Marine furanocembranoids-inspired macrocycles enabled by Pd-catalyzed unactivated C(sp3)-H olefination mediated by donor/donor carbenes

Biomimetic modularization and function-oriented synthesis of structurally diversified natural product-like macrocycles in a step-economical fashion is highly desirable. Inspired by marine furanocembranoids, herein, we synthesize diverse alkenes substituted furan-embedded macrolactams via a modular biomimetic assembly strategy. The success of this assembly is the development of crucial Pd-catalyzed carbene coupling between ene-yne-ketones as donor/donor carbene precursors and unactivated Csp3‒H bonds which represents a great challenge in organic synthesis. Notably, this method not only obviates the use of unstable, explosive, and toxic diazo compounds, but also can be amenable to allenyl ketones carbene precursors. DFT calculations demonstrate that a formal 1,4-Pd shift could be involved in the mechanism. Moreover, the collected furanocembranoids-like macrolactams show significant anti-inflammatory activities against TNF-α, IL-6, and IL-1β and the cytotoxicity is comparable to Dexamethasone.


Method A:
2-tert-Butylaniline (2.50 g, 16.7 mmol) was added to 13 mL 48% w/w hydrobromic acid. The thus formed pale pink suspension was cooled to -56 ℃. Then, sodium nitrite (1.68 g, 24.4 mmol) was added portion-wise. After stirring for 1 hour, 20 mL Et2O was added slowly. The temperature was adjusted to -8 ℃ over 2 hours and kept at -8 ℃ for an additional 2 hours. Next, the system was cooled to -40 ℃ again. 3.06 g Na2CO3 was added. The reaction was then warming up slowly to room temperature over 3 hours and stirred overnight. The reaction was diluted with water and extracted with EtOAc. The organic layer was washed with water, NaHCO3 (aq.), brine, dried over Na2SO4, concentrated in vacuo, and purified by silica gel column chromatography with petroleum ether to afford 1-bromo-2-(tert-butyl)benzene as a colorless oil.(1.49 g, 42% yield)

Method B:
Under nitrogen atmosphere, TiCl4 (1 M in CH2Cl2, 2.2 equiv.) was added to anhydrous CH2Cl2 (0.3 M) and cooled to -40 ℃. Then, a solution of Me2Zn (1 M in hexane, 2.2 equiv.) was added dropwise. After stirring for 15 minutes, a solution of aryl ketone in anhydrous CH2Cl2 (1 M) was added dropwise. The reaction was warm to 0 ℃ slowly (over 3 hours), stirred at 0 ℃ overnight, and poured into ice water. The aqueous phase was extracted with CH2Cl2. The combined organic layers were washed with NaHCO3 (aq.), brine, dried over Na2SO4, filtered and concentrated in vacuo. (If necessary, the crude product was dissolved in CH2Cl2, treated with 3chloroperoxybenzoic acid (m-CPMA) and stirred at room temperature to remove the aryl propene, a by-product of the reaction, which is not easy to be separated from the desired product by silica gel column chromatography). The crude product was purified by silica gel column chromatography with petroleum ether/ethyl acetate to afford the corresponding aryl bromide. (45-67% yield)

Method C:
To a stirred solution of HMDS (4.25 mL, 20.0 mmol) in THF (36.8 mL) was added n-BuLi (1.60 M in hexane, 12.5 mL, 20.0 mmol) at 0 °C. After the mixture was stirred for 1.5 h, a solution of methyl 2-(2-bromophenyl)acetate (1.2 g, 5.0 mmol) in THF (25.0 mL) was added, and the mixture was stirred for 2 h at room temperature. Then CH3I (1.20 mL, 20.0 mmol) was added at 0 °C, and the mixture was stirred for 12 h at room temperature. Then the reaction was quenched by addition of 1 M HCl at 0 °C. The crude mixture was extracted with EtOAc (x4) and the combined organic extracts were washed with brine, dried (Na2SO4) and concentrated in vacuo. The crude product was purified by silica gel column chromatography with petroleum ether/ethyl acetate to afford the corresponding product as a colorless oil. (1g, 78% yield)

