Absolute stereochemistry of citrinadins A and B from marine-derived fungus

Article abstract of DOI:10.1021/jo051499o

Citrinadin A (2) is a pentacyclic indolinone alkaloid isolated from the cultured broth of a fungus, Penicillium citrinum, which was separated from a marine red alga. The absolute stereochemistry of the pentacyclic core in 2 and its new congener, citrinadi

Full text of DOI:10.1021/jo051499o

Absolute Stereochemistry of Citrinadins A and B from
Marine-Derived Fungus
Takao Mugishima,† Masashi Tsuda,† Yuu Kasai,† Haruaki Ishiyama,† Eri Fukushi,‡
Jun Kawabata,‡ Masayuki Watanabe,§ Kenichi Akao,§ and Jun’ichi Kobayashi*,†
Graduate School of Pharmaceutical Sciences, Hokkaido University, Sapporo 060-0812, Japan,
Graduate School of Agriculture, Hokkaido University, Sapporo 060-8589, Japan, and Spectrometry
Instrument Division, JASCO Corporation, Hachioji 192-0032, Japan
jkobay@pharm.hokudai.ac.jp
Received July 19, 2005
Citrinadin A (2) is a pentacyclic indolinone alkaloid isolated from the cultured broth of a fungus,
Penicillium citrinum, which was separated from a marine red alga. The absolute stereochemistry
of the pentacyclic core in 2 and its new congener, citrinadin B (1), was elucidated by analysis of
the ROESY spectrum for the chlorohydrin derivative (3) of 1 as well as comparison of the electronic
circular dichroism (ECD) spectra for 1 and 2 with those of known spirooxiindole alkaloids. On the
other hand, the absolute configuration at C-21 bearing an epoxide ring was assigned as S by
comparison of the vibrational circular dichroism (VCD) spectra of 1 with those of model compounds
2S- and 2R-2,3-epoxy-3,3-dimethyl-1-phenylpropan-1-one (4a and 4b, respectively).
Introduction
Marine-derived fungi have proven to been a rich source
of new compounds with high chemical diversity.1 In our
search for new metabolites from marine-derived fungi,2
a novel pentacyclic spiroindolinone alkaloid, citrinadin
A (2), with an N,N-dimethylvaline residue and an R,â-
epoxy carbonyl unit was isolated from the cultured broth
of a fungus, Penicillium citrinum N059 strain, which was
separated from a marine red alga.3 The gross structure
and relative stereochemistry for the pentacylic core have
been elucidated by 2D NMR data, whereas the absolute
configurations remained unsolved. Recently, we have
isolated a new citrinadin congener, citrinadin B (1), from
the same strain and elucidated the absolute stereochem-
* To whom correspondence should be addressed. Phone: +81 11 706
4985. Fax: +81 11 706 4989.
† Graduate School of Pharmaceutical Sciences, Hokkaido University.
‡ Graduate School of Agriculture, Hokkaido University.
§ JASCO Corp.
(1) Bugni, T. S.; Ireland, C. M. Nat. Prod. Rep. 2004, 21, 143-163
and references therein.
istry of citrinadins A (2) and B (1) on the basis of NMR,
ECD, and VCD4 data.
(2) (a) Kasai, Y.; Komatsu, K.; Shigemori, H.; Tsuda, M.; Mikami,
Y.; Kobayashi, J. J. Nat. Prod. 2005, 68, 777-779. (b) Shigemori, H.;
Kasai, Y.; Komatsu, K.; Tsuda, M.; Mikami, Y.; Kobayashi, J. Marine
Drugs 2004, 2, 164-169.
(3) Tsuda, M.; Kasai, Y.; Komatsu, K.; Sone, T.; Tanaka, M.; Mikami,
Y.; Kobayashi, J. Org. Lett. 2004, 6, 3087-3089.
Results and Discussion
Isolation and Structure of Citrinadin B (1). The
fungus P. citrinum (strain N059) was separated from a
10.1021/jo051499o CCC: $30.25 © 2005 American Chemical Society
Published on Web 10/07/2005
9430
J. Org. Chem. 2005, 70, 9430-9435

Absolute Stereochemistry of Citrinadins A and B
TABLE 1. 1H and 13C NMR Data (ppm) of Citrinadin B
(1) in CDCl3
multiplicity,
position
13C
1H
J (Hz)
1
9.61
s
2
185.3
60.5
3
3a
4
134.6
133.3
122.5
127.6
117.5
142.6
41.7
7.64
7.16
7.75
d, 7.4
dd, 7.4, 8.0
d, 8.0
5
6
7
7a
8
FIGURE 1. Selected 2D NMR data for citrinadin B (1).
spectrum suggested the presence of OH/NH (3390 and
3301 cm-1) and carbonyl group(s) (1701 and 1671 cm-1).
