N-Aryl nitrones

Ein analoger Reaktionsweg wird bei der Cyclisierung des N-Aryl-nitrons IV mit Bromcyan zu I-(Ethoxycarbonyl-imino-methyl)-2-hydroxy-benzimidazol(66% Schmp. 127-132°) beobachtet591 ... 

N-Aryl nitrones were generated by visible light-mediated oxidation of N-substituted Af-aryl hydroxylamines e.g. 89 in the presence of a catalytic amount of an Ir(III) polypyridyl complex and the resulting 90 trapped in situ with suitable dipolarophiles. For example, isoxazolidine 91 was synthesized in high yield and good cf5-stereoselectivity in the presence of an excess of ethyl vinyl ether (140L2872). 

A number of a-aryl-A-alkyl nitrones and contrast enhancement compositions, which can be used to make contrast enhancement layer photoresist composites (230, 231), and inhibitors of free radical polymerization of monomers in nonexposed regions of the photoresist layer at selective actinic radiation (232). Histidine was used as a catalyst in the synthesis of a, A-diaryl nitrones in situ (233). To study diphenylborate chelates with mono- and bidentate ligands, a series of hydroxyl-containing nitrones have been synthesized (Fig. 2.7) (234-237). 

Imidazole synthesis. A new synthesis of 4,5-diarylimidazoles (2) involves reaction of catalytic amounts of aqueous elhanolic K.CN with N-methyl-C-aryl nitrones (1), prepared by condensation of aryl aldehydes with N-methyl-hydroxylamine. The reaction involves an intermediate cyanoimine (n). 

A method was proposed [196] for the production of 4-phosphazenyl-2,3-dihydropyrrol-2-ones and 5-phosphazenyl-2-pyridones from p-(N-acylphosphazenyl)enamines of dimethyl acetylenedicarboxylate. The authors of [197] examined the chemoselective cycloaddition of C-aryl-N-phenyl nitrones at the 1,2 double bond of allenylphosphonates, leading to a mixture of adducts possessing pyrrolidine and isoxazolidine structures. 

Ambident reactivity was shown by oximate anions normally, Oarylation predominated over N-arylation, with ratios of oxime ethers to nitrones ranging from 9 1 (for the benzophenone oxime anion) to 1.7 1 (for the fluorenone oxime anion) [77]. The arylation of two heterocyclic oximes was performed under mild conditions and led mainly to the corresponding oxime ethers which served as good precursors for the generation of unstable aryl fulminates, ArONC [79,80],... 

Similar reactions have been carried out with variously substituted pyrroline 1-oxides, imidazole 1-oxides, isoxazoline N-oxides (nitronic esters) and 3,4-diazacyclopentadienone AT-oxides in combination with a large variety of alkenic and alkynic dipolarophiles, aryl isocyanates, aryl isothiocyanates and N- sulfinylamines, leading to pyrrolidinoisoxazoles, pyrrolo[l,2-6][l,2,4]oxadiazoles, pyrrolo[2,l-d][l,2,3,5]oxathiadiazoles, isoxazolo[2,3-b ]isoxazoles and isoxazolo[l, 2-6 ]pyrazoles. 

Pyrroloindoles/ A simple route to this system, which is present as the corresponding quinone in mitomycins, can be obtained by reaction of N-aryl-hydroxylamines (1) with ethyl 6-oxo-2-hexynoate (2) to form a nitrone that is reduced without isolation with NaBHsCN (1.2 equiv.). 

Azinemonoxides are cleaved by two distinct photochemical pathways. One involves oxygen migration and the other involves a pericyclic ring closure. 4) Both processes are represented in the mass spectra of the compounds. Similar rearrangements occur in both the photochemistry and mass spectrometry of aryl nitrones, aryl N-oxides and aromatic azoxybenzenes. )... 

An addition to the earlier cycloadditions is the self condensation of an N-methyl-C-aryl nitrone (18) in the presence of potassiunt cyanide (Scheme 4.2.8) [22, 23[. The reactions are carried out in mild conditions (room temperature overnight in aqueous ethanol), but although the yields are good there arc no obvious advantages over other methods which are available for making 4,5-diarylimidazoles. Oie nitrones (18) are, however, readily available from Af-aryl- or iV-alkylhydroxylamines and aryl aldehydes [24], and so there may be occasions when the approach could be useful. 

