Phosphorylation is a prime example of a reaction that’s hugely important in biochemistry that organic synthesis struggles with terribly (as opposed to the efficiency and finesse with which it’s handled in by enzymatic processes). The methods used to attach phosphate esters in the flask are frankly pretty crude (all the way up, or down, to good ol’ phosphorus oxychloride), and generally involve hydrolyzing intermediates to reveal the free phosphate groups at the end. You see high temperatures, acid/base conditions, oxidizing reagents, and less-than-impressive yields – as opposed to the biological systems which are whisking around selectively adding and removing phosphate groups from specific protein protein and carbohydrate residues. As usual, watching the enzymes at work is like watching a ballet being performed in mid-air on trapeze; down here on the ground we can only shake our heads.
This new paper, though, gets us a bit closer. The authors (from the Univ. of Tokyo and the Nara Inst. of Science and Technology) are using a biomimetic strategy with phophoenolpyruvate (PEP) as the phosphate donor. This needs to be activated (which is what the enzyme active sites will do for you), and they’ve found that the combination of DMF as solvent and tetrabutylammonium hydrogen sulfate as acid does the trick quite well. The reaction optimization is pretty interesting – a lot of other acids barely work, or not at all. The usual organic-chemist thinking is that protic acids are all fairly similar once you hit the same pKa (as opposed to Lewis acids, which definitely have widely varying behaviors), but this is a good example of how fine-tuning H-acid conditions really can be crucial.
Detailed mechanistic work shows that the real activated phosphorylating agent is the odd-looking (and previously undescribed) mixed anhydride shown at right. The reagent system phosphorylates primary and secondary alcohols in the presence of a wide variety of other functional groups (aldehydes, carboxylic acids, alkynes, phenols, protecting groups such as Fmoc, Boc, and trityl, and more. That phenolic selectivity means that you can take an oligopeptide and phosphorylate serines and/or threonines in the presence of tyrosine residues, and the authors show several examples of this and of carbohydrate substrates as well, yields 50 to 80%.
This makes a person wonder what other phosphoryl donor species might be out there that we haven’t come across. It’s for sure that the authors weren’t aiming at the POSOP system when they started out – they noticed that acidic ion-exchange resin gave them some product (test alcohol plus PEP), and investigated a whole list before finding that the hydrogen sulfate salts stood out, and so on. But that’s how good reagents get found! And this one looks like it will be widely adopted.