title: Competitive retro-cycloaddition reactions in heterocyclic fullerene bis-adducts ions: selective removal of the heterocyclic moieties.
abstract: BACKGROUND
  We have investigated the fragmentation reactions of ions from bis-adducts containing isoxazolino-, pyrrolidino- and methanofullerene moieties.
  
  
  METHODS
  The fragmentation reactions induced by collision-induced dissociation (CID) of ions generated under electrospray ionization (ESI) in positive and negative modes of detection using an ion-trap spectrometer have been investigated.
  
  
  RESULTS
  The competitive retro-cycloaddition process between isoxazoline and pyrrolidine rings fused to [60]fullerene reveals that it is strongly dependent on the experimental negative or positive ESI experimental conditions. Thus, whereas retro-cycloaddition reaction is favored in the pyrrolidine ring under negative conditions, the protonation occurring on the nitrogen atom of the pyrrolidine ring under positive conditions precludes its retro-cycloaddition and, therefore, only the isoxazoline ring undergoes the retro-cycloaddition process. The obtained experimental results are different from those reported when the reaction is carried out under thermal conditions. Competitive retro-cycloaddition reactions of isoxazolino- and methanofullerenes show that the heterocyclic ring undergoes cycloelimination, leaving the methanofullerene moiety unchanged. In this case, the same selectivity is observed under thermal and gas-phase conditions.
  
  
  CONCLUSIONS
  The observed selectivity in the heterocyclic removal in these [60]fullerene derivatives is reversed from negative conditions (radical anions) to positive conditions (protonated molecules). Moreover, the retro-cycloaddition reaction behaves differently under spectrometric and thermal conditions.
authors.affiliation: Laboratorio de Síntesis Orgánica, Facultad de Química, Universidad de La Habana, 10400, Ciudad Habana, Cuba.
authors.forename: Margarita
authors.initials: M
authors.lastname: Suárez
authors.affiliation: Departamento de Química Orgánica, Facultad de Ciencias Químicas, Universidad Complutense, E-28040, Madrid, Spain.
authors.forename: Enrique E
authors.initials: EE
authors.lastname: Maroto
authors.affiliation: Departamento de Química Orgánica, Facultad de Ciencias Químicas, Universidad Complutense, E-28040, Madrid, Spain.
authors.forename: Salvatore
authors.initials: S
authors.lastname: Filippone
authors.affiliation: Departamento de Química Orgánica, Facultad de Ciencias Químicas, Universidad Complutense, E-28040, Madrid, Spain.|IMDEA-Nanociencia, Campus de Cantoblanco, E-28049, Madrid, Spain.
authors.forename: Nazario
authors.initials: N
authors.lastname: Martín
authors.affiliation: Departamento de Química Orgánica, Facultad de Ciencias Químicas, Universidad Complutense, E-28040, Madrid, Spain.
authors.forename: Roberto
authors.initials: R
authors.lastname: Martínez-Álvarez