Intrinsic Gas-Phase Reactivity of Ionized 6-(Oxomethylene)cyclohexa-2,4-dienone: Evidence Pointing to Its Neutral r-Oxoketene Counterpart as a Proper Precursor of Various Benzopyran-4-ones and Analogues Paulo Sergio de Carvalho, † Fabiane M. Nachtigall, ‡ Marcos Nogueira Eberlin,* ,‡ and Luiz Alberto Beraldo Moraes* ,§ Sa ˜ o Francisco UniVersity, Braganc ¸ a Paulista 12916-900, ThoMSon Mass Spectrometry Laboratory, Institute of Chemistry, State UniVersity of Campinas, 13083-970 Campinas, and Department of Chemistry, Faculty of Philosophy, Science and Languages of Ribeira ˜ o Preto, UniVersity of Sa ˜ o Paulo, 14040-901 Ribeira ˜ o Preto, Sa ˜ o Paulo, Brazil eberlin@iqm.unicamp.br ReceiVed March 1, 2007 Despite its unique structure and potential use as an important building block in organic synthesis, the title R-oxoketene 1 has been formed mostly under very special conditions as a short-lived species. The reactivity of 1 is, therefore, nearly unexplored. In great contrast, it seemed that its ionized gaseous form 1 •+ is stable and easily accessible. In this study, we used multiple-stage pentaquadrupole mass spectrometry to probe the formation of gaseous 1 •+ and explore its stability and intrinsic reactivity. With water and methanol, gaseous 1 •+ was found to react similarly to solvated 1, which indicates that there is a close parallel between their reactivities. Gaseous 1 •+ was also found to react promptly via polar [3 + 2] cycloadditons with various dienophiles including alkenes, alkynes, isocyanates, ketones and esters, thus forming a series of benzopyran- 4-ones (flavones, 4-chromanones, 4-chromenones, benzo[1,3]dioxin-4-ones, and analogues) that are common structural units in many natural products. The present availability of 1 at room temperature and the gas- phase findings reported herein for gaseous 1 •+ indicate that solvated 1 should undergo many [4 + 2] cycloadditions and functions as a versatile precursor of a variety of biologically active molecules. Introduction R-Oxoketenes are molecules of great synthetic utility. 1 Many of these rather reactive molecules have been found to act as versatile precursors and to participate promptly in a variety of reactions including nucleophilic additions, cycloadditions, un- usual rearrangements, and DNA cleavage. 2 However, unless stabilized sterically or electronically, 1b these interesting mol- ecules are short-lived; hence, they are difficult to form and isolate in the condensed phase under ordinary reaction condi- † Sa ˜o Francisco University. ‡ State University of Campinas. § University of Sa ˜o Paulo. 5986 J. Org. Chem. 2007, 72, 5986-5993 10.1021/jo070371z CCC: $37.00 © 2007 American Chemical Society Published on Web 07/13/2007