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Org. Synth. 2019, 96, 494-510

DOI: 10.15227/orgsyn.096.0494

Synthesis of 5-Hydroxy-4-methoxy-2-methylpyrylium Trifluoromethanesulfonate from Kojic acid

Submitted by Nana B. Agyemang and Ryan P. Murelli*¹

Checked by Richard K. Jackson III and John L. Wood

1. Procedure (Note 1)

A. 2-(Chloromethyl)-5-hydroxy-4H-pyran-4-one. (2). An oven-dried 1000 mL, 24/40 three-necked, round-bottomed flask is charged with a 4-cm oval Teflon-coated stir bar. Kojic acid (1, 50.0 g, 0.352 mol, 1 equiv) (Note 2) is then added to the flask through a glass powder funnel on the middle neck. A thermometer with adaptor is connected to the left neck and a gas-adaptor connecting to a HCl scrubber is attached to the right neck (Note 3). The solid is then stirred at 400 rpm for 10 min. The glass powder funnel is removed and replaced by a 250 mL addition funnel topped with a septum (Figure 1A). Thionyl chloride (205 mL, 2.81 mol, 8 equiv) (Note 4) is added by plastic syringe to the addition funnel and then added dropwise to the round-bottomed flask over the course of 1 h. During the first 45 min of thionyl chloride addition, gaseous sulfur dioxide and HCl are generated; after addition is complete, the internal temperature is 35 °C. The reaction is stirred for an additional 1 h at room temperature (23 °C). After this time TLC analysis indicates the reaction is complete (Figure 1B) (Note 5). The resultant heterogeneous reaction mixture is vacuum filtered through a 10.5-cm diameter Büchner funnel lined with a 90 mm Whatman qualitative filter paper. The filtrate is collected in a 500 mL filter flask and the light yellow product is washed with chilled hexanes (0 °C) (2 x 100 mL) (Notes 6 and 7) to obtain an off-white/yellow solid (Figure 1C).

Figure 1.A) Reaction setup for step A; B) TLC analysis of compound 2 after 1 h; C) Recrystallization setup

The solid is transferred through a glass powder funnel to a 2 L Erlenmeyer flask equipped with a 4-cm oval Teflon-coated stir bar. Chloroform (1.5 L) (Note 8) is added to the Erlenmeyer flask and the resultant heterogeneous mixture is stirred at 800 rpm and heated to reflux (61 °C) in a heating mantle filled with sand (Note 9). Once the solid is fully dissolved (3 h), heating is ceased and the Erlenmeyer flask is removed from the heating mantle and allowed to cool at ambient temperature (23 °C) for 1 h. The Erlenmeyer flask is then transferred to an ice bath for an additional 3 h to drive crystallization. The crystalline solid is collected by vacuum filtration using a clamped 500 mL filter flask and a 10.5 cm diameter Büchner funnel lined with a 90 mm Whatman qualitative filter paper. The residual crystallized product is rinsed from the 2L flask with chloroform (2 x 25 mL). The product is transferred to a tared 500 mL, single-necked, 24/40 round-bottomed flask, and the residual solvent is removed under vacuum (1.40 mmHg) for 12 h to afford compound 2 (38.1 g, 67% yield, 97.1% purity) as off-white plates (Figure 2) (Notes 10 and 11).

Figure 2. A small sample of recrystallized compound 2 after vacuum drying for 12 h

B. 5-Hydroxy-2-methyl-4H-pyran-4-one. (3). An oven-dried 1 L, 24/40 three-necked, round-bottomed flask is equipped with a 4-cm oval Teflon-coated stir bar. The flask is clamped into an aluminum block and the gaps between the flask and block are filled with sand. A thermometer with adaptor is fitted into the left neck (Note 12), a 10-cm wide plastic powder funnel is placed in the middle neck, and a septum on the remaining neck (Figure3).

