Structure elucidation and chiral-total synthesis of a new indole alkaloid, (-)-9-methoxymitralactonine, isolated from Mitragyna speciosa in Malaysia

Article abstract of DOI:10.1016/S0040-4020(00)00235-0

A new Corynanthe-type indole alkaloid, (-)-9-methoxymitralactonine (1), having a highly conjugated system was isolated from the young leaves of Mitragyna speciosa in Malaysia, and its structure was first deduced by spectroscopic analysis and then confirmed by chiral-total synthesis starting from optically pure epoxy-ketone and 5-methoxy-3,4-dihydro-β-carboline. The chiral HPLC analysis demonstrated that the natural 9-methoxymitralactonine contained predominantly the (-)-enantiomer over the (+)-enantiomer in the ratio of 62:38. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4020(00)00235-0

TETRAHEDRON
Pergamon
Tetrahedron 56 (2000) 3145±3151
Structure Elucidation and Chiral-Total Synthesis of a New Indole
Alkaloid, (2)-9-Methoxymitralactonine, Isolated from
Mitragyna speciosa in Malaysia
a
a
b
a
Hiromitsu Takayama,a, Mika Kurihara, Mariko Kitajima, Ikram M. Said and Norio Aimi
*
aFaculty of Pharmaceutical Sciences, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
bChemistry Department, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia
Received 23 February 2000; accepted 21 March 2000
AbstractÐA new Corynanthe-type indole alkaloid, (2)-9-methoxymitralactonine (1), having a highly conjugated system was isolated from
the young leaves of Mitragyna speciosa in Malaysia, and its structure was ®rst deduced by spectroscopic analysis and then con®rmed by
chiral-total synthesis starting from optically pure epoxy-ketone and 5-methoxy-3,4-dihydro-b-carboline. The chiral HPLC analysis demon-
strated that the natural 9-methoxymitralactonine contained predominantly the (2)-enantiomer over the (1)-enantiomer in the ratio of 62:38.
q 2000 Elsevier Science Ltd. All rights reserved.
Introduction
The new compound 1 was obtained as an orange amorphous
1
powder, exhibiting [a]2D5ˆ2123 (c 0.19, CHCl3). The H
Recently, we have reported the structure elucidation and
total synthesis of a new indole alkaloid, (2)-mitralactonine
(2),1 obtained from the young leaves of Malaysian
Mitragyna speciosa Korth. (Rubiaceae), which is an original
plant for traditional folk medicine in Malay Peninsula, used
for a stimulant like coca or as a substitute for opium.2 By
continuous effort for further investigation of the minor
constituents of this plant,3 we were able to ®nd a new mitra-
lactonine-related alkaloid. In this paper, we describe the
isolation, structure elucidation, chiral-total synthesis, and
analysis of the enantiomeric excess of this new Corynanthe-
type indole alkaloid, (2)-9-methoxymitralactonine (1).
and 13C NMR spectra of 1 showed the presence of a
9-methoxyindole nucleus, an ethane-bridge at C5±C6, an
ethyl group at C-20, and a methoxycarbonyl group, which
are the fundamental structural units in the common
Corynanthe-type monoterpenoid indole alkaloids. The UV
spectrum exhibited a long wavelength absorption at 462 nm,
indicating a long conjugation in the molecule. The 13C NMR
and HMBC spectra disclosed the presence of six conjugated
sp2 carbons including an ester and a lactone carbonyl
carbons, besides the aromatic carbons due to the indole
nucleus. The quite characteristic proton signal observed at
d 6.39 (1H, singlet) was unambiguously assigned to be the
proton at C14 by the HMQC spectrum, and this signal has
the HMBC connectivities between the C2, C3, C15, C16
and C20 carbons. The molecular formula (C22H22O5N2)
obtained from high-resolution mass spectrum, as well as
the fact that the carbon signal at C20 resonated at d
Results and Discussion
From the ethyl acetate extract of the young leaves of M.
speciosa in Malaysia, 10 Corynanthe-type indole
alkaloids, i.e. mitragynine, speciogynine, speciociliatine,
paynantheine, 7a-hydroxy-7H-mitragynine,3a mitragynaline,2b
corynantheidaline, corynantheidine, isocorynoxeine, and
mitralactonine (2) were isolated. In addition, a porphine
derivative, phaeophorbide a,4 was obtained. Furthermore,
a new indole alkaloid (1) could be isolated as a minor
component and its structure was elucidated as follows
(Fig. 1).
Keywords: alkaloids; indoles; asymmetric synthesis.
