Total Synthesis of Ophiorrhine A, G and Ophiorrhiside E Featuring a Bioinspired Intramolecular Diels–Alder Cycloaddition

Abstract We report the first total synthesis of the monoterpene indole alkaloids ophiorrhine A via a late stage bioinspired intramolecular Diels–Alder cycloaddition to form the intricate bridged and spirannic polycyclic system. Several strategies were investigated to construct the indolopyridone moiety of ophiorrhiside E, the postulated biosynthetic precursor of ophiorrhine A. Eventually, the Friedel–Crafts‐type coupling of N‐methyl indolyl‐acetamide with a secologanin‐derived acid chloride delivered ophiorrhine G. Cyclodehydration of a protected form of the latter was followed by the desired spontaneous intramolecular Diels–Alder cycloaddition of protected ophiorrhiside E leading to ophiorrhine A.


Acylation of indolylacetic acid derivatives with a carboxylic acid
1.5 Cyclodehydration of protected ophiorrhine G and tandem Diels-Alder cycloaddition into protected ophiorrhine A.

Experimental procedures and data of all compounds.
Compound 28a: 1-isobutyl-9H-pyrido [3,4-b]indole Inspired from a known a procedure. 1 To a solution of the L-tryptophan 12 (1.02 g, 5.00 mmol) in CH2Cl2 (25 mL) at room temperature was added 3-methylbutanal 27 (1.07 mL, 10 mmol), and then TFA (1.48 mL, 20 mmol) dropwise at 0 o C. The temperature was then allowed to warm up to room temperature and the reaction mixture was stirred at this temperature overnight. Then, the reaction mixture was concentrated under reduced pressure to remove TFA and CH2Cl2. The mixture was dissolved in 50 mL of DMF, followed by addition of NCS (
To a solution of pyridinium 29a (5.0 mg, 0.0137 mmol) in DMSO (0.5 mL) was added NaH (1 mg, 4.0 eq) at room temperature. The reaction mixture was heated to 150 ℃ and stirred at this temperature overnight. The reaction mixture was cooled down to room temperature and diluted with 2 mL of CH2Cl2 and was washed with a saturated aqueous solution of NaCl. The organic layer was dried over Na2SO4, filtered and concentrated under reduced pressure. The residue was purified by preparative TLC (CH2Cl2/MeOH:10/1) to afford 31 (2 mg) as colorless solid in 74% yield.

Rf
Inspired from a known procedure. 1

S11
To a solution of L-tryptophan 12 (46 mg, 0.228 mmol) in CH2Cl2 (3 mL) at room temperature was added (±)-16 (29 mg, 0.114 mmol), which was prepared according our previous adaptation of Tietze procedures. 4 TFA (65 mg, 0.571 mmol) was then added dropwise at 0 o C under argon. Then, the temperature was allowed to warm up to room temperature and the reaction mixture stirred at this temperature overnight. The reaction mixture was concentrated under reduced pressure to remove TFA and CH2Cl2. The mixture was dissolved in 2 mL of DMF, followed by addition of NCS (64 mg, 0.479 mmol) and Et3N (58 mg, 0.571 mmol) dropwise at 0 o C. Then the mixture was stirred at room temperature for 3 h and quenched with water. The mixture was diluted with EtOAc and washed 3 times with a saturated aqueous solution of NaCl. The organic phase was dried over Na2SO4, filtered and concentrated under reduced pressure. The residue was purified by preparative TLC on silica gel (CH2Cl2/MeOH 18:1) to give 13 mg of (±)-28b as a pale yellow solid (29% yield).

Rf
Inspired from a known procedure. 2 To a solution of (±)-28b (12.6 mg, 0.0321 mmol) in CH3CN (1.5 mL) was added CH3I (229 mg, 1.607 mmol) under argon. The mixture was heated at 80 °C (heated by oil bath) for 21 h and then cooled to room temperature. Removal of the solvent under reduced pressure afforded the crude product, which was purified by preparative TLC (CH2Cl2/MeOH:10/1) to give 11 mg of (±)-29b as a red solid (64% yield).

