Unambiguously confirmed structure of 2‐phenacylidene‐1,2‐dihydro‐4<i>H</i>‐pyrido[2,3‐<i>b</i>]pyrazin‐3‐one and 3‐phenacylidene‐3,4‐dihydro‐1<i>H</i>‐pyrido[2,3‐<i>b</i>]pyrazin‐2‐one

<jats:title>Abstract</jats:title><jats:p>To determine the structures of two isomeric products, 2‐phenacylidene‐1,2‐dihydro‐4<jats:italic>H</jats:italic>‐pyrido[2,3‐<jats:italic>b</jats:italic>]pyrazin‐3‐one <jats:bold>(2)</jats:bold> and 3‐phenacylidene‐3,4‐dihydro‐1<jats:italic>H</jats:italic>‐pyrido[2,3‐<jats:italic>b</jats:italic>]pyrazin‐2‐one <jats:bold>(3)</jats:bold> obtained by condensation of 2,3‐diaminopyridine (1) with ethyl benzoylpyruvate [1–3], these compounds were hydrolyzed to give 2‐methyl‐4<jats:italic>H</jats:italic>‐pyrido[2,3‐<jats:italic>b</jats:italic>]pyrazin‐3‐one <jats:bold>(4)</jats:bold> and 3‐methyl‐1<jats:italic>H</jats:italic>‐pyrido[2,3‐<jats:italic>b</jats:italic>]pyrazin‐2‐one <jats:bold>(5)</jats:bold>, respectively [4,5]. Both hydrolysates <jats:bold>4</jats:bold> and <jats:bold>5</jats:bold> were hydrogenated to afford 2‐methyl‐1,2‐dihydro‐4<jats:italic>H</jats:italic>‐pyrido[2,3‐<jats:italic>b</jats:italic>]pyrazin‐3‐one <jats:bold>(6)</jats:bold> and 3‐methyl‐3,4‐dihydro‐1<jats:italic>H</jats:italic>‐pyrido[2,3‐<jats:italic>b</jats:italic>]pyrazin‐2‐one <jats:bold>(7)</jats:bold>. The latter compound was identical with an unequivocally synthesized compound providing proof for the structures of all these compounds.</jats:p>