Total Synthesis of the Natural Products Ulmoside A and (2 R,3 R)-Taxifolin-6- C -β- d -glucopyranoside

Article abstract of DOI:10.1055/s-0040-1707971

An efficient first total synthesis of highly polar ulmoside A and (2 R,3 R)-taxifolin-6- C -β- d -glucopyranoside, useful for the prevention of metabolic disorders, has been described. Key elements of the synthesis include a Sc(OTf) ₃-catalyzed regio- and stereoselective C -glycosidation on taxifolin in 35percent yield with d -glucose and chiral semipreparative reverse-phase high-performance liquid chromatography (HPLC) for the separation of both taxifolins and the diastereomeric mixture of taxifolin-6- C -β- d -glucopyranosides. Correlation of the analytical data of synthetic ulmoside A and its diastereomer with a natural ulmoside A sample confirmed the assigned absolute stereochemistry of the natural products.

Full text of DOI:10.1055/s-0040-1707971

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© Georg Thieme Verlag Stuttgart · New York
2020, 31, A–E
letter
A
en
L. Macha et al.
Letter
Synlett
Total Synthesis of the Natural Products Ulmoside A and (2R,3R)-
Taxifolin-6-C--D-glucopyranoside
Lingamurthy Machaa#
Aravind Reddy Dorigundlaa,b#
Raju Gurrapua
Umamaheswara Sarma Vankaa
Venkateswara Rao Batchu*a,b
OH
OH
OH
OH
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O
a Organic Synthesis and Process Chemistry Division, CSIR-Indian
Institute of Chemical Technology,, Hyderabad-500007, India
venky@csiriict.in
ulmoside A
protecting-group-free
one-step convergent synthesis from aglycon and glycon in 35% yield
(2R,3R)-taxifolin-6-C-b-D-glucopyanoside
drb.venky@gmail.com
b Academy of Scientific and Innovative Research, New Delhi, India
# Both authors contributed equally.
Received: 04.02.2020
Accepted after revision: 19.02.2020
Published online: 02.03.2020
wallichiana plant2a–c (Figure 1). The bark of this plant is
known in practice in some parts of India to treat bone frac-
tures.2b Mourya and co-workers evaluated their activity in
stimulating osteoblast differentiation, which is a bone ana-
bolic function needed for osteoporosis therapy. They also
developed a composition for osteoporosis therapy with
these compounds. Among them, ulmoside A (1) shows ex-
cellent bone deflecting activity and efficacy in treating ste-
roid-induced metabolic disorders.2d Interestingly, it has
shown excellent adiponectin agonist activity, useful for the
cure and prevention of diabetes, and also antioxidative and
antiapoptotic activity.2e Stark et al. also isolated ulmoside A
(1) and (2R,3R)-taxifolin-6-C--D-glucopyranoside (2) and
other flavonoid C-glycosides from the bark of medicinal
plant Garcinia buchananii.2c,3a The H2O2 scavenging and ox-
ygen radical absorbance capacity (ORAC) assays demon-
strated that compounds 1 and 2 show extraordinary antiox-
idative power. Earlier, (2R,3R)-taxifolin-6-C--D-glucopyra-
noside (2) was also isolated from Garcinia epuntata (Figure 1).3b
The above compounds can only be obtained from natu-
ral sources in very small quantities. Moreover, evaluation of
their biological activity and their development into a drug
requires more quantities of the compounds. Therefore, a
simple, short, and scalable strategy is indeed essential. To
meet the above requirements we have undertaken the total
synthesis of those compounds. Interestingly, these two
compounds show similar chemical shifts in their NMR
spectra, and both have a positive optical rotation. They are
only distinguished by their CD spectra. So far, there are no
reports on the synthesis of these compounds in the litera-
ture. Hence, their total synthesis also helps confirming the
assigned stereochemical structures. The important issue to
be addressed for designing a synthesis for this class of
compounds is the C–C bond formation between the aglycon
and carbohydrate moiety to give corresponding aryl C-gly-
cosides.