Rf
Inspired from known procedures. 5 A solution of pyridine N-oxide 33a (15 mg, 0.0625 mmol) in acetic anhydride (3.5 mL) was refluxed at 165 o C for 3 h. Then, the reaction mixture was cooled down and a saturated aqueous solution of NaHCO3 was added and the mixture was stirred at 0 o C for about 10 min. The resulting mixture was extracted 3 times with CH2Cl2. The combined organic layers were dried over Na2SO4, filtered and concentrated under reduced pressure The residue was purified by column chromatography on silica gel (CH2Cl2/MeOH 50:1 to 30:1) to give 11 mg of (±)-34a as a colorless solid (62% yield).

Rf
Inspired from known procedures. 5 A solution of pyridine acetate (±)-34a (150 mg, 0.531 mmol) in CHCl3\EtOH (1:1) 15 mL was treated with m-CPBA (450 mg, 2.65 mmol) at room temperature after which the reaction mixture was heated to 80 o C for about 18 h. Then, the reaction mixture was cooled down and a saturated aqueous solution of NaHCO3 was added and the mixture was stirred at 0 o C for about 10 min. The resulting mixture was extracted 3 times with CH2Cl2. The combined organic layers were dried over Na2SO4, filtered and concentrated under reduced pressure. The residue was purified by column chromatography on silica gel (CH2Cl2/MeOH 50:1 to 30:1) to give 124 mg of 1-(1-acetoxy-2-methylpropyl)-9H-pyrido[3,4-b]indole 2-oxide as a colorless solid (78% yield). A solution this pyridine N-oxide (120 mg, 0.402 mmol) in acetic anhydride (18 mL) was reflux at 165 o C for 3 h. Then the reaction mixture was cooled down and NaHCO3 (aq.) was added and stirred at 0 o C for about 10 min. The resulting mixture was extracted 3 times with CH2Cl2. The combined organic layers were dried over Na2SO4, filtered and concentrated under reduced pressure. The residue was purified by column chromatography on silica gel (CH2Cl2/MeOH 50:1 to 30:1) to give 32 mg of (±)-35a as a colorless solid (23% yield).

Rf
Inspired from a known procedure. 5b To a solution of acetoxypyridine (±)-35a (13 mg, 0.0382 mmol) in MeOH (3 mL) was added K2CO3 (131.3 mg, 0.95 mmol) at room temperature. Then the mixture was stirred at 70 o C for 2 h. Then, MeI (5 ml) and acetone (24 mL) were added into the mixture which was stirred at 70 o C overnight. The reaction mixture was then concentrated directly under reduced pressure, and was then purified by preparative TLC on silica gel (CH2Cl2) to give 3 mg of (±)-36a as a yellow solid (29% yield).
To a solution of pyridine N-oxide 33a (24 mg, 0.100 mmol) in CH2Cl2 (1 mL), was added Tf2O (28.2 mg, 0.5 mmol) and Et3N (102 mg, 1 mmol) at room temperature. The reaction mixture was stirred at room temperature for about 30 min and was then quenched with water after which it was extracted 3 times with CH2Cl2. The combined organic layers were dried over Na2SO4, filtered and concentrated under reduced pressure. The residue was purified by column chromatography on silica gel (CH2Cl2) to give 28 mg of 37a as a colorless oil (74% yield).

Rf
To a solution of triflyloxypyridine 37a (15 mg, 0.0403 mmol) in acetone (5 mL) was added MeI (25 mL) and K2CO3 (12 mg, 0.088 mmol) at room temperature. Then, the mixture was stirred at 60 o C for 48 h. The reaction mixture was then directly concentrated under reduced pressure, and was purified by preparative TLC on silica gel (CH2Cl2/petroleum ether 1:1) to give 6 mg of 38a as a solid (30% yield).

