Here’s a remarkable paper that shows how enzymes can be engineered to turn out some very unnatural-looking structures. Frances Arnold’s group at CalTech has published on a great deal of work in this area in the past, and this latest variation is something to see. They’ve been working with various heme proteins, since that reactive center is known to be able to generate carbenes, and they noted one variant, in the presence of ethyl diazoacetate as a carbene source and an aryl acetylene as reactant, gave furan products.
These are already reported as coming from a carbene mechanism, so the group thought that they must be on to something. Further screening of variants around that particular enzyme isoform turned up some that allowed a second carbene reaction to cycle around, giving the unusual bicyclobutanes shown at right. Freeing up some room around the active site was essential, otherwise the cyclopropene intermediate goes off to the furan product instead. A systematic screen of amino acid variations at the key residues of the enzyme in E. coli, with monitoring of product production, furnished an enzyme that was capable of high turnover (enough to produce hundreds of milligrams of product). A variety of different R groups are tolerated on the phenyl, with yields from 30 to 80%.
In this case, the phenylcyclopropene intermediate was not isolable, but the team also look for variants that could produce those products as well (in the non-aryl cases). Similar mutation screens against promising enzymes produced another one that could indeed furnish a variety of cyclopropenes, as shown at right, in yields of 35 to 85%. Variants were found that could produced enhanced ratios of either enantiomer, to boot.
These sorts of reactions are known in the literature, up to a point (here’s a review presentation from the Wipf group a few years back on the topic). But such things are mostly seen on very electron-deficient alkynes and often go in lower yields than what are reported here (and I think you can largely forget about enantioselectivity). The fact that enzymes can be made to do such chemistry is remarkable, and there are surely many more such things waiting to be discovered. The metal centers in these proteins do a lot of extraordinary chemistry, and many groups are in the process of harnessing them for synthetic purposes. Seeing unusual strained systems like this being formed so readily just shows how much must be possible!