I wanted to call attention to this new paper because the structures it covers have been showing up a lot over the last few years in the medicinal chemistry literature. Oxetanes are attractive compounds in many ways – you get a group with an unusual shape that often adds solubility to a larger molecule, and there’s nothing else that looks quite like it from a stereoelectronic perspective. Small polar rings in general have been popular in modern med-chem for these sorts of reasons, and you can’t forget that recent synthetic routes to some of them make them unlikely to be represented in the older prior art, too. Instant novel structures! The paper linked above notes that over 200 med-chem manuscripts and patents contain these things. So what’s not to like?
Instability, apparently. As shown in the scheme at right, the authors have found that many of these structures isomerize to products that you may or may not have wanted, but which at any rate you probably didn’t realize that you had! You’d think that this would be a problem in the presence of Lewis-acidic reagents, and it sure is, but you don’t need to go that far. And you might think that this would be a problem on heating, and it sure is, but you don’t need to go that far, either. The parent oxetane-methylene-carboxylic acid building block, freshly synthesized, is already 7% converted to that green hydroxymethyllactone structure after sitting at room temperature for one week, and after a year of storage it’s 100% isomerized.
The paper goes into detail on reaction conditions that speed up this problem, and investigates a range of oxetane/COOH structures. Some of them are more stable than others, but the isomerization problem is more common than not, and appears to have gone unnoticed in the literature until now. So it would be prudent, if you have some of these building blocks on the shelf, to go have a look at them and make sure that they’re still what the label says they are. The paper says that storing these compounds as sodium or potassium salts completely stops the isomerization problem, so that’s something to consider. But a larger problem might be if you’ve incorporated such groups into your larger drug candidate molecules. Having them isomerize into new structures with the same molecular weight would be something you’d want to watch for!
