Novel cyclization reactions of mono‐ and dichloroazodienes. A new synthesis of substituted pyridazines

Cyclization of the in—situ generated chloroazodienes with a variety of enamines was found to give chloro‐substituted tetrahydropyridazines which could be aromatized to pyridazines by base treatment. This sequence appears to be a formal 4 + 2 hetero Diels‐Alder reaction with a high degree of regio and stereo control and constitutes a new synthesis of substituted pyridazines. However, cyclization of the corresponding dichloroazodienes with acyclic enamines gave not only the expected pyridazine product, but also gave an N ‐aminopyrrole product. Combination of the dichloroazodiene with cyclic enamines gave bicyclic dihy‐dropyridazines, bicyclic pyridazines, and acyclic enamines which could be forced to close to the bicyclic dihydropyridazines upon further heating. These results would indicate a stepwise mechanism. The scope and mechanistic speculations on these reactions will be presented. While exploring the novel cyclization reactions of 4‐chloroazodienes with electron rich olefins we developed a new and general synthesis of substituted 3‐phenypyridazines. A number of these analogs were found to exhibit bleaching herbicidal activity (phytoene desaturase inhibition). Further methodology development coupled with analog synthesis led to the preparation of 3‐heteroaryloxy and 3‐aryloxypyridazines with increased unit activity and selectivity as well as good environmental properties. These compounds were found to be more active than current commercial standards on a number of important weed species with selectivity in corn in the US and small grains in Europe. Greenhouse activity of the most active analogs ranged from 17–140 g/hectare on important narrow‐leaf species.