In a
125-ml. Claisen flask, equipped with a capillary for vacuum distillation and a
thermometer, are placed
50 g. (0.3 mole) of isodehydroacetic acid (p. 549) and
2 g. of copper powder (Note
1). A 125-ml. simple distilling flask, cooled by a
water jet, is used as a vacuum receiver and is attached to a
water aspirator. The Claisen flask is heated at atmospheric pressure in an
oil bath at 230–235° for 45 minutes or until the decarboxylation has ceased. The pressure on the system is then slowly reduced, and the
dimethylcoumalin is distilled directly from the reaction flask (Note
2). The crude
dimethylcoumalin (
34–35 g.,
92–95%) is redistilled from a Claisen flask. The yield of
4,6-dimethylcoumalin is
30–32 g. (
81–87%); b.p.
140–142°/35 mm. (Note
3); m.p.
50–51°.