Synthesis of dimeric terpenoyl glycoside side chains from cytotoxic saponins

Article abstract of DOI:10.1002/anie.200352895

A bottleneck in the synthesis of the model compound 1, which bears a dimeric terpenoyl glycoside side chain from cytotoxic saponins, was overcome by the stepwise construction of the terpene part by regioselective acylation of the sugar with a functionalized phosphonate followed by a Horner-Wadsworth-Emmons olefination.

Full text of DOI:10.1002/anie.200352895

Angewandte
Chemie
Glycoside Synthesis
Synthesis of Dimeric Terpenoyl Glycoside Side
Chains from Cytotoxic Saponins**
Sabine Reicheneder and Carlo Unverzagt*
Dedicated to Professor Axel Zeeck
on the occasion of his 65th birthday
Saponins are natural products from plants and marine
organisms exhibiting a wide variety of biological activities,
[
1]
for example, anticancer properties. Most saponins consist of
a lipophilic steroid or triterpene core and hydrophilic
oligosaccharide side chains. Despite their structural diversity,
only few saponins contain an additional monoterpene glyco-
[
2]
side side chain (A, Scheme 1) like that found in julibrosides,
[
3]
[4]
elliptosides and avicins. These compounds selectively
[
4a,c]
Scheme 2. Functionalization of linalool: a) Ac O, NEt , DMAP (75%);
2 3
inhibit tumor cell growth by induction of apoptosis
À1
b) 1. MCPBA, 08C; 2. HIO ·2H O, THF/H O (b: 65%); c) 3, NaH,
4 2 2
(
IC : 0.2 mgmL for Jurkat cells) and prevent chemically
5
0
THF (87%, E:Z=95:5). DMAP=4-dimethylaminopyridine,
MCPBA=m-chloroperbenzoic acid.
[
4d,e]
induced carcinogenesis in mice.
The occurrence of the
terpenoyl glycoside side chain A appears to enhance the
[
2a]
cytotoxic effect, which encouraged us to develop a syn-
thetic path to this unique motif for subsequent bioactivity
studies.
Our initial approach to terpenoyl glycosides was to
functionalize linalool 1 with a carboxylate followed by
glycosylation (Scheme 2). The tertiary alcohol function of
racemic linalool (Æ )-1 was acetylated, and the more sub-
stituted double bond was selectively cleaved to give the
aldehyde 2 by epoxidation with MCPBA, periodic-acid-
catalyzed diol formation, and oxidative cleavage of the vicinal
[
5]
diol. The volatile aldehyde 2 was extended to furnish the
[
6]
desired menthiafolic acid ethyl ester 4 in a Horner–Wads-
[
7]
worth–Emmons (HWE) reaction using phosphonate 3.
Scheme 1. Retrosynthesis of terpenoyl glycoside side chain A from cytotoxic saponins.
[
*] S. Reicheneder, Prof. C. Unverzagt
Bioorganische Chemie, Gebäude NW1
Universität Bayreuth
However, the tertiary alcohol could not be deprotected
without undesired side reactions. In the presence of base in
methanol, the free hydroxy group participated in an intra-
molecular 1,4-addition, resulting in a mixture of eight possible
diastereomers of the tetrahydrofuran derivative 5. In addition
transesterification to the methyl ester and hydrolysis to the
free acid were observed.
95440 Bayreuth (Germany)
Fax: (+49)921-555365
E-mail: carlo.unverzagt@uni-bayreuth.de
[
**] This work was supported by the Fonds der Deutschen Chemischen
Industrie.
In order to circumvent the use of a protective group for
the OH function, the tertiary alcohol was glycosylated prior to
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2004, 43, 4353 –4357
DOI: 10.1002/anie.200352895
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4353

Communications
[
8]
the construction of the a,b-unsaturated ester. Glycosylation
of linalool (À)-1 with the trichloroacetimidates of d-quino-
vose (6 ) and d-glucose (7 ) gave the corresponding linalyl
(Scheme 4), a 3-O-acylated regioisomer of the naturally
occurring 4-O-acyl side chain A. Acyl groups tend to undergo
migration to neighboring hydroxy groups under basic con-
ditions; however, isolated 17 did not undergo rearrangement,
presumably due to the bulky and deactivated (a,b-unsatu-
rated) terpenoic acid moiety.