Synthesis of Ene-Yne-Ketones
The ene-yne-ketones used in this study were prepared by two steps starting from terminal alkynes and 1,3-dicarbonyl compounds.
The alkyne (10 mmol) was dissolved in dry THF (15 mL) and the solution was cooled to -40 °C under nitrogen, n-butyllithium (1.6 M in hexanes, 6.8 mL, 11 mmol) was added dropwise over 2 minutes while maintaining the temperature between -35 and -40 °C. After completion of the addition, anhydrous DMF (1.55 mL, 20 mmol) was added in one portion and the cold bath was removed. The reaction mixture was warm to room temperature and aged for 30 minutes. The THF solution was poured into a vigorously stirred biphasic solution prepared from aqueous solution of KH2PO4 (50 mL, 30 mmol) and Et2O (30 mL) cooled over ice to about 5 °C. Layers were separated and the organic extract was washed with water (2 x 30 mL). Combined aqueous layers were back extracted with Et2O (30 mL). Combined organic layers were dried over Na2SO4 and filtered. Then solvent was removed in vacuo carefully under 0 °C to leave a crude acetylenic aldehyde.
The crude product was then dissolved in THF (8 mL), and 1,3-dicarbonyl compounds (10 mmol) was added into the solution. Then AcOH (2 mmol) and MgSO4 (2 mmol) was added to the reaction mixture. The mixture was stirred at room temperature for about one hour. When the reaction was completed as monitored by TLC, filtration through celite and removal of the solvent by rotary evaporation gave the crude product.
The ene-yne-ketones was purified by chromatography on silica gel with the appropriate mixture of PE and EA (PE = petroleum ether, EA = ethyl acetate) with about 50-70% yields (two steps). 4

Synthesis of 6a
Synthesis of 3qa:

Please refer to 3ba
From 3qa to S1: To a solution of 3qa (50 mg, 0.14 mmol) in DCM (2 mL) were added EDCI (51.5 mg, 0.27 mmol), HOBt (36.6 mg, 0.27 mmol), Boc-Phe-OH (42.4 mg, 0.16 mmol). The stirring of the solution was continued at r.t. for 18 h. Then the solution was washed with water, and the organic phase was evaporated under reduced pressure and the residue was purified by FCC to get the desired product S1 (54.9 mg, yield: 66%).
From S1 to S2: The S1 (54.9 mg, 0.09 mmol) was dissolved in DCM (2 mL) and TFA (0.2 mL) were added slowly. The stirring of the solution was continued at r.t. for 4 h. Then the solution was evaporating under reduced pressure. And the residue was used in the next step without further purification.
The residue was dissolved in DCM (5 mL). And EDCI (51.5 mg, 0.27 mmol), HOBt (36.6 mg, 0.27 mmol), Boc-NH-(CH2)11-COOH (56.7 mg, 0.18 mmol) were added. The stirring of the solution was continued at r.t. for 24 h. Then the solution was washed with water, and the organic phase was evaporated under reduced pressure and the residue was purified by FCC to get the desired product S2 (54.2 mg, 74% yield in two steps from S1).
From S2 to 6a: The S2 (54.2 mg, 0.07 mmol) was dissolved in DCM (2 mL) and TFA (0.2 mL) were added slowly. The stirring of the solution was continued at r.t. for 4 h. Then the solution was evaporating under reduced pressure. And the residue was used in the next step without further purification.
The residue was dissolved in MeOH (2 mL) and the solution of NaOH (100 mg) in H2O were added slowly. The stirring of the solution was continued at 50 o C for 4 h.
Then the solution was acidified by HCl, followed by evaporating under reduced pressure. And the residue was used in the next step without further purification.
The residue was dissolved in DCM (40 mL). And EDCI (76.4 mg, 0.4 mmol), HOBt (54.0 mg, 0.4 mmol) were added. The stirring of the solution was continued at r.t. for 24 h. Then the solution was washed with water, and the organic phase was evaporated under reduced pressure and the residue was purified by FCC to get the desired product 6a as a white solid (11.8 mg, 27.2% yield in three steps from S2).

