A facile and convergent synthesis of sarmentosin

Article abstract of DOI:10.1081/SCC-120023993

A concise synthesis of sarmentosin (1) starting from butane-1,2,4-triol-1,2-acetonide is described. This convergent route can also be employed for the preparation of sarmentosin analogues for structure-activity relationship studies.

Full text of DOI:10.1081/SCC-120023993

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SYNTHETIC COMMUNICATIONSÕ
Vol. 33, No. 19, pp. 3355–3363, 2003
A Facile and Convergent Synthesis
of Sarmentosin
*
Rui Xu and Donglu Bai
Shanghai Institute of Materia Medica, Shanghai
Institutes for Biological Sciences, Chinese Academy of
Sciences, Shanghai, China
ABSTRACT
A concise synthesis of sarmentosin (1) starting from butane-1,2,4-
triol-1,2-acetonide is described. This convergent route can also
be employed for the preparation of sarmentosin analogues for
structure-activity relationship studies.
Key Words: Sarmentosin; Synthesis; Glycosylation.
*Correspondence: Donglu Bai, Shanghai Institute of Materia Medica, Shanghai
Institutes for Biological Sciences, Chinese Academy of Sciences, 555 Zuchongzhi
Road, Shanghai 201203, China; E-mail: dlbai@mail.shcnc.ac.cn.
3355
DOI: 10.1081/SCC-120023993
Copyright & 2003 by Marcel Dekker, Inc.
0039-7911 (Print); 1532-2432 (Online)
www.dekker.com

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3356
Xu and Bai
INTRODUCTION
Sarmentosin (1) is the active principle isolated from the folk medicine,
Sedum Sarmentosum Bunge, which is used for treatment of hepatitis in
China.[1] Clinical trials showed that sarmentosin had a significant effect
on lowering serum glutamate-pyruvate transaminase (SGPT) level of
patients suffering from chronic virus hepatitis.[2] It was later found that
sarmentosin also had immunomodulating activity.[3]
The cyano-ethylene b-glucoside structure of 1 was elucidated by
spectral and chemical methods.[2] It is know that cyanogenic glycosides
are readily hydrolyzed in aqueous media, so sarmentosin is chemically
rather unstable. The content of sarmentosin in plant varies greatly with
the growing region and the collecting season. Although the structure of
sarmentosin seems rather simple, the reports on the synthesis of sarmen-
tosin and its analogues are limited due to the instability of the aglycon.
We have previously reported the first synthesis of sarmentosin.[4] The
aglycon instability problem was avoided by attaching the glucose com-
ponent first to the precursor of aglycon and then converting intermediate
2 to sarmentosin by functional group transformations (Fig. 1a). The
overall yield of sarmentosin obtained by this method was low. In addi-
tion, we supposed that the aglycon was the pharmacophoric group of
sarmentosin. Thus, it is desirable to develop a convergent synthetic
approach to this target molecule, and we can also utilize it to prepare
analogues for further structure-activity relationship studies. Herein, we
report a new synthesis of sarmentosin, in which the protected aglycon 3
was successfully prepared, and 3 was then condensed with a glycosyl
donor to give the target compound (Fig. 1b).
Figure 1. Retrosynthetic analysis of sarmentosin.