* Corresponding author. Tel.: 181-43-290-2902; fax: 181-43-290-2902;
e-mail: htakayam@p.chiba-u.ac.jp
Figure 1. RˆOMe: 9-methoxymitralactonine (1); RˆH: mitralactonine (2).
0040±4020/00/$ - see front matter q 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020(00)00235-0

3146
H. Takayama et al. / Tetrahedron 56 (2000) 3145±3151
Scheme 1.
77.4 ppm, showed the presence of a lactone function
constructed between the oxygen atom on C20 and the
carbonyl group at the C17 position. All the above ®ndings,
as well as biogenetic consideration enabled us to compose
the molecular structure of the new alkaloid to be the formula
1 having a highly conjugated pentacyclic 9-methoxy-
Corynanthe-skeleton. In the 13C NMR spectrum, the sp2
carbons at C14 and C16 resonate at unusually high positions
at d 87.08 and 94.91 ppm, respectively. This abnormal
phenomenon, as well as the novel chemical structure of 1,
prompted us to synthesize the new alkaloid in a chiral
manner in order to con®rm the structure including the
absolute con®guration due to one chiral center at the C20
position.
(c 0.35, CHCl3)} was obtained in 97% ee. The enantiomeric
excess of 7 was determined by chiral HPLC analysis of the
p-nitrobenzoate derivatives and the absolute con®guration
was deduced to be (R) by the proposed reaction mecha-
nism.6b Next, the allylic alcohol (7) (97% ee) was subjected
to Sharpless asymmetric epoxidation under kinetic resolu-
tion conditions7 (1.0 equiv. of Ti(O-i-Pr)4, 1.2 equiv. of
diisopropyl d-tartrate, t-BuOOH, 2408C, 40.5 h) to give
the (2)-epoxide (8) {[a]2D3ˆ212.4 (c 0.56, CHCl3)} with
.99% ee. The enantiomeric excess of 8 was also deter-
mined by chiral HPLC analysis of the p-bromobenzoate
derivatives and the absolute con®guration of the quaternary
center was assigned to be (S) by using the well-established
enantioselectivity principle.7b The secondary carbinol in 8
was then converted to a ketone by Swern oxidation to give
the (2)-epoxy-ketone (4) {[a]2D4ˆ258 (c 0.54, CHCl3)}
(Scheme 2).
Our basic approach to 1, which features the assembly of
three fragments, i.e. 5-methoxy-3,4-dihydro-b-carboline
(3), a chiral epoxy-ketone (4), and dimethyl malonate, is
outlined in Scheme 1.
The thus-obtained epoxide (2)-(4) and 5-methoxy-3,4-
dihydro-b-carboline (11), which was prepared from
4-methoxytryptamine (9)8 through N-formylation and
subsequent Bischler±Napieralski reaction, were condensed9
in heated MeOH to afford two diastereomeric tetracyclic
compounds (5a and 5b) in 33 and 17% yields, respectively.
The C3 con®guration of the major and minor product was
deduced by comparison of their CD spectra, as follows. In
general, the Cotton effect at the long-wave region (around
300 nm) of the tetracyclic Corynante-type indole alkaloids
having the C3(S) con®guration exhibits a positive Cotton
First, we started with the preparation of the chiral epoxy-
ketone (4), which is an essential synthon for the construction
of the functionalized tetracyclic compound (5). Synthesis of
optically pure 4 was achieved by a combination of Corey
asymmetric reduction and Sharpless asymmetric epoxida-
tion as follows. By reduction of the enone (6)5 using a chiral
oxazaborolidine catalyst (0.7 equiv. of BH3, 0.2 equiv. of
(S)-5,5-diphenyl-2-methyl-3,4-propano-1,3,2-oxazaborolidine,
2208C),6 an optically active alcohol (1)-(7) {[a]2D3ˆ19.5
Scheme 2.

H. Takayama et al. / Tetrahedron 56 (2000) 3145±3151
3147
1
chromatographic behaviors, UV, H and 13C NMR, and
mass spectra. The observed optical rotation of the synthetic
compound showed levorotation similar to the natural
product; however, the speci®c rotation was very different
{[a]2D4ˆ2838 (c 0.10, CHCl3)} from that of natural (1)
{[a]1D8ˆ2123 (c 0.19, CHCl3)}. Then, we synthesized
racemic 9-methoxymitralactonine (1) starting from the
achiral epoxy-ketone (4) and analyzed the enantiomeric
purity of both the synthetic (^)-(1) and (2)-(1) and the
natural product using chiral column chromatography. As a
result, it was found that the natural 9-methoxymitralacto-
nine contained predominantly the (2)-enantiomer over the
(1)-enantiomer in the ratio of 62:38 (Fig. 2).