The UV absorption at 333 nm (ꢀ 3100) was attributed to
a conjugated benzenoid chromophore. The 13C NMR
(Table 1) spectrum disclosed the existence of two car-
bonyls (δC 194.8 and 185.3), three sp2 quaternary carbons
(δC 142.6, 134.6, and 117.5), three sp2 methines (δC 133.3,
127.6, and 122.5), five sp3 quaternary carbons (δC 82.4,
68.0, 61.6, 60.5, and 51.0), three sp3 methines (δC 64.0,
58.2, and 52.5), six sp3 methylenes (δC 51.1, 41.7, 31.7,
29.0, 28.7, and 17.3), and six methyls (δC 30.4, 27.5, 24.3,
21.7, 18.6, and 12.0). Since 5 out of 11 unsaturations were
accounted for, 1 was inferred to contain six rings. The
1H NMR (Table 1) spectrum of 1 disclosed proton signals
due to three D2O-exchangeable ones (δH 10.6, 9.61, and
5.25), three benzenoid methines (δH 7.75, 7.64, and 7.16),
five singlet methyls (δH 2.45, 1.57, 1.36, 1.25, and 0.97),
and a doublet methyl (δH 1.38), which was similar to that
of citrinadin A (2), except for the lack of signals due to
the N,N-dimethylvalyloxy group and a methine signal at
C-14 observed for 2.
R
2.17
2.10
d, 13.9
d, 13.9
â
9
10
68.0
51.1
R
â
3.62
3.18
10.6
d, 11.5
d, 11.5
11
12
13
brs
58.2
28.7
3.61
2.95
1.60
1.70
1.66
2.50
1.66
3.71
2.18
1.66
m
R
â
R
â
R
â
m
m
14
15
17.3
29.0
m
m
m
m
16
17
52.5
31.7
brt, 11.4
R
â
m
m
18
82.4
18-OH
19
5.25
brs
51.0
194.8
64.0
61.6
24.3
18.6
20
21
4.02
s
22
23
1.57
1.25
s
s
24
25
The structure of citrinadin B (1) was elucidated to be
a 14-des(N,N-dimethyl)valyloxy form of 2 on the basis of
spectroscopic data including 2D NMR data such as the
1H-1H COSY, ROESY, and HMBC spectra (Figure 1).
The 1H-1H COSY, TOCSY, and HSQC spectra revealed
three connectivities from C-4 to C-6, from C-12 to C-17
and C-27, and from N-25 to C-26. The presence of a
7-substituted 3-spiroindolinone ring (C-1-C-7a) was sug-
gested by HMBC correlations as follows: NH-1/C-2, NH-
1/C-3, NH-1/C-3a, NH-1/C-7a, and H-4/C-3, H-5/C-3a,
H-5/C-7, and H-6/C-7a. HMBC correlations for H-6/C-20,
H-21/C-20, H3-23/C-21, H3-23/C-22, H3-24/C-21, and H3-
24/C-22 indicated the existence of a 2,3-epoxy-3-methyl-
1-oxobutyl side chain (C-20-C-24) at C-7. A cyclopenta-
[b]quinolizidine moiety (N-11, C-3, and C-8-C-19) was
revealed by HMBC correlations as follows: H-8/C-3, H-8/
C-9, H-8/C-18, H-8/C-19, H-10/C-9, H-10/C-16, H-10/C-
18, H-17/C-9, H3-28/C-18, H3-28/C-19, H3-29/C-3, and H3-
29/C-19. HMBC correlations for a D2O-exchangeable
proton (OH-18) at δH 5.25 to C-17 and C-18 indicated that
a hydroxyl group was attached to C-18. It was revealed
that an N-methylamino group (N-25-C-26) was con-
nected to C-9, since the HMBC correlation was observed
for H3-26/C-9. HMBC correlations for H2-8/C-3 and H3-
29/C-3 indicated that the cyclopenta[b]quinolizidine moi-
ety and the indolinone ring were connected to each other
through the spiro carbon (C-3). Therefore, the gross
structure of citrinadin B was concluded to be 1.