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Facile and Convergent Synthesis of Sarmentosin
RESULTS AND DISCUSSION
3357
Our synthesis started from butane-1,2,4-triol-1,2-acetonide (4),
which was easily prepared from commercially available malic acid in 2
steps.[5] Alcohol 4 was first protected as p-methoxy benzyl (PMB) ether 5.
Ketal hydrolysis of acetonide 5 followed by selective protection of the
primary alcohol with t-butyldiphenylsilyl chloride (TBDPSCl) afforded 6.
The secondary alcohol in 6 was oxidized with Dess-Martin periodinane
to give keone 7. Conversion of 7 into cyanohydrin 8 by treatment with
acetone cyanohydrin in the presence of Et3N, followed by dehydration
with thionyl chloride in pyridine yielded the desired E-olefin 9 as a single
isomer. PMB protecting group in 9 was removed by DDQ to give the
protected aglycon 10 in 90% yield. Glycosylation between 10 and the
trichloroacetimidate 11[6,7] in the presence of 1.2 equiv. of boron trifluor-
ide etherate furnished the desired b-D-glucoside 12 stereoselectively. The
TBDPS group in 12 was removed with TBAF-HOAc to afford 13 in 81%
yield. Finally, deacetylation of 13 with MeOH–Et3N–H2O (8:1:1) led to
sarmentosin in 72% yield (Sch. 1).
Selection of protecting groups for C1 and C4 hydroxy groups in 9
was critical to the success of this synthetic route. Initially we tried to use
acetyl or TBDPS group to protect 4-hydroxy. However, all attempts to
remove these protecting groups failed because of the instability of the
aglycon. Considering that the aglycon is very sensitive to acids and bases,
we finally chose PMB as the protecting group, which was easily removed
under neutral condition by DDQ oxidation to give the key intermediate
10 in 90% yield.
For protection of 1-hydroxy group, the bulky trityl was used at
the beginning for the E-selectivity of the dehydration step. However,
glycosylation between 14 and 11 under Schmidt condition[8] afforded
only the orthoester 16. No desired product 15 was obtained (Sch. 2).
The glycosylation involved an acetoxonium intermediate 17[9]
which was formed via the C2 neighboring group participation (Fig. 2).
Either the glycosyl acceptor could attack C1 to give the desired
b-glucoside or attack the oxonium to give the orthoester. In the case
mentioned above, the bulky aglycon 14 could only attack from the less
steric hindrance face to obtain the kinetic product 16 (Fig. 2). To solve
this problem, excess Lewis acid could be added to make the orthoester
rearranged to the more stable thermodynamic product 15. Therefore,
1.0 equiv. of Lewis acid was used. However, a complex mixture was
obtained because the trityl group could not survive under this condition.
TBDPS group was then chosen as 1-hydroxy protecting group for
its stability under strong Lewis acid condition. This bulky group

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3358
Xu and Bai
Scheme 1. Reagents and conditions: (a) PMBC(¼NH)CCl3, CSA (cat.),
CH2Cl2, rt, 12 h, 91%; (b) i) TsOH, MeOH, rt, 2h; ii) TBDPSCl, imidazole,
CH2Cl2, rt, 30 min, 87%; (c) Dess-Martin periodinane, pyridine, CH2Cl2, rt,
10 h, 89%; (d) acetone cyanohydrin, Et3N, MeOH, rt, 2 h; (e) SOCl2, pyridine,
0ꢀC, 2 h, rt, 4 d, 32%; (f ) DDQ, CHCl3, H2O, rt, 2 h, 90%; (g) Boron trifluoride
etherate (1.2 equiv.), molecular sieve (4 A), CH2Cl2, À20ꢀC, 1 h, 54%; (h) TBAF,
AcOH, THF, rt, 30 min, 81% (i) Et3N, MeOH, H2O, rt, 5 h, 72%.
Scheme 2. Reagents and conditions: Me3SiOTf (0.1 equiv.), molecular sieve
(4 A), CH2Cl2, À40ꢀC, 10 min.