Experimental
General
UV: recorded in MeOH. Hitachi U 3400. 1H and 13C NMR
spectra: recorded at 500 and 125.65 MHz, respectively
(ppm, J in Hz with TMS as int. standard). JEOL JNM
A-500. EI-MS: direct probe insertion at 70 eV. JEOL
JMS-AM20. FAB-MS: JEOL JMS-HX110. CD: JASCO
J-720WI. Optical Rotation: JASCO DIP-140. TLC:
precoated Kieselgel 60 F254 plates (Merck, 0.25 mm
thick). Column chromatography: Kieselgel 60 [Merck,
70±230 (for open chromatography) and 230±400 mesh
(for ¯ash chromatography)], Sephadex LH-20 [Pharmacia
Biotech]. MPLC: silica gel prepacked column Kusano CPS-
HS-221-05. Prep. TLC: silica gel 60 GF254 (Merck 7730,
0.5 mm thick).
Figure 2. Chiral HPLC analysis of natural (2)-9-methoxymitralactonine
(1) (conditions: Chiralcel OD; 20% EtOH/n-hexane; ¯ow rate 0.5 mL/min;
column temperature 308C).
curve; on the contrary, that of C3(R) con®guration shows a
negative curve.10 In fact, the major and minor products,
respectively, show the positive and negative Cotton curves
in the long-wave region, indicating that 5a has S and 5b has
R con®guration, respectively, at the C3 position. The major
isomer (5a) was subjected to Knoevenagel condensation
with dimethyl malonate in re¯uxing toluene in the presence
of AcONH4 and AcOH to give directly the pentacyclic
product (12) having a lactone residue in 51% yield. Inter-
estingly, the same product (12) was obtained starting from
the minor isomer (5b), through isomerization at C3 during
condensation under acidic conditions. Finally, the double
bond was introduced to the C3-14 position in 12 by a two-
step operation: (i) t-BuOCl, Et3N, (ii) ethanolic HCl then
NaHCO3 (Scheme 3). The synthetic compound (1) was
identi®ed with the natural product by comparison of their
Extraction and isolation of 9-methoxymitralactonine
Fresh young leaves (315 g) of Mitragyna speciosa Korth.,
collected in Malaysia were moistened with 10% ammonia
water and allowed to stand overnight. They were then
macerated with ethyl acetate three times and ®ltered. The
Scheme 3.

3148
H. Takayama et al. / Tetrahedron 56 (2000) 3145±3151
combined ®ltrate was concentrated under reduced pressure
to give a crude extract (3.25 g). The extract was roughly
separated by silica gel ¯ash column chromatography. The
column was eluted with n-hexane/AcOEt±AcOEt/MeOH
gradient to give the ten fractions. The 60% ± 80% AcOEt/
n-hexane eluent from the ®rst SiO2 column chromatography
was subjected to Sephadex (LH-20) chromatography
(MeOH) to give a fraction containing 9-methoxymitralacto-
nine, which was further puri®ed by SiO2 medium-pressure
liquid chromatography (50% AcOEt/n-hexane) and then by
ODS column chromatography (25% H2O/MeOH) to afford
1.9 mg of 9-methoxymitralactonine (1).
(±CvCH2), 71.09 (±CH(OH)CH3), 24.24 and 22.19
(±CH2CH3 and ±CH(OH)CH3), 12.26 (±CH2CH3). Owing
to the high volatility of 7, mass spectral data including high
resolution MS could not be obtained. Therefore, the mass
spectral data was obtained for the p-nitrobenzoyl derivative
as described below.
To determine the optical purity of 7, two p-nitrobenzoate
derivatives {FAB-MS (NBA) m/z: 250 ([M11]1). HR-
FABMS (NBA): Calcd for C13H16O4N: 250.1079, found:
250.1072} were, respectively, prepared by conventional
method starting from the racemic and chiral alcohols
obtained by the above reaction. They were analyzed by
HPLC using chiral column chromatography (Chiral Cel
OB, Daicel Chemical Industries, Ltd., solvent; 1% iso-
propanol in n-hexane, ¯ow rate; 0.5 mL/min, column
temperature; 308C). The benzoate derived from optically
active 3 exhibited two peaks at 16.4 and 19.0 min in a
ratio of 98.6:1.4 (97% ee).
