Desmethylubiquinone Q2 from the Far-Eastern ascidian Aplidium glabrum: Structure and synthesis

Article abstract of DOI:10.1016/j.tetlet.2004.11.157

Two new diprenylquinones, glabruquinone A (desmethylubiquinone Q ₂) having cancer preventive properties and its minor isomer Glabruquinone B were isolated from the ascidian Aplidium glabrum. Their structures have been elucidated by NMR and mass spectra and confirmed by synthesis.

Full text of DOI:10.1016/j.tetlet.2004.11.157

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 559–562
Desmethylubiquinone Q2 from the Far-Eastern ascidian
Aplidium glabrum: structure and synthesis
Larisa K. Shubina,a Sergey N. Fedorov,a,b Oleg S. Radchenko,a Nadezhda N. Balaneva,a
Sophia A. Kolesnikova,a Pavel S. Dmitrenok,a Ann Bode,b
Zigang Dongb and Valentin A. Stonika,*
aPacific Institute of Bioorganic Chemistry, Far-Eastern Branch of the Russian Academy of Sciences,
Vladivostok-22, Prospekt 100 let Vladivostoku 159, Russia
bThe Hormehl Institute of the University of Minnesota, 801 16th Avenue NE, Austin, MN 55912, USA
Received 1 October 2004; revised 23 November 2004; accepted 30 November 2004
Abstract—Two new diprenylquinones, glabruquinone A (desmethylubiquinone Q2) having cancer preventive properties and its
minor isomer Glabruquinone B were isolated from the ascidian Aplidium glabrum. Their structures have been elucidated by
NMR and mass spectra and confirmed by synthesis.
Ó 2004 Elsevier Ltd. All rights reserved.
Polyprenylated 1,4-benzoquinones and hydroquinones
such as ubiquinones, plastoquinones, and tocopherols
are widespread in plants and animals, in which they play
important roles in electron transport, photosynthesis,
and as antioxidants.1,2 Naturally occurring marine pren-
yl benzoquinones and hydroquinones having a terpe-
noid portion ranging from one to nine isoprene units
and differing structurally from the above-mentioned
groups have been described from marine organisms,
and especially from brown algae of the order Fucales,
sponges, and ascidians. Many algae contain tetraprenyl,
triprenyl-, and diprenylhydroquinones.3,4 Sponges5–8
contain linear unsubstituted polyprenylated hydroquin-
ones and benzoquinones with longer side chains and
moderate antimicrobial activity9 as well as ATPase
inhibiting sulfated prenylhydroquinones.10–12 Ascidians
of the genus Aplidium have previously yielded about a
dozen prenylated quinones13–16 including the most sim-
ple of them, monoprenylbenzoquinone.14
C18H24O4 304.1675] from the ethanolic extracts of Apli-
dium glabrum were isolated and purified by repeated
HPLC on a silica gel column using a hexane–ethyl ace-
tate system (6:1; a total yield of 0.09% on wet weight).
The ratio of 1 to 2 was 95:5. The EIMS showed the typ-
ical peak for quinones (M++2) at m/z 306 along with the
molecular ion peak at m/z 304. The UV and IR spectra
of 1 and 2 showed a maximum absorption at 264 nm
(e = 15,000) and IR absorption bands at 1675, 1657,
1603 cmꢀ1 corresponding to the p-benzoquinone moi-
1
ety. The H NMR spectrum of 1 was similar to that of
verapliquinone A from the Aplidium sp.13 and differed
in having an additional singlet signal at 4.02 ppm, which
is typical for a MeO group. Quartets at 61.2 and
61.3 ppm in the 13C NMR spectrum indicated the pres-
ence of two methoxyls in 1. The attachment of a terpe-
noid fragment to C-5 of the benzoquinone moiety and
methoxyls at the 2,3-positions were established by a de-
tailed inspection of the NMR spectra, including 1H–1H-
COSY, NOESY, and HMBC (see Scheme 1). Especially
important were the multiplicity of H-6 (a narrow triplet
at 6.34 ppm) and the cross peaks corresponding to H-6/
New diprenylquinones, glabruquinone A 1 [yellow oil,
HREIMS m/z 304.1655 [M]+, calcd for C18H24O4
304.1675] and a minor isomer, glabruquinone B 2
[yellow oil, HREIMS m/z 304.1649 [M]+, calcd for
1
H-10 allylic coupling in H–1H-COSY and H-6/H-10
interaction in NOESY. The signals of two trisubstituted
double bonds, three methyl groups attached to these
bonds and three methylene groups in the 13C NMR
spectrum clearly showed the presence of a diprenyl side
chain. A comparison of the NMR spectra of 1 and 2
showed that glabruquinones A and B contain geranyl
Keywords: Marine natural products; Polyprenylated quinones; Syn-
thesis; Cancer preventive activity.
