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Monomeric glyoxal was generated by heating a mixture of 8.0 g. of glyoxal hydrate (0.117 mol. of monomer) and 48.0 g. of phosphorus pentoxide over a gentle flame over 35 minutes. The liberated monomer was bubbled (argon stream) into 4.8 g. (0.04 mol) of 1-ethoxy-1,3-pentadiene (purity 92.3%) at 120°. After all the monomer had been generated, the solution was heated at 120° for an additional 45 minutes. Distillation afforded 1.32 g. of the pyran (b0.07 =56°, purity 91%) and 1.86 g. of recovered diene (yield based on diene-32%, based on glyoxal=6.6%).

1. temperatureby heating
2. additiona mixture of 8.0 g
3. customThe liberated monomer was bubbled (argon stream) into 4.8 g
4. customat 120°
5. temperaturethe solution was heated at 120° for an additional 45 minutes
6. distillationDistillation
7. customafforded 1.32 g