Synthesis of 6b
Synthesis of 6b: This compound was prepared form 3qa and Boc-Ala-OH using the procedure described for 6b and was isolated as a white solid, 2.5 mg, 2.9% yield in six steps from 3qa.

Synthesis of 6c
Synthesis of 6c: This compound was prepared form 3qa and Boc-Val-OH using the procedure described for 6c and was isolated as a white solid, 4.6 mg, 5.5% yield in six steps from 3qa.

Synthesis of 6d
Synthesis of 6d: This compound was prepared form 3qa and Boc-Pro-OH using the procedure described for 6d and was isolated as a white solid, 9.1 mg, 10.9% yield in six steps from 3qa.

Synthesis of 6e:
This compound was prepared form S1 and Boc-Val-OH using the procedure described for 6e and was isolated as a white solid, 4.1 mg, 6.1% yield in seven steps from S1.

Synthesis of 6f:
This compound was prepared form S1 and Boc-Met-OH using the procedure described for 6f and was isolated as a white solid, 2.8 mg, 3.6% yield in seven steps from S1.

Synthesis of 6g
From S1 to S13: To a solution of S1 (100 mg, 0.27 mmol) in DCM (2 mL) were added EDCI (103 mg, 0.54 mmol), HOBt (73.2 mg, 0.54 mmol), Fmoc-NH-(CH2)11-COOH (141.6 mg, 0.32 mmol). The stirring of the solution was continued at r.t. for 18 h. Then the solution was washed with water, and the organic phase was evaporated under reduced pressure and the residue was purified by FCC to get the desired product S13 (131.2 mg, yield: 13 64%).
From S13 to 6g: The S13 (100 mg) was dissolved in MeOH (2 mL) and the solution of NaOH (100 mg) in H2O were added slowly. The stirring of the solution was continued at 50 o C for 4 h. Then the solution was acidified by HCl, followed by evaporating under reduced pressure. And the residue was used in the next step without further purification.
The residue was dissolved in DCM (40 mL). And EDCI (95.5 mg, 0.5 mmol), HOBt (67.5 mg, 0.5 mmol) were added. The stirring of the solution was continued at r.t. for 24 h. Then the solution was washed with water, and the organic phase was evaporated under reduced pressure and the residue was purified by FCC to get the desired product 6g (17.8 mg, yield: 27%).

Synthesis of 6h
From S13 to S14: To a solution of S13 in DCM were added DBU (10 eq) slowly and stirring of the solution was continued at r.t. for 1 h. Then the solution was washed with water and extracted by EtOAc. Then the organic phase was evaporated under reduced pressure and the residue was purified by FCC (DCM/EtOH = 2:1).
The product purified was dissolved in DCM. And EDCI (3 eq), HOBt (3 eq) were added followed by (((9H-fluoren-9-yl)methoxy)carbonyl)valine. The stirring of the solution was continued at r.t. for 18 h. Then the solution was washed with water, and the organic phase was evaporated under reduced pressure and the residue was purified by FCC (DCM/MeOH = 1% -5%) to get the desired product S14.
From S14 to 6h: The S14 was dissolved in MeOH and the solution of 10% LiOH in H2O were added.
The stirring of the solution was continued at 50 o C for 4 h. Then the pH of solution was adjusted to 7-8. Then the solvent was evaporated under reduced pressure. And the residue was used in the next step without further purification.
The stirring of the solution was continued at r.t. for 24 h. Then the solution was washed with water and extracted by EtOAc. Then the organic phase was evaporated under reduced pressure and the residue was purified by FCC to get the desired product 6h.