[CH2:1]([O:3][C:4](=[O:19])[CH:5]([CH2:10][C:11](=O)[C:12]1[CH:17]=[CH:16][CH:15]=[CH:14][N:13]=1)[C:6](=[O:9])[CH2:7][CH3:8])[CH3:2].[OH-].[Na+].CCOCC>C(O)C>[CH2:1]([O:3][C:4]([CH:5]1[CH2:10][C:11]([C:12]2[CH:17]=[CH:16][CH:15]=[CH:14][N:13]=2)=[C:7]([CH3:8])[C:6]1=[O:9])=[O:19])[CH3:2]A solution of the diketoester 20b (3.1 g, 11.19 mmol) in dry ethanol (35 ml) was added dropwise to a solution of sodium hydroxide (0.447 g, 11.19 mmol) in dry ethanol (15 ml) with vigorous stirring. The solution was stirred at room temperature overnight. Ether (200 ml) was added and the organic phase was washed with 2 N HCl (3×100 ml). The aqueous layer was cooled to 0° C. and made slightly basic by addition of sodium bicarbonate. The aqueous layer was then extracted with ethyl acetate (3×300 ml) and the organic phase was dried over anhydrous sodium sulfate. The solvent was removed in vacuo to give an oil that was purified by flash column chromatography (EtOAc/hexanes, 1:10) to give the cyclic β-ketoester 21b (2.115 g, 73%) as a yellow oil: TLC Rf=0.35 (silica gel, 33% EtOAc/hexanes); 1H NMR (400 MHz, CDCl3) δ 8.59 (s, 1H), 7.62-7.60 (m, 1H), 7.54-7.52 (m, 1H), 4.24 (q, J=7.2 Hz, 2H), 3.56 (dd, J=2.8 Hz, 7.2 Hz, 1H), 3.42-3.25 (m, 2H), 2.41 (s, 3H), 2.12 (s, 3H), 1.31 (t, J=7.2 Hz, 3H); 13C NMR (75 MHz, CDCl3) δ 203.2, 169.5, 164.4, 151.4, 150.6, 136.8, 136.5, 134.2, 123.3, 61.7, 51.1, 32.5, 18.6, 14.3, 10.6; HRMS calcd for C15H17NO3+Na+ 282.1106; found 282.1109 [M+Na+].
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