Total synthesis of solamargine

Article abstract of DOI:10.1016/j.bmcl.2011.03.064

Solamargine, (25R)-3β-{O-α-l-rhamnopyranosyl-(1→2)-[O- α-l-rhamnopyranosyl-(1→4)]-β-d-glucopyranosyloxy} -22α-N-spirosol-5-ene, has been synthesized in 13 steps in a 10.5% overall yield starting from the naturally abundant diosgenin. Condensation of a partially protected glucopyranosyl donor with an oxaza-spiro moiety, which was formed in one-pot azido reduction, significantly improved the synthesis of desired molecule. The target compound exhibited good cytotoxic activities against tumor cells HeLa, A549, MCF-7, K562, HCT116, U87, and HepG2 with IC ₅₀ ranging from 2.1 to 8.0 μM.

Full text of DOI:10.1016/j.bmcl.2011.03.064

Bioorganic & Medicinal Chemistry Letters 21 (2011) 2930–2933
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Total synthesis of solamargine
Guohua Wei a, Jing Wang a, Yuguo Du a,b,
⇑
a State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Sciences (CAS), Beijing 100085, China
b College of Chemistry and Chemical Engineering, Graduate University of Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Solamargine, (25R)-3b-{O-a-L-rhamnopyranosyl-(1?2)-[O-a-L-rhamnopyranosyl-(1?4)]-b-D-glucopyr-
anosyloxy}-22a-N-spirosol-5-ene, has been synthesized in 13 steps in a 10.5% overall yield starting from
the naturally abundant diosgenin. Condensation of a partially protected glucopyranosyl donor with an
oxaza-spiro moiety, which was formed in one-pot azido reduction, significantly improved the synthesis
of desired molecule. The target compound exhibited good cytotoxic activities against tumor cells HeLa,
Received 22 November 2010
Revised 25 February 2011
Accepted 17 March 2011
Available online 23 March 2011
A549, MCF-7, K562, HCT116, U87, and HepG2 with IC50 ranging from 2.1 to 8.0 lM.
Keywords:
Solasodine
Ó 2011 Elsevier Ltd. All rights reserved.
Natural products
Solamargine
Cytotoxic activity
Glycosides
Solamargine is a major glycoalkaloid which occurs in at least
100 Solanum species.1 Pharmacological studies have indicated that
solamargine and its derivatives show anti-tumor activities through
inhibition of tumor cell growth, such as colon (HT-29, HCT-15),
prostate (LNCap, PC-3), breast (T47D, MDA-MB-231), and human
hepatoma (PLC/PRF/5) cells.2 Despite its widespread occurrence
and biological importance, there are surprisingly few reports of
the total synthesis of solamargine or structurally related
compounds.3 Solamargine is structurally composed of two parts:
an oxaza-spiro steroidal aglycone (solasodine) and a chacotriose
would be synthesized using a one-pot reductive ring closure from
the known compound 6.
The synthesis of 4 started with the known compound 6 which
was obtained from the commercially available diosgenin in three
steps and a 50% overall yield.5 Treatment of 6 with K2CO3 in MeOH
then generated spiroketal 7 and its counterpart dione 8. These two
compounds were found to be interconvertible in organic solution
and consequently, the mixture of 7 and 8 was treated directly with
p-toluenesulfonyl chloride/pyridine (?9), followed by azido-sub-
stitution with NaN3, to give 10 in 85% yield over three steps. It
has been well documented that selective reduction of the 16-ke-
tone of the cholestan-16,22-dione with NaBH4 in i-PrOH provided
the corresponding furostan through a concurrent intramolecular
hemiketal formation.6 Applying the same idea, dione 10 was suc-
cessfully converted into the hemiketal 11 in a moderate yield of
62%. The key reductive-cyclization of compound 11 was carried
out smoothly in the presence of Ph3P under refluxing conditions,
and followed by desilylation with 6 N HCl in one-pot, furnished
solasodine 4 in 95% yield over two steps (Scheme 1). The stereo-
chemistry on C-22 is fully controlled as the free amine attacks
C-22 from the less hindered back face, and the structure of 4 was
confirmed by comparison of its physical data with those reported
for the natural solasodine.7,8,14
(2,4-bis-a-L-rhamnopyranosyl-b-D-glucopyranose) attached to the
3-hydroxy group of solasodine. We have previously described an
efficient method for the preparation of natural saponins possessing
2,4-branched oligosaccharides using partially protected glycosyl
donors4 and would like to report here an extension of this strategy
to a facile total synthesis of natural solamargine and its anti-tumor
activities.
