Reaction Steps
To a solution of (6R,6aS,14aR)-1,6,8,14a-tetrahydroxy-11-((3R,4R,5R,6S)-4-hydroxy-3,5-dimethoxy-6-methyltetrahydro-2H-pyran-2-ylamino)-6a-methoxy-3-methyl-7,9,12,14-tetraoxo-5,6,6a,7,9,12,14,14a-octahydrobenzo[a]tetracene-2-carboxylic acid (80 mg, 0.12 mmol) in THF (1 mL) was added polystyrene-carbodiimide (208 mg, 0.24 mmol), 1-hydroxybenzotriazole hydrate (32 mg, 0.24 mmol) and benzyl alcohol (0.5 mL). The reaction mixture was stirred at room temperature under nitrogen for 17 h. The reaction was filtered through a micro filter and concentrated. The crude material was purified by preparative HPLC (10:90 acetonitrile/water to 60:40 acetonitrile/water with 0.05% TFA over 10 min, then isocratic) to afford (6R,6aS,14aR)-benzyl 1,6,8,14a-tetrahydroxy-11-((3R,4R,5R,6S)-4-hydroxy-3,5-dimethoxy-6-methyltetrahydro-2H-pyran-2-ylamino)-6a-methoxy-3-methyl-7,9,12,14-tetraoxo-5,6,6a,7,9,12,14,14a-octahydrobenzo[a]tetracene-2-carboxylate as a mixture of diastereomers (14 mg, 20%) and as a dark red solid: 1H NMR (500 MHz, CDCl3) δ 14.26 (s, 0.3H), 14.17 (s, 0.7H), 12.09 (s, 0.3H), 12.08 (s, 0.7H), 8.32 (br s, 6H), 8.27 (s, 0.3H), 8.21 (s, 0.7H), 6.86 (d, J=8.0 Hz, 1H), 6.52 (s, 1H), 5.87 (s, 0.7H), 5.86 (s, 0.3H), 5.28 (s, 2H), 4.96 (d, J=8.0 Hz, 1H), 4.67 (d, J=8.0 Hz, 1H), 3.80 (s, 3H), 3.74-3.70 (m, 2H), 3.61 (s, 3H), 3.58-3.53 (m, 2H), 3.40 (s, 3H), 3.37-3.30 (m, 3H), 3.13 (t, J=9.0 Hz, 1H), 2.41 (s, 2H), 3.39 (s, 1H), 1.36 (d, J=6.5 Hz, 3H); MS (ESI+) m/z 774 (M+H); HPLC >99% (AUC), tR 15.52 min.
