Process Development and Multikilogram-Scale Synthesis of a TRPV1 Antagonist

The process development and multikilogram preparation of a TRPV1 antagonist, 1, is described. Pyrido[2,3-b]pyrazine 1 was prepared in a convergent manner by the coupling of two key fragments, glyoxal 2 and diamine 3. Glyoxal 2 was synthesized in six chemical steps in 20% overall yield, the key step being a challenging Grignard reaction to install the glyoxalate moiety. Diamine 3 was also prepared in six chemical steps in 46% overall yield, exploiting a regioselective nucleophilic aromatic substitution to obtain the key nitrodiamine intermediate 19.