Total synthesis and biological evaluation of methylgerambullone

Article abstract of DOI:10.1016/j.bmcl.2009.11.040

First total synthesis of methylgerambullone (MGB, 1) isolated from Glycosmis angustifolia was completed via a convergent route. The effect of MGB on the contractile responses of the isolated guinea-pig ileum induced by acetylcholine was investigated. As a result, it showed a potent relaxation rate (78.66 ± 4.30% at 100 mg/L) in a concentration-dependent manner on longitudinal smooth muscle contraction of isolated guinea-pig ileum induced by 1 μM acetylcholine.

Full text of DOI:10.1016/j.bmcl.2009.11.040

Bioorganic & Medicinal Chemistry Letters 20 (2010) 52–55
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Total synthesis and biological evaluation of methylgerambullone
Jong Taik Moon a, Sung Hoon Ha a, So Hyung Lee a, Tae Hui Kwon a, Chun Rim Oh b,
Young Deuk Kim b, Jungahn Kim a, Dong Joon Choo a, Jae Yeol Lee a,
*
a Research Institute for Basic Sciences and Department of Chemistry, College of Sciences, Kyung Hee University, Seoul 130-701, Republic of Korea
b DongWoo Syntech Co., Ltd, Bondae-Ri Geumwang-Eup, Eumsung-Goon, Choongcheungbook-Do 369-901, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
First total synthesis of methylgerambullone (MGB, 1) isolated from Glycosmis angustifolia was completed
Received 31 August 2009
Revised 10 November 2009
Accepted 11 November 2009
Available online 14 November 2009
via a convergent route. The effect of MGB on the contractile responses of the isolated guinea-pig ileum
induced by acetylcholine was investigated. As
(78.66 4.30% at 100 mg/L) in a concentration-dependent manner on longitudinal smooth muscle con-
traction of isolated guinea-pig ileum induced by 1 M acetylcholine.
Ó 2009 Elsevier Ltd. All rights reserved.
a result, it showed a potent relaxation rate
l
Keywords:
Natural product
Methylgerambullone
Guinea-pig ileum
Relaxation
Methylgerambullone (MGB, 1) and methylisogerambullone
(MIGB) were isolated from the methanolic leaf extract of Glycosmis
angustifolia collected from Dambulla rock in Sri Lanka.1 They have
the same sulfone structures derived from methylthiopropenoic
Synthesis of segment B (3) and segment C (4) is shown in
Scheme 2. N-Methyltyramine HCl salt (3) was obtained in high
yield by indirect monomethylation of tyramine 12 as follows:
t
introduction of Boc group, LiAlH4 reduction and treatment with
´
amideandalsocontainthephenethylaminemoietieswith6-oxoger-
SOCl2/MeOH. For the segment C (4), 1,4-addition of CH3SH (gas)
to propiolic acid 13 by Ikegami method5 afforded an unsatisfactory
result, which showed the stereoisomeric mixture (14 and 15) as
well as the side products derived from further addition of CH3SH
due to the longer reaction time. However, the treatment of CH3SNa
at 100 °C also afforded the mixture of cis and trans-isomers (1:1 ra-
tio),6 which was easily converted to all trans-isomer 15 on reflux in
xylene. The oxidation of 15 with excess of mCPBA readily furnished
the desired segment C (4) as a white solid in quantitative yield.
Segment B (3) was coupled with segment C (4) by using EDC to
give amide 16 in 51% yield, which was also obtained by coupling
reaction of segment B (3) with sulfide 15 and subsequent oxidation
with mCPBA in higher yield as shown in Scheme 3. As a next step,
the general Mitsunobu coupling reaction of segment A (2) with
amide 16 was unexpectedly very sluggish (10% yield in 2 days
reaction time). When ultrasound was applied to the reaction mix-
ture,7 however, the coupling reaction was completely gone within
3 h in 61% yield. As a final step, the deprotection of dithiane group
of 17 with basic condition (EtI, CaCO3, aq acetonitrile)8 resulted in
the cleavage of the corresponding MGB (1) into compounds 18 and
16 as shown in Figure 3.9 In addition, the high temperature condi-
tion (TBHP/MeOH, reflux)10 also afforded the same result. This
result implies that MGB (1) cannot be simply converted into MIGB
via a migration of double bond using basic or equilibrium condi-
tion. Eventually, the removal of dithiane group of 17 using Takano
method (CH3I, CH3CN/H2O, rt)11 successfully afforded MGB (1) in
anyloxy groups in para-position except for a position of a double
bond as shown in Figure 1. Especially, MIGB was also isolated in Kor-
ean white radish root (Raphanus sativus L.) and reported to show
stimulates the small bowel motility through the activation of acetyl-
choline receptors.2 The intriguing structure, the low natural abun-
dance, and the clinical potential of MGB, a constitutional isomer of
MIGB, prompted us to develop a synthesis of MGB. Herein, we report
a first convergent total synthesis and biological evaluation of MGB.
Our synthetic strategy is to divide a MGB molecule into three
segments A, B, and C, as shown in Figure 2. We selected Mitsunobu
reaction as a coupling reaction between the segment A (2) and the
segment B (3), and amide coupling reaction between the segment B
(3) and the segment C (4).
Synthesis of segment A (2) is shown in Scheme 1. Prenol (6) pro-
tected by TBDPS group was regioselectively oxidized using SeO2/
TBHP.3 The reaction mixture containing over-oxidized aldehyde
was reduced with LiAlH4 to afford pure allylic alcohol 7 in overall
54% yield. The exact structure of 7 was confirmed by NOE experi-
ment. Allylic alcohol 7 was easily converted to bromide 9 via tos-
ylate 8. Bromide 9 was coupled with dithiane anion of 104 to
provide 11, which was treated with TBAF to give the desired seg-
ment A (2) in overall 80% yield.
* Corresponding author. Tel.: +82 2 961 0726; fax: +82 2 966 3701?
E-mail address: ljy@khu.ac.kr (J.Y. Lee).
0960-894X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2009.11.040