26
30.4
12.0
21.7
27.5
2.45
1.38
0.97
1.36
s
27
d, 6.7
28
s
s
29
marine red alga, Actinotrichia fragilis, collected at Hedo
Cape, Okinawa Island, and grown in PYG liquid medium
containing seawater for 14 days at 25 °C. The mycelium
(258 g) of the culture broth (12 L) was extracted with
MeOH. The extract was partitioned between hexane and
90% aqueous MeOH, and the MeOH-soluble portion was
extracted with n-BuOH. The n-BuOH-soluble portions
were subjected to LH-20 and SiO2 column chromatogra-
phies to afford citrinadin B (1, 4.1 mg, 0.0016%, wet
weight) together with citrinadin A (2, 6.7 mg) and a
known mycotoxin, citrinin.5
Citrinadin B {1, [R]20D +8° (c 1.0, MeOH)} showed the
pseudomolecular ion peak at m/z 482 in the FABMS, and
the molecular formula was revealed to be C28H40O4N3 by
HRFABMS [m/z 482.3022, (M + H)+, 0.4 mmu]. The IR
(4) (a) Sugeta, H. Jasco Report 1991, 33, 99-107. (b) Dukor, R. K.;
Nafie, L. A. In Encyclopedia of Analytical Chemistry: Instumentation
and Aplications; Meyer, R. A., Ed.; John Wiley and Sons: Chichester,
U.K., 2000; pp 662-676. (c) Nafie, L. A.; Freedman, T. B. In Infrared
and Raman Spectroscopy of Biological Materials; Gremlich, H.-U., Yan,
B., Eds.; Marcel Dekker: New York, 2000; pp 15-54. (d) Keiderling,
T. A. In Circular Dichroism: Principles and Applications, 2nd ed.;
Berova, N., Nakanishi, K., Woody, R. W., Eds.; Wiley-VCH: New York,
2000; pp 621-666. (e) Kuppens, T.; Bultinck, P.; Langenaeker, W. Drug
Discovery Today: Technologies 2004, 1, 269-275. (f) Taniguchi, T.;
Miura, N.; Nishimura, S.; Monde, K. Mol. Nutri. Food Res. 2004, 48,
246-254.
The relative stereochemistry of the pentacyclic core in
citrinadin B (1) was elucidated on the basis of ROESY
1
data and H-1H coupling constants (Figure 2). ROESY
(5) Turner, W. B. In Fungal Metabolites; Academic Press: London,
1971; p 121.
correlations for H-4/H3-26 and H-4/H3-29 indicated that
J. Org. Chem, Vol. 70, No. 23, 2005 9431

Mugishima et al.
SCHEME 1
FIGURE 2. Selected ROESY correlations and relative stereo-
chemistry for citrinadin B (1).
configuration. Therefore, the absolute configurations at
six chiral centers, C-3, C-9, C-12, C-14, C-16, and C-18,
in citrinadin A (2) were assigned as S, S, R, R, S, and R,
respectively. Citrinadin B (1) showed a similar Cotton
curve to that of 2, thus indicating that the absolute
configurations at C-3, C-9, C-12, C-16, and C-18 in 1 were
S, S, R, S, and R, respectively.
FIGURE 3. Selected ROESY correlations for chlorohydrin
derivative (3) of citrinadin A (2).
Absolute Stereochemistry of C-21 in Citrinadin
A (2). As described above, the absolute configuration at
C-21 remained unsolved. To determine the absolute
configuration at C-21, model compounds (4a and 4b) for
the 2,3-epoxy-3-methyl-1-oxobutyl side chain in 1 and 2
were synthesized from benzaldehyde 5 as shown in
Scheme 1. Grignard reaction of 5 with isobutenylmag-
nesium bromide afforded aryl alcohol, which was con-
verted into an aryl ketone 6 by oxidation with manganese-
(IV) oxide. Compound 6 was subjected to asymmetric
dihydroxylation9 with AD-mix-R to give a diol 7a in a 58%
yield and 95% ee. Enantiomer excesses for all chiral
synthetic products were determined by chiral HPLC
analysis. Tosylation of the diol 7a gave 8a, which was
treated with potassium carbonate to afford 2S-2,3-epoxy-
3,3-dimethyl-1-phenylpropan-1-one (4a) in a 26% yield
and 71% ee in five steps total. By a manner similar to
that described above, 2R-2,3-epoxy-3,3-dimethyl-1-phen-
ylpropan-1-one (4b) was prepared in the 40% yield and
90% ee from compound 6 using AD-mix-â.
one (C-29) of two methyl groups at C-19, the C-3-C-3a
bond, and the methylamino group (C-25-C-26) were
R-oriented. On the other hand, ROESY correlations for
H-10â/H3-27, H-10â/OH-18, H-16/H3-27, and OH-18/H3-
28 and proton signal patterns for H-16 (brt, J ) 11.4 Hz)
suggested â-axial orientations for H-10â, H-16, OH-18,
and H3-27. Since ROESY correlations were observed for
H-10R/H-12R, H-12R/H-13â, and H-17â/H3-28, H-10R,
H-12R, H-13â, and H-17â were considered to have
equatorial orientations. Therefore, the relative configu-
ration of the cyclopenta[b]quinolizidine moiety in 1 was
elucidated to be anti/syn/anti and chair forms for the two
six-membered rings.