9-Methoxymitralactonine (1). A dark orange amorphous
powder. Rf value; 0.3 [SiO2, solvent system: AcOEt/n-
hexane 2:1]. [a]2D5ˆ2123 (c 0.19, CHCl3); UV (MeOH)
lmax (log e): 462 (4.51), 435 (sh), 340 (3.72), 267 (sh),
220 (4.21) nm. EIMS m/z (%): 394 (M1, 20), 365 (14),
350 (74), 319 (17), 292 (35), 291 (39), 275 (14), 97 (100).
HR-EIMS: Calcd for C22H22O5N2: 394.1529, found:
1
394.1531. H NMR (500 MHz, CDCl3) d: 8.64 (1H, s,
Preparation of (2)-epoxide (8) by asymmetric epoxida-
tion of (1)-allylic alcohol (7). To a stirred solution of
freshly distilled diisopropyl d-tartrate (7.77 g, 33.2 mmol)
and dry CH2Cl2 (120 mL), freshly distilled Ti(O-i-Pr)4
(8.0 mL, 27.1 mmol) was added under argon atmosphere
and the mixture was cooled to 2408C. To this solution a
mixture of (1)-7 (97% ee, 2.66 g, 26.6 mmol) and dry
CH2Cl2 was added and the resulting mixture was stirred at
2408C for 45 min. A cold solution of t-BuOOH in toluene
(3.5 M, 7.7 mL) diluted with dry CH2Cl2 (10 mL) was
added dropwise to the above reaction mixture for 20 min
and the mixture was stirred at 2408C for 40.5 h under argon
atmosphere. The reaction was quenched by addition of 10%
l-tartaric acid solution (110 mL) at 2408C and the whole
mixture was introduced into a separatory funnel. After
separation of the organic layer, the remaining aqueous
layer was extracted with Et2O three times. The combined
organic layer was washed with water, dried over MgSO4 and
concentrated to ca. 50 mL under 180 mmHg at 238C. After
dilution with Et2O (50 mL) a 1 N NaOH solution (50 mL)
was added and the whole mixture was stirred at 08C for
45 min. The solution was introduced into the separatory
funnel. After separation of the organic layer, the remaining
aqueous layer was extracted with Et2O three times. The
combined organic layer was washed with water, dried
over MgSO4 and concentrated to ca. 150 mL under
200 mmHg at 218C. The residue was passed through a
short column of silica gel and again concentrated to ca.
20 mL under 180 mmHg at 228C. The residue was distilled
under reduced pressure (60±968C, 65 mmHg) to afford
1.71 g (56%) of epoxide 8 as a colorless oil. [a]2D3ˆ212.4
(c 0.56, CHCl3). IR nmax (neat): 3500, 2975 cm21. 1H NMR
(400 MHz, CDCl3) d: 3.94 (1H, dddd, Jˆ6.3, 6.3, 6.3 Hz,
1.7, ±CH(OH)CH3), 2.88 (1H, d, Jˆ4.6 Hz, methylene
proton of oxiran ring), 2.65 (1H, d, Jˆ4.6 Hz, methylene
proton of oxiran ring), 2.08 (1H, br.s, ±CH(OH)CH3), 1.62±
1.83 (2H, m, ±CH2CH3), 1.24 (3H, dd, Jˆ6.3, 0.8 Hz,
±CH(OH)CH3), 0.93 (3H, dd, Jˆ7.6, 7.6 Hz, ±CH2CH3).
13C NMR (125 MHz, CDCl3) d: 65.86, 63.04, 47.84,
23.71, 18.50, 7.97. Owing to the high volatility of 8, mass
spectral data including high resolution MS could not be
obtained. Therefore, the mass spectral data was obtained
for the p-bromobenzoyl derivative as described below.
Na-H), 7.32 (1H, dd, Jˆ8.1, 8.1 Hz, H-11), 6.97 (1H, d,
Jˆ8.1 Hz, H-12), 6.49 (1H, d, Jˆ8.1 Hz, H-10), 6.39 (1H,
s, H-14), 3.93 (3H, s, 9-OCH3), 3.88 (3H, s, 22-OCH3), 3.66
(1H, d, Jˆ12.2 Hz, H-21), 3.56 (1H, d, Jˆ12.2 Hz, H-21),
3.50±3.68 and 3.28±3.41 (4H, m, H2-5 and H2-6), 1.80±
1.94 (2H, m, H2-19), 0.95 (3H, dd, Jˆ7.4, 7.4 Hz, H3-18).
13C NMR (125 MHz, DMSO-d6) d: 169.74 (C-15), 168.61
(C-17), 163.14 (C-22), 154.91 (C-9), 149.24 (C-3), 140.07
(C-13), 126.46 (C-11), 125.52 (C-2), 116.80 (C-7), 115.74
(C-8), 105.17 (C-12), 99.74 (C-10), 94.91 (C-16), 87.08
(C-14), 77.36 (C-20), 55.23 (9-OCH3), 54.86 (C-21),
50.49 (22-OCH3), 50.41 (C-5), 29.60 (C-19), 21.75 (C-6),
7.12 (C-18).
