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Cyclization of the title compounds with sodium ethoxide may proceed in various ways to afford differently fused pyridazino ring systems. Theoretical considerations based on frontier molecular orbital (FMO) analysis of 4-chloro-5-hydroxyalkylamino-6-nitropyridazinones were in agreement with experimental results in most cases. Formation of pyridazino[3,4] annelated systems was predicted and observed from 5-hydroxyethylamines and their N-benzyl analogs. However, in the ring closure reaction of the 5-N-benzyl-N-hydroxypropylamino derivative, besides [3,4] annelation, another cyclization also occurred to form a pyridazino[4,5-b]oxazepine derivative as minor product, whereas the 5-hydroxypropylamino derivative without an N-benzyl group gave a [3,4]-fused bicyclic compound, and a 6-ethoxy monocyclic derivative. The next homologue, i.e. the 5-hydroxybutylamino derivative, underwent only an intermolecular nucleophilic substitution reaction to give 6-ethoxypyridazinone derivative. In the cases of the 4-regioisomers, the N-benzyl derivatives could only be cyclized, and pyridazino[4,5] annelated systems were obtained. Structures of new compounds were proven by spectroscopic methods and microanalytical data. For a regioisomeric pair of pyridazino[4,5-b]oxazepines, X-ray analyzes were also carried out. |
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