27/-Azirine-3-carboxylates cycloaddition

The 3//-azepines obtained by cycloaddition of azirines to cyclopentadienones (see Section 3.1.1.1.2.) are thought to arise from the initially formed 2/7-azepines by [1,5]-H suprafacial sigmatropic shifts.31-108 In contrast, 1/Z-azepine 9 results from the thermal rearrangement of the nonisolable 2//-azepine-2-carboxylate 8.13 Presumably, the 1 //-azepine is stabilized, relative to the 3//-isomer, by intramolecular hydrogen bonding between the NH and the adjacent ester group. 

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. 

Aziridines can also be synthesized from their unsaturated azirine counterparts or existing aziridine rings. The first examples of enantiopure 2-substituted 2Z/-azirine 3-carboxylates 46 were prepared via dehydrochlorination of methyl 2-chloroaziridine-2-carboxylates. Bicyclic and tricyclic aziridines such as 47 were then generated via an aza-Diels-Alder reaction with the corresponding dienes <070L1707>. A related azomethine cycloaddition in the presence of an aziridine has also been reported <07JOC8506>. 

The reaction of 27/-azirine-3-carboxylates unsubstituted at C2 with diazomethane occurs to produce a 4,5-dihydro-3/f-pyrazole derivative 185. This reaction represents an interesting example of the imino group acting as a 27t-component in a 1,3-dipolar cycloaddition reaction <03TL6313>. The process seemingly involves the reaction of 2//-azirine 182 with... 

Scheme 6.33 Cycloaddition of benzyl 2H-azirine-3-carboxylate to furan ° ...

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