A possibility for azomethine ylide generation from 4-isoxazolines was first suggested by Baldwin, who demonstrated a thermal N—O bond cleavage of 4-isoxazoline systems (68JA5325). Cycloaddition of N-methylene(t-butyl)-amine V-oxide to dimethyl acetylenedicarboxylate takes place at 0°C, rapidly and quantitatively, to give 4-isoxazoline 146 (R = t-Bu, =R = COOMe), which isomerizes at 80°C into 4-oxazoline 148 (R = f-Bu, R = R = COOMe). The nitrone bearing an N-aryl substituent and the same acetylene directly afford 2-acylaziridine 147 (R = 2,4,6-trimethylphenyl, R = R = COOMe). Reaction of the N-t-butylnitrone with 3-methylbutyn-... 

MacMillan s imidazolidinone salts, used successfully in the organocatalysed Diels-Alder reaction (see Section 8.1) also function as effective catalysts in the asymmetric nitrone cycloaddition with simple monodentate dipolarophiles. Thus acrolein (8.63) and crotonaldehyde (8.99) both react with acyclic C-aryl, N-benzyl nitrones and C-aryl N-alkyl nitrones such as (8.198) with high ees ranging from 90 to 99% in the presence of the perchlorate salt of imidazolidinone (8.91). 

Other Electrophiles. In addition to carbonyl compounds, ester enolate (1) also reacts with other electrophiles. With nitrones, the product is dependent upon the structure of the nitrone a,N-dialkyl nitrones provide alkenes, while a-aryl-A-alkyl nitrones or aW-diaryl nitrones usually give aziridines. With the phenylhy-drazone of a 1,2-dicarbonyl compound, reaction with (1) provides a convenient preparation of 3(2f/)-P3 dazinones (eq 3). ... 

A series of 3-substituted-2-isoxazoles are prepared by the following simple procedure in situ conversion of nitroalkane to the silyl nitronate is followed by 1,3-dipolar cycloaddition to produce the adduct, which undergoes thermal elimination during distillation to furnish the isoxazole (Eq. 8.74). 5 Isoxazoles are useful synthetic intermediates (discussed in the chapter on nitrile oxides Section 8.2.2). Furthermore, the nucleophilic addition to the C=N bond leads to new heterocyclic systems. For example, the addition of diallyl zinc to 5-aryl-4,5-dihydroi-soxazole occurs with high diastereoselectivity (Eq. 8.75).126 Numerous synthetic applications of 1,3-dipolar cycloaddition of nitronates are summarized in work by Torssell and coworker.63a... 

Z- Configuration is typical of the majority of a-aryl(hetaryl)-/V-alkylaldo-nitrones. The isolation of -isomers in the condensation of aromatic aldehydes with iV-j3-]ihenyletli Tliydroxylamine has been described (155). The synthesis of a, N -diary lnitrones gives best results if acidic catalysis is employed (156), or when clay is used as a catalyst (157). Significant reduction of reaction time and increase in the yields of nitrones can be achieved if microwave irradiation is used (158, 159). On the basis of polymeric arylaldehydes, the synthesis of polymeric a,-diarylnitrones has been described (160). 

Cycloaddition of a-aryl-A-phenylnitrones to the C16-C17 n-bond in 16-dehydropregnenolone-3P-acetate (545) involves only the minor rotamer (A-form) of the nitrones. It proceeds regio-, stereo- and Jt-facial-selectively to give steroido[16,17-d]isoxazolidines (546) in high yield (Scheme 2.257), (Table 2.24) (760). Similarly the cycloaddition of a,N -diphenylnitrones proceeds with five-membered heterocyclic enones (761). 

Nitrones of several aryl methyl ketones were hydrogenated by a catalyst prepared in situ from [IrCl(COD)]2, (S)-BINAP, and (n-C4H9)4NBH4 in THF under 80 atm of H2 and at 0 °C to give the corresponding Af-hydroxylamines in up to 86% ee (Scheme 6) [14]. Substitution of halides at the 3 or 4 position of the ar-... 

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