Régiosélectivité de l'hydrogénation catalytique de quelques pyrido[2,3-b]pyrazines

Catalytic hydrogenation on palladium–carbon in ethanol of several pyrido[2,3-b]pyrazines leads to mixtures of the 1,2,3,4- and 5,6,7,8-tetrahydro derivatives except in two cases: the reduction of the nonsubstituted heterocycle takes place on the pyrazine ring and, in the case of the 2,3-diphenyl derivative, the pyridine ring is reduced. In pure acetic acid this reduction provides only 1,2,3,4-tetrahydropyrido[2,3-b]pyrazines. A study of the reduction of 2,3-diphenylpyrido[2,3-b]pyrazine and several di- and tetrahydro derivatives as a function of solvent acidity suggests that the observed selectivity could result from the isomerization of partially hydrogenated intermediates. [Journal Translation]