Mechanism, origin of diastereoselectivity and factors affecting reaction efficiency of serine/threonine ligation: A computational study

Abstract The mechanism of serine/threonine ligation was investigated by density functional theory. The reaction sequentially proceeds through imine capture, 5-endo-trig cyclization and [1,5] O-to-N acyl transfer steps, all of which occur with the assistance of pyridinium. The imine capture and 5-endo-trig cyclization steps are reversible whereas the [1,5] O-to-N acyl transfer step is irreversible. The [1,5] O-to-N acyl transfer step proceeds through a stepwise addition-elimination mechanism with the C–O bond cleavage as the rate-determining step of the overall reaction. The diastereoselectivity for the exclusive formation of the S-configured N,O-benzylidene acetal-linked peptide originates from the disfavored steric repulsion between the leaving phenolic oxygen and the side chain of the C-terminal peptide in the R-configured C–O bond cleavage transition state in the acyl transfer step. A suitable ring size as well as a good leaving group in acyl transfer are responsible for the high efficiency of the serine/threonine ligation reaction.