was isolated in moderate yield. Its structure was carefully confirmed
by NMR spectroscopy. Thus, 1H NMR spectrum of adduct 5
shows three groups of signals characteristic to the moiety CH2eCH
at d¼4.15e4.55 ppm. In its 13C NMR spectrum, the signal of carbonyl
group appears at d¼200.4 ppm. Instead of the signals of the
double bond carbons, doublet of the tertiary carbon atom CHBr
(d¼47.6 ppm, J¼154 Hz) and a doublet of triplets of the secondary
carbon joined to phenol moiety (d¼69.3 ppm, J¼150 and 4.4 Hz) are
observed.
We have found that adduct 5 is a precursor of pushepull aroxyenone
6 in view of its ability to undergo dehydrobromination
under the treatment with both mineral and organic base. In fact,
when we used K2CO3 or Et3N as a base the reaction proceeded
slowly and the formation of the (0.3:1) mixture of the target
pushepull aroxyenone 6 and initial ketone 5 was registered by
NMR spectroscopy after 3e5 h of the reaction (Scheme 6). Its
structure was confirmed by NMR and mass spectrometry as well as
by comparison with the previously reported data.10 Thus, the olefinic
protons appeared as doublets at 6.39 and 6.77 ppm with JHH
¼16.3 Hz. In the 13C NMR spectrum the signals of olefinic carbons
are observed at 148 and 108 ppm while carbonyl carbon expectedly
shifted to low frequency field (d¼192 ppm). Finally, in the EIMS
spectrum a very intensive molecular ion peak 192 (100) was presented.
This can be regarded as a strong evidence for reaction via
mechanistic pathway B (Scheme 4).