Teresa M. V. D. Pinho e Melo and Antonio M. d'A. Rocha Gonsalves Pages 275 - 292 ( 18 )
2H-Azirines are the smallest unsaturated heterocyclic compounds containing nitrogen. Despite their strain many stable azirines are known. Due to the high reactivity 2H-azirines have been explored extensively for various synthetic purposes. The ring system occurs naturally in the antibiotics azirinomycin, isolated from Streptomyces auras cultures. Moreover, both enantiomers of (R)-(+)- and (S)-(-)-dysidazirine and (S)-(+)- antazirine were isolated from the marine sponge Dysidea fragilis. The chemistry of 2H-azirines is dominated by ring strain and processes in which the strain is relieved are very favoured. 2H-Azirines undergo reactions in which they can function either as a nucleophile or as an electrophile. They can participate in cycloadditions as 2π-components and undergo ring cleavage on photochemical excitation to give nitrile ylides which can act as 4π-component in dipolar cycloadditions. Thermal ring cleavage originates vinyl nitrenes which allows for ring expansion reactions. X-Ray crystallographic structure determinations reported to highly substituted 2H-azirines indicate an unusually long C2-N bond explaining the preferential C2-N cleavage in thermal reactions of 2H-azirines. The photolysis of azirines bearing a conjugative substituent (e.g. phenyl, vinyl) at C-2 leads to nitrile ylides with conjugative groups which can participate in 1,5-dipolar electrocyclic ring closure. The study of the reactivity of 2-halo-2H-azirines is of particular interest since this system bears an extra functional group. In contrast with other 2H-azirine derivatives the reactivity of 2-halo-2H-azirines has been less explored. However, the development of a general route to these heterocycles raised the interest on their use in organic synthesis. This review will focus on the chemistry of 2-halo-2H-azirines (1) taking into account the known chemistry of the non-halogenated-2H-azirines.
2H-Azirines, non-halogenated-2H-azirines, 2-halo-2H-azirines
Departamento de Quimica,Faculdade de Ciências e Tecnologia, Universidade de Coimbra, 3004-535Coimbra, Portugal