Reaction Steps
A solution of n-butyllithium (1.57 M, 18.3 mL, 28.7 mmol, 1.61 equiv) in hexanes was added dropwise to a solution of freshly distilled diisopropylamine (4.02 mL, 28.7 mmol, 1.61 equiv) in tetrahydrofuran (4.28 mL) at −78° C. After 5 min, the solution was allowed to warm to 0° C. After 35 min, the solution of lithium diisopropylamide was transferred via cannula to a solution of N-Boc-indole (6.24 g, 28.7 mmol, 1.61 equiv) and dimethyldichlorosilane (3.25 mL, 26.8 mmol, 1.50 equiv) in tetrahydrofuran (47.3 mL) at 0° C. After 8 h, the reaction mixture was transferred via cannula to a solution of tetracyclic bromide (+)-E2-16 (8.78 g, 17.83 mmol, 1 equiv) and 4-dimethylaminopyridine (4.68 g, 38.28 mmol, 2.15 equiv) in tetrahydrofuran (63.3 mL) at 23° C. After 14 h, the reaction mixture was diluted with ethyl acetate (300 mL) and washed with saturated aqueous sodium bicarbonate solution (500 mL). The aqueous layer was further extracted with ethyl acetate (2×200 mL). The combined organic layers were dried over anhydrous sodium sulfate, were filtered, and were concentrated under reduced pressure. The resulting orange foam was purified by flash column chromatography on silica gel (eluent: gradient, 33→50% ethyl acetate in hexanes) to provide the indole-tethered tetracycle E2-21 (10.1 g, 74.3%) as a white foam. Structural assignments were made using additional information from gCOSY, HSQC, and gHMBC experiments. 1H NMR (600 MHz, CDCl3, 20° C.): δ 8.03 (d, J=7.6, 2H, SO2Ph-o-H), 7.96 (d, J=8.3, 1H, C8′H), 7.63 (d, J=7.7, 1H, C5′H), 7.57 (d, J=8.1, 1H, C8H), 7.53 (t, J=7.4, 1H, SO2Ph-p-H), 7.44 (app-t, J=7.9, 2H, SO2Ph-m-H), 7.31 (app-t, J=7.4, 1H, C7′H), 7.27 (app-t, J=7.9, 1H, C7H), 7.26-7.21 (m, 1H, C5H), 7.26-7.21 (m, 1H, C6′H), 7.26-7.21 (m, 1H, C3′H), 7.00 (app-t, J=7.5, 1H, C6H), 6.33 (s, 1H, C2H), 5.13 (d, J=3.7, 1H, C12H), 4.30 (d, J=3.6, 1H, C1′H), 4.11 (d, J=17.2, 1H, C15Ha), 3.79 (d, J=17.2, 1H, C15Hb), 2.82 (s, 3H, C17H), 1.69 (s, 9H, CO2C(CH3)3), 0.54 (s, 3H, Si(CH3)2), 0.50 (s, 3H, Si(CH3)2). 13C NMR (150 MHz, CDCl3, 20° C.): δ 165.5 (C13), 165.2 (C16), 151.7 (C═Ocarbamate), 139.1 (C2′), 138.9 (C9), 137.8 (SO2Ph-ipso-C), 137.3 (C9—), 133.8 (SO2Ph-p-C), 131.4 (C4′), 131.3 (C4), 131.0 (C7), 129.0 (SO2Ph-m-C), 128.4 (SO2Ph-o-C), 125.6 (C6), 125.4 (C5), 124.9 (C7′), 122.7 (C6′), 121.5 (C5′), 121.1 (C3′), 116.5 (C8), 115.4 (C8′), 85.5 (C2), 84.6 (CO2C(CH3)3), 74.6 (C12), 69.6 (C3), 66.5 (C11), 54.3 (C15), 33.6 (C17), 28.3 (CO2C(CH3)3), 0.5 (Si(CH3)2), −0.4 (Si(CH3)2). FTIR (thin film) cm−1: 2957 (w), 1715 (s), 1466 (w), 1447 (w), 1373 (s), 1335 (m), 1251 (m), 1169 (s), 1072 (w), 922 (w), 834 (w), 751 (m), 689 (w). HRMS (ESI) (m/z): calc'd for C3H3BrN4O7SSi [M+H]+: 765.1408. found: 765.1441. TLC (50% ethyl acetate in hexanes), Rf: 0.44 (UV, CAM).
