[CH2:1]([O:3][C:4](=[O:26])[CH:5]([O:23][CH2:24][CH3:25])[CH:6]([C:8]1[CH:13]=[CH:12][C:11]([O:14][CH2:15][C:16]2[CH:21]=[CH:20][CH:19]=[CH:18][CH:17]=2)=[CH:10][C:9]=1[CH3:22])O)[CH3:2].S(=O)(=O)(O)O>CN(C)C=O>[CH2:1]([O:3][C:4](=[O:26])/[C:5](/[O:23][CH2:24][CH3:25])=[CH:6]/[C:8]1[CH:13]=[CH:12][C:11]([O:14][CH2:15][C:16]2[CH:17]=[CH:18][CH:19]=[CH:20][CH:21]=2)=[CH:10][C:9]=1[CH3:22])[CH3:2]To a solution of 3-(4-benzyloxy-2-methyl-phenyl)-2-ethoxy-3-hydroxy-propionic acid ethyl ester (mixture of stereoisomers; 48.8 g, 136.2 mmol) in N,N-dimethylformamid (500 ml) was added sulfuric acid (19.6 ml, 96%). The reaction mixture was heated to 100° C. for 2.5 h, cooled to ambient temperature, poured onto ice water/saturated aqueous NaHCO3 solution 1/1 and extracted two times with ethyl acetate. The combined extracts were washed with ice water/brine 1/1 and dried over sodium sulfate. The solvent was removed under reduced pressure to give 46.1 g (135.4 mmol) of crude (Z)-3-(4-benzyloxy-2-methyl-phenyl)-2-ethoxy-acrylic acid ethyl ester which was used in the next step without further purification.
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