Abstract
Amides are important natural products which occur in a few plant families. Piplartine and piperine, major amides in Piper tuberculatum and P. nigrum, respectively, have shown a typical N–CO cleavage when analyzed by EI-MS or HRESI-MS. In this study several synthetic analogs of piplartine and piperine were subjected to both types of mass spectrometric analysis in order to identify structural features influencing fragmentation. Most of the amides showed an intense signal of the protonated molecule [M + H]+ when subjected to both HRESI-MS and EI-MS conditions, with a common outcome being the cleavage of the amide bond (N–CO). This results in the loss of the neutral amine or lactam and the formation of aryl acylium cations. The mechanism of N–CO bond cleavage persists in α,β-unsaturated amides because of the stability caused by extended conjugation. Computational methods determined that the protonation of the piperamides and their derivatives takes place preferentially at the amide nitrogen supporting the dominant the N–CO bond cleavage.
Highlights
Amides are an important class of natural products found in a few plant families such as Asteraceae, Piperaceae, Rutaceae and Solanaceae, among others.[1,2,3,4] The two most well-known amides are piperine and capsaicin, the pungent principles of black pepper (Piper nigrum, Piperaceae) and chili pepper (Capsicum varieties, Solanaceae), respectively.[5]
Several analogs synthesized were subjected to mass spectrometry studies to determine whether the N–CO a-cleavage could be generalized and used as a criteria for the rapid identi cation of piperamides
Computational studies were carried out to verify the N–CO a-cleavage. Both EI and HRESI-MS experiments were carried out to compare the corresponding fragmentation patterns and, together a calculation of energetic pro les at protonation sites were examined to support the characterization of amides
Summary
Amides are an important class of natural products found in a few plant families such as Asteraceae, Piperaceae, Rutaceae and Solanaceae, among others.[1,2,3,4] The two most well-known amides are piperine and capsaicin, the pungent principles of black pepper (Piper nigrum, Piperaceae) and chili pepper (Capsicum varieties, Solanaceae), respectively.[5]. The conjugation between the amide carbonyl and the b double bond reinforced by the presence of an aryl group at the g or 5 positions from the carbonyl could account for preferable N–CO fragmentation.[15,16] several analogs synthesized were subjected to mass spectrometry studies to determine whether the N–CO a-cleavage could be generalized and used as a criteria for the rapid identi cation of piperamides. Computational studies (proton affinity and bond energies) were carried out to verify the N–CO a-cleavage Both EI and HRESI-MS experiments were carried out to compare the corresponding fragmentation patterns and, together a calculation of energetic pro les at protonation sites were examined to support the characterization of amides
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