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DOI: 10.1055/a-2608-1058
Lipase-Catalyzed Esterification of Protected Amides under Ultrasound Irradiation via N–C(O) Cleavage
Abstract
A novel ultrasound-assisted protocol has been devised for the highly chemoselective esterification of tert-butyloxycarbonyl (Boc)-activated secondary carboxamides with alcohols. This study highlights the catalytic versatility of PPL lipase, which extends beyond their traditional hydrolytic functions to include synthetic applications like esterification and transesterification. The research investigates the enzymatic esterification of protected amides with alcohols, utilizing PPL lipase as a biocatalyst under ultrasound irradiation. This method can achieve high reaction rates and selectivities under mild conditions.
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Amides are essential functional groups and significant structural components in organic molecules.[1] [2] [3] [4] They are the basic building blocks of proteins and are found in a variety of pharmaceuticals,[5] natural products,[6,7] polymers,[8] and synthetic chemicals.[9] Amides have been described as weak electrophiles, owing to the resonance stability of the amide bond.[1] [3] This stability and reactivity are linked to the amide bond’s planar resonance, which ranges between 15 to 20 kcal/mol.[10] [11] [12] [13] [14] However, any modification of the amide bond that breaks planar conjugation causes significant changes in its physical and chemical characteristics. In particular, cleaving amide bonds directly under the necessary conditions has proven challenging due to its resonance, thus far, primarily because of the ester group’s high reactivity. Esters are one of the most important functional groups in organic chemistry and are found in a wide range of natural compounds, synthetic intermediates, polymers, and medications.[15] [16] [17] Figure [1] illustrates the structural diversity of biologically active esters, showcasing their functional groups and potential mechanisms of action.[18] [19] This highlights their versatility and importance in various biological active drugs. Therefore, recent research efforts to manage hypertension have focused on blocking or competing with Angiotensin II (Ang II) for receptor binding. Benzyl benzoate and benzyl 4-methylbenzoate inhibited this binding. In vivo, the anti-hypertensive effects of benzyl benzoate and benzyl 4-methylbenzoate were also evaluated in mice, and both showed inhibitory activity.[19]
Owing to the importance of ester, many methods have been reported for the interconversion of amide to ester. The amide-to-ester interconversion utilizing a metal catalyst for the activation of the amide bond to generate a flexible intermediate was described by Garg et al.[20] [21] [22] and Danoun et al.[23] in 2017. It has been reported recently that amides can be esterified at high temperatures via a fluoride-catalyzed process.[24]
Several transition-metal-free amide esterification reactions have also been reported previously by many techniques. Amide esterification can occur by amide alcoholysis under basic circumstances like K₃PO₄ and Cs₂CO₃ (Figure [2]A, B),[18] [25] or directly through dimethyl sulfate (Figure [2]C).[26] The use of traditional chemical catalysts such as potassium phosphate (K₃PO₄), cesium carbonate (Cs₂CO₃), and dimethyl sulfate in the esterification of amides presents several drawbacks. K₃PO₄ and Cs₂CO₃, being strong bases, can lead to unwanted side reactions, reduced selectivity, and potential degradation of sensitive substrates. Dimethyl sulfate, while effective as a methylating agent, is extremely poisonous, carcinogenic, and has substantial environmental and handling risks. In addition, these chemical catalysts frequently need harsh reaction conditions, such as high temperatures and long reaction durations, which may limit their use in green chemistry. In contrast, the use of lipase as a biocatalyst under ultrasound-assisted conditions in dimethyl sulfoxide (DMSO) solvent offers significant advantages. Lipase is highly selective, operating under mild conditions that prevent substrate degradation and minimize side reactions.
Enzymes are becoming increasingly popular in organic synthesis due to their chemo-, regio-, and stereoselective reactivity under mild reaction conditions. Lipases are hydrolytic enzymes with a surface loop and lid-like shape. The active catalytic site of lipase is comprised of three main residues: serine, histidine, and aspartate, or glutamate.[27] Lipases are versatile enzymes that can catalyze both hydrolytic and synthetic transformations, and they are currently being used extensively in industry. Various lipases, such as Porcine Pancreatic Lipase (PPL), Candida Cylindracea Lipase (CCL), Candida Antarctica Lipase B (CAL-B), Pseudomonas Cepacia Lipase (PCL), and Pseudomonas Stutzeri Lipase (PSL), have been demonstrated to catalyze the aminolysis of esters, resulting in amides.[28] [29] [30] To the best of our knowledge, there has been no report on the esterification of protected amides with lipase as a bio-catalyst.
In view of the above and in order to exploit the catalytic nature of lipase here, we present a generic, transition-metal-free, ultrasound-assisted PPL lipase-catalyzed, easily operable technique for esterifying amides at low temperatures using a highly selective N–C(O) bond cleavage (Figure 2D). In the present technique, esters were formed along with amine as minor product. These mild conditions are especially effective for converting amides into esters using benzyl alcohol, a reaction that is hard to achieve with metal-based methods for activating amide bonds.
Due to the unique ability of ultrasound (US) to accelerate numerous organic reactions through cavitation, it offers shorter reaction times and higher yields compared to conventional heating methods or catalyst-based systems.[31] [32] [33] The cavitation phenomenon, involving bubble formation, growth, and collapse under sound irradiation, generates immense energy and transforms kinetic energy into localized heating.[34,35] Ultrasound irradiation activates lipase by flipping the lid surface, revealing the active binding site.[36,37] Ultrasound-assisted lipase-catalyzed synthesis expands beyond traditional methods by increasing reaction rates.
To synthesize the product benzyl benzoate (3a), unactivated amide, such as N-phenylbenzamide, was first put through the esterification reaction with benzyl alcohol. Lipase was used as a biocatalyst in solvent under ultrasound at 750 W. However, the abovementioned parameters did not result in the expected product of benzyl benzoate (3a) being obtained. Next, using the identical reaction conditions, we performed the reaction using N-Boc-N-phenylbenzamide [tert-butyl benzoyl(phenyl)carbamate, 1a] with benzyl alcohol (2a) in the presence of lipase as a catalyst. We were delighted that this reaction produced a good product yield of 3a. Motivated by this outcome, model substrates N-Boc-N-phenylbenzamide (1a), and benzyl alcohol (2a) were selected, and various reaction conditions were investigated.
