1.2.3- Triazolo pyridazines

The azodiamidine (109) also gives Diels-Alder adducts with simple dienes. The initial adducts readily undergo ring closure to give 1,2,4-triazolo-pyridazines.170... 

The reaction between zwitterion 74 and aryldiazonium salt at room temperature provides the fluoroborate salt 75. Upon treatment with a base the salt 75 undergoes a valence bond isomerization to yield triazolo-pyridazine 76 (Scheme 2) <2000CC1785>. 

Dihydroxypyridazines are readily prepared from the diene 103 and DEAZD or PTAD,159 and the amino acid 104, a constituent of the Monamycin antibiotics, was synthesized from the Diels-Alder adduct of phthalazine-l,4-dione and penta-2,4-dienoic acid.160 These unusual amino acids can also be synthesized from the PTAD and parent HTAD adducts of penta-2,4-dienoic acid,161 and a variety of similar adducts, triazolo-pyridazines 105, has been synthesized from a wide range of 4-substituted 1,2,4-triazole-3,5-diones.162... 

That the benzodiazepine structure was a prerequisite for the characteristic tranquilizer profile and specific binding at the benzodiazepine receptor was a long-held belief. This has now been shown not to be the case by the discovery of a range of different compounds that bind to the benzodiazepine receptor [Figure 29-2). These include the 3-carbolines (e.g., abecarnil), the triazolo-pyridazines [e.g., GL 218,872), the imidazopyridines (e.g., zolpidem), the cyclopyrrol ones [e.g., suriclone), and the pyrazoloquinolines. 

The reaction of several substituted imidazo[4,5-c/]-, pyrazolo[3,4-r/]- and triazolo[4,5-zf]pyrid-azines 3 with ynamines, in competition with [4 + 2] cycloaddition, leads to [2 + 2] derivatives 4, which rearrange to l,2-diazocines5.7 8 The reaction seems to be sensitive to the substituents, as replacement of the electron-withdrawing group R on the pyridazine ring of the pyrazolo compound (A = N, B = CH) by chlorine completely inhibits both the [4 + 2] and [2 + 2] cycloaddition reactions. The X-ray structure of the imidazo derivative 5 (R = Ms, A = CH, B = N) reveals a tub conformation of the eight-membered ring. 

Die Reduzierbarkeit cyclischer Hydrazone mit Hydriden ist stark strukturabhangig. 7,8-Dihydro-( 1,3,4-triazolo-[4,3-b]-pyridazin) (I) und auch l,3,4-Triazolo-[4,3-b ]-pyridazin (II) werden durch 10 Mol-Aquiva-lente Natriumboranat in siedendem Athanol mit guten Ausbeuten unter Sattigung des Pyridazin-Rings zu 5,6,7,8-Tetrahydro- (1,3,4-triazolo-[4,3-b]-pyridazin) reduziert2 ... 

Another interesting scavenger is polymer-supported anthracene, developed by Porco for the scavenging of dienophiles [109]. An example of its application to the synthesis of a complex 5,8-dihydro-(l,2,4)triazolo[l,2-a]pyridazine-l,3-diones via hetero-Diels-Alder reaction followed by removal of the excess of triazole-3,5-dione under microwave irradiation is depicted in Scheme 24. For this particular example, moving from thermal heating (toluene, 100 °C) to a microwave-assisted protocol (DCE, 150 °C) reduced scavenging time from 3 h to just 15 min. 

The silylation-amination of l-benzyl-4,7-dihydroxy-l,2,3-triazolo[4,5-d]pyridazine 272 with N-methylpiperazine, HMDS 2, and (NH4)2S04 gives, after 24 h at... 

Likewise, l-[4-methylbenzyl]-4,7-dihydroxy-l,2,3-triazolo[4,5-d]pyridazine 275 reacts with phenylhydrazine, HMDS 2, and (NH4)2S04 to give the monoamino product 276 in 60% yield [83]. Reaction of l-benzyl-3-n.-butyl-4-hydroxy-l,2,3-triazo-lo[4,5-d]-pyrimidine 277 for 8 h at 120°C with cyclopentylamine, HMDS 2, and (NH4)2S04 affords the aminated product 278 in 49% yield [84] (Scheme 4.33). 

Methyl-2,4,6-triazatricyclo[5.2.2.02 6]undeca-8-ene-3,5-dione (5,8-Dihydro-2-methyl-5,8-ethano-17/-[l,2,4]triazolo[l,2a] pyridazine-l,3(2//)dione)... 

Compound 248 treated with (EtOhP in refluxing xylene provides an 89% yield of 19 (Equation 37) <2005AGE7089>. Similarly, [l,2,3]triazolo[4,5- /]pyridazine 249 provided a 68% yield of the corresponding mesomeric betaine 250 (Equation 38). However, reductive cyclizations of the analogous 3-(2-nitrophenyl)-377-[l,2,3]triazolo[4,5-//Jpyrimidine, 3-(3-nitropyridin-2-yl)-3//-[l,2,3]triazolo[4,5-. 

Photoproducts arising by a [3 + 2] cycloaddition of s-triazolo[4,3-b]-pyridazine (269) to alkenes have been described.222 Addition to cyclohexene, for example, led to the formation of adducts 270 and 271, and the proposed mechanism is outlined in Scheme 8. The reaction has been extended to include addition to cis- and rans-hex-3-ene,223 cyclooctene,224 and furan.225... 

Two types of addition to pyrimidine bases appear to exist. The first, the formation of pyrimidine photohydrates, has been the subject of a detailed review.251 Results suggest that two reactive species may be involved in the photohydration of 1,3-dimethyluracil.252 A recent example of this type of addition is to be found in 6-azacytosine (308) which forms a photohydration product (309) analogous to that found in cytosine.253 The second type of addition proceeds via radical intermediates and is illustrated by the addition of propan-2-ol to the trimethylcytosine 310 to give the alcohol 311 and the dihydro derivative 312.254 The same adduct is formed by a di-tert-butyl peroxide-initiated free radical reaction. Numerous other photoreactions involving the formation by hydrogen abstraction of hydroxyalkyl radicals and their subsequent addition to heterocycles have been reported. Systems studied include 3-aminopyrido[4,3-c]us-triazine,255 02,2 -anhydrouri-dine,256 and sym-triazolo[4,3-fe]pyridazine.257 The photoaddition of alcohols to purines is also a well-documented transformation. The stereospecific addition of methanol to the purine 313, for example, is an important step in the synthesis of coformycin.258 These reactions are frequently more... 

Several interesting 1,2,4-triazole fused-ring systems have been reported. A facile synthesis of 3,5-dihydro-677-imidazo[l,2-fc]-l,2,4-triazol-6-ones 162 was obtained by an iminophosphorane-mediated annulation <06EJ04170>. 8-Trifluoromethyl-l,2,4-triazolo[4,3- >]pyridazines 163 has been prepared from 4-trifluoromethyl-4,5-dihydropyridazin-3-one... 

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