D. N. Kozhevnikov

Transformations of 1,2,4-Triazines in Reactions with Nucleophiles: V. SNH and ipso-Substitution in the Synthesis and Transformations of 5-Cyano-1,2,4-triazines

The scope of functionalization of 1,2,4-triazines can be considerably extended via successive nucleophilic substitution of hydrogen (SNH) and ipso-substitution. A convenient procedure has been developed for direct cyanation of 1,2,4-triazine 4-oxides with acetone cyanohydrin in the presence of triethylamine. The cyano group in the resulting 5-cyano-1,2,4-triazines is readily replaced by reactions with various aliphatic alcohols and amines.

From 3-chloromethyl-1,2,4-triazine 4-oxides to various substituted pyridines and 1,2,4-triazines

An efficient strategy for the synthesis of new pyridine and 1,2,4-triazine derivatives starting from available 6-aryl-3-chloromethyl-1,2,4-triazine 4-oxides was proposed. The deoxygenative nucleophilic hydrogen substitution in the triazine-oxide ring, nucleophilic substitution of the chlorine atom in the side chain, and transformations of the 1,2,4-triazine ring into the pyridine ring via the inverse-electron-demand Diels-Alder reactions, being used in different orders, are a rather flexible tool for the functionalization of the titled heterocycles. The cyanide anion, indoles, thiophenols, amines, and triphenylphosphine were used as nucleophiles. The direct introduction of indole residues into the 1,2,4-triazine ring followed by the substitution of the chlorine atom by a residue of the primary or secondary aliphatic amine was found to be the most convenient method for the library synthesis.

SNH reactions of pyrazine N-oxides and 1,2,4-triazine 4-oxides with CH-active compounds

Nucleophilic substitution of hydrogen in pyrazine N-oxides under the action of CH-active compounds requires activation with acylating agents. This activation facilitates aromatization of intermediate σH adducts via elimination of the acid residue to form substituted pyrazines. More electrophilic 1,2,4-triazine 4-oxides react with carbanions derived from CH-active compounds without additional activation according to a scheme, which has previously been unknown for azine N-oxides. This scheme involves aromatization of σH adducts through elimination of water by the E1cb mechanism. The reaction products occur in DMSO-d6 solutions predominantly as 6-methylene-1,6-dihydropyrazines and 5-methylene-4,5-dihydro-1,2,4-triazines.

SNH reactions of 1,2,4-triazine N-oxides, pyrazine N-oxides, and pterin N-oxides with arenethiols*

Abstract1,2,4-Triazine 4-oxides were found to enter into the reactions of nucleophilic substitution of hydrogen with S-nucleophiles (arenethiols) in the presence of acylating agents and trifluoroacetic acid. The reactions proceeded with loss of the N-oxide function to form 5-arylthio-1,2,4-triazines. 2-Amino-3-ethoxycarbonylpyrazine 1-oxides and 2-amino-4-oxopterin 8-oxides react with arenethiol analogously.

Nucleophilic substitution or dipolar 1,3-cycloaddition in reactions of cyanamide with 4-arylpyrimidine 1-oxides

Pyrimidine 1-oxides with cyanamide afforded 2-ureidopyrimidines as the result of the nucleophilic substitution of hydrogen, whereas the formation of similar 2-trichloroacetylaminopyrimidines occurs as dipolar 1,3-cycloaddition of the same oxides to trichloroacetonitrile under much more drastic conditions and in lower yields.

Nucleophilic substitution of hydrogen in the reaction of 1,2,4-triazin-4-oxides with cyanamide

It was shown that cyanamide can successfully be used in reactions of nucleophilic substitution of hydrogen with 1,2,4-triazin-4-oxides in the presence of a base to give 5-cyanoimino-1,2,4-triazines. It was found by13C NMR spectroscopy that these compounds and their alkylation products at the cyclic nitrogen atom exist in the form of 5-cyanoimino-2,5-dihydro-1,2,4-triazines.