Investigations on electrochemical α-methoxylation of selected dipeptides

Voltamperometric studies on the indirect electrochemical alpha-methoxylation of Boc-Pro-Gly-OMe and Boc-Val-Gly-OMe in MeOH in the presence of NaCl or NaBr as the mediator suggested that the first reaction step was a direct N-halogenation of the dipeptide by active chlorine or bromine adsorbed on the electrode surface. The kind of mediator (NaCl or NaBr), its concentration, the current density, and the applied electric charge had a significant influence on the reaction course. In the case of Boc-Pro-Gly-OMe, the use of sodium bromide was necessary to obtain a relatively high ratio of alpha-monomethoxylation to alpha,alpha-dimethoxylation. For Boc-Val-Gly-OMe, the selectivity for alpha-monomethoxylation was close to 100%, independently of the mediator. Optimisation of the selected electrolysis parameters allowed us to significantly improve the yield and selectivity of the alpha-methoxylation of Boc-Pro-Gly-OMe (Kardassis et al. Tetrahedron 54: 3471, 1998) and to obtain good results in the alpha-methoxylation of Boc-Val-Gly-OMe.