Absolute Stereochemistry of the Pentacyclic Core
in Citrinadins A (2) and B (1). To determine the
absolute stereochemistry of six stereogenic centers in the
pentacyclic core of citrinadin A (2), ROESY correlations
from protons of the L-N,N-dimethylvalyl group to those
of the pentacylic core in 2 were analyzed. Measurement
of the ROESY spectrum was performed using the chlo-
rohydrin derivative (3) of citrinadin A (2), which was
obtained by treatment of 2 with 50 mM HCl in MeOH.
In the ROESY spectrum of 3, ROE correlations were
observed for H-15â/H3-6′ and H3-27/H3-8′ (Figure 3).
Considering the S-configuration at C-2′, the absolute
stereochmistry at C-14 was elucidated to be R. The ECD
spectrum of citrinadin A (2) showed the negative first
Cotton effect at λext 340 nm (∆ꢀ -1.5), which was longer
in wavelength by conjugation with a carbonyl group than
those (λext ca. 280 nm) of usual spirooxindole alkaloids.6-8
This suggested that the spiro carbon at C-3 had S-
The VCD spectra of compounds 4a and 4b showed
weak Cotton effects at 1230 cm-1 and were the mirror
(6) ECD spectra of uncarines C and F possessing 3R-configuration
showed the positive first Cotton effect, while those of uncarines D and
E possessing 3S-configuration disclosed the negative first Cotton effect.
Beecham, A. F.; Hart, N. K.; Johns, S. R.; Lamberton, J. A. Aust. J.
Chem. 1968, 21, 491-504.
(7) (a) Pousset, J.-L.; Poisson, J.; Legrand, N. Tetrahedron Lett.
1966, 6283-6289. (b) Pousset, J.-L.; Poisson, J.; Shine, R. J.; Shamma,
M. Bull. Soc. Chim. Fr. 1967, 2766-2779.
(8) Takayama, H.; Shimazu, T.; Sada, H.; Harada, Y.; Kitajima, M.;
Aimi, N. Tetrahedron 1999, 55, 6841-6846.
(9) Sharpless, K. B.; Amberg, W.; Bennani, Y. L.; Crispino, G. A.;
Hartung, J.; Jeong, K.-S.; Kwong, H.-L.; Morikawa, K.; Wang, Z.-M.;
Zu, D.; Zhang, X.-L. J. Org. Chem. 1992, 57, 2768-2771.
9432 J. Org. Chem., Vol. 70, No. 23, 2005

Absolute Stereochemistry of Citrinadins A and B
FIGURE 5. Two plausible biogenetic precursors (a and b) of
citrinadins. Diketopiperazine and dipeptide portions and the
isoprene unit are shown in magenta, green, and blue.
fungi of the genuses Penicillium or Aspergillus, the
pentacyclic skeleton of 1 is unique. The citrinadin
skeleton may be generated by loss of amide carbonyl
carbon (C-18) from the marcfortine-type skeleton, such
as a, in which C-15 and C-20 are oxidized and C-17 is
methylated (Figure 5). Alternatively, a dipeptide-like
precursor such as b may be converted into citrinadins.
Although some alkaloids with an epoxy isoprene unit at
C-4 position of a indole ring have been isolated from
Penicillium spp.,15 alkaloids possessing those at C-7
position of a indole ring such as 1 and 2 were rare.16
Citrinadin B (1) showed modest cytotoxicity against
murine leukemia L1210 cells (IC50, 10 µg/mL).
FIGURE 4. IR (upper) and VCD (lower) spectra of 1) model
compounds 4a (green) and 4b (blue) and 2) citrinadin A (2).
image of each other, although undurations were observed
for the baselines, probably due to a limitation of the
instrument. Compound 4a possessed a negative Cotton
effect for the absorption at 1230 cm-1 due to symmetrical
ring expansion of an epoxide, while compound 4b showed
a positive Cotton effect at 1230 cm-1 (Figure 4). The
Cotton curve at 1245 cm-1 for citrinadin A (2) revealed
the negative sign, thus indicating that the absolute
configuration at C-21 was S. Although the VCD spectrum
of citrinadin B (1) could not be measured, due to the small
amount of sample, citrinadin B (1) probably possessed
the same absolute configuration at C-21 as that of 2, since
the ECD spectra of 1 and 2 were similar.
Plausible Biosynthetic Pathway and Bioactivity.
Citrinadins A (2) and B (1) belong to a novel class of
pentacyclic spiroindolinone alkaloids with an epoxy iso-
prene unit for 1 and 2 and an N,N-dimethylvaline residue
for 2. Although several spiroindolinone alkaloids such as
brevianamides,10 paraherquamides,11 marcfortines,12 scle-
rotamide,13 and asperparalins14 have been isolated from
Experimental Section
Citrinadin B (1): pale yellow solid; [R]20 +8° (c 1.0,
D
MeOH); UV (MeOH) λmax 333 (ꢀ 3100), 248 (9800), 223 (7600),
and 203 nm (12 500); IR (neat) νmax 3390, 3301, 1702, and 1671
1
cm-1; H and 13C NMR (Table 1); FABMS m/z 482 (M + H)+;
HRFABMS m/z 482.3022 [(M + H)+, calcd for C28H40O4N3,
482.3018].