Figure 3. Reaction setup for step B

Compound 2 (38.1 g, 0.237 mol, 1 equiv) is added to the flask through the powder funnel, followed by de-ionized water (286 mL), and the resulting mixture is stirred under ambient atmosphere and temperature (23 °C) at 500 rpm. Zinc powder (31.1 g, 0.476 mol, 2 equiv) (Note 13) is slowly added to the opaque heterogeneous solution through the powder funnel over 30 min. The rate of zinc powder addition is adjusted to ensure the internal temperature does not rise above 40 °C. After addition of the zinc is complete, the reaction mixture becomes a light gray heterogeneous solution (Figure 4A). The powder funnel is removed and replaced by a 125 mL addition funnel which is then charge with 12 M hydrochloric acid (60 mL, 0.71 mol, 3 equiv) (Note 14). The addition funnel is left uncapped and the septum on the right neck is removed. The HCl is added to the reaction dropwise over 30 min, during which time the internal temperature increases to 60 °C (Figure 4B). After complete addition of hydrochloric acid, TLC analysis reveals the absence of starting material and the formation of a new spot (Figure 5A) (Note 15).

Figure 4. A) After addition of zinc powder; B) During addition of hydrochloric acid

Figure 5. A) TLC analysis of compound 3 after addition of 12 M HCl; B) Hot filtration setup for step B

At this point the reaction mixture is heated to 63 °C and held at this temperature for 1h. During the heating process the solution takes on an amber color and all solids dissolve into the aqueous solution (Note 16). Heating and stirring are then halted and the reaction flask is removed from the aluminum block. After 2 min of standing at 23 °C, the addition funnel is removed and the hot solution is filtered into a 1 L Erlenmeyer flask using a 10-cm diameter glass powder funnel and folded 24-cm Whatman qualitative filter paper (Figure 5B). After standing for 3 h under ambient conditions (23 °C), the flask containing the filtrate is cooled on an ice bath for an additional 3 h to drive crystallization (Figure 6A). The crystalline solid is collected by vacuum filtration using a 500 mL filter flask equipped with a 10.5 cm diameter Büchner funnel lined with a 90 mm Whatman qualitative filter paper. The crystalline solid is washed with chilled isopropyl alcohol (0 °C) (2 x 50 mL) (Note 6). The product is transferred to a tared 50 mL single-necked, 14/20 round-bottomed flask, and the residual solvent is removed under vacuum (1.40 mmHg) for 12 h to obtain compound 3 (9.7 g, 32% yield, 97.0% purity) as a white solid (Notes 17 and 18) (Figure 6B).

Figure 6. A) Recrystallization of compound 3 from acidic water; B) Compound 3 after vacuum drying for 12h

C. 5-hydroxy-4-methoxy-2-methylpyrylium trifluoromethanesulfonate. (4). An oven-dried, 24/40 single-necked, 300 mL round-bottomed flask is equipped with a 4-cm oval Teflon-coated stir bar. Compound 3 (5.00 g, 0.0397 mol, 1 equiv) (Note 19) is transferred to the flask through a glass powder funnel and dissolved in dichloromethane (100 mL) (Note 20). Methyl trifluoromethanesulfonate (6.50 mL, 0.06 mol, 1.5equiv) (Note 21) is carefully transferred to the flask by syringe. The round-bottomed flask is fitted with a reflux condenser (Figure 7A) and heated to reflux using an aluminum block with sand while stirring (800 rpm) under ambient atmosphere. After 1 h of heating, the resulting yellow solution is removed from the aluminum block and allowed to cool for 40 min (Figure 7B) (Note 22).

