We are trying to make a diester of dibromosuccinic acid from the free acid. We would prefer to make something that will deprotect easily in the presence of a vinyl sulfone, but at this point I would settle for just about any ester. The two problems we envision are low solubility in some common organic solvents and the possible replacement of bromine by a nucleophile. When we tried DCC/catalytic DMAP coupling in acetonitrile, the crude yield was very low, possibly because the acid did not want to go into solution. When we tried DCC/catalytic DMAP in DMF, I was unable to interpret the NMR. So far our literature searching has not been very successful, but we did turn up a protocol of an acid-catalyzed esterification of a di-acid that used cation-exchange resin as the catalyst, which I thought was promising.
If we try acid-catalyzed esterification, the only way that I can think to avoid a substitution entirely is to use HBr (not HCl) as the catalyst. However, cation-exchange resin and para-toluenesulfonic acid are not very nucleophilic to the best of my knowledge. If we try DCC-promoted coupling again, it occurred to me that we could try dissolving the acid in an excess of the alcohol that we wish to use, then add DCC and catalytic DMAP. Once the reaction is done, we could add ether or DCM to precipitate out the dicyclohexylurea. Does anyone have any advice for us?