Ionic Transacetalization with Acylium Ions: A Class-Selective and Structurally Diagnostic Reaction for Cyclic Acetals Performed under Unique Electrospray and Atmospheric Pressure Chemical Ionization I

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• 作者：Eduardo C. Meurer ; Adã ; o A. Sabino ; and Marcos N. Eberlin
• 刊名：Analytical Chemistry
• 出版年：2003
• 出版时间：September 1, 2003
• 年：2003
• 卷：75
• 期：17
• 页码：4701 - 4709
• 全文大小：158K
• 年卷期：v.75,no.17(September 1, 2003)
• ISSN：1520-6882

文摘

Ionic transacetalization of cyclic acetals with the gaseous(CH₃)₂NCO⁺ acylium ion has been performed underunique in-source ion-molecule reaction (in-source IMR)conditions of electrospray (ESI) and atmospheric pressurechemical ionization (APCI). In-source IMR under ESI andAPCI greatly expands the range of neutral molecules thatcan be brought to the gas phase to react by ionic transacetalization, a general, class-selective and structurallydiagnostic reaction for cyclic acetals (Moraes, L. A. B.;Gozzo, F. C.; Vainiotalo, P.; Eberlin, M. N. J. Org. Chem.1997, 62, 5096). Heavier, more polar, and less volatilecyclic acetals than those previously employed in quadrupole collision cells are shown to react efficiently by ionictransacetalization under the ESI and APCI in-source IMRconditions. Tetramethylurea (TMU) acts as an efficientdopant, being co-injected with the acetal in either benzene,toluene, methanol, or water/methanol solutions. UnderAPCI or ESI, the basic TMU dopant is protonated preferentially, and the labile protonated TMU then undergoesdissociation to (CH₃)₂NCO⁺, the least acidic and the mosttransacetalization-reactive acylium ion so far tested. Underthe relatively high-pressure, low-energy collision conditions set to favor associative reactions, (CH₃)₂NCO⁺ reactscompetitively both with TMU to form acylated TMU andwith the acetal via ionic transacetalization to form therespective cyclic ionic acetals. Spectrum subtractionremoves the ionic products of the dopant (TMU) self-reactions, thus providing clean ion-molecule reactionproduct ion mass spectra, which are used for the selective, structurally diagnostic detection of cyclic acetals.Information on ring substituents comes from characteristic mass shifts resulting from aldehyde/ketone by acylium ion replacement. Enhanced selectivity in structuralcharacterization or chemical recognition for cyclic acetalmonitoring is gained by performing on-line collision-induced dissociation via tandem mass spectrometricexperiments. Most cyclic ionic acetals dissociate exclusively or nearly exclusively to re-form the reactant (CH₃)₂NCO⁺ acylium ion whereas the presence of additionalfunctional groups with increased structural complexitytends to favor other specific but likewise selective dissociation channels.