Proof of delocalization-stabilization by sulfur in enolate formation during racemisation of sulfur containing amino acid residues.

The present kinetic data on the accelerating action of sulfur in the base catalyzed racemisation of cyclic dipeptides ($\text{3}$) support the explanation of Barber and Jones ($\text{4}$) for the α center and suggest further a 1,3- and 1,4-interaction of -S- with the amide N-atom ($\text{8}$) for the α′ center.