Figure 7. A) Reaction setup for step C; B) Completed reaction cooling to ambient temperature

After cooling to ambient temperature, the dichloromethane is removed in vacuo using a rotary evaporator (Note 23) to obtain a yellow oil. Crystallization can be initiated by blowing air over the top of the derived oil. Once an appreciable amount of crystallization occurs residual solvent is removed by placing the flask under high vacuum for 40 min (1.40 mmHg), during which time the orange color fades to yield a white/orange solid (cf., Figures 8A and 8B) (Note 24). The crystalline solid is removed from the round-bottomed flask with a spatula and collected by vacuum filtration using a 500 mL filter flask equipped with a 6.5-cm diameter Büchner funnel lined with a 55-mm Whatman qualitative filter paper. The crystalline solid is washed with chilled ethyl acetate (-78 °C) (4 x 25 mL) (Notes 6 and 25). The product is transferred to a tared 100 mL single-necked, 24/40 round-bottomed flask and the residual solvent removed under vacuum (1.40 mmHg) for 2 h to obtain compound 4 (9.72 g, 84% yield, 97.3% purity) as a peach/white solid (Notes 26, 27, and 28) (Figure 8C).

Figure 8. A) Compound 4 as oil;. B) After 40 min. under high vacuum; C) After ethyl acetate wash and drying for 2 h under high vacuum

2. Notes

1. Prior to performing each reaction, a thorough hazard analysis and risk assessment should be carried out with regard to each chemical substance and experimental operation on the scale planned and in the context of the laboratory where the procedures will be carried out. Guidelines for carrying out risk assessments and for analyzing the hazards associated with chemicals can be found in references such as Chapter 4 of "Prudent Practices in the Laboratory" (The National Academies Press, Washington, D.C., 2011; the full text can be accessed free of charge at https://www.nap.edu/catalog/12654/prudent-practices-in-the-laboratory-handling-and-management-of-chemical. See also "Identifying and Evaluating Hazards in Research Laboratories" (American Chemical Society, 2015) which is available via the associated website "Hazard Assessment in Research Laboratories" at https://www.acs.org/content/acs/en/about/governance/committees/chemicalsafety/hazard-assessment.html. In the case of this procedure, the risk assessment should include (but not necessarily be limited to) an evaluation of the potential hazards associated with, as well as the proper procedures for Kojic acid, thionyl chloride, hexane, chloroform, zinc powder, hydrochloric acid, dichloromethane, methyl trifluoromethane sulfonate, and ethyl acetate.

2. Kojic acid (99.5 %) was obtained from Chem-Impex International and used as received.

3. The submitters performed this reaction without a scrubber and allowed SO₂ and HCl to vent into the fume hood. The checkers found it convenient to scrub the emitted HCl gas by venting through a 6 M NaOH solution, which was cooled in an ice water bath (e.g., see step A and associated photographs in Caron, S.; Wei, L. Org. Synth. 2013, 90, 174-181).

4. Thionyl chloride (≥99%) was obtained from Sigma-Aldrich and used as received. Thionyl chloride is highly toxic. As it serves as both reagent and solvent, it is used in excess. Care should be taken when disposing of excess thionyl chloride on large scale. Consult with waste management and/or Environmental Health and Safety department to establish appropriate disposal procedures for your laboratory prior to experiment.

5. TLC analysis was performed on a small aliquot that was diluted with dichloromethane (ca. 0.5 mL). The product (2) has an R_f = 0.80 in ethyl acetate (Figure 1B, KA = Kojic acid, Co = Co-spot, R = Reaction). All TLC's were run with ethyl acetate on TLC Silica gel 60 F-254 (2.5 x 4.75 cm glass plates) purchased from EMD Millipore Corporation.

6. Solvents were cooled in 500 mL squirt bottles to either 0 °C using an ice water bath or -78 °C with a dry ice/acetone bath.

7. Hexanes (>99.9%) were obtained from Fisher Chemical and used as received.

8. Chloroform (≥99.8%, stabilized with amylene) was obtained from Sigma-Aldrich and used as received.

9. The checkers found complete dissolution of the solid required 3 h at reflux, during which time the volume of chloroform was maintained with additional chloroform (ca.500 mL).

