1-Dimensional Selective Nuclear Overhauser Effect NMR

Laboratory Experiment Cite This: J. Chem. Educ. XXXX, XXX, XXX−XXX

pubs.acs.org/jchemeduc

1‑Dimensional Selective Nuclear Overhauser Effect NMR Spectroscopy To Characterize Products from a Two-Step Green Chemistry Synthesis Russell Hopson,† Po Yin Bowie Lee, and Kathleen M. Hess*,† Department of Chemistry, Brown University, Providence, Rhode Island 02912, United States S Supporting Information *

ABSTRACT: One dimensional (1-D) 1H and 13C NMR experiments are common tools used in undergraduate organic laboratories to characterize synthesized molecules. However, 1-D NMR spectra cannot always provide unambiguous structure determination. In research laboratories, advanced NMR spectroscopy methods are employed for structural assignments of synthesized molecules. This experiment is designed to introduce undergraduate students to the application of an advanced method of 1H NMR spectroscopy, the 1-D selective nuclear Overhauser effect (NOE) experiment, to fully characterize the structure of two compounds. The experiment features a two-step green chemistry synthesis to produce the two compounds. In the first step of the synthesis, 4-methylaniline (para-toluidine) is acetylated to form 4′-methylacetanilide (para-acetotoluidide). The 1-D selective NOE method is used to identify the two different methyl groups present in 4′-methylacetanilide and to assign the aromatic protons. For the second step of the synthesis, bromination of the aromatic ring of 4′-methylacetanilide is accomplished using Oxone and ammonium bromide in an aqueous solvent. The 1-D selective NOE method is applied to determine the regioselective product of the bromination reaction. The 1-D selective NOE experiment is readily accessible and easily implemented. Successful interpretation of the 1-D selective NOE results leaves students confident in making structural assignments that would otherwise be based solely on theory or software prediction. KEYWORDS: Second-Year Undergraduate, Laboratory Instruction, Organic Chemistry, Inquiry-Based/Discovery Learning, Electrophilic Substitution, NMR Spectroscopy, Green Chemistry

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INTRODUCTION Electrophilic aromatic substitution reactions are a cornerstone of organic chemistry laboratory experiments emphasizing the understanding of key concepts such as reaction mechanisms, aromatic directing groups, resonance and inductive effects, and regioselectivity. Many of these reactions require the use of toxic and hazardous reagents1−4 and do not follow current trends in undergraduate laboratory experiments of implementing green chemistry principles.5 This experiment was designed to utilize green methods to complete a two-step synthesis that included an electrophilic aromatic substitution. In addition, the experiment was designed to incorporate an element of investigation6 by addressing the question of regioselectivity of the electrophilic aromatic substitution. Finally, the application and understanding of the one dimensional (1-D) selective nuclear Overhauser effect (NOE) method to distinguish between acetyl and aryl methyl groups (step 1 of the synthesis) and to identify the regioselective product of the aromatic bromination reaction (step 2 of the synthesis) was the pedagogical goal of the experiment. The design and goal of this experiment is fully aligned with the American Chemical Society’s (ACS) guidelines for bachelor’s degrees.7 © XXXX American Chemical Society and Division of Chemical Education, Inc.

Regioselectivity in aromatic substitutions is typically explored with aromatic substrates that afford unambiguous determination of the regioselectivity of the products by 1H/13C NMR spectra or the students are provided the identity of the various isomers either by GC or IR spectroscopy.8−11 The experiment introduced here illustrates the utility of the 1H 1-D selective NOE experiment for solving structural ambiguities more akin to real life synthetic problems facing today’s chemists. Although there are several two-dimensional (2-D) NOE experiments in the literature for advanced organic/bioorganic structure determination12−15 and also a 1-D NOE difference experiment,16 no examples of 1-D selective NOE experiments were found in this Journal. The two examples of 1-D selective NOE experiments found in other educational literature17,18 involve characterization of more complicated natural products and their derivatives, one designed for upper-division students, and neither involves determining regioselectivity or application to an electrophilic aromatic substitution reaction. 1-D selective Received: July 9, 2017 Revised: January 16, 2018

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DOI: 10.1021/acs.jchemed.7b00494 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

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Laboratory Experiment

EXPERIMENT A detailed procedure for the experiment, including all student handouts, is given in the Supporting Information.

