An alternative total synthesis of solamargine

Article abstract of DOI:10.1007/s11426-011-4476-7

Solamargine, (25R)-3β-{O-α-L-rhamnopyranosyl-(1→2)-[O- α-L-rhamnopyranosyl-(1→4)]-β-D-glucopyranosyloxy} -22α-Nspirosol-5-ene, isolated from the berries of solanum aculeastrum, has been synthesized in 26.8% overall yield. First glycosylation before N-cyclization significantly facilitated synthesis of the desired molecule. We anticipate that this work will provide a new approach to access solamargine and its diversified analogues. Science China Press and Springer-Verlag Berlin Heidelberg 2012.

Full text of DOI:10.1007/s11426-011-4476-7

SCIENCE CHINA
Chemistry
July 2012 Vol.55 No.7: 1247–1251
doi: 10.1007/s11426-011-4476-7
• ARTICLES •
An alternative total synthesis of solamargine
WEI GuoHua, WEI DongBin* & DU YuGuo*
State Key Laboratory of Environmental Chemistry and Ecotoxicology; Research Center for Eco-Environmental Sciences (CAS),
Beijing 100085, China
Received October 10, 2011; accepted October 14, 2011; published online January 5, 2012
Solamargine, (25R)-3-{O--L-rhamnopyranosyl-(12)-[O--L-rhamnopyranosyl-(14)]--D-glucopyranosyloxy}-22-N-
spirosol-5-ene, isolated from the berries of solanum aculeastrum, has been synthesized in 26.8% overall yield. First glycosyla-
tion before N-cyclization significantly facilitated synthesis of the desired molecule. We anticipate that this work will provide a
new approach to access solamargine and its diversified analogues.
solasodine, natural products, solamargine, glycosides
1 Introduction
2 Experimental
General methods
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6]. Despite its widespread occurrence
and biological importance, there are surprisingly few reports
of the total synthesis of solamargine or structurally related
compounds [7]. Recently, we reported the total synthesis of
solamargine using partially protected glucopyranosyl donor
and an oxaza-spiro moiety in 13 steps in a 10.5% overall
yield. The synthetic solamargine exhibited good cytotoxic
activities against tumor cells HeLa, A549, MCF-7, K562,
HCT116, U87, and HepG2 with IC50 ranging from 2.1 to
8.0 µM [8]. To further investigate the preclinical potential,
we need a practical method toward large scale preparation
of solamargine. Herein we would like to report our solution
for this purpose.
Optical rotations were determined at 25 °C with a Perkin-
Elmer Model 241-Mc automatic polarimeter unless other-
1
wise stated. H, 13C NMR spectra were recorded with ARX
400 spectrometers for solutions in CDCl3 or CD3OD.
Chemical shifts are given in ppm downfield from internal
Me4Si. Mass spectra were measured using MALDI-TOF-
MS with dihydroxybenzoic acid (DHB) as the matrix.
High-resolution mass spectrometry was conducted in a pos-
itive mode using ESI-source. Thin layer chromatography
(TLC) was performed on silica gel HF254 with detection by
charring with 30% (v/v) H2SO4 in MeOH or in some cases
by a UV detector.
Synthesis of compound 5
Compound 2 (185 mg, 0.38 mmol) was subjected to
Ac2O/Py. After removal of the solvent, the residue was dis-
solved into wet acetone (2 mL), and NBS (136 mg, 0.76
mmol) was added at 10 °C. Then, the reaction mixture was
stirred for 30 min in the dark, concentrated, and extracted
with methylene chloride (3 × 20 mL). The organic phase
was washed successively with saturated aqueous NaHCO3,
brine, then dried over anhydrous Na2SO4 and concentrated
*Corresponding author (email: weidb@rcees.ac.cn; duyuguo@rcees.ac.cn)
© Science China Press and Springer-Verlag Berlin Heidelberg 2012
chem.scichina.com www.springerlink.com