DOI: 10.1055/s-0040-1707971; Art ID: st-2020-k0073-l
Abstract An efficient first total synthesis of highly polar ulmoside A
and (2R,3R)-taxifolin-6-C--D-glucopyranoside, useful for the preven-
tion of metabolic disorders, has been described. Key elements of the
synthesis include a Sc(OTf)3-catalyzed regio- and stereoselective C-gly-
cosidation on taxifolin in 35% yield with D-glucose and chiral semipre-
parative reverse-phase high-performance liquid chromatography
(HPLC) for the separation of both taxifolins and the diastereomeric mix-
ture of taxifolin-6-C--D-glucopyranosides. Correlation of the analytical
data of synthetic ulmoside A and its diastereomer with a natural ulmo-
side A sample confirmed the assigned absolute stereochemistry of the
natural products.
Key words flavonoids, C-glycosidation, chiral HPLC, metal triflates,
aglycone
Glycosyl flavonoids are ubiquitously distributed in na-
ture and commonly found as secondary metabolites of
plant or bacteria. They exhibit a wide range of very interest-
ing biological activities. The glycosyl flavonoids are mostly
present in nature as O-glycosides and rarely as C-glyco-
sides.1 In general, C-glycosides show excellent and different
bioactivity when compared to the corresponding O-glyco-
sides and aglycons. The C–C bond in these glycosides that
links the sugar to aglycon appears to be of great impor-
tance, because the C-glycosyl bond is stable to enzymatic
hydrolysis under physiological conditions, unlike the corre-
sponding O-glycosides.1c Therefore, some aryl C-glycosides
are on the market as SGL2 inhibitors for treating diabetes.1d
Mourya and co-workers, in 2009, discovered and identi-
fied three new C-glycosylated flavonoids that were consid-
ered as stable glycosides in the biological system, namely
(2S,3S)-taxifolin-6-C--D-glucopyranoside (or) K058 (or)
ulmoside A, (2S,3S)-aromadendrin-6-C--D-glucopyrano-
side, and quercetin-6-C--D-glucopyranoside from Ulmus
© 2020. Thieme. All rights reserved. Synlett 2020, 31, A–E
Georg Thieme Verlag KG, Rüdigerstraße 14, 70469 Stuttgart, Germany

B
L. Macha et al.
Letter
Synlett
OH
OH
OH
OH
OH
OH
OH
OH
HO
O
O
HO
HO
O
O
HO
O
O
O
HO
O
O
HO
HO
HO
O
O
HO
HO
HO
HO
HO
HO
HO
OH
HO
HO
OH
OH
OH
HO
HO
HO
OH
OH
OH
OH
O
1
2
(2S,3S)-taxifolin-6-C-β-D-
glucopyranoside
(ulmoside A / K058)
(2R,3R)-taxifolin-6-C-β-D-glucopyranoside
OH
D-glucose
HO
D-glucose
L.A
L.A
OH
OH
OH
OH
OH
O
HO
O
O
HO
O
O
O
HO
HO
O
O
O
HO
HO
HO
OH
HO
HO
OH
OH
OH
OH
O
HO
OH
HO
OH
(_)-(2S,3S)-taxifolin (4)
OH
(+)-(2R,3R)-taxifolin (3)
(2R,3R)-aromadendrin-6-C-β-D
(2S,3S)-aromadendrin-6-C-β-D
OH
glucopyranoside
glucopyranoside
OH
OH
OH
OH
OH
HO
O
O
OH
OH
HO
O
HO
O
O
O
HO
O
O
O
HO
HO
OH
OH
HO
HO
HO
HO
OH
OH
OH
OH
(
)-taxifolin (6)
HO
HO
OH
OH
(+)-(2R,3S)-catechin (5)
(2S,3R)-taxifolin-6-C-β-D-glucopyranoside quercetin-6-C-β-D-glucopyranoside
OMOM
OMOM
(K012)
OMOM
MOMO
Figure 1 Structures of some natural flavonoid C-glycosides
MOMO
O
7
Formation of the C–C bond between carbohydrate units
and electron-rich aromatic moieties was first achieved by
Friedel–Crafts reaction.4 Later, Suzuki and co-workers5 and
Kometani et al.6 developed a methodology for the C-glyco-
sylation of phenols by a Lewis acid-catalyzed rearrange-
ment of O-glycoside to a C-glycosyl derivative, known as a
Fries-type reaction. In all these general methods, protected
sugar and aromatic moieties have been used for glycosida-
tion that, finally, have to be deprotected to obtain the target
compounds. Recently, Sato and others reported the C-glyco-
sidation of unprotected flavonoids with unprotected sugars
using metal triflates.7–9 Although the yields obtained with
use of this method are lower and although the method was
applied on simpler systems, we wanted to apply this meth-
od for the synthesis of chiral polyphenolic compounds 1
and 2.