NOE experiments require much less time to acquire and process than 2-D NOE and NOE difference experiments, making them suitable for large class sizes. Additionally, interpretation of the 1-D selective NOE experiments is very straightforward given that the strongest signals in the spectrum are between the nucleus irradiated and its closest through-space neighbors. As depicted in Figure 1, para-substituted aromatics with R groups of similar directing strengths may have indistinguishable

Step 1 Synthesis

In an Erlenmeyer flask (50 mL), 4-methylaniline (4.5 mmol) and distilled water (2 mL) are stirred with a stir bar on a hot plate at a temperature not exceeding 50 °C, until the solid melts. A Pasteur pipet is used to add acetic anhydride (6.3 mmol) to the stirring solution. The reaction is stirred for 10 min. The solid from the reaction is filtered and then washed with ice-cold water. The crude product is recrystallized from 50/50 methanol/water. The purified compound is analyzed by TLC with ultraviolet (UV) visualization, melting point determination, IR spectroscopy, and GC−MS. For NMR analysis, each student prepared 0.015 g of 2a dissolved in 0.700 mL of chloroform-d (CDCl3) for 1H, 13C, and 1-D selective NOE NMR experiments. Step 2 Synthesis

The product from step 1 (0.300 g), Oxone (1.1 mmol, molecular mass = 614.8 g/mol), and ammonium bromide (2.2 mmol) are placed into a round-bottom flask (25 mL) with a stir bar. The reaction solvent, 50:50 methanol/water (10 mL) is added to the round-bottom flask with stirring. The reaction is stirred for 15 min, and then, distilled water (10 mL) is added to the round-bottom flask. The solid from the reaction is filtered and washed with ice-cold distilled water. The product is recrystallized with 50:50 ethanol/water. The purified compound is analyzed by TLC with UV visualization, melting point determination, GC−MS, and IR spectroscopy. For NMR analysis, each student prepared 0.015 g of 2b dissolved in 0.700 mL of acetone-d6 (CD3COCD3) for 1H, 13C, and 1-D selective NOE NMR experiments.

Figure 1. Ambiguities in 1H NMR assignments of starting material and products of para-substituted aromatics with similar R groups.

A and B protons in the 1H NMR spectrum appearing as two sets of coupled doublets. Correct assignment of the A and B protons under such circumstances would rely entirely upon predicting the competing substituent effects of the similar R groups using theoretical methods (see Supporting Information for examples of theoretical prediction methods). Most importantly, regardless of the R group on the para-substituted aromatic ring, a subsequent electrophilic aromatic substitution reaction would result in the 1H NMR splitting pattern for both monosubstituted products being the same, a singlet and two doublets. Therefore, the position of the new electrophile, X, on the para-substituted aromatic ring will remain ambiguous. Synthetic organic chemists routinely confront ambiguities like the example shown in Figure 1, where unequivocal structural assignments cannot be made from theory alone and require advanced NMR techniques to assist in structural determination. This experiment introduces one such advanced NMR method, the nuclear Overhauser effect. Theorized in 1953 by Albert Overhauser,19 the NOE is a process by which neighboring nuclei can alter each other’s NMR signal intensity by a “through-space” or dipolar interaction. Unlike “through bond” or scalar interactions between neighboring nuclei that produce visible splitting patterns on the coupled nuclei in the NMR spectrum, the NOE alters the cross-relaxation rates between dipolar-coupled nuclei resulting in a visible change in signal intensity for nuclei within 5 Å of each other without producing any visible splitting pattern and can occur both intra and intermolecularly. NOE methods and variations thereof remain an essential NMR tool for solution state conformational analysis (3-D structure) of both small synthetic molecules and large biomolecules such as proteins and DNA.20−25 Questions in the laboratory report for the experiment were tailored to assess the students’ understanding and application of the NOE method. Analysis of the students’ completed lab reports for the experiment provided data to support achievement of the pedagogical goal of the experiment.