10. The identity of compound 2 was established with the following characterization data. ¹H NMR pdf (400 MHz, DMSO-d6) δ: 4.65 (s, 2H), 6.56 (s, 1H), 8.12 (s, 1H), 9.29 (s, 1H); ¹³CNMR pdf (101 MHz, DMSO-d6) δ: 41.3, 113.3, 140.2, 146.1, 161.7, 173.8; HRMS (ESI+) calc. for C₆H₅ClNaO₃ [M+Na]⁺ 182.9819, found 182.9819; IR (neat): 3187 (w), 3106 (w), 3065 (w), 1650 (m), 1609 (s), 1583 (m), 1452 (m), 1373 (m), 1208 (s), 1163 (m), 1112 (s), 951 (s), 882 (s), 850 (m), 765 (m), 739 (s), 627 (s) cm^-1; mp 162-163 °C (corrected) (submitters found: 166-167 °C); TLC: R_f = 0.67 in ethyl acetate (submitters R_f = 0.80 in ethyl acetate). The purity of the compound 2 was calculated by qNMR pdf with a relaxation delay of 30 seconds using 67.0 mg of 1,3,5-trimethoxybenzene (purity 99%) and 30.1 mg of the compound 2.

11. A second run on half scale provided 22.0 g (78% yield) of compound 2. The purity was found to be 98.9% as calculated by qNMR with a relaxation delay of 30 seconds using 25.3 mg of 1,3,5-trimethoxybenzene (purity 99%) and 37.2 mg of the compound 2. The checkers noted that the yield for step A was typically 5-10% higher on half scale compared to full scale.

12. VWR General Purpose Blue Spirit Thermometer 300 mm (-20 to 110 °C).

13. Zinc powder (99.9%, 100 mesh) was obtained from Alfa Aesar and used as received.

14. Hydrochloric acid (2.5 L, 37% solution, 12 M) was obtained from Fisher Chemical and used as received.

15. The aqueous solution was spotted directly onto the TLC plate. The resulting UV-active spot had an R_f = 0.47 in ethyl acetate. (Figure 5A) Cl = Compound 2, C = Co-spot, R = Reaction spot. All TLC's were performed with ethyl acetate on TLC Silica gel 60 F-254 (2.5 x 4.75 cm glass plates) purchased from EMD Millipore Corporation.

16. The checkers noted in some instances that heating for longer than 1 h was required for complete dissolution of the solids.

17. The identity of compound 3 was established with the following characterization data. ¹HNMR pdf (400 MHz, DMSO-d6) δ: 2.23 (d, J = 0.6 Hz, 3H), 6.23 (d,J = 0.6 Hz, 1H), 7.96 (s, 1H), 8.94 (s, 1H); ¹³CNMR pdf (101 MHz, DMSO-d6) δ: 19.1, 112.0, 139.4, 145.4, 165.3, 173.8; HRMS (ESI+) calc. for C₆H₇O₃ [M+H]⁺ 127.0390, found 127.0390; IR (Neat): 3212 (s), 3045 (w), 1649 (m), 1607 (m), 1583 (m), 1435 (m), 1381 (m), 1362 (m), 1273 (w), 1218 (m), 1147 (m), 1049 (m), 913 (m), 882 (s), 834 (m), 767 (s), 689 (s), 591 (s) cm^-1; mp: 149-151 °C (corrected) (submitters found 152-153 °C). TLC: R_f = 0.47 in ethyl acetate (submitters found TLC: R_f = 0.68 in ethyl acetate). The purity of the compound 3 was calculated by qNMR pdf with a relaxation delay of 30 seconds using 152.0 mg of 1,3,5-trimethoxybenzene (purity 99%) and 107.0 mg of the compound 3.

18. A second run on half scale provided 4.2 g (28% yield) of compound 3. The purity was found to be 98.4% as calculated by qNMR with a relaxation delay of 30 seconds using 20.3 mg of 1,3,5-trimethoxybenzene (purity 99%) and 15.5 mg of the compound 3.