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Wei GH, et al. Sci China Chem July (2012) Vol.55 No.7
I
I
to dryness, and then the residue was subjected to DBU/
Cl3CCN. Purification of the residue by silica-gel column
chromatography (3:1 petroleum ether-EtOAc) afforded 5
H, J1 ,2 8.0 Hz, CH3OPhCH2, H-1I), 5.0 (m, 2 H, H-2I, 4I),
5.32 (d, 1 H, J 5.0 Hz, H-6), 6.827.25 (m, 8 H, Ph). Se-
lected 13C NMR (100 Hz, CDCl3): 13.0, 15.4, 17.7, 19.4,
20.5, 20.9, 20.9, 27.5, 29.7, 30.9, 31.8, 32.9, 36.8, 36.9,
37.2, 38.6, 39.6, 41.7, 42.2, 43.4, 49.8, 51.3, 55.3 (2 C),
57.6, 66.3, 69.7, 71.0, 72.8, 72.9, 73.3, 73.6, 79.5, 79.7,
99.9, 121.1, 140.8, 159.2, 159.3, 169.2, 169.6, 213.2, 218.1.
ESI-HRMS calcd for C53H71N3O12: 941.5038 [M]+; found,
942.5111 [M+H]+.
25
1
(202 mg, 88% for 3 steps); []D –28 (c 2.4, CHCl3); H
NMR (400 MHz, CDCl3): 1.92, 2.01 (2 s, 2 × 3 H, CH3CO),
3.473.55 (m, 2 H), 3.80 (2 s, 2 × 3 H, CH3OPhCH2),
4.034.07 (m, 2 H), 4.44 (q, 2 H, CH3OPhCH2), 4.60 (q, 2
H, CH3OPhCH2), 5.06 (dd, 1 H, J 3.5, 10.0 Hz, H-2), 5.22 (t,
1 H, J 10.0 Hz, H-4), 6.54 (d, 1 H, J 3.4 Hz, H-1),
6.537.24 (m, 8 H, Ph), 8.61 (s, 1 H, NH). 13C NMR (100
MHz, CDCl3): 20.5, 20.7, 55.2, 55.23, 68.3, 69.9, 71.8,
72.18, 73.1, 74.1, 75.9, 90.9, 93.5, 159.2, 159.3, 160.7,
169.2, 169.7. MALDI-TOF-MS calcd for C28H32Cl3NO10:
647.1 [M]+; found, 670.2 [M+Na]+.
Synthesis of compound 11
The mixture of 8 (80 mg, 0.09 mmol) and NaBH4 (100
mg, 2.63 mmol) in i-propanol (2 mL) and CH2Cl2 (0.2 mL)
was stirred at room temperature for 10 h. The reaction mix-
ture was then extracted with methylene chloride (3 × 20
mL). The organic phase was washed successively with sat-
urated aqueous NaHCO3, brine, then dried over anhydrous
Na2SO4 and concentrated. The residue was subjected to
column chromatography on a silica gel column with 3:1
petroleum ether-EtOAc as the eluent to yield crude foamy 9
(50 mg). To the solution of 9 (50 mg, 0.05 mmol) in THF (3
mL) and H2O (0.3 mL) was added Ph3P (41.7 mg, 0.16
mmol). The mixture was refluxed for 4 h, and then
co-evaporated with toluene to dryness to yield 10, which
was used in the next step without further purification.
Compound 10 was dissolved into a mixed solvent (5 mL,
CH2Cl2:MeOH = 1:3) and NaOMe was added at room tem-
perature to make the pH around 10. The above mixture was
stirred at room temperature for 36 h, and neutralized with
Amberlite IR-120 (H+) resin. The liquid phase was evapo-
rated in vacuo and the residue was purified on silica gel
column chromatography with 1:2 petroleum ether-EtOAc→
EtOAc as the eluent to yield 11 as an amorphous solid (38
mg, 53% from 8); []D25 43 (c 0.55, CHCl3). Selected 1H
NMR (400 Hz, CDCl3): δ 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,
Synthesis of compound 7
To the solution of 6 (100 mg, 0.14 mmol) in dried pyri-
dine (3 mL) was added HF-Py complex (0.8 mL) at 0 °C.
The mixture was stirred for 10 h, and extracted with meth-
ylene chloride (3 × 20 mL). The organic phase was washed
successively with saturated aqueous NaHCO3, 1 N HCl,
saturated aqueous NaHCO3, brine, then dried over anhy-
drous Na2SO4 and concentrated. The residue was subjected
to column chromatography on a silica gel column with 3:1
petroleum ether-EtOAc as the eluent to yield 7 as an amor-
25
phous solid (55 mg, 85%); []D –54 (c 2.1, CHCl3). Se-
lected 1H NMR (400 MHz, CDCl3): 0.78 (s, 3 H, CH3), 0.98
(d, 3 H, J 6.5 Hz, CH3), 1.03 (m, 6 H, CH3), 2.552.62 (m,
3 H), 2.752.82 (m, 1 H), 3.16 (dd, 1 H, J 5.4, 12 Hz,
H-26a), 3.27 (dd, 1 H, J 5.4, 12 Hz, H-26b), 3.54 (m, 1 H,
H-3), 5.35 (d, 1 H, J 5.1 Hz, H-6). 13C NMR (100 MHz,
CDCl3): 12.9, 15.3, 17.6, 19.3, 20.5, 27.4, 29.63, 30.9, 31.7,
32.9, 36.5, 36.9, 37.2, 38.6, 39.6, 41.6, 42.1, 43.8, 49.4,
51.2, 57.5, 66.2, 71.5, 120.8, 140.9, 213.2, 218.1.
ESI-HRMS calcd for C27H41N3O3: 455.3148 [M]+; found,
456.3221 [M+H]+.
Synthesis of compound 8
To a solution of compound 5 (170 mg, 0.26 mmol) and
compound 7 (80 mg, 0.18 mmol) in anhydrous CH2Cl2 (2
mL) at 15 °C was added AgOTf (20 mg, 0.08 mmol) under
N2 protection. The mixture was stirred under these condi-
tions for 80 min, at the end of which TLC indicated the re-
action was complete. The mixture was neutralized with
Et3N and concentrated to dryness. The residue was purified
by column chromatography on a silica gel column with 3:1
petroleum ether-EtOAc as the eluent to yield 8 as an amor-
I
I
CH3OPhCH2), 4.32 (m, 1H, H-16), 4.36 (d, 1H, J1 ,2 = 7.8
Hz, H-1I), 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 Hz, CDCl3):
δ 15.4, 16.3, 19.2, 19.4, 20.9, 29.6, 29.7, 31.2, 31.4, 31.8,
32.1, 32.2, 36.9, 37.3, 38.9, 39.8, 40.6, 42.4, 47.7, 50.1,
55.3 (2 C), 56.4, 62.9, 70.3, 71.8, 73.3, 74.1, 74.1, 74.3,
79.1, 83.4 (2 C), 98.4, 101.3, 121.8, 140.4, 159.3, 159.4.
ESI-HRMS calcd for C49H69NO9: 815.4972 [M]+, found:
816.5046 [M+H]+.
25
phous solid (155 mg, 94%); []D  (c 0.9, CHCl3). Se-
lected 1H NMR (400 MHz, CDCl3): 0.79 (s, 3 H, CH3), 0.94
(d, 3 H, J 6.5 Hz, CH3), 1.04 (m, 6 H, CH3), 1.92, 2.01 (2 s,
2×3 H, CH3CO), 2.55-2.61 (m, 3 H), 2.76 (m, 1 H), 3.17 (dd,
1 H, J 5.4, 12 Hz, H-26a), 3.27 (dd, 1 H, J 5.4, 12 Hz,
H-26b), 3.44-3.55 (m, 4 H, H-3, 6aI, 6bI, 5I), 3.66 (t, 1 H, J
9.4 Hz, H-3I), 3.78 (2 s, 2 × 3 H, CH3OPhCH2), 4.48 (m, 5
Synthesis of compound 13
Compounds 11 (22 mg, 0.03 mmol) and 12 (35 mg, 0.10
mmol) and activated 4 Å molecular sieves were pre-dried in

