Selectivities in the 1,3-dipolar cycloaddition of nitrile oxides to dicyclopentadiene and its derivatives

The 1,3-dipolar cycloaddition of nitrile oxides, generated from aldoximes and nitroalkanes, to dicyclopentadiene proceeds with complete chemo- and stereoselectivity. The approach of the dipole takes place exclusively from the exo-face of the bicycloheptane moiety providing a mixture of regioisomers in approximately 55:45 ratio. On the other hand, nitrile oxide cycloaddition to dimethyldicyclopentadiene dicarboxylate (Thiele's ester), besides exhibiting chemo- and stereoselectivity as in the case of dicyclopentadiene, exhibits complete regioselectivity as well providing a single isomer in good yield. The Influence of remote substituents, including sterically ‘sterile’ ones, on the regioselectivity has also been investigated using 8-hydroxy and 1-keto derivatives of dicyclopentadiene. These experimental observations have been investigated through gas phase and solvent model MO calculations on the transition state geometries at semiempirical (PM3) and hybrid ab initio-DFT levels of theory. The Computational methods employed in this study were rigorously tested by performing model calculations on well-established experimental observations.     

Regioselectivity in the 1,3-dipolar cycloaddition of nitrile oxides to 3,3-methylene-5,5-dimethyl-2-pyrrolidinone

Summary Nitrile oxides add regioselectively to the carbon-carbon double bond of 3,3-methylene-5,5-dimethyl-2-pyrrolidinone (1), giving exclusively spiro-isoxazolines: 3-aryl-6-oxo-8,8-dimethyl-1-oxa-2,7-diazaspiro[4.4]non-2-enes3 a–l. The regiochemistry of the 1,3-dipolar cycloaddition to 1 seems to be controlled by the steric effect of the methyl groups at the ring junction and by frontier orbital interactions.Part XXV in the series 1,3-Dipolar Cycloaddition on Heterocycles. Part XXIV Ref. [10]     

Study on the 1,3-dipolar cycloaddition reaction of 4-ethoxy-1,1,1-trifluoro-3-buten-2-one with nitrile oxides

1,3-Dipolar cycloaddition (1,3-DC) reaction of 4-ethoxy-1,1,1-trifluoro-3-buten-2-one 1 with nitrile oxides was studied. It was found that besides its CC participating in the formation of isoxazole rings, trifluoromethyl activated CO also underwent 1,3-DC reaction with nitrile oxides to afford 1,4,2-dioxazole rings. Single crystal diffraction analysis also evidenced the diheterocyclic configuration.     

Stereoselective preparation of 1,2,4-oxadiazole derivatives substituted by pentafluorophenyl by 1,3-dipolar cycloaddition reaction

1,3-Dipolar cycloaddition reactions of chiral imines obtained from optically active amino acids with nitrile oxides afforded 1,2,4-oxadiazole derivatives in moderate to good yields with good stereoselectivity. Investigation on the effect of bases suggested that triethylamine was prone to afford better stereoselectivity, while NaHCO₃ was prone to increase the reaction rates and yields.     

An efficient 1,3-dipolar cycloaddition between aromatic selenoaldehydes and nitrile oxides or nitrile imines: an easy access to selenium-containing five-membered heterocyclic ring system

1,3-Dipolar cycloaddition between aromatic selenoaldehydes, generated by thermal retro Diels-Alder reaction of anthracene cycloadducts, and nitrile oxides or nitrile imines proceeded efficiently to give the corresponding [3+2] cycloadducts as a single isomer in good yields, being 1,4,2-oxaselenazoles or 1,3,4-selenadiazoles, respectively.     

Synthesis of polyfluorophenyl substituted-4,5-dihydropyrazole derivatives via 1,3-dipolar cycloaddition of nitrile imine with ethyl acrylate

Oxidation of aldehyde 4-chloro-2,3,5,6-tetrafluorophenylhydrazones (1) with [bis(acetoxy)iodo]benzene leads to the formation of nitrile imines (2) which reacted in situ with ethyl acrylate to produce 3-substituted-1-(4-chloro-2,3,5,6-tetrafluorophenyl)-5-ethoxycarbonyl-4,5-dihydropyrazoles (3) in moderate to good yields. The structures of new compounds were fully confirmed by their spectral data, elemental analyses and X-ray diffraction (XRD) analysis. A plausible reaction mechanism for the synthesis of title compounds is presented.     

Asymmetric 1,3-dipolar cycloaddition of nitrile oxides with optically active vinylboronic ester

Pinanyldioxy styrylboronic ester (1) was employed for asymmetric 1,3-dipolar cycloaddition with nitrile oxides, optically active △2-isoxazolines (3) or 4-hydroxy-△2-isoxazolines (4) were obtained. The effect of different bases on the selectivity of the cycloaddition reaction was studied.     