On the basis of the assigned stereochemistry, the ret-
rosynthetic route for ulmoside A (1) and (2R,3R)-taxifolin-
6-C--D-glucopyranoside (2) has been designed (Scheme 1).
We envisioned a one-step synthesis of the target molecules
1 and 2 from corresponding taxifolins 3 and 4 by protect-
ing-group-free C-glycosidation with D-glucose in the pres-
ence of metal triflates. The (+)-taxifolin (3) required for the
synthesis of (2R,3R)-taxifolin-6-C--D-glucopyranoside (2)
is commercially available but it is expensive; rather it can
be prepared from (+)-catechin hydrate (5) by using a re-
ported procedure.10 In the case of (–)-taxifolin (4), the agly-
con part of compound 1, to the best of our knowledge, nei-
ther its isolation nor its synthesis is reported in the litera-
ture. Only the racemic synthesis of taxifolin 6 is reported in
the literature.11 Therefore, we decided to synthesize both
HO
O
HO
OH
+
OH
OH
phloroglucinol (8)
OH
caffeic acid (9)
Scheme 1 Retrosynthetic analysis
the taxifolins 3 and 4 from the chalcone compound 7 using
epoxidation followed by a cyclization approach. Compound
7 could be obtained from commercially available phloroglu-
cinol (8) and caffeic acid (9) (Scheme 1).
In order to prepare (+)-taxifolin (3), the hydroxy groups
of (+)-catechin hydrate (5) were benzylated by treatment
with BnBr and K2CO3 to give 5,7,3′,4′-tetra-O-benzyl-(+)-
catechin, which, on further treatment with BnBr and NaH,
afforded 3,5,7,3′,4′-pent-O-benzyl-(+)-catechin 10 in 73%
yield (over two steps) (Scheme 2).10 In the previous re-
port,10b oxidation of 3,5,7,3′,4′-pent-O-benzyl-(+)-catechin
10 by using 2,3-dichloro-5,6-dicyano-1,4-benzoquinone
(DDQ) in a mixed solvent of CH2Cl2/H2O afforded desired
keto compound 11 in 15% yield. To improve the yields in the
oxidation, compound 10 was subjected to excess DDQ in a
mixed solvent of CH2Cl2/1,4-dioxane/H2O (8:4:1), which af-
forded the desired keto compound 11 in 80% yield.12
Further, hydrogenolysis of keto compound 11 in the presence
of H2 and 10% Pd/C in MeOH/THF afforded (+)-taxifolin (3)
in 85% yield.10
© 2020. Thieme. All rights reserved. Synlett 2020, 31, A–E

C
L. Macha et al.
Letter
Synlett
Table 1 Optimization of C-Glycosidation Reaction Conditions
OH
OH
OBn
OBn
HO
O
BnO
O
OH
OH
Ref. 10b
OH
OH
HO
O
O
O
OBn
HO
O
O
HO
OH
OH
OBn
OBn
10
5
OH
MeCN/H2O
reflux
OH
OBn
HO
HO
OH
OH
OH
BnO
O
DDQ
(2R,3R)-taxifolin-6-C-β-D-glucopyanoside (2)
(+)-taxifolin (3)
OBn
10% Pd/C, H2
CH2Cl2/1,4-dioxane/H2O (8:4:1)
THF/MeOH (2:1)
48 h, 85%
0 °C to r.t., 8 h, 80%
Entry Catalyst
Equiv.
MeCN/H2O
Time (h) Yield (%)b
OBn O
11
1
2
3
4
5
6
7
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
Sm(OTf)3
Bi(OTf)3
Yb(OTf)3
0.2
0.2
0.2
0.4
0.4
0.4
0.4
1:1
1:1
2:1
2:1
2:1
2:1
2:1
24
48
48
48
48
48
48
10
OH
HO
O
O
15
OH
20
OH
OH
35
(+)-taxifolin (3)
8
Scheme 2 Synthesis of (+)-taxifolin (3)
5
no reactionc
Having synthesized the required (+)-taxifolin (3), we
next turned our attention to a regio- and stereoselective
protecting-group-free C-glycosidation between (+)-taxifo-
lin (3) and D-glucose. Thus, experiments were carried out
with different catalysts under different solvent ratios as
shown in Table 1. The highest yield (35%) of compound 2
was obtained with scandium triflate (0.4 equiv) with a 2:1
ratio of acetonitrile and water heated to reflux for 48 hours
a Reaction conditions: (+)-taxifolin (3) (1 equiv), D-glucose (2.5 equiv),
M3+(OTf)3, solvent, reflux.