19. The submitters reported that reaction C could be performed successfully with 10.0 g compound 3.

20. Dichloromethane (Not Stabilized/HPLC, ≥99.9%) obtained from Fisher Chemical and was dried using a solvent purification system manufactured by SG Water U.S.A., LLC. The submitters used Dichloromethane (>99.5%) obtained from VWR without additional drying or purification.

21. Methyl trifluoromethanesulfonate (97%) was obtained from Matrix Scientific and used as received. Methyl trifluoromethanesulfonate is a strong methylating agent; Only handle Methyl trifluoromethanesulfonate in a well-ventilated fume hood.

22. Monitoring by TLC analysis was not possible due to extensive hydrolysis back to compound 3. However, monitoring the reaction over multiple runs by NMR (MeCN-d3), the checkers noted that the reaction was always complete after 1 h of heating to reflux.

23. The rotary evaporator was located in a fume hood and with the heating bath was set to 31 °C. The pressure was carefully decreased to 200 mmHg.

24. The submitters found that adding ethyl acetate (50 mL) to the oil caused crystalline solid to precipitate. The checkers found that the ethyl acetate needed to be chilled in a dry ice/acetone bath to initiate precipitation, but the checkers could not achieve the desired yield for step C using this method.

25. ethyl acetate (99.5%) was obtained from Fisher Chemical and used as received.

26. The submitters employed DMSO-d6 4 as the NMR solvent; however, the checkers found that compound 4 invariably decomposes over time in DMSO-d6. Although adequate spectra could be obtained with newly opened ampoules of 99.9% or 99.96% DMSO-d6, qNMR analyses were irreproducible in this solvent. Thus, given the observed sensitivity of compound 4 to DMSO-d6, the checkers determined the preferred NMR solvent to be MeCN-d3. For the sake of completeness NMR data in both solvents are included here (Note 27).

27. The identity of compound 4 was established with the following characterization data. ¹H-NMR pdf (400 MHz, DMSO-d6) δ: 2.75 (s, 3H), 4.27 (s, 3H), 7.93 (s, 1H), 9.00 (s, 1H), 11.68 (br s, 1H);¹³CNMR pdf (101 MHz, DMSO-d6) δ: 20.8, 59.9, 107.9, 120.7 (d, J = 322 Hz), 143.4, 149.4, 170.2, 175.2; ¹HNMR pdf (400 MHz, MeCN-d3) δ: 2.73 (s, 3H), 4.28 (s, 3H), 7.55 (s, 1H), 8.76 (s, 1H), 9.11 (s, 1H); ¹³CNMR pdf (101 MHz, MeCN-d3) δ: 21.5, 61.0, 108.8, 121.9 (d, J = 320 Hz), 144.6, 150.1, 171.5, 177.5; ¹⁹F NMR pdf (565 MHz, MeCN-d3) δ: -77.69 ppm. HRMS (ESI+) calc. for C₇H₉O₃ [M-OTf]⁺⁺ 141.0546, found 141.0554; IR (Neat): 3096 (w), 1638 (m), 1556 (m), 1497 (m), 1467 (w), 1441 (w), 1364 (w), 1280 (m), 1253 (m), 1217 (s), 1163 (s), 1024 (s), 982 (s), 872 (m), 797 (m), 633 (s), 516 (m) cm^-1. Melting point: 85-86 °C (corrected) (Submitters found:50-51 °C). The purity of compound 4 was calculated by qNMR pdf with a relaxation delay of 30 seconds using 16.6 mg of 1,3,5-trimethoxybenzene (purity 99%) and 28.2 mg of the compound 4.

28. A second run on half scale provided 1.69 g (74% yield) of compound 4. On half scale, a second recrystallization of the filtrate was needed in order to obtain this yield. The first crop provided 1.54 g compound 4; the purity of the first crop of crystals was found to be 97.2% as calculated by qNMR with a relaxation delay of 30 seconds using 23.1 mg of 1,3,5-trimethoxybenzene (purity 99%) and 33.1 mg of the compound 4. The second crop provided 0.15 compound 4; the purity of the second crop of crystals was found to be 99.0% as calculated by qNMR with a relaxation delay of 30 seconds using 25.6 mg of 1,3,5-trimethoxybenzene (purity 99%) and 19.6 mg of the compound 4. MeCN-d3 was employed as the deuterated solvent in both cases.