J. T. Moon et al. / Bioorg. Med. Chem. Lett. 20 (2010) 52–55
53
O O
S
O O
S
N
N
O
O
O
O
O
O
Methylgerambullone (MGB, 1)
Methylisogerambullone (MIGB)
Figure 1. Structures of methylgerambullone (MGB, 1) and methylisogerambullone (MIGB).
O O
N
S
O
O
O
Methylgerambullone (1)
Mitsunobu reaction
amide coupling
O O
NH
HO
S
S
S
OH
HO
O
Segment A (2)
Segment B (3)
Segment C (4)
Figure 2. Synthetic strategy for methylgerambullone (MGB, 1).
b
c
HO
NOE (4.5%)
X
OR
OTBDPS
OTBDPS
8
: X = TsO
5
7
: R = H
a
d
6: R = TBDPS
9: X = Br
S
S
H 10
S
S
e
OR
11: R = TBDPS
f
2
: R = H
Scheme 1. Synthesis of segment A (2). Reagents and conditions: (a) TBDPSCl, imidazole, DCM, overnight; (b) TBHP, SeO2 (cat.), then LiAlH4, À78 °C, 3 h, 54% from 5; (c) p-TsCl,
DMAP, TEA, À20 °C, 2 h; (d) LiBr/ THF, À20 to 0 °C, 4 h; (e) 10, t-BuLi, À78 to À40 °C, 4 h; (f) TBAF, THF, rt, overnight, 80% from 7.
NH2
NH.HCl
a
HO
HO
12 (tyramine)
3 (N-methyltyramine)
O
S
O
OH
O
S
OH
b
d
OH
+
S
O
O
OH
O
13
14
15
4
c
Scheme 2. Synthesis of segment B (3) and segment C (4). Reagents and conditions: (a) (tBoc)2O, DCM, rt; LiAlH4, Dioxane, 0 °C to reflux, 48 h; SOCl2/MeOH, 93% from 12; (b)
CH3SNa, 100 °C, 12 h, then HCl, (c) xylene, reflux, 70%; (d) 15, mCPBA (2.2 equiv), 0 °C to rt, >99%.

