Catalytic specificity of p-sulfonatocalix[n]arenes on the alcoholysis of N-Ac-l-amino acids in methanol solution

The specific acid catalysis of p-sulfonatocalix[n]arenes (n=4:Calix-S4, n=6:Calix-S6, n=8:Calix-S8) was observed in the alcoholysis of N-Ac-l-amino acids in methanol. With respect to the alcoholysis of Ala, His, Phe, Trp and Tyr substrates, the alcoholysis-rate of N-Ac-l-His-OH was markedly enhanced in the presence of Calix-SN as compared with that in the presence of p-hydroxybenzenesulfonic acid (pHBS), which is a noncyclic analogue of Calix-SN. Furthermore, in relation to 1H NMR results of N-Ac-l-amino acids and Calix-SN in CD3OD, it was found that the spectrum of the mixture of N-Ac-l-His-OH and Calix-S4 was significantly different from the combined spectra of the respective compounds. These changes in spectra support the formation of inclusion complex of Calix-S4 and N-Ac-l-His-OH. The above results seem to indicate that the catalytic specificity of Calix-SN is originated from the complexation with N-Ac-l-His-OH