We envisaged that solamargine (1) could be constructed from a
fully acetylated L-rhamnopyranosyl bromide 2 and a well-defined
glycoside 3. Compound 3 could be prepared through a coupling
reaction between solasodine 4 and a partially protected thiogluco-
pyranoside 5. In designing this glucosyl donor, PMB (p-methoxy-
benzyl) was selected as the protecting groups, as it not only
enhances the reactivity of the sugar moiety, but it also improves
the selectivity in the final deprotection step (Fig. 1). This building
block could also facilitate the continuous C-20 and C-40 glycosyla-
tion due to the accessible free 2,4-hydroxyl groups. Solasidine 4
The partially protected thioglycoside donor 5 was prepared
from compound 124 and p-methoxybenzylidenation of 12 with
anisaldehyde dimethyl acetal (?13), followed by tin-assisted reg-
ioselective p-methoxybenzylation,9 provided 14 in a yield of 70%
from 12. Selective ring opening10 of 14 with sodium cyanoborohy-
dride and trifluoroacetic acid proceeded smoothly to afford 5 in
85% yield (Scheme 2).
⇑
Corresponding author. Tel./fax: +86 10 62849126.
E-mail address: duyuguo@rcees.ac.cn (Y. Du).
0960-894X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2011.03.064

G. Wei et al. / Bioorg. Med. Chem. Lett. 21 (2011) 2930–2933
2931
H
N
Br
O
AcO
2
O
AcO
OAc
H
N
OH
O
O
O
O
HO
O
OPMB
O
O
HO
O
HO
PMBO
1
HO
HO
O
OH
3
HO
OH
OH
H
O
N
OAc
OPMB
O
O
O
HO
PMBO
S
OH
6
5
TBDPSO
4
HO
Figure 1. Retrosynthetic analysis of solamargine 1.
O
O
O
R
O
a
6
OH
TBDPSO
TBDPSO
8 R = OH
9 R = OTs
7
b
c
10
R = N3
d
H
N
OH
O
N3
e
O
TBDPSO
11
4
HO
Scheme 1. Preparation of solasodine 4. Reagents and conditions: (a) K2CO3, THF/MeOH, rt, 4 h; (b) TsCl, py, rt, 5 h; (c) NaN3, NH4Cl, DMF, 50 °C, 6 h, 85% for three steps; (d)
NaBH4, i-PrOH, rt, 5 h, 62%; (e) Ph3P, THF/H2O, reflux, 2 h; 6 N HCl, EtOH, reflux, 2 h, 95% for two steps.
OH
PMBO
a
PMBO
b
O
O
O
O
O
HO
HO
O
O
S
S
S
5
HO
PMBO
c
OH
12
OH
OH
14
13
Scheme 2. Preparation of partially protected thioglycoside 5. Reagents and conditions: (a) p-MeOPhCH(OMe)2, TsOH, CH3CN, rt, 2 h, 85%; (b) Bu2SnO, MeOH, reflux, 2 h;
PMBCl, Bu4NI, toluene, 80 °C, 6 h, 82% for two steps; (c) NaCNBH3, TFA, 4 Å MS, DMF, 0 °C, 6 h, 85%.
