Stereoselectivity of N-benzyl-C-ethoxycarbonyl nitrone cycloaddition to (S)-5-hydroxymethyl-2(5H)-furanone and its derivatives

Ondrus, V.; Orsag, M.; Fišera, Ľ.; Prónayová, Naďa

doi:10.1016/s0040-4020(99)00568-2

Cited by 27 publications

(21 citation statements)

References 16 publications

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“…In a recent paper [22] it was shown that 1,3-dipolar cycloaddition reactions of N-phenyl-C-arylnitrones to the C16-C17 π-bond in 16-dehydropregnenolone acetate proceed through the minor rotamer (E-isomer; for the formulas of the E-and Z-isomers see Scheme 2) of the nitrones. In another recent work [23] the cycloaddition reactions of N-b e n z y l -C-ethoxycarbonylnitrones in benzene solution have been stated to proceed mainly through the E-isomer of the nitrone. Additionally intramolecular cycloadditions of E-isomers of nitrones to alkenes have been utilized as key steps in the syntheses of a number of important isoxazolidine derivatives [24][25][26][27].…”

Section: Resultsmentioning

confidence: 99%

“…In general, the electron-withdrawing substituents of N-m e t h y l -C-substituted-phenylnitrones should not stabilize the E-isomers (Structure I, Scheme 2), and therefore the Z-isomers should predominate in solution [22,23,25,26]. The endo and exo transition states leading to the trans and cis cycloadducts are now of comparable e n e rgies [5] (exo and e n d o approach of the Z-i s o m e r, Scheme 3).…”

Section: Resultsmentioning

confidence: 99%

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Structural Characterization of Isomeric 2,3,5‐Substituted Tetrahydropyrrolo[3,4‐d]isoxazole‐4,6‐diones Prepared by Cycloaddition of N‐Methyl‐C‐arylnitrones to N‐Phenyl‐ or N‐Methylmaleimide.

et al. 2005

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1,3-Dipolar cycloaddition reactions of N-methyl-C-arylnitrones with N-phenyl-or N-methylmaleimide were studied. The reaction of p-dimethylamino-, 4-benzyloxy-3-methoxy-, p-nitro-and p-chloro-substituted phenylnitrones with N-phenylmaleimide gave c i s and t r a n s cycloadducts but that of the corresponding phenylnitrones with N-methylmaleimides only the cis adducts in the case of p-dimethylamino and 4-benzyloxy-3-methoxy substitution. All cis adducts attain a biased conformation whereas the trans forms are shown (by 1 H NMR at 233 K and 13 C NMR at 208 K) to be mixtures of two invertomers, namely o-(N-lone pair antiperiplanar to 3H; minor) and i-conformations (3H-C-C-3aH dihedral angle close to 90°; major). PM3 and DFT calculations at the B3LYP/6-31G(d) level of theory prove qualitatively that these two conformers of the trans adduct are of comparable stability and represent energy minima.

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Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

Structural Characterization of Isomeric 2,3,5‐Substituted Tetrahydropyrrolo[3,4‐d]isoxazole‐4,6‐diones Prepared by Cycloaddition of N‐Methyl‐C‐arylnitrones to N‐Phenyl‐ or N‐Methylmaleimide.

et al. 2005

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“…However, only small changes in the regio-and stereoselectivity were observed. [29] The synthesis of methyleneisoxazolidine nucleoside analogues by microwave-assisted nitrone cycloaddition was developed by Chiacchio et al in 2007. [30] The authors studied the cycloaddition between C-ethoxycarbonyl-N-methylnitrone (22, Scheme 8) and the protected thiamallene 23a under microwave irradiation conditions over 45 min in CCl 4 as solvent.…”

Section: Cycloadditions Of Nitronesmentioning

confidence: 99%

Microwave‐Assisted 1,3‐Dipolar Cycloaddition: an Eco‐Friendly Approach to Five‐Membered Heterocycles

Piñeiro

Melo

2009

Eur J Org Chem

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The review describes the use of microwave technology to carry out 1,3-dipolar cycloaddition reactions, an important tool for the construction of five-membered heterocycles. Microwave methodology can be applied to generate 1,3-dipoles (nitrones, nitrile oxides, azomethine ylides, azomethine imines, nitrile imines, azides, carbonyl ylides) and to promote the subsequent cycloaddition and usually avoids harsh reaction conditions. This nonconventional energy source is able

