Double transacetalization of diacylium ions

A novel gas-phase reaction of diacylium ions of the O=C=X+=C=O type (X = N, CH) is reported: double transacetalization with cyclic acetals or ketals, The reaction is exothermic and highly efficient, and forms members of a new class of highly charged-delocalized ions: cyclic ionic diketals, Pentaquad rupole double-and triple-stage mass spectrometric (MS2 and MS3) experiments reveal the high double transacetalization reactivity of O=C=N+=C=O and O=C =CH+=C=O, whereas the synthesis of differently substituted cyclic ionic dik etals is performed in MS3 experiments via sequential mono- and double trans acetalization of O=C=N+=C=O and O=C=CH+=C=O with different acetals, With cy clic acetals, the acylium-thioacylium ion O=C=N+=C=S reacts promptly and se lectively by mono-transacetalization at its acylium site, but the free thia cylium site of its cyclic ionic ketal is nearly unreactive by double transa cetalization. Therefore, only the acylium site of O=C=N+=C=S can be efficie ntly protected by transacetalization, Low-energy MS3 collision-induced diss ociation of the cyclic ionic diketals of O=C=N+=C=O and O=C=CH+=C=O sequent ially frees each of the protected acylium site to form the mono-derivatized ion, and then the fully deprotected diacylium ion. Copyright (C) 2000 John Wiley & Sons, Ltd.