Total synthesis of malagashanine: a chloroquine potentiating indole alkaloid with unusual stereochemistry

A stereoselective cascade annulation reaction generates the tetracyclic core of the Malagasy alkaloids, facilitating the total synthesis of malagashanine.


II.
Procedures and Characterization S2 III. Structural assignments for 14 and 26 S25 IV. Attempted reductions of 28 S28 V.
NMR Spectra S30 I. General Information 1 H and 13 C NMR spectra were recorded on a Varian Inova 600 spectrometer (600 MHz 1 H, 150 MHz 13 C), a Varian Unity plus 600 spectrometer (600 MHz 1 H, 150 MHz 13 C), and a Varian Inova 400 spectrometer (400 MHz 1 H, 100 MHz 13 C) at room temperature in CDCl 3 (neutralized and dried using anhydrous K 2 CO 3 ) with internal CHCl 3 as the reference (7.26 ppm for 1 H and 77.23 ppm for 13 C), unless otherwise stated. Chemical shifts (δ values) were reported in parts per million (ppm) and coupling constants (J values) in Hz. Multiplicity was indicated using the following abbreviations: s = singlet, d = doublet, t = triplet, q = quartet, qn = quintet, m = multiplet, b = broad. Infrared (IR) spectra were recorded using Thermo Electron Corporation Nicolet 380 FT-IR spectrometer. High-resolution mass spectra were obtained using a Thermo Electron Corporation Finigan LTQFTMS (at the Mass Spectrometry Facility, Emory University). Melting points were taken using a Fisher Johns melting point apparatus and are uncorrected. Analytical thin layer chromatography (TLC) was performed on precoated glass backed EMD 0.25 mm silica gel 60 plates. Visualization was accomplished with UV light, ethanolic anisaldehyde, or KMnO 4 . Flash column chromatography was carried out using Silicycle SilaFlash® F60 silica gel (40-63 µm). All reactions were conducted using anhydrous solvents in oven dried and nitrogen charged glassware. Anhydrous solvents were obtained by passage through activated alumina using a Glass Contours solvent purification system unless otherwise noted. Solvents used in workup, extraction and column chromatography were used as received from commercial suppliers without further purification. All reagents were purchased from Sigma Aldrich or Strem and used as received unless otherwise noted. 4Å powdered molecular sieves were activated by heating to 100 °C under reduced pressure (0.2 torr) for at least 12 hours. We acknowledge the use of shared instrumentation provided by grants from the NIH and the NSF.

Sulfonamide S-5:
Carbamate S-4 was prepared according to modified version of a previously reported procedure. Et 3 N (6.3 mL, 45.0 mmol, 1.5 equiv.) was added to a solution of tryptamine (4.8 g, 30.0 mmol, 1.0 equiv.) in THF (150.0 mL) at 0 °C. A solution of Boc 2 O (7.2 g, 33.0 mmol, 1.1 equiv.) in THF (50.0 mL) was added over 15 minutes via cannula. The mixture was gradually warmed to room temperature and stirred overnight. The reaction mixture was concentrated under reduced pressure, and the residue was filtered through a pad of silica gel (eluted with 1:1 hexanes/EtOAc). The filtrate was concentrated, and the resultant residue was used in the next step without further purification. A mixture of the crude residue, powdered NaOH (3.0 g, 75.0 mmol, 2.5 equiv.) and Bu 4 NHSO 4 (1.0 g, 3.0 mmol, 0.1 equiv.) in CH 2 Cl 2 (200.0 mL) was cooled to 0 °C. BnBr (5.6 g, 30.0 mmol, 1.0 equiv.) was slowly added. The mixture was gradually warmed to room temperature and stirred overnight. The reaction was quenched with water (40.0 mL), and the layers were separated. The aqueous phase was extracted with CH 2 Cl 2 (2 x 50.0 mL). The combined organic extracts were washed with brine (2 x 30.0 mL), dried over anhydrous Na 2 SO 4 , filtered, and concentrated under reduced pressure. Purification by flash column chromatography on silica gel (10:1 to 4:1 hexanes/EtOAc) provided carbamate S-4 (9.5 g, 87 % over two steps) as a colorless oil. Spectral data matched that previously reported in the literature. R f 0.80(hexanes/EtOAc, 2:1).

Allylsilane 15:
(Trimethylsilyl)methylmagnesium chloride solution (1.0 M in Et 2 O, 40.7 mL, 40.7 mmol, 3.0 equiv.) was added to anhydrous ZnBr 2 (9.79 g, 43.5 mmol, 3.2 equiv.), and the resulting suspension was stirred vigorously for 14 hours. DMF (30.0 mL) was added, followed by Et 2 O (10.0 mL), and the mixture was stirred for ten minutes. A solution of carboxylic acid 18 (4.51 g, 13.6 mmol, 1.0 equiv.) in DMF (25.0 mL) was added via cannula, and the resulting suspension was cooled to 0 °C. A solution of Pd(CH 3 CN) 2 Cl 2 (0.352 g, 1.36 mmol) in DMF (5.0 mL) was added over five minutes, and the resulting mixture was stirred for two hours, warmed to room temperature, and stirred for 30 minutes. The reaction mixture was cooled to 0 °C and quenched with saturated aqueous NH 4 Cl (100.0 mL). EtOAc (200.0 mL) was added, and the mixture was stirred for 15 minutes. The layers were separated, and the aqueous phase was extracted with EtOAc (3 x 125.0 mL). The combined organic layer was washed with brine (300.0 mL), filtered through celite, dried over anhydrous MgSO 4 , filtered, and concentrated under reduced pressure. Crude allylsilane 15 (3.76 g, 93 %) was obtained as a yellow oil, and was used without further purification.

