N-Methyl N-Carboxyanhydride: An Unexpected By-Product When Coupling Boc-N-Methyl Amino Acids

Activation of Boc-amino acids and Boc-N-methyl amino acids leads to the corresponding NCA. This side reaction explains the low yields obtained when coupling N-methyl amino acids. It was not observed with the Z or Fmoc-protective groups.

Concerning the N-methyl amino acids, the activation of Z- or Fmoc-MeVal-OH with PyCloP/DIEA yielded the corresponding symmetrical anhydrideslz. During the course of the coupling reaction, these intermediate were easily aminolyzed, providing good yields (peptides 4 and 8). However, Boc-MeVal-OH led immediatly to a l/l mixture of the symmetrical anhydride and N-methyl-NCA 1113. The formation of the latter product accounts for the low yield of the peptide 6.

The NCA 10 arises from oxazolone 9 (R = tBu); this type of reaction has already been qx~ted.9J4~15 Using the same reasoning, one could asume that N-methyl-NCA 11 was formed from the oxazolonium 12. Similar ions have been obtained by activation of N-benzoyl N-methyl amino acids with DCC.16 The inter- mediates 9 (R = tBu) and 12 could easily lose the r-butyl cation: this was supported by the detection of r-butyl chloride and isobutene in the reaction medium. Thus, the easy loss of the t-butyl group could explain the particular behavior of the Boc-protected amino acids.

The formation of NCA seems to be a general phenomenon. We observed it with Boc-Leu-OH, Boc-Ala-OH, Boc-MeLeu-OH and Boc-MeAla-OH, but not with Boc-Pro-OH. Furthermore, we verified that, in the case of Boc-MeVal-OH, PyBroP reacts like PyClol? Finally, compound 11 was also detected by activating Boc-MeVal-OH with other coupling reagents, namely, DCC and PyBOP@.

It should be noted that the slower the coupling reaction, the larger the amount of NCA formed and the lower the yield of the peptide. This is particularly the case when the C-protected amino acid is N-methyla- ted.1

Based on these results, coupling of Boc-N-methyl amino acids should be avoided. We are currently using a Fmoc strategy for solid phase peptide synthesis.

Aknowlegmentsz We are grateful to Dr Annie Heitz for the 13C NMR spectra and to Dr S. L. Sahli for the revision of the manuscript PyBroP was a generous gift from Novabiochem (Lailfelfingel. Switzerland).