Asymmetric reactions oxaziridine

The vast majority of organocatalytic reactions proceeds via covalent formation of the catalyst-substrate adduct to form an activated complex. Amine-based reactions are typical examples, in which amino acids, peptides, alkaloids and synthetic nitrogen-containing molecules are used as chiral catalysts. The main body of reactions includes reactions of the so-called generalized enamine cycle and charge accelerated reactions via the formation of iminium intermediates (see Chapters 2 and 3). Also, Morita-Baylis-Hillman reactions (see Chapter 5), carbene-mediated reactions (see Chapter 9), as well as asymmetric ylide reactions including epoxidation, cyclopropanation, and aziridination (see Chapter 10), and oxidation with the in situ generation of chiral dioxirane or oxaziridine catalysts (see Chapter 12), are typical examples. 

Another substrate class, for which the outcomes of a radical and a carbocationic process are opposite, are indoles (Fig. 85) [418], Indeed, when oxaziridines 315a or 315c were treated with indoles 314c in the presence of 2 or 10 mol% of C11CI2/ TBAC oxazolidinoindolines 316c were obtained as the exclusive products in 53-90% yield. The reaction is applicable to 2-, 3-, and 2,3-disubstituted indoles. Chiral indole derivatives acylated with (S)-proline units at nitrogen underwent asymmetric diastereoselective aminohydroxylation reactions with 86-91% de. Tricyclic hemiaminals derived from tryptamine derivatives could be transformed to pyrrolidinoindolines, which are core structures of a number of alkaloids. 

Asymmetric sulfide oxidations are reported using oxaziridines other than A -sulfonyloxaziridines, but it is necessary to use a protic acid or Lewis acid to increase their reactivity. For example, -tolyl methyl sulfide 152 with bicyclic oxaziridine 153 in the presence of TFA gave the (A)-sulfoxide 154 in 50% yield and 42% ee in 24 h <1999T155>. It is interesting to note that use of MsOH resulted in much faster reaction with the oxidation complete in less than a minute. Similarly, sulfide 155 with chiral oxaziridine 156 in the presence of zinc chloride afforded sulfoxide 157 in 30% yield and 55% ee <2005JOC301>. 

Asymmetric oxidation of diazene 195 with (+)-(dichlorocamphorsulfonyl)oxaziridine 146 in EtOAc gave the corresponding A -oxide 196 in 75% yield. The reaction is highly enantioselective, affording the product in 92-95% ee <1998CC749>. 

Enantiomeric purities ranging from 20 to 80% have been reported for the acid-promoted asymmetric oxidation of sulfides to sulfoxides by binaphthyl-derived oxaziridines has been described <2007T6232>. A novel amino hydroxylation of olefins has been developed using /ra t-2-phenylsulfonyl-3-phenyloxaziridine 33 <2007JA1866>. The reaction, which is catalyzed by copper(ll) salts, affords good yields of the product. Oxidation of aldimines to oxaziridines using alumina-supported M0O3 catalyst and anhydrous /-butyl hydroperoxide (TBHP) has been described. Yields are excellent. 

Oxaziridines. Davis has developed the use of chiral 2-sulfonyloxaziridines derived from camphorsulfonic acid as chiral auxiliaries in the asymmetric oxidation reactions. Although other oxaziridines may be preferable, the camphor-derived oxaziridines can be used for the oxidation of sulfides and disulfides to sulfoxides and thiosulfinates as well as for the epoxidation of alkenes. On the other hand, the camphoryloxaziridines are the preferred reagents for hydroxylation of lithium enolates of esters, amides, and ketones, as utilized in the synthesis of kjellmanianone (eq 17). ... 

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. 

Related reactions Davis oxaziridine oxidation, Prilezhaev reaction, Sharpless asymmetric epoxidation, Shi asymmetric epoxidation ... 

AE reactions of simple olefins. The Sharpless AE reaction has been supplemented by other approaches to asymmetric epoxide synthesis the most evident goal being to obviate the need for an allylic alcohol. Attempts to carry out asymmetric epoxidation reactions on simple olefins have utilized transition-metal-containing catalysts such as porphyrins as well as stoichiometric chiral reagents (peroxides, dioxiranes, and oxaziridines). These approaches have been summarized [19]. 

The utility of oxaziridines in asymmetric a-hydroxylation also extends to reactions with achiral enolates. This has been made possible by the discovery that certain chiral A -sulfonyl oxaziridines can react with enolates to afford a-hydroxy carbon compounds in excellent yield and enantioselectivity. An application of a highly selective sulfonyloxaziridine derived from camphor to the synthesis of daunomycin is shown in Scheme 8.23. Attack of the oxaziridine presumably occurs such that the enolate ester avoids nonbonded interactions with the exo methoxy group on the bicyclic ring system (cf. Schemes 8.23c and d). This is a very useful reaction of wide scope, and can be carried out on both stabilized enolates derived from keto esters (shown) and simple ketone enolates [99]. 

Asymmetric hydroxylation of etiolates. Davis and Chen1 have reviewed this reaction using in particular (R,R)- and (S,S)-2-phenylsulfonyl)-3-phcnyloxaziridene (1) and (camphorylsulfonyl)oxaziridine (2). Of these reagents, 1 and ( + )- and (—)-2, derived from (lR)-lO-camphorsulfonic acid, provide highest enantioselectivity and in addition are easy to prepare. They are effective for hydroxylatation of ketones, esters, /2-keto esters, amides, lactones, and lactams. 

Sulfenimines undergo asymmetric oxidations to form sulfinimines via a reaction with Davis reagents. The sulfenimine 77 was oxidized by 9 to yield the sulfmimine 78 in 82% yield and 97% ee. Yields and enantiomeric excess varied based on the oxaziridine reagent used.64... 

---