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A facile and scalable synthesis of qinghaosu (artemisinin)
Tetrahedron 69 (2013) 1112e1114
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Tetrahedron journal homepage: www.elsevier.com/locate/tet
A facile and scalable synthesis of qinghaosu (artemisinin) Hui-Jun Chen, Wei-Bo Han, Hong-Dong Hao, Yikang Wu * State Key Laboratory of Bioorganic and Natural Products Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 November 2012 Received in revised form 16 November 2012 Accepted 16 November 2012 Available online 23 November 2012
A very simple and efficient approach for the conversion of dihydro-artemisinic acid into artemisinin is developed, featuring use of a molybdate induced disproportionation of hydrogen peroxide to generate singlet oxygen. The whole synthesis was completed by stirring at ambient temperature without need for any special equipments. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Ene reaction Singlet oxygen Ring-closing Peroxide Antimalarial
1. Introduction Qinghaosu (QHS, 1, artemisinin, Fig. 1) is a sesquiterpene containing a 1,2,4-trioxane core isolated in the early 1970s from a herb (qinghao, Artemisia annua L.) by Chinese scientists.1 Because of its outstanding antimalarial activity, especially in treatment of the multi drug resistant cases, QHS and its derivatives and analogues, has been broadly recognized as the most important weapons against malaria.2 H O O O
H
O 1
HO O
H
H
2
H O
H HO 3
H
H
O
Fig. 1. The structures for QHS 1, dihydro-qinghao acid (dihydro-artemisinic acid) 2, and qinaghao acid (artemisinic acid) 3.
Unfortunately, QHS is only present in very low content in the herb growing in tropic/subtropical areas. And the same herb that grows in the northern areas does not produce QHS at all. Total synthesis of QHS was achieved even back in the 1980s.3,4 However, the length of the routes, the reagents and conditions involved, and
* Corresponding author. E-mail address: [email protected] (Y. Wu). 0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tet.2012.11.056
the yield make it impossible for total synthesis to serve as a source of supply. All these prompted exhaustive efforts in seeking alternative sources for QHS or developing simpler organic peroxides5 with comparable potency. Dihydro-artemisinic acid (2, a natural product,6 also accessible via reduction7 of 3) is long-recognized as a valuable starting material for chemical synthesis of QHS. The possibility for transforming 2 into 1 (featuring using an ene reaction of the photogenerated 1O2 and subsequent trapping of 3O2) was elegantly shown in late 1980s/early 1990s by the pioneers8,9 and subsequently applied by other investigators.10 The novel eye-catching fermentation access11 to acid 3 also relies on this transformation to comprise a prospective alternative supply for QHS. However, the conversion of 2 to 1 could be performed satisfactorily only on small scales and afforded QHS in quantities less than 180 mg (in most cases <30 mg) as seen from the published papers over the years. Presumably, there were some hidden difficulties in there, which made scaling up of the seemingly very promising shortcut unfeasible over the past decades. vesque and SeeRecently, a major advance was made by Le berger.12 They carried out the same photosensitized oxygenationbased transformation on an unprecedented 3-g scale in a specially designed continuous-flow reactor and obtained QHS in 39% yield (before recrystallization). As never before has QHS been synthesized on such a large scale, not to mention the reproducible yield, the novel continuous-flow approach brings forth a suddenly brightened prospect for low cost production of QHS and thus has been highly valued by the scientific community.13
H.-J. Chen et al. / Tetrahedron 69 (2013) 1112e1114
1113
Herein we wish to disclose a useful alternative for achieving the same conversion of acid 2 into QHS 1, which remains the advantages of the continuous-flow technique while the operations are greatly simplified. More importantly, all the special equipments required for the photo reaction are no longer needed.
singlet oxygen in this much-studied transformation and hopefully prompt further investigations that eventually lead to a low cost production of QHS.
2. Results and discussion
In summary, developing a cost-effective and scalable synthesis of QHS, a long-sought after goal in seeking alternative sources of supply for the natural QHS, represents a lasting challenge since the 1980s. Transformation of 2 into QHS discovered some 20 years ago is broadly considered to be the most promising candidate, especially after the access to 3 by fermentation11 was developed. However, despite many successful small scale precedents, further elaboration into a scalable synthesis has proven to be much more difficult than the published papers implied. However, despite many successful small scale precedents, further elaboration into a scalable synthesis has proven to be much more difficult than the published papers implied. Now, we show that by using the Na2MoO4/H2O2 protocol to replace the so far almost unanimously employed photosensitized oxygenation to generate 1O2, the feasibility and scalability of the synthesis can be dramatically improved; the operations become very simple with no need for any special equipments, while the reproducible overall yield remain among the best so far documented. This procedure should favorably complement the existing ones10e,12 and illustrate an additional possible entry to the eventual low cost production of QHS.