No reaction happened even after a long period when the model reaction was conducted at 35 °C without a catalyst (Table [1], entry 1). After that, the model reaction was carried out at 35 °C temperature with a catalyst and in the presence of several polar protic solvents, such as water, methanol, and ethanol, resulting in the product 3a in 20–25% yield (entries 2–4). A 40% yield of benzoic acid (5a) was formed when the reaction was carried out in water solvent. However, using polar aprotic solvents, including acetonitrile, DCM, DMF, and DMSO, resulted in the product 3a in 45–70% yield (entries 5–8). On the other hand, when non-polar solvents such as hexane, benzene, toluene, and 1,4-dioxane were used, the product 3a was produced in 40–48% yield (entries 9–12). Out of all the solvents tested, DMSO demonstrated the highest yield 70% of our desired product 3a and 10% tert-butyl phenylcarbamate (4a) in 120 minutes under ultrasound-irradiation conditions. After that, we looked at how temperature affected the yield of product 3a (entries 13–17). When the temperature was increased from 35 to 40 °C, we noted a 70 to 75% increase in the yield of product 3a (entry 13) in a short period. On the other hand, the yield of product 3a dropped to 60% when the reaction temperature was lowered to 25 °C (entry17).
a Reaction conditions: N-Boc-N-phenylbenzamide (1a; 1.0 mmol), benzyl alcohol (2a; 1.0 mmol), and lipase (100–400 mg) in solvent (10 mL) under ultrasound irradiation at 750 W, 5000 J, 20% amplitude, pulse 3 sec on and 1 sec off, at 25–50 °C temperature.
b Isolated yield.
The lipase load (100, 150, 200, 300, and 400 mg) was also changed to optimize the reaction conditions. The results are displayed in Table [1]. We found that adding 300 mg of lipase in the presence of DMSO produced 87% yield of product 3a in shorter time (Table [1], entry 20). The temperature at which enzymatic reactions occur plays a crucial role in affecting enzymes’ catalytic efficiency and stability. In this experiment, the model reaction was carried out at six different temperatures, ranging from 25 to 50 °C, showing significant differences in yield at various temperatures (Table [1]). Consequently, 40 °C was determined to be the optimal temperature for achieving the highest yield.
The following optimal conditions were determined after experimentation: activated amide (1a; 1.0 mmol) and benzyl alcohol (2a; 1.0 mmol) were reacted in the presence of lipase (300 mg) as the biocatalyst at 40 °C, using DMSO as the solvent, under ultrasound irradiation. The application of ultrasound significantly accelerated the reaction, yielding a high product output. To explore the effect of ultrasound energy on reaction time and yield, the model reaction was tested at energy levels ranging from 500 to 10 000 J. The results are summarized in Table [2].
After determining the optimal conditions (Table [1], entry 20), we investigated the scope and limitations of the developed esterification reaction. A range of N-Boc-activated aromatic amides were tested with various alcohols and reacted under the optimized conditions. It was observed that the reaction proceeded smoothly with both the nucleophilic aliphatic and aromatic alcohol, resulting in good product yields (Schemes 1, 2). To evaluate the electronic influence of substituents on the aromatic ring of alcohols, satisfactory yields were obtained in the presence of both electron-donating and electron-withdrawing groups (Scheme [1, 3f–l]). We have synthesized two biological drugs benzyl benzoate (3a), and benzyl 4-methylbenzoate (3b) in 87% and 82% yield, respectively.[19]
a Isolated yield.
The aromatic alcohols containing electron-donating groups were well tolerated and afforded the ester products in good yields (Scheme [2], 3af–ai). On the other hand, electron-withdrawing groups produced moderate to good yields because of their decreased nucleophilicity (Scheme [2], 3aj–ao). The current method can be directly applied to the transition-metal-free conversion of amides to thioesters without altering the reaction conditions (Scheme [3], 3ap,aq).
We tested the methodology’s feasibility in multigram-scale synthesis (Scheme S1 in SI). Under the optimal reaction conditions, N-Boc-N-phenylbenzamide (1a; 1.480 g, 5.0 mmol), and phenol (2k; 0.439 mL, 5.0 mmol) reacted to produce the desired product phenyl benzoate (3aa; 0.879 g, 83%) and tert-butyl phenyl carbamate (4a; 0.096 g, 10%).
Competition Reaction
Knowing the selectivity pattern of alcohols and amides under ultrasonic esterification may be useful. Various amides and alcohols were subjected to intermolecular competition in these experiments. Scheme [4] A shows that electron-withdrawing amides are more reactive than electron-donating amides due to their competition between electron-donating and electron-withdrawing amides with benzyl alcohol, which produced 3c as a significant product. Subsequent intermolecular competition reactions suggested that alcohol’s nucleophilicity is the dominant factor between amide and various alcohols (Scheme [4] B).
We focused our research on mechanistic pathways with optimal standard reaction conditions, along with some control experiments (Scheme [5]). Initially, we performed the reactions of activated amide 1a and benzyl alcohol (2a) in the absence of lipase at US conditions and found no reaction (Scheme [5]A), indicating the importance of lipase in the process. Next, we carried out our reaction with lipase in the absence of ultrasound and obtained only a trace amount of the desired product (Scheme [5]B). We also observed no response in the absence of lipase and ultrasound (Scheme [5]C). We changed our reaction toward fundamental conditions such as K2CO3, K3PO4, and Cs2CO3, but in all cases, only a trace amount of our supposed products was received (Scheme [5]D).
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Mechanistic Studies
A plausible mechanism is suggested for this reaction, as illustrated in Scheme [6], based on the reaction’s results and comparable transformations documented in the literature.[25] [38] First, three hydrogen bonds polarize N-Boc-N-phenylbenzamide (1a) as they enter the enzyme’s binding pocket, forming intermediate I. Next, a proton is moved from benzyl alcohol (2a) to the imidazolyl of histidine (His) residue of lipase. Then the alcohol oxide anion immediately attacks intermediate I to create intermediate II, and further intermediate III is created after abstracting the proton from imidazolyl of His. The intermediate III is produced when benzyl alcohol and N-Boc-N-phenylbenzamide combine. After eliminating tert-butyl phenylcarbamate (4a), the intended product benzyl benzoate (3a) was produced by cleavage of the C–N bond in this unstable intermediate III.
In conclusion, we have presented an innovative, eco-friendly method for PPL lipase-catalyzed esterification of activated amide under ultrasound irradiation. In recent years, the US has become a viable alternative energy source for organic synthesis. The main advantages of this approach over traditional ones are transition metal-free esterification, shorter reaction times, increased yields, fewer stages in the reaction, and lower by-product formation. US acts as a reaction accelerator and synergizes with solvents and catalysts.