With building blocks 4 and 5 in hand, the efficient synthesis of
solamargine 1 was initiated. Coupling of solasodine 4 and a par-
tially protected glycosyl donor 5 was carried out in dry methylene
dichloride in the presence of AgOTf and N-iodosuccinimide (NIS) at
ꢀ50 °C generating the desired key saponin 3 in 65% isolated yield
after a preparative HPLC purification.11 In this glycosylation, AgOTf
was found to be a more effective promoter than the more com-
monly used TMSOTf (trimethylsilyl trifluoromethanesulfonate),
probably because of the existence of the secondary amine in 4. A
doublet at 4.36 ppm (J = 7.8 Hz) in 1H NMR spectrum clearly indi-
generated saponin 15 in a yield of 81%. Finally, removal of the
PMB groups from compound 15 using 10% TFA13 in CH2Cl2 at
ꢀ15 °C, followed by global deacylation with 0.3 M NaOH in MeOH
afforded the target compound 1 in 80% yield over two steps
(Scheme 3). Remarkably, this complex natural saponin was pre-
pared convergently in 13 steps and in a 10.5% overall yield. The
analytic data14 13C NMR, MS, optical rotation) for the synthetic
(
solamargine 1 were identical to those reported for the natural
product.1
The cytotoxic activities of synthetic solamargine 1 on tumor
cells HeLa, A549, MCF-7, K562, HCT116, U87, and HepG2, as well
as two normal cell lines (HL7702 and H9C2), were evaluated fol-
cated the b-configuration of 3. Condensation of 3 and L-rhamno-
pyranosyl bromide 212 in the presence of AgOTf at ꢀ10 °C

2932
G. Wei et al. / Bioorg. Med. Chem. Lett. 21 (2011) 2930–2933
H
N
H
N
O
O
OR1
b
a
4
5
OPMB
O
O
O
O
HO
R1O
O
PMBO
O
OH
O
R2O
15 R1 = PMB, R2 = Ac
1
O
R2O
c
3
R2O
R
1 = R2 = H
R2O
R2O
OR2
Scheme 3. Synthesis of solamargine 1. Reagents and conditions: (a) NIS, AgOTf, CH2Cl2, ꢀ50 °C, 2 h, 65%; (b) 2, AgOTf, CH2Cl2, ꢀ10 °C, 2 h, 81%; (c) 10% TFA in CH2Cl2, ꢀ15 °C,
1 h; 0.3 M NaOH, MeOH, rt, 4 h, 80% for two steps.
6. Mazur, Y.; Danieli, N.; Sondheimer, F. J. Am. Chem. Soc. 1960, 82, 5889.
7. Bird, G. J.; Collins, D. J.; Eastwood, F. W.; Exter, R. H.; Romanelli, M. L.; Small, D.
D. Aust. J. Chem. 1979, 32, 783.
8. Puri, R.; Wong, T. C.; Puri, R. K. Magn. Reson. Chem. 1993, 31, 278.
9. Nashed, M. A.; Anderson, L. Tetrahedron Lett. 1976, 17, 3530.
10. Garegg, P. J. Pure Appl. Chem. 1984, 56, 849.
11. A general procedure for the glycosylation was described as follows: Silver triflate
(AgOTf, 0.3 equiv of 5) was added to a solution of N-iodosuccinimide (NIS,
1.5 equiv of 5) in a mixture of CH2Cl2/toluene (10:1, v/v). To a mixture of 5
(1.2 equiv of 4) and 4 in dry CH2Cl2 at ꢀ50 °C under nitrogen was added
dropwise the above solution of NIS/AgOTf and the progress was monitored by
TLC analysis. After complete disappearance of the starting materials, the
reaction was quenched by adding a 10% aq Na2S2O3 solution. The organic layer
was washed with 10% aq Na2S2O3 solution, dried over Na2SO4, and
concentrated in vacuo. The residue was purified by preparative HPLC to
obtain the pure product.
Table 1
Cytotoxicity of compound 1 on seven tumor cell lines (IC50, l
M)a
HeLa
A549
MCF-7
K562
HCT116
U87
HepG2
1
6.0
8.0
2.1
5.2
3.8
3.2
>30
2.5
9.8
Cisplatin
26.3
>30
17.0
23.1
18.5
a
Values are means of three independent experiments.
Table 2
Cytotoxicity of compounds 1 on two normal cell lines (IC50, l
M)a
HL7702
H9C2
>20
12. Hansen, T.; Daasbjerg, K.; Skrydstrup, T. Tetrahedron Lett. 2000, 41, 8645.
13. Zhan, Z. Y.; Ollmann, I. R.; Ye, X. S.; Wischnat, R.; Baasov, T.; Wong, C. H. J. Am.
1
13.5
a
Chem. Soc. 1999, 121, 734.
Values are means of three independent experiments.
14. The selected physical data. Compound 4: ½a D25
ꢁ ꢀ112 (c 0.81, CHCl3) [lit.
