Mechanisms of antioxidant action: rubber bound antioxidants based on nitrones—I. Non-sulphur vulcanizates

The antioxidant activities of a number of N-phenyl nitrones containing phenolic functions were compared with N-methyl nitrones and conventional antioxidants in peroxide and TMTD sulphur-less vulcanizates. The N-phenyl nitrones were found to be effectively bound during vulcanization whereas the N-methyl nitrones were not. Unhindered phenols reacted to a higher degree and were relatively more effective than the typical hindered phenol antioxidant structures. A conventional bisphenol was effective in the sulphurless vulcanizate after solvent extraction but this is considered to be due to the formation of an insoluble zinc salt. Synergism was observed in the TMTD sulphurless vulcanizate both before and after extraction.     

2-Substituted nitrones and isomeric hydroxylamines - obtained via aluminium amalgam reduction of nitro nitriles and ketones—a new access to convenient intermediates for nitroso carbonyl compounds preparation

Substituted five-membered cyclic nitrones (pyrroline N-oxides) have been obtained in good to high yields from tertiary γ-nitro ketones and nitriles employing aluminium amalgam as a reducing agent in moist diethyl ether or THF. Attempts to obtain cyclic amino nitrones from α- or β-nitro nitriles failed and only the corresponding hydroxylamines have been isolated. Both nitrones and hydroxylamines have been used for synthesis of tertiary C-nitroso nitriles or ketones.     

Increased formation of oxepanes in non-aqueous medium in the cycloaddition of 3-O-allyl-1,2-isopropylidenefuranose N-Ph nitrones

Cycloaddition of 3-O-allyl-1,2-isopropylidene N-Ph nitrones afforded appreciably increased yields of oxepanes compared to the corresponding N-Me or N-Bn nitrones. Higher yields permitted some useful further transformations of the oxepanes.     

H-N HMBC as a valuable tool for the identification and characterization of nitrones

¹H-¹⁵N HMBC has been evaluated as an efficient and high-speed method to determine ¹⁵N chemical shifts for nitrones, which can be used to identify aromatic nitrones and to extract structural information by comparison with reference data. Substituent effects have been measured on C and N aryl groups separately, showing up the influence of electronic effects on C-aryl groups rather than N-aryl groups on the ¹⁵N chemical shifts. Steric effects are remarkable in the case of C-aryl-N-alkyl nitrones. Depending on N or C substitution, chemical shift changes in such an additive way that it is possible to predict chemical shifts for unknown nitrones.     

Oxazolinyloxiranyllithium-mediated synthesis of highly strained heterocyclic compounds

The addition reaction of α-lithiated oxazolinyloxiranes to nitrones has been investigated. 1,5,9-Trioxa-8,10-diazadispiro[2.0.4.3]undecanes formed in a completely diastereoselective manner upon treatment of α-lithiated oxiranes with nitrones. The lithiation of optically active trans and cis-oxazolinyloxiranes followed by the addition of a nitrone resulted in the formation of the same dispirocyclic compound. An explanation for the observed stereoselectivity is provided.     

Enantioselective synthesis of homocarbocyclic-2′-oxo-3′-azanucleosides

The enantioselective synthesis of homocarbocyclic-2′oxo-3′-azanucleosides has been performed by cycloaddition reaction of the N-glycosyl nitrones with allyl nucleobases. The use of nitrones originated from two different carbohydrates, the N-ribosyl nitrone and the N-mannosyl nitrone, proceeded in a stereocontrolled and predictable manner with a good degree of enantioselectivity, so allowing an easy entry to both enantiomers.     

Diastereoselective addition of alkynylalanes to carbohydrate-derived nitrones

Propargylic N-hydroxypyrrolidines were prepared by diastereoselective addition of pre-formed alkynylalanes to various highly functionalized carbohydrate-derived endocyclic nitrones. Excellent diastereoisomeric excesses were obtained using dimethyl-2-phenylethynylalane. Addition of other alkynylalane derivatives to such type of nitrones is also reported.     

