Enantioselective synthesis of phyllanthurinolactone, a leaf-closing substance of Phyllanthus urinaria L., and its analogs toward the development of molecular probes

Article abstract of DOI:10.1016/j.tetlet.2004.05.129

We report enantioselective synthesis of phyllanthurinolactone (1), a leaf-closing substance of Phyllanthus urinaria L., and its analogs with sugars other than D-glucose. Structure-activity relationship study using them revealed that the structure of the sugar moiety did not affect their bioactivity at all. This result is very important for the development of molecular probes based on the structure of 1.

Full text of DOI:10.1016/j.tetlet.2004.05.129

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 5885–5888
Enantioselective synthesis of phyllanthurinolactone, a
leaf-closing substance of Phyllanthus urinaria L., and
its analogs toward the development of molecular probes
Yoshifumi Urakawa,a Takanori Sugimoto,b Hirotaka Satob and Minoru Uedab,*
aKeio University, Department of Applied Chemistry, Hiyoshi, Yokomhama 223-8522, Japan
bTohoku University, Department of Chemistry, Aoba-ku, Sendai 980-8578, Japan
Received 12 May 2004; revised 24 May 2004; accepted 27 May 2004
Available online 19 June 2004
Abstract—We report enantioselective synthesis of phyllanthurinolactone (1), a leaf-closing substance of Phyllanthus urinaria L., and
its analogs with sugars other than D-glucose. Structure–activity relationship study using them revealed that the structure of the sugar
moiety did not affect their bioactivity at all. This result is very important for the development of molecular probes based on the
structure of 1.
ꢀ 2004 Elsevier Ltd. All rights reserved.
Most leguminous plants close their leaves in the evening,
as if to sleep, and open them early in the morning
according to the circadian rhythm controlled by the
biological clock.1 Charles Darwin, well-known for his
theory of evolution, was the first to establish the basis of
this field in the 19th century.2 Nyctinastic leaf movement
is induced by the swelling and shrinking of motor cells in
the pulvini, a small organ located in the joint of the leaf
to the stem. Motor cells play a key role in plant leaf-
movement. Flux of potassium ions across the plasma
membranes of the motor cells is followed by massive
water flux, which results in swelling and shrinking of
these cells.3 An issue of great interest is the regulation of
the opening and closing of the potassium channels in-
volved in nyctinastic leaf movement. We have revealed
that nyctinasty is controlled by a pair of leaf-movement
factors: leaf-opening and leaf-closing substances.4
Recently, we revealed that the target cell of the leaf-
opening substance is motor cells,5 which play a key role
in nyctinastic leaf-movement.3 On the other hand, no
attempt was carried out to clarify the target cell of the
leaf-closing substances because the structure of most
leaf-closing substance is so simple that structural mod-
ification toward a molecular probe, such as a fluores-
cence-labeled one, seems to be difficult because large
fluorescence dye or a photoaffinity unit would decrease
the bioactivity of the synthetic probe seriously.
Phyllanthurinolactone (1), a leaf-closing substance of
Phyllanthus urinaria L., is a glycoside-type leaf-closing
substance.6 The structure of 1 is comparatively large
enough for the structure modification essential for a
synthetic probe. The synthesis of 1 was completed by
Audran and Mori in 1998.7 They synthesized all stereo-
isomers on the aglycon moiety of 1 and determined its
absolute stereochemistry. By using these isomers, they
also showed that the stereochemistry of natural 1 is
essential for its bioactivity: No stereoisomer of 1 was
biologically active. However, structure–activity rela-
tionship study on the sugar moiety of 1 was not
undertaken. Thus, we synthesized the analogs of 1 with
sugars other than
D-glucose and studied the structure–
activity relationship, which is an essential data for the
development of biologically active molecular probes,
such as fluorescence-labeled or photoaffinity labeling
probes.
Keywords: Bioactive substance; Plant; Nyctinasty.
* Corresponding author. Tel./fax: +81-222176553; e-mail: ueda@
org.chem.tohoku.ac.jp
0040-4039/$ - see front matter ꢀ 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.05.129

