1.2.4.5- Tetrazinedicarboxylic ester

Finally, a Diels-Alder reaction with inverse electron demand, in which the thiophene acts as the 27r-component, has been reported (78AP728). Tetrazinedicarboxylic ester adds to thiophenes to give the adduct (290) loss of nitrogen from this is followed by oxidation, yielding the thieno[2,3-rf]pyridazines (291) in 10-15% yield. With 2,5-dimethyl-thiophene as the substrate, aromatization is blocked, and the product (292) is obtained in 57% yield. 

Tetrazinedicarboxylic ester is quite sensitive to protic solvents (H2O, ROH) especially in the presence of base, as well as to ammonia and secondary amines. A thorough study by Seitz and coworkers showed that the primary step is the addition of the nucleophile, with subsequent loss of nitrogen and ring opening <82CB683>. A more detailed discussion of other addition reactions is found in <84CHEC-l(3)531>. 

Amino-1,2,4-triazole has been obtained from orthoformic ester and hydrazine hydrate in a sealed tube at 120° 1 by heating formylhydrazine at 150-210° 2-3-4 by heating N,N -diformyl-hydrazine at 160° 5 by decarboxylation of 4-amino-1,2,4-tria-zoldicarboxylic acid 6 by fusion of l,2-dihydro-l,2,4,5-tetrazine 6 and by heating l,2-dihydro-l,2,4,5-tetrazinedicarboxylic add above its melting point.4-6-7... 

Aromatization also occurs easily if alcohol, thiol, or amine can be eliminated from a trans-arrangement of substituents. The cycloaddition step is especially fast if ketene aminals, enamines, enol ethers, enol esters, or trimethylsilyloxyalkenes are used as 27t-compounds. The reaction enthalpy A/f for the eycloaddition of 1,3-eyelopentadiene or 1-hexene to dimethyl tetrazinedicarboxylate is in the range of 60-70 kcal mol <93ZORi7i9>. 

Dihydrotetrazines (340), which can easily be oxidized to 1,2,4,5-tetrazines, can be formed by dimerization of thiohydrazides (337) or amidrazones (338). The ring closure of hydrazidines (339) in a [5 + 1] fashion proceeds well with activated carboxylic acid derivatives such as imidates (341), orthocarboxylates (342) or dithiocarboxylates (343). The [4 + 2] procedure is found in the transformation of 1,3,4-oxadiazoles (346) or 1,4-dichloroazines (345) with hydrazine. Finally diazoalkanes (344) can be dimerized in a [3 - - 3] manner under the influence of a base the dimerization of diazoacetic ester is an early example, leading to 3,6-tetrazinedicarboxylate (48), which is frequently used in (4 -I- 2) cycloaddition reactions with inverse electron demand. Nitrile imines, reactive intermediates which are formed from many precursors, can dimerize in a [3 -I- 3] fashion to form 1,3,4,6-tetrasubstituted 1,4-dihydrotetrazines. These reactions are summarized in Scheme 57. 

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