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HAZARDS 4-Methylaniline is a suspected carcinogen and can generate a strong odor if left open on the benchtop for an extended period of time. Keep this solid capped when not in use. Avoid contact of the compound with skin since it can cause irritation. Acetic anhydride is corrosive and has a vinegar-like odor. Keep the reagent capped. Methanol and ethanol are volatile and flammable liquids. Chloroform-d (CDCl3) is a potential carcinogen and should be handled with care. Avoid contact with skin and keep the bottle in the fume hood at all times. The oxidizing agent, Oxone, is corrosive and can be irritating to the eyes, respiratory system, and skin. Ammonium bromide is also an irritant. The product of step 1 synthesis (2a) can cause skin and eye irritation. Acetone-d6 (CD3COCD3) is flammable and can cause eye irritation. The product of step 2 synthesis, 2b, is not a hazardous substance. Ethyl acetate and n-hexane are volatile, flammable, and can cause dizziness upon prolonged exposure or being used without proper environmental controls. Keep these solvents covered during the TLC analysis. n-Hexane is a neurotoxin. After dispensing or using any of these chemicals listed, wash hands thoroughly with soap and water. All chemicals are toxic if ingested. Instructors should consult Safety Data Sheets (SDS) for all chemicals to ensure knowledge of all hazards. Proper laboratory attire and environmental controls (fume hoods) as specified by university protocols are required during the experiment. B



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RESULTS AND DISCUSSION The two-step synthesis for this experiment is shown in Scheme 1. The experiment is completed over 2 weeks. Since both

Table 1. Comparative Student Melting Point Data for Products 2a, 2b, and 2c Melting Point Values, °C

Scheme 1. Two-Step Synthesis Showing Intermediate 2a and the Two Monobrominated Regioisomers 2b and 2c a

Products

Literature

Av., Initial

Av., Final

2a (N = 185) 2b (N = 187) 2c (N = 187)

151a 114b 114c

147 ± 9 115 ± 6 115 ± 6

150 ± 4 119 ± 8 119 ± 8

See ref 28. bSee ref 29. cSee ref 30.

consistent with the literature value. The larger standard deviation for the initial melting point could be the result of the uncertainty in students’ ability to recognize the first appearance of melting. Representative student spectra are available in the Supporting Information for the other types of spectroscopy performed on 2a. In the second step of the synthesis, completed in week 2, the bromination of the aromatic ring was accomplished using Oxone and ammonium bromide in an aqueous environment.31 Oxone is well documented in the literature, and several papers have been published in this Journal about the use of Oxone as a safer and greener alternative for many reactions.32−34 After recrystallization, the average percentage yield of the purified product was 36% with a standard deviation of 15% for 204 students. As with step 1 synthesis, no attempt was made to increase the yield since enough material was obtained to complete all necessary analysis. TLC analysis with UV visualization was completed for this reaction as well (see Supporting Information for details). For the melting point analysis, students were required to search the Reaxys database to obtain the most recent journal article that contained melting points for both possible regioisomers, 2b and 2c. Table 1 shows the literature melting points of both regioisomers and the students’ experimental values. Although the average student melting point values were slightly above the literature values, the melting point values strongly indicate the brominated product was isolated. As shown in Table 1, the literature melting points available for both regioisomers are identical. Consequently, students had to rely on the NOE results to identify the regioselective product. Representative student spectra of the other forms of spectroscopy performed on this product can be found in the Supporting Information. Of the numerous versions of NOE methods available on modern NMR spectrometers, the 1-D selective NOE is by far the most easily implemented and interpreted. In the 1-D selective NOE experiment, the frequency of a single NMR peak is selected for saturation, a process that disrupts the natural Boltzmann distribution of the selected nuclei’s spins in the magnet allowing for cross-relaxation to neighboring spins. Following a series of shaped and gradient pulses, the resulting NMR spectrum shows the NMR signal of the selected nucleus as a strong positive peak in the spectrum, while any neighboring nuclei within 5 Å will have their NMR signal inverted. Figure 2 displays a representative student’s 1H NMR spectrum and 1-D selective NOE spectra obtained on the product of the first step of the synthesis, 2a. Figure 2A displays the 1H NMR spectrum of 2a showing two sets of aromatic doublets and two methyl group singlets. In Figure 2B, the methyl resonance at δ 2.14 ppm has been selectively targeted and appears as a large positive phase peak in the 1H NMR spectrum. The largest negative intensity peak in the spectrum corresponds to the amide proton at δ 7.6 ppm affording an

