A solution of (40) (0.53 g, 1.50 mmol) in isobutyric acid (1 mL) was added to a mixture of isobutyric acid (0.96 mL, 10.8 mmol) and N-methylmorpholine (1.14 mL, 10.4 mmol) at room temperature. The resulting reaction mixture was stirred at room temperature for 48 h. The reaction mixture was diluted with dichloromethane, washed successively with water, 10% aqueous NaHCO3 solution and brine, then dried over anhydrous Na2SO4. After filtration and removal of the solvent in vacuo, the residue was purified by flash chromatography on silica gel, eluting with 5% ethyl acetate in hexane to afford the title compound (41) as a mixture of two diastereomers (400 mg, 66%). 1H—NMR (CDCl3, 400 MHz): δ 0.85 (m, 6H), 1.14 (m, 8H), 1.44 (m, 3H), 1.62 (m, 1H), 2.13 (m, 1H), 2.32 (m, 2H), 2.51 (m, 1H), 3.06 (m, 1H), 3.24 (m, 1H) 4.95 (m, 1H), 5.10 (s, 2H), 6.76 (q, 1H).