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Facile and Convergent Synthesis of Sarmentosin
3359
Figure 2.
also played an important role in controlling the olefin configuration in
dehydration of cyanohydrin 8.
This convergent synthetic approach to sarmentosin has proved to be
more facile for the preparation of various analogues of sarmentosin.
EXPERIMENTAL
General Procedures
Melting points were measured on a Buchi510 apparatus and were
uncorrected. Elemental analyses were performed on a Carlo-Erba 1106
instrument. Infrared spectra were obtained on a Nicolet Magna 750
spectrometer. NMR spectra were measured on Bruker AMX-400 or
Gemini-300 MHz spectrometers for 1H and 100 or 75 MHz spectrometers
for 13C, respectively, with tetramethylsilane as internal standard.
Chemical shifts are reported in ꢀ (ppm) and coupling constants in Hz.
Specific rotations were measured on a Perkin-Elmer 241 MC. Mass
spectra were determined on a Varian MAT-711 mass spectrometer.
4-[2-(4-Methoxy-benzyloxy)ethyl]-2,2-dimethyl-[1,3]dioxolane (5). To
a
stirred solution of butane-1,2,4-triol-1,2-acetonide (4) (4.10 g,
28.1 mmol) in CH2Cl2 (60 mL) was added p-methoxybenzyl trichloroace-
timidate (8.60 mL, 41.2 mmol) and a catalytic amount of camphorsulfo-
nic acid (0.37 g). After stirring for 12 h at room temperature, the reaction
mixture was diluted with ether (300 mL), washed with saturated aqueous
NaHCO3, water and brine and dried. Concentration followed by purifi-
cation of the residue by column chromatography on silica gel (petroleum
ether:ether, 15:1) afforded 5 (6.83 g, 91%) as a colorless oil. IR (ꢁ, cmÀ1
,
film): 1612, 1514, 1369, 1302, 1248, 1094. 1H NMR (300 MHz, CDCl3): ꢀ
7.24 (2H, d, J ¼ 8.5 Hz), 6.86 (2H, d, J ¼ 8.5 Hz), 4.41 (2H, s), 4.19 (1H,
m), 4.03 (1H, m), 3.79 (3H, s), 3.53 (3H, m), 1.88 (2H, m), 1.38 (3H, s),

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3360
Xu and Bai
1.34 (3H, s). EI-MS (m/z): 266 (Mþ, 1), 251 (1), 199 (12), 135 (24), 121
(100). Anal. calcd. C15H22O4: C, 67.65; H, 8.33. Found: C, 67.37; H, 8.41.
1-(tert-Butyldiphenylsilyloxy)-4-(4-methoxy-benzyloxy)-butan-2-ol (6).
To a solution of acetonide 5 (1.00 g, 3.76 mmol) in methanol (15 mL)
was added p-toluenesulfonic acid monohydrate (0.075 g, 0.40 mmol).
The mixture was stirred at room temperature for 2 h, before NaHCO3
