Construction of Spirocyclic Tetrahydro-β-carbolines via Cross

Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX

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Construction of Spirocyclic Tetrahydro-β-carbolines via CrossAnnulation of Phenols with Tryptamines in Water Zemin Wang,† Jiabin Niu,† Huiying Zeng,*,† and Chao-Jun Li*,‡ †

The State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, P. R. China Department of Chemistry and FQRNT Centre for Green Chemistry and Catalysis, McGill University, 801 Sherbrooke Street West, Montreal, QC H3A 0B8, Canada

‡

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S Supporting Information *

ABSTRACT: Phenols are readily available by degradation of lignin resource. Palladium-catalyzed conversion of phenols to tetrahydro-βcarboline skeletons bearing a spirocycle at the C-1 position in water is reported. Various substituted phenols are successfully cross-annulated with different tryptamines via sequential C(Ar)−O bond cleavage of phenols, C−H bond activation of tryptamines, and C−N/C−C bond formations. This method provides a new protocol of converting lignin phenols into high-value-added compounds, such as natural product Komavine.

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cross-coupling of phenols to generate cyclohexyl compounds (Scheme 1b);7 and cross-coupling of phenols with different nucleophilic reagents to form aryl compounds (Scheme 1c).8 However, up to now, only one new C−hetero or C−C bond (except for C−H bonds) was formed in the above-mentioned works, which limits the scope and efficiency in converting phenols into more complex compounds, such as natural products. In addition, some natural products and pharmaceutical agents include a spirocyclic tetrahydro-β-carboline skeleton, such as the natural products Komavine (1),9 Acetylkomavine (2),9 and Tabertinggine (3)10 (Figure 1). It is highly desirable

ignins are the second most abundant organic resource on Earth as an underutilized renewable matter.1 They are phenolic polymers linked by C−O−C bonds and C−C bonds.2 As phenols are readily available by degradation of the lignins,3 converting phenols into high-value-added fine chemicals as alternatives to using the fossil-based resources will be highly desirable for sustainable development.4 Consequently, many important achievements have been made by chemists: examples include converting phenol into cyclohexanone,5 cyclohexanol, and cyclohexane6 (Scheme 1a); reductive Scheme 1. Different Strategies for Converting Phenols into Value

Figure 1. Representative tetrahydro-β-carboline natural products.

to synthesize spirocyclic tetrahydro-β-carboline skeletons from phenols. We envisioned that phenol can be converted to cyclohexanone via hydrogenation in situ, which then condenses with amine to form imine. Further cyclization of the imine with an indole ring via our recently developed tryptamine C−H functionalization11 will generate the spirocycle products. However, there are two major competing reactions in this transformation: (1) under the reductive conditions, the double bond of the imine intermediate is more easily reduced to form amine than the reaction with the indole ring of tryptamine at the C-2 position to form a C−C bond;7j Received: July 25, 2019

© XXXX American Chemical Society

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DOI: 10.1021/acs.orglett.9b02613 Org. Lett. XXXX, XXX, XXX−XXX

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(74%) significantly (Table 1, entry 16). Changing the reaction temperature decreased the yields (Table 1, entries 17 and 18). The details of the effects of solvent, reaction time, hydride sources, and others were shown in the Supporting Information. After obtaining the optimized reaction conditions, different phenols were explored with tryptamine and 15.0 equiv of sodium formate using Pd(OH)2/C(10 mol %) in aqueous solvent (1.0 mL) at 100 °C under an argon atmosphere together with a small amount of toluene (0.2 mL) for 6 h. As shown in Scheme 2, phenol (5a) and halogen-containing

(2) competing N-cyclohexylation of the nitrogen of the indole ring via a dearomatization−rearomatization process.12 Herein, overcoming these challenges, we report the palladiumcatalyzed conversion of phenols into tetrahydro-β-carboline bearing a spirocycle at the C-1 position via sequential C(Ar)− O bond cleavage of phenols, C−H bond activation of tryptamines, and C−N/C−C bond formations (Scheme 1d). Tryptamine (4a) and phenol (5a) were selected as model compounds to evaluate the reaction conditions. When those compounds were catalyzed by Pd(OH)2/C (10 mmol %) and refluxed at 100 °C for 6 h using sodium formate as a reductant under an argon atmosphere in water (1.0 mL), a moderate yield (47%) of the cross-annulation product Komavine (1) was obtained (Table 1, entry 1). Increasing the load of sodium

