Azirines, reduction

The iodo azide undergoes stereospecific trans dehydrohalogenation to give a vinyl azide, which on photolysis or pyrolysis gives the 2H-azirine. Reduction of the 2H-azirine with lithium aluminium hydride gives the m-aziridine in good yield. 

The pyrazole ring is resistant to oxidation and reduction. Only ozonolysis, electrolytic oxidations, or strong base can cause ring fission. On photolysis, pyrazoles undergo an unusual rearrangement to yield imidazoles via cleavage of the N —N2 bond, followed by cyclization of the radical iatermediate to azirine (27). 

In some cases, the /3-haloamine undergoes spontaneous cyclization to the aziridine <70TL1125>. Most of these routes, however, require either reductive or base-induced cyclization to the aziridine. Access to a vast number of aziridines and 1-azirines has been... 

A number of nonfused steroidal aziridines have been synthesized. The reduction of the azirine (118) over a platinum catalyst yields the spiro aziridine (119). Several aziridine derivatives of the type (120) have been... 

Unsaturated oximes are attractive substrates for aziridine synthesis. Treatment of oxime 77 with Red-Al yielded vinylaziridines 78, 79, and 80, in various ratios depending on the E/Z ratio of the starting oxime 77 (Scheme 2.22) [38]. This reaction should proceed through abstraction of HA, Hb, or He in the intermediate 81, followed by hydride reduction of the resulting 2H-azirines 82-84. 

Reduction of azirine-2-phosphonates 99 (Scheme 3.32) with NaBH4 in ethanol exclusively gave ris-aziridine-2-phosphonates 100 in 81-82 % yield [86, 87]. A Diels-Alder reaction between azirine-2-phosphonate 101 and trans-piperylene 102... 

The highly strained and reactive 2iT-azirines have been extensively studied for various synthetic purposes, such as ring expansion reactions, cycloaddition reactions, preparation of functionalized amines and substituted aziridines. The older literature on azirines in synthesis has extensively been reviewed [69]. Concerning azirines with defined chirality only scarce information is available. Practically all reactions of azirines take place at the activated imine bond. Reduction with sodium borohydride leads to cz5-substituted aziridines as is shown in Scheme 48 [26,28]. 

The photochemistry of aryl azides is quite complex, suggesting that the nitrene 14 may not be the only reactive intermediate and that insertion reactions may not be the only route to form photoconjugates.Although aryl nitrenes are much less susceptible to rearrangements than acyl nitrenes, they may still occur and lead to the formation of reactive intermediates such as azepines, which may go on to react with nucleophiles.[911 141 Addition of nitrenes to double bonds will generate azirines, while dimerization will produce azobenzenesJ11 Aryl azides are stable to most of the procedures used in the course of peptide synthesis except for reduction reactions. Non-photochemical reduction of aryl azides to the primary amines by thiols has been reported by Staros et al.[15]... 

A common reaction intermediate (59) has been invoked79 to account for the isomerization of 5-alkoxyisoxazoles to azirine derivatives, and the reductive cleavage of 5-alkyl(aryl)isoxazoles to enamino ketones, in the presence of a catalytic amount of... 

Grignard reagents have been shown to react with 1-azirines to give aziridines.23,62 Furthermore, the attack of the Grignard reagent undoubtedly occurs stereospecifically from the less sterically hindered side of the azirine.03 This is analogous to the stereospecific reduction of azirines to aziridines by lithium aluminum hydride. 

Catalyt c hydrogenation (palladium or Raney nickel catalyst) surprisingly results in reduction of the carbon-nitrogen single bond rather than the double bond.4,12,40 The imines, or possibly enamines, are usually not isolated and their existence has only been inferred in most instances. Harvey and Ratts have shown that this reaction with azirine (165) does not proceed first to the aziridine which is then reduced to 166, since aziridine (167) is inert to hydrogen and palladium on carbon.40... 

Oximes, hydrazones, semicarbazones, diazines and carbodiimides all undergo reactions involving addition to a carbon-nitrogen double bond (A, D], but these reactions are of limited value, with the exception of the reaction with ketoximes. This last reaction, with an excess of Grignard at elevated temperatures, is a useful route to aziridines, (4), although yields are rarely high. In some cases, reduction of the intermediate azirine (2) leads to an alternative aziridine (5). 

Interestingly, the reaction of the more heavily substituted diphenyl 2//-azirine afforded azetine 808 in 73% yield when it was allowed to react with diazo malonate in the presence of Rh2(OAc>4. The structure of azetine 808 was established by reduction to diol 809 under the action of LLAIH4 (Scheme 197). It would appear as though the reactivity of the initially formed azirinium ylide is dependent on the degree of substitution about the 2//-azirine ring. 

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