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16.8.4.3.1.2 Method 2: Modification of Ketones, Aldehydes, and Derivatives

DOI: 10.1055/sos-SD-016-00192

Haider, N.; Holzer, W.Science of Synthesis, (200416215.

Pyridazinecarbaldehydes and pyridazinyl ketones generally show a high degree of carbonyl reactivity, as a consequence of the pronounced electron deficiency of the 1,2-diazine ring system. For example, pyridazine-4-carbaldehyde forms a stable hydrate just on standing in moist air.[‌626‌] Pyridazine-derived ketones as well as aldehydes can be easily transformed into oximes,[‌261‌,‌456‌,‌472‌,‌581‌,‌626‌,‌627‌] hydrazones,[‌471‌,‌626‌,‌628‌‌631‌] thiosemicarbazones,[‌622‌,‌626‌,‌632‌] imines,[‌600‌,‌626‌,‌628‌,‌633‌] and similar derivatives.[‌588‌] Reduction to the corresponding alcohols is best performed with sodium borohydride,[‌35‌,‌259‌,‌350‌,‌406‌,‌407‌,‌634‌] and oxidation of aldehydes to the carboxylic acids can be achieved, e.g. with silver oxide.[‌333‌,‌336‌] Pyridazinecarbaldehydes are known to undergo the Cannizzaro reaction to afford a mixture of the hydroxymethyl compound and the corresponding carboxylic acid.[‌626‌,‌628‌] Various examples of CC bond formation have been described; the synthetic potential may be illustrated by the WittigHorner alkenation of phenyl pyridazin-4-yl ketone (Scheme 70), affording the Z-configured compounds 181 (with phenyl pyridazin-3-yl ketone, mixtures of E- and Z-isomers are obtained).[‌635‌] Knoevenagel condensation of the aldehydes 182 with 1,3-dimethylbarbituric acid affords compounds 183 in good yields (Scheme 70);[‌636‌] similar products are obtained with Meldrum's acid or malononitrile as methylene-activated reagents.[‌435‌,‌636‌] Further examples are given in Table 9.[‌333‌,‌336‌,‌350‌,‌406‌,‌626‌,‌627‌,‌631‌,‌637‌‌639‌]

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