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All chemicals were purchased from Sigma-Aldrich suppliers and used without further purification. TLC was conducted with analytical thin layer percolated E. Merck 60 GF254 silica gel plates and spots were visualized using UV light and column chromatography was carried out using silica gel of 60–120 mesh size. The melting points were determined in an open capillary melting point apparatus and are uncorrected. Ultrasonic irradiation was performed using Sonics Vibra Cell Ultrasonic Processor Model VCX750 (Sonics & Materials, Inc.) with a fixed power of 750 W and amplitude variation from 20–80%, and a tapered micro tip was used as an ultrasonic probe operating at a frequency of 20 kHz. 1H, 13C, and 19F NMR spectra were recorded on a Bruker Avance 500 MHz spectrometer in CDCl3 and DMSO-d 6 using TMS as the internal standard at 500 and 126 MHz frequency, respectively. HRMS (m/z) were recorded in an electron ionization or electrospray ionization (ESI) mode on Waters-Q-TOF Premier-HAB213 and Sciex X500R QTOF instruments. All products synthesized were confirmed by using melting point, 1H, 13C, 19F NMR, and mass spectra, and the reported compounds were compared with the literature data.
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Synthesis of N-Boc-Activated Secondary Amides 1; General Procedure
N-Boc-activated amides were synthesized according to the reported method.[39] In an oven-dried round-bottom flask, amide (1.0 equiv.), DMAP (0.1 equiv.), and DCM were added, and the reaction temperature was maintained at 0 °C. Boc anhydride (1.5 equiv.) was then added dropwise. The reaction mixture was stirred at rt for 24 h after the addition. The reaction’s progress was monitored by TLC. Upon completion, the mixture was concentrated under reduced pressure, and purified by column chromatography, and the product was obtained in excellent yield.
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tert-Butyl Benzoyl(phenyl)carbamate (1a)[39]
Yield 96%; white solid; mp 96 °C.
1H NMR (500 MHz, CDCl3): δ = 7.74 (d, J = 7.4 Hz, 2 H), 7.53 (t, J = 7.2 Hz, 1 H), 7.46–7.42 (m, 4 H), 7.35 (t, J = 7.3 Hz, 1 H), 7.28 (d, J = 7.8 Hz, 2 H), 1.24 (s, 9 H).
13C NMR (126 MHz, CDCl3): δ = 172.91, 153.42, 139.21, 137.09, 131.83, 129.33, 128.40, 128.25, 128.07, 127.92, 83.62, 27.59.
HRMS (ESI): m/z calcd for C18H19NO3 [M + H]+: 298.1443; found: 298.1369.
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tert-Butyl (4-Methylbenzoyl)(phenyl)carbamate (1b)
Yield: 92%; white solid; mp 220 °C.
1H NMR (500 MHz, DMSO-d 6): δ = 8.94 (t, J = 4.6 Hz, 1 H), 7.80 (d, J = 6.5 Hz, 2 H), 7.35–7.27 (m, 6 H), 3.32 (s, 9 H), 2.36 (s, 3 H).
13C NMR (126 MHz, DMSO-d 6): δ = 172.80, 141.94, 138.27, 133.40, 130.17, 129.76, 128.59, 128.22, 127.48, 81.36, 21.92.
HRMS (ESI): m/z calcd for C19H21NO3 [M + H]+: 312.1600; found: 312.1598.
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tert-Butyl (4-Fluoromethyl)(benzoyl)(phenyl)carbamate (1c)
Yield: 95%; white solid; mp 75 °C.
1H NMR (500 MHz, DMSO-d 6): δ = 7.99 (d, J = 2.6 Hz, 2 H), 7.35–7.28 (m, 7 H), 1.13 (s, 9 H).
13C NMR (126 MHz, DMSO-d 6): δ = 174.69, 165.62, 164.36 (d, J = 248.22 Hz), 163.38, 140.05, 131.29 (d, J = 2.52 Hz), 130.35 (d, J = 8.82 Hz), 128.75, 127.45 (d, J = 57.96 Hz), 115.70 (d, J = 21.42 Hz), 81.48, 28.40.
19F NMR (471 MHz, DMSO-d 6): δ = –100.68.
HRMS (ESI): m/z calcd for C18H18FNO3 [M + H]+: 316.1349; found: 316.1342.
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tert-Butyl (2-Chlorobenzoyl)(phenyl)carbamate (1d)
Yield: 94%; white solid; mp 235 °C.
1H NMR (500 MHz, CDCl3): δ = 7.51–7.46 (m, 3 H), 7.40–7.33 (m, 6 H), 1.21 (s, 9 H).
13C NMR (126 MHz, CDCl3): δ = 169.69, 152.15, 138.40, 137.75, 130.83, 130.21, 129.51, 129.35, 128.80, 128.42, 128.38, 127.11 84.04, 27.51.
HRMS (ESI): m/z calcd for C18H18ClNO3 [M + H]+: 332.1053; found: 332.1045.
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tert-Butyl (3-Nitrobenzoyl)(phenyl)carbamate (1e)
Yield: 95%; white solid; mp 225 °C.
1H NMR (500 MHz, CDCl3): δ = 8.43 (s, 1 H), 8.27 (d, J = 6.0 Hz, 1 H), 7.93 (d, J = 6.1 Hz, 1 H), 7.55 (t, J = 6.4 Hz, 1 H), 7.36 (t, J = 6.1 Hz, 2 H), 7.29 (d, J = 5.8 Hz, 1 H), 7.17 (d, J = 5.7 Hz, 2 H), 1.18 (s, 9 H).
13C NMR (126 MHz, CDCl3): δ = 170.31, 152.92, 148.01, 138.45, 133.79, 129.62, 129.47, 128.44, 128.12, 126.00, 123.03, 84.49, 28.45, 27.68.
HRMS (ESI): m/z calcd for C18H18N2O5 [M + H]+: 343.1294; found: 343.1289.
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PPL Lipase-Catalyzed Esterification of N-Boc-Activated Secondary Amides with Alcohols; Benzyl Benzoate (3a);[40] Typical Procedure
In a beaker, were added the N-Boc-N phenylbenzamide (1.0 mmol), benzyl alcohol (2a; 1.0 mmol), lipase (300 mg), and DMSO (10 mL). The mixture was irradiated under ultrasonication at 750 W of power, 5000 J energy, 20% amplitude, pulse 3 sec on and 1 sec off, at 40 °C temperature for the necessary time. TLC was used to monitor the reaction’s completion. The reaction mixture was then diluted with EtOAc and the EtOAc layer was washed with H2O. To obtain the desired product, the organic layer was dried (Na2SO4) and the mixture was concentrated under reduced pressure. Finally, the target product 3a and side product 4a were obtained by column chromatography on silica gel with n-hexane/EtOAc as the eluent; yield: 87%; colorless liquid.