Working with Hazardous Chemicals

The procedures in Organic Syntheses are intended for use only by persons with proper training in experimental organic chemistry. All hazardous materials should be handled using the standard procedures for work with chemicals described in references such as "Prudent Practices in the Laboratory" (The National Academies Press, Washington, D.C., 2011; the full text can be accessed free of charge at http://www.nap.edu/catalog.php?record_id=12654). All chemical waste should be disposed of in accordance with local regulations. For general guidelines for the management of chemical waste, see Chapter 8 of Prudent Practices.

In some articles in Organic Syntheses, chemical-specific hazards are highlighted in red "Caution Notes" within a procedure. It is important to recognize that the absence of a caution note does not imply that no significant hazards are associated with the chemicals involved in that procedure. Prior to performing a reaction, a thorough risk assessment should be carried out that includes a review of the potential hazards associated with each chemical and experimental operation on the scale that is planned for the procedure. Guidelines for carrying out a risk assessment and for analyzing the hazards associated with chemicals can be found in Chapter 4 of Prudent Practices.

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3. Discussion

Oxidopyrylium ylides are versatile and reactive species that have been used widely in total synthesis, particularly due to their ability to undergo complexity-forming [5+2] cycloaddition reactions.² One particularly valuable precursor to oxidopyrylium ylides are 3-hydroxy-4-pyrones,³ which can undergo tautomerization or group transfer chemistry to access oxidopyrylium ylides (Scheme 1A).³ Group transfer strategies have been generally limited to intramolecular cycloaddition reactions, however, likely due to the short lifetime of the active ylide. However, in 1992 Wender and Mascareñas found that this limitation can be overcome through the use of methyl triflate based pre-ylide salt 4 (Scheme 1B).⁴

Scheme 1. 3-Hydroxy-4-pyrone-based oxidopyrylium [5+2] cycloaddition overview. (A) Group-transfer process typical used for intramolecular oxidopyrylium cycloaddition reactions employing 3-hydroxy-4-pyrones. (B) Methyl-triflate-based process that is effective for intermolecular cycloaddition reactions.

Our lab has found that one of the main reasons why 4 is so effective in intermolecular cycloadditions is that its dimer is not a major impediment to the [5+2] cycloaddition reaction,⁵ as has often described with other oxidopyrylium ylides. This appears to be due to the reversibility of the dimerization process, which allows for regeneration of the active ylide. Adding to this value, the dimer can be readily generated purely through simple treatment of a solution of 4 in dichloromethane to triethylamine, followed by an ammonium chloride wash (Scheme B).^6,7 This purified dimer is also effective in intermolecular cycloaddition reactions, and as such can provide a 'clean source' of oxidopyrylium ylide, free of conjugate acid or residual base that would otherwise accompany the formation of the ylide. The use of the dimer as the source of oxidopyrylium ylide has had numerous benefits, including optimizing a three-component oxidopyrylium cycloaddition,⁷ suppressing Brønsted acid-mediated racemic background cycloadditions in order to increase enantioselectivities in asymmetric oxidopyrylium cycloadditions,⁸ and enabling chromatography free α-‑hydroxytropolone syntheses (Scheme 2).⁹

Scheme 2. Synthesis of oxidopyrylium dimer from salt 4 and established uses thereof

Throughout our studies on reagent 4, we have found that we can synthesize it on large scale (>100 g) from kojic acid (1) without any need for chromatography, and that it can be stored for long periods of time (> 1 year) without any special precautions other than a freezer. For these reasons, salt 4 is considered a de facto starting material in our lab. As such, a detailed, scalable procedure of salt 4, including the synthesis of allomaltol (3) from starting material kojic acid¹⁰ was warranted. Our hope is that this procedure will enable more widespread use of oxidopyrylium salt 4, and accelerate the development of this highly valuable reagent.

