Structures of the radical (DPPH) oxidation products of dihydrocapsaicin

Article abstract of DOI:10.1016/S0040-4039(02)01813-0

Dihydrocapsaicin was oxidized with 2,2-diphenyl-1-picrylhydrazyl (DPPH), a free radical, and the products were analyzed by spectroscopic means.

Full text of DOI:10.1016/S0040-4039(02)01813-0

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 8181–8183
Structures of the radical (DPPH) oxidation products of
dihydrocapsaicin
Tetsuya Nakamura, Takashi Ooi, Kentaro Kogure, Miki Nishimura, Hiroshi Terada and
Takenori Kusumi*
Faculty of Pharmaceutical Sciences, Tokushima University, Tokushima 770-8505, Japan
Received 20 June 2002; revised 19 August 2002; accepted 23 August 2002
Abstract—Dihydrocapsaicin was oxidized with 2,2-diphenyl-1-picrylhydrazyl (DPPH), a free radical, and the products were
analyzed by spectroscopic means. © 2002 Elsevier Science Ltd. All rights reserved.
The pungent principal of hot pepper has long been used
as an ingredient of spices all over the world. Capsaicin
(1) and dihydrocapsaicin (2) are the main components
of hot pepper, constituting more than 90% of the total
pungent principle.1 The capsaicins are known to possess
potent antioxidative activity. It has been reported that
the oxidation of oleic acid at cooking temperatures was
inhibited by the presence of capsaicin.2 The antioxida-
tion effect of capsaicin is also discussed.3 Recently,
Kogure et al. exhibited that the radical scavenging
activity of capsaicins was more potent4 than that of
tocopherol by the experiments using 2,2-diphenyl-1-
dihydrocapsaicin by means of spectroscopic analyses
and to deduce the antioxidation process of capsaicinoid
on the structural basis of the products (Fig. 1). Dihy-
drocapsaicin (2) was preferred to capsaicin (1), because
(i) 1 and 2 have the same biological activity5 and (ii) the
absence of the olefinic bond, which might be attacked
by a radical, on the side-chain of 2 will simplify the
composition of the oxidation product.
Dihydrocapsaicin (2) was treated with 2 equivalents4 of
DPPH in chloroform at room temperature for 1 h.
After the reaction was completed, the solvent was evap-
1
picrylhydrazyl
[DPPH:
(Ph2N-N(·)-2,4,6-trinitro-
orated to dryness. The residue was then subjected to H
phenyl)], a free radical, although the oxidation course
of capsaicins was not clarified. Detection of a capsaicin
dimer by HPLC and MS/MS in the oxidation product
of 1 has been reported.3 The objective of the present
study is to identify the radical oxidation products of
NMR analysis. The spectrum showed that the signals
due to the aromatic (l 6.75–6.85), benzyl (l 4.34), and
methoxy (l 3.87) protons of 2 remarkably decreased in
comparison with the side chain proton signals. This
indicates
that
the
4-hydroxy-3-methoxyphenyl-
Figure 1. Chemical structures of capsaicin (1), dihydrocapsaicin (2) and the radical oxidation products (3, 4, 5 and 6) isolated in
the present experiments.
Keywords: capsaicin; dihydrocapsaicin; oxidation; radical scavenger.
* Corresponding author.
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)01813-0

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T. Nakamura et al. / Tetrahedron Letters 43 (2002) 8181–8183
methylene moiety of 2 underwent decomposition and
the product possessing the 8-methylnonyl group might
be the main radical oxidation product. The major sig-
nals apparently resemble those of 8-methylnonanamide
(4), which was actually isolated from the reaction mix-
ture (vide infra), although the recovery of 4 by silica gel
chromatography was poorer than expected possibly
because of its high polarity and partial adsorption on
silica gel.
the oxidative coupling site, the coupling patterns of the
aromatic proton signals were analyzed (Fig. 2(a)) with
the help of 2D NMR techniques. Signals a and c were
assigned as 2% and 6%, respectively, on the basis of the
facts that they were meta-coupled by 1.6 Hz and that
C-2% and 6% [assignable by HSQC] showed correlation
peaks (3JH,C) with the aldehyde proton in the HMBC
spectrum. Proton a (H-2%) was distinguishable from c
(H-6%) because the former showed an NOE [NOESY]
with the adjacent methoxy protons. Protons e (H-6)
and d (H-2) are meta-coupled by 1.6 Hz, the former at
the same time being coupled with b (H-5) by 8.0 Hz. In
the HMBC spectrum, the carbons (C-3 and C-3%) have
correlations with adjacent methoxy protons, and d (H-
2) and e (H-6) show cross peaks with the benzyl pro-
tons. Two methoxy protons were distinguished by the
NOE with protons a (H-2%) and d (H-2), respectively.
An NOE was observed between protons b (C-5) and c
(C-6%) (NOESY spectrum), which ensures that dihydro-
capsaicin and vanillin were coupled at 4 and 5% posi-
tions, as shown in structure 5.
Chromatographic separation of the reaction mixture
afforded unreacted 2 (13%) and compounds 3 (10%
based on consumed 2), 4 (26%), 5 (10%) and 6 (2%),
together with DPPH-H (ꢀ100%).
Compound 3 was identified as vanillin by comparison
of its NMR properties and Rf value on the TLC with
those of the authentic sample.
The 1H and 13C NMR properties of compound 46
(HRMS: calcd for C10H21NO 171.1623, found m/z
171.1628) were very close to those of 2 except for the
signals of vanillyl moiety. The 1H NMR spectrum
showed a broad signal (2H) at l 5.38 due to -CONH2
and the 13C NMR spectrum showed a singlet at l 175.3
ascribable to an amide carbonyl group. These data
indicated that 4 was 8-methylnonanamide, the side-
chain part of 2.
The molecular formula of compound 66 (HRMS: calcd
for C16H14O6 302.0790, found m/z 302.0766) suggested
that 6 was an oxidative self-coupling product of vanillin
(C8H8O3). It’s 1H NMR spectrum showed two aldehyde
signals at l 9.75 (1H, s) and 9.90 (1H, s), two methoxy
signals at l 3.96 (3H, s) and 4.01 (3H, s), a phenolic
hydroxyl signal at l 6.33 (1H, s), and aromatic proton
signals exhibited in Fig. 2(b). Signals c and d were
assigned as H-2% and 6%, respectively, by their mutual
meta-coupling (1.6 Hz) and the HMBC correlations of
C-2% and 6% (assignable by HSQC) from an aldehyde
proton (l 9.75). Proton c was distinguished from d by
an NOE from the adjacent methoxy group (l 4.01).
The molecular formula of compound 56 (HRMS: calcd
for C26H35NO6 457.2464, found m/z 457.2491) indi-
cated that 5 was an oxidative coupling product of
1
dihydrocapsaicin with vanillin. It’s H NMR spectrum
appeared to be the combination of the spectra of 2 and
3, except for the signals of aromatic region. To confirm
Figure 2. The patterns of the aromatic proton signals and 2D correlations (HMBC and NOESY) found for 5 (a) and 6 (b)
(CDCl3, 400 MHz). Multiplicities and coupling constants (J in Hz) in parentheses.

