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“Stereocontrolled organocatalytic synthesis of prostaglandin PGF2α in seven steps” Coulthard, G.; Erb, W.; Aggarwal, V. K. Nature 2012, online view. DOI: 10.1038/nature11411
In my very un-scientific survey of the green chemistry-branded journals, I see way more new methodologies than I see total syntheses. I hope to single-handedly change this, and show how green a total synthesis can be by writing about the awesome recent synthesis of prostaglandin PGF2α by Aggarwal and coworkers. First, a few words on the target molecule. Being hormones, prostaglandins such as PGF2α are involved in tons of biological processes. Interestingly, instead of being synthesized by some important gland and acting in far-off regions of the body as are endocrine hormones, they are autocrine or paracrine hormones and are synthesized “on-site.” The first structural characterizations of prostaglandins came in the 1960s, some 30 years after their initial discovery. Soon after, they became the subject of numerous syntheses, the first of which was achieved by E. J. Corey in 1969. A series of syntheses followed, but even 40 years later, the structurally-related glaucoma drug latanoprost is synthesized in 20 steps using Corey’s 1969 prostaglandin strategy.
That’s right, the prostglandin structural motif is medicinally relevant. So, not only would an improved synthesis be cool from a fundamental science perspective, it might actually be moved into industrial production and have an immediate impact! (more…)
“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.
Savile, Janey, Mundorff, Moore, Tam, Jarvis, Colbeck, Krebber, Fleitz, Brands, Devine, Huisman, and Hughes. Biocatalytic Asymmetric Synthesis of Chiral Amines from Ketones Applied to Sitagliptin Manufacture. Science, 2010, 329, 305-309. DOI: 10.1126/science.1188934.
Here’s a short follow-up on this previous post, which covered a biocatalytic reaction developed by Codexis to make the key intermediate in the synthesis of Merck’s drug montelukast (aka Singulair). The 2010 Presidential Green Chemistry Awards were just announced, and the award for “Greener Reaction Conditions” went to Merck and Codexis for developing an enantioselective biocatalyst for the synthesis of sitagliptin, Merck’s blockbuster anti-diabetes drug (aka Januvia). This work is also the subject of a recently-published Science paper from Merck and Codexis.
The paper describes the development of an enzyme-catalyzed replacement for the final reaction in the synthesis of sitagliptin, in which a ketone functionality is converted into an amine. (more…)
Liang, Lalonde, Borup, Mitchell, Mundorff, Trinh, Kochrekar, Cherat, Pai. Development of a Biocatalytic Process as an Alternative to the (−)-DIP-Cl-Mediated Asymmetric Reduction of a Key Intermediate of Montelukast. Org. Process Res. Dev. 2010, 14, 193-198. DOI: 10.1021/op900272d
This article from researchers at Codexis describes the development of a biocatalytic (i.e. enzyme-catalyzed) method for creating the lone stereocenter in the synthesis of montelukast sodium, aka Merck’s asthma drug Singulair. The original Merck process route includes an enantioselective ketone reduction using a boron reagent derived from alpha-pinene called (-)-DIP-Cl. The reaction works well: high yield, high enantioselectivity (although still requiring a recrystallization step to upgrade from ~95% to 99% ee), and (-)-DIP-Cl is made in one step from cheap starting materials. The downside is that at least 1.5 equivalents of (-)-DIP-Cl must be used, and the reagent is moisture sensitive and corrosive. Codexis, being in the enzyme business, decided to find an enzyme that would catalyze this same reaction. (more…)