Diastereoselective intermolecular radical addition to nitrones

The intermolecular radical addition to chiral nitrones 2, 4, 5, and 16 was studied. The isopropyl radical addition to Oppolzer's camphorsultam derivative 2 of glyoxylic nitrone proceeded with excellent diastereoselectivity to give the desired isopropylated product 3a accompanied by the diisopropylated product 3b. A high degree of stereocontrol in the reaction of cyclic nitrone 4 was achieved. The ethyl radical addition to nitrone 4 with triethylborane afforded the desired ethylated product 9a accompanied by the diethylated product 10a and the ethylated nitrone 11a. To evaluate the utility of cyclic nitrone 4, several alkyl radicals were employed in the addition reaction, which afforded the alkylated products 9b-d with excellent diastereoselectivities. In the presence of Mg(ClO(4))(2), the ethyl radical addition to BIGN 16 afforded selectively syn isomers. In contrast, the alkyl radical addition to 16 took place even in the absence of Lewis acid to give anti isomers.     

1,3-Dipolar cycloadditions of new 2,5-bifunctionalized five-membered cyclic nitrones

A new family of five-membered cyclic nitrones bearing functional groups at positions 2 and 5 were tested in 1,3-cycloadditions with various dipolarophiles. These reactions gave the corresponding cycloadducts as single diastereomers in high yields. The reaction scope was examined, 23 compounds were isolated and fully characterized. The comparison of the reactivity was carried out by a study of reaction conversions while different nitrones or dipolarophiles were used. In addition, the exceptional reactivity of nitrone bearing benzylamino group in position 2 was studied. The formation of the nitrone complex and evaluation of its reactivity in 1,3-DC is presented.     

Nitrone Directing Groups in Rhodium(III)‐Catalyzed C−H Activation of Arenes: 1,3‐Dipoles versus Traceless Directing Groups

Functionalizable directing groups (DGs) are highly desirable in C?H activation chemistry. The nitrone DGs are explored in rhodium(III)‐catalyzed C?H activation of arenes and couplings with cyclopropenones. N‐tert‐butyl nitrones bearing a small ortho substituent coupled to afford 1‐naphthols, where the nitrone acts as a traceless DG. In contrast, coupling of N‐tert‐butyl nitrones bearing a bulky ortho group follows a C?H acylation/[3+2] dipolar addition pathway to give bicyclics. The coupling of N‐arylnitrones follows the same acylation/[3+2] addition process but delivers different bicyclics.     

Action de l'eau oxygénée et de l'acide p-nitroperbenzoïque sur des imines,des oxazirannes et des nitrones cycliques stéroïdiques dérivés de la conanine

Hydrogen peroxide and a peracid react with pyrrolidinic imines, oxaziridines and nitrones of the steroïdal series, to give α-hydroxylated nitrones, α-peroxyimines, hydroxylactones or hydroxamic acids. Analogies and differences between reactivities of both oxidants are shown. Thus, a peracid oxidizes a nitrone as well as an oxaziridine, whereas hydrogen peroxide which oxidizes a nitrone, by contrast, deoxygenates an oxaziridine. Differences appear also, depending on the structure of the intermediates formed which carry either hydroperoxide or perester groups. The presence of a better leaving group in the latter case may modify the course of reactions effected on a α-hydroxylated nitrone or imine. Furthermore the acylation of an aldonitrone facilitates its oxidation and leads with a peracid to an O-acylhydroxamic acid protected from further oxidation.     

