Oxaziridines oxidative cleavage

Direct conversion of diol 395 to cytisine 31a was accomplished by a one-pot procedure. Oxidative cleavage to give the dicarboxylic acid, followed by treatment with aqueous ammonia and catalytic reduction provided racemic cytisine 31a in good yield <20000L4205>. Photolysis in benzene at 254 nm of the nitrone 396 afforded smooth rearrangement to 17-oxosparteine 195 likely through the intermediacy of an oxaziridine derivative (Scheme 80) <2002OL2577>. 

Direct proof of an oxaziridine intermediate was achieved in photolysis experiments in an organic glass at 77 K (80JA5643). Oxaziridine (75), formed by photolysis of A/-oxide (74) and evidenced by UV spectroscopy under the above conditions, decomposed at higher temperature to form the imino ether (76) by N—O bond cleavage and C -> O migration of an aryl group. 

The principal product of prolonged irradiation of the oxaziridine (175), formed from 3,5,5-trimethyl-l-pyrroline 1-oxide, is 3,5,5-trimethyl-2-pyrrolidone (176). In this instance, the 1,2-shift of a hydrogen atom, possible in pyrrolidines not containing an alkyl substituent on C-2, is preferable to pyrrolidine ring cleavage. A similar effect is observed in the photolysis of 5,5-dimethyl-l-pyrroline 1-oxide.147... 

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. 

Diarylnitrone (31) formation from N-substituted, diaromatic imines has been recognized to require the presence of NADPH/O2, and has been proposed to proceed via the intermediacy of an oxaziridine3 possibly arising from reaction of the parent imines with the putative P-450 [FeO]3+ species in analogy to the oxidation of olefins118. Ring cleavage of the oxaziridine then yields nitrone or amide (equation 10). 

Acetalization of thiochroman-3-one gives a 1 1 diastereomeric mixture and subsequent oxidation with Davis reagent, W-(phenylsulfonyl)(3,3-dichlorocamphoryl)oxaziridine, yielded the sulfoxides each with a 4 1 enantioselectivity. Chiral chromatographic separation of the diastereomers preceded isolation of the major enantiomers. (3-Elimination and isomerization of the double bond then produced the individual thiochromene 1-oxide diastereomers. The generation of an a-sulfinyl carbanion effects the cleavage of one of the acetal C-O bonds with the protected diol released in a final ozonolysis step. The stereochemical results indicate that it is the C-O bond syn to the sulfoxide function that is cleaved (Scheme 63) <1996TA29>. 

Acylations of lithium enolates of JV-acyloxazolidinones 5.30 and 5.31 by acetyl or benzoyl chloride at -78°C are highly diastereoselective (de > 90%) [167]. Such is also the case for reactions of Samp or Ramp 1.76 hydrazone enolates with CICOOMe [1077] or MeNCS [1078], Sodium enolates of iV-acyloxazolidinones 5.30 and 5.31 can be oxidized on the least hindered face at -78°C by an oxaziridine [741], After cleavage of the chiral auxiliary by magnesium methoxide, a-hydro-xyesters are obtained with an excellent enantiomeric excess [742] (Figure 5.39). Similar results are obtained from Samp and Ramp 1.76 hydrazone enolates [742] (Figure 539). Oppolzer and coworkers [147] carried out the stereoselective... 

Enantiomerically pure aldehydes 287 were obtained by DIBAL reduction of the corresponding o-alanine methyl esters. Coupling with t-butyl glycinate quantitatively gave the ( )-imines 288 (Scheme 85). Peracid oxidation did not allow the isolation of oxaziridines 289 which spontaneously decomposed to give complex mixtures of products [156]. This was probably due to a deprotonation a to the ester group accompanied by cleavage of the weak N-O bond. Imines 290, derived from aldehydes 287 and P-alanine esters, could be indeed oxidised into stable oxaziridines 291 which were isolated as mixtures of two dia-stereoisomers (Scheme 86) [157]. 

In the same year Widmer and KELLER-ScfflERLEiN 194) used an oxaziridine for the synthesis of N -hydroxyarginine (224) (Scheme 48). However, in their synthesis m-chloroperbenzoic acid was used for oxidation of Schiff base and Dowex 50 X2 for cleavage of the oxaziridine. The resulting derivative of N -hydroxyornithine (245) was transformed... 

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