Chlorohydrin (3) of Citrinadin A (2). A solution of
citrinadin A (2, 0.5 mg) in 50 mM HCl/MeOH (1 mL) was
stirred at room temperature for 30 min. After evaporation of
the solvent, chlorohydrin (3) was afforded as a pale yellow
solid: IR (film) νmax 3323, 2926, 1736, 1701 and 1601 cm-1
;
1H NMR (500 MHz, CDCl3) δH 13.02 (1H, brs, H-1′), 11.41 (1H,
brs, H-11), 9.67 (1H, brs, H-1), 7.87 (1H, d, J ) 8.0 Hz, H-6),
(10) (a) Birch, A. J.; Wright, J. J. J. Chem. Soc. D 1969, 644-645.
(b) Birth, A. J.; Wright, J. J. Tetrahedron 1970, 26, 2329-2344. (c)
Birch, A. J.; Russell, R. A. Tetrahedron 1972, 28, 2999-3008.
(11) (a) Yamazaki, M.; Okuyama, E.; Kobayashi, M.; Inoue, H.
Tetrahedron Lett. 1981, 22, 135-136. (b) Ondeyka, J. G.; Goegelman,
R. T.; Schaeffer, J. M.; Keleman, L.; Zitano, L. J. Antibiot. 1990, 43,
1375-1379. (c) Blanchflower, S. E.; Banks, R. M.; Everett, J. R.;
Manger, B. R.; Reading, C. J. Antibiot. 1991, 44, 492-497. (d)
Blanchflower, S. E.; Banks, R. M.; Everett, J. R.; Reading, C. J.
Antibiot. 1993, 46, 1355-1363.
(14) (a) Hayashi, H.; Nishimoto, Y.; Nozaki, H. Tetrahedron Lett.
1997, 38, 5655-5658. (b) Hayashi, H.; Nishimoto, Y.; Akiyama, K.;
Nozaki, H. Biosci. Biotechnol. Biochem. 2000, 64, 111-115.
(15) (a) Numata, A.; Takahashi, C.; Ito, Y.; Takada, T.; Kawai, K.;
Usami, Y.; Matsumura, E.; Imachi, M.; Ito, T.; Hasegawa, T. Tetra-
hedron Lett. 1993, 34, 2355-2358. (b) Jadulco, R.; Edrada, R. A.; Ebel,
R.; Berg, A.; Schaumann, K.; Wray, V.; Steube, K.; Proksch, P. J. Nat.
Prod. 2004, 67, 78-81. (c) Hayashi, H.; Matsumoto, H.; Akiyama, K.
Biosci. Biotechnol. Biochem. 2004, 68, 753-756. (d) Dalsgaard, P. W.;
Blunt, J. W.; Munro, M. H. G.; Frisvad, J. C.; Christophersen, C. J.
Nat. Prod. 2005, 68, 258-261.
(12) (a) Polonsky, J.; Merrien, M.-A.; Prange´, T.; Pascard, C.;
Moreau, S. J. Chem. Soc., Chem. Commun. 1980, 601-602. (b) Prange´,
T.; Billion, M.-A.; Vuilhorgne, M.; Pascard, C.; Polonsky, J.; Moreau,
S. Tetrahedron Lett. 1981, 22, 1977-1980.
(16) Dom´ınguez, X. A.; Cano, G.; Luna, I.; Dieck, A. Phytochemistry
1977, 16, 1096.
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J. Org. Chem, Vol. 70, No. 23, 2005 9433

Mugishima et al.
7.67 (1H, d, J ) 7.4 Hz, H-4), 7.20 (1H, dd, J ) 7.4 and 8.0
Hz, H-5), 5.44 (1H, brs, H-14), 5.12 (1H, s, H-21), 4.03 (1H, m,
H-16), 3.79 (1H, m, H-10R), 3.67 (2H, m, H-12 and H-2′), 3.50
(1H, m, H-13R), 3.25 (1H, m, H-10â), 3.16 (1H, m, H-15R), 2.99
(3H, brs, H3-7′), 2.91 (3H, brs, H3-8′), 2.53 (3H, s, H3-26), 2.41
(1H, m, H-4′), 2.22 (2H, m, H2-8), 2.18 (1H, m, H-17R), 2.02
(1H, m, H-15â), 1.89 (1H, m, H-13â), 1.79 (1H, m, H-17â), 1.70
(3H, s, H3-23), 1.61 (1H, s, H3-24), 1.58 (3H, d, J ) 6.7 Hz,
H3-27), 1.44 (3H, s, H3-29), 1.38 (3H, d, J ) 6.5 Hz, H3-6′),
1.08 (3H, d, J ) 6.5 Hz, H3-5), and 1.03 (3H, s, H3-29); ESIMS
m/z 661 (M + H)+; HRESIMS m/z 661.3721 [(M + H)+, calcd
for C35H53N4O635Cl, 661.3732].