As far as the formation of QHS 1 from acid 2 is concerned, the chemistry involved in the present work (Scheme 1, the lower path) should be exactly the same as in the previous8,9,12 endeavors. However, the singlet oxygen now was generated by disproportionation of H2O2 (sometimes referred to as ‘dark singlet oxygen’14,15) rather than the traditional photo irradiation in the presence of a sensitizer. (The continuous-flow approach)
Light source, sensitizer O2 from cylinder, pumps special reactor, etc.
(photo-generated 1O2) H
H
H 3
HO 2
H O
39% overall
H
H
O HO HO 4
30% H2O2/Na2MoO4 (cat.) NaOH/MeOH-H2O ambient temperature stirring
O2/H+
O O O 1 QHS
O 1
(dark O2)
H
O O
41% overall
(The present approach)
Scheme 1. The outline of the present approach with the main differences shown in comparison with the continuous-flow one.
Because of this change, the whole synthesis now can be carried out in an ordinary laboratory hood; all the equipments required are essentially no more than a round bottom flask and a magnetic stirrer. The cost for the facilities is hence lowered to the minimum one can imagine, while the scalability is improved to a new level. In a typical run, commercially available 30% H2O2 was added slowly to a mixture of acid 2, a catalytic amount of Na2MoO414c and NaOH in MeOH/H2O stirred at ambient temperature. The 1O2 generated by disproportionation of H2O2 reacted in situ with 2 to form (presumably) the intermediate 4 as described in the previous8,9,12 investigations. The reaction proceeded very smoothly, without any need for special care and close monitoring. Normally, when the characteristic burgundy color faded (over 12e14 h, signaling full consumption of H2O2), TLC showed disappearance of the starting acid 2. The crude intermediate (4) thus obtained after simple conventional workup was stirred in petroleum ether at ambient temperature under an oxygen atmosphere (balloon) in the presence of an acidic catalyst for two days to go through those reactions that were already discussed in detail in the previous studies and eventually afforded the desired end product QHS 1 in 41% isolated yield.16 Some other molybdenum-containing catalysts are also known to be able to induce disproportionation of H2O2 to generate dark 1 O2, such as Mg0.7Al0.3eLDHeMoO414a,b and (NH4)2MoO4.4 However, their performance appeared much less satisfactory than Na2MoO414c in the present context. As part of the expense at which the smoothness and pleasing reproducibility of the new approach were achieved, the overall reaction time required here is relatively long if compared with that for those photo irradiation-based approaches. However, because only stirring at ambient temperature was involved, the longer time does not seem to impose any real problem. The approach presented above by no means is already fully explored. Nevertheless, the preliminary results shown here may help to draw attention to the so far overlooked potential of dark
3. Conclusion
4. Experimental 4.1. General To a solution of acid 2 (3.00 g, 12.7 mmol) in MeOH/H2O (85:15 v/v, 130 mL) stirred at ambient temperature (23e25 C) was added Na2MoO4$2H2O (308 mg, 1.27 mmol) in a 250 mL round bottom open flask, followed by NaOH (508 mg, 12.7 mmol). The mixture was stirred at ambient temperature for ca. 2 min before aq H2O2 (30%, 20 mL, 178 mmol) was added over ca. 1 min. A yellow color developed immediately, which soon became orange and finally looked similar to red-wine with further addition of H2O2. The mixture (essentially solution) was stirred (with speed set to ca. 800 rpm) at ambient temperature (23e25 C) for 12e14 h, over which the burgundy color