Synthesis of novel isoxazolidine analogues of homonucleosides

A general method for the synthesis of nucleobase-derived nitrones 4a–e by treatment of N-(2-oxoethyl)nucleobases with N-methylhydroxylamine is reported. The nitrones 4a–e were applied in the synthesis of isoxazolidine homonucleosides. Moderate diastereoselectivities (de 28–82%) were observed for cycloadditions between nitrones 4a–e and allyl alcohol with cis-isoxazolidines predominating. The stereochemistry of the substituted isoxazolidines was established based on an analysis of 2D NOE experiments for uracil-containing cycloadducts 6a and 7a. Cycloadditions of uracil-based nitrone 4a with vinyl-, allyl-, vinyloxymethyl- and allyloxymethylphosphonates gave the respective phosphonylated cis-isoxazolidines as the major adducts.     

Exploiting microwave-assisted neat procedures: synthesis of N-aryl and N-alkylnitrones and their cycloaddition en route for isoxazolidines

Microwave irradiation allows increasing the speed of several reactions and also offers the possibility of eliminating poisoning organic solvents. In this work we report the microwave-assisted neat synthesis of α-phenyl-tert-butylnitrone (PBN) and other alkyl and aryl nitrones and also the rapid synthesis of isoxazolidines resulting from 1,3-dipolar cycloaddition of nitrones to ethyl trans-crotonate.     

Experimental and theoretical studies on Mannich-type reactions of chiral non-racemic N-(benzyloxyethyl) nitrones

The nucleophilic addition of both silyl ketene acetals and lithium enolates derived from methyl acetate to chiral non-racemic N-(benzyloxyethyl)nitrones has been studied both experimentally and theoretically. Aromatic nitrones showed lower reactivity that aliphatic nitrones and the addition of the silyl ketene acetal led to lower selectivities than the addition of the corresponding lithium enolate. Whereas low selectivity was obtained for the addition of the silyl ketene acetal, only one diastereomer could be detected in all cases for the addition of lithium enolate to aliphatic nitrones. The synthetic utility of the two chiral auxiliaries employed lies in the preparation of enantiomeric compounds. DFT theoretical calculations confirmed the stepwise mechanism for the addition of silyl ketene acetals to nitrones and are in good agreement with the observed experimental results.     

Intramolecular 1,3-dipolar cycloaddition of N-alkenyl nitrones en route to glycosyl piperidines

Stereoselective intramolecular 1,3-dipolar cycloaddition of homochiral N-(alkenylglycosyl)nitrones, prepared by allylation of C-(glycosyl)nitrones and subsequent oxidation, is described. The previously described 2-aza-Cope rearrangement was not observed for these substrates, but evidences of E/Z isomerism during the cycloaddition were obtained. The obtained cycloadducts can serve as key precursors of imino disaccharide analogues. This is exemplified by a short route to a protected 2-furanosyl-4-hydroxy-6-phenyl piperidine.     

Stereoselective synthesis of pyrrolidinyl glycines from nitrones: complementarity of nucleophilic addition and 1,3-dipolar cycloaddition

Epimeric pyrrolidinyl glycines, a sort of conformationally constrained α,β-diaminoacids, were stereoselectively prepared using complementary approaches based on nitrone chemistry. Nucleophilic additions to pyrrolidinyl nitrones and 1,3-dipolar cycloadditions of l-serine derived nitrones to form the corresponding hydroxylamines and isoxazolidines, respectively, provided key intermediates for the synthesis of the target compounds. Whereas the nucleophilic addition route afforded the syn adduct, the 1,3-dipolar cycloaddition approach furnished the precursor for the preparation of the corresponding anti compound.     

Chiral four-membered cyclic nitrones; asymmetric induction in the (4+2)-cycloaddition reaction of chiral ynamines and nitroalkenes

Chiral four-membered cyclic nitrones were synthesized by the asymmetric (4+2)-cycloaddition of nitroalkenes 1 and chiral ynamines 2. The subsequent stereoselective addition of nucleophiles to these nitrones enabled the synthesis of chiral N-hydroxyazetidines.     