*
Corresponding author. Tel.: +7 4232 311168; fax: +7 4232 314050;
e-mail: stonik@piboc.dvo.ru
0040-4039/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.11.157

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L. K. Shubina et al. / Tetrahedron Letters 46 (2005) 559–562
Scheme 1. Structures and synthesis of 1 and 2. Reagents and conditions: (a) H2O2, H2SO4, MeOH, 2 h; (b) geranyl bromide, Na, ether (or benzene),
25 h, reflux; (c) geraniol, BF3ÆEt2O, ether, 18 h; (d) CAN, CH3CN, H2O, 1 h.
and neryl types of side chains, respectively (Table 1). In
fact, these compounds differ from each other by the con-
figurations of the C20, C30-double bond as was estab-
lished on the basis of the chemical shifts of C-100. In
the sterically more congested E-isomers, this signal
was observed at a higher field when compared with the
Z-isomers.17 The C-100 chemical shifts of 1 and 2 differ
significantly, 16.2 and 22.8 ppm in spectra of 1 and 2,
respectively. Note that neryl derivatives like 2 are
very rare in nature whilst the majority of natural lin-
ear benzoquinones from marine organisms are of the
geranyl type.
Table 1. 13C- and 1H NMR data for 1 and 2 in CDCl3 at 75.5 and 300 MHz, respectively
Atom
1
2
dC
dH (J, Hz)
dC
dH (J, Hz)
1
184.38*
s
184.38* s
144.91**
145.16**
184.54* s
146.92 s
130.45 d
27.27 t
2
144.91**
145.16**
s
s
s
s
3
4
184.54*
146.92 s
130.45 d
27.17 t
s
5
6
6.34 t, J = 1.7, 1H
3.10 dd, J = 7.3, 1.7, 2H
5.13 t sext, J = 7.3, 1.2, 1H
6.34 t, J = 1.7, 1H
3.11 br d, J = 7.1, 1.2, 2H
5.13 m, 1H
10
20
30
40
50
60
70
80
90
100
OCH3
117.78 d
140.17 s
39.72 t
117.78 d
140.17 s
32.01 t
2.08 m, 2H
2.09 m, 2H
5.08 t sept, J = 6.8, 1.2, 1H
2.04 m, 2H
2.04 m, 2H
5.07 m, 1H
26.52 t
26.52 t
123.98 d
1131.95 s
25.77 q
17.79 q
16.2 q
123.98 d
131.95 s
25.77 q
17.79 q
22.76 q
1.70 d, J = 1.2, 3H
1.60 br s, 3H
1.66 br d, J = 1.2, 3H
1.59 d, J = 1.2, 3H
1.75 q, J = 1.2, 3H
4.00 s; 4.02 s
1.62 d, J = 1.2, 3H
4.00 s; 4.02 s
61.3 q; 61.2 q
61.3 q; 61.2 q
*,**—Values can be interchanged.

L. K. Shubina et al. / Tetrahedron Letters 46 (2005) 559–562
561
3. Ochi, M.; Kotsuki, H.; Inooue, M.; Taniguchi, M.;
Tokoroyama, T. Chem. Lett. 1979, 831–832.
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chemistry 1981, 20, 2598–2600.