faded to orange and finally almost colorless. When TLC showed full consumption of the starting 2 (if necessary, 2e5 mL portions of H2O2 might be added), most of the MeOH was removed at ambient temperature on a rotary evaporator. The residue was diluted with EtOAc (30 mL), acidified with HOAc (1.1 mL, 19 mmol). Water (5 mL) was then added. The mixture was extracted with EtOAc (350 mL). The combined organic layers were washed with brine (5 mL). The organic layer was concentrated at ambient temperature on a rotary evaporator. To the colorless sticky oily residue (in a 500 mL round bottom flask) were added petroleum ether (bp 60e90 C, 250 mL) and CF3CO2H (0.19 mL, 2.5 mmol). The mixture (the sticky intermediate remained ‘insoluble’ at the bottom) was stirred (egg-shaped stirring bar, 3.5 cm in length, with stirrer speed set to ca. 1000 rpm) at ambient temperature under O2 (balloon) for 2 d. The mixture was poured into another flask and thus separated from the white solids (QHS) precipitated on the flask wall. To the liquid mixture was added aq satd NaHCO3 (10 mL). The biphasic mixture was stirred at ambient temperature for 30 min. The organic solvent was removed on a rotary evaporator. The residue was extracted with EtOAc (350 mL). The combined organic layers were washed with brine and concentrated by rotary evaporation. The residue together with the precipitated white solids was chromatographed (6:1:0.1 petroleum ether/EtOAc/CH2Cl2) on silica gel to give QHS 1 as a white solid (1.482 g, 5.25 mmol, 41%). Further purification by
1114
H.-J. Chen et al. / Tetrahedron 69 (2013) 1112e1114
recrystallization in cyclohexane gave white needles. Mp¼153e154 C (nat. mp¼153e154 C). [a]26 D þ68.0 (c 0.91, CHCl3), 1 (nat. [a]20 D þ66.6 (c 0.90, CHCl3)). H NMR (400 MHz, CDCl3): d¼5.84 (s, 1H), 3.38 (dq, J¼7.3, 5.7 Hz, 1H), 2.41 (ddd, J¼14.6, 13.3, 3.9 Hz, 1H), 2.08e1.94 (m, 2H), 1.92e1.83 (m, 1H), 1.80e1.70 (m, 2H), 1.52e1.31 (m, 3H), 1.42 (s, 3H), 1.19 (d, J¼7.3 Hz, 3H), 1.14e1.00 (m, 2H), 0.98 (d, J¼6.0 Hz, 3H). 13C NMR (100 MHz, CDCl3): d¼172.0, 105.3, 93.7, 79.5, 50.0, 45.0, 37.5, 35.9, 33.6, 32.8, 25.1, 24.8, 23.3, 19.8, 12.5. IR (film): n¼2956, 2935, 2860, 1732, 1113, 994 cm1. ESIMS: m/z 283.3 ([MþH]þ). Use of EtOH (first step) and air (i.e., open flask, in the second step) instead of MeOH and O2, respectively, gave more or similar results (with the overall yields 1e3% lower), even on 6 g scales, without any discernible difficulties. Acknowledgements This work was supported by the National Basic Research Program of China (the 973 Program, 2010CB833200), and the National Natural Science Foundation of China (21172247, 21032002, 20921091, 20621062).
6. 7. 8.
Supplementary data Supplementary data related to this article can be found online at http://dx.doi.org/10.1016/j.tet.2012.11.056. These data include MOL files and InChIKeys of the most important compounds described in this article.
9.
10.
References and notes 11. 1. (a) Liu, J. M.; Ni, M. Y.; Fan, J. F.; Tu, Y. Y.; Wu, Z. H.; Wu, Y. L.; Chou, W. S. Acta Chim. Sinica 1979, 37, 129e143; Chem. Abstr. 1980, 92, 94594w; (b) Klayman, D. L. Science 1985, 228, 1049e1055; (c) Haynes, R. K.; Vonwiller, S. C. Acc. Chem. Res. 1997, 30, 73e79; (d) Vroman, J. A.; Alvim-Gaston, M.; Avery, M. A. Curr. Pharm. Des. 1999, 5, 101e138; (e) Robert, A.; Dechy-Cabaret, O.; Cazalles, J. M.; Meunier, B. Acc. Chem. Res. 2002, 35, 167e174; (f) O’Neill, P. M.; Posner, G. H. J. Med. Chem. 2004, 47, 2945e2964; (g) Posner, G. H.; O’Neill, P. M. Acc. Chem. Res. 2004, 37, 397e404; (h) Tang, Y.; Dong, Y.; Vennerstrom, J. L. Med. Res. Rev. 2004, 24, 425e448; (i) Muraleedharan, K. M.; Avery, M. A. Comprehensive Medicinal Chemistry II. In Therapeutic Areas II: Cancer, Infectious Diseases, Inflammation & Immunology and Dermatology; John, B. T., David, J., Eds.; 2007; Vol. 7, pp 765e814; (j) Li, Y.; Wu, Y. L. Front. Chem. China 2010, 5, 357e422. 