Abstract
Gas‐phase reactions of four acylium ions and a thioacylium ion with three isomeric α‐, β‐ and γ‐hydroxy ketones are performed by pentaquadrupole mass spectrometric experiments. Novel structurally diagnostic reactions are observed, and found to correlate directly with interfunctional group separation. All five ions tested (CH~3~CO^+^, CH~2~CHCO^+^, PhCO^+^, (CH~3~)~2~NCO^+^ and (CH~3~)~2~NCS^+^) react with the γ‐hydroxy ketone (5‐hydroxy‐2‐pentanone) to form nearly exclusively a cyclic oxonium ion of m/z 85 that formally arises from hydroxy anion abstraction. With the β‐hydroxy ketone (4‐hydroxy‐2‐pentanone), CH~2~CHCO^+^, PhCO^+^ and (CH~3~)~2~NCO^+^ form adducts that undergo fast cyclization via intramolecular water displacement, yielding resonance‐stabilized cyclic dioxinylium ions. With the α‐hydroxy ketone (3‐hydroxy‐3‐methyl‐2‐butanone), PhCO^+^, (CH~3~)~2~NCO^+^ and (CH~3~)~2~NCS^+^ form stable adducts. Evidence that these adducts display cyclic structures is provided by the triple‐stage mass spectra of the (CH~3~)~2~NCS^+^ adduct; it dissociates to (CH~3~)~2~NCO^+^ via a characteristic reaction–dissociation pathway that promotes sulfur‐by‐oxygen replacement. If cyclizations are assumed to occur with intramolecular anchimeric assistance, relationships between structure and reactivity are easily recognized. Copyright © 2001 John Wiley & Sons, Ltd.