Electron Ionization and Chemical Ionization Mass Spectra of Some N^α-(t-Butoxycarbonyl)-O-(diorganylphosphono)-L-serines

Abstract

N ^α -(t- Butoxycarbonyl )-O-( diorganylphosphono )L- serines underwent a complex fragmentation under both electron ionization and chemical ionization ( isobutane ) with the dominant pathway being a function of the phosphono organyl group (phenyl, ethyl, methyl, benzyl, t-butyl). In the case of the phenyl-, ethyl- and methyl-substituted derivatives, both electron ionization (15 eV) and chemical ionization mass spectra showed that β-eliminative loss of the seryl residue was the major pathway, and that the two-step loss of the t- butoxycarbonyl group from the [MH]⁺ ion was a inor pathway. In the case of the benzyl- and t-butyl-substituted derivatives, the poor mass spectra obtained under electron ionization conditions indicated that extensive thermal decomposition of the sensitive dibenzyl or di-t-butyl phosphate functionality occurred on the heated probe. Under the softer chemical ionization conditions, the mass spectrum of the benzyl-substituted derivative showed that loss of the phosphono benzyl group as the tropylium ion was the major fragmentation, and that both loss of the seryl residue by β-elimination and the cleavage of the t- butoxycarbonyl group were minor events. However, the poor chemical ionization spectrum obtained for the t-butyl derivative indicated that the di-t-butyl phosphate functionality was still sensitive to these milder conditions, with the compound undergoing decomposition on the heated probe.