Five-membered ring heterocycles triazole

Five-Membered Ring Heterocycles with Three or More Heteroatoms Sydnones, Triazoles, and Benzotriazoles... 

Among the five-membered ring heterocyclics with more than one hetero-atom, the derivatives of imidazole, oxazole, thiazole and 1,2,4-triazole are particularly important. 

Five-membered ring heterocyclic compoxmds containing nitrogen are one of the most common heterocycles. In fact, the synthesis of a great variety of that such as pyrrole, pyrazole, imidazole, triazole, tetrazole, lactams, and imides involving porous catalytic systems have been investigated. 

Figure 2.19) exhibited the most potent antiviral activity among the evaluated five-membered ring heterocycles (1,2,4-triazole and imidazole) carbocychc nucleosides (Figure 2.20 and Table 2.12). 

The conversion of a nontriazole ring system into a triazole most commonly involves the substitution of a nitrogen for another heteroatom in a five-membered ring. The process usually involves nucleophilic ring opening of the heterocycle followed by ring closure and loss of the other atom. Such reactions thus often involve the same intermediates as those described in the reactions of acyclic systems (Scheme 33). 

Azoles are heterocyclic compds characterized by a five membered ring contg nitrogen. They include diazoles, triazoles and tetrazoles, as well as compds contg other atoms such as O and S in the ring oxazoles, dioxazoles, thiazoles, thiadiazoles etc. Some azoles or their derivs are expl, for example the tri-azoles and the tetrazoles Refs l)K.A.Jensen A.Friediger, Kgl DanskeVidenskabSelskab, Math-FysMedd... 

Most diazotized heterocyclic amines undergo normal coupling reactions with common aromatic coupling components, as well as with some C—H acidic compounds . This pertains for five-membered ring systems such as pyrrols 46 pyrazoles imidazoles 45 , l,2,4-(49) and 1,2,3-triazoles 50... 

Dipolar cycloadditions can, of course, only produce five-membered rings. Addition of dipolarophiles can generate tetrahydro, dihydro or aromatic oxidation level heterocycles, as illustrated above. Alkene dipolarophiles, with a group that can be eliminated following cycloaddition, give the same result as equivalent alkyne dipolarophiles, for example enamines as the dipolarophile, interact with azides, as the 1,3-dipole, with subsequent elimination of the amine, affording 1,2,3-triazoles. ... 

The A-imines of s-triazoles have been investigated the most thoroughly of all the heterocyclic five-membered rings. All 1,2,4-triazole-A-imines prepared contain the A-imino group in the 4-position of the triazole ring. 

Triazoles are five-membered aromatic heterocycles that contain three nitrogen atoms located at the 1,2,3 or 1,2,4 positions in the ring. A large munber of ionic compounds that contain a triazole derivative are described as energetic materials. 

Oxidative procedures have been utilized for the synthesis of both monocyclic five-membered heterocycles and their ring-fused analogs, although the ease of synthesis of the precursors for the latter ring closures results in wider application of this procedure. A variety of oxidizing agents have been used and the conversion of the benzylidene hydrazidines (221) into the 4-arylamino-l,2,4-triazole (222) was effected with mercury(II) oxide (77BCJ953). 

The diazotization of heteroaromatic amines is basically analogous to that of aromatic amines. Among the five-membered systems the amino-azoles (pyrroles, diazoles, triazoles, tetrazoles, oxazoles, isooxazoles, thia-, selena-, and dithiazoles) have all been diazotized. In general, diazotization in dilute mineral acid is possible, but diazotization in concentrated sulfuric acid (nitrosylsulfuric acid, see Sec. 2.2) or in organic solvents using an ester of nitrous acid (ethyl or isopentyl nitrite) is often preferable. Amino derivatives of aromatic heterocycles without ring nitrogen (furan and thiophene) can also be diazotized. 

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