b Isolated yield.
c Based on TLC analysis.
phy (HPLC) allowed us to successfully separate 3 and 4
(Scheme 4). The separated enantiomers have the same
physical data with opposite optical rotation values. The
spectral and analytical data of 3 and 4 were in full agree-
ment with the literature values, and also the values of the
product obtained from (+)-catechin hydrate (5) in Scheme
2.10c
(Table 1, entry 4).13 All analytical data including 1H and 13
C
NMR spectra of the synthetic (2R,3R)-taxifolin-6-C--D-glu-
copyranoside (2) were in agreement with reported values.3
Then, we decided to synthesize (±)-taxifolin (6) from
chalcone 7 as shown in Scheme 3. For this purpose, a new
and biomimetic approach was developed to obtain 7. Treat-
ment of phloroglucinol (8) with methoxymethyl chloride
(MOMCl) and NaH in dry DMF gave compound 12 in 90%
yield. Esterification of caffeic acid (9) with MeOH in the
presence of H2SO4 as a catalyst and protection of the pheno-
lic groups with MOMCl provided the ester compound. Hy-
drolysis of the resultant ester with NaOH in MeOH/THF af-
forded the acid compound 13 in 90% yield. Further, acyla-
tion of compound 12 with acid 13 in the presence of
trifluoroacetic anhydride (TFAA) in CH2Cl2 afforded chal-
cone 7 in 85% yield. Treatment of compound 7 with alkaline
hydrogen peroxide led to the formation of racemic epoxide
14, which was subjected to concd HCl in MeOH/THF to af-
ford (±)-taxifolin (6) in 85% yield (Scheme 3).11 Our proce-
dure gave better overall yields (72%) when compared to the
earlier reported procedure11 (40%) from MOM-protected in-
termediates, and also it involves cheaply available starting
materials. Our approach has an additional advantage: The
caffeic acid can be converted into a chiral epoxy or dihy-
droxy derivative, which, upon acylation on 12 followed by
cyclization, can lead to chiral taxifolins.
HO
OH
MOMO
OMOM
NaH, MOMCl
DMF, 90%
OH
8
OMOM
12
O
O
OH
i. MeOH, H2SO4
OH
ii. NaH, MOMCl, DMF
iii. NaOH, MeOH/THF
90%
OMOM
OH
OH
OMOM
9
13
OMOM
OMOM
OMOM
MOMO
H2O2, NaOH
MeOH
TFAA, CH2Cl2
12 + 13
–5 °C, 30 min
85%
7
MOMO
O
OH
OH
OMOM
OMOM
OMOM
HO
O
MOMO
concd HCl
O
OH
MeOH/THF
85%
OH
O
MOMO
O
(
)-taxifolin (6)
14
Having (±)-taxifolin (6) in hand, our next step was to re-
solve it to obtain enantiomers 3 and 4. Chiral semiprepara-
tive reverse-phase high-performance liquid chromatogra-
Scheme 3 Synthesis of (±)-taxifolin (6)
© 2020. Thieme. All rights reserved. Synlett 2020, 31, A–E

D
L. Macha et al.
Letter
Synlett
OH
OH
OH
OH
OH
HO
OH
HO
O
O
HO
O
O
O
O
chiral HPLC separation
of enantiomers
+
OH
OH
OH
OH
OH
OH
(
)-taxifolin (6)
(−)-taxifolin (4)
(+)-taxifolin (3)
Scheme 4 Chiral HPLC separation of racemic taxifolins 6
To accomplish the second target molecule ulmoside A
(1), we initiated our synthesis from the (–)-taxifolin (4) as
shown in Scheme 5. Thus, C-glycosidation of (–)-taxifolin
(4) with D-glucose in the presence of Sc(OTf)3 in MeCN/H2O
furnished ulmoside A (1) in 35% yield.11 Conducting the re-
action for 48 hours helped us to obtain 35% yield of 1,
whereas Sato’s conditions gave 25% yield of the product in
glycosidation. During the column purification of the prod-
uct, the unreacted starting material was also recovered.