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28, 1315–1324.
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7. Pouchus, Y. F.; Verbist, J. F.; Biard, J. F.; Boukef, K. J.
Nat. Prod. 1988, 51, 188–189.
The structures 1 and 2 were confirmed by synthesis from
commercially available 2,3,4–trimethoxybenzaldehyde 3
using the route shown in Scheme 1. The corresponding
trimethoxyphenol 4 was obtained by the previously de-
scribed method.18 The base-catalyzed alkylation of phe-
nol 4 with geranyl bromide led to geranylphenol 519
(18% total yield) purified by HPLC. The yield was less
than in other reported similar transformations.9,20,21
Modification of the reaction conditions did not increase
the yield, but led to the formation of O-alkylated side
products (7, 8).22
8. Lumsdon, D.; Capon, R. J.; Thomas, S. G.; Beveridge, A.
A. Aust. J. Chem. 1992, 45, 1321–1325.
9. De Rosa, S.; De Giulio, A.; Iodice, C. J. Nat. Prod. 1994,
57, 1711–1716.
10. Fusetani, N.; Sugano, M.; Matsunaga, S.; Hashimoto, K.;
Shikama, H.; Ohta, A.; Nagano, H. Experientia 1987, 43,
1233–1234.
11. Stonik, V. A.; Makarieva, T. N.; Dmitrenok, A. S. J. Nat.
Prod. 1992, 55, 1256–1260.
12. Bifulco, G.; Bruno, I.; Minale, L.; Riccio, R.; Debitus, S.;
Bourdy, G.; Vassa, A.; Lavayre, J. J. Nat. Prod. 1995, 58,
1444–1449.
13. Guella, G.; Mancini, I.; Pietra, F. Helv. Chim. Acta 1987,
70, 621–626.
14. Howard, B. M.; Clarkson, K.; Bernstein, R. L. Tetrahe-
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16. Faulkner, D. J. Nat. Prod. Rep. 1993, 93, 1771–1791.
17. Guella, G.; Mancini, I.; Guerriero, A.; Pietra, F. Helv.
Chim. Acta 1985, 68, 1276–1282.
A higher yield (55%) was achieved using the conditions
c, when a partial isomerization of 5 into 6 takes place
(85:15). Oxidative demethylation of geranyl phenol 5
by CAN yielded glabruquinone 1 (33%), which was
identical to the natural product (comparison of NMR
spectra and biological activities). A similar oxidative
demethylation of the mixture of 5 and 6 resulted in a
mixture of quinones 1 and 2. Synthetic 2 was separated
by HPLC from this mixture and identified with natural
glabruquinone B by comparison of their NMR spectra.
Recently, 1 was synthesized from 2,3-dimethoxybenz-
aldehyde in five steps. However, the total yield of 1
was not given.23,24
In our opinion, of special interest is the observation that
glabruquinone A is structurally more closely related to
the ubiquinones than other linear polyprenyl quinones
from sponges and ascidians. Because glabruquinone A
does not contain a methyl group in the quinoid moiety
in contrast with ubiquinones, 1 can be named desmeth-
ylubiquinone Q2. Glabruquinone A showed cancer
preventive activity in the anchorage-independent
transformation assay against mouse JB6 P+ Cl 41 cells
transformed with epidermal growth factor, inhibiting
the number of colonies with an IC50 (INCC50) of
7.3 lM. Its INCC50 were of 12.7, 17.5, and 50.5 lM
against HCT-116, MEL-28, and HT-460 human tumor
cells, respectively. At 10 lM concentration, 1 increased
the UVB-induced p53-dependent transcriptional activity
of JB6 P+ Cl 41 cells 2.5 times as much. Results of the
studies on the biological activities of 1 will be published
in detail elsewhere.
18. Matsumoto, M.; Kobayashi, H.; Hotta, Y. J. Org. Chem.
1984, 49, 4740–4741.
