3.10. Lactone 11

PCC (10 mol%, 150 μL of a 0.01 M stock solution in acetonitrile) was added to a vigorously stirred suspension of H₅IO₆ (15 eq., 0.022 mmol, 49.8 mg) in acetonitrile (100 μL) at room temperature. After 5 min., compound 4 (9.0 mg, 0.015 mmol) dissolved in acetonitrile (100 μL + 2 × 100 μL rinse) was added to give a yellow cloudy mixture. After 16 h TLC analysis (n-hexane-EtOAc, 8:2) revealed the presence of a product at Rf = 0.3. No further progress of the process was observed after 2 h. Therefore, freshly prepared CCP (12 mol%, 180 μL of a 0.01 M stock solution in acetonitrile) was added to the reaction mixture. After 15 min., TLC analysis revealed the further progress of the process, but successive TLC analyses indicated no further consumption of the starting material. Two successive additions of the same amounts of CCP were required to drive the process to completion (overall further 2 h). Then, EtOH (1 mL) was added and the mixture was taken to dryness. Filtration of this material on a short pad of silica gel (eluent: CHCl₃-MeOH, 95:5) followed by HPLC purification (n-hexane-EtOAc, 7:3) gave pure acetate 11 (4.1 mg, 55%) as a colourless oil. [α]D20 = +27.7 (c = 0.13, CHCl₃). IR (neat) λ_max 1794, 1745, 1225 cm⁻¹. ¹H-NMR: (700 MHz, CDCl₃): δ 5.46 (1H, dd, J = 5.1, 3.3), 4.85 (1H, dd, J = 9.8, 3.3,), 4.75 (1H, dd, J = 9.8, 1.7), 4.19 (1H, ddd, J = 12.1, 9.5, 4.6), 3.75 (1H, ddd, J = 12.0, 10.5, 4.3), 3.42 (1H, ddd, J = 10.5, 8.4, 2.5), 3.26 (1H, dd, J = 9.5, 1.7), 3.01 (1H, ddd, J = 12.9, 4.6, 4.3), 2.95 (1H, dd, J = 18.1, 5.1), 2.69 (1H, d, J = 18.1), 2.41 (1H, ddd, J = 12.9, 12.1, 12.0), 2.17 (3H, s, acetate), 2.04 (1H, m), 1.59 (1H, m), 0.96 (3H, t, J = 7.4). ¹³C-NMR: (176 MHz, CDCl₃): δ 171.9, 169.8, 83.7, 83.0, 79.3, 69.5, 49.8, 46.7, 46.1, 44.9, 37.9, 25.7, 21.0, 9.5. HRESIMS m/z: calcd for C₁₄H₁₉⁷⁹Br₃NaO₅ 526.8680 [M + Na]⁺, found: 525.8671.