reactions are completed within 20 min, there is time for purification and analysis of each product during the length of an undergraduate organic laboratory period (3−4 h). The experiment was individually completed by over 200 students in the second semester of an organic chemistry laboratory. The first step of the reaction, completed in week 1, was acylation of 4-methylaniline to form 2a, 4′-methylacetanilide.26 The reaction employed the green technique of isolating the solid product in the absence of any organic solvents. After purification of the sample by recrystallization, the product was used in the second step. The consumption of the product in the second reaction reduces waste and is another application of green chemistry.27 The percentage yield for the pure product, 2a, was an average of 59% with a standard deviation of 14% (207 individual student results). No attempt was made to improve the yield of the reaction nor was emphasis placed on obtaining high yields during the experiment. The focus of the experiment was maintained on the analysis of the product and the critical thinking aspect of the NOE method. (See Instructors’ Notes for suggestions for the recrystallization in the Supporting Information.) In addition to the 1-D NMR spectroscopy analysis, 2a was analyzed by TLC with UV visualization (see Supporting Information for details) and melting point determination. Melting point analysis for pure 2a is shown in Table 1 with the literature value. The students’ melting point values are C




Figure 2. 300 MHz 1H NMR spectrum (A) and 1-D selective 1H NOE NMR spectra (B,C) of 2a.

results in the strongest observed NOE to the amide proton at δ 8.44 ppm as previously observed for 2a when irradiating this methyl group. Selective irradiation of the other methyl group at δ 2.3 ppm (Figure 3C) reveals the strongest NOEs to the singlet at δ 7.44 ppm and the doublet at δ 7.16 ppm, which indicates the bromine added ortho to the amide substituent, 2b. Had the bromine added meta to the amide, 2c, a single NOE from the aryl methyl group to a doublet would have been observed. Typical artifacts in 1-D NOE experiments are signals with dispersive line-shape and weak NOEs from partial saturation to resonances in close proximity to the selectively saturated resonance. Students were instructed to ignore the dispersive

unequivocal distinction between the acetyl and aryl methyl groups. In Figure 2C, the methyl resonance at δ 2.3 ppm is selectively targeted and appears as a large positive peak in the spectrum. The largest negative peak, a doublet at δ 7.1 ppm, clearly distinguishes the aromatic resonance ortho to the aryl methyl group. The product of the green bromination reaction of 2a was subjected to an analogous set of NMR experiments as illustrated in the representative student’s spectra in Figure 3. Figure 3A shows the 1 H NMR spectrum indicating monobromination of the aromatic ring by the presence of two doublets and a singlet, each integrating to one. In Figure 3B, selective irradiation of the methyl group at δ 2.17 ppm D




Figure 3. 300 MHz 1H NMR spectrum (A) and 1-D selective 1H NOE NMR spectra (B,C) spectra of bromination product, 2b.

