title: Gas-phase reactivity of protonated 2-oxazoline derivatives: mass spectrometry and computational studies.
abstract: BACKGROUND
  Oxazolines have attracted the attention of researchers worldwide due to their versatility as carboxylic acid protecting groups, chiral auxiliaries, and ligands for asymmetric catalysis. Electrospray ionization tandem mass spectrometric (ESI-MS/MS) analysis of five 2-oxazoline derivatives has been conducted, in order to understand the influence of the side chain on the gas-phase dissociation of these protonated compounds under collision-induced dissociation (CID) conditions.
  
  
  METHODS
  Mass spectrometric analyses were conducted in a quadrupole time-of-flight (Q-TOF) spectrometer fitted with electrospray ionization source. Protonation sites have been proposed on the basis of the gas-phase basicity, proton affinity, atomic charges, and a molecular electrostatic potential map obtained on the basis of the quantum chemistry calculations at the B3LYP/6-31 + G(d,p) and G2(MP2) levels.
  
  
  RESULTS
  Analysis of the atomic charges, gas-phase basicity and proton affinities values indicates that the nitrogen atom is a possible proton acceptor site. On the basis of these results, two main fragmentation processes have been suggested: one taking place via neutral elimination of the oxazoline moiety (99 u) and another occurring by sequential elimination of neutral fragments with 72 u and 27 u. These processes should lead to formation of R(+).
  
  
  CONCLUSIONS
  The ESI-MS/MS experiments have shown that the side chain could affect the dissociation mechanism of protonated 2-oxazoline derivatives. For the compound that exhibits a hydroxyl at the lateral chain, water loss has been suggested to happen through an E2-type elimination, in an exothermic step.
authors.affiliation: Núcleo de Pesquisa em Produtos Naturais e Sintéticos, Departamento de Física e Química, Faculdade de Ciências Farmacêuticas de Ribeirão Preto, Universidade de São Paulo, Ribeirão Preto, SP, Brasil. vessecchi@bol.com.br
authors.forename: Ricardo
authors.initials: R
authors.lastname: Vessecchi
authors.forename: José Carlos
authors.initials: JC
authors.lastname: Tomaz
authors.forename: Guilherme Purcote
authors.initials: GP
authors.lastname: dos Santos
authors.forename: Alfredo R Marques
authors.initials: AR
authors.lastname: de Oliveira
authors.forename: Norberto Peporine
authors.initials: NP
authors.lastname: Lopes
authors.forename: Giuliano Cesar
authors.initials: GC
authors.lastname: Clososki
mesh_terms: D005740:Gases; D007477:Ions / Q000737:chemistry; D008958:Models, Molecular; D009584:Nitrogen; D010080:Oxazoles / Q000737:chemistry*; D011522:Protons; D021241:Spectrometry, Mass, Electrospray Ionization / Q000379:methods*; D013816:Thermodynamics
chemical_list: D005740:Gases; D007477:Ions; D010080:Oxazoles; D011522:Protons; D009584:Nitrogen