Pyridazines. XXIII. A novel pyridazine into pyrazole ring transformation

Treatment of phenyl(4-pyridazinyl)methanols 1a and 1b with p-toluenesulfonic acid at elevated temperatures causes transformation into the C-4 substituted pyrazole derivatives 8a and 8b . Limitations of this novel rearrangement reaction as well as mechanistic considerations are discussed. Furthermore, the preparation of hitherto unknown 4-pyridazinylmethanols 1b , 3 , and 7 is described.     

Studies on pyridazine compounds,XIX. Mesylation of 5-aminopyrazoles and related compounds

Summary Acylation of 5-amino-1-substituted pyrazoles gave — depending on the substituents in position 3 and agents used — mono-, di- and triacylated products, respectively.Poster presented at the 10^th International Congress on Heterocyclic Chemistry, Waterloo/Canada, August 11–16, 1985.     

Photo-enhanced reduction of carbonyl compounds by sodium borohydride

The reduction of ketones and some esters by sodium borohydride is dramatically accelerated by uv irradiation. The reaction seems to proceed from the (n, π) excited state of the carbonyl compound. The photocatalytic effect is dependent on the solvent polarity and substituents on aromatic carboxylic acids and alcohols of esters.     

Studies of pyridazine compounds, XXV. Reinvestigation of acylation of pyridazinylhydrazones

The acylation of morpholino substituted pyridazinylhydrazones afforded triazolo[4,3- ]pyridazinium salts. Structure elucidation by ir, ¹H- and ¹³C-nmr spectroscopy, reaction mechanism and ring - chain tautomerism are discussed.     

Microdetermination of carbonyl compounds by reduction with alkaline sodium borohydride

Summary Alkaline sodium borohydride has been employed as a reducing reagent for the titrimetric determination of compounds containing the carbonyl function. The sample is dissolved in methanol and is then reduced with alkaline sodium borohydride. After completion of the reaction, the excess sodium borohydride is back titrated against standardized hydrochloric acid solution using methyl red as indicator. The stoichiometry between the carbonyl function and sodium borohydride is 41.

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Zusammenfassung Natriumborhydrid dient als Reduktionsmittel zur Titration von Carbonylverbindungen. Die Probe wird in Methanol gelöst und dann mit alkalischem Natriumborhydrid reduziert. Nach Ablauf der Reaktion wird der Überschuß mit Salzsäure gegen Methylrot zurücktitriert. Carbonylverbindung und Natriumborhydrid reagieren im Verhältnis 41.

     

Pyridazines. LXX. Reactions of azidoazolopyridazines with unsaturated compounds

Cycloaddition reactions of azidoazolopyridazines with unsaturated esters, cyclic enol ethers, styrene, 2-vinyl-pyridine, bicyclo[2.2.1]heptene, dicyclopentadiene and dehydrobenzene were investigated. The reaction proceed via the intermediate Δ² -1,2,3-triazolines which were in most cases termolabile and decomposed further to give the final products which were enamines, imines of fused aziridines.     

Pyridazines. XLI synthesis of novel pyrido[3,4-d]pyridazine derivatives from 4,5-disubstituted pyridazines

Pyrido[3,4-d]pyridazines 2–5, 21 bearing one, two, or three aryl substituents in the pyridine moiety are shown to be conveniently accessible from 4-aroyl-5-methylpyridazines or 4,5-diaroylpyridazines, respectively, by condensation reactions with appropriate C-N fragments. In addition, the novel pyridazine-annelated pyridones 24, 25 were found to be easily available from ethyl 5-methyl-4-pyridazinecarboxylate.     

Effect of borohydride reduction on antibodies

The effect of borohydride reducing reagents on monoclonal and polyclonal antibodies was examined by enzyme-linked immunosorbent assay (ELISA). Each antibody showed different stability characteristics to the reducing reagents. Sodium cyanoborohydride was at least five times milder toward immunological activity than sodium borohydride, however, sodium cyanoborohydride with a catalytic amount of metal ion (Zn²⁺ or Al³⁺ ) can be as harsh as sodium borohydride. Activated hydrophobic borohydrides, 9BBN-pyridine, did not have any advantages in respect to the stabilities of antibodies. Antibodies to be used for immunosorbent purification must be evaluated individually to determine whether their structure is stable to immobilization reagents and conditions prior to their linkage to the column support.     