54
J. T. Moon et al. / Bioorg. Med. Chem. Lett. 20 (2010) 52–55
O O
S
NH.HCl
N
4
2
a (51%)
O
c
HO
HO
3
16
15, a; b (82%)
O O
N
S
d
S
S
methylgerambullone (1)
O
O
17
Scheme 3. Synthesis of methylgerambullone (MGB, 1). Reagents and conditions: (a) EDC, DMAP, TEA, DMF, 48 h; (b) mCPBA, DCM, À5 °C, 12 h; (c) DIAD, PPh3, THF, rt,
ultrasound, 3 h, 61%; (d) CH3I, CH3CN, H2O, rt, 1 day, 54%.
O O
S
O O
S
N
N
O
O
+
O
O
O
HO
H
1
18
16
Base
Figure 3. Supposed mechanism for cleavage of methylgerambullone (1) under basic condition.
54% without decomposition. The spectroscopic data (1H, 13C NMR,
and HRMS) for synthetic MGB (1) were identical to those of the
authentic natural product: the presence of two conformers due
to restricted rotation about the amide C–N bond (s-cis and
s-trans).1,12
Based on the traditional remedy that radishes help to aid diges-
tion by stimulating the gut and the reported research about the in-
creased effect of MIGB on gastrointestinal contraction and rat
ileum motility,2 we tested the biological effect of MGB (1) on the
acetylcholine-induced contractions in the isolated longitudinal
smooth muscle of guinea-pig ileum using the experimental proto-
col13 as follows: The stable longitudinal smooth muscle of guinea-
In summary, we have successfully achieved the total synthesis
of methylgerambullone (MGB, 1) via a convergent route involving
a coupling of three segments A, B, and C for the first time and
MGB showed the relaxatory effect on guinea-pig isolated ileum
precontracted with acetylcholine in a concentration-dependent
manner. The exact mechanism of relaxatory action of MGB on
the precontracted smooth muscle of guinea-pig ileum is in
progress.
Acknowledgements
This project was supported by a Grant from Graduate School,
Kyung Hee University and a Grant from Korea Foundation for the
Advancement of Science & Creativity (2009-04-448).
pig ileum was prepared and treated with 1 lM of acetylcholine in
order to induce its contraction. After then, the precontracted
smooth muscle of guinea-pig ileum was treated with MGB (1) at
three concentrations, respectively. As shown in Table 1, the pre-
contracted smooth muscle of guinea-pig ileum was relaxed in a
concentration-dependent manner and showed a potent relaxation
rate (78.66 4.30%) at 100 mg/L concentration. Interestingly, this
biological effect was contrary to that of MIGB, which caused a sig-
nificant increase the isolated rat ileum contraction in a concentra-
tion-dependent manner.2 Therefore, it seems that the combination
treatment of MGB and MIGB could strongly stimulate a gastroin-
testinal mobility via complementary action each other and thus
become a potential remedy for dysfunction of gastrointestinal
mobility such as constipation.
References and notes
1. Greger, H.; Hofer, O.; Zechner, G.; Hadacek, F.; Wurz, G. Phytochemistry 1994,
37, 1305.
2. Jeong, S. I.; Lee, S.; Kim, K. J.; Keum, K. S.; Choo, Y. K.; Choi, B. K.; Jung, K. Y. J.
Pharm. Pharmacol. 2005, 57, 1653.
3. Umbreit, M. A.; Sharpless, K. B. J. Am. Chem. Soc. 1977, 99, 5526.
4. Compound 10 was obtained by coupling with prenal and 1,3-propanedithiol in
the presence of BF3ÁOEt2 at À10 °C in 89% yield.
5. Ikegami, F.; Sekine, T.; Duangteraprecha, S.; Matsushita, N.; Matsuda, N.;
Ruangrungsi, N.; Murakoshi, I. Phytochemistry 1989, 28, 881.
6. Cis and trans-isomers were differentiated by chemical shifts and coupling
constants of vinyl protons in 1H NMR as follows: for cis-isomer (14), d 7.20 and
5.87, J = 10.1 Hz; for trans-isomer (15), d 7.88 and 5.66, J = 14.8 Hz.
7. Lepore, S. D.; He, Y. J. Org. Chem. 2003, 68, 8261.
8. Clarke, P. A.; Cridland, A. P. Org. Lett. 2005, 7, 4221.
Table 1
9. (a) Perry, N. B.; Foster, L. M.; Lorimer, S. D.; May, B. C. H.; Weavers, R. T. J. Nat.
Prod. 1996, 59, 729; (b) Baek, S. H.; Perry, N. B.; Weavers, R. T. J. Nat. Prod. 1998,
61, 1143.
Relaxatory effect of methylgerambullone (1) on the isolated smooth muscle of
guinea-pig ileum precontracted with 1
l
M of acetylcholine
10. Barhate, N. B.; Shinde, P. D.; Mahajan, V. A.; Wakharkar, R. D. Tetrahedron Lett.
2002, 43, 6031.
Group
Number of animal (n)
Dose (mg/L)
Relaxation rate (%)
G1
G2
G3
8
8
8
30
60
100
45.31 3.30
69.95 4.18
78.66 4.30
11. Takano, S.; Hatakeyama, S.; Ogasawara, K. J. Chem. Soc., Chem. Commun. 1977,
68.
12. s-cis conformer of methylgerambullone (1): 1H NMR (400 MHz, CDCl3): d 7.31
(1H, d, J = 14.4 Hz), 7.27 (1H, d, J = 14.4 Hz), 7.07 (2H, d, J = 8.8 Hz), 6.79 (2H,
J = 8.8 Hz), 6.07 (1H, t, J = 1.2 Hz), 5.53(1H, m), 4.51 (1H, d, J = 6.4 Hz), 3.59 (2H,
t, J = 7.2 Hz), 3.10 (2H, br s), 2.97 (3H, s), 2.95 (3H, s), 2.78 (2H, t, J = 6.4 Hz),
2.10 (3H, br s), 1.84 (3H, br s), 1.70 (3H, br s); 13C NMR (100 MHz, CDCl3): d
The results were statistically analyzed by Student’s t-test; Data are expressed as
Mean S.E; Relaxations induced by MGB (30–100 mg/L) were significant compared
to equivalent volume of normal saline (P <0.01).