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“…However, attempts to accelerate the cycloadditions by microwave irradiation, using 1,4-dioxane as the solvent, were successful and the reaction time decreased from hours to less than 10 min with only a small change in the stereoselectivity (in favor of the exo-162 isomer). Moreover, the use of microwaves also led to the formation of a small amount of the unexpected syn adduct 165 [101]. When the aldehyde 137 and N-methylhydroxylamine (166) were mixed with silica gel and irradiated with microwaves for 15 min the adduct obtained was 168 (82 % yield).…”

Section: Nitronesmentioning

confidence: 99%

Microwaves in Cycloadditions

Hoz

Díaz‐Ortiz

Langa³

2002

Microwaves in Organic Synthesis

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The first examples of microwave-induced reactions were cycloadditions and pericyclic reactions performed under pressure, because of the use of domestic ovens, to avoid the risk of igniting flammable solvents (Scheme 9.1) [11,12].Reactions were performed in sealed thick-walled glass tubes or in Teflon acid-digestion vessels, in domestic microwave ovens [13]. Teflon vessels can be used at pressures up to 14 atm, at temperatures below 250 8C, and are resistant to most commonly used chemicals, although they deform at temperatures >250 8C.The lack of temperature and pressure control in these systems could lead to safety problems, because of overpressure resulting from the heating rate caused by microwaves. These problems can be reduced by using solvent-free methodology, which also enables the use of larger quantities of reagents. Yields have been greatly improved, and reaction times reduced, in comparison with conventional procedures in solvents under reflux.These reactions were next revisited by Strauss and it was shown that, under identical conditions in xylene, the reaction rates were strictly identical within experimental error [14]. Reactions under RefluxReactions involving a solvent under reflux require the use of modified commercial microwave ovens. In these modified systems the oven is perforated on the top to accommodate a reflux condenser and a 10 cm pipe is used to avoid microwave leakage; the turnable dish is replaced by a magnetic stirrer or by monomode reactors especially designed for chemical synthesis [15].The advantage of reactions involving solvent is that the reaction temperature is controlled by the reflux temperature of the solvent [16]. It should, however, be re-296 9 Microwaves in Cycloadditions CO 2 Me MeO 2 C MeO 2 C CO 2 Me MICROWAVE 10 min p-xylene, 87% 325º<361 ºC CONTROL 4 h p-xylene, 67% 138 ºC 1 2 3 Scheme 9.1 297 9.3 Solvent-free Conditions 5 6 7 8 Scheme 9.2 silica gel [29] and alumina [30]have also been used to generate 1,3-dipoles and, in this way, heterocyclic compounds can be obtained in a one-pot procedure. In these reactions changes in the regioselectivity of the reaction have not been observed under the action of microwave irradiation. Reactions without SupportAn alternative to the use of supported reagents is the use of uncatalyzed ªneat reactionsº. Under these conditions the radiation is absorbed directly by the reagents and this results in spectacular acceleration, higher yields and purity of the reaction products, a simple workup procedure, and sometimes changes in the selectivity of the cycloaddition. Diels±Alder reactions [31] and 1,3-dipolar cycloadditions [32,33] have been performed by use of this methodology. For example, DÌaz-Ortiz described the hetero-Diels±Alder and 1,3-dipolar cycloaddition reactions of ketene acetals. The reactions were improved and products were isolated directly from the crude reaction mixture without polymerization of the ketene acetals [34].When the enantiomerically pure ketene acetal 16 was used, cycloaddition with N,a-diphenylnitrone (11) or ...

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Stereoselectivity of N-benzyl-C-ethoxycarbonyl nitrone cycloaddition to (S)-5-hydroxymethyl-2(5H)-furanone and its derivatives

Cited by 27 publications

References 16 publications

Structural Characterization of Isomeric 2,3,5‐Substituted Tetrahydropyrrolo[3,4‐d]isoxazole‐4,6‐diones Prepared by Cycloaddition of N‐Methyl‐C‐arylnitrones to N‐Phenyl‐ or N‐Methylmaleimide.

Structural Characterization of Isomeric 2,3,5‐Substituted Tetrahydropyrrolo[3,4‐d]isoxazole‐4,6‐diones Prepared by Cycloaddition of N‐Methyl‐C‐arylnitrones to N‐Phenyl‐ or N‐Methylmaleimide.

Microwave‐Assisted 1,3‐Dipolar Cycloaddition: an Eco‐Friendly Approach to Five‐Membered Heterocycles

Microwaves in Cycloadditions

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