(0.04 g) was added. The solution was concentrated and ethyl acetate
was added to the residue. The mixture was filtered and the filtrate
concentrated. The crude diol thus obtained was dissolved in CH2Cl2
(25 mL). To the solution was added imidazole (0.56 g, 8.4 mmol) and
tert-butylchlorodiphenylsilane (1.10 mL, 4.26 mmol) at 0ꢀC. The mixture
was stirred at 0ꢀC for 30 min and then diluted with ether (75 mL),
washed with water (10 mL) and dried. After concentration, the residue
was purified by flash chromatography on silica gel (petroleum ether:-
ethyl acetate, 12:1) to give 6 (1.51 g, 87% for two steps) as a colorless
oil. IR (ꢁ, cmÀ1, film): 3481, 1612, 1587, 1514, 1427, 1248, 1113.
1H NMR (300 MHz, CDCl3): ꢀ 7.3–7.7 (10H, m), 7.20 (2H, d,
J ¼ 8.5 Hz), 6.84 (2H, d, J ¼ 8.5 Hz), 4.41 (2H, s), 3.89 (1H, m), 3.78
(3H, s), 3.58 (4H, m), 1.74 (2H, m), 1.04 (9H, s). EI-MS (m/z): 199 (50),
149 (60), 121 (100). Anal. calcd. C28H36O4Si: C, 72.38; H, 7.81. Found:
C, 72.14; H, 7.94.
1-(tert-Butyldiphenylsilyloxy)-4-(4-methoxy-benzyloxy)-butan-2-one
(7). To a stirred solution of alcohol 6 (1.41 g, 3.04 mmol) in CH2Cl2
(30 mL) was added pyridine (1.9 mL) and Dess-Martin periodinane
(1.65 g, 3.89 mmol). After stirring at room temperature for 10 h,
ether (150 mL), saturated aqueous NaHCO3 solution (30 mL) and satu-
rated aqueous Na2S2O3 solution was added successively to the reaction
mixture. The organic layer was separated. The aqueous phase was
extracted with ether. The combined organic layers were washed with
brine, dried and concentrated. The residue was purified by column
chromatography on silica gel (petroleum ether:ethyl acetate, 12:1) to
give 7 (1.25 g, 89%) as a colorless oil. IR (ꢁ, cmÀ1, film): 1736, 1612,
1
1514, 1427, 1248, 1113, 1036. H NMR (300 MHz, CDCl3): ꢀ 7.3–7.7
(10H, m), 7.22 (2H, d, J ¼ 8.5 Hz), 6.84 (2H, d, J ¼ 8.5 Hz), 4.39 (2H,
s), 4.18 (2H, s), 3.78 (3H, s), 3.69 (3H, t, J ¼ 6.3 Hz), 2.78 (3H, t,
J ¼ 6.3 Hz), 1.07 (9H, s). EI-MS (m/z): 405 (MþÀ57, 3), 327 (3), 199
(17), 121 (100). Anal. calcd. C28H34O4Si: C, 72.69; H, 7.41. Found: C,
72.37; H, 7.53.
(E )-1-(tert-Butyldiphenylsilyloxymethyl)-4-(4-methoxy-benzyloxy)-2-
butenenitrile (9). To a solution of ketone 7 (1.17 g, 2.53 mmol) in metha-
nol (35 mL) was added triethyl amine (0.49 mL, 3.55 mmol) and acetone
cyanohydrin (1.7 mL, 18 mmol). The reaction mixture was stirred at