J. T. Moon et al. / Bioorg. Med. Chem. Lett. 20 (2010) 52–55
55
198.13, 162.29, 156.5, 157.5, 139.5, 135.2, 133.3, 130.5, 129.7, 124.3, 122.8,
114.9, 64.7, 55.0, 50.6, 42.5, 36.5, 32.5, 27.7, 20.8, 17.0. s-tran conformer of
methylgerambullone (1): 1H NMR (400 MHz, CDCl3) d 6.69–6.95 (2H, m), 6.85–
6.78 (2H, m), 6.07 (1H, t, J = 1.2 Hz), 5.53 (1H, m), 4.50 (1H, d, J = 6.4 Hz), 3.55
(2H, t, J = 6.4 Hz), 3.10 (2H, br s), 3.00 (3H, s), 2.76 (2H, t, J = 6.4 Hz), 2.10 (3H, br
s), 1.84 (3H, br s), 1.70 (3H, br s); 13C NMR (100 MHz, CDCl3): d 198.11, 162.85,
157.9, 156.5, 138.6, 135.2, 132.4, 130.0, 129.2, 124.2, 122.8, 115.2, 64.7, 55.0,
52.2, 42.5, 34.2, 34.0, 27.7, 20.8, 17.0; HRMS (FAB): m/z calcd for C23H32NO5S
[M+H]+: 434.2001, found 434.2007.
13. Fatehi, M.; Farifteh, F.; Fatehi-Hassanabad, Z. J. Ethnopharmacol. 2004, 91, 321.