Functionalization of Oxazolo[4,5-b]pyrazines by Deprotometallation

Abstract

Different 2-arylated oxazolo[4,5-b]pyrazines, obtained by palladium(II)-catalyzed domino reaction from 2,3-dichloropyrazine and the corresponding carboxamides, were functionalized by deprotometallation. Employing lithium 2,2,6,6-tetramethylpiperidine, in order to form a lithio derivative, and then trapping it by iodolysis, proved to be inefficient. However, the presence of a zinc-based in situ trap allowed most substrates to be functionalized. Deprotonation of the pyrazine ring was observed in the presence of tolyl and anisyl groups at the oxazole 2-position. In contrast, with chlorophenyl and thienyl groups in this 2-position, deprotonation rather occurred on these groups either competitively or exclusively. The regioselectivities were discussed in the light of calculated pKa values of the substrates in THF. Finally, in the case of 2-phenyloxazolo[4,5-b]pyrazine, we converted the mixture of 5- and 6-iodinated products into the corresponding 5,6-diiodide, which was further functionalized by a double Suzuki coupling.