Break Out the Purple Carrots

OK, there’s a lot of stuff going on out there in biopharma, but as for me, I’m going to take a day to talk about purple carrots. Seriously, because there is a paper on them in synthesis in ACS Medicinal Chemistry Letters, and it makes me wish I had a need to use them in the lab.

Some of the synthetic organic chemists in the crowd will have guessed what I’m referring to. There are a number of reductase enzymes in common foodstuffs that are present in enough concentration to make them useful reagents, and being enzymes, they tend to produce a single enantiomeric product. (For the non-chemists, those are the “right-handed” or “left-handed” isomeric forms, and since enzyme active sites have that handedness themselves they can pass it along to their products). A classic in this line is using yeast to reduce carbonyl groups to alcohols. There have been a number of yeast strains developed over the years with varying substrate preferences and selectivities, but good ol’ baker’s yeast from the grocery store will do the trick for less demanding applications. I once saw a Japanese post-doc set off what I can only describe as a “yeast volcano” when he tried to do this on too large a scale.

But there are a number of shredded vegetables that have useful enzymatic activity in them as well. And just to clear things up for those who might not know this part (what with the popularity of freshly squeezed juices and all) these enzymes are doing you personally no good whatsoever when you consume said vegetables or their expensively pressed constituents. They’re proteins, and your body will digest them as proteins as soon as they hit your stomach and intestines. You can find all sorts of pretty ridiculous claims about the amazing effects of active enzymes from fresh fruits and vegetables, but those effects only apply if you are doing organic synthesis with them.

The paper linked in the first paragraph is from a group at AbbVie who were looking for ways to do reductions of heterocyclic ketones enantioselectively. It’s worth investigation, because many transformations reported in the literature (traditional reagents and enzymes alike) can go off the rails a bit when you start introducing heteroatoms – you can’t assume anything without testing it. The carrot enzymes seemed to handle a test heteroaryl ketone pretty well, though, producing the (S) secondary alcohol from a thiazole derivative in very high enantioselectivity. As always with such reactions, you need to optimize the conditions. There have been reports that adding surfactants can increase the yield of some such reactions, but in this case they strongly decreased it (presumably by denaturing the enzyme itself). pH adjustment was also necessary – the reaction ran much better in basic media, with 16% conversion at pH 6 and 90% at 8.5. And I particularly appreciated a table where the authors investigated different carrot sources. Purple carrots performed notably better than the usual orange ones, but the supermarket source of them didn’t seem to matter. The initial runs were performed by taking the purple ones out of a mixed-carrot bag from Trader Joe’s, but it seems that any purple carrots will do the trick (who knows, some of these different brands are probably coming from the same farming operations in California or Peru).

And you’re also going to have to investigate the substrate scope of anything enzymatic, because they tend to have very strong and unpredictable preferences. In this case, thiazole and pyridine ketones of various types worked well, but imidazoles and indoles failed to react at all. Most pyrimidine examples worked (but not all), and quinolines were good for the most part. Turning to the other side of the ketone, going from 2-pyridyl methyl ketone to the ethyl ketone decreased the selectivity from >99:1 to 97:3, while the isopropyl ketone stayed at that enantiomeric ratio but with a much lower yield. The trifluoromethyl ketone (always a good way to puzzle an enzyme, since this group is so far out of the normal biochemical scope) gave similar yields, but the enantiomeric ratio was down to 86:14, which I think is the lowest they saw.

But if you have a heteroaryl ketone to reduce, I would definitely recommend taking a look at the carrot method. It’s mild, it seems easy to run, and it’s a simple way to get a solid enantioselective transformation into your synthesis. And I freely admit that I would love to chop up some purple carrots in the lab and toss them into a big Erlenmeyer flask!