Intrinsic Gas-Phase Reactivity of Ionized 6-(Oxomethylene)cyclohexa-2,4-dienone:  Evidence Pointing to Its Neutral α-Oxoketene Counterpart as a Proper Precursor of Various Benzopyran-4-ones and Analogues

Abstract

Despite its unique structure and potential use as an important building block in organic synthesis, the title alpha-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.

Isolation, Structure Elucidation, and Biosynthesis of an Unusual Hydroxamic Acid Ester-Containing Siderophore from Actinosynnema mirum

“…MS 2 as well as MS 3 fragmentation was carried out by performing collision-induced dissociation (CID) experiments. MS 2 fragmentation yielded a major fragment ion with a m / z of 469.2040 ([M + H] + , calculated m / z 469.2042), which could be associated with the tripeptide ion of 2,3-DHB-Arg-fhOrn (2,3-DHB = 2,3-dihydroxybenzoate; fhOrn = δ- N -formyl-δ- N -hydroxyornithine), resulting from a neutral loss of dehydrated 2,3-DHB (dh-2,3-DHB, observed m / z 136.0160, calculated m / z 136.0160), in accordance with the fact that ortho -hydroxylated benzoic acid derivatives are known to form benzodioxetanes during MS n gas-phase fragmentation …”

“…MS 2 fragmentation yielded a major fragment ion with a m/z of 469.2040 ([M + H] + , calculated m/z 469.2042), which could be associated with the tripeptide ion of 2,3-DHB-Arg-fhOrn (2,3-DHB = 2,3dihydroxybenzoate; fhOrn = δ-N-formyl-δ-N-hydroxyornithine), resulting from a neutral loss of dehydrated 2,3-DHB (dh-2,3-DHB, observed m/z 136.0160, calculated m/z 136.0160), in accordance with the fact that ortho-hydroxylated benzoic acid derivatives are known to form benzodioxetanes during MS n gas-phase fragmentation. 19 Additional fragment ions observed correspond to the consecutive loss of two dh-2,3-DHB moieties (observed m/z 333.1886, calculated m/z 333.1881) and the loss of dh-2,3-DHB along with fhOrn (observed m/z 293.1248, calculated m/ z 293.1244), giving rise to the dipeptide ions Arg-fhOrn and 2,3-DHB-Arg, respectively (Figure 3 and Supporting Information, Figure S1). To gain further information about the respective constituents, the main fragment resulting from MS 2 fragmentation was subjected to MS 3 analysis.…”

Ketene–Ketene Interconversion. 6-Carbonylcyclohexa-2,4-dienone–Hepta-1,2,4,6-tetraene-1,7-dione–6-Oxocyclohexa-2,4-dienylidene and Wolff Rearrangement to Fulven-6-one

“…The molecular ion of methyl salicylate 7 •+ ( m / z 152) disappears at FVT temperatures above 700 °C (Figure ). The ion corresponding to 1 •+ ( m / z 120) is both a fragment peak in the mass spectrum of 7 and a thermal reaction product. This ion, 1 •+ ( m / z 120), reaches maximum intensity at ca.…”

“…The structure of 22 and the assignment of the 13 C NMR data have been established unequivocally. 25 It is interesting to note that benzoxirene (23) is not involved in the rearrangements. Had it been, then labeling of the carboxylate carbon would have occurred, and this was not detectable (Scheme 9).…”