Preparation of (R)-(1)-alcohol (7) by asymmetric reduc-
tion of enone (6). A solution of borane±THF complex
(28 mL, 28 mmol) was added dropwise at room temperature
to a solution of (S)-5,5-diphenyl-2-methyl-3,4-propano-
1,3,2-oxazaborolidine (2.14 g, 7.72 mmol) in dry THF
(40 mL) under argon atmosphere. After stirring at the
same temperature for 40 min, the mixture was cooled to
2208C. To this solution, a cold mixture of 6 (4.16 g,
42.5 mmol) in dry THF (40 mL) was added dropwise for
30 min, then the reaction mixture was stirred for 17.5 h at
2208C. MeOH (14 mL) was added to the reaction mixture
and the resulting solution was gradually warmed to room
temperature. The mixture was then poured into saturated
NH4Cl solution (400 mL) and the whole mixture was
extracted with Et2O four times. The organic layer was
washed with water, dried (MgSO4) and concentrated to ca.
100 mL under 180 mmHg at 208C. The residue was passed
through a short column of silica gel and again concentrated
to ca. 20 mL under 180 mmHg at 308C. The residue was
distilled under reduced pressure (45±878C, 70 mmHg) to
afford 2.77 g (65%) of alcohol 7 as a colorless oil.
[a]2D3ˆ19.5 (c 0.35, CHCl3). IR nmax (neat): 3426,
1
1642 cm21. H NMR (500 MHz, CDCl3) d: 5.04 (1H, dd,
Jˆ1.1, 1.1 Hz, vCH), 4.81 (1H, d, Jˆ1.5 Hz, vCH), 4.28
(1H, ddd, Jˆ6.5, 6.5, 6.5 Hz, ±CH(OH)CH3), 2.02±2.15
(2H, m, ±CH2CH3), 1.30 (3H, d, Jˆ6.5 Hz, ±CH(OH)CH3),
1.08 (3H, dd, Jˆ7.4, 7.4 Hz, ±CH2CH3).13C NMR
(125 MHz, CDCl3) d: 154.91 (±CvCH2), 107.09

H. Takayama et al. / Tetrahedron 56 (2000) 3145±3151
3149
The 1H NMR spectrum of distilled 8 indicated the presence
of a single isomer. The presence of the diastereomer of 8,
i.e. (2R, 3R)-8 and/or (2S, 3S)-8, could not be observed by
comparison with that of the diastereomeric mixture
prepared by m-CPBA oxidation of the racemic alcohol 7.
To determine the optical purity of 8, p-bromobenzoyl
derivative {FABMS (NBA) m/z: 301 ([M13]1), 299
([M11]1). HR-FABMS (NBA): Calcd for C13H16O379Br:
299.0283; found: 299.0288, calcd for C13H16O381Br:
301.0262; found: 301.0250} were prepared by conventional
method. They were analyzed by HPLC using a chiral
column chromatography (Chiral Cel OB, Daicel Chemical
Industries Ltd, solvent; 1% iso-propanol in n-hexane, ¯ow
rate; 0.5 mL/min, column temperature; 308C). The benzoate
derived from optically active 8 obtained by above reaction
exhibited one peak at 26.0 min; on the other hand, the
benzoate derivative containing (2S, 3R)-8 and (2R, 3S)-8
gave two peaks at 20.9 min and 26.0 min, revealing that
the enantiomeric excess of 8 obtained by the Sharpless
epoxidation was .99%.
6.48 (1H, d, Jˆ8.0 Hz, H-6), 3.92 (3H, s, ±OCH3), 3.88
(2H, dd, Jˆ7.6, 7.6 Hz, H2-3), 3.09 (2H, dd, Jˆ7.6,
7.6 Hz, H2-4). 13C NMR (100 MHz, CDCl3) d: 155.51,
151.29, 138.21, 126.97, 125.57, 116.64, 116.20, 104.99,
99.94, 55.23, 48.56, 20.67. EIMS m/z (%): 200 (M1, 100),
199 (92), 184 (24), 149 (26), 105 (48). HR-EIMS: Calcd for
C12H12ON2: 200.0950, found: 200.0959.