Rf
Inspired from a known procedure. 6 Inspired from known procedures. 5 To solution of pyridine (±)-28b (12 mg, 0.0306 mmol) in CHCl3 (2 mL) was added m-CPBA (25.8 mg, 0.15 mmol) at room temperature and the reaction was stirred at this temperature for 1 h. Then the reaction mixture was quenched with a saturated aqueous solution of NaHCO3 and the mixture was stirred at 0 o C for about 10 min. The resulting mixture was extracted 3 times with CH2Cl2. The combined organic layers were dried with Na2SO4, filtered and concentrated under reduced pressure. The residue was purified by column chromatography on silica gel (CH2Cl2 to EtOAc to CH2Cl2/MeOH 10:1) to give 5 mg of (±)-33b as a colorless solid (40% yield).
To a solution of pyridine N-oxide (±)-33b (10 mg, 0.0245 mmol) in CH2Cl2 (1 mL) was added Tf2O (35 mg, 0.125 mmol) and Et3N (25 mg, 0.25 mmol) at 0℃. Then the reaction mixture was stirred at 0 o C for about 5 min and was quenched with water. The aqueous layer was extracted with CH2Cl2 for 3 times. The combined organic layers were dried with Na2SO4, filtered and concentrated in vacuo. The residue was purified by column chromatography on silica gel (Petroleum ether/ EtOAc 10:1) to give 1 mg of (±)-37b as a colorless oil (7% yield).  Prepared according to a known procedure. 7

S17
To a solution of indole-3-acetic acid 15c (2.10 g, 12.0 mmol) in methanol (24 mL) under argon at 0 °C was slowly added SOCl2 (4.28 g, 36 mmol). The resulting mixture was heated to 68 °C over 12 h. The reaction was poured into a saturated aqueous solution of NaHCO3 (40 mL). The resulting mixture was extracted three times with EtOAc (3 × 50 mL) and the combined organic layers were dried over Na2SO4, filtered and concentrated under reduced pressure. The residue was purified by column chromatography on silica gel (Petroleum ether/ EtOAc 3:2) to give 2.064 g of 15b as a colorless solid (91% yield).  Prepared according to a known procedure. 8 To a solution of indole-3-acetic acid 15c (2.16 g, 12.33 mmol) in EtOH (24 mL) was added dropwise SOCl2 (4.28 g, 36 mmol). The reaction was stirred to reflux for overnight. Upon completion of the reaction (monitoring by TLC), the mixture was concentrated to dryness and the crude material was directly treated for next step without isolation. The crude reaction mixture was dissolved in a solution of MeNH2 (25 mL of 33% in EtOH). The mixture was stirred at 25 ºC for 24 h until ethyl 3-indoleacetate was fully consumed and was concentrated under reduced pressure. The crude product was purified by flash chromatography on silica gel (Petroleum ether/EtOAc 1:3) to give 2.264 g of 15a as a colorless solid (98% yield).  Compound (±)-40: 2,2'-((3-methylbutane-1,1-diyl)bis(1H-indole-2,3-diyl))bis(N-methylacetamide)

S19
Prepared according to a known procedure. 11 To a solution of tert-butyl N-hydroxycarbamate (402 mg, 3.02 mmol), Et3N (420 μL, 3.02 mmol) in CH2Cl2 (5 mL) was added 4-trifluoromethylbenzoyl chloride (314 mg, 1.51 mmol) dropwise at 0 o C for 30 min and stirred at 0 o C for another 10 min. Then, the mixture was poured into a saturated aqueous solution of NaHCO3. The resulting mixture was extracted twice with CH2Cl2. The combined organic phases were then dried over Na2SO4, filtered and concentrated under reduced pressure. The residue was then dissolved in dry CH2Cl2 (1 mL). Trifluoroacetic acid (2 ml) was then added dropwise at room temperature and the reaction mixture was stirred at 0 o C for another 30 min. Then, the mixture was poured into a saturated aqueous solution of NaHCO3 and the resulting mixture was extracted twice with 25 mL of CH2Cl2. The combined organic layers were then dried over Na2SO4 and concentrated under reduced pressure. The residue was purified by column chromatography on silica gel (Petroleum ether/ EtOAc 5:1) to give 132 mg of 26 as a white solid (42% yield).

