Terpenoids—CXXXVII: Fragmentation of constunolide and its derivatives under electron‐impact

Abstract

Abstract-An attempt has been made to correlate the fragmentation modes and structure of costunolide and some lactones derived from it, containing a ten-membered carbocyclic system. The possible origins for the major fragments have been determined, using high resolution mass spectrometry and deuterium labelling.MASS spectra of aliphatic lactones,lr2 (-)santonin and its derivative^^,^ have been recently investigated. However, the behaviour under electron-impact of sesquiterpenic y-lactones, with a ten-membered carbocyclic system, does not seem to have received much attention. Costunolide is the parent lactone of such a group and the present communication deals with the fragmentation of some of its derivatives. From costunolide (I), the lactones (11): (III),t (IV),t (V)5 and (VI)6 were prepared by known procedures.A comparative study of the mass spectra of lactones I to IV, has revealed some common fragmentation modes and a few significant differences of diagnostic value in structure elucidation. The thermal rearrangement' of I1 and the transannular acid catalysed c y c l i z a t i~n~~~ of I and TI are known. In order to explain satisfactorily some electron-impact induced fragmentation modes, observed in I, I1 and IV, it is necessary to assume that similar processes are operating in the vapour state. Table 1 gives the relative intensities of some significant ions observed in the spectra of these lactones.In unsaturated lactones I to IV, the double bond appears to trigger the primary fragmentation processes. Depending on its location, differences are observed in the fragmentation patterns or in the relative intensities of some fragments. Sometimes, the migration of double bonds induced by electron-impact, alters the expected fragmentation reactions leading to unexpected cleavages.1°The insignificant value of the [M -CH,] ion in V, suggests that the contribution from the methjjl groups at C,, C,, and C,,, to the formation of this ion is negligible. This ion is, however, significant in the lactones I to IV, which is difficult to rationalise. The intensities of these ions in II and IV are comparable with that observed in (-)santonin (33 %), where the major contribution has been proposed to come from the angular methyl group. A concerted cyclization mechanism leading to the elimination of a methyl radical, appears to contribute considerably to the formation of this ion in I1 and IV (Scheme 1).

Biosynthesis of Costunolide, Dihydrocostunolide, and Leucodin. Demonstration of Cytochrome P450-Catalyzed Formation of the Lactone Ring Present in Sesquiterpene Lactones of Chicory

“…The ion peak of this compound was shifted from 234 to 236 amu, whereas the [M-Me] ϩ peak was shifted from 219 to 221 amu. Genuine costunolide (15) and dehydrosaussurea lactone (16) have an almost similar mass spectrum (Jain et al, 1971), and the small peak of 204 amu in their mass spectra arises from the loss of carbon monoxide from the lactone moiety of the molecular ion (Sathe et al, 1969(Sathe et al, , 1971. Because in the 18 O 2 -labeling experiment, the ion peak of 204 amu from dehydrosaussurea lactone is shifted to 206 amu (Fig.…”

Essential oils and related products

“…The mass spectrometry fragmentation pattern of costunolide was related to its structure by Sathe et al (594).…”

Mass spectrometry

“…GC/ MS has been used to quantitatively analyze mixtures of diterpene hydrocarbons (687). Other studies include hydrocarbons of the carane and menthane series (688), triterpenoid dehydration products (689), aromatic diterpenes (690), triterpenes related to serratenediol (691), custunolide and its derivatives (692), and two isomers of camphor (693).…”