Azirines special

H-Azirine-2-carboxylates such as 57 (Scheme 3.19) are a special class of imines that undergo additions to their C=N double bonds to give aziridine-2-carboxylates... 

The regiochemistry of this elimination reaction resembles that observed by Davis et al. (see Scheme 9) [23]. The special nature of the bonds in three-mem-bered rings is probably responsible for this exclusive regiochemistry. It is of interest to note that 3,3-dimethylaziridine-2-carboxylic ester indeed leads to the corresponding 3H-azirine ester upon Swern oxidation here there is, of course, no choice. 

The normal reactivity of nitrile ylides is determined by their ability to undergo 1,3-dipolar cycloadditions.Special nitrile ylides possessing a suitable olefinic double bond react as alkylideneaminocarbenes with cyclopropanation of the double bond. " ° Hence, the nitrile ylide 3, generated either by elimination of hydrogen chloride from benzoyl chloride imides 1, or photochemically from a 2H-azirine 2, reacts via intramolecular [2 -I-1 ] or [2 +- 3] cycloaddition to the incorporated double bond. Intermolecular [2-f 1] cycloadditions of such nitrile ylides have not been observed. 

Monoalkylhydrazines can also be prepared by hydrolysis of diaziridines that are obtained from diazirines and Grignard reagents.287 Pentamethylenedi-azirine (2,3-diazaspiro[2.5]oct-2-ene)288 is a particularly suitable starting material for this synthetic route, which is of special value when the alkyl halide is more accessible than the amine. 

Azirine/oxazolone method, a synthetic method for the introduction of sterically highly hindered a,a-disubstituted a-amino acids into peptides. The synthesis of e.g., Aib-rich peptides requires either highly reactive coupling reagents (e.g., amino acid halides) or special derivatives (such as 3-amino-2H-azirines or a-azido carboxylic acid chlorides). The azirine/oxazolone method utilizes an amino component that... 

Perhaps the most widespread variation of the Neber rearrangement is its utility in preparing 2//-azirines. The first reported isolation of such a product (18), albeit in a special case, was reported by Neber in his seminal studies of this reaction, Since then, numerous reports have appeared which prepare these moieties in good yields under predictably selective conditions, usually... 

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