Nitrogen pyrimidines

Cytosine (C) A nitrogenous pyrimidine base, which bonds with guanine (G) to form the G-C base pair. 

Thymine (T) A nitrogenous pyrimidine base found in DNA but not in RNA it bonds with adenine (A) to form the A-T base pair. 

When there is an N-oxide function present in an azine, another site becomes available for proton attack. Although pyridazine iV-oxides often react at the free nitrogen, pyrimidine oxides frequently form the hydroxy salts (83CHE1012). 

Amitrole heterocyclic nitrogen, triazole Ampropylfos phosphoro organic, phosphonic acid Ancymidol heterocyclic nitrogen, pyrimidine Anilazine heterocyclic nitrogen, triazine Anilofos phosphoro organic, phosphoro dithioate ANTU thiourea... 

Bronopol (other) nitrodiol Bupirimate heterocyclic nitrogen, pyrimidine, sulfamate... 

Dimethametryn heterocyclic nitrogen, triazine Dimethazone see Clomazone Dimethenamid heterocyclic sulfur, thiophene, amide Dimethipin heterocyclic sulfur, dithiin, sulfone Dimethirimol heterocyclic nitrogen, pyrimidine Dimethoate phosphoro organic, phosphoro dithioate, amide... 

Ethion phosphoro organic, phosphoro dithioate Ethirimol heterocyclic nitrogen, pyrimidine Ethofumesate benzofuran, sulfonate Ethoprop (Ethoprophos) phosphoro organic, phosphoro dithioate... 

Hexazinone heterocyclic nitrogen, triazine Hexythiazox thiazolidine, amide Hydramethylnon heterocyclic nitrogen, pyrimidine Hydroprene dienedodecanoate 8-Hydroxyquinoline sulphate heterocyclic nitrogen, quinoline... 

Mefluidide amide, sulfonamide Mepanipyrim heterocyclic nitrogen, pyrimidine Mephosfolan phosphoro organic, phosphoro amido thioate... 

Nitrothal-isopropyl aromatic carboxylic acid Norbormide heterocyclic nitrogen, pyridine Norflurazon heterocyclic nitrogen, pyridazinone Nuarimol heterocyclic nitrogen, pyrimidine... 

Pyridate heterocyclic nitrogen, pyridazine Pyrifenox oxime, pyridine Pyrimethanil heterocyclic nitrogen, pyrimidine Pyriproxifen phenyl-ether, pyridine Pyrithiobac-sodium heterocyclic nitrogen, pyrimidine... 

The nucleophilicity of amine nitrogens is also differentiated by their environments. In 2,4,5,6-tetraaminopyrimidine the most basic 3-amino group can be selectively converted to a Schiff base. It is meta to both pyrimidine nitrogens and does not form a tautomeric imine as do the ortho- and /xira-amino groups. This factor is the basis of the commercial synthesis of triamterene. 

Treatment of 2-imino-3-phenyl-4-amino-(5-amido)-4-thiazoline with isocyanates or isothiocyanates yields the expected product (139) resulting from attack of the exocyclic nitrogen on the electrophilic center (276). Since 139 may be acetylated to thiazolo[4,5-d]pyrimidine-7-ones or 7-thiones (140). this reaction provides a route to condensed he erocycles (Scheme 92). 

Two nitrogen containing heterocyclic aromatic compounds—pyrimidine and purine— are the parents of the bases that constitute a key structural unit of nucleic acids... 

The most important derivatives of pyrimidines and purines are nucleosides Nucleosides are N glycosides m which a pyrimidine or purine nitrogen is bonded to the anomeric carbon of a carbohydrate The nucleosides listed m Table 28 2 are the mam building blocks of nucleic acids In RNA the carbohydrate component is d ribofuranose m DNA It IS 2 deoxy d ribofuranose... 

Ring- or side-chain fluoriaated nitrogen heterocycHcs have been iacorporated iato crop-protection chemicals, dmgs, and reactive dyestuffs. Key iatermediates iaclude fluoriaated pyridines, quiaolines, pyrimidines, and tria2iaes. Physical properties of some fluoriaated nitrogen heterocycHcs are Hsted ia Table 13. 

The primary stmcture of DNA is based on repeating nucleotide units, where each nucleotide is made up of the sugar, ie, 2 -deoxyribose, a phosphate, and a heterocycHc base, N. The most common DNA bases are the purines, adenine (A) and guanine (G), and the pyrimidines, thymine (T) and cytosine (C) (Fig. 1). The base, N, is bound at the I -position of the ribose unit through a heterocycHc nitrogen. 

Nitrogen nucleophiles used to diplace the 3 -acetoxy group include substituted pyridines, quinolines, pyrimidines, triazoles, pyrazoles, azide, and even aniline and methylaniline if the pH is controlled at 7.5. Sulfur nucleophiles include aLkylthiols, thiosulfate, thio and dithio acids, carbamates and carbonates, thioureas, thioamides, and most importandy, from a biological viewpoint, heterocycHc thiols. The yields of the displacement reactions vary widely. Two general approaches for improving 3 -acetoxy displacement have been reported. One approach involves initial, or in situ conversion of the acetoxy moiety to a more facile leaving group. The other approach utilizes Lewis or Brmnsted acid activation (87). 

Although all the rings in Figure 1 contain six tt-electrons, the accumulation of electronegative nitrogen atoms in the polyaza structures leads to hydrolytic as well as thermal instability. This is noticeable in pyrimidine, and marked in the triazines and tetrazine. Some stability can be conferred by appropriate substitution, as we shall outline later. 

Interesting structures can be formed by combinations of ring and side-chain substituents in special relative orientations. As indicated above, structures (28) contain the elements of azomethine or carbonyl ylides, which are 1,3-dipoles. Charge-separated species formed by attachment of an anionic group to an azonia-nitrogen also are 1,3-dipoles pyridine 1-oxide (32) is perhaps the simplest example of these the ylide (33) is another. More complex combinations lead to 1,4-dipoles , for instance the pyrimidine derivative (34), and the cross-conjugated ylide (35). Compounds of this type have been reviewed by Ramsden (80AHCl26)l). 

Alkylation of pyrimidin-2(or 4)-amine on a ring-nitrogen gives an imine, e.g. (8), of quite high basic strength (pjSTa 10.7) because its cation, e.g. (13 R = Me), has typical and effective resonance stabilization indeed, methylation of pyrimidine-2,4-diamine gives a still stronger base (pjSTa> 13) due to an even more resonance-stabilized cation (14). 

The UV absorption of pyrimidine occurs in two bands centred at 243 and 298 nm in cyclohexane. The second band is ascribed to the electronic transition from a nitrogen lone pair non-bonding orbital to an empty ring tt-orbital, in short an n transition, on... 

Much of the reactivity shown by the ring atoms and substituents of pyrimidine is akin to that of the corresponding parts of 1,3-dinitrobenzene and 3-nitropyridine. This arises from the quantitatively similar electron-withdrawing effects of doubly-bound ring nitrogen atoms and of nitro groups in reducing sharply the aromaticity of the cyclic system. 

Despite considerable localization of tt-electrons at the nitrogen atoms of pyrimidine, the ring system is still sufficiently aromatic to possess substantial stability. This is a great advantage in the primary synthesis of pyrimidines, in the synthesis of pyrimidines from the breakdown or modification of other heterocyclic systems and in the myriad of metatheses required to synthesize specifically substituted pyrimidines. 

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