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Y. Urakawa et al. / Tetrahedron Letters 45 (2004) 5885–5888
After deprotection of the benzylidene group, iodine was
introduced to the 6-position of resulting 10 (Scheme 2).
After protection of the 4-position by MOM, the product
was treated with t-BuOK to give olefin 13. Ferrier
cyclization of 13 by catalytic Hg(OCOCF3)2 gave 14.10
Compound 14 was treated by MsCl to give a,b-unsat-
urated ketone 15. a-Hydroxyl group of 15 was epimer-
ized with DBU. Subsequent deprotection of MOM by
using various acids, such as TsOH, TFA, ended in the
recovery of 16. However, 0.1 equiv of AcCl in MeOH at
0 ꢁC gave 17 in 35% yield with recovered 16 in high yield
(64%). When the amount of AcCl was increased to
0.5 equiv, the reaction gave a mixture of decomposed
products. Then, we synthesized 17 by repeated depro-
tection of recovered 16. Next, we examined the acylation
of the resulting free hydroxyl group in 17 for the fol-
lowing intramolecular Hornor–Emmons reaction. But
all attempts to introduce diethyl phosphonoacetic acid
to a-hydroxyl group of 17 ended in failure. Thus, we
introduced bromoacetyl chloride to the hydroxyl group
of 17, and then the resulting 18 was treated with triethyl
phosphite to give 19. Intramolecular Hornor–Emmons
reaction of 19 was examined using various conditions
(Table 1). Mild conditions using DBU gave a deacylated
product (17) or a complex mixture. However, NaH gave
the best result to give 20 in 43% yield. After deprotection
by DDQ, agalycon with naturally occurring stereo-
chemistry (21) was obtained. Glycosidation of 21 was
carried out according to the method reported by Audran
and Mori.7 Coupling of 21 and 2,3,4,6-tetra-O-acetyl-a-
Scheme 1. Synthesis of compound 10.
We planned the asymmetric synthesis of 1 using
D-glu-
cose as a chiral starting material for the aglycon moiety.
Ferrier’s carbocyclization reaction and intramolecular
Horner–Emmons reaction would give the aglycon of 1
with natural absolute stereochemistry.
Methyl a-D-glucopyranoside (3) was protected with a
benzylidene group, and then treated with N-tosylim-
idazole (1.0 equiv) and NaH (1.2 equiv) to give tosylate
(5) (Scheme 1). When this reaction was carried out with
p-toluenesulfonyl chloride (1.2 equiv) and 2 equiv of
NaH, ditosylate (6) was obtained.
Compound 5 was treated with NaH to give epoxide (7).8
Epoxide 7 was then reduced by LiAlH4 according to the
method by Taillefumier and Chapleur,9 and the inversed
20-hydroxyl group was protected by a p-methoxybenzyl
group to give 9.
D
-glucopyranosyl bromide (22) was carried out with Ag2
CO3 and AgOTf to give 23 with recovered 21 (Scheme
3). Acetylated aglycon 24, resulted by nucleophilic at-
tack of the hydroxyl group in 21 to the orthoester
Scheme 2. Synthesis of aglycon 21.
Table 1. Reaction conditions for intramolecular Horner–Emmons reaction of 19
Entry
19 (equiv)
Conditions
Product (Yield %)
111
2
1.0
1.0
1.0
1.0
1.0
LiBr (2.0 equiv), DBU (1.0 equiv), THF, )78 to )30 ꢁC
DBU (1.0 equiv), THF, rt
17 (59%)
Complex mixture
Complex mixture
20 (29%)
3
NaHMDS (1.0 equiv), THF, )78 ꢁC
K2CO3 (1.2 equiv), THF, rt
4
5
NaH (0.5 equiv), THF, 0 ꢁC
20 (42%)

