Pyridazines , preparation

From extensive studies in the field of pharmacologically active pyridazines performed in France, interesting novel anticonvulsant agents emerged. Out of a series of 6-aryl-3-(hydroxypolymethyleneamino)pyridazines prepared [169, 184], compounds CM 40907 (CAS 93181-81-8) (54, R = H) and SR 41378 (CAS 93181-85-2) (55, R = Cl) appear to be of particular interest. 

Electrophilic substitution of 4-(tributystannyl)pyridazines, prepared from tributylstannyl acetylenes and tetrazines, has been demonstrated (Scheme 80) with iodination, and palladium-catalyzed benzoylation and phenylation <91H(32)1387>. 

Some of these compounds are used as potential intermediates for the preparation of 4,7-dioxo-4,5,6,7-tetrahydrothiazolo[4,5d]pyridazines (78). The diesters (77) are hydrolyzed under appropriate conditions to free acids (79), whose monopotassium salts (80) yield the cyclic anhydrides (81) under the influence of thionylchloride. Pyrolysis of 79, Rj = a-thienyl, results in its decarboxylation to 82. 

When nitration of pyridazine iV-oxides is carried out with acyl nitrates (prepared in situ from acyl chlorides and silver nitrate) the reaction takes place at the /3-position relative to the iV-oxide group. Under these circumstances only mononitro derivatives are formed. For example, nitration of pyridazine 1-oxide with acetyl nitrate yields 3-nitropyridazine 1-oxide (17%) and 5-nitropyridazine 1-oxide (0.8%), whereas with benzoyl nitrate a better yield of 5-nitropyridazine 1-oxide is obtained. 

Alkylthio- and arylthio-pyridazines can be prepared from the corresponding halo-substituted pyridazines by using appropriate alkyl and aryl thiolates. 

Amino groups in pyridazine A-oxides can be diazotized and the diazonium group further replaced by halogens, hydroxy group or hydrogen. So, 3-, 4-, 5- and 6-bromopyridazine 1-oxides can be prepared from the corresponding amino A-oxides. 

Alkyl- and aryl-pyridazines can be prepared by cross-coupling reactions between chloropyridazines and Grignard reagents in the presence of nickel-phosphine complexes as catalysts. Dichloro[l,2-bis(diphenylphosphino)propane]nickel is used for alkylation and dichloro[l,2-bis(diphenylphosphino)ethane]nickel for arylation (78CPB2550). 3-Alkynyl-pyridazines and their A-oxides are prepared from 3-chloropyridazines and their A-oxides and alkynes using a Pd(PPh3)Cl2-Cu complex and triethylamine (78H(9)1397). 

Homolytic acylation of ethyl pyridazine-4-carboxylate is a convenient general method for preparation of 4-acylpyridazines (Scheme 42) (79M365). 

Since the pyridazine ring is generally more stable to oxidation than a benzene ring, oxidation of alkyl and aryl substituted cinnolines and phthalazines can be used for the preparation of pyridazinedicarboxylic acids. For example, oxidation of 4-phenylcinnoline with potassium permanganate yields 5-phenylpyridazine-3,4-dicarboxylic acid, while alkyl substituted phthalazines give pyridazine-4,5-dicarboxylic acids under essentially the same reaction conditions. 

Side-chain lithiation with lithium diisopropylamide and subsequent alkylation or acylation is a practical method for the preparation of various alkyl-, alkenyl- and acyl-methyl-pyridazines 78CPB2428, 78CPB3633, 79CPB916) (Scheme 47). 

Pyridazinecarbohydrazides are prepared in the normal way from an ester or acid chloride and hydrazine or a substituted hydrazine, generally in good yields. Pyridazines with two ortho alkoxycarbonyl groups give cyclic hydrazides with hydrazine, which are pyridazinopyridazines. 

Reduction of nitroaminopyridazines yields the corresponding aminopyridazines. Reductive cleavage of hydrazinopyridazines to give amino compounds is of practical significance in cases when halogen atoms are resistant to ammonolysis. Many substituted 3,4-diamino-, 4,5-diamino- and 3,5-diamino-pyridazines can be prepared in this way. 

The most commonly used methods for the preparation of pyridazinesulfonamides are the condensation of aminopyridazines with p-acylaminobenzenesulfonyl chloride and the reaction of halosubstituted pyridazines with sulfanilamide by fusion or in an appropriate solvent. 

Amino-pyridazines and -pyridazinones react with monomethyl- or iV,A-dimethyl-formamide and other aliphatic amides in the presence of phosphorus trichloride, thionyl chloride, phosgene or benzenesuUonyl chloride to give mono- or di-alkylaminomethyl-eneamino derivatives. The same compounds can be prepared conveniently with A,iV-dimethylformamide dimethyl acetal in high yield (Scheme 50). 

Several 3-substituted 6-methylmercuriothiopyridazines and complexes of perfluoro-pyridazine with metal carbonyl anions have been prepared <67MI21200). 

Pyridazine-3,6-diones (diazaquinones) are prepared from cyclic hydrazides by oxidation with lead tetraacetate or other oxidizing agents, such as r-butyl hypochlorite, chlorine or nickel peroxide. 