Synthesis of the tetracyclic ketone (5a and 5b). A solution
of (2)-epoxide (4) (194 mg, 1.70 mmol) and 5-methoxy-
3,4-dihydro-b-carboline (11) (167 mg, 0.84 mmol) in dry
MeOH (3 mL) was heated under re¯ux for 5 h under
argon atmosphere. The reaction mixture was cooled and
then poured onto the saturated aqueous NaHCO3. The
whole was extracted with CHCl3 three times. The combined
organic layer was washed with brine, dried over MgSO4 and
evaporated. The residue was separated by SiO2 column
chromatography (n-hexane/AcOEt 3:1) to give 5a
(85.3 mg, 33%) and 5b (43.4 mg, 17%). Major isomer 5a
(more polar compound); colorless amorphous powder. UV
(MeOH) lmax (log e): 293 (3.82), 286 (sh, 3.84), 268 (3.96),
225 (4.58) nm. IR nmax (KBr): 3316, 2943, 2700±2840,
Preparation of (2)-ketone (4) by Swern oxidation of car-
binol (8). To a stirred solution of oxalyl chloride (0.75 mL,
8.60 mmol) in dry CH2Cl2 (20 mL) was added dropwise
DMSO (1.2 mL, 16.91 mmol) in dry CH2Cl2 (7 mL) at
2788C under argon atmosphere. After stirring the reaction
1
1721, 1621, 1569, 1510, 1462, 1348, 1257, 1104 cm21. H
NMR (500 MHz, CDCl3) d: 7.74 (1H, br.s, Na-H), 7.06 (1H,
dd, Jˆ7.9, 7.9 Hz, H-11), 6.93 (1H, d, Jˆ7.9 Hz, H-12),
6.49 (1H, d, Jˆ7.9 Hz, H-10), 4.05 (1H, s, ±OH), 3.89
(3H, s, ±OCH3), 3.82 (1H, m, H-3), 3.11±3.20 (2H, each
m, H-5 and H-6), 3.07 (1H, d, Jˆ11.9 Hz, H-21), 3.04±3.09
(1H, m, H-6), 3.03 (1H, dd, Jˆ13.7 Hz, 10.4, H-14), 2.84±
2.91 (1H, m, H-5), 2.77 (1H, dd, Jˆ13.7 Hz, 4.1, H-14),
2.65 (1H, d, Jˆ11.9 Hz, H-21), 1.89±1.97 (1H, m, H-19),
1.62±1.70 (1H, m, H-19), 0.96 (3H, dd, Jˆ7.5, 7.5 Hz,
H-18). 13C NMR (125 MHz, CDCl3) d: 207.92 (C-15),
154.52 (C-9), 137.49 (C-13), 130.24 (C-2), 122.89 (C-11),
117.03 (C-8), 108.47 (C-7), 104.38 (C-12), 100.02 (C-10),
79.38 (C-20), 63.33 (C-21), 58.56 (C-3), 55.24 (-OCH3),
52.05 (C-5), 41.96 (C-14), 25.47 (C-19), 23.16 (C-6), 7.08
(C-18). EIMS m/z (%): 314 (M1, 100), 313 (63), 214 (95),
200 (70), 186 (46). HR-FABMS: Calcd for C18H23O3N2:
315.1709, found: 315.1700. [a]2D4ˆ187.6 (c 0.27, CHCl3).
CD (0.41 mM, MeOH, 268C), lnm (De): 337 (0), 310
(11.3), 296 (0), 291 (sh, 21.2), 284 (sh, 21.4), 267
(22.5), 246 (21.4), 234 (22.6), 230 (0), 219 (112.3),
203 (0). Minor isomer 5b (less polar compound); pale
yellow amorphous powder. UV (MeOH) lmax (log e): 293
(3.90), 282 (sh, 3.91), 266 (4.02), 225 (4.59) nm. IR nmax
(KBr): 3316, 2943, 2700±2830, 1721, 1621, 1510, 1462,
mixture for 30 min,
a solution of (2)-8 (800 mg,
6.90 mmol) in dry CH2Cl2 (12 mL) was added dropwise,
and the reaction mixture was stirred for 1.5 h at 2788C.
Then Et3N (4.8 mL, 34.44 mmol) was added, and the reac-
tion mixture was allowed to warm to room temperature
gradually. Water (25 mL) was added to the reaction
mixture. The organic layer was separated, and the aqueous
layer was extracted with Et2O three times. The combined
organic layer was washed with water, dried over MgSO4 and
concentrated to ca. 20 mL under 200 mmHg at 15±218C.
