EIMS OF CURCUMIN ANALOGUES
327
derived therefrom. The calculations were limited to the
case of the [2 ] 2] cycloaddition with the
bicyclo[3.2.0]heptane-type intermediate. The calcu-
lation results are given in Scheme 5 (results obtained for
the fully 12C compound), which also shows the approx-
imate conÐguration (14b) of the reactant 14`~, the pos-
sible intermediates and the products. First the
ÏstretchedÏ conÐguration (cf. Table 4, for the dione) was
brought to the possible conÐguration preceding ring
closure. In the case of the dione, this reacting conÐgu-
ration was not a local minimum and collapsed to give
the distonic Ðve-membered ring intermediate, 14b,
whereas the enolone has a distinct local minimum for
both structures 14aº and 14bº. The relative stability of
the distonic 14b may imply that it can also act as an
intermediate in other fragmentations, e.g. in the loss of
CO or in the net loss of the mass of [aryl ] CH];
however, given the complexity of the system, it would
be difficult to prove that this intermediate is the key in
some rate determining isomerization. The two
bicyclo[3.2.0]heptane-type intermediates, 14c and 14cº,
are both local minima. The overall reaction is thermon-
eutral and the envisaged intermediates have an enthalpy
of formation below that of the reactant ions and that of
the products. Hence it is clear that the intermediates
required for the oleÐn metathesis reaction are accessible
once ionization of 14, be it in the enolone or the dione
form, has taken place. The calculations show that the
overall oleÐn metathesis reaction can proceed once the
neutral molecules, be they of dione or enolone type, are
ionized.
CONCLUSION
Although much of the fragmentation behaviour of sub-
stituted curcumins can be explained by conventional
fragmentation mechanisms, all 24 curcumin-type com-
pounds investigated in this study after ionization
undergo a rearrangement to form substituted stilbenes.
13C labelling shows that the stilbene product ions are
probably formed via a [2 ] 2] cycloaddition and a sub-
sequent retro-[2 ] 2] cycloaddition, formally an oleÐn
metathesis-type reaction. The possible involvement of a
bicyclo[3.2.0]heptane-type intermediate, analogous to
particular organic synthetic pathways, was conÐrmed
by MNDO/PM3 calculations.
Acknowledgement
A. N. NurÐna and A. M. Supardjan (Department of Pharmacy, Uni-
versitas Gadjah Mada, Yogyakarta, Indonesia) are gratefully acknow-
ledged for the synthesis of unlabelled compounds. The authors thank
the CAOS/CAMM Centre of the University of Nijmegen for the use
of calculation facilities. B.L.M.v.B. also thanks Privat Doz. Dr D.
Kuck for stimulating discussions.
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( 1998 John Wiley & Sons, Ltd.
JOURNAL OF MASS SPECTROMETRY, VOL. 33, 319È327 (1998)
Van Baar, Ben L. M.