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Facile and Convergent Synthesis of Sarmentosin
3361
room temperature for 2 h and then concentrated in vacuo. The crude
cyanohydrin 8 was dissolved in pyridine (15 mL). Thionyl chloride
(0.41 mL, 5.6 mmol) was added to this solution at 0ꢀC. The reaction
mixture was stirred at 0ꢀC for 2 h and then at room temperature for
4 days. The mixture was diluted with ethyl acetate and washed with
water, saturated aqueous CuSO4 solution, water and brine successively.
The organic layer was dried and concentrated. The residue was purified
by column chromatography on silica gel (petroleum ether:ethyl acetate,
20:1 to 15:1) to give the olefin 9 (0.38 g, 32% for two steps) as a colorless
1
oil. IR (ꢁ, cmÀ1, film): 2222, 1612, 1514, 1427, 1250, 1113. H NMR
(300 MHz, CDCl3): ꢀ 7.3–7.7 (10H, m), 7.24 (2H, d, J ¼ 8.5 Hz), 6.88
(2H, d, J ¼ 8.5 Hz), 6.55 (1H, t, J ¼ 6.3 Hz), 4.45 (2H, s), 4.28 (2H, d,
J ¼ 6.3 Hz), 4.22 (2H, d, J ¼ 1.6 Hz), 3.78 (3H, s), 1.08 (9H, s). Anal.
calcd. C29H33NO3Si: C, 73.85; H, 7.05; N, 2.97. Found: C, 73.38; H,
7.09; N, 2.49.
(E )-1-(tert-Butyldiphenylsilyloxymethyl)-4-hydroxy-2-butenenitrile
(10). To a solution of PMB ether 9 (0.220 g, 0.467 mmol) in CHCl3
(20 mL) and water (1 mL) was added DDQ (0.310 g, 1.37 mmol). After
stirred at room temperature for 2 h, the reaction mixture was diluted with
ethyl acetate (120 mL) and washed with water, 0.5% aqueous NaHCO3
solution, water and brine successively. The organic layer was dried and
concentrated. The residue was purified by column chromatography on
silica gel (petroleum ether:ethyl acetate, 8:1) to give 10 (0.148 g, 90%) as a
colorless oil. The aglycon 10 was not stable and was carried forward to
1
the next step immediately. H NMR (300 MHz, CDCl3): ꢀ 7.3–7.7 (10H,
m), 6.53 (1H, t, J ¼ 6.3 Hz), 4.43 (2H, d, J ¼ 6.3 Hz), 4.23 (2H, d,
J ¼ 1.5 Hz), 1.08 (9H, s).
(E )-1-(tert-Butyldiphenylsilyloxy)-2-cyano-4-b-D-tetraacetylglucopyr-
anosyloxy-2-butene (12). A suspension of the aglycon 10 (141 mg,
0.40 mmol), the glycosyl donor trichloroacetimidate 11 (390 mg,
0.79 mmol) and 4 A molecular sieve (1.2 g) in CH2Cl2 (3 mL) was stirred
at room temperature for 30 min. The reaction mixture was cooled to
À20ꢀC, and boron trifluoride etherate (0.060 mL, 0.48 mmol) was added
dropwise. After stirred at À20ꢀC for 1 h, the reaction was quenched by
triethyl amine (2 drops). The reaction mixture was filtered, and the filtrate
was concentrated in vacuo. The residue was purified by column chro-
matography on silica gel (petroleum ether:ethyl acetate, 4:1) to give 12
20
(147 mg, 54%) as a colorless oil. ½ꢂꢁD À4.2 (c 0.59, CHCl3). IR (ꢁ, cmÀ1
,
film): 2234, 1757, 1429, 1367, 1223, 1051. 1H NMR (400 MHz, CDCl3): ꢀ
7.3–7.7 (10H, m), 6.50 (1H, t, J ¼ 6.3 Hz), 5.17 (1H, t, J ¼ 9.5 Hz), 5.07
(1H, t, J ¼ 9.8 Hz), 5.00 (1H, dd, J ¼ 9.1, 8.0 Hz), 4.45–4.55 (3H, m),
4.20–4.30 (3H, m), 4.13 (1H, dd, J ¼ 12.4, 1.5 Hz), 3.70 (1H, m), 2.06

