Add time:08/02/2019         Source:sciencedirect.com

Publisher SummaryThis chapter reviews the chemistry of 8-azapurines (1,2,3-triazolo[4,5-d]pyrimidines). Two approaches to the preparation of 8-azapurines, from pyrimidines or from triazoles are presented in the chapter. Later, the crystallography, electron density, covalent hydration, ultraviolet spectra, NMR spectra, infrared spectra, mass spectra, and polography of 8-azapurines is described. The two rings of 8-azapurine molecules are individually flat and, at least in the derivatives, inclined to one another at very small angles. Melting points are low and sharp in the absence of hydrogen-bonding structures such as NH2 and (in the ring) CONH. The simplest examples examined by X-ray diffraction are 6-amino-8-azapurine (azaadenine) hydrochloride (anhydrous), 6-amino-7-methyl-8-azapurine (anhydrous), and 8-azapurin-6-one (azahypoxanthine). 8-azapurine has two pKa values, an acidic one at 4.84 and a basic one at 2.05. The change from equilibrium titration to rapid-reaction technique raised the pKa of 8-methyl-8-azapurine much less (from 3.18 to 4.2), which signifies much greater hydration (about 10%) in the neutral species. Typical UV spectra of 8-azapurines show a prominent, rather featureless envelope with λmax 255–275 nm (log є 3.8-4.0) and a second absorption area around 200 nm.

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