19. Compound 5: pale yellow oil, IR (CCl4): 3541, 2935, 1498,
1
1464 cmꢀ1; H NMR (250 MHz, CDCl3) d: 6.44 (s, 1H),
5.45 (s, 1H), 5.31 (m, 1H), 5.11 (m, 1H), 3.95 (s, 3H), 3.86
(s, 6H), 3.79 (s, 3H), 3.31 (br d, J = 7.1, 2H), 2.07 (m, 4H),
1.72 (d, J = 1.2, 3H), 1.67 (d, J = 1.2, 3H), 1.60 (d, J = 0.7,
13
3H); C NMR (CDCl3, 62.9 MHz) d: 16.12 (q, C-100),
17.66 (q, C-90), 25.66 (q, C-80), 26.73 (t, C-50), 27.90 (t, C-
10), 39.75 (t, C-40), 56.62 (q, OMe), 60.89 (q, OMe), 61.16
(q, OMe), 108.30 (d, C-6), 121.61 (s, C-5), 121.98 (d, C-20
or C-60), 124.20 (d, C-60 or C-20), 128.89 (s, C-4), 131.41 (s,
C-70), 136.59 (s, C-1), 140.04 (s, C-30), 140.81 (s, C-2 or C-
3), 146.14 (s, C-3 or C-2).
20. Buozbouz, S.; Kirschleger, B. Synthesis 1994, 714–718.
21. Manners, G. D.; Wong, R. J. J. Chem. Soc., Perkin Trans.
1 1981, 1849–1854.
22. Compound 7: yellow oil, IR (CDCl3) 2937, 1491,
;
1435 cmꢀ1 1H NMR (250 MHz, CDCl3) d: 6.61 (d,
J = 9.0), 6.55 (d, J = 9.0, 1H), 5.50 (m, 1H), 5.09 (m,
1H), 4.53 (d, J = 6.6, 2H), 3.91 (s, 3H), 3.90 (s, 3H), 3.82
(s, 3H), 2.08 (m, 4H), 1.60 (d, J = 1.0, 3H), 1.68 (d, J = 1.2,
3H), 1.71 (d, J = 1.2, 3H); 13C NMR (62.9 MHz, CDCl3)
d: 16.62 (q, C-100), 17.71 (q, C-90), 25.69 (q, C-80), 26.33 (t,
C-50), 39.54 (t, C-40), 56.41 (q, OMe), 61.14 (q, OMe),
61.22 (q, OMe), 66.66 (t, C-10), 106.45 (d, C-5 or C-6),
109.05 (d, C-6 or C-5), 120.11 (d, C-20 or C-60), 123.88 (d,
C-60 or C-20), 131.74 (s, C-70), 140.59 (s, C-30), 143.20 (s,
C-1 or C-2, or C-3, or C-4), 144.21 (s, C-2, or C-1, or C-3,
or C-4), 146.94 (s, C-3, or C-1, or C-2 or C-4), 147.94 (s,
C-4, or C-1, or C-2, or C-3). Compound 8: yellow oil, IR
Acknowledgements
We are grateful to B. B. Grebnev for identification of the
ascidian. The research described in this publication was
made possible in part by Grant NS-725.2003.4 of RFBR
and a Grant from the Program ꢀMolecular and cell
biology of RASꢁ.
(CDCl3) 2935, 1489, 1459, 1434, 1415 cmꢀ1 1H NMR
;
References and notes
(250 MHz, CDCl3) d: 6.45 (s, 1H), 5.55 (m, 1H), 5.28 (m,
1H), 5.10 (m, 2H), 4.46 (d, J = 7.1, 2H), 3.93 (s, 3H), 3.87
(s, 3H), 3.81 (s, 3H), 3.33 (d, J = 7.3, 2H), 2.08 (m, 8H),
1.72 (d, J = 1.2, 3H), 1.70 (d, J = 1.2, 3H), 1.69 (d, J = 1.0,
3H), 1.67 (d, J = 1.0, 3H), 1.61 (d, J = 0.7, 3H), 1.60 (d,
J = 0.7, 3H).
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2. Pennock, J. F. In Terpenoids in Plants; Pridham, J. B.,
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