2. (a) Mann, A. Nature 2010, 466, 672e673; (b) Enserik, M. Science 2010, 328, 842; (c) Cyranoski, D. Nature 2004, 432, 259; (d) White, N. J. Science 2008, 320, 330e334. 3. Total synthesis of QHS: (a) Schmid, G.; Hofheinz, W. J. Am. Chem. Soc. 1983, 105, 624e625; (b) Xu, X. X.; Zhu, J.; Huang, D. Z.; Zhou, W. S. Tetrahedron 1986, 42, 819e828; (c) Avery, M. A.; Chong, W. K. M.; White, C. J. J. Am. Chem. Soc. 1992, 114, 974e979; (d) Avery, M. A.; White, C. J.; Chong, W. K. M. Tetrahedron Lett. 1987, 28, 4629e4632; (e) For a review, see: Zhou, W. S.; Xu, X. X. Acc. Chem. Res. 1994, 27, 211e216 Semi-synthesis of QHS (f) Ye, B.; Wu, Y. L. J. Chem. Soc., Chem. Commun. 1990, 726e727; (g) Nowak, D. M.; Lansbury, P. T. Tetrahedron 1998, 54, 319e336; (h) Sy, L. K.; Brown, G. D. Tetrahedron 2002, 58, 897e908; (i) Hao, H. D.; Li, Y.; Han, W. B.; Wu, Y. K. Org. Lett. 2011, 13, 4212e4215. 4. For a very recent gram-scale synthesis of QHS using a [2þ2] cycloaddition of 1 O2 to introduce the peroxy bond, see: Zhu, C. Y.; Cook, S. P. J. Am. Chem. Soc. 2012, 134, 13577e13579. 5. (a) Posner, G. H.; Oh, C. H.; Wang, D. S.; Gerena, L.; Milhous, W. K.; Meshnick, S. R.; Asawamahasadka, W. J. Med. Chem. 1994, 37, 1256e1258; (b) Haraldson, C. A.; Karle, J. M.; Freeman, S. G.; Duvadie, R. K.; Avery, M. A. Bioorg. Med. Chem. Lett. 1997, 7, 2357e2362; (c) Jefford, C.; Velarde, J.; Bernardinelli, G.; Bray, D. H.; Warhurst, D. C.; Milhous, W. K. Helv. Chim. Acta 1993, 76, 2775e2788; (d) Cazelles, J.; Camuzat-Dedenis, B.; Provot, O.; Robert, A.; Mayrargue, J.; Meunier, M. J. Chem. Soc., Perkin Trans. 1 2000, 1265e1270; (e) O’Neill, P. M.; Rawe, S. L.; Borstnik, K.; Miller, A.; Ward, S. A.; Bray, P. G.;
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Davies, J.; Oh, C. H.; Posner, G. H. ChemBioChem 2005, 6, 2048e2054; (f) Kim, H.-S.; Begum, K.; Ogura, N.; Wataya, Y.; Nonami, Y.; Ito, T.; Masuyama, A.; Nojima, M.; McCullough, K. J. J. Med. Chem. 2003, 46, 1957e1961; (g) Vennerstrom, J. L.; Arbe-Barnes, S.; Brun, R.; Charman, S. A.; Chiu, F. C. K.; Chollet, J.; Dong, Y.; Dorn, A.; Hunziker, D.; Matile, H.; McIntosh, K.; Padmanilayam, M.; Santo Tomas, J.; Scheurer, C.; Scorneaux, B.; Tang, Y.; Urwyler, H.; Wittlin, S.; Charman, W. N. Nature 2004, 430, 900e904; (h) Martyn, D. C.; Ramirez, A. P.; Beattie, M. J.; Cortese, J. F.; Patel, V.; Rush, M. A.; Woerpel, K. A.; Clardy, J. Bioorg. Med. Chem. Lett. 2008, 18, 6521e6524; (i) Ghorai, P.; Dussault, P. H.; Hu, C. Org. Lett. 2008, 10, 2401e2404; (j) Singh, C.; Verma, V. P.; Naikade, N. K.; Singh, A. S.; Hassam, M.; Puri, S. K. J. Med. Chem. 2008, 51, 7581e7592; (k) Haynes, R. K.; Fugmann, B.; Stetter, J.; Rieckmann, K.; Heilmann, H.-D.; Chan, H.-W.; Cheung, M.-K.; Lam, W.-L.; Wong, H.-N.; Croft, S. L.; Vivas, L.; Rattray, L.; Stewart, L.; Peters, W.; Robinson, B. L.; Edstein, M. D.; Kotecka, B.; Kyle, D. E.; Beckermann, B.; Gerisch, M.; Radtke, M.; Schmuck, G.; Steinke, W.; Wollborn, U.; Schmeer, K.; Romer, A. Angew. Chem., Int. Ed. 2006, 45, 2082e2088; (l) Charman, S. A.; Arbe-Barnes, S.; Bathurst, I. C.; Brun, R.; Campbell, M.; Charman, W. N.; Chiu, F. C. K.; Chollet, J.; Craft, J. C.; Creek, D. J.; Dong, Y.; Matile, H.; Maurer, M.; Morizzi, J.; Nguyen, T.; Papastogiannidis, P.; Scheurer, C.; Shackleford, D. M.; Sriraghavan, K.; Stingelin, L.; Tang, Y.; Urwyler, H.; Wang, X.; White, K. L.; Wittlin, S.; Zhou, L.; Vennerstrom, J. L. Proc. Natl. Acad. Sci. U.S.A. 2011, 108, 4400e4405. Wallaart, T. E.; van Uden, W.; Lubberink, H. G. M.; Woerdenbag, H. J.; Pras, N.; Quax, W. J. J. Nat. Prod. 1999, 62, 430e433. For reduction of acid 2 to 3, see: Jung, M.; ElSohly, H. N.; Croom, E. M.; McPhail, A. T.; McPhail, D. R. J. Org. Chem. 1986, 51, 5417e5419. (a) Roth, R. J.; Acton, N. J. Nat. Prod. 1989, 52, 1183e1185; (b) Acton, N.; Roth, R. J. J. Org. Chem. 1992, 57, 3610e3614. (a) Haynes, R. K.; Vonwiller, S. C. J. Chem. Soc., Chem. Commun. 