9388 J. Agric. Food Chem., Vol. 53, No. 24, 2005
Brenna et al.
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Figure 3. Conversion of 10 into 13 by hydrogenation, reduction, and
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Table 3. Positional and Global
δ
18O Values (‰) for Coumarin 10
sample (origin)
Cd
O
ArO
total
10.1 (natural from F. tonka)
10.2 (natural from Charabot)a
10.3 (natural from Charabot)a
10.4 (synthetic from Fluka)
38.3
46.7
45.7
19.6
2.3
0.5
20.3
23.6
22.6
12.6
−0.5
5.6
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a Different lots.
Determination of the Positional δ18O Values of Coumarin.
To complete the isotopic characterization of coumarin 10, the
positional δ18O values of extractive and synthetic samples were
determined. For this purpose, 10 was converted to chroman 13,
which retains the aromatic oxygen atom of 10. The transforma-
tion was performed chemically, hydrogenating 10 to dihydro-
coumarin 11. The latter compound, on LiAlH4 reduction,
provided 3-(2-hydroxyphenyl)propan-1-ol. This material, treated
with 4-toluenesulfonyl chloride in pyridine, gave the primary
tosylate, whose ring closed to chroman 13 under basic conditions
(Figure 3). Samples 10.1-10.4 and samples 13.1-13.4 were
subsequently submitted to the isotopic oxygen determinations.
The bulk and positional δ18O values of coumarin samples 10.1-
10.4 are reported in Table 3. The numerical data representing
the δ18O values of the carbonyl carbon were calculated from
the δ18O values of 10 and 13.
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A closer look at the whole set of values of 10 allows the
identification of two distinct groups, represented by samples
10.1-10.3 of extractive origin (mean value 22.1‰) and by the
sample 10.4 produced by chemical synthesis (12.6‰). Taking
into consideration also the positional values, the first set is
characterized by an isotope ratio mean value of 0.6‰ of the
aromatic oxygen atom, while a value of 5.6‰ is observed for
the synthetic material 10.4. Moreover, the carbonyl oxygen in
samples 10.1-10.4 displays a mean value of 43.5‰, consider-
ably higher than the value of 19.6‰ of sample 10.4. Therefore,
the positional δ18O values are relevant for the determination of
the origin of coumarin. Additionally, the present data provide
information on the possible mechanism of introduction of the
oxygen function in the aromatic ring of cinnamic acid 4 during
the conversion into o-coumaric acid 7. The mean δ18O value
of 0.6‰ here observed for the aromatic oxygen atom of
extractive 10 is consistent with the values of phenolic oxygen
of p-coumarate-derived products such as raspberry ketone 2,
-0.8 and +0.6‰ (6), and vanillin 1, 5.3 and 6.3‰ (5),
respectively, thus suggesting a similar mechanism for the oxygen
activation, i.e., the direct introduction of the O-atom into the
aromatic ring from O2 through the action of a specific enzyme.
Seen together, the results of the multiple isotope characterization
of coumarin 10 and related materials so far reported are not
only useful for differentiating extractive natural products from
the synthetic ones but also provide insights into the means of
formation of natural products.
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2
Natural abundance H nuclear magnetic resonance study of the
origin of raspberry ketone. J. Agric. Food Chem. 1998, 46, 248-
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Received for review July 28, 2005. Revised manuscript received October
5, 2005. Accepted October 5, 2005. COFIN “Aromi e Fragranze” is
acknowledged for partial financial support.
JF0518507