Purine, aromaticity basicity

Purine has three basic, pyridine-like nitrogens with lone-pair electrons in sp2 orbitals in the plane of the ring. The remaining purine nitrogen is nonbasic and pyrrole-like, with its lone-pair electrons as part of the aromatic i- electron system. 

There are several classes of alkaloids. Among these are purines such as xanthine and caffeine, ter-penes (Chapter 22), polyketides (Chapter 21), and alkaloids derived from amino acids. The basic amino acids ornithine, arginine, histidine, and lysine as well as the aromatic amino acids, anthranilate, and nicoti-nate are some of the starting materials.199 201 Robinson202 203 in 1917 recognized that many alkaloids are derived directly from aromatic amino acids. He proposed that alkaloids arise from Mannich reactions (Eq. 25-12) in which an amine and an aldehyde (probably through a Schiff base) react with a nucleophilic carbon such as that of an enolate anion. Many of the... 

Nitrogenous base. An aromatic nitrogen-containing molecule with basic properties. Such bases include purines and pyrimidines. 

The alkaloids are basic compounds in which an N atom is typically part of a heterocyclic ring but in some cases is merely a substituent of an alicyclic or aromatic ring system (as for example with colchicine, some peptide alkaloids and some Amaryllidaceae alkaloids). Various N-based heterocyclics such as the purine and pyrimidine bases of DNA and RNA (see Chapter 2) and the methylxanthine purine derivatives variously found in tea and coffee (caffeine, theobromine and theophylline) are sometimes referred to as pseudoalkaloids and for consistency will be included as alkaloids in this classification. Indeed all plant heterocyclics with a ring N will be conveniendy lumped in with the alkaloids in the tables for didactic simplicity and consistency. 

Alkaloids can be divided into different t q3es according their pure chemical structures pointing first at the alkaloid base, a basic chemical nucleus. The following are basic types of alkaloids acridones, aromatics, carbo-lines, ephedras, ergots, imidazoles, indoles, bisindoles, indolizidines, manza-mines, oxindoles, quinolines, quinozolines. quinolizidines, phenylisoquinolines, phenylethylamines, piperidines, purines, pyrolidines, pyrrolizidines, pyrro-loindoles, pyrydines, sesquiterpenes, simple tetrahydroisoquinolines, stereoids, tropanes, terpenoids, diterpenes, and triterpenes. 

The most frequently reported reactivity of 2-fluoropurines involves aromatic nucleophihc substitution (SnAt) with amine nucleophiles under basic conditions, and has been amply demonstrated with simple 2-fluoropurines and 2-fluQropurine nucleosides as substrates. The reaction of 2-fluoropurine nucleosides with amines is successful with a wide range of fimctionahty present at the purine 6-position, including hydrogen... 

The nueleobases are heterocyclic aromatic organic compounds that are basic parts of the nucleic adds. These heterocycles are, in general, the purine bases guanine (G) and adenine (A), and the pyrimidine bases cytosine (Q, as well as thymine (T, DNA) or uracil (U, RNA) (Fig. 1). 

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