Recognition of Cyclic, Acyclic, Exocyclic, and Spiro Acetals via Structurally Diagnostic Ion/Molecule Reactions with the (CH 3 ) 2 N-C + dO Acylium Ion Mario Benassi, † Luiz Alberto B. Moraes, ‡ Liliane G. Cabrini, † Luiz Carlos Dias, § Andrea M. Aguilar, | Gilberto A. Romeiro, ⊥ Livia S. Eberlin, † and Marcos N. Eberlin* ,† ThoMSon Mass Spectrometry Laboratory, State UniVersity of Campinas, Campinas-SP, Brazil 13083-970, UniVersity of Sa ˜o Paulo, Ribeira ˜o Preto, SP, Brazil, Synthetic Organic Chemistry Laboratory, State UniVersity of Campinas, Campinas-SP, Brazil, Federal UniVersity of Sa ˜o Paulo, Diadema Campus, 09972-270, Diadema, SP, Brazil, and Institute of Chemistry, Fluminense Federal UniVersity, Rio de Janeiro, RJ, Brazil eberlin@iqm.unicamp.br ReceiVed April 14, 2008 Reactions of the model acylium ion (CH 3 ) 2 N-C + dO with acyclic, exocyclic, and spiro acetals of the general formula R 1 O-CR 3 R 4 -OR 2 were systematically evaluated via pentaquadrupole mass spectrometry. Characteristic intrinsic reactivities were observed for each of these classes of acetals. The two most common reactions observed were hydride and alkoxy anion [R 1 O - and R 2 O - ] abstraction. Other specific reactions were also observed: (a) a secondary polar [4 + + 2] cycloaddition for acetals bearing R,- unsaturated R 3 or R 4 substituents and (b) OH - abstraction for exocyclic and spiro acetals. These structurally diagnostic reactions, in conjunction with others observed previously for cyclic acetals, are shown to reveal the class of the acetal molecule and its ring type and substituents and to permit their recognition and distinction from other classes of isomeric molecules. Introduction Reactivity is governed by molecular structure. Chemists have therefore relied heavily on structurally diagnostic reactions to investigate the structure (connectivity and tridimensional ar- rangement) of molecules of many classes, sizes, and shapes 1 and their reactivities. In solution, however, reactivity is influ- enced by the environment, that is, mostly by properties of the solvent, solutes, and counter-ions. 2 In the diluted gas-phase environment of mass spectrometers, solvent effects are elimi- nated 3 and the intrinsic reactivity of isolated molecules and ions can therefore be properly accessed most particularly via tandem † ThoMSon Mass Spectrometry Laboratory. ‡ University of Sa ˜o Paulo. § Synthetic Organic Chemistry Laboratory. | Federal University of Sa ˜o Paulo. ⊥ Fluminense Federal University. (1) Shriner, R. L.; Hermann, C. K. F.; Morrill, T. C.; Curtin D. Y.; Fuson, R. C. The Systematic Identification of Organic Compounds; John Wiley & Sons: New York, 2003. (2) Tapia, O.; Bertro ´n, J. SolVent Effects and Chemical ReactiVity; Springer: New York, 1999. (3) Futrell, J. H. Gaseous Ion Chemistry and Mass Spectrometry; Wiley: New York, 1986. 10.1021/jo8008269 CCC: $40.75  2008 American Chemical Society J. Org. Chem. 2008, 73, 5549–5557 5549 Published on Web 06/21/2008