[CH3:1][O:2][C:3]([NH:5][C@@H:6]([CH:19]([CH3:21])[CH3:20])[C:7]([N:9]1[CH2:13][C:12](=[O:14])[CH2:11][C@H:10]1[C:15]([O:17][CH3:18])=[O:16])=[O:8])=[O:4].[CH2:22](O)[CH2:23][OH:24].C1(C)C=CC=CC=1.O.C1(C)C=CC(S(O)(=O)=O)=CC=1>CCOC(C)=O>[CH3:1][O:2][C:3]([NH:5][C@@H:6]([CH:19]([CH3:21])[CH3:20])[C:7]([N:9]1[C@H:10]([C:15]([O:17][CH3:18])=[O:16])[CH2:11][C:12]2([O:24][CH2:23][CH2:22][O:14]2)[CH2:13]1)=[O:8])=[O:4]
In a 50 mL round-bottomed flask, (S)-methyl 1-((S)-2-(methoxycarbonylamino)-3-methylbutanoyl)-4-oxopyrrolidine-2-carboxylate (230 mg, 766 μmol, Eq: 1.00) and ethane-1,2-diol (238 mg, 214 μl, 3.83 mmol, Eq: 5) were combined with toluene (15 ml) to give a colorless solution. p-toluenesulfonic acid monohydrate (29.1 mg, 153 μmol) was added and the reaction mixture was heated to 110° C. with a Dean-Stark apparatus for 20 hr. After cooling, it was diluted with EtOAc (50 ml), washed with NaHCO3 (sat solution, 200 ml), dried and concentrated in vacuo. The crude mixture was purified on a silica gel column (CH2Cl2, 20%, 30%, 40% and 60% EtOAc/CH2Cl2) to afford (S)-methyl 7-((S)-2-(methoxycarbonylamino)-3-methylbutanoyl)-1,4-dioxa-7-azaspiro[4.4]nonane-8-carboxylate as an oil (110 mg, 41.7%). 1H NMR (400 MHz, CHLOROFORM-d) 6 ppm 0.95 (d, J=6.78 Hz, 8H) 1.05 (d, J=6.78 Hz, 7H) 1.57 (s, 2H) 2.22 (dd, J=13.18, 6.90 Hz, 3H) 2.37 (dd, J=13.05, 8.78 Hz, 2H) 3.62-3.70 (m, 11H) 3.71-3.87 (m, 10H) 3.93-4.03 (m, 11H) 4.25 (d, J=2.76 Hz, 4H) 4.59-4.78 (m, 2H) 5.23-5.51 (m, 2H) 5.26-5.51 (m, 2H)
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