Oxaziridines solvent

Diaryloxaziridines are even less stable. With oxaziridines (66 Ar = Ph or 4-Me2NC6H4) acid amide formation at 25 °C proceeded in the course of 66 and 10 h respectively in the latter case there were equal amounts of H and aryl migration. Ethanol as solvent again accelerated the reaction, in the latter case by a factor of lO. ... 

Optically active oxaziridines are useful reagents for the enantioselective oxidation of olefins 37 39). The following three preparative methods to make this reagent available have been reported enantioselective oxidation of an imine by (-)-peroxycam-phoric acid 37,38), photocyclization of a nitrone which has a chiral substituent39), and photocyclization of a nitrone in an optically active solvent 39). However, an... 

The use of chiral solvents in this photorearrangment has been shown to promote asymmetric synthesis of oxaziridines,54 and application of the cyclization to highly substituted azoxy compounds provides a route to oxadiaziridines.55... 

Photoexcited nitrobenzene may be used for benzylic hydroxylation (at C-9) of 17/3-acetoxy-3-methoxyoestra-l,3,5(10)-triene. The photochemistry of the 17/3-nitro-steroid (217) is markedly solvent dependent, the major products being in ether the 17-desnitro-compound (218), in propan-2-ol the hydroxylamine (219), and in EtOH-NaOEt the hydroxamic acid (220) and the cyclopropane (221). The hydroxamic acid (220) is probably formed through the oxaziridine (Scheme 7). Although there are analogies to this in the photochemistry of nitrones and oximes, the photoreduction of a nitroalkane in propan-2-ol to an alkyl-hydroxylamine appears to have no precedent. Further studies of photochemistry of conjugated... 

The heteroaromatic A-oxides have been the subject of more recent investigation. The structure of the photoproduct appears to be dependent on a number of factors including the nature of the hetero-cycle and its substituents, and the solvent employed in the photolysis. The product can almost always be interpreted as arising via an intermediate unstable oxaziridine, but in no instance has this been isolated. 

Of peracids, MCPBA and (+)-MPCA are used most often. Oxidation is usually carried out in aprotic solvents, mostly in CH2CI2, CHCI3, or with phase-transfer catalysis. Periluorinated oxaziridines are prepared in acetonitrile212,213. The acid side-product, m-chlorobenzoic acid, is insoluble in the solvent and the desired oxaziridine may be prepared in good yield. MCPBA, however, is expensive, and large-scale oxidations are sometimes contaminated with bis(m-chlorobenzoyl) peroxide, which complicates product purification206. 

In all the following examples, the targeted double bonds were activated by suitable substituents to increase the efficacy of the desired cyclization mode. For the total synthesis of acutumine (26), an activated a,p-unsaturated ketone 27 was chosen as precursor (Scheme 10) [74, 75], Aryl radical additions to this type of alkenes are known to proceed about ten times faster than to comparable allylic alcohols. In a radical-polar crossover reaction, the spirocyclic product 28 was obtained in the presence of triethylaluminum as promoter and an oxaziridine as hydroxylating agent. The fact that even the efficient hydrogen donor tetrahydrofuran could be used as solvent nicely demonstrates the high efficacy of the cyclization step. 

Aldimines are oxidized to oxaziridines by tetrabutylammonium monoperoxysulfate (TBAO) and was investigated by Mohajer et al. <2004TL631>. The reaction gave the corresponding E)- or a mixture of ( )- and (Z)-oxaziridines with high yields (>90%) and good to excellent selectivities (75-100%). The ( /Z)-ratio depends critically on the stereoelectronic nature of the imine substituents, the solvent, and the presence of Lewis acids and bases. For example, oxidation of imine 333 with TBAO afforded ( )-oxaziridine 334 in quantitative yield. 

The procedure consists of the addition of peracetic acid to a solution of the imine. The reaction temperature varies between room temperature and — 78°C. The selectivity of the reaction increases with the lowering of the reaction temperature. Ether and dichloromethane have often been used as solvents in the reaction, with oxaziridine yields ranging between 35-90%. Tables of some oxaziridines prepared by this method are included in two reviews, and these will not be listed in Table 1 except where new physical constants, better yields, and/or different methods of preparation have been reported. 

Further studies of asymmetric induction in the synthesis of optically active oxaziridines via oxidation of imines with chiral acids and the degree of stereoselectivity of these reactions have been reported. It was found that the degree of stereoselectivity in the conversion of aldimines and ketimines to oxaziridines by (+)-monopercamphoric acid (MPCA) is dependent on the solvent and the reaction temperature (Tables 4 and 5). The stereoselectivity of the reaction does not seem to depend on the nature of the alkyl group attached to nitrogen. ... 

The influ nee of reaction conditions on the stereochemistry of the oxidation of optically act ve or racemic Af-diphenylmethylene a-methylbenzylamines 40 with chiral or achiral peroxy acids to oxaziridines was investigated. It was found that asymmetric induction at the ring nitrogen in the resulting oxaziridine from optically active imine 40 does not depend on the nature of the peroxy acid or the solvent. However, the diastereoselectivity seems to be dependent only on the reaction temperature. The ratio of the resulting oxaziridine diastereomers changed by 10% when the reaction temperature was raised from — 30°C to -t-40°C. On the other hand, the enantioselectivity was found to depend on the chirality of the peroxy acid, the temperature, and the solvent. For example, the optical yield of the major oxaziridine diastereomer decreased from 3.4 to 1.3 when the solvent was changed from chloroform to methanol in the oxidation of racemic 40. 

A number of optically active oxaziridines have been synthesized by the photolysis of the corresponding nitrones in (+) or (—)2,2,2-trifluoro-l-phenylethanol/ fluorotrichloromethane as solvents (Table 6). ... 

A similar study using w-chloroperbenzoic acid to oxidize iminesin chiral alcohols gave optically active oxaziridines though in low optical purity (Table 7). It appears that the optical yield is dependent on the nature of the chiral solvent, the highest yield being in chiral alcohols with aromatic substituents. Recently, greater success has been achieved. ... 

R " = H, alkyl, aryl, substituted alkyl and aryl SIRs = SIMes, SiMe2(t-Bu), SIEts solvent CHoCH. pentane, toluene n = 1-3 chiral oxidant Davis chiral oxaziridine, Shi s D-fructose derived ketone/Oxone, (Salen)manganese(lll)-complexes/NaOCI or PhIO... 

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