Heterocycles. XXV Sodium borohydride reduction of flavanonols

Solvent effects on the stereochemistry in the sodium borohydride reduction of (±)-flavanonols have been examined. The effects observed for the (±)-flavanonols with 5-OMe in 2-propanol, dioxane and methanol are explainable by the differences between the steric interactions inherent in the product-like transition states A and B. It has been also found that 5-OAc peculiarly affects the stereochemistry in the reduction to produce the (±)-catechin-type compounds in a one-pot process. The solvent and temperature effects are examined using a model analogous to the above.     

Studies on pyridazine derivatives. IV Mesylation reaction of pyridazinylpyrazoles

On mesylation, 1-pyridazinylpyrazoles ( 1 ), give, depending on the substituents and reaction conditions, O-mesylpyrazoles ( 2 ) and O-mesyl-4-N-mesyl-1,4-dihydro-4-pyridyl-pyrazole derivatives ( 3 ). The structures of these compounds were confirmed by ir and ¹H nmr spectral data.     

Studies on the syntheses of heterocyclic compounds. Part DCXLIV . Catalytic reduction of homoproaporphine related compounds

Hydrogenation of the homoproaporphine (IV) over platinum oxide gave a mixture of the cyclohexanols VIIa and VIIb. In contrast, reduction of IV on palladium provided the cyclo-hexanone VI, which was also obtained from VIIb by oxidation. Treatment of the cyclohexanols VIIa and VIIb under hydrogen on platinum oxide afforded the same mixture of VIIa and VIIb.     

Studies on pyridazine azide cyclisation reactions

Reaction of sodium azide with 4-methyl-3,5,6-tribromopyridazine results in the formation of 3,5,6-triazide intermediate which could cyclise to give two possible bicyclic products while ab initio calculations show that the formation of a tricyclic compound is extremely energetically unfavourable. However, experimentally, only one major product is isolated. The structure of this unstable product has been conclusively established by X-ray crystallography as 3,5-diazido-4-methyl[1,5-b]tetrazolopyridazine confirming theoretical predictions.     

Recent methodologies mediated by sodium borohydride in the reduction of different classes of compounds

Reduction is a fundamental transformation in organic synthesis. Since its discovery by Brown and co‐workers, sodium borohydride is the most frequently hydride used in reduction processes. Owing to the importance of this reagent in modern organic synthesis, the aim of this review is to highlight recent methodologies (2000–2006) mediated by sodium borohydride in the reduction of different classes of compounds. Copyright © 2006 John Wiley & Sons, Ltd.     

Pyridazines. XLVI. Ring Opening of Some Azolo and Azino Pyridazines

A facile cleavage of the five-membered ring of certain quaternary s-triazolo (4, 3-b)pyridazines under basic conditions was recorded recently². We report now several interesting cases of the six-membered ring opening in some related systems under similar conditions.     

Pyridazines. XXXV. Preparation of some novel pyrimido[4,5-c]pyridazine derivatives from 3-alkylamino- and 3-arylamino-4-pyridazinecarboxamides

Facile syntheses of pyrimido[4,5-c]pyridazine-5,7(6H,8H)diones 4 , pyrimido[4,5-c]pyridazin-5(8H)-ones 7–10 , and dihydropyrimido[4,5-c]pyridazin-5(6H)ones 5,6 starting from 3-chloro-4-pyridazinecarbonitrile 1 via aminocarbonitriles 2 and aminocarboxamides 3 are described. In addition, a convenient access to the new aminopyridazinecarbonitrile 11 from the chloronitrile 1, employing the tetrazolo[1,5-b]pyridazine 12 as the key intermediate, is reported.     

Pyridazines XXIV: Application of radicalic ethoxycarbonylation to the synthesis of pyridazine mono- and polycarboxylic acid esters

Reactivity of pyridazines $\text{1}$, $\text{2}$, $\text{3}$, $\text{16}$ towards ethoxycarbonylradical (generated by redox decomposition of oxyhydroperoxide of ethylpyruvate) was studied. Application of this type of homolytic substitution for synthesis of hitherto not accessible pyridazine carboxylic acid esters $\text{6}$, $\text{8}$, $\text{9}$, $\text{12}$, $\text{13}$, $\text{14}$, $\text{15}$, $\text{17}$ is demonstrated. In addition improved synthesis of diethyl 4,5-pyridazinedicarboxylate ($\text{5}$) is proposed.