Y. Urakawa et al. / Tetrahedron Letters 45 (2004) 5885–5888
5887
probes based on 1. The syntheses of molecular probes
based on 1 are now in progress.
Acknowledgements
This work was supported by the Ministry of Education,
Science, Sports and Culture (Japan) Grant-in-Aid for
Scientific Research (No 15681013) and JSPS Grant for
T.S. This work was also supported by Grant-in-Aid for
the 21st Century COE program of Tohoku University
from the Ministry of Education, Culture, Sports, Sci-
ence, and Technology, Japan.
Scheme 3. Synthesis of phyllanthurinolactone (1) and its analogs.
References and notes
1. Schildknecht, H. Angew. Chem., Int. Ed. Engl. 1983, 22,
695–710.
2. Darwin, C. The Power of Movement in Plants. Third
Thousand; John Murray: London, 1882.
3. Satter, R. L.; Gorton, H. L.; Vogelmann, T. C. The
Pulvinus: Motor Organ for Leaf Movement, American
Society of Plant Physiologists, 1990.
4. Ueda, M.; Yamamura, S. Angew. Chem., Int. Ed. 2000, 39,
1400–1414.
5. Sugimoto, T.; Wada, Y.; Yamamura, S.; Ueda, M.
Tetrahedron 2001, 57, 9817–9825.
6. Ueda, M.; Shigemori-Suzuki, T.; Yamamura, S. Tetra-
hedron Lett. 1995, 36, 6267–6270.
intermediate from 22, was also obtained in 52% yield as
the main product of this reaction. Resulting 23 was
deprotected with KCN to give 1 (59%) together with its
epimer 257 (37%) as a by-product. The a-proton in 23
would be easily abstracted by KCN and resulting stable
furan-type intermediate gave a mixture of 1 and 25 by
protonation. Physical data of synthetic 1 were com-
pletely identical to that of the naturally occurring 1.12
Fortunately, 24 can also be deacetylated by using KCN
to give aglycon 1, which can be used for coupling
reaction with the sugar unit.
7. Audran, G.; Mori, K. Eur. J. Org. Chem. 1998, 57–62.
8. Hicks, D. R.; Fraser-Reid, B. Synthesis 1974, 203.
9. Taillefumier, C.; Chapleur, Y. Can. J. Chem. 2000, 78,
708–712.
Similarly, we synthesized analogs of 1 with sugars other
than
with 2,3,4,6-tetra-O-acetyl-a-
mide (26) or 2,3,4,6-tetra-O-acetyl-a-
D
-glucose (Scheme 3). Aglycon 21 was coupled
-galactopyranosyl bro-
-glucopyranosyl
D
10. Amano, S.; Takemura, N.; Ohtsuka, M.; Ogawa, S.;
Chida, N. Tetrahedron 1999, 55, 3855–3870.
11. Guerrab, Z.; Daou, B.; Fklih-Tetouani, S.; Ahmar, M.;
Cazes, B. Tetrahedron Lett. 2003, 44, 5727–5730.
L
bromide (27). The coupling products were then depro-
tected with KCN to give galacto-1 (28)13 or -1 (31),14
L
respectively.
1
12. 1: H NMR (400 MHz, D2O, rt): 6.75 (1H, dd, J ¼ 10:0,
2.4 Hz), 6.47 (1H, br d, J ¼ 10:0 Hz), 5.95 (1H, br s,), 5.15
(1H, ddd, J ¼ 13:6, 5.2, 2.0 Hz), 4.85 (1H, dddd, J ¼ 10:4,
5.2, 2.4, 2.0 Hz), 4.69 (1H, d, J ¼ 8:0 Hz), 3.91 (1H, dd,
J ¼ 12:4, 2.4 Hz), 3.72 (1H, dd, J ¼ 12:4, 5.6 Hz), 3.50
(1H, t, J ¼ 8:8 Hz), 3.48 (1H, ddd, J ¼ 8:8, 5.6, 2.4 Hz),
3.39 (1H, t, J ¼ 8:8 Hz), 3.27 (1H, dd, J ¼ 8:8, 8.0 Hz),
3.03 (1H, dt, J ¼ 10:4, 5.2, Hz), 1.78 (1H, dt, J ¼ 13:6,
10.4 Hz); 13C NMR (100 MHz, D2O, rt) 178.3, 167.1,
142.1, 122.1, 112.1, 103.5, 80.9, 77.5, 77.1, 75.7, 74.9, 71.1,
62.6, 39.5 ppm.; HR FAB MS (positive): [M+H]þ Found
Bioassay of synthetic 1, 28, and 31 using leaves of P.
urinaria was carried out. Despite the variety in the
structure of the sugar moiety, all of these analogs were
effective at 1 · 10ꢀ7 M. This result showed that the
structure of the sugar moiety did not affect the bioac-
tivity of 1. Similar result15 was obtained in the case of
potassium isolespedezate (32), a leaf-opening substance
of Cassia mimosoides L.16 In the case of 32, we devel-
oped biologically active molecular probes based on 32
such as a fluorescence-labeled probe or a photoaffinity
labeling probe by the introduction of large fluorescence
dye5 or a large photoaffinity labeling unit17 into the
sugar moiety. Similar design of a molecular probe would
make it possible to develop biologically active molecular
m=z 315.1054, C14H19O8 requires m=z 315.1080; IR (film) m:
25
3388, 1735, 1637, 1585, 1387 cmꢀ1; ½aꢁ )30.0 (c 0.11,
D
21
H2O); ½aꢁ )6.0 (c 0.20, H2O) in naturally occurring 1.
D
13. 28: 1H NMR (400 MHz, D2O, 40 ꢁC): 6.75 (1H, dd,
J ¼ 2:2, 10.3 Hz), 6.47 (1H, d, J ¼ 10:3 Hz), 5.95 (1H, s),
5.14 (1H, ddd, J ¼ 1:8, 4.8, 13.2 Hz), 4.87–4.83 (1H, m),

