of 5-endo-dig cyclisations of O-propargylic hydroxylamine derivatives, leading to 2,5-dihydroisoxazoles (3-isoxazolines)

O-Propargylic hydroxylamines undergo smooth 5-endo-dig cyclisations upon exposure to 3 equiv of molecular iodine to give respectable yields of the corresponding 4-iodo-2,5-dihydroisoxazoles, which should find a number of applications as intermediates for syntheses amongst this useful class of heterocycles.     

Synthesis of functionalized 2,3-dihydroisoxazoles by domino reactions in water and unexpected ring-opening reactions of 2,3-dihydroisoxazoles

α,α-Dicyanoolefins react with hydroxylamine to afford 2,3-dihydroisoxazoles (2,3-dihydroisoxazoles can be easily isolated by filtration) in excellent yields under mild and environmentally benign conditions. A one-pot reaction in tandem with an unexpected ring-opening of 2,3-dihydroisoxazoles has been developed as well.     

Synthesis of 4,5-dihydroisoxazole and isoxazolin-5-ones

The synthesis of 5-hydrazono-4,5-dihydroisoxazoles 3 , from N'-(3-imino-3-aryl-1-methyl-1-alquenyl) hydrazones 1 and hydroxylamine hydrochloride, is described. These 4,5-dihydroisoxazoles can be transformed into oximes 5 and isoxazolones 6 by treatment with reducing agents and ethyl chloroformate, respectively.     

Synthesis of 5-(thiazol-5-yl)-4,5-dihydroisoxazoles from 3-chloropentane-2,4-dione

Condensation of 3-chloropentane-2,4-dione with thioamides gives 1-(thiazol-5-yl)ethanones and subsequent Wittig olefination, followed by nitrile oxide 1,3-dipolar cycloaddition to the resulting prop-1-en-2-yl moiety, delivers racemic 5-(thiazol-5-yl)-4,5-dihydroisoxazoles. When this thiazole and isoxazoline diheterocyclic scaffold has a carboethoxy substituent at C2 of the thiazole ring, aminolysis provides for effective diversification. A 50-member library of various 5-(thiazol-5-yl)-4,5-dihydroisoxazoles is reported.     

5-Amino-4,5-dihydroisoxazoles. Part I. 5-Amino-3-aryl-4-methylene-4,5-dihydroisoxazoles and 4-aminomethyl-3-arylisoxazoles from 5-amino-4-aminomethyl-3-aryl-4,5-dihydroisoxazoles

5-Amino-4-aminomethyl-3-aryl-4,5-dihydroisoxazoles 2 were obtained by cycloaddition of nitrile oxides to 1,3-diaminopropenes 1. On reaction with methyl iodide the corresponding 4-(quaternary)-ammoniomethyl iodides 3 were formed. These compounds, on reaction with bases, afforded 5-amino-3-aryl-4-methylene-4,5-dihydroisoxazoles 4. The acid-catalyzed deamination of compounds 2 afforded 4-aminomethyl-3-arylisoxa-zoles 5 and 3-arylisoxazoles as retro-Mannich products. The deamination of 2 to yield 5 was also obtained by base catalysis.     

The reaction of 3,5-dichloro-2,4,6-trimethylbenzonitrile oxide with chiral fluoro-substituted allyl sulfoxides

The title reaction allowed the synthesis, with total regio- and moderate stereo-selectivity, of chiral fluorosubstituted 4,5-dihydroisoxazoles.     

Reduction of 2,3-dihydroisoxazoles to beta-amino ketones and beta-amino alcohols

[chemical reaction: see text]. We report the reduction of 2,3-dihydroisoxazoles to beta-amino ketones and beta-amino alcohols. The latter are obtained in high diastereoselectivity with preference for the syn isomer.     

