Chiral oxaziridines, reactions

En gros, the N insertion reactions can be subdivided into a Beckmann type and a Schmidt-type rearrangement part. Furthermore, some photochemical rearrangements of chiral oxaziridines are known to generate a range of optically active lactams. 

Pentamethyleneoxaziridine (6) also acts as an aminating agent, giving quite good yield of aziridines (Scheme 2). > Remarkably, none of the corresponding epoxide was reported in this reaction. Cyclohexene yields instead the hydrazino alcohol (8), probably via the V-aminoaziridine (7). In view of the recent use of chiral oxaziridines for chiral oxidations, it will be interesting to see whether chiral aziridination is possible when chiral V-unsubstituted oxaziridines become available. ... 

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>. 

Scheme 8.23. (a) Application of enolate oxidation reactions of a chiral oxaziridine to the synthesis of an AB ring synthon of daunomycin [100]. ffc) Structure of the oxaziridine used. Proposed (c) favored and (d) disfavored transition structures (see also ref. [101]). 

Enolates are hydroxylated by fluorooxaziridines rapidly under very mild conditions, as exemplified by the reaction of the Z-enolate 303 of propiophenone with the chiral oxaziridine 299. Hydroxyketone 304 was obtained in 94% ee and 71 % yield. ... 

A -Arylimincs (21a) can be oxidatively rearranged to fonnamides (21b) with sodium perborate.42 The reaction works best for secondary or aryl R groups. An oxaziridine intermediate is proposed. Results with chiral secondary R groups indicate epimeriza-tion, suggested to occur via equilibration of (21a) with its enamine tautomer. 

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. 

Oxidation of sulfur-containing azoles quite often leads to the formation of sulfones and sulfoxides. Thus, 3-alkyl-1,2-benzisothiazoles 142 with MCPBA give the oxaziridines 143, and the use of a chiral 3-alkyl substituent leads to pure diastereomers. Reaction of 1,2,3-benzothiadiazole with 30% hydrogen peroxide in a mixture of acetic acid and methanol for 45 days affords 144 in 60% yield. 

Enders and coworicers have shown that deprotonation of chiral SAMP/RAMP hydrazones (or their substituted analogs) derived from ketones or aldehydes, followed by reaction with Davis oxaziridine reagent provides the a-hydroxy hydrazones in moderate yield but with high diastereoselectivity. Direct unmasking or protection followed by unmasking provides the corresponding a-hydroxy ketones or aldehydes respectively (Scheme 24). Both antipodes of the hydroxylated compounds are available by appropriate choice of (5)- or (R)-proline-deiived auxiliaries. The direction of induction is predictable, if not wholly uniform (R substitution alters the a-stereochemistry for aldehyde hydrazones). The process clearly provides a valuable approach to both systems. 

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