The stereodynamics of macrocyclic succinimide-thioethers.

Maleimide-thiol coupling is a popular bioconjugation strategy, but little is known about the stereoselectivity and the stereodynamics of the succinimide thioether formed in a biopolymer environment. We used thiol 1,4-addition for the macrocyclisation of 5 designed pentapeptides with the ringsize of hexapeptides because they incorporate the succinimide thioether (4-8). Both succinimide diastereomers are observed in the constrained macrocyclic rings in each case. In spite of the low diastereoselectivity of the macrocyclisation reaction, there is a significant influence of the amino acid environment on the epimerization rate of the succinimide. Its half life can be as short as several hours at room temperature when Gly is the amino acid following the succinimide (peptide 8). On the contrary, no epimerization is detectable even after several weeks in the case of d-Phe C-terminal to the succinimide in peptide 4. Already the small selection of examples shows how big the differences in epimerization rates can be and that the local environment has a significant influence. The variation of amino acids in the vicinity of the ligation site points the way towards the synthesis of bioconjugates which are obtained as stable and separable diastereomers.