7
½
a 2D6
ꢁ
ꢀ116.7 (c 0.62, CHCl3)]. Selected 1H NMR (400 MHz, CDCl3): d 0.82 (s, 3H,
H-18), 0.84 (d, 3H, J = 6.2 Hz, H-27), 0.96 (d, 3H, J = 7.2 Hz, H-21), 1.03 (s, 3H, H-
lowing the standard MTT assay.15 Tables 1 and 2 show the inhibi-
tion of tumor cell growth by solamargine 1 with IC50 ranging from
2.1 to 8.0 lM, but exhibited a lower cytotoxicity to the normal
hepatocyte cell HL7702.
In conclusion, the natural product solamargine has been
chemically synthesized in 13 steps in a 10.5% overall yield starting
from the natural abundant diosgenin. Application of one-pot
reductive-cyclization to form the spirosolan derivative followed
by condensation with a partially protected glucopyranosyl donor
has significantly simplified the saponin synthesis. The approach
described here should be valuable for the related molecule16
design, synthesis, and bioactivity screening.
19), 2.20–2.33 (m, 2H, H-4a, H-4b), 2.58–2.68 (m, 2H, H-26a, H-26b), 3.48–3.55
(m, 1H, H-3a
), 4.28–4.30 (m, 1H), 5.35 (d, 1H, J = 5.1 Hz, H-6); selected 13C
NMR (100 MHz, CDCl3): d 15.08, 16.22, 19.07, 19.27, 20.86, 30.16, 31.46, 31.66,
32.02, 32.11, 32.12, 33.99, 36.62, 37.24, 39.90, 40.52, 41.30, 42.30, 47.63, 50.19,
56.52, 62.91, 71.65, 78.98, 98.14, 121.24, 140.84. ESI-HRMS calcd for
C
27H43NO2: 413.3294 [M]+, found: 414.3364 [M+H]+. Compound 3: ½a 2D5
ꢁ ꢀ43
(c 0.55, CHCl3); selected 1H NMR (400 MHz, CDCl3, 25 °C): d 0.81 (s, 3H, H-18),
0.86 (d, 3H, J = 6.2 Hz, H-27), 0.96 (d, 3H, J = 7.2 Hz, H-21), 1.03 (s, 3H, H-19),
2.60–2.66 (m, 2H, H-26a, H-26b), 3.36–3.49 (m, 3H), 3.55–3.60 (m, 2H), 3.66
(dd, 1H, J = 5.4, 10 Hz, H-6aI), 3.72 (dd, 1H, J = 5.4, 10 Hz, H-6bI), 3.80 (s, 6H,
I
I
I
CH3OPhCH2), 4.32 (m, 1H, H-16), 4.36 (d, 1H, J1;2 = 7.8 Hz, H-1 ), 4.50 (s, 2H,
CH3OPhCH2), 4.72 (d, 1H, J = 11 Hz, one proton of CH3OPhCH2), 4.88 (d, 1H,
J = 11 Hz, one proton of CH3OPhCH2), 5.34 (d, 1H, J = 5.0 Hz, H-6), 6.85–7.32 (m,
8H, Ph); selected 13C NMR (100 MHz, CDCl3): d 15.35, 16.36, 19.19, 19.39,
20.87, 29.68, 29.71, 31.20, 31.43, 31.76, 32.09, 32.15, 36.87, 37.26, 38.90, 39.80,
40.62, 42.38, 47.67, 50.10, 55.28 (2 C), 56.45, 62.94, 70.30, 71.81, 73.29, 74.10,
74.13, 74.26, 79.04, 83.42 (2 C), 98.40, 101.26, 121.79, 140.40, 159.31, 159.39.
ESI-HRMS calcd for
C
49H69NO9: 815.4972 [M]+, found: 816.5042 [M+H]+.
Acknowledgments
Compound 15: ½a 2D5
ꢁ
+ 68 (c 0.6, CHCl3); selected 1H NMR (400 MHz, CDCl3): d
0.81 (s, 3H, H-18), 0.88 (d, 3H, J = 6.2 Hz, H-27), 1.01 (m, 6H, H-21, 19), 1.19–
1.26 (m, 6H, H-6II, 6III), 1.71, 1.72, 1.87, 2.01, 2.03, 2.05 (6 s, 6 ꢂ 3H, CH3CO),
2.62–2.68 (m, 2H, H-26a, H-26b), 3.36–3.57 (m, 7H), 3.67–3.72 (m, 2H), 3.78 (2
This work was supported in partial by NNSF of China (projects
30701043, 20732001, 20890112) and project 2009ZX09501-011.
