The software combines long syntheses of compounds into shorter and more economical routes, and identifies suspicious chemical recipes that could lead to chemical weapons.

Their main trick seems to be to combine multiple steps into steps that can happen at once, what they call “one pot”.
2022-02-23: Nice analogy on the energy landscape for reactions, twisty pathways along the edges of steep ridges.

What would it take to have software that showed you the best synthesis for a given compound, though? Now that’s a dream that even I think is out of reach for us, at least for as far out into the future as I can imagine. And this paper illustrates why! Look at all the tiny variations that end up making a difference, and sometimes a big difference. If we could model or compute our way to the answers in such situations, believe me, we would do that rather than set up endless arrays of reactions just to see what happens. Everyone who’s done research-level synthetic organic chemistry has experienced this: you flip one chiral center in your molecule, or made a chain 1 carbon longer, you change the solvent from one ether to another, raise or lower the temperature a bit, switch a sodium salt for a potassium one, change a ligand on your palladium catalyst, whatever, and all sorts of craziness breaks loose. And it’s often not easy to see why things changed so much. Ex post facto you can sometimes come up with hypotheses, and use those to fix things up if you’re right. But there are plenty of throw-your-hands-up moments that just never get explained at all.

Organic chemistry wobbles and teeters across an energy landscape that (from the viewpoint of any given reaction) is full of huge hills, deep valleys, and twisty little pathways that are followed by walking along the edges of steep, crumbling ridges. But from a distance, all that topography is compressed into a pretty narrow thermodynamic range. The differences between a reaction working and not working, between it giving you mostly Product A, mostly Product B, mostly returned starting material, or mostly scorched pan drippings are energetically very small. All sorts of little changes can send things off in different directions, and these can be largely inside the error bars of our attempts to model them. Organic chemistry is indeed a mature science, but don’t confuse that with thinking that it’s a solved problem. If you just want the molecules, damn the cost, to answer other questions we can generally provide them. But if you want them made elegantly, you’ll need to take a seat – and you better have packed some lunch with you.

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