2//-Azirines reaction with carboxylic acids

Eremeev, A. V., Fiskunova, I. F., El kinson, R. S. Synthesis of 2-amino-1-azirines and their reactions with carboxylic acids. Khim. 

Intramolecular addition of trialkylboranes to imines and related compounds have been reported and the main results are part of review articles [94, 95]. Addition of ethyl radicals generated from Et3B to aldimines affords the desired addition product in fair to good yield but low diaster control (Scheme 40, Eq. 40a) [96]. Similar reactions with aldoxime ethers [97], aldehyde hydrazones [97], and N-sulfonylaldimines [98] are reported. Radical addition to ketimines has been recently reported (Eq. 40b) [99]. Addition of triethylborane to 2H-azirine-3-carboxylate derivatives is reported [100]. Very recently, Somfai has extended this reaction to the addition of different alkyl radicals generated from trialkylboranes to a chiral ester of 2ff-azirine-3-carboxylate under Lewis acid activation with CuCl (Eq. 40c) [101]. 

Sato et al. observed that the reaction of 2-phenyl-1-azirine with acid chlorides and anhydrides in the presence of triethylamine gives the oxazole directly.57 They have reported that azirines also react with carboxylic anhydrides to give oxazoles (136).57 Aziridine (135) is suggested as a likely intermediate in the reaction of acetic anhydride with azirine (42a), since carrying out the reaction at lower temperature and for a shorter reaction time gave a compound to which they assigned structure 135. 

Activation of 3-alkyl and 3-phenyl-2H-aziiines by Lewis acids also promotes their participation in hetero Diels-Alder reactions with a variety of dienes. This methodology circumvents the previous requirement of needing an electron-withdrawing carboxyl moiety at the 3-position of the 2H-azirine ring (01TL9289). Thus, the reaction of 2H-azirine 125 with Danishefsky s diene gave the endo-cycloadduct 126... 

The e.e. for the stoichiometric reaction was 60% in SCCO2 (200 bar with 7 vol% toluene cosolvent) and 70% in liquid toluene (126 Brown and Jessop, unpublished data, 2002). Without toluene cosolvent, the reaction had poor enantioselectivity. Several azirine carboxylic acids and esters related to the product are antibiotics (126). 

Lewis acid-mediated asymmetric Diels-Alder reactions between 2H-azirines 59, bearing chiral auxiliaries, with enophiles such as 60 afforded mixtures of bicyclic aziridine-2-carboxylates 61 (Scheme 3.20) [68]. 8-Phenylmenthol appeared to be the auxiliary of choice in this reaction in terms of yield and diastereoselectivity. 

There has been some investigation of auxiliary-controlled cycloadditions of azir-ines. Thus, camphor-derived azirine esters undergo cycloaddition with dienes, with poor diastereoselectivity [70]. The same azirines were also observed to react unselectively with phenylmagnesium bromide. Better selectivities were obtained when Lewis acids were used in the corresponding cycloaddition reactions of 8-phe-nylmenthyl esters of azirine 2-carboxylates (Scheme 4.48) [71]. The same report also describes the use of asymmetric Lewis acids in similar cycloadditions, but mediocre ees were observed. 

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