Theoretical study of the spin trapping of hydroxyl radical by cyclic nitrones: a density functional theory approach

The hydroxyl radical (*OH) is an important mediator of biological oxidative stress, and this has stimulated interest in its detection. 5,5-Dimethyl-1-pyrroline N-oxide (DMPO) and its alkoxycarbonyl and alkoxyphosphoryl analogues have been employed as spin traps for electron paramagnetic resonance (EPR) spectroscopic radical detection. Energies of optimized geometries of nitrones and their corresponding *OH adducts were calculated using density functional theory (DFT) at the B3LYP/6-31+G//B3LYP/6-31G level. Calculations predict that the trans adduct formation is favored in alkoxycarbonyl nitrones, while cis adducts with intramolecular H-bonding is favored for alkoxyphosphoryl nitrones. Addition of *OH to a phosphoryl-substituted nitrone is more exoergic than the carbonylated nitrones. Charge and spin densities on the nitrone spin traps were correlated with their rates of addition with *OH, and results show that the charge density on the nitronyl C, the site of *OH addition, is more positive in phosphorylated nitrones compared to DMPO and the alkoxycarbonyl nitrones. The dihedral angle between the beta-H and nitroxyl O bonds is smaller in phosphorylated nitrones, and that aspect appears to account for the longer half-lives of the spin adducts compared to those in DMPO and alkoxycarbonyl nitrones. Structures of nitrones with trifluoromethyl-, trifluoromethylcarbonyl-, methylsulfonyl-, trifluoromethylsulfonyl-, amido-, spiropentyl-, and spiroester substituents were optimized and their energies compared. Amido and spiroester nitrones were predicted to be the most suitable nitrones for spin trapping of *OH due to the similarity of their thermodynamic and electronic properties to those of alkoxyphosphoryl nitrones. Moreover, dimethoxyphosphoryl substitution at C-5 was found to be the most efficient substitution site for spin trapping of *OH, and their spin adducts are predicted to be the most stable of all of the isomeric forms.     

A General Method for the Vinylation of Nitrones. Synthesis of Allyl Hydroxylamines and Allyl Amines

An examination of the vinylation of several nitrones is presented. Whereas a complete diastereofacial discrimination was observed upon the addition of vinyl organometallic reagents to α-alkoxy nitrones, the same reaction with α-amino nitrones gave syn adducts in all cases, with the only exception of a L-serine-derived α-amino monoprotected nitrone. The obtained allyl hydroxylamines were easily transformed into synthetically valuable allyl amines.     

Approach to Fully Substituted Cyclic Nitrones from N‐Hydroxylactam Derivatives: Development and Application to the Total Synthesis of Cylindricine C

An approach to cyclic nitrones from N‐hydroxylactam derivatives is documented. The nucleophilic addition of an organolithium reagent to an N‐OSEM [SEM=2‐(trimethylsilyl)ethoxymethyl] lactam forms a five‐membered chelated intermediate, which undergoes both elimination and deprotection to give a fully substituted nitrone in a one‐pot process. When combined with the N‐oxidation of easily available chiral lactams, this method becomes especially useful for the quick synthesis of chiral nitrones in enantio‐pure form, enabling the concise total synthesis of cylindricine C.     

N-benzyl aspartate nitrones: unprecedented single-step synthesis and [3 + 2] cycloaddition reactions with alkenes

N-benzyl aspartate nitrones 2, prepared by addition of N-benzylhydroxylamine to dialkyl acetylenedicarboxylates 1, underwent [3 + 2] thermal cycloaddition with a wide range of alkenes to afford isoxazolidines 4 bearing a polyfunctionalized quaternary center. Under these uncatalyzed conditions, the trans stereocontrol observed with vinyl ethers is higher than that obtained with all acyclic activated nitrones reported to date. The first asymmetric access to a type-4 pure adduct was achieved starting from the chiral aspartate nitrone derived from (S)-alpha-methylbenzylhydroxylamine.     

Rearrangements of bicyclic nitrones to lactams: comparison of photochemical and modified Barton conditions

The rearrangement of nitrones to lactams can be carried out by photochemical activation or by treatment with Tf(2)O followed by KOH-promoted rearrangement (a modification of conditions originally introduced by Barton). Substrates in which the nitrone is part of a fused bicyclic ring system have traditionally proven problematic for this kind of reaction. In this study, a series of mono-, bi-, and tricyclic ring-fused nitrones were prepared to investigate the dependence of products on nitrone ring size and tether length. Results indicated that photochemical rearrangement of nitrones in benzene afforded reasonably good yields (30-68%) of lactams, while the two-step nonphotochemical process provided slightly better average yields (30-95%) of the same targets.     