Based on the recovery the yield of the product is around
93%. All analytical data including 1H and 13C NMR spectra of
the synthetic ulmoside A (1) were in full agreement with
the data of a natural sample of ulmoside A (1).2a,13,14
A simple and scalable first synthesis of highly polar ul-
moside A (1) and (2R,3R)-taxifolin-6-C--D-glucopyrano-
side (2), useful for the prevention of metabolic disorders,
has been described. Scandium triflate proved to be a suit-
able catalyst for the regio- and stereoselective C-glycosyla-
tion on taxifolins (3) and (4) in 35% yield with unprotected
D-glucose. Despite this, an achiral semipreparative HPLC
method was developed for the separation of both (±)-taxi-
folin (6) and the diastereomeric mixture of taxifolin-6-C--
D-glucopyranosides 1 and 2. The absolute configuration of
aglycon of ulmoside A (1) and its diastereomer (2) was con-
firmed as 2S,3S and 2R,3R. This strategy is also useful to
build a library of compounds with different sugars and
polyphenols for better therapeutic activity. Most of these
flavonoid C-saccharides are present in dietary supplements
and also in the extracts used in Indian traditional medi-
cines. These compounds are less toxic and have a high prob-
ability to become drugs. Therefore, we are planning to ap-
ply this approach to the synthesis of other flavonoid C-sac-
charides. Further, chiral epoxidation of chalcone 7 or
converting caffeic acid into chiral epoxy or hydroxy deriva-
tives for acylation can lead to a chiral synthesis of taxifolins.
OH
OH
OH
HO
O
O
HO
O
HO
OH
OH
OH
Sc(OTf)3, D-glucose
HO
HO
OH
OH
O
OH
O
(_)-taxifolin (4)
MeCN/H2O (2:1)
reflux, 48 h, 35%
ulmoside A (1)
Scheme 5 Total synthesis of ulmoside A (1)
Next, we proceeded with similar conditions as men-
tioned in Scheme 5, to obtain the mixture of ulmoside A (1)
and (2R,3R)-taxifolin-6-C--D-glucopyranoside (2) from
(±)-taxifolin (6) and D-glucose (Scheme 6). Chiral semipre-
parative reverse-phase HPLC allowed us to separate suc-
cessfully both ulmoside A (1) and its diastereomer 2 with-
out any loss of compound. The HPLC chromatogram of syn-
thetic ulmoside A (1) revealed a peak at 23.6 min, which
was very well correlated with that of natural ulmoside A
(see Supporting Information). The complete analysis of
HPLC data clearly confirmed that the absolute configuration
of the aglycon part of ulmoside A (1) is 2S,3S and that of its
diastereomer is 2R,3R.
Funding Information
M.L.M and G.R. thank the Council of Scientific and Industrial Research
(CSIR). A.R.D thanks the University Grants Commission (UGC), New
Delhi for financial support as part of XII Five Year Plan programme
under the titles ORIGIN (CSC-0108) and DENOVA (CSC-0205); Manu-
script communication number: IICT/Pubs./2019/342.
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n
c
i
l of
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c
i
e
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n
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ustri
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i
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ersityGrants
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(C
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Acknowledgment
We thank Dr. T. K. Chakraborthy, former Director of CSIR-CDRI, for
providing the sample of natural ulmoside A. The authors also thank
the Director of CSIR-IICT for the constant support and encourage-
ment.
OH
OH
HO
O
O
O
HO
O
O
HO
OH
Sc(OTf)3, D-glucose
OH
OH
MeCN/H2O (2:1)
reflux, 48 h, 35%
OH
HO
HO
OH
OH
OH
ulmoside A (1)
(
)-taxifolin (6)
and
(2R,3R) taxifolin-6-C-β-D-glucopyranoside (2)
Scheme 6 Direct C-glycosidation on (±)-taxifolin (6)
© 2020. Thieme. All rights reserved. Synlett 2020, 31, A–E

E
L. Macha et al.
Letter
Synlett
Supporting Information
(6) Kometani, T.; Kondo, H.; Fujimori, Y. Synthesis 1988, 1005.
(7) (a) Sato, S.; Akiya, T.; Nishizawa, H.; Suzuki, T. Carbohydr. Res.
2006, 341, 964. (b) Van Der Westhuizen, J. H.; Ferreira, D.;
Joubert, E.; Bonnet, S. L. WO2011064726A1, 2011. (c) Sato, S.;
Koide, T. Carbohydr. Res. 2010, 345, 1825.
