The Monoacetalization of Glyoxal: A Direct Synthesis of 2, 2-Dimethoxy and Diethoxy Ethanals

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• A- A Soxhlet type extractor filled with anhydrous magnesium sulfate (60 g) or 4A molecular sieves (45 g) is fitted on a flask in which are stirred and refluxed 150 ml of chloroform, 14.5 g of 40 % aqueous glyoxal solution (0.1 mole), one mole of alcohol and 0.008 mole of zirconium sulfate. The reaction is monitored by GC and stopped after 3 h (ethanol) or 10 hours (methanol). The mixture is cooled, filtered, stirred a few minutes with solid NaHCO3 (15 g), filtered again and flash distilled (polymerized residue). Fractional distillation (oil bath, GC control) affords a 60–68 % yield of monoacetal containing a few percents of alcohol, followed by diacetal (15 to 20% yield). With 10 (ethanol) or 24 (methanol) hours reflux, diacetals are produced with 80% yield.
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• 2,2-diethoxyethanal: bp 60–65° C/4 mm. IR (neat) 1740 cm−1 1H NMR : 1.2 (t, 6H), 3.75 (d × q, 4H), 4.5 (d, J = 2Hz, 1H), 9.5 (d, J = 2Hz, 1H) 5a. 13C NMR: 14.5 (CH3), 62.3 (CH2O), 101.8 (OCHO), 196.0 (CH =O)
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• 2,2-dimethoxyethanal: bp 30° C/12 mm. IR (neat) 1740 cn−1 1H NMR : 3.4 (s, 6H), 4.48 (d, J = 1.5 Hz, 1H), 9.48 (d, J = 1.5 Hz, 1H) 5b. 13C NMR : 54.34 (CH3O), 102.5 (OCHO), 196.7 (CH =O).
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• B- A 40% aqueous solution of one mole of glyoxal is evaporated under reduced pressure, mixed with 10 moles of alcohol and 0,02 mole of zirconium sulfate, GC monitored, stirred and refluxed two (ethanol) or four (methanol) hours, treated as above (in the case of methanol the diacetal can be azeotropically extracted with cyclohexane) and affords monoacetal (45 to 57% yield), partly combined with alcohol (5 to 10 %), along with diacetal (3 to 9% yield).
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• French Patent Sté Française Hoechst 1986, 86–07957, 87–06106
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