1H NMR (500 MHz, CDCl3): δ = 8.15 (d, J = 7.1 Hz, 2 H), 7.60 (t, J = 7.4 Hz, 1 H), 7.52–7.38 (m, 7 H), 5.43 (s, 2 H).
13C NMR (126 MHz, CDCl3): δ = 166.49, 136.13, 133.10, 130.20, 129.81, 128.68, 128.45, 128.31, 128.24, 66.75.
HRMS (ESI): m/z calcd for C14H12O2 [M + H]+: 213.0916; found: 213.1026.
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Benzyl 4-Methylbenzoate (3b)[41]
Yield: 82%; colorless liquid.
1H NMR (500 MHz, CDCl3): δ = 8.1 (d, J = 8.2 Hz, 2 H), 7.55–7.39 (m, 5 H), 7.28 (d, J = 8.0 Hz, 2 H), 5.45 (s, 2 H), 2.44 (s, 3 H).
13C NMR (126 MHz, CDCl3): δ = 166.48, 143.75, 136.41, 129.91, 129.84, 129.23, 128.70, 128.28, 127.60, 66.56, 21.65.
HRMS (ESI): m/z calcd for C15H14O2 [M + H]+: 227.1072; found: 227.1065.
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Benzyl 4-Fluorobenzoate (3c)
Yield: 85%; colorless liquid.
1H NMR (500 MHz, CDCl3): δ = 8.14 (dd, J = 8.9, 5.5 Hz, 2 H), 7.50 (d, J = 7.2 Hz, 2 H), 7.46–7.38 (m, 3 H), 7.15–7.12 (m, 2 H), 5.41 (s, 2 H).
13C NMR (126 MHz, CDCl3): δ = 166.87, 166.44, 165.86 (d, J = 254.52 Hz), 165.47, 164.85, 136.00, 132.39, 132.31, 132.23, 128.69, 128.33 (d, J = 11.34 Hz), 126.45 (d, J = 2.52 Hz), 115.58 (d, J = 21.42 Hz), 66.87.
19F NMR (471 MHz, CDCl3): δ = –105.36.
HRMS (ESI): m/z calcd for C14H11FO2 [M + H]+: 231.0821; found: 231.0819.
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Benzyl 2-Chlorobenzoate (3d)
Yield: 85%; colorless liquid.
1H NMR (500 MHz, CDCl3): δ = 7.89 (d, J = 7.0 Hz, 1 H), 7.50 (d, J = 7.2 Hz 2 H), 7.48–7.37 (m, 5 H), 7.30 (td, J = 7.6, 1.3 Hz, 1 H), 5.42 (s, 2 H).
13C NMR (126 MHz, CDCl3): δ = 165.47, 135.68, 133.88, 132.72, 131.64, 131.17, 130.01, 128.70, 128.44, 126.67, 67.32.
HRMS (ESI): m/z calcd for C14H11ClO2 [M + H]+: 247.0526; found: 247.0521.
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Benzyl 3-Nitrobenzoate (3e)
Yield: 85%; colorless liquid.
1H NMR (500 MHz, CDCl3): δ = 8.90 (s, 1 H), 8.43–8.40 (m, 2 H), 7.66 (t, J = 8.0 Hz, 2 H), 7.50–7.39 (m, 5 H), 5.54 (s, 2 H).
13C NMR (126 MHz, CDCl3): δ = 164.35, 148.30, 135.43, 135.32, 131.91, 129.66, 128.78, 128.67, 128.52, 127.54, 124.68, 124.65, 67.64.
HRMS (ESI): m/z calcd for C14H11NO4 [M + H]+: 258.0766; found: 258.0762.
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4-Bromobenzyl Benzoate (3f)[25]
Yield: 85%; colorless liquid.
1H NMR (500 MHz, CDCl3): δ = 8.09 (d, J = 7.1 Hz, 2 H), 7.59 (t, J = 7.4 Hz, 1 H), 7.54 (d, J = 8.4 Hz, 2 H), 7.47 (t, J = 7.8 Hz, 2 H), 7.35 (d, J = 8.5 Hz, 2 H), 5.34 (s, 2 H).
13C NMR (126 MHz, CDCl3): δ = 166.34, 135.10, 133.21, 131.77, 129.89, 129.76, 128.47, 122.32, 65.92.
HRMS (ESI): m/z calcd for C14H11BrO2 [M + H]+: 291.0021; found: 291.0018.
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4-Chlorobenzyl Benzoate (3g)[41]
Yield: 85%; white solid; mp 59 °C.
1H NMR (500 MHz, CDCl3): δ = 8.10 (d, J = 7.9 Hz, 2 H), 7.59 (t, J = 7.4 Hz, 1 H), 7.48–7.38 (m, 6 H), 5.36 (s, 2 H).
13C NMR (126 MHz, CDCl3): δ = 166.32, 134.64, 134.17, 133.19, 129.98, 129.77, 129.61, 128.83, 128.47, 65.88.
HRMS (ESI): m/z calcd for C14H11ClO2 [M + H]+: 247.0526; found: 247.0520.
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4-Chlorobenzyl 4-Methylbenzoate (3h)[41]
Yield: 82%; white solid; mp 135 °C.
1H NMR (500 MHz, CDCl3): δ = 7.98 (d, J = 8.2 Hz, 2 H), 7.42–7.37 (m, 4 H), 7.26 (d, J = 8.0 Hz, 2 H), 5.34 (s, 2 H), 2.43 (s, 3 H).
13C NMR (126 MHz, CDCl3): δ = 166.40, 143.91, 134.75, 134.09, 129.79, 129.54, 129.17, 128.79, 127.21, 65.69, 21.65.
HRMS (ESI): m/z calcd for C15H13ClO2 [M + H]+: 261.0682; found: 261.0679.
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4-Chlorobenzyl 2-Chlorobenzoate (3i)
Yield: 83%; white solid; mp 154 °C.
1H NMR (500 MHz, CDCl3): δ = 7.86 (d, J = 6.7 Hz, 1 H), 7.48–7.31 (m, 7 H), 5.36 (s, 2 H).
13C NMR (126 MHz, CDCl3): δ = 165.39, 134.30, 134.10, 133.89, 134.06, 132.78, 131.58, 131.50, 131.18, 129.78, 128.84, 126.65, 66.45.
HRMS (ESI): m/z calcd for C14H10Cl2O2 [M + H]+: 281.0136; found: 281.0132.