Compound 46b: methyl-2-(2-(hex-5-enoyl)-1H-indol-3-yl)acetate
Inspired from known procedures. 13 To a solution of hex-5-enoic acid 44 (13.7 mg, 0.120 mmol) in CH2Cl2 (0.5 mL) at 0 °C, was slowly added oxalyl chloride (19.8 mg, 0.156 mol). The resulting mixture was allowed to warm up to room temperature, while stirring overnight. The reaction was concentrated under reduced pressure and used for next step without any purification. Then the reaction mixture was dissolved in 1 mL of Et2O, methyl 2-(1H-indol-3-yl)acetate 15b (18.9 mg, 0.1 mmol) was added and then cooled to 0 °C. A solution of tin chloride (0.012 mL of 1 M in heptane ) was added dropwise at 0 °C. The resulting mixture was allowed to warm up to room temperature and stirred at room temperature for overnight. The reaction mixture was quenched with a saturated aqueous KF solution and the resulting mixture was extracted 5 times with CH2Cl2. The combined organic layers were dried with Na2SO4 and concentrated under reduced pressure. The crude product was purified by preparative TLC on silica gel (Petroleum ether/EtOAc 7:3) to give 23 mg of 46b as a white solid (80% yield).

Compound 46a: 2-(2-(hex-5-enoyl)-1H-indol-3-yl)-N-methylacetamide
Inspired from known procedures. 13 To a solution of hex-5-enoic acid 44 (11.4 mg, 0.100 mmol) in CH2Cl2 (0.5 mL) at 0 °C, oxalyl chloride (16.5 mg, 0.13 mmol) was added slowly and the resulting mixture was allowed to warm up to room temperature, while stirring overnight. The reaction mixture was concentrated under reduced pressure and used for next step without any purification. To the residue dissolved in 0.5 mL of Et2O and 0.5 mL of CH2Cl2, 15a (23 mg, 0.12 mmol) was added and the mixture was cooled to 0 °C. A solution of tin chloride (0.1 mL of 1 M in CH2Cl2) was added dropwise at 0 °C. The resulting mixture was allowed to warm up to room temperature and stirred at room temperature for 4 h. The reaction mixture was quenched with a saturated aqueous solution of KF and the resulting mixture was extracted 5 times with CH2Cl2. The combined organic layers were dried over Na2SO4, filtered and concentrated under reduced pressure. The crude products were purified by preparative TLC on silica gel (CH2Cl2/  Inspired from known procedures. 14,15 Et3N (27 µL, 0.19 mmol) was added to a solution of 46a (11 mg, 0.0387 mmol) in 1 mL of acetic acid and the reaction mixture was stirred at reflux for 1.5 h. The mixture was then cooled-down and then poured into water, and a solution of ammonium hydroxide was added until it reached pH ≥ 7. Then the resulting mixture was extracted with 5 times CH2Cl2. The combined organic layers were dried over Na2SO4, filtered and concentrated under reduced pressure. The crude products were purified by preparative TLC on silica gel (CH2Cl2/Methanol 15:1) to give 9 mg of 47 as a fluorescent yellow solid (87% yield).
A solution of 46a (14 mg, 0.0492 mmol) in 1 mL of acetic acid was stirred at reflux for 1.5 h. The mixture was then cooled-down and then poured into water, and a solution of ammonium hydroxide was added until it reached pH ≥ 7. Then the resulting mixture was extracted with 5 times CH2Cl2. The combined organic layers were dried over Na2SO4, filtered and concentrated under reduced pressure. The crude products were purified by preparative TLC on silica gel (CH2Cl2/Methanol 15:1) to give 11 mg of 47 as a fluorescent yellow solid (85% yield).

