Propanal → 4,4-Dimethoxy-2-methylbut-2-enal

Temperatur

40 °C

Propionic acid (24.7 mL) was added carefully to an aqueous solution of dimethylamine (40%, 41.6 mL), stirring and cooling (ice-water bath) to ensure the temperature of the mixture did not exceed 10° C. A pale yellow solution formed and the cooling bath was replaced with a heating mantle. From this point on, the temperature was controlled by a digital controller; a temperature probe was placed in the flask and the mantle was connected to the controller. The mixture was warmed to 40° C., the solution became colorless and dimethoxyacetaldehyde (aq. 60%, 100 mL) was added. The mixture was heated to 60° C. and propionaldehyde (42.26 g) was added dropwise at such a rate that the temperature did not exceed 68° C. (ca. 30 min). During the addition the color of the mixture changed, first from yellow to orange, and then to brown. The mixture was vigorously stirred and the temperature was maintained at 70° C. for 24 hours. The reaction mixture was allowed to cool down to a room temperature and then was extracted three times with dichloromethane (100 mL). The combined dichloromethane extracts were washed with water (150 mL). The organic layer was dried over anhydrous sodium sulfate (3 g) and the solvents were evaporated on a rotary evaporator at the room temperature. 117 g of a dark reddish-brown material was obtained. The crude product was distilled under reduced pressure (<0.5 mm Hg) using a modified version of a falling film apparatus at 78° C. (the b.p. of ethyl acetate, which was used as the heating medium). The clear distillate (of formula II, R1 and R2 are methyl; 101 g) was collected in a trap cooled in a dry ice bath.