The reaction of dihalohydrazones with Hünig's base gives 1-carbethoxy-3-phenyl-4-haloazodienes in-situ, which were found to combine with a variety of electron rich olefins to yield halo-substituted tetrahydropyridazines (Scheme 2 and Table 1 ). These haloazodiene cyclizations are best characterized as inverse electron demand, 4 + 2 hetero Diels-Alder reactions that maintain a high degree of regio- and stereochemical control (Schemes 5 and 6). The chloro-substituted tetrahydropyridazines that are formed give high yields of substituted pyridazines upon treatment with base (Table 1). The sequence of a chloroazodiene cyclization to a tetrahydropyridazine followed by an aromatization constitutes a new and general synthesis of substituted pyridazines. In contrast to the haloazodiene cyclizations, the novel cyclization reactions of the in-situ generated 1-carbethoxy-3-phenyl-4,4-dichloroazodiene were found to give N-aminopyrroles and pyridazines when combined with acyclic enamines (Table 3 ). However, reactions with cyclic enamines gave the N-aminopyrroles, pyridazines, a dihydropyridazine as products as well as the noncyclized enamine intermediates (Table 4 ). The noncyclized enamines could be converted to the N-aminopyrroles simply upon heating to higher temperatures, indicating a stepwise mechanism (Schemes 8 and 9). The examples described here are the first reported cyclization reactions for dichloroazodienes.
1996 ring closure reactions ring closure reactions O 0130
-069Novel Cyclization Reactions of Dichloroazodienes.-Treatment of the trichlorohydrazones (I) with Huenig's base generates highly colored dichloroazodienes which react in situ with the electron-rich acyclic enamines (II) to give the cyclized products (III) and (IV) . However, reactions with the cyclic enamines (V) may yield the non-cyclized enamine intermediates (VIII) as well. -(SOUTH, M. S.; JAKUBOSKI, T. L.; WESTMEYER, M. D.; DUKESHERER, D. R.; Tetrahedron Lett. 37 (1996) 9, 1351-1354; Ceregen Div., Monsanto Corp., St. Louis, MO 63167, USA; EN)
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