5888
Y. Urakawa et al. / Tetrahedron Letters 45 (2004) 5885–5888
4.63 (1H, d, J ¼ 7:9 Hz), 3.93 (1H, d, J ¼ 3:3 Hz), 3.80–
m), 3.33 (1H, t, J ¼ 9:3 Hz), 3.21 (1H, dd, J ¼ 7:8, 9.3 Hz),
3.05 (1H, dt, J ¼ 4:6, 10.9 Hz), 1.74 (1H, dt, J ¼ 10:9,
13.2 Hz) ppm; 13C NMR (100 MHz, D2O, 40 ꢁC): 178.6,
166.6, 143.3, 122.5, 112.0, 102.7, 80.9, 78.6, 77.3, 75.5,
75.4, 71.0, 62.3, 38.0 ppm; HR FAB MS (positive):
3.69 (3H, m), 3.66 (1H, dd, J ¼ 3:3, 9.9 Hz), 3.52 (1H, dd,
J ¼ 7:9, 9.9 Hz), 3.04 (1H, dt, J ¼ 4:8, 10.6 Hz), 1.78 (1H,
dt, J ¼ 10:6, 13.2 Hz) ppm; 13C NMR (100 MHz, D2O,
40 ꢁC): 178.5, 167.1, 142.4, 122.2, 112.0, 103.9, 80.9, 76.8,
75.9, 74.3, 72.2, 70.1, 62.4, 39.3 ppm; HR FAB MS
(positive): [M+H]þ found m=z 315.1086, C14H19O8 re-
[M+H]þ found m=z 315.1070, C14H19O8 requires m=z
25
315.1080; IR (film) m: 3354, 1739, 1593 cmꢀ1; ½aꢁ )9.3 (c
D
quires m=z 315.1080; IR (film) m: 3365, 1738, 1591 cmꢀ1
½aꢁ )34.5 (c 0.11, H2O).
;
1.1, H2O).
15. Ueda, M.; Sawai, Y.; Yamamura, S. Tetrahedron Lett.
1999, 40, 3757–3760.
25
D
14. 31: 1H NMR (400 MHz, D2O, 40 ꢁC): 6.75 (1H, dd,
J ¼ 2:5, 9.8 Hz), 6.45 (1H, d, J ¼ 9:8 Hz), 5.95 (1H, s),
5.13 (1H, ddd, J ¼ 1:3, 4.6, 13.2 Hz), 4.90–4.84 (1H, m),
4.71 (1H, d, J ¼ 7:8 Hz), 3.85 (1H, dd, J ¼ 2:5, 12.4 Hz),
3.66 (1H, dd, J ¼ 6:4, 12.4 Hz), 3.43 (1H, m), 3.41 (1H,
16. Shigemori, H.; Sakai, N.; Miyoshi, E.; Shizuri, Y.;
Yamamura, S. Tetrahedron 1990, 46, 383–394.
17. Sugimoto, T.; Fujii, T.; Idutu, Y.; Yamamura, S.; Ueda,
M. Tetrahedron Lett. 2004, 45, 335–338.