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.     

Cycloaddition dipolaire 1,3 sur des composes a liaison ethylenique activee—XVII : Addition des nitrones aux olefines α,β-disubstituees; stereochimie des isoxazolidines obtenues

The addition of nitrones to varions disubstituted olefine has been achieved, in order to discuss the different factors which determine the orientation of the 1,3 dipolar cycloaddition, and the way in which the two partners approach one another. The C,N-diphenyl nitrone leads to two epimeric isoxazolidines, but the C-benzoyl N-phenyl nitrone gives only one heterocycle. The determination of the stereochemistry of the resulting isoxazolidines allows the way in which the dipole is approaching the dipolarophile to be discerned.     

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.     

Rhodium‐Catalyzed Cyclization of Diynes with Nitrones: A Formal [2+2+5] Approach to Bridged Eight‐Membered Heterocycles

N‐aryl‐substituted nitrones were employed as five‐atom coupling partners in the rhodium‐catalyzed cyclization with diynes. In this reaction, the nitrone moiety served as a directing group for the catalytic C? H activation of the N‐aryl ring. This formal [2+2+5] approach allows rapid access to bridged eight‐membered heterocycles with broad substrate scope. The results of this study may provide new insight into the chemistry of nitrones and find applications in the synthesis of other heterocycles.     

带取代基硝酮化合物光环化反应的研究

本工作合成了一系列取代的硝酮类化合物。其中部分化合物仅一侧的苯环上带有取代基,而另部分化合物则两个苯环上均带有取代基。研究工作表明:取代基处于一侧的硝酮类化合物,不论取代基是给出电子或接受电子的,均有利于化合物光环化反应的进行。而当两个苯环分别带有给电子及接受电子的取代基形成推拉结构时,则大大减弱了化合物光环化反应的发生。文章对上述结果进行了初步讨论。     

1,3-Addition of silyl enol ethers to nitrones catalyzed by mesoporous aluminosilicate

Mesoporous aluminosilicate (Al-MCM-41) was found to be an effective and reusable catalyst for 1,3-addition of silyl enol ethers to nitrones. The reaction proceeded under mild reaction conditions to afford the corresponding β-(siloxyamino)ketones in high yields. Furthermore, a unique chemoselectivity of a nitrone over an aldehyde and an acetal, which are more reactive toward silyl enol ether in the presence of Al-MCM-41 than a nitrone, was observed.     

Efficient stereoselective nucleophilic addition of pyrroles to chiral nitrones

Regioselective additions of pyrroles to a variety of optically active nitrones under smooth acidic conditions lead to chiral pyrrolic N-hydroxylamines in good to excellent yields. Depending on the position of the chirality on the nitrone partner, the addition products have been isolated with high diastereoselectivity levels. Reaction of glyoxylate based chiral nitrones either at the C-2 or at the C-3 position of the pyrrole nucleus afforded N-hydroxyamino esters in high yields as single diastereoisomers. These adducts allow access to enantio-enriched non proteinogenic 2'- and 3'-pyrrolylglycines (13 and 19 respectively).     

Formation of intermolecular hydrogen bonds in light-sensitive crystals of C-2′-(o-oxyphenyl)vinyl-N-p-methylphenyl nitrone,C-2′-(2″-oxy-5″-bromophenyl)vinyl-N-p-methylphenyl nitrone,C-2′-(2″-oxy-5″-bromophenyl)-vinyl-N-phenyl nitrone,and C-2′-(o-oxyphenyl)vinyl-n-methyl nitrone

