The Mass Spectral Fragmentation of Perezoneand Related Compounds

Eduardo Cortes*, Benjamın Ortiz, Ruben Sanchez-Obregon, Fernando Walls and Francisco Yuste*Instituto de Quımica, Universidad Nacional Autonoma de Mexico, Circuito Exterior, Ciudad Universitaria, Coyoacan,04510, Mexico D.F.

Electron impact mass spectra of six isomeric compounds, namely perezone, isoperezone, α-pipitzol, â-pipitzol,â-isopipitzol and dihydroisoperezinone have been recorded. The proposed fragmentation pathways leading tothe formation of a number of important daughter ions have been confirmed from the corresponding parent ionspectra by collision-induced dissociation (CID) experiments. The elemental composition of the principalfragment ions was determined by accurate mass measurements. © 1997 by John Wiley & Sons, Ltd.

Received 17 December 1996; Revised 24 March 1997; Accepted 24 March 1997Rapid. Commun. Mass Spectrom. 11, 904–906 (1997)No. of Figures: 1 No. of Tables: 3 No. of Refs: 5

Perezone (1), a sesquiterpene quinone known since1852, occurs along with several related quinones invarious species of Perezia.1 The thermal treatment of 1produces an equimolecular mixture of the cedranoidsα-pipitzol (3a) and â-pipitzol (3b).2

Recently, we have reported the isomerization ofperezone (1) into isoperezone (2) by means of a1,2-carbonyl transposition catalyzed by 3,4,5,6-tetra-hydro-2-pyrimidinethiol.3 The ZnBr2 catalyzed intra-molecular cycloaddition4 of 2 produced â-isopipitzol(4), perezinone2,5 and dihydroisoperezinone (5).3 Theuse of Eu(fod)3 as catalyst4 affords a mixture of 4 and5, whereas the treatment of 2 with BF3·Et2O leads to 5as the only product.3 As structures 1–5 have somesimilarities, consideration of the limited data availablein the literature on mass spectra of these compoundsprompted us to record their mass spectral fragmenta-tion patterns. In this paper we describe the resultsobtained.

EXPERIMENTAL

The compounds examined have been described pre-viously.1–4 The electron impact mass spectra wererecorded on a JEOL JMS-AX505HA mass spectrome-ter. The accurate mass determination of the molecularion and main fragment ions was made using a JEOLJMS-5X102A high resolution mass spectrometer. Thedirect inlet system was used for all the samples. Thespectra were determined in an ionization chamberoperated at 190 °C and 70 eV ionizing voltage. Parentand daughter ions spectra were recorded by collision-induced dissociation (CID) experiments.

RESULTS AND DISCUSSION

The relative abundances of the principal fragments ionsof compounds 1–5 are given in Table 1. The elementalcomposition of molecular ion and main fragment ions

*Correspondence to: E. Cortes or F. Yuste

RAPID COMMUNICATIONS IN MASS SPECTROMETRY, VOL. 11, 904–906 (1997)

CCC 0951–4198/97/080904–03 $17.50 © 1997 by John Wiley & Sons, Ltd.

Table 1. Relative abundances (%) of principal fragments in the mass spectra of compounds 1–5

Compound No. M+.[M-15]+ [M-29]+ [M-43]+ [M-57]+ [M-68]+ [M-71]+

m/z 167 m/z 166 m/z 138

1 19 3 2 3 7 5 4 22 100 22 46 8 5 9 6 9 7 100 85 703a 100 12 25 39 33 5 18 76 70 33b 100 13 33 51 48 7 27 93 90 184 61 14 11 22 13 6 16 100 39 295 66 12 8 12 17 8 9 100 40 33

in the electron impact mass spectrum of perezone (1),determined by exact mass measurements, are given inTable 2. The proposed fragmentation pattern forcompounds 1–5, displayed in Scheme 1 for perezone(1), confirms the major part of the daughter ionsderived from the molecular ion (Table 3). As can beseen from Table 1, compounds 1–5 have some commonfragments but with different relative abundances. In 3aand 3b the molecular ion is the base peak, whereas in 2,4 and 5 the ion at m/z 167 is the base peak and in 1, it

is the ion at m/z 166. All mass spectra exhibit an intensemolecular ion, which probably reflects the stable natureof compounds 1–5 under electron impact.

The main fragmentation pathways of the isomericcompounds 1–5 was rationalized to occur by elimina-tion from the molecular ion. Thus, the [M – 15]+ ion isproduced by loss of CH3 and the [M – 29]+ ion isproduced by loss of C2H5. Another significant fragmen-tation to produce the [M – 29]+ ion is the loss of CH2(14 amu) from the [M – 15]+ ion. It was established that

Scheme 1.