T. Nakamura et al. / Tetrahedron Letters 43 (2002) 8181–8183
8183
Figure 3. A presumed mechanism of oxidation of dihydrocapsaicin producing 3, 4, 5 and 6.
Similarly, signals a and b were assigned as H-2 (d, 1.6
Hz) and 6 (dd, 8.0, 1.6 Hz), respectively, by their
coupling patterns and the HMBC correlations of C-2
and 6 from the aldehyde proton (l 9.90). Proton e was
ortho-coupled with b by 8.0 Hz. Two methoxy protons
were distinguished by the NOE with protons a and c.
An NOE was observed between d (H-6%) and e (H-5) in
the NOESY spectrum.
3. Henderson, D. E.; Slickman, A. M.; Henderson, S. K. J.
Agric. Food Chem. 1999, 47, 2563–2570.
4. Kogure, K.; Goto, S.; Nishimura, M.; Yasumoto, M.;
Abe, K.; Chiwa, C.; Sassa, H.; Kusumi, T.; Terada, H.
Biochim. Biophys. Acta, in press.
5. Iwai, K.; Watanabe, T. Red Peppers, Sciences and Pun-
gency; Saiwai Shobo: Tokyo, 2000; p. 95.
6. 1H (400 MHz: J in Hz) and 13C NMR (100 MHz) spectra
1
were recorded in CDCl3 solutions. 4: H l 0.85 (6H, d, J
6.8; Me%s on C-8), 1.09–1.19 (2H, m), 1.19–1.39 (6H, m),
1.50 (1H, m; H-8), 1.63 (2H, quint, J 7.6; 3), 2.21 (2H, t,
J 7.6; 2), 5.38 (2H, brs; -NH2). 13C l 22.5 (q×2), 25.4 (t),
27.1 (t), 27.8 (d), 29.2 (t), 29.5 (t), 35.8 (t), 38.8 (t), 175.3
(s). 5: 1H l 0.84 (6H, d, J 6.4; H-16, 17), 1.06–1.18 (m; 14),
1.18–1.39 (m; 11, 12, 13), 1.49 (m; 15), 1.66 (quint, J 7.6;
10), 2.22 (t, J 7.6; 9), 3.82 (s, 3-OMe), 3.98 (s, 3%-OMe),
4.43 (d, J 6.0; 7), 5.73 (brs; NH), 6.48 (s; OH), 6.83 (dd, J
8.0, 1.6; 6), 6.93 (d, J 1.6; 2), 6.99 (d, J 1.6; 6%), 7.00 (d, J
8.0; 5), 7.20 (d, J 1.6; 2%), 9.70 (s; CHO). 13C l 22.5 (C-16,
17), 25.7 (10), 27.1 (12 or 13), 29.3 (11), 27.8 (15), 29.5 (13
or 12), 36.7 (9), 38.8 (14), 43.2 (7), 55.9 (3-OMe), 56.4
(3%-OMe), 106.5 (2%), 112.4 (2), 113.6 (6%), 120.3 (6), 120.8
(5), 128.3 (1%), 136.1 (1), 142.3 (4%), 143.8 (4), 145.2 (5%),
Based on the structures of the products, 3, 4, 5, and 6,
formed by the DPPH oxidation of 2, we assumed a
plausible mechanism leading to the products, as shown
in Fig. 3. The DPPH radical attacks the benzyl hydro-
gen of 2 forming a benzyl radical (a). By the action of
another DPPH that abstracts a hydrogen of NH, an
immino ketone (b), which tautomerizes with a methyl-
ene quinone (c) may be formed. The CꢀN bond is
hydrolytically cleaved by water present in the solvent to
give 3 and 4. The resulting vanillin (3) is converted to
radical 3% by DPPH. This radical reacts with c and 3 to
afford 5 and 6, respectively.
1
148.2 (3%), 151.0 (3), 172.9 (8), 190.3 (CHO). 6: H l 3.96
(s; 3-OMe), 4.01 (s; 3%-OMe), 6.33 (s; OH), 6.94 (d, J 8.0;
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