3-Methyl-1-phenylbut-2-en-1-one (6). A solution of 1-bro-
mo-2-methylpropene (7.60 g, 56.5 mmol) in THF (5 mL) was
added slowly to a suspension of magnesium (1.35 g, 56.3 mmol)
in THF (10 mL). This solution was kept at reflux by heating
for 1 h, and then THF (10 mL) was added to this mixture after
magnesium disappeared. After the mixture was cooled in an
ice bath, a solution of benzaldehyde (2.09 g, 19.7 mmol) in THF
(2 mL) was added dropwise to the mixture, and stirring was
continued for 1 h. The reaction mixture was poured onto
saturated aqueous NH4Cl, and the mixture was extracted with
Et2O. The organic layer was washed with water and brine,
dried over MgSO4, and evaporated. The crude product was
subjected to silica gel column chromatography (hexane/EtOAc,
3:1) to afford an aryl alcohol (3.19 g, 19.6 mmol, 99%) as a
pale yellow oil: UV (MeOH) λmax 216 (sh ꢀ 9000) and 205
(13 200) nm; IR (neat) νmax 3349 and 1027 cm-1; 1H NMR (400
MHz, CDCl3) δ 7.40-7.32 (4H, m), 7.26 (1H, tt, J ) 2.0 and
7.3 Hz), 5.44 (1H, d, J ) 9.1 Hz), 5.41 (1H,m), 1.80 (3H, s),
and 1.76 (3H, s); 13C NMR (100 MHz, CDCl3) δ 144.1, 134.8,
128.2, 127.6, 127.0, 125.7, 70.6, 25.8, and 18.2; EIMS m/z 162
(M+); HREIMS m/z 162.1044 (M+, calcd for C11H14O, 162.1044).
tion at 270 nm). Major and minor constituents of 7a were
found at tR 20.8 and 21.8 min, respectively.
(2S)-2,3-Dihydroxy-3-methyl-1-phenylbutan-1-one (7b).
Compound 6 (300 mg, 1.9 mmol) was treated with AD-mix-â
(3.7 g) by the same procedure as described above to afford
compound 7b (205.6 mg, 1.1 mmol, 57%, 95% ee) as colorless
solid: [R]21D +51° (c 1.0, CHCl3); UV (MeOH) λmax 247 (ꢀ 7900)
and 203 nm (1200); IR (film) νmax 3535, 3339, 1673, and 1089
1
cm-1; H NMR (400 MHz, CDCl3) δ 7.93 (2H, d, J ) 8.0 Hz),
7.63 (1H, t, J ) 7.6 Hz), 7.50 (2H, brt, J ) 8.0 Hz), 4.95 (1H,
d, J ) 7.2 Hz), 1.17 (3H, s), and 1.14 (3H, s); 13C NMR (100
MHz, CDCl3) δ 201.9, 136.2, 134.0, 128.9, 128.7, 78.1, 72.7,
26.4, and 25.9; FABMS m/z 217 (M + Na)+; HRFABMS m/z
217.0844 [(M + Na)+, calcd for C11H14O3Na, 217.0841].
The enantiomeric excess was determined by the same
method as described above. Major and minor constituents of
7b were found at tR 21.8 and 20.8 min, respectively.
(2R)-3-Hydroxy-3-methyl-1-phenyl-2-(toluenesulfonyl-
oxy)butan-1-one (8a). Toluenesulfonyl chloride (504.0 mg,
2.62 mmol) was added to a solution of diol 7a (173.3 mg, 0.89
mmol) in pyridine (7 mL), and stirring was continued at room
temperature for 6 h. After addition of saturated aqueous
CuSO4, the mixture was extracted with CHCl3, and the organic
layer was washed with water and brine, dried over MgSO4,
and evaporated. The residue was subjected to silica gel column
chromatography (hexane/EtOAc, 4:1) to afford compound 8a
(210.4 mg, 0.60 mmol, 68%, 73% ee) as pale yellow oil: [R]21
D
-37° (c 1.0, CHCl3); UV (MeOH) λmax 251 (ꢀ 8500), 228 (11 600),
and 203 nm (20 100); IR (neat) νmax 3524, 1687, 1363, and 1181
1
cm-1; H NMR (400 MHz, CDCl3) δ 7.87 (2H, d, J ) 8.5 Hz),
7.67 (2H, d, J ) 8.5 Hz), 7.57 (1H, t, J ) 7.6 Hz), 7.43 (2H,t,
J ) 7.6 Hz), 7.19 (2H, d, J ) 8.5 Hz), 5.56 (1H, s), 2.37 (3H,
s), 1.28 (3H, s), and 1.26 (3H, s); 13C NMR (100 MHz, CDCl3)
δ 195.1, 145.1, 136.1, 133.7, 132.6, 129.6, 129.0, 128.5, 128.0,
83.6, 72.0, 26.6, 26.4, and 21.6; FABMS m/z 349 (M + H)+;
HRFABMS m/z 349.1111 [(M + H)+, calcd for C18H21O5S,
349.1110].
