Oxaziridines, absolute configuration

For oxaziridines the N-inversion barrier is considerably higher than that for similar aziridines. Af-Alkyl-3,3-dialkyloxaziridines are resolvable and absolute configurations have been determined (Section 5.08.2.3.1). 

X-ray analysis of an optically active oxaziridine substituted at nitrogen with the 1-phenylethyl group of known configuration led to the absolute configuration (+)-(2R,3R)-2-(5-l-phenylethyl)-3-(p-bromophenyl)oxaziridine of the dextrorotatory compound as expected, C-aryl and A-alkyl groups were trans to each other (79MI50800). 

Nitrogen chirality may also be produced by the action of an achiral peroxyacid on a Schiff base containing a chiral amine (75JOC3878). In this case the oxaziridine contains a configurationally known centre of chirality relative to this, absolute configurations of the centres of chirality at nitrogen and carbon, and thus the complete absolute configuration of the molecule, can be determined (see Section 5.08.2.2). 

Fluoroalcohols 1 have been employed in assigning absolute configuration of amines (30), benzothiophene oxides (45), lactones (33,46), oxaziridines (41), and an amine oxide (47). 

There are many examples of the asymmetric and diastereoselective hydroxylation of metal enolates using the enantiomerically pure camphor-derived A -sulfonyloxaziridines (- -)-146 and (+)-202, commercially available in both enantiomerically pure forms <20030R1>. The enantiomeric purity of the a-hydroxy products is good to excellent and can be obtained in both enantiomeric forms, as the absolute configuration of the oxaziridine controls the stereoinduction. Examples are given in Table 21. 

More definitive studies were carried out of analogous reactions including x-ray analysis and, therefore, the absolute configurations of the new chiral centers in the resulting oxaziridines 30-33 were determined. 

X-ray analysis showed that oxaziridine 30 is (2R,3R)-2-(S)-l-phenylethyl-3-p-bromophenyloxaziridine. Furthermore, thermal isomerization at 120°C converted oxaziridine 33 into 30 and oxaziridine 32 into 31. Hence the absolute configurations of the chiral centers in 30-33 were established." Moreover, a rule analogous to Freudenberg s rule of shift was proposed that relates the value of the optical rotation of oxaziridines to the absolute configuration, and therefore predicts the latter in a number of oxaziridines. 

Triethoxy-l,2A5-oxaphospholene (112) reacts at room temperature with 2-(p-tolyl)-3-phenyl-oxaziridine (63d) to give a /Miydroxy-y-ketophosphate (113) in 82% yield (Equation (26)) (91TL5313). It seems likely that an oxirane intermediate is initially formed which rearranges on work-up to (113). Asymmetric oxidation with (+ )-[(8,8-dichlorocamphoryl)sulfonyl]oxaziridine (74) gave (113) in 49% ee and 88% isolated yield. The absolute configuration of (113) was not established. 

Modest to low ees are reported for the oxaziridine mediate asymmetric hydroxylation of 4,5-dihydroisoxazoles (177) to 4,5-dihydro-4-isoxazolols (178) (Equation (42)) <93JOC759i>. Dichloro oxaziridine (+)-(74) and the lithium enolate, generated in the presence of TMEDA, gave the best results. Transition state TS-2 predicts the absolute configuration of the product which was confirmed by chemical correlation. 

The absolute configuration of an oxaziridine made from p-bromobenzyl-idene-a-S-phenylethylamine has been determined by X-ray analysis." ... 

We have recently reported that [(8,8-dimethoxycamphoryl)sulfonyl]oxaziridine is a particularly effective reagent for asymmetric sulfide oxidation, especially in non-aryl sulfide substrates.11 Here we report a three-step chemical synthesis of 1,3-dithiane 1-oxide with very high ee that is based upon such an oxidation as the key step. The procedure is effective for production of multigram quantities of material of either absolute configuration. The sequence is illustrated for the preparation of 1S-(-)-1,3-dithiane 1-oxide. 

Davis et al. used a chiral oxaziridine for the asymmetric oxidation of sulfides to sulfoxides.Oxidation of isopropyl- -tolyl sulfide (427) with oxaziridine 428, for example, gave 60.3% ee (S) of 429 (at -78°C in chloroform).5 0 The absolute configuration of the sulfide is determined by approach of the sulfide to the oxaziridine oxygen, as illustrated by 430. Steric factors appear to be the primarily reason for the chiral recognition. 5 1 In this model, attack by sulfur minimizes the Rl and Rs interactions with the oxaziridine... 

The photochemical synthesis of optically active oxaziridines from prochiral nitrones was driven by an interest in examining the configurational stability of the oxaziridin nitrogen atom. Nitrone 7 was irradiated at — 78°C in a 1 1 mixture of ( + )- or (— )-2,2,2-trifluorophenylethanol (8) and fhiorotrichloromethane yielding nitrone 9lent-9 in 30% ee (absolute stereochemistry unknown) [17,18]. At room temperature this selectivity decreased to 5% ee. Less bulky residues also reduced the selectivity. An exchange of f-butyl by /-propyl led to 20% ee at — 78°C (Scheme 5). 

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