THE SEARCH FOR THE GAS-PHASE NEGATIVE-ION PINACOL REARRANGEMENT

Deprotonated 1,2-diols and beta-methoxyhydrins often eliminate ROH (R = H, Me) on collisional activation in the gas phase. These losses do n ot involve a negative ion pinacol rearrangement in acyclic systems in which there are no conformational restraints on the relative positions of the reacting groups. In the case of beta-methoxyhydrins, labeling studies show that the product ion is formed by the losses of a methoxi de ion and a proton from adjacent positions. The absence of a deuteriu m isotope effect for this process precludes the operation of an elimin ation process: we propose that the loss of methanol proceeds via an ep oxide cyclization viz. Me2C(OMe)C(O-)(Me)2 --> (MeO-)-[GRAPHICS] CH2=C (Me)C(O-)(Me)2 + MeOH. In contrast, epoxide cyclization does not occur when the two oxygenated substituents cannot adopt an anti orientation . Thus deprotonated cis-2-methoxycyclohexanol loses methanol via a pin acol rearrangement, while loss of methanol from the trans isomer produ ces deprotonated cyclohex-2-en-1-ol, presumably by an epoxide mechanis m.