• 71% of students accurately explained the ambiguity of the 1 H NMR spectrum for 2b and 2c. Although amide formation and electrophilic aromatic substitution reactions were covered in lecture, the NOE method was not discussed in lecture. The directing ability of the groups on the aromatic ring were briefly discussed in a prelaboratory lecture but not emphasized. Therefore, the laboratory provided the platform for the students to acquire the NOE methodology and application. Students were given a lecture with a PowerPoint presentation (see Supporting Information) that provided context for the NOE method and implementation in the experiment. In the laboratory report (see Supporting Information), students answered questions to assess

peaks and focus only on the largest of the inverted signals as the reliable NOE. The following synopsis of results was determined by reviewing the lab reports of 218 students. • 91% of students correctly assigned the methyl groups 2a. • 50% of students accurately explained the assignment methyl groups based on NOE. • 98% of students correctly assigned the position bromine on 2b. • 72% of students accurately explained the assignment bromine on 2b.

of of of of E




John Geleney, Fred Guerzon, and undergraduate assistants from the undergraduate teaching laboratories chemistry stockroom.

their understanding of the NOE method for structural assignments of the products of both reactions. The majority of students, 91%, successfully used the NOE results to distinguish the two methyl groups of 2a (acetyl = δ 2.14 ppm, aryl = δ 2.3 ppm). However, the ability of the students to provide a reasonable explanation for the identification based only on the NOE results was only 50%. The remaining students relied on arguments relating to the shielding effects of the aromatic ring versus the carbonyl to identify the methyl groups. Students’ familiarity with electronegativity and shielding effects provided a plausible reason for the frequency of shielding effects in their explanation. However, students that relied on the electronegativity argument still successfully used the NOE results to make the assignment for the relationship of the aromatic protons on the ring. For the identification of the regioselective isomer of the aromatic bromination reaction, 98% of students identified 2b as the product. A large percentage of students, 72%, were able to successfully explain the location of the bromine on the aromatic ring using the NOE results. A common assumption in the explanations was that bromination would not change the chemical shifts of the two methyl groups. This resulted in sole reliance on the aryl methyl NOE result for predicting the regioisomer. In addition, 71% of students understood that the 1 H NMR spectrum alone could not provide data that would unambiguously identify the bromination product as 2b or 2c. Overall, students’ ability to comprehend and apply the NOE method in both steps of the reaction was successful, and the pedagogical goal of the experiment was achieved: students completed a two-step green chemistry synthesis and applied an advanced method of NMR analysis for structural assignments of the products of both reactions.