Synthesis of functionalized polyhedral oligomeric silsesquioxane (POSS) macromers by microwave assisted 1,3-dipolar cycloaddition

In this paper we report the first synthesis of isoxazolidine and isoxazoline-POSS macromers by 1,3-dipolar cycloaddition reactions of vinyl- and styryl-POSS with N-methyl-C-ethoxycarbonylnitrone and ethoxycarbonyl nitrile oxide, promoted by microwave irradiation. The nature of the resulting cycloadducts has been determined by NOE experiments and supported by computational studies at PM3 and DFT B3LYP/6-31G* levels.     

Base-promoted 1,3-dipolar cycloaddition reaction of nitrile oxides with methyl 1,4-dioxo-1,4-dihydronaphthalene-2-carboxylate for the construction of naphtho[2,3-d]isoxazole-4,9(3aH,9aH)-diones

A base mediated 1,3-dipolar cycloaddition reaction of methyl 1,4-dioxo-1,4-dihydronaphthalene-2-carboxylate with nitrile oxides in situ generated from N-hydroximoyl chlorides was achieved. With this developed protocol, a range of structurally diverse naphtho[2,3-d]isoxazole-4,9(3aH,9aH)-dione derivatives were smoothly obtained in high yields (up to 95%) with up to >20:1 regioselectivities and >20:1 diastereoselectivities under mild conditions. The promising applicability of the protocol was also demonstrated by the further transformations.     

A facile synthesis of pyrazoles with multi-point structural diversity by 1,3-dipolar cycloaddition

We describe the synthesis of diverse pyrazoles by 1,3-dipolar cycloaddition of ethyl diazoacetate with various acetylenes in refluxing toluene. The product pyrazoles are useful starting points for preparing a diverse collection of trisubstituted pyrazole carboxamides. For aryl and heteroaryl alkynes a single product is obtained while alkyl alkynes afford a ca. 6:1 mixture of regioisomers. The observed regioselectivity for the cycloaddition step and the ease of reaction are consistent with predictions derived from computing the HOMO-LUMO energies of the reactants.     

Ruthenium-catalyzed 1,3-dipolar cycloaddition of trifluoromethylated propargylic alcohols with azides

The 1,3-dipolar cycloaddition of trifluoromethylated propargylic alcohols 1 with azides in the presence of catalytic [Cp*RuCl₂]_n afforded exclusively 4-trifluoromethyl-1,4,5-trisubstituted-1,2,3-triazoles 2 in high yields.     

The synthesis and biological activity of novel spiro-isoxazoline C-disaccharides based on 1,3-dipolar cycloaddition of exo-glycals and sugar nitrile oxides

A series of novel spiro-isoxazoline C-disaccharides were synthesized by the key step of 1,3-dipolar cycloaddition reactions of exo-glycals and sugar nitrile oxides, and followed by catalytic debenzylation in the presence of Pd(OH)₂/C. The cycloaddition reactions were carried out stereoselectively and afforded α-isomers exclusively except in the case of galactose. The biological activities of the novel disaccharides against the glycosidases (α-amylase, α-glucosidase, and β-glucosidase) and HIV and BVDV were evaluated.     

Stereoselective intramolecular 1,3-dipolar nitrile oxide cycloaddition reaction of N-formyl-β-nitroamides

The stereoselective intramolecular nitrile oxide cycloaddition (INOC) reaction was achieved from N-formyl-β-nitroamides, which were prepared through the Michael addition of allylic formamides to nitroalkenes, and cis-pyrroroisoxazoles and trans-piperidinoisoxazoles were obtained in a stereoselective manner.     

Regio- and stereoselective 1,3-dipolar cycloaddition of arylnitrile oxides to 5-acetoxy-2(5H)-furanone

Summary Arylnitrile oxides undergo regio- and stereo-specific 1,3-dipolar cycloaddition reactions with 5-acetoxy-2(5H)-furanone. In each case a single product3a–3g results from ananti approach to the 5-acetoxy substituent, the oxygen of the 1,3-dipole being attached to C-4 of furan. Under similar conditions 5-benzoyloxy-2(5H)-furanone yields3h–3i. The structures of the adducts were determined by¹H- and¹³C-NMR spectroscopy.

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Regio- und stereoselektive 1,3-dipolare Cycloaddition von Arylnitriloxiden mit 5-Acetoxy-2(5H)-furanon

Zusammenfassung Arylnitriloxide reagieren mit 5-Acetoxy-2(5H)-furanon in einer regio- und stereoselektiven 1,3-dipolaren Cycloaddition. In jedem der untersuchten Fälle ergaben sich die einheitlichen Produkte3a–3g als Folge eineranti-Annäherung an den 5-Acetoxysubstituenten, wobei der Sauerstoff des 1,3-Dipols an das C-4 des Furans addiert. Unter ähnlichen Bedingungen ergab 5-Benzoyloxy-2(5H)-furanon die Produkte3h–3i. Die Strukturen der Addukte wurden mittels¹H-und¹³C-NMR bestimmt.