[
9]
[10]
[
11]
b-glycosides 8 and 9 in satisfactory yields (Scheme 3). A
general advantage of this strategy was the increased chemical
stability of the terpene glycosides and their nonvolatility.
The linalyl glycosides 8 and 9 were conveniently con-
verted into the aldehydes 11 (61%) and 12 (81%) by
epoxidation and periodate cleavage. Subsequent HWE reac-
Thus, we developed a different strategy to 4-O-acylated
quinovosides by first introducing levulinic acid at OH-3 as an
orthogonal temporary protecting group (18, 19) using levu-
linoic anhydride and DMAP catalysis. In contrast to previous
[
12]
tion of 11 with phosphonate 13 led to the quinovoside 14 in
3% yield (E/Z = 85:15; isomers can be separated by
[
16]
8
acylation studies of b-quinovosides OH-4 was more reac-
tive than OH-2 of the menthiafolyl quinovosides 18 and 19.
repeated chromatography) without affecting the acetylated
carbohydrate moieties. Selective deacetylation of compounds
[
17]
By using optimized Yamaguchi conditions we could couple
the model compound tiglic acid (a-methyl crotonic acid) to
OH-4 of compounds 18 and 19, albeit in low yields and with
concomitant overacylation. However, when we tried to attach
the glucosylated acid 16, only traces of regioisomeric products
were detected, rendering this direct approach ineffective. This
can be rationalized by the significantly reduced reactivity of
a,b-unsaturated acids in esterifications compared with that of
9
and 14 (containing an allyl ester) was achieved by a variant
of the ZemplØn deacylation using catalytic sodium allylate in
allyl alcohol. With the deprotected menthiafolyl quinovoside
1
5, a monomeric building block for the side chain (A) of
julibrosides was obtained.
To avoid the extensive use of protective groups for the
sugar part we attempted the regioselective esterification of
compound 15 at OH-4. However, acylation of the unprotected
quinovosides 10 and 15 with a variety of anhydrides was found
to proceed with good to excellent regioselectivity for OH-3.
Previously, 3-monoacylated quinovosides were obtained only
[
18]
their saturated counterparts. An additional a-methyl group
decreases the reactivity (acidity) further and increases the
steric hindrance. In the case of compounds 18 and 19, the
levulinoyl protection at O-3 limits the accessibility of OH-4
for the complex and unreactive acid 16.
[
13]
by enzymatic reactions, whereas stannylene acetals yielded
[
14]
mixtures.
Following the observed regioselectivity, the
To overcome this bottleneck in the synthesis we antici-
pated that we could reduce the steric hindrance by assembling
the terpenoic acid part stepwise from a sugar functionalized
esterification of quinovoside 10 with glucosyl menthiafolic
[
15]
acid 16 gave the dimeric terpene glycoside 17 in 43% yield
Scheme 3. Functionalization of terpene glycosides: a) 5 equiv (À)-1, BF ·OEt , CH Cl , À408C to RT (8: 64%, 9: 55%); b) NaOMe, MeOH (91%);
3
2
2
2
c) 1. MCPBA, 08C; 2. HIO ·2H O, THF/H O (c: 11: 61%, 12: 81%); d) 13, NaH, THF, E/Z=85:15 (83%); e) NaOAll, AllOH/THF (99%).
4
2
2
All=allyl.
4
354
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Angewandte
Chemie
Scheme 4. Regioselective acylation and HWE elongation: a) 16, EDCI, NEt , DMAP, CH Cl (43%) b) Lv O, pyridine, DMAP, CH Cl , 08C (18:
3
2
2
2
2
2
9
0%, 19: 84%); c) 20, pyridine, CH Cl , 08C (R=Me: 54%, R=CO All: 53%); d) Lv O, pyridine, DMAP, CH Cl (21: 87%, 22: 93 %); e)12
2 2 2 2 2 2
(
1 equiv), NaH, THF (23: 40%, 24: 72%). EDCI=N-ethyl-N’-(-dimethylaminopropyl)carbodiimide, Lv=levulinoyl.