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4-Methylbenzyl Benzoate (3j)[42]
Yield: 87%; colorless liquid.
1H NMR (500 MHz, CDCl3): δ = 8.12 (d, J = 7.2 Hz, 2 H), 7.59 (t, J = 7.4 Hz, 1 H), 7.47 (t, J = 7.7 Hz, 2 H), 7.40 (d, J = 7.9 Hz, 2 H), 7.24 (d, J = 7.8 Hz, 2 H), 5.38 (s, 2 H), 2.41 (s, 3 H).
13C NMR (126 MHz, CDCl3): δ = 166.54, 138.14, 133.09, 133.01, 130.27, 129.78, 129.70, 129.32, 128.41, 66.72, 21.30.
HRMS (ESI): m/z calcd for C15H14O2 [M + H]+: 227.1072; found: 227.1070.
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2,4,6-Trimethylbenzyl Benzoate (3k)[40]
Yield: 80%; colorless liquid.
1H NMR (500 MHz, CDCl3): δ = 7.54 (d, J = 7.6 Hz, 2 H), 7.46 (t, J = 7.7 Hz, 1 H), 7.41 (t, J = 7.7 Hz, 2 H), 6.92 (s, 2 H), 5.43 (s, 2 H), 2.43 (s, 6 H), 2.30 (s, 3 H).
13C NMR (126 MHz, CDCl3): δ = 170.53, 166.79, 162.42, 138.33, 134.67, 133.47, 132.95, 130.61, 130.20, 129.71, 129.16, 129.06, 128.80, 128.35, 61.63, 21.03, 19.69.
HRMS (ESI): m/z calcd for C17H18O2 [M + H]+: 255.1385; found: 255.1383.
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4-Isopropylbenzyl Benzoate (3l)
Yield: 83%; colorless liquid.
1H NMR (500 MHz, CDCl3): δ = 8.14 (d, J = 7.1 Hz, 2 H), 7.59 (t, J = 7.4 Hz, 1 H), 7.49–7.44 (m, 4 H), 7.31 (d, J = 8.1 Hz, 2 H), 5.40 (s, 2 H), 3.02–2.95 (m, 1 H), 1.33 (s, 3 H), 1.31 (s, 3 H).
13C NMR (126 MHz, CDCl3): δ = 166.55, 149.10, 133.48, 133.02, 130.29, 129.80, 129.73, 128.47, 128.40, 126.72, 66.72, 33.90, 23.99.
HRMS (ESI): m/z calcd for C17H18O2 [M + H]+: 255.1385; found: 255.1380.
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Pyridine-2-ylmethyl 4-Methylbenzoate (3m)
Yield: 80%; white solid; mp 57 °C.
1H NMR (500 MHz, CDCl3): δ = 8.64 (d, J = 7.3 Hz, 1 H), 8.03 (d, J = 8.2 Hz, 2 H), 7.74 (td, J = 7.7, 1.8 Hz, 1 H), 7.47 (d, J = 7.9 Hz, 1 H), 7.29–7.26 (m, 3 H), 5.50 (s, 2 H), 2.44 (s, 3 H).
13C NMR (126 MHz, CDCl3): δ = 166.32, 156.14, 149. 40, 143.98, 136.93, 129.90, 129.18, 127.09, 122.88, 121.72, 66.98, 21.67.
HRMS (ESI): m/z calcd for C14H13NO2 [M + H]+: 228.1025; found: 228.1020.
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Ethyl 3-Nitrobenzoate (3n)
Yield: 84%; white solid; mp 142 °C.
1H NMR (500 MHz, CDCl3): δ = 8.81 (s, 1 H), 8.39–8.34 (m, 2 H), 7.64 (t, J = 8.0 Hz, 1 H), 4.42 (q, J = 7.1 Hz, 2 H), 1.42 (t, J = 7.1 Hz, 3 H).
13C NMR (126 MHz, CDCl3): δ = 164.41, 148.20, 135.26, 135.20, 132.19, 129.63, 129.56, 127.27, 127.21, 124.45, 124.44, 66.91, 14.17.
HRMS (ESI): m/z calcd for C9H9NO4 [M + H]+: 196.0610; found: 196.0602.
#
Allyl Benzoate (3o)
Yield: 85%; colorless liquid.
1H NMR (500 MHz, CDCl3): δ = 8.08 (d, J = 8.2 Hz, 2 H), 7.55 (t, J = 7.4 Hz, 1 H), 7.43 (t, J = 7.8 Hz, 2 H), 6.09–6.01 (m, 1 H), 5.44–5.28 (m, 2 H), 4.83 (d, J = 5.6 Hz, 2 H).
13C NMR (126 MHz, CDCl3): δ = 166.18, 133.00, 132.97, 132.31, 130.18, 129.68, 129.58, 128.38, 128.37, 118.18, 65.50.
HRMS (ESI): m/z calcd for C10H10O2 [M + H]+: 163.0759; found: 163.0692.
#
2-(Trimethylsilyl)ethyl 4-Methylbenzoate (3p)
Yield: 85%; colorless liquid.
1H NMR (500 MHz, CDCl3): δ = 7.96 (d, J = 8.2 Hz, 2 H), 7.24 (d, J = 7.9 Hz, 2 H), 4.45–4.41 (m, 2 H), 2.41 (s, 3 H), 1.17–1.13 (m, 2 H), 0.11 (s, 9 H).
13C NMR (126 MHz, CDCl3): δ = 166.82, 143.30, 129.57, 129.51, 129.03, 127.96, 62.98, 21.66, 17.41, –1.44.
HRMS (ESI): m/z calcd for C13H20O2Si [M + H]+: 237.1311; found: 237.1307.
#
Phenyl Benzoate (3aa)[40]
Yield: 85%; white solid; mp 70 °C.
1H NMR (500 MHz, CDCl3): δ = 8.25 (d, J = 7.2 Hz, 2 H), 7.67 (t, J = 7.4 Hz, 1 H), 7.55 (t, J = 7.8 Hz, 2 H), 7.49–7.45 (m, 2 H), 7.31 (t, J = 7.5 Hz, 1 H), 7.26 (d, J = 7.7 Hz, 2 H).
13C NMR (126 MHz, CDCl3): δ = 165.23, 151.00, 133.64, 130.24, 129.62, 129.53, 128.60, 121.76.
HRMS (ESI): m/z calcd for C13H10O2 [M + H]+: 199.0759; found: 199.0866.
#
Phenyl 4-Methylbenzoate (3ab)[18]
Yield: 80%; white solid; mp 82 °C.
