A study of the electron impact mass spectrometry is reported for eight, stable, previously synthesized, enolic isomers of 2-aroyl-4-aracyl-l,3-~yclopentanediones.As part of the research on P-tricarbonyl derivatives of cyclopentane, a number of 2,4-disubstituted 1,3-cyclopentanediones were prepared: completely enolized stable enolic isomers la-d forming a conjugated chelate system (dienoles) and 2a-d enolized preferentially in the five-membered ring (monoenoles).'Formulae of enolic isomers of 2-aroyl-4-aracyl-l,3-~yclopentane-diones These compounds exist in the solid state as stable tautomers while in solution they form tautomeric mixtures.2In this work we have examined the electron impact (EI) ionization mass spectra of enolic 2-aroyl-4-aracyl-1,3-~yclopentanedione isomers. So far as we know, no mass spectral data concerning similar compounds have been reported, although the mass spectra of some substituted 1,3-cyclo-pentanediones related to compounds encountered in the chemistry of the bitter constituents of hops have been p~b l i s h e d .~-~
EXPERIMENTALThe mass spectra were recorded with the aid of Varian-MAT CH-7 (Bremen, Germany) and Kratos 25RF (Manchester, UK) mass spectrometers using electron energies of 70 eV and ion source temperature of 105 "C. Samples were introduced into the ion source via a direct-inlet system using a glass crucible and programmable heating up to about 105 "C.Mass spectral data of the compounds 2a-d are pre-* Author for correspondence.sented in Table 1 and proposed fragmentation routes are given in Scheme 1. An asterisk (*) indicates the observed position of a metastable ion. There are only minor differences in the percentage relative abundances of ions in the mass spectra of compounds la-d (dienoles) and 2a-d (monoenoles). The study of metastable transitions pointed towards completely identical fragmentation pathways of these isomers. Thermal isomerization prior to ionization has occurred, which was also proved by interconversion of isomer 1 to isomer 2 by heating. ' In all cases, the predominant feature is cleavage of the side chain / 3 to the ring giving aroyl ions (e) as base peak while the expulsion of aryl groups from molecular ions is of minor importance (ions c). Loss of carbon monoxide from the ions e gives rise to ions f; a metastable peak at m/z56.5 (for a transition 105++77+) supports the proposed mechanism.A McLafferty rearrangement of molecular ions in 2-aroyl-4-aracyl-l,3-cyclopentanediones 2a-d leads to ions a (Ar = C6H5; m/z 275; 6.6% relative abundance; For explanation see Scheme 1.