A novel route to substituted 4-methylene-4,5-dihydroisoxazoles mediated by hafnium(IV) chloride

Heating alkyl vinyl ketones and N-tert-butylarylmethylideneamine N-oxides in the presence of HfCl4 results in the formation of 4-methylene-4,5-dihydroisoxazoles in good yield.     

Synthesis and Mesomorphic Properties of New 3-Aryl-5-cyano-4,5-dihydroisoxazoles

New 3-aryl-5-cyano-4,5-dihydroisoxazoles possessing liquid crystalline properties were synthesized in two ways. The key stage in both procedures is 1,3-dipolar cycloaddition of nitrile oxides to acrylonitrile.     

Isomerization of 3-unsubstituted 4,5-dihydroisoxazoles over alumina. A new synthesis of β-hydroxy nitriles

3-Unsubstituted 4,5-dihydroisoxazoles obtained by nitrosation of arylcyclopropanes are capable of undergoing efficient isomerization into 3-aryl-3-hydroxypropanenitriles during chromatography on alumina.     

A novel strategy for the synthesis of 4,5-dihydroisoxazoles via conjugate additions to α-nitroalkenes

Michael condensation of α-nitroalkenes with various nucleophiles, followed by the addition of trimethylchlorosilane, led to novel syntheses of 4,5-dihydroisoxazoles via cycloadditions of silyl nitronates.     

Reaction of 3,5-Disubstituted 4,5-Dihydroisoxazoles with Hexacarbonylmolybdenum

Depending on the reaction conditions and structure of the 5-substituent, reactions of substituted 4,5-dihydroisoxazoles with hexacarbonylmolybdenum involve cleavage of the heteroring at the N-O bond, its aromatization, or/and 1,3-decyclization.     

5-Amino-4,5-dihydroisoxazoles. Part II . Reactions of 5-amino-3-aryl-4-methylene-4,5-dihydroisoxazoles with Nucleophiles

Several 5-amino-3-aryl-4-methylene-4,5-dihydroisoxazoles were reacted with methoxide, benzenethiolate, benzenethiol, carboxylic acids and secondary amines. Addition and/or addition-elimination products were obtained. Reaction mechanisms are discussed.     

Reaction of Allyl Phenylcarbamate with Benzaldehyde Oximes in the Presence of N-Chlorobenzenesulfonamide Sodium Salt

Allyl phenylcarbamate reacts with benzaldehyde oxime and ring-substituted benzaldehyde oximes in ethanol in the presence of N-chlorobenzenesulfonamide sodium salt to give the corresponding 3-aryl-5-(phenylcarbamoyloxymethyl)-4,5-dihydroisoxazoles.     

A new multicomponent approach to highly functionalized 2,3-dihydroisoxazoles

The synthesis of highly functionalized 2,3-dihydroisoxazoles by a one-pot multicomponent reaction of an aldehyde, hydroxamic acid, and malononitrile or methyl cyanoacetate is described.     

Haloacetylated enol ethers. 5 [5]. Heterocyclic ring closure reactions of β-alkoxyvinyl dichloromethyl ketones with hydroxylamine

The β-alkoxyvinyl dichloromethyl ketones 1a-d are cyclocondensed with hydroxylamine hydrochloride in pyridine to afford the 5-hydroxy-5-dichloromethyl-4,5-dihydroisoxazoles 2a-d in good yield. The cyclo-condensation of compound 1c gave, together with 2c , 3-cyano-2-hydroxy-2-dichloromethyltetrahydrofuran 5c. The dehydratation of compounds 2a,b , derived from acyclic enol ethers, with concentrated sulfuric acid at 30°, led the corresponding 5-dichloromethylisoxazoles 3a,b. The dehydratation of compounds 2c,d , derived from cyclic enol ethers, with concentrated sulfuric acid at 30°, led the bicyclic 4,5-dihydroisoxazoles 4c,d , and at 55°, a competitive rearrangement reaction gives the 3-cyano-2-hydroxy-2-dichloromethyl-2H-pyran 5d.