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(1) Santos Santos, E.; Gavilán García, I. C.; Lejarazo Gómez, E. F.; Vilchis-Reyes, M. A. Synthesis of Aryl-Substituted 2,4-Dinitrophenylamines: Nucleophilic Aromatic Substitution as a Problem-Solving and Collaborative-Learning Approach. J. Chem. Educ. 2010, 87 (11), 1230−1232. (2) Avila, W. B.; Crow, J. L.; Utermoehlen, C. M. Nucleophilic Aromatic-Substitution: A Microscale Organic Experiment. J. Chem. Educ. 1990, 67 (4), 350−351. (3) McCullagh, J. V.; Daggett, K. A. Synthesis of Triarylmethane and Xanthene Dyes Using Electrophilic Aromatic Substitution Reactions. J. Chem. Educ. 2007, 84 (11), 1799−1802. (4) Forbes, D. C.; Agarwal, M.; Ciza, J. L.; Landry, H. A. Zeroing in on Electrophilic Aromatic Substitution. J. Chem. Educ. 2007, 84 (11), 1878−1881. (5) ACS Green Chemistry Institute. Greening the Lab (and Beyond!): A Guide to Applying Green Chemistry to Practical Settings and Creating Displays to Spread the Word; American Chemical Society: Washington, DC, 2014; https://www.acs.org/content/dam/acsorg/ greenchemistry/education/greening-the-lab.pdf (accessed Jan 2018). (6) Mohrig, J. R. The Problem with Organic Chemistry Labs. J. Chem. Educ. 2004, 81 (8), 1083. (7) American Chemical Society, Office of Professional Training. Undergraduate Professional Education in Chemistry: ACS Guidelines and Evaluation Procedures for Bachelor’s Degree Programs; American Chemical Society: Washington, DC, 2015; https://www.acs.org/ content/dam/acsorg/about/governance/committees/training/2015acs-guidelines-for-bachelors-degree-programs.pdf (accessed Jan 2018). (8) Ballard, C. E. pH-Controlled Oxidation of an Aromatic Ketone: Structural Elucidation of the Products of Two Green Chemical Reactions. J. Chem. Educ. 2010, 87 (2), 190−193. (9) Cardinal, P.; Greer, B.; Luong, H.; Tyagunova, Y. A Multistep Synthesis Incorporating a Green Bromination of an Aromatic Ring. J. Chem. Educ. 2012, 89 (8), 1061−1063. (10) Smith, R. E.; McKee, J. R.; Zanger, M. The Electrophilic Bromination of Toluene: Determination of the Ortho, Meta, and Para Ratios by Quantitative FTIR Spectrometry. J. Chem. Educ. 2002, 79 (2), 227−229. (11) Treadwell, E. M.; Lin, T. Y. A More Challenging Interpretative Nitration Experiment Employing Substituted Benzoic Acids and Acetanilides. J. Chem. Educ. 2008, 85 (11), 1541−1543. (12) Huggins, M. T.; Billimoria, F. Nuclear Overhauser Effect Spectroscopy: An Advanced Undergraduate Experiment. J. Chem. Educ. 2007, 84 (3), 471−474. (13) Sales, E. S.; Silveira, G. P. Synthesis and 1H NMR Spectroscopic Elucidation of Five- and Six-Membered Ribonolactone Derivatives. J. Chem. Educ. 2015, 92 (11), 1932−1937. (14) Walsh, E. L.; Ashe, S.; Walsh, J. J. Nature’s Migraine Treatment: Isolation and Structure Elucidation of Parthenolide from Tanacetum parthenium. J. Chem. Educ. 2012, 89 (1), 134−137. (15) Rehart, A. M.; Gerig, J. T. Proton NMR Studies of the Conformation of an Octapeptide: An NMR Exercise for Biophysical Chemistry. J. Chem. Educ. 2000, 77 (7), 892−894. (16) LeFevre, J. W. Isolating Trans-Anethole from Anise Seeds and Elucidating Its Structure: A Project Utilizing One- and TwoDimensional NMR Spectrometry. J. Chem. Educ. 2000, 77 (3), 361− 363. (17) Andersh, B. Utilization of One-Dimensional Gradient NOE, HMQC, and COSY in Undergraduate Laboratories: Hydroboration− Oxidation of (1R)-(+)-α-Pinene Revisited. Chem. Educ. 2003, 8 (1), 28−32. (18) Chou, S.-C.; Mercier, J. E.; Wilson, K. A.; Beck, J. J. Complete Proton and Carbon Assignment of (+)-Catechin via One- and TwoDimensional Nuclear Magnetic Resonance (NMR) Analysis: A Hands-

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.7b00494. Detailed procedure for the experiment; list of chemicals used in the experiment; instructors’ notes (including representative student spectra for IR spectroscopy, GC− MS, and 13C NMR spectroscopy) (PDF, DOCX ) Lab report and the PowerPoint presentation (Introduction to NOEs) for explaining the NOEs (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Kathleen M. Hess: 0000-0002-1203-1838 Author Contributions †

R.H. and K.M.H. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to thank Brown University for the Undergraduate Teaching and Research Award for Po Yin Bowie Lee for summer 2016. We would like to thank the students and teaching assistants of the second semester organic chemistry laboratory course at Brown University. Also, a special thanks to F