The residue was distilled under reduced pressure (44±
818C, 82±96 mmHg) to afford 327 mg (41%) of the ketone
4 as a colorless oil. [a]2D4ˆ258 (c 0.54, CHCl3). IR nmax
(neat): 3750, 1704, 1642 cm21. 1H NMR (400 MHz, CDCl3)
d: 2.93 (1H, d, Jˆ4.9 Hz, H-1), 2.89 (1H, d, Jˆ4.9 Hz,
H-1), 2.04 (3H, s, ±COCH3), 2.14, 1.64 (2H, each m,
H2-3), 0.94 (3H, ddd, Jˆ7.5, 7.5 Hz, 0.8, H3-4). 13C NMR
(125 MHz, CHCl3) d: 207.7, 63.29, 50.44, 23.87, 23.00,
8.63.
Preparation of 5-methoxy-3,4-dihydro-b-carboline (11).
To a solution of Nb-formyl-4-methoxytryptamine (10)
(869 mg, 3.99 mmol) (which was prepared by condensation
of 4-methoxytryptamine (9) and formic acid, in CH3CN
(10 mL)) POCl3 (500 mL, 5.36 mmol) was added at 08C
and the reaction mixture was heated under re¯ux for 1 h
under argon atmosphere. The reaction mixture was concen-
trated under reduced pressure and the residue was dissolved
in aqueous 1 N HCl solution. The insoluble material was
®ltered off and the ®ltrate was alkali®ed with aqueous conc.
NH3 solution. By suction ®ltration of the resulting precipi-
tate, 11 (518 mg, 65%) was obtained as yellow amorphous
powder. UV (MeOH) lmax: 349, 317, 249, 220, 208,
205 nm. IR nmax (KBr): 3185, 1623, 1580, 1544, 1519,
1
1348, 1104 cm21. H NMR (500 MHz, CDCl3) d: 7.66
(1H, br.s, Na-H), 7.06 (1H, dd, Jˆ8.1, 8.1 Hz, H-11), 6.93
(1H, d, Jˆ8.1 Hz, H-12), 6.49 (1H, d, Jˆ8.1 Hz, H-10),
3.89 (3H, s, ±OCH3), 3.79 (1H, s, OH), 3.53±3.57 (1H,
m, H-3), 3.22 (1H, d, Jˆ11.3 Hz, H-21), 3.13±3.18 (1H,
m, H-6), 3.08±3.12 (1H, m, H-5), 3.03±3.07 (1H, m,
H-6), 2.82 (1H, dd, Jˆ13.3, 3.2 Hz, H-14), 2.75 (1H, dd,
Jˆ13.3, 11.9 Hz, H-14), 2.64±2.69 (1H, m, H-5), 2.53 (1H,
d, Jˆ11.3 Hz, H-21), 2.21±2.30 (1H, m, H-19), 1.81±1.89
(1H, m, H-19), 0.82 (3H, dd, Jˆ7.5, 7.5 Hz, H-18). 13C
NMR (125 MHz, CDCl3) d: 211.22 (C-15), 154.54 (C-9),
137.51 (C-13), 130.63 (C-2), 122.82 (C-11), 117.08 (C-8),
108.99 (C-7), 104.29 (C-12), 100.01 (C-10), 79.35 (C-20),
65.97 (C-21), 59.74 (C-3), 55.25 (-OCH3), 52.15 (C-5),
42.97 (C-14), 30.67 (C-19), 23.94 (C-6), 6.94 (C-18).
EIMS m/z (%): 314 (M1, 96), 313 (52), 214 (100), 200
1
1425, 1373, 1262, 1174, 1107 cm21. H NMR (400 MHz,
CDCl3) d: 8.55 (1H, br.s, Na-H), 8.33 (1H, br.s, H-1), 7.17
(1H, dd, Jˆ8.0, 8.0 Hz, H-7), 6.97 (1H, d, Jˆ8.0 Hz, H-8),

3150
H. Takayama et al. / Tetrahedron 56 (2000) 3145±3151
(48), 186 (36). HR-FABMS: Calcd for C18H23O3N2:
315.1709, found: 315.1690. [a]2D4 2145 (c 0.35, CHCl3).
CD (0.43 mM, MeOH, 26.08C), lnm (De): 319 (0), 296
(23.7), 287 (sh, 22.7), 279 (sh, 22.0), 270 (0), 258
(12.3), 245 (11.7), 233 (13.5), 229 (0), 220 (211.3),
204 (0).
and evaporated. The residue was separated by SiO2 column
chromatography (n-hexane/AcOEt 1:1) to give (2)-1
(19 mg, 81%). Orange amorphous powder, [a]2D4ˆ2838 (c
0.1, CHCl3). CD (0.58 mM, MeOH, 26.08C), lnm (De): 507
(0), 475 (25.1), 463 (20.1), 427 (25.1), 368 (0), 337
(13.9), 294 (10.6), 261 (18.9), 237 (sh, 1.6), 234 (0),
222 (25.2), 212 (0). The chromatographic behaviors and
UV, 1H and 13C NMR, and mass spectral data were identical
with those of the natural product.