FRAGMENTATION OF PEREZONE AND RELATED COMPOUNDS 905

© 1997 by John Wiley & Sons, Ltd. Rapid Communications in Mass Spectrometry, Vol. 11, 904–906 (1997)

Table 2. Elemental composition of the ions in the electronimpact mass spectrum of perezone (1)

Ion Elemental Calculated Measured ∆m (mmu)composition mass mass

M+ C15H20O3 248.1412 248.1425 –1.3[M-15]+ C14H17O3 233.1178 233.1181 –0.3[M-29]+ C13H15O3 219.1021 219.1025 –0.4[M-43]+ C13H17O2 205.1229 205.1228 +0.1[M-57]+ C11H11O3 191.0708 191.0712 –0.4[M-68]+ C10H12O3 180.0786 180.0794 –0.8[M-71]+ C10H9O3 177.0552 177.0561 –0.9m/z 167 C9H11O3 167.0708 167.0701 +0.7m/z 166 C9H10O3 166.0630 166.0639 –0.9m/z 138 C8H10O2 138.0681 138.0667 +1.4

Table 3. Daughter ions in the electron impact mass spectraof compounds 1–5 in collision-induced dissociationexperiments

A Parent ion at m/z 248 [M]+ A’ Daughter ions at m/z:a. 233 [M-15]+, b. 219 [M-29]+

c. 191 [M-57]+, d. 180 [M-68]+

e. 177 [M-71]+

f. 167, g. 166B Parent ion at m/z 233 [M-15]+ B’ Daughter ion at m/z:

a. 205 [M-43]+, b. 219 [M-29]+

C Parent ion at m/z 167 C’ Daughter ion at m/z:a. 138

the [M – 43]+ ion originated by loss of CO (28 amu)from the [M – 15]+ ion. The fragment ions [M – 57]+ ,[M – 68]+ and [M – 71]+ involves the loss, from the sidechain, of C4H9, C5H8 and C5H11 units, respectively. Themost remarkable feature of the mass spectra of

compounds 1–5 is the presence of the abundant ions atm/z 167 and 166. These ions originate from the loss ofC6H9 and C6H10 side chain units, respectively. The ionat m/z 167 further loses 29 amu, corresponding to theelimination of CHO, to yield the ion at m/z 138. In theisomeric compound 1 the single hydrogen transferleading to the m/z 166 ion (B.P.) is more important thanthe double hydrogen transfer to produce the m/z 167ion from the molecular ion. On the contrary, in theisomeric compound 2 the double hydrogen transfer ismore important to produce the m/z 167 ion (B.P.). Thisdifference is useful to help distinguish between theisomeric compounds 1 and 2 (see Table 1). Both m/z167 and 166 ions of compounds 1–5 have diagnosticvalue and may be useful for a selected-ion monitoringassay of biological samples. The results presentedclearly show that the molecular ion was the majorprecursor of the fragment ions in the electron impactmass spectra of compounds 1–5.

Acknowledgments

We wish to thank J. Perez and L. Velasco for their assistance in theacquisition of the mass spectral data.

REFERENCES

1. P. Joseph-Nathan and R. L. Santillan, The chemistry of perezoneand its consequences. In Studies in Natural Products Chemistry,vol. 5 Structural Elucidation (Part B), Atta-ur-Rahman (Ed.),Elsevier, Amsterdam (1989), pp. 763–813.

2. F. Walls, J. Padilla, P. Joseph-Nathan, F. Giral, M. Escobar and J.Romo, Tetrahedron 22, 2387–2399 (1966).

3. A. Rodrıguez-Hernandez, H. Barrios, O. Collera, R.G. Enrıquez,B. Ortiz, R. Sanchez-Obregon, F. Walls, F. Yuste, W. F. Reynoldsand M. Yu, Nat. Prod. Lett. 4, 133–139 (1994).

4. F. Yuste, H. Barrios, E. Dıaz, B. Ortiz, R. Sanchez-Obregon and F.Walls, Tetrahedron Lett. 35, 9329–9332 (1994).

5. D. A. Archer and R. H. Thomson, J. Chem. Soc. (C) 1710–1716(1967).

906 FRAGMENTATION OF PEREZONE AND RELATED COMPOUNDS

© 1997 by John Wiley & Sons, Ltd. Rapid Communications in Mass Spectrometry, Vol. 11, 904–906 (1997)