Reaction Steps
95% TFA (3 mL) was added drop-wise to dimer compound 15a (75 mg, 0.08 mmol) at 0° C. This was then stirred for 1 h and the mixture was poured into saturated NaHCO3 (30 mL) solution to naturalize the reaction mixture. The mixture was extracted with chloroform (3×20 mL). The organic layer was then washed water (20 mL), brine (20 mL) then dried (MgSO4) and filtrated. 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 16a (30 mg, 0.05 mmol, 70%) as a solid: [α]20D=+477° (c=0.11, CHCl3); 1H NMR (CDCl3, 400 MHz): δ 2.01-2.06 (m, 4H, 2-H, 2′-H), 2.28-2.31 (m, 4H, 1-H, 1′-H), 2.39-2.42 (m, 2H, 13-H), 3.55-3.60 (m, 2H, 3-H, 3′-H), 3.67-3.73 (m, 2H, 11a-H, 11a′-H), 3.77-3.87 (m, 2H, 3-H, 3′-H), 3.91 (s, 6H, 7-OMe, 7′-OMe), 4.23-4.31 (m, 4H, 12-H, 12′-H), 6.84 (s, 2H, 9-H, 9′-H), 7.50 (s, 2H, 6-H, 6′-H), 7.64 (d, 2H, J=4, 11-H, 11′-H); 13C NMR (CDCl3, 400 MHz): δ 24.1, 28.4, 29.6, 46.6, 53.6, 56.1, 58.4, 65.4, 110.8, 111.6, 120.4, 140.6, 147.8, 150.6, 162.3, 164.6; IR (neat): 3350, 2951, 1600, 1505, 1434, 1262, 1217, 1021 cm−1; MS (FAB) m/z (relative intensity) 597 ([M+2×MeOH]+., 16), 565 ([M+MeOH]+., 5), 533 (M+., 100).
