A mixture of
400 g. of polyphosphoric acid and
250 ml. of glacial acetic acid is heated to reflux in a
2-l. round-bottomed flask equipped with a stirrer, reflux condenser, and thermometer.
Dibenzyl ketone (42.0 g., 0.200 mole) (Note
1) is then added, and the reaction mixture is heated at reflux (130–135°) for 1.5 hours. The solution is cooled to 30° in an
ice water bath, and 1 l. of water is added slowly with stirring. The brown precipitate which forms is collected by filtration, washed with 1 l. of water, and dissolved in
1 l. of hot benzene. The hot
benzene solution is treated with
2 g. of activated carbon, filtered hot through a pad of diatomaceous earth, dried with
10 g. of magnesium sulfate, decanted from the
magnesium sulfate, and concentrated to 500 ml. On addition of
450 ml. of hexane and cooling to 5–10°, tan crystals of the crude pyranone separate. Filtration affords
25–27 g. (
45–49%) of product melting at
202–206°. For purification this material is dissolved in
500 ml. of hot benzene, treated with
1 g. of activated carbon as before, and precipitated from solution by the addition of
250 ml. of hexane and cooling of the mixture to 5–10°. On filtration
19–21 g. (
34–38%) of
2,6-dimethyl-3,5-diphenyl-4H-pyran-4-one is obtained. This material melts sharply at
207–209° (Note
2). An additional quantity (
3–4 g.,
5–7%) of somewhat less pure product (m.p.
204–206°) may be recovered by evaporation of the filtrate and recrystallization of the residue from
200 ml. of benzene-hexane (50% benzene by volume).