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;
References
5), 7.17 (d, J 1.6; 6%), 7.31 (d, J 1.6; 2%), 7.40 (dd, J 8.0, 1.6;
6), 7.53 (d, J 1.6; 2), 9.75 (s; 1%-CHO), 9.90 (s; 1-CHO). 13
C
1. (a) Iwai, K.; Suzuki, T.; Fujiwake, H. Agric. Biol. Chem.
1979, 43, 2493–2498; (b) Suzuki, T.; Kawada, T.; Iwai, K.
J. Chromatogr. 1980, 198, 217–223.
l 56.1 (3-OMe), 56.5 (3%-OMe), 106.6 (2%), 110.7 (2), 116.8
(6%), 117.3 (5), 125.5 (6), 128.6 (1%), 132.7 (1), 142.4 (5%),
143.0 (4%), 148.3 (3%), 150.5 (3), 150.9 (4), 189.8 (1%-CHO),
190.5 (1-CHO).
2. Henderson, D. E.; Henderson, S. K. J. Agric. Food Chem.
1992, 40, 2263–2268.