References and Notes

Department of Chemistry, Brooklyn College, The City of New York, 2900 Bedford Avenue, Brooklyn College 11210, United States. E-mail: rpmurelli@brooklyn.cuny.edu. ORCID for Ryan Murelli: 0000-0002-4247-3936. This work is supported by the National Institute of General Medicinal Sciences of the United States National Institutes of Health in the form of a grant to R.P.M. (SC1GM111158).
(a) Bejcek, L. P.; Murelli, R. P. Tetrahedron 2018, 74, 2501-2521. (b)Singh V.; Krishna, U.M.; Vikrant, Trivedi G. K. Tetrahedron 2008, 64, 3405-3428.
Mascareñas, J. L. Advances in Cycloaddition; Harmata, M., Ed.; Jai: Stamford, CT, 1999; Vol. 6, pp 1-54.
Wender, P. A.; Mascareñas, J. L. Tetrahedron Lett. 1992, 33, 2115-2118.
Meck, C.; Mohd, N.; Murelli, R. P. Org. Lett. 2012, 14, 5988-5991.
Lee, H-Y.; Kim, H-Y.; Kim, B. G.; Kee, J. M. Synthesis 2007 , 15, 2360-2364.
D'Erasmo, M. P.; Meck, C.; Lewis, C. A.; Murelli, R. P. J. Org. Chem. 2016, 81, 3744-3751.
Fuhr, K. N.; Hirsch, D. R.; Murelli, R. P.; Brenner-Moyer, S. E. Org. Lett. 2017, 19, 6356-6359.
Berkowitz, A. J.; Abdelmessih, R. G.; Murelli, R. P. Tetrahedron Lett. 2018, 59, 3026-3028.
Various labs have reported on the synthesis of allomaltol 3 from kojic acid using thionyl chloride chlorination followed by Zn/HCl reduction. The specific procedure carried out in our lab has been derived from that reported by Hider. See: Ma, Y.; Luo, W.; Quinn, P. J.; Liu, Z.; Hider, R. C. J. Med. Chem. 2004, 47, 6349-6362.

Appendix
Chemical Abstracts Nomenclature (Registry Number)

Kojic acid: 4H-Pyran-4-one, 5-hydroxy-2-(hydroxymethyl)-; (501-30-4)

Thionyl chloride: Thionyl chloride; (7719-09-7)

Chloroform: Methane, trichloro-; (67-66-3)

Zinc powder: Zinc dust; (7440-66-6)

Methyl trifluoromethane sulfonate: Methanesulfonic acid, trifluoro-, methyl ester; (333-27-7)

	Nana B. Agyemang received his B.S. in chemistry under the supervision of Kathlyn A. Parker from Stony Brook University, N.Y., in 2012. He worked as an R&D Scientist at Pall Corporation, Port Washington, NY and then later as a Laboratory Technician for Ryan Murelli at Brooklyn College, Brooklyn N.Y. He is currently pursuing his doctoral studies at the City University of New York Graduate Center under the co-mentorship of Profs. Ryan Murelli and Mark Biscoe, where he is studying stereoselective cross-coupling reactions on densely functionalized troponoids.
	Ryan P. Murelli is a Professor in the Chemistry Department at CUNY Brooklyn College. He also holds a joint appointment in the PhD Program in Chemistry at the CUNY Graduate Center, where he serves as the Chair of the Organic Chemistry Subdiscipline. Before joining the faculty at CUNY, Ryan carried out his PhD studies with Marc Snapper at Boston College from 2002-2007, and was a postdoctoral associate with David Spiegel at Yale from 2007-2010.
	Richard K. Jackson received his B.S. in Chemistry from the University of Dallas in 2014. He is currently a graduate student at Baylor University where his research focuses on the total synthesis of complex natural products in the laboratory of John L. Wood.

Notes

References/EndNotes

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