Scheme 2. Cross-Annulation of Different Phenols with Tryptaminea

Table 1. Screening Conditionsa

entry

catalyst

5a (equiv)

[HCO2Na] (equiv)

time (h)

1b (%)

1 2 3 4 5 6 7 8 9 10c 11d 12e 13d 14d 15d 16d,f 17d,g 18d,h

Pd(OH)2/C Pd(OH)2/C Pd(OH)2/C Pd(OH)2/C Pd(PPh3)4 Pd/C Pd(OAc)2 PdCl2 Pd(OH)2 Pd(OH)2/C Pd(OH)2/C Pd(OH)2/C Pd(OH)2/C Pd(OH)2/C Pd(OH)2/C Pd(OH)2/C Pd(OH)2/C Pd(OH)2/C

2 2 2 2 2 2 2 2 2 2 2 2 1.5 2.5 3 3 3 3

5 10 15 20 15 15 15 15 15 15 15 15 15 15 15 15 15 15

6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6

47 67 73 52 n.p. 67 trace 2 trace 78 84 84 67 86 92 (89) 74 85 73

a

General conditions: 4a (0.2 mmol), 5a, HCO2Na, palladium catalyst (10 mol %), and water (1.0 mL) at 100 °C under Ar. bNitromethane was used as an internal standard to determine the yields by 1H NMR; isolated yield is shown in brackets. cToluene (0.1 mL). dToluene (0.2 mL). eToluene (0.3 mL). fPd(OH)2/C (5 mol %). g80 °C. h120 °C.

a

Reaction conditions: 4a (0.2 mmol), 5 (3.0 equiv), Pd(OH)2/C (10 mol %), HCO2Na (15.0 equiv), and PhMe (0.2 mL) in H2O (1.0 mL) at 100 °C for 6 h under Ar; isolated yields. bFor 10 h. c Pd(OH)2/C (15 mol %). dHCO2Na (20.0 equiv). e3-Methoxyphenol was used as substrate.

formate to 15 equiv raised the yield to 73% (Table 1, entries 2 and 3). Further elevating the load to 20 equiv decreased the yield to 52% (Table 1, entry 4), which increased the competing reductive amination reaction to generate the N-cyclohexyl tryptamine byproduct. Other catalysts, such as Pd(PPh3)4, Pd/ C, Pd(OAc)2, PdCl2, and Pd(OH)2, gave lower yields (Table 1, entries 5−9). Interestingly, while Pd(OH)2 without an activated carbon support gave only trace product (Table 1, entry 9), for Pd(OH)2 with an activated carbon support, a 73% yield of product was obtained (Table 1, entry 3). It means that the activated carbon was important to this transformation. A small amount of toluene was added to increase the solubility of phenol, and the yield was elevated slightly (Table 1, entry 10). Increasing the amount of toluene or phenol increased the yields (Table 1, entries 11 and 12; Table 1, entries 13−15). Reducing palladium catalyst to 5 mol % lowered the yield

phenols (F, Cl, and Br, 5b−d) reacted smoothly to afford the same natural product 1 (with dehalogenation for the latter) in good to high yields. Phenols bearing various linear alkyls also reacted successfully to form the cross-annulation products 6b− e in good yields with a moderate diastereomeric ratio. 4Propylphenol, being one of the lignin degradation products, could form the product 6f in 79% yield. Interestingly, when the bulky tert-butyl group was substituted at the C-4 position of phenol, the sole diastereoisomer was generated with moderate yield (Scheme 2, 6g). para-Cyclohexyl phenol also reacted smoothly with good yield and moderate diastereoselectivity (Scheme 2, 6h). Having a methoxy group substituted at the C3 position of phenol, 3-methoxyphenol reacted with tryptophanol to give demethoxylation product 6i in good B