On Learning Experiment for Upper-Division Undergraduate Chemistry Students. Chem. Educ. 2006, 11 (1), 15−22. (19) Overhauser, A. W. Polarization of Nuclei in Metals. Phys. Rev. 1953, 92 (2), 411−415. (20) Gajarsky, M.; Zivkovic, M. L.; Stadlbauer, P.; Pagano, B.; Fiala, R.; Amato, J.; Tomaska, L.; Sponer, J.; Plavec, J.; Trantirek, L. Structure of a Stable G-Hairpin. J. Am. Chem. Soc. 2017, 139 (10), 3591−3594. (21) Lichtenecker, R.; Ludwiczek, M. L.; Schmid, W.; Konrat, R. Simplification of Protein NOESY Spectra Using Bioorganic Precursor Synthesis and NMR Spectral Editing. J. Am. Chem. Soc. 2004, 126 (17), 5348−5349. (22) Mizuki, K.; Iwahashi, K.; Murata, N.; Ikeda, M.; Nakai, Y.; Yoneyama, H.; Harusawa, S.; Usami, Y. Synthesis of Marine Natural Product (−)-Pericosine E. Org. Lett. 2014, 16 (14), 3760−3763. (23) Shen, Y.; Atreya, H. S.; Liu, G.; Szyperski, T. G-Matrix Fourier Transform NOESY-Based Protocol for High-Quality Protein Structure Determination. J. Am. Chem. Soc. 2005, 127 (25), 9085−9099. (24) Usami, Y.; Takaoka, I.; Ichikawa, H.; Horibe, Y.; Tomiyama, S.; Ohtsuka, M.; Imanishi, Y.; Arimoto, M. First Total Synthesis of Antitumor Natural Product (+)- and (−)-Pericosine A: Determination of Absolute Stereo Structure. J. Org. Chem. 2007, 72 (16), 6127−6134. (25) Zeng, T.; Wu, X. Y.; Yang, S. X.; Lai, W. C.; Shi, S. D.; Zou, Q.; Liu, Y.; Li, L. M. Monoterpenoid Indole Alkaloids from Kopsia of f icinalis and the Immunosuppressive Activity of Rhazinilam. J. Nat. Prod. 2017, 80 (4), 864−871. (26) Pavia, D. L.; Lampman, G. M.; Kriz, G. S.; Engel, R. G. Introduction to Organic Laboratory Techniques, A Microscale Approach, 4th ed.; Thomson Higher Education: Belmont, CA, 2007; p 890. (27) Anastas, P. T.; Warner, J. C. Green Chemistry: Theory and Practice; Oxford University Press: New York, 1998; p 152. (28) Haynes, W. M., Ed. CRC Handbook of Chemistry and Physics, 97th ed.; CRC Press: Boca Raton, FL, 2016. (29) Kulangiappar, K.; Anbukulandainathan, M.; Raju, T. Nuclear Versus Side-Chain Bromination of 4-Methoxy Toluene by an Electrochemical Method. Synth. Commun. 2014, 44 (17), 2494−2502. (30) Peltier, D.; Pichevin, A.; Bonnin, A. Étude de la Structure des Acetanilides Substitues par Spectrographie Infrarouge. Bull. Soc. Chim. Fr. 1961, 1619−1623. (31) Arun Kumar, M. A.; Rohitha, C. N.; Kulkarni, S. J.; Narender, N. Bromination of Aromatic Compounds Using Ammonium Bromide and Oxone. Synthesis 2010, 2010 (10), 1629−1632. (32) Gandhari, R.; Maddukuri, P. P.; Vinod, T. K. Oxidation of Aromatic Aldehydes Using Oxone. J. Chem. Educ. 2007, 84 (5), 852− 854. (33) Lang, P. T.; Harned, A. M.; Wissinger, J. E. Oxidation of Borneol to Camphor Using Oxone and Catalytic Sodium Chloride: A Green Experiment for the Undergraduate Organic Chemistry Laboratory. J. Chem. Educ. 2011, 88 (5), 652−656. (34) Nalliah, R. E. Oxone/Fe2+ Degradation of Food Dyes: Demonstration of Catalyst-Like Behavior and Kinetic Separation of Color. J. Chem. Educ. 2015, 92 (10), 1681−1683.

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1-Dimensional Selective Nuclear Overhauser Effect NMR

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