Oxaziridines review

Hydroxylamines may be acyclic (1) with one or two substituents or cyclic (4). Cyclic compounds 5 where both the nitrogen and the oxygen atoms are ring members like oxaziridines (6), 1,2-oxazetidines (7), tetrahydro-l,2-oxazoles (8) (usually named isoxa-zolidines) and tetrahydro-l,2-oxazines (9) will not be considered in this chapter. Each of these compounds is regarded as a compound class on their own and their chemistry has usually been reviewed individually. 

MBoyd Tetrahedron Lett. 1968, 4561 Boyd Spratt Jenna J. Chem. Soc. C1969, 2650 Montanari Moretti Torre Chem. Common. 1968, 1694.1969, 1086 Bucciarelli Forni Moretti Torre Prosyanik Kostyanovsky/. Chem. Soc., Chem. Commun. 1985, 998 Bucciarelli Forni Moretti Torre Bruckner Malpczzi J. Chem. Soc., Perkin Tram. 2 1988, 1595. Sec also Mannschreck Linss Seitz Liebigs Ann. Chem. 1969, 727, 224 Forni Moretti Torre Bruckner Malpczzi Di Silveslro J. Chem. Soc.. Perkin Trans. 2 1984, 791. For a review of oxaziridines. see Schmitz Adv. Heterocycl. Chem. 1979,24, 63-107. 

Applications of oxaziridine rearrangements in asymmetric syntheses have been reviewed,596 and the formation of A,A-disubstituted formamides (462) on sodium perborate oxidation of alkyl A-arylaldimines (460) has been rationalized597 in terms of an intermediate oxaziridine (461) that rearranges via acid-catalysed O—N cleavage. 

The research work of recent years includes predominantly the epoxidation of alkenes9,200, asymmetric hydroxylations209,224-228 and the asymmetric oxidation of sulfides to sulfoxides205,209,229,230. Optical yields of practical significance were obtained (>90%). A detailed review published in 1991231 reports about the versatile use of oxaziridines in the field of the electrophilic amination. 

A review of nonenzymatic asymmetric epoxidations covering the literature through 1983 has been published elsewhere. Improved enantioselectivity (to as high as 64% ee) for epoxidations of schiral oxaziridines has been described and results are included in a review of synthetic tq>pli-cations of oxaziridines. A summary of catalytic asymmetric epoxidations of alkenes is present in Table 12, together with brief comments on each method. 

Vidal et al. have recently reviewed the use of A -alkyloxycarbonyl-3-aryloxaziridines as electrophilic aminating reagents <1997CEJ1691>. These reactions are believed to take place by attack of the nucleophile at the oxaziridine nitrogen atom. 

Epoxidation of alkenes with oxaziridines gives the corresponding epoxides in a rjiw-stereospecific manner and has been the subject of several reviews <1996CHEC-II(1A)365, 1999RCR183>. 

Examples of photorearrangement in nitrones and heteroaromatic N-oxides have again been described, and the subject has been critically reviewed. The hydroxyoxaziridines (81) have been prepared by irradiation of the corresponding 1-pyrroline 1-oxides (82), ° whereas photorearrangement of the dinitrone (83) gave the oxaziridines (84) and (85) and the lactam (86) oxaziridines (84), (85), and (86) were converted into the piperazine-2,5-dione (87) by further irradiation. ... 

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. 

The photolysis of nitrones has been used as a method of preparing oxaziridines. A table of fairly stable oxaziridines prepared from the photolysis of the corresponding nitrones has been included in an extensive review. The photochemical method, though limited in its scope, often serves as an alternative to the peroxidation of imines. The synthetic usefulness of this method is demonstrated by the conversion of nitrones 7 and 9 into oxaziridines 8 and 10 in yields of 78% and 74%, respectively. (Apparently, the phototoxicity of benzodiazepine A(-oxides, e.g., 7, 9, is due to this isomerization. ) Similarly nitrones 11a, h, c yielded oxaziridines 12a, b, c (25-30%). ... 

Secondary amines reacted to form hydrazines in yields of 90% and better. Aniline can be converted to phenylhydrazine. Sometimes the addition was followed by the elimination of water rather than the aminated nucleophile. For example, treatment of 3,3-pentamethyleneoxaziridine with methoxide ion produced cyclohexanone oxime (9-methyl ether. Formanilide reacted with 3-phenyloxaziridine in the presence of sodium ethoxide to produce benzaldehyde phenyUiydrazone. " Of considerable interest is the reaction of Schiff bases with these oxaziridines to produce diaziridines. Other examples are given in Schmitz recent review. One intramolecular version of this kind of amination is known oxaziridine 57 was converted to benzophenone and 5,6-dihydro-4H-l, 2-thiazine 58 upon heating. ... 

There are a number of thermal and photochemical reactions for which oxaziridine intermediates have been proposed but never isolated. These include, among others, the photochemical Beckmann rearrangement of oximes, many photochemical reactions of aromatic A -oxides, and the thermal rearrangement of nitrones to amides. A brief discussion of the first two seems warranted in this review because they have been studied extensively and some strong inferential evidence for oxaziridine intermediates has been obtained. 

All attempts to supply physical evidence for the formation of oxaziridines as intermediates in the photolysis of aromatic A -oxides have met with scanty success, so investigators have relied on indirect evidence. Theoretical calculations seem to agree that, in aromatic A -oxides, oxaziridines do not arise from the ground state but very likely from the first excited singlet state. A comprehensive critical review of this subject has appeared. ... 

The formal addition of an oxygen atom across the carbonyl group gives rise to dioxiranes (equation 33). In practice, this reaction is effected with Oxone, and dimethyldioxirane (30) and other dioxiranes have been generated in solutions of their parent ketones.Dioxirane (30) has been implicated in oxidations of alkenes, sulfides and iinines. The formal addition of nitrogen across a carbon-oxygen double bond to afford oxaziridines has been reviewed (equation 34).There are also many methods available for the indirect conversion of carbonyl compounds to aziridines > and thiiranes using multi-step conversions. 

The addition of oxygen across the C—N double bond is a common and synthetically useful process. The product oxaziridines are of interest for theoretical reasons (largely due to the high configurational stability of the chiral nitrogen), as reagents (such as oxygen transfer moieties), and as synthetic intermediates. The stereochemical aspects of oxaziridine synthesis and reactivity have been reviewed. ... 

Compounds containing bivalent sulfur react rapidly with N—H and iV-methyl oxaziridines to form sulfenamides (R1—S—NH—R2), the stability of which depends on the reactivity of the substrate and the product. Ring closure and sulfenimine (R—S—N=CR2) formation via intermediate (52), are frequently observed products with thioamides and thioureas. Sulfinates (ArS02 Na+) are aminated to sulfonamides in excellent yield. These animations, explored in detail by Schmitz and co-workers with spirooxaziridine (51), have been reviewed <91S327> and only a few illustrative examples are given in Table 4. 

The synthesis of oxaziridines has been the subject of several comprehensive reviews and therefore is only briefly discussed here <84CHEC-I(7)195, 85MI 112-01, 89T5703,91S327,92CRV919). 

The oxidation of chiral imines with peracids and the oxidation of achiral imines with chiral peracids to give optically active A-alkyl oxaziridines has been reviewed <84MI 112-01 >. The resolution of chiral A-alkyloxaziridine-3,3-dicarboxylic esters through the enzymatic hydrolysis of the racemic diesters, in moderate ees, has been reported <88CC1614>. 

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