Deprotonation of the peptide NH groups and diastereoselective hydrogenation of N-acetyl-alpha,beta-dehydrodipeptides

The peptide protons in N-acetyl -alpha,beta -dehydrodipeptides (DHDP) disso ciate in aqueous methanol in the presence of magnesium salts upon the addit ion of alkali, which favors the diastereoselectivities of their hydrogenati on over Pd/C. By contrast, the N-H bonds of the N-acetyl groups in DHDP see m not to dissociate under these conditions. The dissociation of the peptide N-H bonds in strongly alkaline solutions was studied by F-19 NMR spectrosc opy for model compounds containing the 4-FC6H4 fragment.