[OH:1][C:2]1[C:3]([CH3:18])=[C:4]([CH3:17])[C:5]2[O:10][C@@:9]([CH3:14])([C:11]([OH:13])=[O:12])[CH2:8][CH2:7][C:6]=2[C:15]=1[CH3:16].O.[C:20]1(C)C=CC(S(O)(=O)=O)=CC=1>CO.O>[OH:1][C:2]1[C:3]([CH3:18])=[C:4]([CH3:17])[C:5]2[O:10][C@@:9]([CH3:14])([C:11]([O:13][CH3:20])=[O:12])[CH2:8][CH2:7][C:6]=2[C:15]=1[CH3:16]A solution of 2 g (8 mmol) of optically pure (R)-(+)-3,4-dihydro-6-hydroxy-2,5,7,8-tetramethyl2 H-1-benzopyran-2-carboxylic acid (1) and 0.1 g of p-toluenesulphonic acid monohydrate in 40 ml methanol were stirred and re-fluxed for 4 hours. After cooling, the solution was diluted with water, extracted three times into ether that was subsequently washed with brine and aqueous sodium bicarbonate solution. The ether solution was washed, dried with MgSO4 and evaporated to give (R)-(+)-methyl 3,4-dihydro-6-hydroxy-2,5,7,8-tetramethyl-2 H- 1-benzopyran-2 carboxylate (2).
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