Engineering, Synthesis

Green Chemistry via Continuous Flow

“Development of a Continuous Flow Scale-Up Approach of Reflux Inhibitor AZD6906” Gustafsson, T.; Sörensen, H.; Pontén, F. Org. Proc. Res. Dev. 2012, ASAP. DOI: 10.1021/op200340c

“Continuous-Flow Synthesis of the Anti-Malaria Drug Artemisinin.” Lévesque, F.; Seeberger, P. H. Angew. Chem. Int. Ed.. 2012, 51, 1706-1709. DOI: 10.1002/anie.201107446

“Monitoring and Control of a Continuous Grignard Reaction for the Synthesis of an Active Pharmaceutical Ingredient Intermediate Using Inline NIR spectroscopy” Cervera-Padrell, A. E.; Nielsen, J. P.; Pedersen, M. J.; Christensen, K. M.; Mortensen, A. R.; Skovby, Dam-Johansen, T. K.; Kiil, S.; Gernaey, K. V. Org. Proc. Res. Dev. 2012, ASAP. DOI: 10.1021/op2002563

A little while back I wrote about an aerobic oxidation which was greatly improved by switching from a traditional round bottom flask setup to a continuous flow reactor – basically, continuous flow reactors are much better at handling oxygen, especially on scale.  But most of the advantages of the flow reactor were specific to that reaction, and it wasn’t clear to me how a flow process would improve a reaction that doesn’t use oxygen, or some other gas.  Fortunately, a lot has been published since then to help me get a handle on how continuous flow reactions can contribute towards greener processes.  In particular, this review covers continuous processing within a green chemistry context, and Organic Process Research and Developement has a continuous flow themed issue in their ASAP section, including this process-oriented review (speaking of OPRD, check out this recent editorial concerning solvent selection and green chemistry).  It turns out that flow chemistry can improve processes in a bunch of different ways, and it’s hard to get a sense for how this can work by just looking at one reaction.  So I’ll cover a few different reactions that illustrate different green aspects of continuous flow reactors.

One benefit of flow reactors is improved control over reaction temperature, due to reduced reaction volume at a given time, higher surface area, and the movement of the reaction mixture.  This is particularly helpful for very exothermic reactions, which often require cryogenic cooling to prevent runaway reactions – this type of cooling is very expensive and resource-intensive on a large scale.  One such reaction is described in a recent paper from AstraZeneca, in which a phosphinate anion adds into a glycine derivative.  The product of this reaction is an intermediate in the synthesis of a gastroesophageal reflux inhibitor drug candidate called AZD6906.

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Engineering, Synthesis

High-Yielding Semi-Synthesis of an Artemisinin Precursor

“Production of amorphadiene in yeast, and its conversion to dihydroartemisinic acid, precursor to the antimalarial agent artemisinin” Westfall, P.J.; Pitera, D.J.; Lenihan, J.R.; Eng, D.; Woolard, F.X.; Regentin, R.; Horning, T.; Tsuruta, H.; Melis, D.J.; Owens, A.; Fickes, S.; Diola, D.; Benjamin, K.R.; Keasling, J.D.; Leavell, M.D.; McPhee, D.J.; Renninger, N.S.; Newman, J.D.; Paddon, C.J. Proc. Natl. Acad. Sci. U.S.A. 2012109, E111-E118. DOI: 10.1073/pnas.1110740109.

Malaria, caused mainly by the parasite Plasmodium falciparum, leads to nearly a million deaths and 250 million new infections each year. The sesquiterpene lactone endoperoxide artemisinin, derived from Artemisia annua, is very effective as an antimalarial drug, and widespread resistance hasn’t yet developed. Artemisinin is the only high-volume drug that is still isolated by extraction from its native plant producer in a low-yielding (around 10 μg per g plant material), resource-intensive process that uses volatile solvents (most commonly hexane).

Artemisia annua. Photo credit: Jorge Ferreira via Wikimedia Commons.

As a result, supplies of the drug are short, and those who need it often can’t afford it. The development of new processes for artemisinin production would therefore advance both public health and green chemistry interests. Total synthesis of the drug hasn’t been considered as a viable alternative because of low yields, but a lot of effort has been directed toward developing semi-synthetic sources of artemisinin using a combination of microbial fermentation and chemical synthesis. Toward this end, the Keasling lab reported a few years ago that they had constructed a biosynthetic pathway for the artemisinin precursor amorpha-4,11-diene in yeast with yields of ~200 mg/L—already impressive given the complexity of the molecule. Amorphadiene synthase (ADS) comes from Artemisia annua; the rest of the genes are from yeast. Here is the existing pathway:

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