To a stirred solution of aryl alcohol (0.6 g, 3.70 mmol) in
CH2Cl2 (20 mL) was added MnO2 (5.0 g, 57.1 mmol). The
reaction mixture was stirred at room temperature for 3 h. After
filtration and then evaporation of the solvent, a residue was
subjected to silica gel column chromatography (hexane/EtOAc,
8:1) to give compound 6 (477.6 mg, 2.98 mmol, 81%) as
colorless liquid: UV (MeOH) λmax 260 (ꢀ 12 600) and 203 nm
The enantiomeric excess was determined by the same
method as described above. Major and minor constituents of
8a were found at tR 30.6 and 28.8 min, respectively.
(2S)-3-Hydroxy-3-methyl-1-phenyl-2-(toluenesulfonyl-
oxy)butan-1-one (8b). Compound 7b (200.1 mg, 1.03 mmol)
was treated with toluenesulfonyl chloride (3.7 g) by the same
procedure as described above to afford compound 8b (212.3
mg, 0.61 mmol, 59%, 92% ee) as pale yellow oil: [R]21D +33° (c
1.0, CHCl3); UV (MeOH) λmax 251 (ꢀ 8500), 228 (11 600), and
203 nm (20 100); IR (neat) νmax 3521, 1687, 1363, and 1181
(10 200); IR (neat) νmax 1662 and 1615 cm-1 1H NMR (400
;
MHz, CDCl3) δ 7.93 (2H, d, J ) 7.2 Hz), 7.52 (1H, t, J ) 7.2
Hz), 7.43 (2H, t, J ) 7.2 Hz), 6.75 (1H, s), 2.22 (3H, s), and
2.03 (3H, s); 13C NMR (100 MHz, CDCl3) δ 190.7, 156.4, 138.8,
131.8, 128.0, 127.7, 120.7, 27.6, and 20.8; EIMS m/z 160 (M+);
HREIMS m/z 160.0886 (M+, calcd for C11H12O, 160.0888).
(2R)-2,3-Dihydroxy-3-methyl-1-phenylbutan-1-one (7a).
To a suspension of AD-mix-R (3.1 g) in t-BuOH/H2O (1:1, 8
mL) were added potassium osmate dihydrate (6.0 mg, 16
µmol), NaHCO3 (392 mg, 4.7 mmol), and methanesulfonamide
(155 mg, 1.6 mmol) at room temperature, and the mixture was
stirred for 10 min. A solution of compound 6 (249 mg, 1.6
mmol) in t-BuOH/H2O (1:1, 2 mL) was added to this mixture
at 4 °C, and stirring was continued at 4 °C for 15 h. After
addition of Na2SO3 (4.0 g, 31.7 mmol), the reaction mixture
was stirred at room temperature for 1 h. After addition of
CHCl3 (20 mL), the reaction mixture was filtrated. The filtrate
was washed with water and brine, dried over MgSO4, and
evaporated. The residue was subjected to silica gel column
chromatography (CHCl3/EtOAc, 95:5) to give compound 7a
1
cm-1; H NMR (400 MHz, CDCl3) δ 7.87 (2H, d, J ) 8.5 Hz),
7.67 (2H, d, J ) 8.5 Hz), 7.57 (1H, t, J ) 7.6 Hz), 7.43 (2H,t,
J ) 7.6 Hz), 7.19 (2H, d, J ) 8.5 Hz), 5.56 (1H, s), 2.37 (3H,
s), 1.28 (3H, s), and 1.26 (3H, s); 13C NMR (100 MHz, CDCl3)
δ 195.1, 145.1, 136.1, 133.7, 132.6, 129.6, 128.9, 128.5, 128.0,
83.6, 72.0, 26.5, 26.4, and 21.6; FABMS m/z 349 (M + H)+;
HRFABMS m/z 349.1101 [(M + H)+, calcd for C18H21O5S,
349.1110].
The enantiomeric excess was determined by the same
method as described above. Major and minor constituents of
8b were found at tR 28.8 and 30.6 min, respectively.