     

Asymmetric 1,3-dipolar cycloaddition of nitrile oxides to chiral acryloyl esters bearing glucofuranose as auxiliary

Asymmetric 1,3-dipolar cydoaddition of nitrite oxides to an acryloyl ester (1) derived from 1,2:5,6-di-O-isopropylidene glucose (6) was studied. Solvent and temperature effect was discussed. The single diastereoisomer was isolated with high diastereoselective excess.     

Unexpected opening of benzimidazole derivatives during 1,3-dipolar cycloaddition

Hydroxylamine hydrochloride adds via a Michael reaction on the acetonylidene moiety of (4Z)-(2-oxopropylidene)-1,2,4,5-tetrahydro-2H-1,5-benzodiazepin-2-one, leading only to 2-[(5-methyl-isoxazol-3-yl)]methyl]benzimidazoles. This report describes the difference of reactivity of C-phenyl-N-phenyl formohydrazonoyl chloride with benzimidazole. For the N-substituted benzimidazole, an unexpected opening of the azole ring occurs, which was confirmed by single crystal X-ray diffraction analysis.     

Synthesis of 5-aminoisoxazolines from N-allyl compounds and nitrile oxides via tandem isomerization-1,3-dipolar cycloaddition

A new strategy for the synthesis of derivatives of 5-aminoisoxazolines via tandem catalytic isomerization (of N-allyl systems to N-(1-propenyl) systems)—1,3-dipolar cycloaddition (of a stable nitrile oxide to N-(1-propenyl) systems) is presented. Rhodium and ruthenium complexes, Verkade’s superbase, and 18-crown-6/KOH system were used for the syntheses of the N-(1-propenyl) systems. 4-P-substituted isoxazoline was also synthesized via cycloaddition of diphenyl(1-propenyl)phosphine (prepared via isomerization of allyldiphenylphosphine) to 2,6-dichlorobenzonitrile oxide. All cycloadditions were regioselective but not stereoselective and not concerted. Cycloaddition to all N-(1-propenyl) systems yielded 5-N-substituted isoxazolines, but cycloaddition to P-(1-propenyl) system lead to the formation of a 4-P-regioisomer. This difference in regioselectivity is predicted by opposite FMO reactivity indices calculated for model compounds: N-(1-propenyl)amine and N-(1-propenyl)phosphine.     

Tricyclic-isoxazolidine analogues via intramolecular 1,3-dipolar cycloaddition reactions of nitrones

The tricyclic-isoxazolidine analogues tetrahydrothiochromenoisoxazoles, hexahydroisoxazolequinolines and tetrahydroisoxazolepyranopyridines were prepared by an intramolecular 1,3-dipolar cycloaddition reaction of a nitrone with an alkene. For N-alkylated hexahydroisoxazolequinolines, reduction of the reaction time from two days to 40 min was achieved using microwave heating. The cyclization to form tetrahydroisoxazolepyranopyridines only proceeded when the alkene was substituted with an electron withdrawing group.     

Regioselectivity in the 1,3-dipolar cycloaddition of nitrile oxides to N-(3,5-dichlorophenyl)itaconimide

Summary The regioselectivity of the nitrile oxide cycloaddition with 3,3-methylene-1-(3,5-dichlorophenyl)-2,5-pyrrolidindione (1) is discussed. Arylnitrile oxides add regioselectively to the carboncarbon double bond of1, giving exclusively spiro-isoxazolines3a–k, whereas acetonitrile oxide gives both adducts3l and4l. AM1 calculations of the reactants and cycloadducts were performed, the regiochemistry of the cycloaddition seems to be controlled by steric effects. Reduction of3a with NaBH₄ in the presence of magnesium perchlorate at –20 °C was regio-and stereoselective to yield the hydroxylactams9a and10a, whereas hydroxymethylisoxazoline14a was obtained in the absence of Mg(ClO₄)₂.Part XXX in the series 1,3-Dipolar Cycloaddition on Heterocycles. Part XXIX [27]     

Synthesis of novel pyrano[2,3-b]quinolines from simple acetanilides via intramolecular 1,3-dipolar cycloaddition

Some novel isoxazole and pyrazole fused pyrano[2,3-b]quinolines were synthesized from simple acetanilides via intramolecular 1,3-dipolar cycloaddition reactions involving nitrones, nitrile oxides and nitrile imines as 1,3-dipoles, in a regioselective manner.