Wei GH, et al. Sci China Chem July (2012) Vol.55 No.7
1249
one flask under vacuum for 4 h. The mixture was then dis-
solved in CH2Cl2 (1 mL), and AgOTf (30 mg, 0.12 mmol)
and 2,6-lutidine (15 L, 0.12 mmol) were added under an
N2 atmosphere at 30 °C. The mixture was stirred under
these conditions for 3 h, neutralized with triethylamine,
concentrated under reduced pressure, and purified on a sili-
ca gel column with 1:2 petroleum ether-EtOAc→EtOAc as
Amberlite IR-120 (H+). The solvents were filtered, and the
filtrate was concentrated to dryness under diminished pres-
sure. The residue was subjected to column chromatography
on silica gel with 5:2 CH2Cl2-MeOH as the eluent to yield 1
as a white solid (15.7 mg, 80% for 2 steps); []D25 –88 (c 0.
5, MeOH : CHCl3 = 1:1) [lit. [4] []D24 –91 (c 0. 2, MeOH:
CHCl3 = 1:1)]; The diagnostic NMR signals are identical
to those reported for the natural product [1]. Selected 1H
NMR (400 Hz, CDCl3 : CD3OD = 1:1): δ 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 Hz, CDCl3 : CD3OD = 1:1): δ 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, 79.1, 79.5, 79.6, 98.8, 100.4, 101.3, 102.4,
121.8, 141.3. ESI-HRMS calcd for C45H73NO15: 867.4980
[M]+, found: 868.5053 [M+H]+.
25
the eluent to yield 13 as a white solid (30 mg, 90%); []D
+68 (c 0.6, CHCl3). Selected 1H NMR (400 Hz, CDCl3): δ
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 × 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 s, 2 × 3H, CH3OPhCH2), 4.324.34 (m, 1H, H-16),
I
I
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
Hz, CDCl3) δ 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 C73H101NO23: 1359.6764 [M]+, found:
1360.6838 [M+H]+.
3 Results and discussion
In our previous method [8], although treatment of so-
lasodine 3 with a partially protected glycosyl donor 2 in dry
methylene dichloride in the presence of AgOTf and
N-iodosuccinimide (NIS) at 50 °C afforded the desired key
saponin 4 in an optimized yield around 65% (Scheme 1),
significantly simplifying the solamargine synthesis, a large
amount of expensive donor 2 was quickly consumed in the
presence of cyclic amine (oxaza-spiro structure). Based on
the phenomenon, herein we explored a new method for the
total synthesis of solamargine 1 as shown in Scheme 2. First
glycosylation before N-cyclization would favor synthesis of
the desired molecule.
PMB (p-methoxybenzyl) in Schmidt reagent 5 was se-
lected as the protecting groups, as it not only enhances the
reactivity of the sugar moiety, but also improves the selec-
tivity in the final deprotection step. Thus, the known com-
pound 2 [8] was first acetylated with acetic anhydride in
pyridine, which was further converted into trichloroacetim-
Synthesis of compound 1
A solution of 13 (30 mg, 0.02 mmol) in TFA: CH2Cl2
(1:4, V/V, 1 mL) was stirred at 15 °C for 0.5 h, neutralized
with saturated aqueous sodium bicarbonate, and extracted
with dichloromethane (3  15 mL). The organic phases
were combined, dried over anhydrous sodium sulfate and
concentrated, which was used in the next step without fur-
ther purification. To a solution of the above residue in a
mixed solvent (5 mL, CH2Cl2 : MeOH = 1:3) aqueous 0.3 M
NaOH was added until the pH was 10. The mixture was
stirred at room temperature for 6 h, and neutralized with
H
N
H
N
O
O
PMBO
HO
PMBO
PMBO
HO
PMBO
O
O
NIS, AgOTf
65%
O
S
OH
HO
OH
4
2
3
Scheme 1 The previous method of synthesis of Solamargine 1.

