Reactions of Unsaturated Azides; Part 27: Synthesis of 1,4-Diazidobuta-1,3-dienes

Experimental and Theoretical Characterization of the Aromatization, Epimerization, and Fragmentation Reactions of Bi‐2H‐azirin‐2‐yls Prepared from 1,4‐Diazidobuta‐1,3‐dienes

“…When a solution of diazide (Z,Z)-10 [11] was irradiated by a high-pressure mercury lamp at low temperature, only the mono-azirine (Z)-11 (yield: 26 %) but not the corresponding biazirinyls 12 could be detected (Scheme 2). Prolonged photolysis gave only traces of the known [12] pyridazine 13, whereas heating of (Z,Z)-10 in dichloromethane or chloroform resulted in the formation of 13 in 27 % yield besides small amounts of (Z)-11 (up to 2 %).…”

“…It is well-known from the work of Gilchrist et al [13] and others [14] that 2H-azirines with an electron-withdrawing group in the 3-position are unstable but good dienophiles in stereoselective Diels-Alder reactions. In the case of (Z,Z)-10, the ester groups made it possible to prepare the diazide by simple nucleophilic substitution of the corresponding dichloride, [11] but these electron-withdrawing groups increase the electron deficiency at the C=N carbon atoms of the heterocycles 12, which are strongly destabilized. Thus, we looked for other types of 1,4-diazidobuta-1,3-dienes to generate bi-2H-azirin-2-yls.…”

“…When we photolyzed a solution of the diazide (E,Z)-17, [11] a mixture of two diastereomeric, highly unstable 2H-azirines 18, formed with 60 and 26 % yield, was detected by 1 H NMR spectroscopy, whereas the analogous treatment of a symmetrical isomer of 17 [11] gave only a single stereoisomer of 18 in 67 % yield (Scheme 3). Even irradiation at À85 8C did not bring about signals of the desired biazirinyls 19.…”

“…The highly strained heterocycles 7 could not be isolated because they isomerized nearly quantitatively to give pyridazines 8 [11].…”

“…Very recently, however, large-scale synthesis of novel 1,4-diazidobuta-1,3-dienes was presented. [11] We describe here the complete characterization of isolable stereoisomeric bi-2H-azirin-2-yls prepared from such diazides. Thus, it was possible to investigate not only aromatization but also epimerization and fragmentation processes of such heterocycles.…”