with a phosphonate group at O-4, which is then used in a
HWE elongation. The required phosphonate functionality
was selectively installed at OH-4 of compounds 18 and 19 by
esterification with anhydride 20. Some overreaction at OH-2
was observed but could be minimized when DMAP was
omitted. Subsequently, OH-2 was blocked with levulinic acid
to give the fully protected terpenoyl quinovoside phospho-
nates 21 and 22. These compounds set the stage for the
envisioned HWE olefination with aldehyde 12. To our delight,
the coupling of the two glycosylated building blocks in the
presence of sodium hydride gave the desired dimeric terpene
glycosides 23 and 24 in yields of 40% and 72%, respectively
olefination. The required aldehyde 27 was obtained by
regioselective epoxidation and periodate cleavage of the
most activated double bond of the linalyl model compound
23. The HWE olefination of diosgenyl phosphonate 28
(obtained from diosgenin and phosphonate 20) and aldehyde
27 gave the complex neosaponin 26 with significantly higher
yields in the coupling step (52%, E diastereomer only).
Finally, the deprotection of the glycoconjugate 26 was
examined. Brief exposure to sodium methylate in methanol
selectively removed the acetyl moieties without affecting the
remaining ester bonds. The levulinates were cleaved with
[
20]
hydrazine acetate to furnish the deprotected neosaponin 29
(
E diastereomer only).
carrying a variant of the dimeric side chain (A) of julibrosides.
With the accessibility of synthetic terpenoyl glycoside con-
jugates (29) the biological properties of these unusual side
chains from cytotoxic saponins can be investigated.
To obtain conjugates of the dimeric terpenoyl glycoside
side chain A with other steroids, compound 24 was first
deallylated by palladium(0) catalysis (Scheme 5).
[
19]
The
resulting carboxylic acid 25 was successfully coupled to
diosgenin by means of a Yamaguchi esterification. However,
the yield of the complex neosaponin conjugate 26 was
moderate (37%), probably due to the intrinsically low
reactivity of the a,b-unsaturated, a-branched carboxylic
acid 25. For this reason we attempted elongation by a HWE
The presented synthetic strategy provides the first chem-
ical access to terpenoyl glycoside side chains from cytotoxic
saponins. A Horner–Wadsworth–Emmons elongation of
selectively phosphonate-functionalized glycosides with ter-
pene-derived aldehydes was identified as a key step in the
[
21]
construction of the glycoconjugates. The adaptation of this
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Communications
Scheme 5. Coupling of complex terpenoyl glycosides to diosgenin and deprotection: a) [Pd(PPh ) ], dimedone, THF (74%); b) Yamaguchi esterifi-
3
4
cation: 2,4,6-trichlorobenzoyl chloride, NEt , DMAP, diosgenin, 1,2-dichloroethane, reflux (37%); c) 1. MCPBA, 08C; 2. HIO ·2H O, THF/H O (c:
3
4
2
2
4
6%); d) 28, NaH, THF (52%); e) 1. NaOMe, MeOH/CH Cl , 5 min; 2. hydrazine acetate (e: 93%).
2
2
flexible strategy to the natural variations in stereochemistry
and functionalization of the terpene moieties is in progress.
11556; e) V. Haridas, C. J. Arntzen, J. U. Gutterman, Proc. Natl.
Acad. Sci. USA 2001, 98, 11557 – 11562.
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[
Received: September 17, 2003
Revised: April 2, 2004 [Z52895]
[
[
6
3.
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Keywords: antitumor agents · carbohydrates · natural products ·
olefination · saponins
.
2
[
1
[
[
[
[
[
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1
Nohara, T. Miyoshi, J. Nat. Prod. 1997, 60, 102 – 107; c) K. Zou,
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[
15] Compound 16 was obtained from aldehyde 12 by olefination
0
[19]
with phosphonate 13 followed by Pd -catalyzed deallylation.