1H NMR (500 MHz, CDCl3): δ = 8.14 (d, J = 8.2 Hz, 2 H), 7.47 (t, J = 7.9 Hz, 2 H), 7.36–7.29 (t, 3 H), 7.25 (d, J = 8.0 Hz, 2 H), 2.49 (s, 3 H).
13C NMR (126 MHz, CDCl3): δ = 165.35, 151.04, 144.48, 130.26, 129.52, 129.35, 126.81 125.85, 121.82, 21,81.
HRMS (ESI): m/z calcd for C14H12O2 [M + H]+: 213.0916; found: 213.0913.
#
Phenyl 4-Fluorobenzoate (3ac)[43]
Yield: 83%; pale yellow solid; mp 125 °C.
1H NMR (500 MHz, CDCl3): δ = 8.25 (dd, J = 8.9, 5.4 Hz, 2 H), 7.46 (t, J = 7.5 Hz, 2 H), 7.31 (t, J = 7.5 Hz, 1 H), 7.25–7.19 (m, 4 H).
13C NMR (126 MHz, CDCl3): δ = 166.82 (d, J = 279.72 Hz), 165.15, 164.25, 150.84, 132.79 (d, J = 8.82 Hz), 129.53, 126.00, 125.83 (d, J = 2.52 Hz), 121.66, 115.79 (d, J = 21.42 Hz).
19F NMR (471 MHz, CDCl3): δ = –104.31.
HRMS (ESI): m/z calcd for C13H9FO2 [M + H]+: 217.0665; found: 217.0661.
#
Phenyl 2-Chlorobenzoate (3ad)
Yield: 83%; off-white solid; mp 125 °C.
1H NMR (500 MHz, CDCl3): δ = 7.86 (d, J = 7.8 Hz, 1 H), 7.49–7.42 (m, 5 H), 7.39–7.32 (m, 3 H).
13C NMR (126 MHz, CDCl3): δ = 165.39, 150.34, 134.30, 134.07, 133.88, 132.76, 131.51, 131.16, 129.77, 128.82, 126.62.
HRMS (ESI): m/z calcd for C13H9ClO2 [M + H]+: 233.0369; found: 233.0366.
#
Phenyl 3-Nitrobenzoate (3ae)
Yield: 83%; white solid; mp 80 °C.
1H NMR (500 MHz, CDCl3): δ = 9.06 (s, 1 H), 8.56–8.51 (m, 2 H), 7.76 (t, J = 8.0 Hz, 1 H), 7.50–7.47 (m, 2 H), 7.34 (t, J = 7.5 Hz, 1 H), 7.26 (d, J = 7.5 Hz, 1 H).
13C NMR (126 MHz, CDCl3): δ = 163.14, 150.52, 148.43, 135.79, 131.43, 129.91, 129.69, 128.00, 126.42, 125.12, 121.46.
HRMS (ESI): m/z calcd for C13H9NO4 [M + H]+: 244.0610; found: 244.0602.
#
2-Methylphenyl Benzoate (3af)[44]
Yield: 82%; colorless oil.
1H NMR (500 MHz, CDCl3): δ = 8.28 (d, J = 7.1 Hz, 2 H), 7.69 (t, J = 7.5 Hz, 1 H), 7.57 (t, J = 7.8 Hz, 2 H), 7.34–7.29 (m, 2 H), 7.26–7.22 (m, 1 H), 7.20–7.19 (m, 1 H), 2.29 (s, 3 H).
13C NMR (126 MHz, CDCl3): δ = 164.92, 149.60, 133.67, 131.23, 130.25, 130.18, 129.53, 128.69, 127.04, 126.14, 122.06, 16.27.
HRMS (ESI): m/z calcd for C14H12O2 [M + H]+: 213.0916; found: 213.0912.
#
p-Tolyl Benzoate (3ag)[44]
Yield: 82%; white solid; mp 122 °C.
1H NMR (500 MHz, CDCl3): δ = 8.24 (d, J = 7.1 Hz, 2 H), 7.66 (t, J = 7.4 Hz, 1 H), 7.54 (t, J = 7.7 Hz, 2 H), 7.26 (d, J = 8.1 Hz, 2 H), 7.13 (d, J = 8.4 Hz, 2 H), 2.41 (s, 3 H).
13C NMR (126 MHz, CDCl3): δ = 165.44, 148.74, 135.55, 133.53, 130.21, 130.04, 129.70, 128.57, 121.41, 20.98.
HRMS (ESI): m/z calcd for C14H12O2 [M + H]+: 213.0916; found: 213.0910.
#
p-Tolyl 4-Methylbenzoate (3ah)[44]
Yield: 80%; white solid; mp 105 °C.
1H NMR (500 MHz, CDCl3): δ = 8.12 (d, J = 8.2 Hz, 2 H), 7.33 (d, J = 8.0 Hz, 2 H), 7.25 (d, J = 8.2 Hz, 2 H), 7.12 (d, J = 8.4 Hz, 2 H), 2.48 (s, 3 H), 2.41 (s, 3 H).
13C NMR (126 MHz, CDCl3): δ = 165.50, 148.80, 144.33, 135.42, 130.26, 130.00, 129.28, 126.95, 121.45, 21.73, 20.89.
HRMS (ESI): m/z for C15H14O2 [M + H]+: 227.1072; found: 227.0989.
#
p-Tolyl 2-Chlorobenzoate (3ai)[44]
Yield: 82%; colorless liquid.
1H NMR (500 MHz, CDCl3): δ = 8.07 (dd, J = 7.8, 1.5 Hz, 1 H), 7.56–7.49 (m, 2 H), 7.43–7.39 (m, 1 H), 7.27 (d, J = 8.2 Hz, 2 H), 7.18 (d, J = 8.5 Hz, 2 H), 2.42 (s, 3 H).
13C NMR (126 MHz, CDCl3): δ = 164.34, 148.51, 135.81, 134.30, 133.14, 131.91, 131.29, 130.09, 129.55, 126.79, 121.32, 20.92.
HRMS (ESI): m/z calcd for C14H11ClO2 [M + H]+: 247.0526; found: 247.0512.
#
2-Nitrophenyl Benzoate (3aj)
Yield: 72%; colorless oil.
1H NMR (500 MHz, CDCl3): δ = 8.23 (dd, J = 8.4, 1.3 Hz, 2 H), 8.16 (dd, J = 8.2, 1.6 Hz, 1 H), 7.74–7.68 (m, 2 H), 7.55 (t, J = 7.8 Hz, 2 H), 7.48–7.44 (m, 1 H), 7.41 (dd, J = 8.1, 1.3 Hz, 1 H).
