Nitrogen heterocycles covalent hydration

Nitrogen heterocycles covalent hydration in, 20, 117 reactions of acetylenecarboxylic esters with, 23, 263... 

Considering the four potential monohydroxypteridines, pteridin-4- and -7-one 56JCS3443) behave normally whereas pteridin-2- and -6-one (25) form covalent hydrates. The reversible hydration of nitrogen heterocycles was actually discovered with pteridin-6-one (52JCS1620),... 

Reversible covalent hydration across C=N bonds occurs in a number of nitrogen-containing heterocycles, including pteridine and its 2- and 6-hydroxy derivatives, quinazoline (as the cation), and 1,4,6-triazanaphthalene (as the cation). Among bases giving this reaction, the neutral molecule exists predominantly as the anhydrous form, whereas the cation contains an increased proportion of the... 

Tables V and VI contain all the equilibrium constants so far reported for nitrogen-containing heterocycles that undergo reversible covalent hydration. Table V comprises equilibria involving hydration in cations and neutral molecules, and Table VI deals with systems of neutral molecules and anions.

A. Albert, Covalent hydration in nitrogen heterocycles, Adv. Heterocyc. Chem. 20 117... 

Covalent Hydration of Nitrogen-Containing Heterocycles and Heterocyclic Pseudobases... 

Increasing numbers of nitrogen atoms increase not only the kinetic susceptibility toward attack but also the thermodynamic stability of the adducts. Reversible covalent hydration of C = N bonds has been observed in a number of heterocyclic compounds (76AHC(20)117). Pyrimidines with electron-withdrawing groups and most quinazolines show this phenomenon of covalent hydration . Thus, in aqueous solution the cation of 5-nitropyrimidine exists as (164) and quinazoline cation largely as (165). These cations possess amidinium cation resonance. The neutral pteridine molecule is covalently hydrated in aqueous solution. Solvent isotope effects on the equilibria of mono- (166) and dihydration (167) of neutral pteridine as followed by NMR are near unity (83JOC2280). The cation of 1,4,5,8-tetraazanaphthalene exists as a bis-covalent hydrate (168). 

Covalent Hydration in Nitrogen Heterocycles A. Albert, Adv. Heterocycl. Chem., 1976, 20, 117-143. 

Covalent Hydration in Nitrogen-Containing Heteroaromatic Compounds I, Qualitative Aspects A. Albert and W. L. F. Armarego, Adv. Heterocycl. Chem., 1964, 4, 1-42. Covalent Hydration in Nitrogen Heteroaromatic Compounds II, Quantitative Aspects D. D. Perrin, Adv. Heterocycl. Chem., 1964, 4, 43-73. 

Complex metal hydrides, reduction of nitrogen heterocycles with, 6, 45 Covalent hydration... 

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