Reaction Steps
95% TFA (2 mL) was added dropwise to dimer compound 28a (77 mg, 0.074 mmol) at 0° C. which was stirred for 1 h and then poured into saturated aqueous NaHCO3 (30 mL). The mixture was extracted with chloroform (3×20 mL) and the organic layer was then washed with water (20 mL), brine (20 mL), dried (MgSO4) and filtered. The excess solvent was removed under reduced pressure to give the crude product, which was subjected to flash column chromatography (SiO2, 2% methanol-chloroform). Removal of excess eluent under reduced pressure without heating afforded the final product 29a (39 mg, 0.062 mmol, 84%) as a solid: [α]29D=+428° (c=0.07, CHCl3); 1H NMR (CDCl3, 400 MHz): δ 1.33-1.54 (m, 8H, 14-H, 15-H), 1.78-1.93 (m, 4H, 13-H), 2.86-2.97 (m, 2H, 1-H), 3.04-3.17 (m, 2H, 1-H), 3.83-3.89 (m, 2H, 11a-H), 3.93 (s, 6H, 7-OMe), 3.99-4.12 (m, 4H, 12-H), 4.28 (s, 4H, 3-H), 5.16-5.19 (m, 4H, 2a-H), 6.79 (s, 2H, 9-H), 7.49 (s, 2H, 6-H), 7.66 (d, 2H, J=4.42 Hz, 11-H); 13C NMR (CDCl3, 100 MHz): δ 25.2, 25.8, 28.8, 29.2, 35.5, 51.3, 53.8, 56.1, 58.4, 69.0, 109.3, 110.4, 111.4, 119.6, 140.7, 141.7, 147.9, 151.0, 162.4, 164.8; IR (neat): 3310, 2932, 2855, 1598, 1503, 1463, 1451, 1429, 1381, 1260, 1216, 1096, 1013, 911, 786, 729 cm−1; MS (ES+) m/z (relative intensity) 627 ([M+H]+., 100), 628 ([M+2H]+., 53), 332 (92), 323 (61).