1990, 451e452; (b) Vonwiller, S. C.; Warner, J. A.; Mann, S. T.; Haynes, R. K. J. Am. Chem. Soc. 1995, 117, 11098e11105. For synthesis of QHS relying on this strategy, see: (a) Liu, H. J.; Yeh, W. L.; Chew, S. Y. Tetrahedron Lett. 1993, 34, 4435e4438; (b) Constantino, M. G.; Beltrame, M., Jr.; da Silva, G. V. J. Synth. Commun. 1996, 26, 321e329; (c) Yadav, J. S.; Babu, R. S.; Sabitha, G. Tetrahedron Lett. 2003, 44, 387e389; (d) Yadav, J. S.; Thirupathaiah, B.; Srihari, P. Tetrahedron 2010, 66, 2005e2009; (e) Dhainaut, J.; Dlubala, A.; Guevel, R.; Medard, A.; Oddon, G.; Raymond, N.; Turconi, J. U.S. Patent Appl. 2011/ 0065933, 2011. (a) Martin, V. J. J.; Pitera, D. J.; Withers, S. T. J.; Newman, D.; Keasling, J. D. Nat. Biotechnol. 2003, 21, 796e802; (b) Ro, D.-K.; Paradise, E. M.; Ouellet, M.; Fisher, K. J.; Newman, K. L.; Ndungu, J. M.; Ho, K. A.; Eachus1,, R. A.; Ham, T. S.; Kirby, J.; Chang, M. C. Y.; Withers, S. T.; Shiba, Y.; Sarpong, R.; Keasling, J. D. Nature 2006, 440, 940e943. vesque, F.; Seeberger, P. H. Angew. Chem., Int. Ed. 2012, 51, 1706e1709. Le (a) Kupferschmidt, K. Science 2012, 336, 798e799 (a news report), where it was mentioned that Seeberger’s yield was raised to 65% without mentioning the scale and purity of the product; (b) Booker-Milburn, K. Nat. Chem. 2012, 4, 434e435 (a news report). (a) van Laar, F.; De Vos, D.; Vanoppen, D.; Sels, B.; Jacobs, P. A.; Del Guerzo, A.; Pierard, F.; Kirsch-De Mesmaeker, A. Chem. Commun. 1998, 267e268; (b) Sels, B. F.; De Vos, D. E.; Grobet, P. J.; Pierard, F.; Kirsch-De Mesmaeker, F.; Jacobs, P. A. J. Phys. Chem. B 1999, 103, 11114e11123; (c) Aubry, J. M. J. Am. Chem. Soc. 1985, 107, 5844e5849; (d) Jin, H.-X.; Liu, H.-H.; Wu, Y.-K. Chin. J. Chem. 2004, 22, 999e1002. The dark singlet oxygen approach was first examined by the Amyris chemists. In an initial attempt performed on a 40 mg scale, they did not obtain any isolated QHS but a mixture containing 40% QHS (1) as measured by 1H NMR, see: (a) K.K. Reiling, N.S. Renninger, D.J. Mcphee, K.J. Fisher, D.A. Ockey, PCT Int. Appl., 2006, WO 2006128126 A1 20061130. In a subsequent improvement featuring use of methyl ester of 2 instead of 2 as the starting material, they isolated QHS with a highest yield of 44% on a 200 mg scale. The yield for the run affording 1.6 g of QHS (the largest scale reported therein) was only 15%, see: (b) K. Fisher, D.J. Mcphee, F.X. Woolard, WO 2009088404A1. We thank the referees encountered in the previous submission (on Sept. 5, 2012, with an ID of anie.201207200) to another journal for bringing these patents to our attention. For a very recent patent disclosed on Oct. 10, 2012, see: (c) W. Zhang, D. Liu, Q. Yuan, Faming Zhuanli Shenqing, CN 102718773 A (Scifinder AN 2012: 1493460). Differences between the Amyris procedure and the present one include: in the first step, the Amyris procedure used 50% H2O2 (added via a syringe pump), whereas the present one used 30% H2O2 and a smaller amount of Na2MoO4$2H2O as catalyst. Also, a Cu(II) species was required in the Amyris procedure, but not in the present one. Solvents were also different.