Similarly, reduced pyridazines have been prepared by pertungstate oxidation of the corresponding diamines, as shown in Scheme 60 (75JOC1395). 

Unsaturated hydrazones, unsaturated diazonium salts or hydrazones of 2,3,5-triketones can be used as suitable precursors for the formation of pyridazines in this type of cyclization reaction. As shown in Scheme 61, pyridazines are obtainable in a single step by thermal cyclization of the tricyanohydrazone (139), prepared from cyanoacetone phenylhydrazone and tetracyanoethylene (76CB1787). Similarly, in an attempted Fischer indole synthesis the hydrazone of the cyano compound (140) was transformed into a pyridazine (Scheme 61)... 

With the saturated analogs, i.e. succinic anhydride and its derivatives, pyridazines are formed in only a few cases. The reaction has been applied to the preparation of perhydro-pyridazines and their 3,6-diones (68MI21200, 70JOC1468). For the synthesis of 4,5-dihalopyridazinones, /3-formylacrylic acids, for example mucochloric acid, are useful syn-thons (Scheme 80). 

A large number of pyridazines are synthetically available from [44-2] cycloaddition reactions. In one general method, azo or diazo compounds are used as dienophiles, and a second approach is based on the reaction between 1,2,4,5-tetrazines and various unsaturated compounds. The most useful azo dienophile is a dialkyl azodicarboxylate which reacts with appropriate dienes to give reduced pyridazines and cinnolines (Scheme 89). With highly substituted dienes the normal cycloaddition reaction is prevented, and, if the ethylenic group in styrenes is substituted with aryl groups, indoles are formed preferentially. The cycloadduct with 2,3-pentadienal acetal is a tetrahydropyridazine derivative which has been used for the preparation of 2,5-diamino-2,5-dideoxyribose (80LA1307). 

This synthetic appproach has been used in a few cases for the preparation of pyridazines from diazo compounds and cyclopropenes. In general, cycloadducts (176) are formed first and these rearrange in the presence of acid or alkali to pyridazines (Scheme 98) (69TL2659, 76H(5)40l). Tetrachlorocyclopropene reacts similarly and it was found that the stability of the bicyclic intermediates is mainly dependent on substitution (78JCR(S)40, 78JCR(M)0582>. 

The most useful syntheses of pyridazines and their alkyl and other derivatives begins with the reaction between maleic anhydride and hydrazine to give maleic hydrazide. This is further transformed into 3,6-dichloropyridazine which is amenable to nucleophilic substitution of one or both halogen atoms alternatively, the halogen(s) can be replaced by hydrogen as shown in Scheme 110. In this manner a great number of pyridazine derivatives are prepared. 

Pyridazine itself is best prepared (in about 60-67% yield) from 2,5-diacetoxy- or 2,5-dimethoxy-2,5-dihydrofuran (50ACS1233, 56JOC764) or by hydrodehalogenation of 3-chloro-... 

Hydroxyphthalazin-l(2//)-one is obtained in a smooth reaction between phthalic anhydride and hydrazine hydrate and this is again the starting compound for many 1-substituted and/or 1,4-disubstituted phthalazines. The transformations of 1,4-dichloro-phthalazine, which is prepared in the usual manner, follow a similar pattern as shown for pyridazines in Scheme 110. On the other hand, phthalonitrile is the preferential starting compound for amino- and hydrazino-phthalazines. The most satisfactory synthesis of phthalazine is the reaction between a,a,a, a -tetrachloro-o-xylene and hydrazine sulfate in sulfuric acid (67FRP1438827), alt iough catalytic dehalogenation of 1-chloro- or 1,4-dichloro-phthalazine or oxidation of 1-hydrazinophthalazine also provides the parent compound in moderate yield. 

The cleavage of fused pyrazines represents an important method of synthesis of substituted pyrazines, particularly pyrazinecarboxylic acids. Pyrazine-2,3-dicarboxylic acid is usually prepared by the permanganate oxidation of either quinoxalines or phenazines. The pyrazine ring resembles the pyridine ring in its stability rather than the other diazines, pyridazine and pyrimidine. Fused systems such as pteridines may easily be converted under either acidic or basic conditions into pyrazine derivatives (Scheme 75). 

As in the pyridopyrimidines, the MacFadyen-Stevens degradation of tosylhydrazino derivatives has been used to prepare the parent [2,3-tf] compound (63JCS6073), and 1,4-bishydrazinopyrido[3,4-tf] pyridazines are reduced to the 1,4-diamino derivatives by Raney nickel (68AJC1291). 

Pyrazolo[3,4-d]pyridazines (555) can be prepared readily from hydrazines and pyrazoles substituted in positions 4 and 5 with an acyl and an ester group, or with two ester groups. 4,5-Pyrazolinediones have been used as starting materials for the synthesis of the quinoxaline derivatives (548) (see above) and of pyrazolo[3,4-e][l,2,4]triazines (556)... 

Attempts to prepare bis-quaternaiy salts from pyridazine and methyl iodide, ethyl bromide, or l,4-dibromobut-2-ene were unsuccessful. ... 

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