1250
Wei GH, et al. Sci China Chem July (2012) Vol.55 No.7
O
O
N3
N3
PMBO
O
O
O
AcO
PMBO
a
b
2
AcO
OC(NH)CCl3
TBDPSO
HO
6
7
5
c
OH
O
O
N3
N3
O
PMBO
AcO
PMBO
PMBO
d
O
O
O
AcO
PMBO
O
AcO
9
8
H
AcO
e
H
N
N
O
O
PMBO
RO
PMBO
OPMB
O
O
O
O
O
PMBO
RO
g
Me
AcO
O
O
10
11
R = Ac
R = H
f
AcO
Me
AcO
Br
O
AcO
13
Me
AcO
O
AcO
OAc
h
AcO
OAc
12
1
Scheme 2 Reagents and condition: (a) Ac2O, Pyridine; NBS, wet DCM, dark, 0 °C; DBU, Cl3CCN, DCM, 88% (for 3 steps); (b) HF/Py, 85%; (c) AgOTf,
DCM, 40 °C, 94%; (d) NaBH4, i-PrOH, rt; (e) Ph3P, THF/H2O (v/v, 10:1), reflux; (f) NaOMe, MeOH, 53% overall yield for three steps from 4; (g) 12,
AgOTf, 2,6-lutidine, DCM, 30 °C, 90%; (h) 10% TFA in DCM, 15 °C; 0.3 M NaOH, MeOH, 80% for 2 steps.
idate 5. Treatment of compound 6 [8] with HF/Py complex
provided acceptor 7. After coupling of glycosyl donor 5 and
aglycone precursor 7 in anhydrous methylene chloride with
mild catalyst AgOTf [9], key intermediate 8 was obtained in
94% isolated yield. Selective reduction of 16-ketone 8 with
NaBH4/i-PrOH [10] (9), followed by reductive cycliza-
tion with Ph3P/THF/H2O under refluxing (10) and re-
moval of the acetyl group with NaOMe in MeOH, afforded
acceptor 11 in 53% overall yield for three steps [8]. Con-
densation of 11 and rhamnopyranosyl bromide 12 using
AgOTf and 2,6-lutidine as catalysts [11] afforded trisaccha-
ride alkaloid 13 in excellent yield (90%). Removal of the
PMB groups of 13 with 10% TFA in DCM at 15 °C, fol-
lowed by deacetylation with 0.3 M NaOH in MeOH, af-
forded the final solamargine 1 in 80% overall yield for 2
steps [8]. The analytic data (NMR, MS, optical rotation) for
the synthesized solamargine 1 were identical to those of the
natural product [1, 8].
fore N-cyclization significantly facilitated synthesis of the
desired molecule. We anticipate that this work will provide
a new approach to access solamargine and its diversified
analogues.
This work was supported partially by the National Basic Research Pro-
gram of China (2011CB936001) and National Natural Science Foundation
of China (20732001, 20872172, 21072217).
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