DOI: 10.1021/acs.orglett.9b02613 Org. Lett. XXXX, XXX, XXX−XXX

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successfully reacted with phenol to generate the spirocyclic 5,6,7,8-tetrahydro-1,7-naphthyridine skeleton (6u) via C-2 position C−H functionalization of pyridine. Considering that the phenols might be reduced to form cyclohexanone as an intermediate in this reaction system, cyclohexanone was reacted with tryptamine under standard conditions. To our delight, 80% yield of cyclization product 1 was isolated (Scheme 4). Combining our recent research work

yield. It is noteworthy that the methoxy group remained intact at the C-4 position of phenol (Scheme 2, 6j). tert-Butyl carbamate was also tolerated in this reaction system, giving 78% yield of the desired product 6k. A wide range of substituted tryptamines was also investigated (Scheme 3). Electron-donating groups or Scheme 3. Cross-Annulation of Phenol with Different Tryptaminesa

Scheme 4. Control Experiments

on C−H activation of tryptamine with the above experiments,11 a possible mechanism was proposed in Scheme 5: Scheme 5. Tentative Mechanism for the Cross-Annulation

a

Reaction conditions: 4 (0.2 mmol), phenol (5a) (3.0 equiv), Pd(OH)2/C (10 mol %), HCO2Na (15.0 equiv), and PhMe (0.2 mL) in H2O (1.0 mL) at 100 °C for 6 h under Ar; isolated yields. b Toluene was not added. cHCO2Na (25.0 equiv). dPd(OH)2/C (20 mol %). e1-(6-(4-Methoxyphenyl)pyridin-3-yl)propan-2-amine (0.2 mmol), phenol (5a) (3.0 equiv), HCO2Na (10.0 equiv), Pd(OH)2/C (20 mol %), and benzene (0.2 mL) in water (1.0 mL) at 100 °C for 12 h.

Palladium-catalyzed reduction of phenol A with HCO2Na in water generates cyclohexanone B.13 Then, imine D is formed by condensation with tryptamine C. Imine D plays a role of transient directing group to selectively activate C−H of tryptamine. Intermediate E is generated by coordinating the lone pair of Schiff base to form intermediate E.14 Finally, the C−C bond is formed via addition of a C−N double bond to generate the spiro-product F. In conclusion, a palladium-catalyzed conversion of phenols to tetrahydro-β-carbolines bearing a spirocycle at the C-1 position was developed. Different substituted phenols were successfully cross-annulated with various substituted tryptamines via a sequential C−O bond cleavage of phenol, C−H bond activation of tryptamine, and C−N/C−C bond formations to construct spiro-ring tetrahydro-β-carboline derivatives. Notably, high chemoselectivity was obtained by tolerating various functional groups including ethers, esters, alcohols, and amides. The presented work developed a new protocol of converting phenolic lignin model monomers into high-value-added compounds, such as the natural product Komavine.

electron-withdrawing groups substituted at different positions of tryptamines were reacted smoothly to obtain crossannulation products (Scheme 3, 6l−n). Interestingly, when the C-1 position of the chain of substrates beared an alkyl substitution, excellent yields were obtained (Scheme 3, 6o−p). The spirocyclic product 6q was generated with good yield by controlling competitive ester−amide exchange when methyl tryptophanate hydrochloride was chosen for this transformation. It is noteworthy that an excellent yield of spirocyclic product 6i was obtained when tryptophanol was selected as substrate excluding the competitive hemiaminal ether product. Moreover, good to excellent yields were obtained when various tryptophanamide derivatives were used under the standard conditions (Scheme 3, 6r−t). Notably, pyridine substrate also C

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.9b02613. General experimental procedures, characterization details, and copies of 1H and 13C NMR spectra of new compounds (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Huiying Zeng: 0000-0002-2535-111X Chao-Jun Li: 0000-0002-3859-8824 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the NSFC (No. 21971093) and Fundamental Research Funds for the Central Universities (lzujbky-2018-62), the International Joint Research Centre for Green Catalysis and Synthesis, Gansu Provincial Sci. & Tech. Department (Nos. 2016B01017, 18JR3RA284, and 18JR4RA003), and Lanzhou University for support of our research. We also thank the Canada Research Chair (Tier I) foundation, the E.B. Eddy endowment fund, the CFI, NSERC, and FQRNT (to C.-J.L).

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