Five-membered ring heterocycles tetrazole

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. 

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... 

One group of heterocycles made almost exclusively by 1,3-dipolar cycloadditions is the tetrazoles 87. They have four nitrogen atoms in the five-membered ring and therefore only one carbon atom and, inevitably, no C-C bonds. Tetrazoles are as acidic as carboxylic acids (pKt about 5) and disconnection of the anion 88 into a nitrile and azide ion is the usual approach. 

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. 

Azide-tetrazole isomerism, or valence tautomerism, was not discussed for [5.6]bicyclic systems in the previous survey (76AHCS1). During recent years, this type of ring-chain tautomerism has been extensively studied for both six- and five-membered heterocyclic azides. The tautomerism of [5.5]bicyclic tetrazole systems is covered in Section II,C. We discuss the tautomerism of the six-membered heterocyclic azides in this section. 

The number of five-membered heterocycles with four hetero atoms is limited to those with nitrogen atoms. Thus, the tetrazole acyclonucleo-sides were formed from sugars N,N -diphenylformazanes by the action of AT-bromosuccinimide or lead tetraacetate [133,134]. The synthesis of the tetra-O-benzoyl-D-lyxononitrile and its conversion to (o-lyxo-tetritol-l-yl)-tetrazole derivatives upon reaction with sodium azide was reported [133, 134]. A pseudo C-nucleoside including a tetrazole ring was achieved by conversion of 3- 0-benzyl- 1,2-O-isopropylidene-D-ribo-pentodialdehydo-1,4-furanose to the respective nitrile and then reaction with sodiiun azide [86]. 

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