Parallel and four-step synthesis of natural-product-inspired scaffolds through modular assembly and divergent cyclization

By emulating the universal biosynthetic strategy, which employs modular assembly and divergent cyclizations, we have developed a four-step synthetic process to yield a collection of natural-product-inspired scaffolds. Modular assembly of building blocks onto a piperidine-based manifold 6, having a carboxylic acid group, was achieved through Ugi condensation, N-acetoacetylation and diazotransfer, leading to cyclization precursors. The rhodium-catalyzed tandem cyclization and divergent cycloaddition gave rise to tetracyclic and hexacyclic scaffolds by the appropriate choice of dipolarophiles installed at modules 3 and 4. A different piperidine-based manifold 15 bearing an amino group was successfully applied to demonstrate the flexibility and scope of the unified four-step process for the generation of structural diversity in the fused scaffolds. Evaluation of in vitro antitrypanosomal activities of the collections and preliminary structure–activity relationship (SAR) studies were also undertaken.


S2
General: NMR spectra were recorded on JEOL JNM-ECX 400 ( 1 H/400 MHz, 13 C/100 MHz) and JNM-ECX 600 ( 1 H/600 MHz, 13 C/150 MHz) spectrometers. Chemical shifts are reported in  (ppm) using chloroform as an internal standard at  7.26 and 77.16 for 1 H and 13 C NMR, respectively. IR spectra were recorded on a JASCO FT/IR 660 Plus infrared spectrometer. Mass spectra were recorded on JEOL JMS-T100CS (ESI) spectrometer. The medium-pressure liquid chromatography (MPLC) purifications were performed on a YAMAZEN YFLC-AI-580. Where necessary, solvents were distilled from appropriate drying agents prior to use. Flash column chromatography was performed using Kanto Silica Gel 60N.

Synthesis of manifold 15:
To a solution of 6 (715 mg, 4.55 mmol) in t-BuOH (22.8 mL) with molecular sieves 4 Å, diphenylphosphoryl azide (1.47 mL, 6.83 mmol) and Et 3 N (1.14 mL, 8.19 mmol) were added at room temperature. After stirring at room temperature for 50 minute and heating under reflux for 24 h, the mixture was cooled to room temperature and filtrated through Celite ® . To the filtrate was added water, and the mixture was extracted with EtOAc. The combined organic extracts were washed with 1 N HCl (aq), water, saturated NaHCO 3 (aq) and brine, dried over Na 2 SO 4 , filtrated through Celite ® , and concentrated. The residue was purified by silica-gel column chromatography to afford 57 (795 mg, 3.48 mmol, 77%) as a white solid. Then, 57 (242 mg, 1.06 mmol) was treated with 3 N HCl/AcOEt (2 mL) at 0 °C and stirred overnight at room temperature. After being concentrated, the residue was basified with saturated NaHCO 3 (aq) and extracted with CHCl 3 .
Combined organic extracts were dried over Na 2 SO 4 and concentrated to afford 15 (132 mg, 1.03 mmol, 98%) as a white solid.
A solution of 60 (275 mg, 0.440 mmol) and 23 (117 μL, 0.880 mmol) in toluene (2.2 mL) was heated for 3.5 h under reflux. After being cooled to room temperature, the resulting mixture was concentrated and purified by silica-gel column chromatography to afford 61 (286 mg, 0.403 mmol, 91% for a 1:1 mixture of diastereomers) as a yellow amorphous solid.
A solution of 61 (108 mg, 0.152 mmol) in acetonitrile (1.0 mL) was treated with Et 3 N (63 μL, 0.451 mmol) and mesyl azide (26.0 μL, 0.308 mmol) and stirred for 3 h at room temperature. To the S4 mixture was added water, and it was then extracted with EtOAc. The combined organic extracts were washed with water and brine, dried over Na 2 SO 4 , and concentrated. The residue was purified by silica-gel column chromatography to afford 62 (88.9 mg, 0.121 mmol, 81% for a 1:1 mixture of diastereomers) as a pale yellow amorphous solid.
Further incubation was conducted at 37 °C under 5% CO 2 -95% air for 7 days and then 20 μL of MTT-PBS solution (5 mg/mL) was added to each well. The microplate was incubated at 37 °C for 4 h under 5% CO 2 -95% air. Then, the incubation medium was aspirated and 100 μL of DMSO was added to solubilize the MTT formazan product. After mixing, absorbance at 540 nm was measured with iEMS microplate reader MF. The IC 50 value was estimated from the dose-response curve. Figure S1: 1 H NMR spectrum of 63a in CDCl 3 S8 Figure S2: 13 C NMR spectrum of 63a in CDCl 3 S9 Figure S3: 1 H NMR spectrum of 63b in CDCl 3 S10 Figure S1. A 13 C-NMR spectrum of 30a in CDCl 3 Figure S2. A 1 H-NMR spectrum of 18a in CDCl 3 Figure S4: 13 C NMR spectrum of 63b in CDCl 3