Diaryl nitrones

Badoiu A, Bemardinelh G, Mareda J, Kiindig EP, Viton F (2008) Iron- and ruthenium-Lewis acid catalyzed asymmetric 1,3-dipolar cycloaddition reactions between enals and diaryl nitrones. Chem Asian J 3 1298-1311... 

It is noteworthy that quick and effective formation of diaryl nitrones can be achieved through oxidation of diaryl imines with Oxone (potassium peroxy-monosulfate) in such media as aqueous solution of NaHCC>3 in acetonitrile or acetone. When oxidized under such conditions, dialkyl or monoaryl imines give oxaziridines (17). Oxidation of 3,4-dihydroisoquinoline (9) with Oxone initially leads to the formation of oxaziridine (10) which is easily transformed into the corresponding 3,4-dihydroisoquinoline A-oxide (11) upon treatment with catalytic amounts of p-toluenesulfonic acid (Scheme 2.4) (18). 

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). 

Nucleophilic Perfluoroalkylation of Nitrones The reaction of a,N-diaryl nitrones with (trifluoromethyl)trimethylsilane (TMSCF3) gives O-trimethylsilyl ethers of a-(trifluoromethyl)-hydroxylamines. This reaction is initiated by potassium ten -butoxide. Removal of the trimethylsilyl group on acid treatment leads to a-(trifluoromethyl)hydroxylamines, whereas catalytic hydrogenation gives a-(trifluoromethyl)amines (Scheme 2.194). 

Radical copolymerization of diaryl nitrones, such as a-(2-hydroxyphenyl)-A-(2,6-dimethylphenyl) nitrone (HDN), a-(2-hydroxy-4-methacryloyloxyphenyl)-N -(2,6-dimethylphenyl) nitrone (HMDN), and a-(2-hydroxy-4-methacryloyloxy-phenyl)-A-phenylnitrone (HMPN) (Fig. 2.30), with methyl methacrylate leads to copolymers in good yields with considerable quantities of hydroxy substituted diaryl nitrone pendants. The presence of photoactive nitrone pendants in these copolymers allows one to control photochemically the refractive index of polymethyl methacrylate films (468, 700, 701). 

In 2002, Kiindig et al. [23, 24] developed catalytic DCR between diaryl nitrones and a,(3-unsaturated aldehydes in the presence of Binop-F iron and ruthenium complexes as chiral Lewis-acid catalysts (Scheme 6). The corresponding cycloadducts were obtained in good yields with complete endo selectivity and up to 94% ee. The isoxazolidine products were obtained as a mixture of regioi-somers in molar ratios varying from 96 4 to 15 85. Experimental and computational data show that the regioselectivity correlates directly with the electronic properties of the nitrone. 

Cationic chiral Rh2 carboxamidates show catalytic activity for the DCR between diaryl nitrones and methacrolein (Scheme 11). Cycloadducts were obtained as 3,4-endo/3,5-endo mixtures with molar ratios ranging from 13/87 to 90/10. The enantioselectivity of the 3,4 isomers is higher than that of the 3,5 adducts and increases when the steric bulk of the ligand ester group increases [36]. 

Scheme 11 OCR between diaryl nitrones and methacrolein catalyzed by cationic Rh2 carbox-amidates...

Yamamoto also applied N-triflyl phosphoramide 13b to the 1,3 dipolar addition of various electron-deficient diaryl nitrones and ethyl vinyl ether (Scheme 5.33)... 

In general, a Lewis acid catalyzed 1,3 dipolar cycloaddition reaction of nitrones with vinyl ethers provides exo isomers predominantly. In fact, the MeAl BINOL catalyzed cycloaddition reaction of ethyl vinyl ether with diaryl nitrone furnished the corresponding product vhth high exo selectivity [121]. However, phosphoramide 6b as the chiral Bronsted acid catalyst exhibited high e-ndo selectivity in the same cycloaddition reaction (see Scheme 3.27). 

An addition-dehydration reaction s< C,A/-diaryl nitrone are mixed with Cui-... 

Kinugasa Reaction The aUcynes-nitrones cycloaddition, commonly known as the Kinugasa reaction, has emerged as a powerful tool for the synthesis of diverse types of P-lactams. Treatment of the chiral propargyl alcohols 45 with diaryl nitrones 46 furnished mainly the cis-fi-lactams 47 (Scheme 3.18) [51]. The... 

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). ... 

Chiral Brpnsted acid catalysis of organic reactions has become a rapidly growing area of research, as it offers operational simplicity together with mild reaction conditions. However, the first Brpnsted acid-catalyzed 1,3-DC of diaryl nitrones 17 to ethyl vinyl ether 18 was demonstrated by Yamamoto and coworkers in 2008 [16]. Only 5 mol% of chiral phosphoramide catalyst 20 was enough for this transformation. Similar to some Lewis acid-catalyzed 1,3-DC reaction, this protocol provided the endo products as the major diastereomers (Scheme 2.7). 

---