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Sci. USA 2001, 98, 5821 – 5826; b) G. S. Jayatilake, D. R. Free-
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[20] 29: R
2
D
7
= 0.34 (silica gel, CH
Cl
2
/MeOH 10:1); [a] = À36.3 (c =
f
2
1
0.80, CH Cl ); H NMR (500 MHz, [D ]DMSO, D = diosgenin,
2
2
6
4
356
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Angew. Chem. Int. Ed. 2004, 43, 4353 –4357

Angewandte
Chemie
G = glucose, Q = quinovose, T= monoterpene): d = 6.69 (t, J =
Hz, 1H, T-3), 6.66 (t, J = 7 Hz, 1H T-3’), 5.98 (dd, Jtrans = 17.6,
7
Jcis = 11.0 Hz, 1H, T-7), 5.92 (dd, Jtrans = 17.6, Jcis = 11.0 Hz, 1H,
T-7’), 5.37–5.30 (m, 1H, D-6), 5.22–5.02 (m, 6H, 2T-8, Q-OH-2,
Q-OH-3), 4.87 (d, J2,OH = J3,OH = 4.4 Hz, 2H, G-OH-2, G-OH-3),
4
4
.82 (d, J4,OH = 4.4 Hz, 1H, G-OH-4), 4.52–4.45 (m, 1H, D-3),
.42 (dd, J3,4 = J4,5 = 9.3 Hz, 1H, Q-4), 4.32–4.24 (m, 3H, D-16,
Q-1, G-6OH), 4.17 (d, J1,2 = 7.7 Hz, 1H, G-1), 3.64–3.56 (m, 1H,
G-6a), 3.44–3.34 (m, 4H, G-6b, D-26a, Q-5, Q-3), 3.19 (dd, J =
1
J2 = 11 Hz, 1H, D-26b), 3.15–2.88 (m, 5H, G-3, Q-2, G-4, G-5,
G-2), 2.32–2.16 (m, 6H, D-4, 2T-4), 2.03–1.71 (m, 11H, D-15a,
D-7a, D-1a, D-20, D-2a, 2T-10), 1.71–1.43 (m, 14H, D-12a, D-17,
2
2
T-5, D-8, D-24a, D-2b, D-7b, D-25, D-11a, D-23), 1.43–0.87 (m,
2H, D-11b, D-24b, 2T-9, D-15b, D-12b, D-14, D-1b, D-19, Q-6,
1
3
D-9, D-21), 0.75–0.70 ppm (m, 6H, D-18, D-27); C NMR
125.8 MHz, [D ]DMSO): d = 166.6, 166.5 (2T-1), 143.5, 143.3,
(
6
142.9, 142.5 (2T-3, 2T-7), 139.5 (D-5), 127.0, 126.8 (2T-2), 121.9
(
D-6), 114.1, 113.8 (2T-8), 108.4 (D-22), 97.70 (G-1), 97.56 (Q-1),
8
7
6
0.2 (D-16), 78.7, 78.6 (2T-6), 77.0 (G-3), 76.6 (G-5), 75.9 (Q-4),
3.8 (Q-3, Q-2), 73.5 (G-2), 73.2 (D-3), 70.1 (G-4), 68.8 (Q-5),
5.9 (D-26), 61.8 (D-17), 61.1 (G-6), 55.7 (D-14), 49.4 (D-9), 41.1
(
1
(
D-20), 39.1 (D-12), 37.68, 37.64, 37.57 (2T-5, D-4), 37.1, 36.5 (D-
0, D-13), 36.2 (D-1), 31.48, 31.43 (D-7, D-15), 30.92 (D-8), 30.90
D-23), 29.8 (D-25), 28.5 (D-24), 27.4 (D-2), 24.05, 24.02 (2T-9),
2
2
2.79, 22.65 (2T-4), 20.3 (D-11), 19.0 (D-19), 17.7 (Q-6), 17.1 (D-
7), 16.0 (D-18), 14.6 (D-21), 12.31, 12.21 ppm (2T-10); ESI-MS:
m/z calcd for [C H O + Na]: 1077.6, found: 1077.7 [M+Na].
5
9
90 16
[
21] The esterification of phosphonate 20 with triterpene alcohols
bearing a secondary neopentyl OH group has been demon-
strated and subsequent HWE olefination is in progress.
Angew. Chem. Int. Ed. 2004, 43, 4353 –4357
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