We would like to thank Dr. Vicky Gibson of Carbosynth for helping
with the manuscript preparation and Dr. Jianjun Zhang of CAU for
providing compounds 2 and 12 as the starting materials.
I
I
s, 2 ꢂ 3H, CH3OPhCH2), 4.32–4.34 (m, 1H, H-16), 4.39 (d, 2H, J1;2 = 7.8 Hz, H-
1I), 4.48 (s, 2H, CH3OPhCH2), 4.57 (d, 1H, J = 10.0 Hz, one proton of CH3OPhCH2),
4.63 (dd, 1H, J = 2.6, 4.0 Hz, H-2II), 4.68 (dd, 1H, J = 2.6, 4.0 Hz, H-2III), 4.74 (d,
1H, J = 10.0 Hz, one proton of CH3OPhCH2), 4.98–5.04 (m, 3H, H-3III, 4II, 4III),
5.10 (dd, 1H, J = 4.0, 8.6 Hz, H-3II), 5.19 (d, 1H, J = 2.0 Hz, H-1II), 5.23 (d, 1H,
J = 2.0 Hz, H-1III), 5.34 (d, 1H, J = 5.0 Hz, H-6), 6.85–7.32 (m, 8H, Ph). 13C NMR
(100 MHz, CDCl3) d 15.3, 16.4, 17.5, 17.7, 19.3, 19.4, 20.5, 20.7, 20.8, 20.9, 22.7
25.7, 26.1, 29.7 (2 C), 29.9, 31.4, 31.9, 32.1, 32.2, 36.9, 37.3, 38.6, 39.9, 40.6,
41.3, 47.5, 50.1, 55.3 (2 C), 56.5, 62.5, 69.2, 69.3, 69.5, 70.1, 70.3, 70.4, 70.8,
72.3, 72.9 (2 C), 74.6, 74.9, 75.0, 76.0, 79.0, 81.9 (2 C), 97.1 (C-1II), 97.4 (C-1III),
98.3 (C-22), 100.7 (C-1I), 121.7 (C-6), 140.7 (C-5), 159.1 (2 C), 169.7 (2 CH3CO),
169.8 (2 CH3CO), 170.2 (CH3CO), 170.3 (CH3CO). ESI-HRMS calcd for
References and notes
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Phytochemistry 2002, 59, 79.
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C73H101NO23
:
1359.6764 [M]+, found: 1360.6833 [M+H]+. Compound 1:
1
½
a 2D5
ꢁ
ꢀ88 (c 0. 5, MeOH/CHCl3 = 1:1) [lit.
½ ꢁ ꢀ91 (c 0. 2, MeOH/
a 2D4
CHCl3 = 1:1)]. Selected 1H NMR (400 MHz, CDCl3/CD3OD = 1:1): d 0.84 (s, 3H,
H-18), 0.87 (d, 3H, J = 6.2 Hz, H-27), 0.97 (d, 3H, J = 7.0 Hz, H-21), 1.05 (s, 3H, H-
19), 1.22–1.26 (m, 6H, H-6II, 6III), 1.42–2.47 (m, 24H), 2.63–2.67 (m, 2H, H-26),
3.21-4.09 (m, 15H), 4.31–4.33 (m, 1H, H-16), 4.50 (d, 1H, J = 7.8 Hz, H-1I), 4.90
(s, 1H, H-1II), 5.24 (s, 1H, H-1III), 5.40 (d, 1H, J = 4.0 Hz, H-6). Selected 13C NMR
(100 MHz, CDCl3:CD3OD = 1:1): d 15.6, 16.9, 17.4, 17.9, 19.2 (2 C), 20.8, 28.4,
28.7, 29.2, 29.6, 30.2, 31.5, 33.4, 36.2, 38.2, 39.1, 40.5, 41.5, 42.1, 47.3, 51.1,
56.0, 61.4, 62.8, 68.2, 68.9, 69.2, 69.4, 70.5, 71.5, 73.3, 73.5, 75.5, 76.8, 77.1,
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G. Wei et al. / Bioorg. Med. Chem. Lett. 21 (2011) 2930–2933
2933
79.1, 79.5, 79.6, 98.8, 100.4, 101.3, 102.4, 121.8, 141.3. ESI-HRMS calcd for
45H73NO15: 867.4980 [M]+, found: 868.5051 [M+H]+.
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C