Industrialized artemisinin

Besides their essential roles in nature, isoprenoids are of commercial importance in industry. Some isoprenoids have been used as flavors, fragrances, spices, and food additives, while many are used as pharmaceuticals to treat an array of human diseases, such as cancer (Taxol), malaria (artemisinin), and HIV (coumarins). In contrast to the huge market demand, isoprenoids are present only in low abundance in their host organisms. Thus, isolation of the required isoprenoids consumes a large quantity of natural resources. Furthermore, owing to their structural complexity, total chemical synthesis is often not commercially feasible. For these reasons, metabolic engineering may provide an alternative to produce these valuable isoprenoids [88,89]. 

According to Coimbra et solvents play a central role in the majority of chemical and pharmaceutical industrial processes. The most used method to obtain artemisinin (1) from A. annua is through the use of organic solvents such as toluene, hexane, cyclohexane, ethanol, chloroform and petroleum ether. Rodrigues et al described a low-cost and industrial scaled procedure that enables artemisinin (1) enhanced yields by using inexpensive and easy steps. Serial extraction techniques allowed a reduction of 65% in solvent consumption. Moreover, the use of ethanol for compound extraction is safer when compared to other solvents. Flash column pre-purification employing silicon dioxide (Zeosil ) as stationary phase provided an enriched artemisinin (1) fraction that precipitated in hexane/ethyl acetate (85/15, v/v) solution. These results indicate the feasibility of producing artemisinin (1) at final cost lowered by almost threefold when compared to classical procedures. 

The bisquinoline known as piperaquine (137) was synthesized in 1966 at the Shanghai Pharmaceutical Industry Research Institute. Piperaquine was found to be active against chloroquine-resistant falciparum malaria. In order to delay resistance, it is used in combination therapy, including with dihydroartemisinin, a derivative of the Chinese plant product artemisinin, first isolated in 1972. 

In this chapter, we first describe the peculiar electronic structure of 2 and its impact on its chemical reactivity that is opposite from that of ordinary oxygen 02-Then, we compare the respective advantages and limitations of photochemical and chemical methods to generate 2 in a context of industrial development. In particular, we detail the criteria for choosing a reaction medium compatible with both the organic substrate and water-soluble chemical sources of 02- Finally, the main reactions of 2 in organic chemistry are listed and illustrated with two industrially relevant examples recently developed in the fields of perfumery (synthesis of rose oxide) and pharmacy (synthesis of artemisinin). 

Combinatorial biosynthesis, that is, the combination of metabolic pathways in different organisms on a genetic level allowing the use of precursors of the host cells is another promising strategy for the synthesis and industrial production of important classes of natural products, including alkaloids (vinblastine, vincristine), terpenoids (artemisinin, pacUtaxel), and flavonoids [59],... 

On an industrial scale, artemisinin is obtained from plant extracts of sweet wormwood, and by fermentation combined with partial synthesis. 

Among the worldwide total of 30000 known natural products, about 80% stems from plant resources. The number of known chemical structures of plant secondary metabolites is four times the number of known microbial secondary metabolites. Plant secondary metabolites are widely used as valuable medicines (such as paclitaxel, vinblastine, camptothecin, ginsenosides, and artemisinin), food additives, flavors, spices (such as rose oil, vanillin), pigments (such as Sin red and anthocyanins), cosmetics (such as aloe polysaccharides), and bio-pesticides (such as pyrethrins). Currently, a quarter of all prescribed pharmaceuticals compounds in industrialized countries are directly or indirectly derived from plants, or via semi-synthesis. Furthermore, 11% of the 252 drugs considered as basic and essential by the WHO are exclusively derived from plants. According to their biosynthetic pathways, secondary metabolites are usually classified into three large molecule families phenolics, terpenes, and steroids. Some known plant-derived pharmaceuticals are shown in Table 6.1. 

Recently, further improved yeast systems were reported [395]. These included, among others, CYP71AV1 and an alcohol dehydrogenase and aldehyde dehydrogenase (Adhl and AldHl) from A. annua for the respective conversion of artemisinic alcohol and aldehyde [396]. Titers of artemisinic acid of up to 25 g L were achieved in fermentation experiments and could be further converted to artemisinin by means of classical chemistry or photochemistry [395]. This semisynthetic process is now used at Sanofi for the industrial production of artemisinin. 

Very recently, a further improved yeast system was reported, which induded, among other optimizations and besides CYP71AV1, an alcohol and aldehyde [119] dehydrogenases (ADHl and ALDHl) from A. annua for artemisinic alcohol and aldehyde conversion, respectively (Scheme 5.28). Artemisinic acid titers of up to 25gl i were achieved in fermentation set-up [120]. A process based on the developed artemisinic acid-producing yeast strain is now used for the industrial production of artemisinin at Sanofi (www.rsc.org/chemistryworld/2013/04/sanqfi-launches-malaria-drug-production). 

Others have reported a single continuous-flow process that makes artemisinin from dihydroartemisinic acid, itself easily made from artemisinic acid. It is possible that these processes could be scaled up to produce artemisinin industrially in large quantities, thereby reducing the cost of ACT to a level where it could be afforded by Third World economies. 

During the last decade, microbial platforms for industrial production of plant terpenoids have been developed the biotechnological production of artemisinin precursors in yeast and . coli is a relevant milestone [186,187]. Thanks to these technologies, even terpenes occurring in low amounts in plants can be produced at commercial levels, provided that terpene synthases that perform well in the chosen heterologous host can be found. 

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