Contents lists available at SciVerse ScienceDirect
Tetrahedron journal homepage: www.elsevier.com/locate/tet
A facile and scalable synthesis of qinghaosu (artemisinin) Hui-Jun Chen, Wei-Bo Han, Hong-Dong Hao, Yikang Wu * State Key Laboratory of Bioorganic and Natural Products Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 November 2012 Received in revised form 16 November 2012 Accepted 16 November 2012 Available online 23 November 2012
A very simple and efficient approach for the conversion of dihydro-artemisinic acid into artemisinin is developed, featuring use of a molybdate induced disproportionation of hydrogen peroxide to generate singlet oxygen. The whole synthesis was completed by stirring at ambient temperature without need for any special equipments. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Ene reaction Singlet oxygen Ring-closing Peroxide Antimalarial
1. Introduction Qinghaosu (QHS, 1, artemisinin, Fig. 1) is a sesquiterpene containing a 1,2,4-trioxane core isolated in the early 1970s from a herb (qinghao, Artemisia annua L.) by Chinese scientists.1 Because of its outstanding antimalarial activity, especially in treatment of the multi drug resistant cases, QHS and its derivatives and analogues, has been broadly recognized as the most important weapons against malaria.2 H O O O
H
O 1
HO O
H
H
2
H O
H HO 3
H
H
O
Fig. 1. The structures for QHS 1, dihydro-qinghao acid (dihydro-artemisinic acid) 2, and qinaghao acid (artemisinic acid) 3.
Unfortunately, QHS is only present in very low content in the herb growing in tropic/subtropical areas. And the same herb that grows in the northern areas does not produce QHS at all. Total synthesis of QHS was achieved even back in the 1980s.3,4 However, the length of the routes, the reagents and conditions involved, and
* Corresponding author. E-mail address: [email protected] (Y. Wu). 0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tet.2012.11.056
the yield make it impossible for total synthesis to serve as a source of supply. All these prompted exhaustive efforts in seeking alternative sources for QHS or developing simpler organic peroxides5 with comparable potency. Dihydro-artemisinic acid (2, a natural product,6 also accessible via reduction7 of 3) is long-recognized as a valuable starting material for chemical synthesis of QHS. The possibility for transforming 2 into 1 (featuring using an ene reaction of the photogenerated 1O2 and subsequent trapping of 3O2) was elegantly shown in late 1980s/early 1990s by the pioneers8,9 and subsequently applied by other investigators.10 The novel eye-catching fermentation access11 to acid 3 also relies on this transformation to comprise a prospective alternative supply for QHS. However, the conversion of 2 to 1 could be performed satisfactorily only on small scales and afforded QHS in quantities less than 180 mg (in most cases <30 mg) as seen from the published papers over the years. Presumably, there were some hidden difficulties in there, which made scaling up of the seemingly very promising shortcut unfeasible over the past decades. vesque and SeeRecently, a major advance was made by Le berger.12 They carried out the same photosensitized oxygenationbased transformation on an unprecedented 3-g scale in a specially designed continuous-flow reactor and obtained QHS in 39% yield (before recrystallization). As never before has QHS been synthesized on such a large scale, not to mention the reproducible yield, the novel continuous-flow approach brings forth a suddenly brightened prospect for low cost production of QHS and thus has been highly valued by the scientific community.13
H.-J. Chen et al. / Tetrahedron 69 (2013) 1112e1114
1113
Herein we wish to disclose a useful alternative for achieving the same conversion of acid 2 into QHS 1, which remains the advantages of the continuous-flow technique while the operations are greatly simplified. More importantly, all the special equipments required for the photo reaction are no longer needed.
singlet oxygen in this much-studied transformation and hopefully prompt further investigations that eventually lead to a low cost production of QHS.