This paper studies the crystal structure of new substituted light-sensitive azomethine N-oxides (nitrones): C-2′-(o-oxyphenyl)vinyl-N-p-methylphenyl nitrone (1), C-2′-(2″-oxy-5″-bromophenyl)vinyl-N-p-methylphenyl nitrone (2), C-2′-(2″-oxy-5″-bromophenyl)-vinyl-N-phenyl nitrone (3), and C-2′-(o-oxyphenyl)vinyl-N-methyl nitrone (4). In contrast to the compounds studied earlier [1, 2], C-2′-(β-oxy-α-naphthyl)vinyl-N-p-methylphenyl nitrone (5), C-2′-(β-oxy-α-naphthyl)vinyl-N-phenyl nitrone (6), C-2′-(o-oxyphenyl) vinyl-N-phenyl nitrone (7), and C-2′-(o-oxyphenyl)vinyl-N-p-bromophenyl nitrone (8), the nitrones studies in this work have anti-rather than syn-orientations of the nitrone and hydroxyl groups. Due to this spatial arrangement of the proton-donating hydroxyl and proton-accepting nitrone groups, molecules in crystals 1–4 are bonded by intermolecular hydrogen bonds (IHB) to form chains but not centrosymmetric dimeric associates (CDA). Two types of chain arrangements were revealed: “head-to-tail” and “head-to-tail, tail-to-head”. It is shown that the introduction of an alkyl substituent instead of an aryl one at the nitrogen atom of the nitrone group in 4 leads to a change in the geometry of the IHB in the H-associate. It is proven that the hydroxyl proton can undergo an intermolecular O→O transfer in the chain of hydrogen bonds in crystals 1–4, which can give rise to photochemical transformations in these crystals. Institute of Chemical Physics in Chernogolovka, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 37, No. 2, pp. 349–362, March–April, 1996. Translated by L. Smolina     

Alpha-(trifluoromethyl)amine derivatives via nucleophilic trifluoromethylation of nitrones

(Trifluoromethyl)trimethylsilane (TMSCF(3)) reacts with nitrones to afford alpha-(trifluoromethyl)hydroxylamines protected as O-trimethylsilyl ethers. Potassium t-butoxide initiates the nucleophilic trifluoromethylation. The reaction works best with alpha,N-diaryl nitrones, and the conditions are compatible with a range of substituents on the aryl groups. Acidic deprotection of the nitrone/TMSCF(3) adducts generates alpha-(trifluoromethyl)hydroxylamines. Catalytic hydrogenation of the adducts produces alpha-(trifluoromethyl)amines. Nitrone/TMSCF(3) adducts with strong electron-withdrawing groups on the alpha-aryl ring or heterocyclic alpha-aryl groups undergo an elimination/addition sequence to generate alpha,alpha-bis(trifluoromethyl)amines. Nitrones with alkyl groups bound directly to the 1,3-dipolar moiety fail to react with TMSCF(3), but trifluoromethylation of beta,gamma-unsaturated nitrones followed by reduction of the double bond can circumvent this limitation.     

Stereoselective vinylogous Mannich reaction of 2-trimethylsilyloxyfuran with N-gulosyl nitrones

Stereoselective vinylogous Mannich reaction of 2-trimethylsilyloxyfuran with L-gulose-derived chiral nitrones in the presence of a catalytic amount of trimethylsilyl trifluoromethanesulfonate was investigated. The selectivity was strongly influenced by the bulkiness of the C-substituent of the nitrone: for example, C-benzyloxymethyl nitrone afforded four stereoisomers, whereas bulky C-[(4S)-2,2-dimethyl-1,3-dioxolan-4-yl]nitrone gave a single stereoisomer. The latter product was elaborated to afford key synthetic intermediates for polyoxin C and dysiherbaine.     

SmI2-mediated cross-coupling of nitrones with β-silyl acrylates: synthesis of (+)-australine

The SmI(2)-mediated cross-coupling of nitrones with β-silyl-α,β-unsaturated esters, followed by zinc reduction, allows an efficient and highly diastereoselective preparation of β-silyl lactams, which are precursors of β-hydroxy lactams through Tamao-Fleming oxidation. By applying the method to a cyclic, carbohydrate-derived nitrone, a new synthesis of (+)-australine has been realized in only 11 steps and in 21% overall yield from L-xylose.