Mass Spectral Fragmentation of Monocyclic γ-Pyrones

Abstract

The mass spectra of six monocyclic γ-pyrones are recorded (Fig. 1-6), and their fragmentation reactions are described. The characteristic features of the fragmentation pattern are: i) the elimination of carbon monoxide from the molecular ion, and ii) the retro-Diels-Alder cleavage of the γ-pyrone ring. As a pertinent example, the fragmentations of 2, 6-dimethyl-γ-pyrone (II) are discussed in detail, where the meta-stable ion peaks (m*) for most reaction steps were clearly observed (Scheme I and II) . The primary process of the fragmentation of II is an expulsion of carbon monoxide from the molecular ion A (m/e 124) with generation of a corresponding furan ion B (m/e 96) (Reaction 1) . The subsequent loss of a hydrogen atom from B would give an oxonium ion C (m/e 95) (Reaction 2) . A skeletal rearrangement of the furan ion B may afford an equivalent ion B' (m/e 96) (Reaction 3), which leads to ions of D (m/e 43) and E (m/e 81) through plausible mechanisms (Reaction 4 and 5) . The loss of carbon monoxide from E yields methylcyclopropenyl cation F (m/e 53) (Reaction 6) . The intermediacy of the furan ion in the fragmentation is supported by the similarity of the spectrum of 2, 6-dimethyl-γ-pyrone(II)to that of 2, 5-dimethyl-furan. Another important fragmentation pathway of II is the retro-Diels-Alder cleavage of the γ-pyrone ring with formation of an ion G (m/e 84) (Reaction 7) . A further loss of a methyl radical from G produces an ion H (m/e 69) (Reaction 8) . Several minor modifications and some additional fragmentation processes are required to interprete the spectra of other γ-pyrones. For methoxyl derivatives (IV and V), the expulsion of carbon monoxide from the molecular ion is followed by the loss of a methyl radical from the methoxyl group to give an oxonium ion, from which carbon monoxide is further eliminated (Scheme III) . When 2 and 6 positions of γ-pyrones are not substituted as in I and III, the hydrogen migration takes place after the retro-Diels-Alder cleavage of the ring, and carbon monoxide expulsion from the resulting ion gives the corresponding ketene radical cation (Scheme IV) . In the case of bromo derivative VI, the fragmentation involves also the expulsion of bromine radical(Scheme V). γ-Pyrones are isoelectronic with α-pyrones and tropones, and their mass spectral behaviors are expected to be similar in general. Comparisons of their spectra, however, indicate slight differences, which are well accounted for by the mechanistic scheme discussed above.