2. Results and discussion
In summary, developing a cost-effective and scalable synthesis of QHS, a long-sought after goal in seeking alternative sources of supply for the natural QHS, represents a lasting challenge since the 1980s. Transformation of 2 into QHS discovered some 20 years ago is broadly considered to be the most promising candidate, especially after the access to 3 by fermentation11 was developed. However, despite many successful small scale precedents, further elaboration into a scalable synthesis has proven to be much more difficult than the published papers implied. However, despite many successful small scale precedents, further elaboration into a scalable synthesis has proven to be much more difficult than the published papers implied. Now, we show that by using the Na2MoO4/H2O2 protocol to replace the so far almost unanimously employed photosensitized oxygenation to generate 1O2, the feasibility and scalability of the synthesis can be dramatically improved; the operations become very simple with no need for any special equipments, while the reproducible overall yield remain among the best so far documented. This procedure should favorably complement the existing ones10e,12 and illustrate an additional possible entry to the eventual low cost production of QHS.
As far as the formation of QHS 1 from acid 2 is concerned, the chemistry involved in the present work (Scheme 1, the lower path) should be exactly the same as in the previous8,9,12 endeavors. However, the singlet oxygen now was generated by disproportionation of H2O2 (sometimes referred to as ‘dark singlet oxygen’14,15) rather than the traditional photo irradiation in the presence of a sensitizer. (The continuous-flow approach)
Light source, sensitizer O2 from cylinder, pumps special reactor, etc.
(photo-generated 1O2) H
H
H 3
HO 2
H O
39% overall
H
H
O HO HO 4
30% H2O2/Na2MoO4 (cat.) NaOH/MeOH-H2O ambient temperature stirring
O2/H+
O O O 1 QHS
O 1
(dark O2)
H
O O
41% overall
(The present approach)
Scheme 1. The outline of the present approach with the main differences shown in comparison with the continuous-flow one.
Because of this change, the whole synthesis now can be carried out in an ordinary laboratory hood; all the equipments required are essentially no more than a round bottom flask and a magnetic stirrer. The cost for the facilities is hence lowered to the minimum one can imagine, while the scalability is improved to a new level. In a typical run, commercially available 30% H2O2 was added slowly to a mixture of acid 2, a catalytic amount of Na2MoO414c and NaOH in MeOH/H2O stirred at ambient temperature. The 1O2 generated by disproportionation of H2O2 reacted in situ with 2 to form (presumably) the intermediate 4 as described in the previous8,9,12 investigations. The reaction proceeded very smoothly, without any need for special care and close monitoring. Normally, when the characteristic burgundy color faded (over 12e14 h, signaling full consumption of H2O2), TLC showed disappearance of the starting acid 2. The crude intermediate (4) thus obtained after simple conventional workup was stirred in petroleum ether at ambient temperature under an oxygen atmosphere (balloon) in the presence of an acidic catalyst for two days to go through those reactions that were already discussed in detail in the previous studies and eventually afforded the desired end product QHS 1 in 41% isolated yield.16 Some other molybdenum-containing catalysts are also known to be able to induce disproportionation of H2O2 to generate dark 1 O2, such as Mg0.7Al0.3eLDHeMoO414a,b and (NH4)2MoO4.4 However, their performance appeared much less satisfactory than Na2MoO414c in the present context. As part of the expense at which the smoothness and pleasing reproducibility of the new approach were achieved, the overall reaction time required here is relatively long if compared with that for those photo irradiation-based approaches. However, because only stirring at ambient temperature was involved, the longer time does not seem to impose any real problem. The approach presented above by no means is already fully explored. Nevertheless, the preliminary results shown here may help to draw attention to the so far overlooked potential of dark
3. Conclusion