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Xu and Bai
(3H, s), 2.01 (3H, s), 2.00 (3H, s), 1.99 (3H, s), 1.06 (9H, s). FAB-MS (m/z):
681 (Mþþ1, 78), 656 (6), 625 (8), 604 (11), 185 (100), 133 (54).
(E)-2-cyano-4-b-D-tetraacetylglucopyranosyloxy-2-buten-1-ol (13). To
a solution of TBDPS ether 12 (95 mg, 0.14 mmol) in THF (3 mL) was
added acetic acid (0.040 mL) and tert-butyl ammonium fluoride (90 mg,
0.28 mmol). The reaction mixture was stirred at room temperature for
30 min. Saturated aqueous NH4Cl solution (10 mL) was added. The
reaction mixture was extracted with ether. The combined organic
layers were washed with brine, dried, and concentrated. The residue
was purified by column chromatography on silica gel (petroleum
ether:ether 1:10) to give 13 (50 mg, 81%) as a white solid. M.p.
20
90–92þC; ½ꢂꢁD À4.8 (c 0.74, CHCl3); IR (ꢁ, cmÀ1, KBr): 3500, 2220,
1
1763, 1379, 1252, 1213, 1045. H NMR (400 MHz, CDCl3): ꢀ 6.54 (1H,
t, J ¼ 6.3 Hz), 5.20 (1H, t, J ¼ 9.5 Hz), 5.08 (1H, t, J ¼ 9.5 Hz), 4.98
(1H, dd, J ¼ 9.4, 8.0 Hz), 4.57 (1H, d ¼ 7.8 Hz), 4.52 (2H, d ¼ 6.5 Hz),
4.32 (1H, dd, J ¼ 12.4, 2.3 Hz), 4.24 (2H, s), 4.14 (1H, dd, J ¼ 12.4,
4.1 Hz), 3.69 (1H, m), 2.10 (3H, s), 2.05 (3H, s), 2.02 (3H, s), 2.00 (3H,
s). 13C NMR (100 MHz, CDCl3): ꢀ 171.0, 170.2, 169.4 ꢂ 2, 142.6, 116.8,
115.3, 100.8, 72.7, 72.2, 71.1, 68.2, 67.6, 62.5, 61.4, 20.8, 20.7, 20.5 ꢂ 2.
EI-MS (m/z): 443 (Mþ), 383 (4), 346 (7), 323 (6), 243 (23), 200 (37),
169 (38), 157 (78), 145 (55), 140 (45), 115 (100), 98 (83). Anal.
calcd. C19H25NO11: C, 51.46; H, 5.68; N, 3.16. Found: C, 51.30; H,
5.66; N, 3.14.
Sarmentosin (1)
A solution of compound 13 (40 mg, 0.09 mmol) in a mixture of
MeOH-Et3N-H2O (8:1:1, 5 mL) was stirred at room temperature for 5 h.
After evaporation of the solvent in vacuo, toluene was added to the residue
and removed again in vacuo. This process was repeated several times to
remove water. The residue was purified by column chromatography on
silica gel (CHCl3:MeOH, 8:3) to give sarmentosin (1) (18 mg, 72%) as a
20
syrup. ½ꢂꢁD À18.3 (c 0.91, MeOH). IR (ꢁ, cmÀ1, film): 3540–3240, 2227,
1643, 1100, 1076, 1050. 1H NMR (400 MHz, D2O): ꢀ 6.70 (1H, t,
J ¼ 6.4 Hz), 4.5–4.7 (2H, m), 4.49 (1H, t, J ¼ 7.9 Hz), 4.24 (2H, s), 3.90
(1H, dd, J ¼ 12.4, 1.9 Hz), 3.71 (1H, dd, J ¼ 12.4, 5.4 Hz), 3.25–3.52 (4H,
m). 13C NMR (100 MHz, D2O): ꢀ 145.3, 118.0, 117.6, 103.2, 77.4, 77.1,
74.4, 70.9, 68.6, 63.0, 62.0. FAB-MS (m/z): 276 (Mþþ1, 10), 207 (17), 185
(62), 93 (100). The above data are consistent with those of the authentic
natural sample reported in the literature.[1,2,4]

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Facile and Convergent Synthesis of Sarmentosin
ACKNOWLEDGMENTS
3363
This work was financially supported by the National Natural Science
Foundation of China.
REFERENCES
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isolation and structure of the active principle sarmentosin. Chinese
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Sin. 1982, 40, 273–280.
3. Zhai, S.K.; Shen, M.L.; Xiong, Y.L.; Li, J.F.; Ding, Y.E.; Gao, Y.S.
A preliminary report on the immuno-suppressive activity of sarmen-
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6. Mikamo, M. Facile 1-O-deacylation of per-O-acyladoses. Carbo-
hydr, Res. 1989, 191, 150.
7. Urban, F.J.; Moore, B.S.; Breitenbach, R. Synthesis of tigogenyl b-
O-cellobioside heptaacetate and glycoside tetraacetate via Schmidt’s
trichloroacetimidate method; some new observations. Tetrahedron
Lett. 1990, 31, 4421.
8. Schmidt, R.R. New methods for the synthesis of glycosides and oli-
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Angew. Chem. Int. Ed. Engl. 1986, 25, 212.
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gosine glycosylation in glycosphingolipid synthesis. J. Carbohydr.
Chem. 1988, 7, 435.
Received in the USA April 3, 2002

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