Reaction SMILES: [CH2:1]([N:5]1[C:9](=[O:10])[C:8]2=[CH:11][C:12]([CH3:15])=[CH:13][CH:14]=[C:7]2[C:6]1=[O:16])[CH:2]([CH3:4])[CH3:3].O>CO.[BH4-].[K+]>[OH:10][CH:9]1[C:8]2[C:7](=[CH:14][CH:13]=[C:12]([CH3:15])[CH:11]=2)[C:6](=[O:16])[N:5]1[CH2:1][CH:2]([CH3:4])[CH3:3].[OH:16][CH:6]1[C:7]2[C:8](=[CH:11][C:12]([CH3:15])=[CH:13][CH:14]=2)[C:9](=[O:10])[N:5]1[CH2:1][CH:2]([CH3:4])[CH3:3]
Reactant: Cc1ccc2c(c1)C(=O)N(CC(C)C)C2=O
Agent: [BH4-]
Agent: [K+]
Solvent: O
Solvent: CO
Product: Cc1ccc2c(c1)C(=O)N(CC(C)C)C2O
Product: Cc1ccc2c(c1)C(O)N(CC(C)C)C2=O
Temperature: 20 °C
Reaction time: 16 h

Procedure:
3-Hydroxy-2-isobutyl-5-methyl-2,3-dihydroisoindol-1-one and 3-hydroxy-2-isobutyl-6-methyl-2,3-dihydroisoindol-1-one are prepared as described in Example 1, starting with 6.7 g of N-isobutyl-4-methylphthalimide in 65 cm3 of methanol and 1.7 g of potassium borohydride. The reaction mixture is stirred at a temperature in the region of 20° C. for 16 hours, 0.3 g of potassium borohydride is then added, and the reaction mixture is stirred at a temperature in the region of 20° C. for two hours. The mixture is then cooled to a temperature, in the region of 0° C., and 40 cm3 of distilled water are added dropwise. The precipitate obtained is filtered off, and the filtrate is then concentrated to dryness under reduced pressure (2 kPa) at a temperature in the region of 25° C. The residue is purified by chromatography under a pressure of argon (50 kPa), on a column of silica gel (particle size 40-63 μm), eluting successively with mixtures of cyclohexane/ethyl acetate (70/30; 60/40 by volume). The fractions containing a mixture of the two expected products are combined and concentrated to dryness under reduced pressure (2 kPa) at a temperature in the region of 25° C. 2.27 g of a mixture of 3-hydroxy-2-isobutyl-5-methyl-2,3-dihydroisoindol-1-one and 3-hydroxy-2-isobutyl-6-methyl-2,3-dihydroisoindol-1-one are thus obtained in the form of a white solid (Rf=0.54, thin layer chromatography on silica gel, eluent: cyclohexane/ethyl acetate (50/50 by volume)). A second batch starting with 3.5 g of N-isobutyl-4-methylphthalimide in the same way yields 2.2 g of a mixture of 3-hydroxy-2-isobutyl-5-methyl-2,3-dihydroisoindol-1-one and 3-hydroxy-2-isobutyl-6-methyl-2,3-dihydroisoindol-1-one in the form of a white solid. The two batches are combined and then separated by HPLC chromatography on a system comprising two columns of 60 mm in diameter in series containing, respectively, 700 g and 475 g of CHIRALPAK AS chiral stationary phase with a particle size of 20 μm, eluting with a heptane/isopropanol mixture (90/10 by volume) at a flow rate of 90 ml/min. The first and second isomers eluted correspond to the first pair of regioisomers. The third and fourth isomers eluted correspond to the second pair of regioisomers. Three injections of 1 g, 1.6 g and 1.7 g, respectively, were performed under these conditions. Concentrating the fractions under reduced pressure (1 kPa) at a temperature in the region of 40° C. gives the following: 0.77 g of (+)-3hydroxy-2-isobutyl-5-methyl-2,3-dihydroisoindol-1-one in the form of a white solid, (αD20=+17.30°±0.80° in methanol, at a concentration of 0.5%), 1.09 g of (+)-3-hydroxy-2-isobutyl-6-methyl-2,3-dihydroisoindol-1-one in the form of a white solid, (αD20=+23.20°±0.70° in methanol, at a concentration of 0.5%), 1.17 9 of (−)-3-hydroxy-2-isobutyl-6-methyl-2,3-dihydroisoindol-1-one in the form of a white solid (αD20=−20.1°±1.80° in methanol, at a concentration of 0.5%), and 0.85 g of (−)-3-hydroxy-2-isobutyl-5-methyl-2,3-dihydroisoindol-1-one in the form of a white solid, (αD20=−15.60°±0.60° in dichloromethane, at a concentration of 0.5%).