(2S)-2,3-Epoxy-3-methyl-1-phenylbutan-1-one (4a). To
a solution of tosylate 8a (200.4 mg, 0.58 mmol) in MeOH (2.2
mL) was added K2CO3 (100 mg), and stirring was continued
at 0 °C for 3 h. After filtration, the filtrate was evaporated in
vacuo. The crude product was purified by silica gel column
chromatography (hexane/EtOAc, 4:1) to afford compound 4a
(82.7 mg, 0.47 mmol, 82% yield, 71%ee) as colorless solid:
(249.0 mg, 0.90 mmol, 58%, 95% ee) as a colorless solid; [R]21
D
-58° (c 1.0, CHCl3); UV (MeOH) λmax 247 (ꢀ 7900) and 203 nm
1
(12 000); IR (film) νmax 3534, 3339, 1672, and 1087 cm-1; H
NMR (400 MHz, CDCl3) δ 7.92 (2H, d, J ) 8.0 Hz), 7.60 (1H,
t, J ) 7.6 Hz), 7.47 (2H, brt, J ) 8.0 Hz), 4.94 (1H, s), 1.16
(3H, s), and 1.12 (3H, s); 13C NMR (100 MHz, CDCl3) δ 201.9,
136.2, 133.9, 128.9, 128.7, 78.1, 72.7, 26.4, and 25.9; FABMS
m/z 217 (M + Na)+; HRFABMS m/z 217.0849 [(M + Na)+, calcd
for C11H14O3Na, 217.0841].
The enantiomeric excess was determined by HPLC analysis
using a CHIRALCEL OD column (Dicel Chemical, 0.46 × 250
mm; 2-propanol/hexane, 1:9; flow rate 0.5 mL/min; UV detec-
[R]17 -11° (c 1.0, CHCl3); UV (MeOH) λmax 248 (ꢀ 8500) and
D
1
203 nm (12300); IR (film) νmax 1691 and 1231 cm-1; H NMR
(400 MHz, CDCl3) δ 7.98 (2H, d, J ) 8.5 Hz), 7.62 (1H, t, J )
7.3 Hz), 7.50 (2H, t, J ) 7.3 Hz), 4.04 (1H, s), 1.60 (3H, s), and
1.25 (3H, s); 13C NMR (100 MHz, CDCl3) δ 194.1, 135.7, 133.7,
128.8, 128.1, 64.5, 61.2, 24.4, and 18.6; EIMS m/z 176 (M+);
HREIMS m/z 176.0833 (M+, calcd for C11H12O2,176.0829).
9434 J. Org. Chem., Vol. 70, No. 23, 2005

Absolute Stereochemistry of Citrinadins A and B
The enantiomeric excess was determined by the same
method as described above. Major and minor constituents of
4a were found at tR 12.2 and 14.0 min, respectively.
Acknowledgment. We thank M. Kiuchi and S. Oka,
Center for Instrumental Analysis, Hokkaido University,
for mass spectra measurements, and M. Iha, Tropical
Technology Center, for his help with red alga collection.
This work was partly supported by a Grant-in-Aid for
Scientific Research from the Ministry of Education,
Culture, Sports, Science, and Technology of Japan.
(2R)-2,3-Epoxy-3-methyl-1-phenylbutan-1-one (4b). To-
sylate 8b (202.3 mg, 0.58 mmol) was treated with AD-mix-â
(3.7 g) by the same procedure as described above to afford
compound 4b (72.5 mg, 0.41 mmol, 71%, 90% ee) as colorless
solid: [R]17D +12° (c 1.0, CHCl3); UV (MeOH) λmax 248 (ꢀ 8500)
and 204 nm (12300); IR (film) νmax 1691 and 1231 cm-1 1H
;
NMR (400 MHz, CDCl3) δ 7.97 (2H, d, J ) 8.5 Hz), 7.62 (1H,
t, J ) 7.3 Hz), 7.51 (2H, t, J ) 7.3 Hz), 4.04 (1H, s), 1.60 (3H,
s), and 1.25 (3H, s); 13C NMR (100 MHz, CDCl3) δ 194.1, 135.7,
133.7, 128.8, 128.1, 64.5, 61.2, 24.4, and 18.6; EIMS m/z 176
(M+); HREIMS m/z 176.0832 (M+, calcd for C11H12O2,176.0829).
The enantiomeric excess was determined by the same
method as described above. Major and minor constituents of
4b were found at tR 14.0 and 12.2 min, respectively.
Supporting Information Available: Spectral data for
citrinadin B (1), 4a, 4b, 6, 7a, 7b, 8a, and 8b. This material
is available free of charge via the Internet at http://pubs.acs.org.
JO051499O
J. Org. Chem, Vol. 70, No. 23, 2005 9435