Rf
Inspired from known procedure. [13][14][15] To a solution of (±)-23 (170 mg, 0.629 mmol) in CH2Cl2 (3 mL) at 0 ºC was added dropwise oxalyl chloride (97 mg, 0.767 mmol) and then the reaction mixture was allowed to stir at room temperature overnight. It was then concentrated to dryness and the crude material was directly treated for next step without isolation. To the crude reaction mixture dissolved in Et2O (6 mL) was added 15b (133.8 mg, 0.708 mmol) followed by a dropwise addition of a solution of SnCl4 (0.06 mL of 1 M in heptane, 0.06 mmol) at 0 ºC. The reaction mixture was then allowed to stir at room temperature for 2 h. The reaction mixture was quenched with a saturated aqueous solution of KF and the resulting mixture was extracted 5 times with CH2Cl2. The combined organic layers were dried over Na2SO4, filtered and concentrated under reduced pressure. The crude product was purified by flash chromatography on silica gel (CH2Cl2/Petroleum ether 10:1 to 5:1) to give 120 mg of acylation product in mixture with an unknown compound as a colorless oil. To a solution of 13 mg of this mixture containing the acylation product in toluene (1 mL) were added H2NMe (0.2 mL of 33% in EtOH) and NH4OAc (7 mg, 0.09 mmol) at room temperature and the reaction mixture was stirred at 95 ºC for about 6 h. Upon completion of the reaction (monitoring by TLC), the reaction mixture was allowed to cool down to room temperature and was then concentrated under reduced pressure. The crude product was purified by flash chromatography on silica gel (CH2Cl2/MeOH 20:1 to10:1) to give 4 mg of (±)-50 as a yellow solid (14% yield over 2 steps).  Inspired from a known procedure. 16 A solution of (±)-51 (5 mg, 0.0118 mmol) in bromobenzene (1 mL) was stirred at 156 ºC for about 5 h. Upon completion of the reaction (monitoring by TLC), the reaction mixture was allowed to cool down to room temperature and was then concentrated under reduced pressure. The crude product was purified by preparative TLC (CH2Cl2/MeOH 10:1) to give 3 mg of (±)-53 as a white solid (60% yield). [M + Na] + calcd. for C24H26N2NaO5 + 445.1734, found 445.1712.  Prepared according to a known procedure. 17 To secologanin tetraacetate (−)-17 (60 mg, 0.108 mmol, prepared according to our modifications of Ishikawa's procedures 18 ) in acetone (2 mL) at 0 ºC was added dropwise a solution of Jones Reagent (0.06 mL of 2.67 M in water) and the reaction mixture was stirred at 0 ºC for about 30 min. Upon completion of the reaction (monitoring by TLC), the mixture was quenched with isopropanol at 0 ºC and was filtered over celite and concentrated under reduced pressure. The crude product was purified by preparative TLC on silica gel (CH2Cl2/MeOH 10:1) to give 58 mg of (−)-24 as a white solid (93% yield).   Inspired from known procedures. 13 To a solution of (−)-24 (47 mg, 0.0821 mmol) in CH2Cl2 (0.5 mL) at 0 ºC was added dropwise oxalyl chloride (20 mg, 0.16 mmol) and then, the reaction mixture was allowed to stir at room temperature overnight. The reaction mixture was concentrated to dryness under reduced pressure and the crude material was directly treated for next step without isolation. To the crude reaction mixture dissolved in Et2O (1 mL) at 0 ºC were successively added 15b (30 mg, 0.16 mmol) and dropwise a solution of SnCl4 (0.008 mL of 1 M in CH2Cl2, 0.008 mmol). The reaction mixture was allowed to stir at room temperature for 2 h. The reaction mixture was quenched with a saturated aqueous solution of KF. The resulting mixture was extracted 5 times with CH2Cl2. The combined organic layers were dried over Na2SO4, filtered and concentrated under reduced pressure. The crude product was purified by preparative TLC on silica gel (Dichloromethane/Ethyl acetate 5:1) to give 28 mg of (−)-50b as a colorless oil (46% yield).   Inspired from a known procedure. 13 To a solution of (−)-24 (23 mg, 0.04017 mmol) in CH2Cl2 (0.5 mL) at 0 ºC, was added dropwise oxalyl chloride (7 µL, 0.08 mmol) and then the reaction mixture was allowed to stir at room temperature overnight. The reaction mixture was concentrated to dryness under reduced pressure and the crude material was directly treated for next step without purification. To the crude reaction mixture dissolved in Et2O (0.4 mL) and CH2Cl2 (0.1 mL) at 0 ºC, was added 15a (15 mg,0.08 mmol) followed by the dropwise addition of a solution of tin chloride (0.04 mL, 1 M in CH2Cl2, 0.04 mmol). The reaction mixture was allowed to stir at room temperature overnight. The reaction mixture was quenched with a saturated aqueous solution of KF and the resulting mixture was extracted 5 times with CH2Cl2. The combined organic layers were dried over Na2SO4 and concentrated under reduced pressure. The crude product was purified by preparative TLC on silica gel (EtOAc) to give 13 mg of (−)-50a as a colorless oil (44% yield). To a solution of (−)-50a (13 mg, 0.0175 mmol) of methanol (1 mL) at 0 °C, was added K2CO3 (7 mg, 0.05 mmol). The reaction mixture was stirred at 0 °C for 20 min and was then directly purified by silica gel preparative TLC (CH2Cl2[saturated with ammonium hydroxide]/MeOH 5:1) to give 10 mg of ophiorrhine G (−)-7a as a white solid (quant.).  Inspired from known procedures. 14,15 To (−)-50a (18 mg, 0.0242 mmol) in AcOH (1.0 mL) at room temperature was added Et3N (50 µL, 0.364 mmol) and then the reaction mixture was stirred at 80 ºC for 1 h, after which an addition of Et3N (50 µL, 0.364 mmol) was effected followed by a third addition of Et3N (50 µL, 0.364 mmol) after an additional 1 h and the reaction mixture was stirred for an additional 14 h. Then the mixture was cooled to room temperature, diluted with CH2Cl2, and then quenched with a solution of ammonium hydroxide until it reached pH ≥ 7. The resulting mixture was extracted 5 times with CH2Cl2 in. The combined organic layers were dried over Na2SO4, filtered and concentrated under reduced pressure. The crude product was used directly in the next step without purification. It was dissolved in MeOH (1.0 mL) onto which K2CO3 (10 mg, 0.073 mmol) was added at 0 °C. The reaction mixture was stirred at 0 °C for 30 min and was then directly purified by silica gel preparative TLC (CH2Cl2[saturated with ammonium hydroxide]/EtOAc[saturated with ammonium hydroxide] /MeOH 3:2:1) to give 8 mg of ophiorrhine G (−)-7a as a white solid (57%) and 4 mg of ophiorrhiside E (−)-6a as a yellow oil (30%).