13C NMR (126 MHz, CDCl3): δ = 164.43, 144.35, 141.99, 134.80, 134.24, 130.58, 130.51, 128.78, 128.45, 126.70, 125.91, 125.42.
HRMS (ESI): m/z calcd for C13H9NO4 [M + H]+: 244.0610; found: 244.0603.
#
4-Nitrophenyl Benzoate (3ak)
Yield: 75%; white solid; mp 210 °C.
1H NMR (500 MHz, DMSO-d 6): δ = 8.27 (d, J = 8.7 Hz, 2 H), 8.06 (d, J = 8.6 Hz, 2 H), 7.97 (d, J = 7.6 Hz, 2 H), 7.65–7.55 (m, 3 H).
13C NMR (126 MHz, DMSO-d 6): δ = 166.39, 145.45, 142.60, 134.24, 132.35, 128.67, 128.01, 124.92, 120.00, 119.92.
HRMS (ESI): m/z calcd for C13H9NO4 [M + H]+: 244.0610; found: 244.0606.
#
4-Acetylphenyl Benzoate (3al)[44]
Yield: 75%; white solid; mp 143 °C.
1H NMR (500 MHz, CDCl3): δ = 8.23 (d, J = 7.1 Hz, 2 H), 8.08 (d, J = 8.8 Hz, 2 H), 7.69 (t, J = 7.5 Hz, 1 H), 7.55 (t, J = 7.8 Hz, 2 H), 7.36 (d, J = 8.8 Hz, 2 H), 2.65 (s, 3 H).
13C NMR (126 MHz, CDCl3): δ = 196.89, 164.65, 154.72, 134.83, 133.94, 130.27, 129.08, 128.70, 121.95, 26.63.
HRMS (ESI): m/z calcd for C15H12O3 [M + H]+: 241.0865; found: 241.0961.
#
4-Acetylphenyl 4-Methylbenzoate (3am)
Yield: 78%; pale yellow solid; mp 167 °C.
1H NMR (500 MHz, CDCl3): δ = 8.11–8.04 (m, 4 H), 7.37–7.29 (m, 2 H), 2.66 (s, 3 H), 2.46 (s, 3 H).
13C NMR (126 MHz, CDCl3): δ = 197.13, 172.48, 164.73, 154.86, 144.66, 134.70, 130.32, 130.24, 129.23, 126.66, 121.99, 26.67, 21.72.
HRMS (ESI): m/z calcd for C16H14O3 [M + H]+: 255.1021; found: 255.1014.
#
Naphthalen-2-yl Benzoate (3an)
Yield: 85%; white solid; mp 105 °C.
1H NMR (500 MHz, CDCl3): δ = 7.81–7.77 (m, 3 H), 7.71 (d, J = 8.2 Hz, 2 H), 7.47–7.44 (m, 2 H), 7.37–7.35 (m, 2 H), 7.18 (d, J = 2.4 Hz, 1 H), 7.13 (dd, J = 8.8, 2.5 Hz, 2 H).
13C NMR (126 MHz, CDCl3): δ = 165.25, 153.33, 134.59, 129.91, 128.96, 127.82, 127.75, 126.55, 126.34, 123.65, 117.75, 117.72, 109.51, 109.48.
HRMS (ESI): m/z calcd for C17H12O2 [M + H]+: 249.0916; found: 249.0907.
#
Naphthalen-2-yl 4-Fluorobenzoate (3ao)
Yield: 82%; white solid; mp 115 °C.
1H NMR (500 MHz, CDCl3): δ = 8.31 (dd, J = 8.8, 5.4 Hz, 2 H), 7.95–7.86 (m, 3 H), 7.73 (s, 1 H), 7.56–7.51 (m, 2 H), 7.39 (dd, J = 8.8, 2.1 Hz, 1 H), 7.24 (t, J = 8.6 Hz, 2 H).
13C NMR (126 MHz, CDCl3): δ = 166.23 (d, J = 255.78 Hz), 164.41, 148.50, 133.83, 132.86 (d, J = 10.08 Hz), 131.57, 129.54, 127.71, 126.67, 125.84 (d, J = 3.78 Hz), 121.15, 118.70, 115.85 (d, J = 21.42 Hz).
19F NMR (471 MHz, CDCl3): δ = –104.14.
HRMS (ESI): m/z calcd for C17H11FO2 [M + H]+: 267.0821; found: 267.0802.
#
S-(p-Tolyl) Benzothioate (3ap)
Yield: 82%; white solid; mp 124 °C.
1H NMR (500 MHz, DMSO-d 6): δ = 7.98 (d, J = 7.2 Hz, 2 H), 7.74 (t, J = 7.4 Hz, 1 H), 7.65–7.57 (m, 2 H), 7.41 (d, J = 8.1 Hz, 2 H), 7.33 (d, J = 7.9 Hz, 2 H), 2.38 (s, 3 H).
13C NMR (126 MHz, DMSO-d 6): δ = 189.98, 140.12, 136.39, 135.45, 130.57, 129.82, 129.67, 127.61, 127.43, 123.59, 21.28.
HRMS (ESI): m/z calcd for C14H12OS [M + H]+: 229.0687; found: 229.0606.
#
S-(4-Chlorophenyl) Benzothioate (3aq)
Yield: 80%; white solid; mp 142 °C.
1H NMR (500 MHz, DMSO-d 6): δ = 7.99 (d, J = 8.2 Hz, 2 H), 7.76 (t, J = 7.4 Hz, 1 H), 7.64–7.56 (m, 6 H).
13C NMR (126 MHz, DMSO-d 6): δ = 189.24, 137.30, 137.22, 137.12, 136.10, 135.36, 130.02, 129.91, 129.85, 129.72, 127.69, 127.50, 126.17.
HRMS (ESI): m/z calcd for C13H9ClOS [M + H]+: 249.0140; found: 249.0109.
#
tert-Butyl Phenylcarbamate (4a)
Yield: 10%; white solid; mp 135 °C.
1H NMR (500 MHz, CDCl3): δ = 7.38 (d, J = 7.9 Hz, 2 H), 7.33–7.30 (m, 2 H), 7.06 (t, J = 7.3 Hz, 1 H), 6.51 (s, 1 H), 1.55 (s, 9 H).
13C NMR (126 MHz, CDCl3): δ = 152.76, 138.33, 129.00, 123.04, 118.52, 80.52, 28.32.
HRMS (ESI): m/z calcd for C11H15NO2 [M + H]+: 194.1181; found: 194.1102.