Knoevenagel reaction of 5a. A mixture of 5a (56 mg,
0.18 mmol), dimethyl malonate (330 mL, 2.89 mmol),
AcONH4 (30.5 mg, 0.40 mmol) and AcOH (21 mL,
0.37 mmol) in dry toluene (2.5 mL) was re¯uxed under
argon atmosphere with Dean±Stark apparatus for 3.5 h.
The reaction mixture was cooled and then poured onto the
saturated aqueous NaHCO3. The whole was extracted with
CHCl3 three times. The combined organic layer was washed
with brine, dried over MgSO4 and evaporated. The residue
was separated by SiO2 column chromatography (n-hexane/
AcOEt 2:1) to give 12 (36 mg, 51%). Pale yellow amor-
phous powder. UV (MeOH) lmax (log e): 293 (3.85), 285
(sh, 3.91), 270 (4.02), 226 (4.67) nm. IR nmax (KBr): 3365,
2951, 2700±2840, 1768, 1652, 1511, 1438, 1334,
Chiral HPLC analysis of synthetic (2)-9-methoxymitra-
lactonine, (^)-9-methoxymitralactonine and natural
9-methoxymitralactonine. Three samples were analyzed
by HPLC using a chiral column chromatography (Chiral
Cel OD, Daicel Chemical Industries Ltd, solvent; 20%
EtOH in n-hexane, ¯ow rate; 0.5 mL/min, column tempera-
ture; 308C). The synthetic racemate, which was prepared by
the same procedure described above starting from racemic
epoxy-ketone (4), exhibited two peaks at 23.6 and 25.5 min
in a ratio of 50:50. The synthetic and natural (2)-mitra-
lactonine exhibited corresponding peaks in a ratio of
100:0 and 62:38, respectively.
1
1255 cm21. H NMR (500 MHz, CDCl3): 8.02 (1H, br.s,
Na-H), 7.06 (1H, dd, Jˆ7.9, 7.9 Hz, H-11), 6.93 (1H, d,
Jˆ7.9 Hz, H-12), 6.49 (1H, d, Jˆ7.9 Hz, H-10), 4.12 (1H,
dd, Jˆ12.7, 2.9 Hz, H-14), 3.94 (3H, s, 22-OCH3), 3.89 (3H,
s, 9-OCH3), 3.43 (1H, d, Jˆ10.7 Hz, H-21), 3.37 (1H, br.d,
Jˆ9.4 Hz, H-3), 3.03±3.09 (1H, m, H-5), 3.03±3.15 (2H, m,
H2-6), 2.72 (1H, ddd, Jˆ11.4, 11.4, 4.6 Hz, H-5), 2.49 (1H,
dd, Jˆ12.7, 9.4 Hz, H-14), 2.42 (1H, d, Jˆ10.7 Hz, H-21),
2.16±2.25 and 2.08±2.16 (2H, each m, H-19), 0.81 (3H, dd,
Jˆ7.5, 7.5 Hz, H3-18). 13C NMR (125 MHz, CDCl3) d:
180.47 (C-15), 167.51 (C-17), 162.05 (C-22), 154.54
(C-9), 137.60 (C-13), 130.30 (C-2), 122.90 (C-11), 117.06
(C-16), 116.93 (C-8), 109.10 (C-7), 104.35 (C-12), 99.98
(C-10), 85.67 (C-20), 64.96 (C-21), 59.94 (C-3), 55.24
(9-OCH3), 52.55 (C-5), 52.39 (22-OCH3), 32.36 (C-14),
28.25 (C-19), 23.97 (C-6), 7.05 (C-18). EI-MS m/z (%):
396 (M1, 100), 395 (94), 337 (17), 213 (92). HR-FABMS:
Calcd for C22H24O5N2: 396.1686, found: 396.1667.
[a]2D6ˆ163.7 (c 1.29, CHCl3). CD (0.28 mM, MeOH,
26.08C), lnm (De): 328 (0), 283 (12.1), 264 (11.0), 231
(158.1), 223 (0), 216 (235.4).
Acknowledgements
This work was supported in part by Grant-in-Aid
(Nos. 10557229 and 10044325) for Scienti®c Research
from the Ministry of Education, Science, Sports and
Culture, Japan.
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