Rf
To ophiorrhine G (−)-7a (6 mg, 0.0104 mmol) in AcOH (0.5 mL) at room temperature, was added Et3N (22 µL, 0.15 mmol) and then the reaction mixture was stirred at 80 ºC for 1 h, after which an addition of Et3N (22 µL, 0.15 mmol) was effected followed by a third addition of Et3N (22 µL, 0.15 mmol) after an additional 1 h and the reaction mixture was stirred for an additional 14 h. Then the mixture was cooled to room temperature, diluted with CH2Cl2, and then quenched with a solution of ammonium hydroxide until it reached pH ≥ 7. The resulting mixture was extracted 5 times with CH2Cl2. The combined organic layers were dried over Na2SO4, filtered and concentrated under reduced pressure. The  To (−)-50a (7 mg, 0.00943 mmol) in AcOH (0.5 mL at room temperature) was added Et3N (59 µL, 0.425 mmol) and the reaction mixture was stirred at 125 ºC for about 6 h. Then the mixture was cooled down to room temperature, diluted with CH2Cl2, and then quenched with a solution of ammonium hydroxide until pH ≥ 7. The resulting mixture was extracted 5 times with CH2Cl2. The combined organic layers were dried over Na2SO4 and concentrated under reduced pressure. The crude product was purified by silica gel preparative TLC (EtOAc) to give 2.0 mg of (+)-55 (29%) as a white solid and 4.0 mg of (−)-54 as a white solid (58%). To a solution of (+)-55 (2 mg, 0.00277 mmol) in MeOH (0.5 mL) at 0 °C was added K2CO3 (1 mg, 0.07 mmol). The reaction mixture was stirred at 0 °C for 30 min and was then directly purified by silica gel preparative TLC (CH2Cl2 [saturated with ammonium hydroxide]/MeOH 5:1) to give 1.5 mg of (+)-56 as a white solid (98% 50.5, CH3 51.9, CH3 51.9, CH3 a) the methanol-d4 peak was at 47.6 ppm on the provided 13 C NMR of natural ophiorrhine A 21