#
Benzoic Acid (5a)
Yield: 40%; white solid; mp 120 °C.
1H NMR (500 MHz, CDCl3): δ = 11.68 (s, 1 H), 8.16 (d, J = 7.0 Hz, 2 H), 7.67–7.63 (m, 1 H), 7.53–7.50 (m, 2 H).
13C NMR (126 MHz, CDCl3): δ = 172.56, 133.86, 130.25, 129.39, 128.53, 76.82.
HRMS (ESI): m/z calcd for C7H6O2 [M + H]+: 123.0446; found: 123.0411.
#
#
Conflict of Interest
The authors declare no conflict of interest.
Acknowledgment
Priya Mahaur acknowledges the University Grants Commission (UGC) for a Senior Research Fellowship (SRF). The authors are thankful to the Central Instrumental Facility Center (CIFC), IIT (BHU) for the NMR facilities.
Supporting Information
- Supporting information for this article is available online at https://doi-org.accesdistant.sorbonne-universite.fr/10.1055/a-2608-1058.
- Supporting Information
-
References
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Corresponding Authors
Publication History
Received: 04 April 2025
Accepted after revision: 14 May 2025
Accepted Manuscript online:
14 May 2025
Article published online:
04 June 2025
© 2025. Thieme. All rights reserved
Georg Thieme Verlag KG
Oswald-Hesse-Straße 50, 70469 Stuttgart, Germany
-
References
- 1 Clayden J. Angew. Chem. Int. Ed. 2003; 42: 1788
- 2 Cupido T, Tulla-Puche J, Spengler J, Albericio F. Curr. Opin. Drug Discov. Dev. 2007; 10: 768
- 3 Greenberg A, Breneman CM, Liebman JF. The Amide Linkage: Structural Significance in Chemistry, Biochemistry, and Materials Science. Wiley; New York: 2000
- 4 Bode JW. Curr. Opin. Drug Discov. Dev. 2006; 9: 765
- 5 Roughley SD, Jordan AM. J. Med. Chem. 2011; 54: 3451
- 6 Kudo F, Miyanaga A, Eguchi T. Nat. Prod. Rep. 2014; 31: 1056
- 7 Walsh CT, O’Brien RV, Khosla C. Angew. Chem. Int. Ed. 2013; 52: 7098
- 8 Marchildon K. Macromol. React. Eng 2011; 5: 22
- 9 Vitaku E, Smith DT, Njardarson JT. J. Med. Chem. 2014; 57: 10257
- 10 Szostak R, Shi S, Meng G, Lalancette R, Szostak M. J. Org. Chem. 2016; 81: 8091
- 11 Pace V, Holzer W, Meng G, Shi S, Lalancette R, Szostak R, Szostak M. Chem. Eur. J. 2016; 22: 14494
- 12 Szostak R, Meng G, Szostak M. J. Org. Chem. 2017; 82: 6373
- 13 Meng G, Shi S, Lalancette R, Szostak R, Szostak M. J. Am. Chem. Soc. 2018; 140: 727
- 14 Singh V, Rajput K, Mishra A, Singh S, Srivastava V. Chem. Commun. 2023; 59: 14009
- 15 Ishihara K. Tetrahedron 2009; 65: 1085
- 16 Otera J, Nishikido J. Esterification: Methods, Reactions, and Applications. Wiley; New York: 2009
- 17 Kim I, Lee C. Angew. Chem. Int. Ed. 2013; 52: 10023
- 18 Li G, Lei P, Szostak M. Org. Lett. 2018; 20: 5622
- 19 Ohno O, Ye M, Koyama T, Yazawa K, Mura E, Matsumoto H, Ichino T, Yamada K, Nakamura K, Ohno T, Yamaguchi K, Ishida J, Fukamizu A, Uemura D. Bioorg. Med. Chem. 2008; 16: 7843
- 20 Hie L, Baker EL, Anthony SM, Desrosiers J, Senanayake C, Garg NK. Angew. Chem. Int. Ed. 2016; 55: 15129
- 21 Hie L, Fine Nathel NF, Shah TK, Baker EL, Hong X, Yang Y.-F, Liu P, Houk KN, Garg NK. Nature 2015; 524: 79
- 22 Dander JE, Garg NK. ACS Catal. 2017; 7: 1413
- 23 Bourne-Branchu Y, Gosmini C, Danoun G. Chem. Eur. J. 2017; 23: 10043
- 24 Wu H, Guo W, Daniel S, Li Y, Liu C, Zeng Z. Chem. Eur. J. 2018; 24: 3444
- 25 Ye D, Liu Z, Chen H, Sessler JL, Lei C. Org. Lett. 2019; 21: 6888
- 26 Qin H, Han Z, Bonku EM, Sun H, Odilov A, Zhu F, Abduahadi S, Zhu W, Shen J, Aisa HA. Commun. Chem. 2024; 7: 93
- 27 Hasan F, Shah AA, Hameed A. Enzyme Microb. Technol. 2006; 39: 235
- 28 Petchey MR, Grogan G. Adv. Synth. Catal. 2019; 361: 3895
- 29 Pintor A, Lavandera I, Volkov A, Gotor-Fernández V. ACS Sustainable Chem. Eng. 2023; 11: 10284
- 30 van Rantwijk F, Hacking MA. P. J, Sheldon RA. Monatsh. Chem. 2000; 131: 549
- 31 Kaur N. Mini-Rev. Org. Chem. 2018; 15: 520
- 32 Shabalala NG, Pagadala R, Jonnalagadda SB. Ultrason. Sonochem. 2015; 27: 423
- 33 Mahaur P, Rajput K, Singh V, Srivastava V, Singh S. RSC Adv. 2024; 14: 21213
- 34 Patil R, Bhoir P, Deshpande P, Wattamwar T, Shirude M, Chaskar P. Ultrason. Sonochem. 2013; 20: 1327
- 35 Mason TJ. Ultrason. Sonochem. 2007; 14: 476
- 36 Jadhav SH, Gogate PR. Ultrason. Sonochem. 2014; 21: 1374
- 37 Wang F, Zhang H, Wang J, Chen G, Fang X, Wang Z, Wang L. Molecules 2012; 17: 10864
- 38 Yang Z.-J, Wang N, He W.-X, Yu Y, Gong Q.-T, Yu X.-Q. Catal. Lett. 2020; 150: 1268
- 39 Liu Y, Shi S, Achtenhagen M, Liu R, Szostak M. Org. Lett. 2017; 19: 1614
- 40 Huang C, Li J, Wang J, Zheng Q, Li Z, Tu T. Sci. China Chem. 2021; 64: 66
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