[CH3:1][C:2]1[CH:3]=[C:4]([CH:7]=[CH:8][C:9]=1[CH3:10])[CH:5]=O.C(O)(=O)[CH2:12][C:13]([OH:15])=[O:14].N1CCCCC1.Cl>N1C=CC=CC=1.O>[CH3:1][C:2]1[CH:3]=[C:4]([CH:5]=[CH:12][C:13]([OH:15])=[O:14])[CH:7]=[CH:8][C:9]=1[CH3:10]
A suspension of 3,4-dimethylbenzaldehyde (15.000 g; 111.793 mmol) and malonic acid (22.103 g; 212.410 mmol) in pyridine (85 ml) was heated to 50° C., under nitrogen. Then piperidine (8.5 ml; 86.079 mmol) was added dropwise (over 5 minutes) and the resulting suspension was heated to 75° C. for 2 h. The reaction mixture was cooled to 0° C., and poured into an ice-cooled solution of concentrated hydrochloric acid (12 N; 96 ml) in water (1200 ml). The precipitated colorless product was filtered off, and washed with water (3×100 ml). Remaining water was evaporated under reduced pressure, then under HV to give the dried product 3-(3,4-dimethyl-phenyl)-acrylic acid as a colorless solid (19.230 g; 98%). LC-MS: tR=0.88 min; [M+H]+: no ionisation.
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