4. Experimental 4.1. General To a solution of acid 2 (3.00 g, 12.7 mmol) in MeOH/H2O (85:15 v/v, 130 mL) stirred at ambient temperature (23e25 C) was added Na2MoO4$2H2O (308 mg, 1.27 mmol) in a 250 mL round bottom open flask, followed by NaOH (508 mg, 12.7 mmol). The mixture was stirred at ambient temperature for ca. 2 min before aq H2O2 (30%, 20 mL, 178 mmol) was added over ca. 1 min. A yellow color developed immediately, which soon became orange and finally looked similar to red-wine with further addition of H2O2. The mixture (essentially solution) was stirred (with speed set to ca. 800 rpm) at ambient temperature (23e25 C) for 12e14 h, over which the burgundy color faded to orange and finally almost colorless. When TLC showed full consumption of the starting 2 (if necessary, 2e5 mL portions of H2O2 might be added), most of the MeOH was removed at ambient temperature on a rotary evaporator. The residue was diluted with EtOAc (30 mL), acidified with HOAc (1.1 mL, 19 mmol). Water (5 mL) was then added. The mixture was extracted with EtOAc (350 mL). The combined organic layers were washed with brine (5 mL). The organic layer was concentrated at ambient temperature on a rotary evaporator. To the colorless sticky oily residue (in a 500 mL round bottom flask) were added petroleum ether (bp 60e90 C, 250 mL) and CF3CO2H (0.19 mL, 2.5 mmol). The mixture (the sticky intermediate remained ‘insoluble’ at the bottom) was stirred (egg-shaped stirring bar, 3.5 cm in length, with stirrer speed set to ca. 1000 rpm) at ambient temperature under O2 (balloon) for 2 d. The mixture was poured into another flask and thus separated from the white solids (QHS) precipitated on the flask wall. To the liquid mixture was added aq satd NaHCO3 (10 mL). The biphasic mixture was stirred at ambient temperature for 30 min. The organic solvent was removed on a rotary evaporator. The residue was extracted with EtOAc (350 mL). The combined organic layers were washed with brine and concentrated by rotary evaporation. The residue together with the precipitated white solids was chromatographed (6:1:0.1 petroleum ether/EtOAc/CH2Cl2) on silica gel to give QHS 1 as a white solid (1.482 g, 5.25 mmol, 41%). Further purification by
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H.-J. Chen et al. / Tetrahedron 69 (2013) 1112e1114
recrystallization in cyclohexane gave white needles. Mp¼153e154 C (nat. mp¼153e154 C). [a]26 D þ68.0 (c 0.91, CHCl3), 1 (nat. [a]20 D þ66.6 (c 0.90, CHCl3)). H NMR (400 MHz, CDCl3): d¼5.84 (s, 1H), 3.38 (dq, J¼7.3, 5.7 Hz, 1H), 2.41 (ddd, J¼14.6, 13.3, 3.9 Hz, 1H), 2.08e1.94 (m, 2H), 1.92e1.83 (m, 1H), 1.80e1.70 (m, 2H), 1.52e1.31 (m, 3H), 1.42 (s, 3H), 1.19 (d, J¼7.3 Hz, 3H), 1.14e1.00 (m, 2H), 0.98 (d, J¼6.0 Hz, 3H). 13C NMR (100 MHz, CDCl3): d¼172.0, 105.3, 93.7, 79.5, 50.0, 45.0, 37.5, 35.9, 33.6, 32.8, 25.1, 24.8, 23.3, 19.8, 12.5. IR (film): n¼2956, 2935, 2860, 1732, 1113, 994 cm1. ESIMS: m/z 283.3 ([MþH]þ). Use of EtOH (first step) and air (i.e., open flask, in the second step) instead of MeOH and O2, respectively, gave more or similar results (with the overall yields 1e3% lower), even on 6 g scales, without any discernible difficulties. Acknowledgements This work was supported by the National Basic Research Program of China (the 973 Program, 2010CB833200), and the National Natural Science Foundation of China (21172247, 21032002, 20921091, 20621062).
6. 7. 8.
Supplementary data Supplementary data related to this article can be found online at http://dx.doi.org/10.1016/j.tet.2012.11.056. These data include MOL files and InChIKeys of the most important compounds described in this article.
9.
10.
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Soc. 1985, 107, 5844e5849; (d) Jin, H.-X.; Liu, H.-H.; Wu, Y.-K. Chin. J. Chem. 2004, 22, 999e1002. The dark singlet oxygen approach was first examined by the Amyris chemists. In an initial attempt performed on a 40 mg scale, they did not obtain any isolated QHS but a mixture containing 40% QHS (1) as measured by 1H NMR, see: (a) K.K. Reiling, N.S. Renninger, D.J. Mcphee, K.J. Fisher, D.A. Ockey, PCT Int. Appl., 2006, WO 2006128126 A1 20061130. In a subsequent improvement featuring use of methyl ester of 2 instead of 2 as the starting material, they isolated QHS with a highest yield of 44% on a 200 mg scale. The yield for the run affording 1.6 g of QHS (the largest scale reported therein) was only 15%, see: (b) K. Fisher, D.J. Mcphee, F.X. Woolard, WO 2009088404A1. We thank the referees encountered in the previous submission (on Sept. 5, 2012, with an ID of anie.201207200) to another journal for bringing these patents to our attention. For a very recent patent disclosed on Oct. 10, 2012, see: (c) W. Zhang, D. Liu, Q. Yuan, Faming Zhuanli Shenqing, CN 102718773 A (Scifinder AN 2012: 1493460). Differences between the Amyris procedure and the present one include: in the first step, the Amyris procedure used 50% H2O2 (added via a syringe pump), whereas the present one used 30% H2O2 and a smaller amount of Na2MoO4$2H2O as catalyst. Also, a Cu(II) species was required in the Amyris procedure, but not in the present one. Solvents were also different.