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Simon Muller
Simon Muller

The ecological footprint of all kinds of our daily life activities and materials has received attention in the sustainability discussion: be it in the field of mobility), in agriculture (check out the “fart tax”), or in architecture. You can even calculate your own personal ecological footprint here. So why not talk about the ecological footprint of chemical compounds, for example of natural products?

Natural products are pretty much what you would expect from their name. They are compounds being produced by living organisms (plants, microorganisms, marine organisms etc), usually playing an important role in the metabolism of its organism. Remarkably, they are extremely useful for us as well! More than half of all released medicinal drugs are based on natural products[i].

There are two ways of getting these natural products. The first option is to isolate them through extraction and purification from their natural resources, the second is the so- called total synthesis of the compounds in a chemical laboratory. Surely, the first method sounds greener, closer to nature, doesn’t it? Let’s  take  a closer look…

When you want to isolate a compound from its natural resource, you have to consider at least three things:

1)      How high is the concentration of the compound in the natural resource?

2)      How difficult is it to purify the compound/separate it from the other compounds in the extract?

3)      How ubiquitous is the natural resource?

The answer to the first question is crucial. In the case of the famous anticancer compound Taxol (Paclitaxel) you need to harvest a whole Pacific Yew tree (which is not very big though) to get one bottle of Taxol[ii]. That is not very efficient and it also leads to an incredibly wasteful process of destroying natural resources.

The second question also points to an ecological drawback in natural product isolation. Litres  of partially toxic solvents have to be used to isolate and purify the natural product from the extract containing hundreds of chemical compounds. The isolation process gets even more problematic, if the source of your compound is limited or precious.

 

While our lifespan increases thanks to Paclitaxel,

the Pacific Yew tree population might think differently[iii]

So should we synthesize every molecule in a lab? At some point in the future, this will be ecologically desirable. Processes get more and more sustainable, total synthesis (the ways to get to the desired structure) more elegant and facile, the methods available for organic chemists highly catalytic and selective. However, the yields of total synthesis have just been too low to make them cheaply accessible to everyone (we’re  talking about medicine, so this is very relevant!). Yields for complex molecules can range around 5%, which means from 100 molecules you start with, you get five  of your desired compound in the end.

So if neither isolation nor synthesis is the perfect way to get to medicinal drugs, why not combine both methods? Search for a molecule with a similar chemical core as your desired product but more ubiquitous in nature and easily accessible. Then, modify it synthetically in the lab and get your desired molecule cheap and green. That’s  exactly what’s  done, in  is a process called semi-synthesis. So- called precursors of your target molecule are isolated and then chemically modified in the lab.

Organic chemists keep on improving their methods for green total synthesis. For the time being, it depends on the specific natural product if isolation from natural resources, total synthesis or semi-synthesis is the most sustainable method.

 



[i] Edited by K. Ding, L.-X. Dai: Organic Chemistry – Breakthroughs and Perspectives, Wiley-VCH, p. 1

[ii] http://en.wikipedia.org/wiki/Natural_product, visited 2.2.2013

[iii] Picture taken from: http://en.wikipedia.org/wiki/Taxus_brevifolia, visited 2.2.2013