Diastereoselective syntheses of pyrazolyl isoxazolidines via 1,3-dipolar cycloaddition

Synthesis of novel 2,3,4,5-tetrasubstituted isoxazolidine and 2,3,5-trisubstituted isoxazolidine by 1,3-dipolar cycloaddition of nitrones with electron-deficient and electron-rich olefins is described. 1,3-Dipolar cycloaddition of nitrones with β-nitrostyrene, gives exclusively endo-diastereoisomer of isoxazolidine and with ethyl vinyl ether, gives exo-diastereoisomer of isoxazolidine with de >95%. The stereochemistry of the products were assigned using extensive NMR studies.     

Synthesis of substituted 2-(thiophen-2-yl)-1<Emphasis Type="Italic">H</Emphasis>-imidazol-1-ols

Substituted 2-(thiophen-2-yl)-1H-imidazol-1-ols were synthesized by cyclization of the corresponding α-thienyl nitrones in alkaline medium. α-Thienyl nitrones were obtained by reaction of N-(1-hydroxyimine-1-R-propan-2-yl)hydroxylamines with thiophene-2-carbaldehyde in methanol. At boiling α-thienyl nitrones in methanol in the presence of sodium methylate 5-aryl(hetaryl)-2-(thiophen-2-yl)-1H-imidazol-1-ols are formed chemoselectively.     

Silyl triflate-accelerated additions of catalytically generated zinc acetylides to N-phenyl nitrones

Terminal alkynes readily form zinc acetylides in the presence of iPr₂NEt and 20 mol % ZnBr₂, then attack N-phenyl nitrones activated by trimethylsilyl trifluoromethanesulfonate. Deprotection with aqueous acid yields N-hydroxyl propargylamine. Yields are generally high for nitrones derived from aromatic aldehydes. Control experiments suggest that silyl triflate has a significant accelerating effect upon the reaction.     

Improved dimethylzinc-promoted vinylation of nitrones with vinylboronic esters

Vinylboronic esters derived from 4,4,6-trimethyl-[1,3,2]dioxaborinane react with nitrones in the presence of dimethylzinc; nucleophilic addition of the vinyl group onto nitrones produces N-allylic hydroxylamines in fair yields. The sequence is compatible with various functional groups on the vinylic moiety. The mechanism and kinetic aspects are discussed on the basis of DFT calculations.     

InBr3-catalyzed direct alkynylation of nitrones with terminal alkynes: an efficient synthesis of N-hydroxy-propargyl amines

An InBr₃-catalyzed direct and efficient alkynylation of nitrones with terminal alkynes was developed. The process enables practical synthesis of a wide range of synthetically useful N-hydroxy-propargyl amine derivatives in good yields under mild conditions. The application of this method to optically active propargylic N-hydroxyamines syntheses was also described. With chiral nitrones, good diastereoselectivities were obtained. Moreover, the first chiral indium(III) complex-catalyzed asymmetric alkynylation of nitrone was achieved with moderate enantioselectivity.     

Efficient approach to the unknown isoxazolo[3,4-d]thieno[2,3-b]pyridine system by regioselective intramolecular nitrone cycloadditions

An effective approach to the new isoxazolo[3,4-d]thieno[2,3-b]pyridine system was provided by way of an intramolecular nitrone cycloaddition. The required nitrones were built in good yields starting from thiophene-2-carboxylic acids. The same skeleton was achieved in optically active form employing chiral nitrones derived from N-α-methylbenzyl- and the N-α-hydroxymethylbenzyl-hydroxylamines. The absolute configuration of the products was assigned by X-ray analysis.     

Nucleophilic additions of lithiated allylphenylsulfone to nitrones: experimental and theoretical investigations

The nucleophilic addition of lithiated allylphenylsulfone to nitrones at −80 °C proceeds exclusively α to the phenylsulfonyl group affording anti adducts in high yield. At 0 °C isoxazolidines are obtained with complete all-trans selectivity. The formation of these compounds involves isomerization of the allylsulphonyl moiety to give a transient vinylsulfone that then undergoes a subsequent intramolecular Michael addition. The addition to several nitrones has been studied and theoretical calculations have been refined to accurately explain the selectivity of the allylation reaction.