S18
Ester 25: To a solution of trifluoromethylketone 24 (97.7 mg, 0.17 mmol, 1.0 equiv.) in benzene (2.5 mL), was added powdered KOH (17.0 mg, 0.30 mmol, 1.8 equiv.) and H 2 O (15.0 μL). The reaction mixture was heated to reflux overnight (oil bath temperature of 100 °C). The light yellow suspension was cooled to room temperature. EtOAc (20.0 mL) was added, and the pH of the mixture was adjusted to 7.0 by the slow addition of 0.5 N HCl. The layers were separated, and the aqueous phase was extracted with EtOAc (3 x 30.0 mL). The combined organic layer was washed with brine (20.0 mL), dried over anhydrous Na 2 SO 4 , filtered, and concentrated under reduced pressure. The crude residue was brought up in MeOH (2.0 mL) and toluene (3.0 mL). Excess TMSCHN 2 (2M in Et 2 O, 170.0 μL) was added drop-wise, and the resulting mixture was stirred at room temperature for one hour. The reaction mixture was quenched by the slow addition of AcOH (1.0 mL). The mixture was concentrated under reduced pressure, and EtOAc (30.0 mL) and saturated NaHCO 3 solution (10.0 mL) were added to the residue. The layers were separated, and the aqueous phase was extracted with EtOAc (3 x 20.0 mL). The combined organic layer was washed with brine (20.0 mL), dried over anhydrous Na 2 SO 4 , filtered, and concentrated under reduced pressure. Purification by flash column chromatography on silica gel (1:1 hexanes/EtOAc) afforded Ester 25 (72.1 mg, 79 % over two steps).

Malagashanine 1:
MeOH (3.0 mL) was added to a vial charged with methylamine 28 (6.0 mg, 0.015 mmol) and Raney nickel (excess). The heterogeneous mixture was then submitted to hydrogen gas (110 bar) for 5 days in a Parr high-pressure vessel. The pressure was carefully released, and the mixture was filtered through celite. The filter cake was washed with MeOH (3 x 10.0 mL). The filtrate was concentrated under reduced pressure, and then EtOAc (15.0 mL) was added. The solution was washed with brine (3 x 10.0 mL), dried over anhydrous Na 2 SO 4 , filtered, and concentrated under reduced pressure. Purification by short flash column chromatography on silica gel (93:7 CH 2 Cl 2 /methanol with 0.5% NH 4 OH (aq) ) afforded malagashanine as a white amorphous solid (5.8 mg, 97 %). 13.2, 6.6 Hz), 1.62 -1.60 (m, 1H), 1.48 -1.47 (m, 1H), 1.23 (d, 3H, J = 6.6 Hz) ppm; 13 C NMR (CDCl 3 , 150 MHz) δ 174. 4, 168.7, 141.0, 138.1, 127.5, 126.0, 124.3, 119.5, 75.0, 68.4, 66.4, 64.6, 55.2, 52.9, 51.8, 48.6, 41.5, 38.7, 37.0, 36.3, 28.3, 24.1, 18.5 ppm; IR (thin film, cm -1 ) 2928,2851,1731,1655,1460,1396,1161,1106,1033,757 The NMR spectra of synthetic malagashanine showed a set of major peaks with a set of minor peaks (major peaks : minor peaks = 5:1). These are assigned as acetamide rotamers (VT 1 H NMR shows broadening, but not full coalescence at 90 °C). The major peaks were compared with the data from the isolation paper (   The 13 C NMR data of our synthetic sample are in excellent agreement with those from the isolation paper (Table 2.2). 1 The 1 H NMR data also matched well with the data reported in the isolation paper. Of particular importance, the data characterizing the syn relationship between C(19) and C(20) matched well with the reported data. 1 Moreover, the small coupling constant (3.0 Hz) between C(19) and C(20) protons was expected for this type of J 3 coupling because the dihedral angle was close to 90 o . However, there were three discrepancies between the data of our synthetic sample and those from the isolation paper. We believe that they are most likely the result of typographical errors in the original paper. The first major inconsistency was the coupling constant for the proton on C(14). The reported data contained a coupling constant of 3.4 Hz while our data contained a coupling constant of 13.2 Hz. This proton is expected to have a J 2 coupling, which is usually larger than 12 Hz (based on dihedral angles from the crystal structure). 2 Therefore, we believe the 3.4 Hz reported from the isolation paper was a typographical error. (We speculate, it might be 13.4 Hz, with the "1" being accidently omitted). The other two inconsistencies were the coupling constants for the two protons on C(17). The reported data contained coupling constants of 2.5 Hz for both protons while our data contained coupling constants of 12.6 Hz. We believe that these discrepancies were caused by the same type of mistake as the proton on C(14) because these protons should also have a large J 2 coupling constants (based on dihedral angles from the crystal structure). In support of the hypothesis that these are likely to be typographical errors, coupling constants are usually reported in descending order. All the other coupling constants reported in the isolation manuscript adhere to this standard formatting. However, these three coupling constants in question are outliers, and do not match this standard convention. On the basis of the analysis above, we conclude that the synthetic sample is identical to the natural product malagashanine.  Crystal data and structure refinement for dm-ii-253_sq.             30  40  50  60  70  80  90  100  110  120  130  140  150  160  170  180  190  200  210  220  230 chemical shift (ppm)