Supporting information for this article is available online at
https://doi.org/10.1055/s-0040-1707971.
S
u
p
p
orti
n
gInformati
o
n
S
u
p
p
orit
n
gInformati
o
n
(8) Santos, R. G.; Xavier, N. M.; Bordado, J. C.; Rauter, A. P. Eur. J. Org.
Chem. 2013, 1441.
References and Notes
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(c) Kikuchi, T.; Nishimura, M.; Hoshino, A.; Mortia, Y.; Saito, N.;
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J. R.; Bhadauria, S.; Trivedi, A. K.; Singh, A. K.; Mishra, J. Y.;
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S. K. D.; Yadav, M.; Dixit, P.; Mishra, D. P.; Sharma, S.; Kamal, R.
A. PCT Patent PCT/IN2014/000464, 2014; (US2016/019324,
2014. (e) Gupta, P.; Singh, A.; Tiwari, S.; Mishra, A.; Maurya, R.;
Singh, S. Neurotoxicology 2019, 73, 100.
(3) (a) Stark, T.; Matsutomo, T.; Losch, S.; Boakye, P.; Balemba, O.;
Pasilis, S.; Hofmann, T. J. Agric. Food Chem. 2012, 60, 2053.
(b) Mbafor, J. T.; Fomum, Z. T.; Promsattha, B.; Sanson, D. R.;
Tempesta, M. S. J. Nat. Prod. 1989, 52, 417.
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C. The Chemistry of C-Glycosides; Pergamon: Oxford, 1995.
(c) Postema, M. H. D. C-Glycoside Synthesis; CRC Press: London,
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Org. Chem. Jpn. 1998, 56, 841. (f) Schmidt, R. R.; Hoffmann, M.
Tetrahedron Lett. 1982, 23, 409.
(14) Ulmoside A (1)
A solution of (–)-taxifolin (4) (0.3 g, 0.98 mmol) and D-glucose
(0.44 g, 2.46 mmol) was stirred in a mixture of solvents
MeCN/H2O (2:1, 10 mL) heated to reflux in an oil bath for 48 h in
the presence of Sc(OTf)3 (0.19 g, 0.39 mmol). Then, the reaction
mixture was subjected to silica gel column chromatography
(acetone/EtOAc/H2O/AcOH 15:30:2:0.1) to afford ulmoside A (1)
as a white amorphous powder yield: 0.16 g (35%); []D25 = +1.56
(c = 0.1, MeOH){lit2a[]D25 = +1.33 (c = 0.098, MeOH)}. IR (neat):
3325, 2945, 2833, 1646, 1450, 1413, 1017 cm–1 1H NMR (500
.
MHz, DMSO-d6):  = 12.47 (s, 1 H, HO-C(5)), 9.03 (s, 1 H, HO-
C(4')), 8.98 (s, 1 H, HO-C(3')), 6.84 (s, 1 H, H-2'), 6.71 (s, 2 H, H-
5', 6'), 5.87 (s, 1 H, H-8), 5.78 (s, 1 H, HO-C(3)), 4.92 (d, J = 10.9
Hz, 1 H, H-2), 4.82 (br s, 2 H, OH), 4.58 (br s, 1 H, OH), 4.46–4.47
(m, 1 H, H-6''b), 4.45 (d, J = 10.0 Hz, 1 H, H-1''), 4.43 (d, J = 10.9
Hz, 1 H, H-3), 3.96 (t, J = 9.1, 9.3 Hz, 1 H, H-2''), 3.63 (d, J = 10.9
Hz, 1 H, H-6''a), 3.04–3.15 (m, 3 H, H-3'', 4'', 5''). 13C NMR (125
MHz, DMSO-d6):  = 198.0 (C-4), 166.0 (C-7), 162.6 (C-5), 161.3
(C-9), 145.8 (C-3'), 145.0 (C-4'), 128.0 (C-1'), 119.4 (C-6'), 115.3
(C-2'), 115.1 (C-5'), 106.0 (C-6), 100.2 (C-10), 94.7 (C-8), 82.9 (C-
2), 81.6 (C-5''), 79.1 (C-3''), 72.9 (C-1''), 71.6 (C-3), 70.7 (C-2''),
70.3 (C-4''), 61.6 (C-6''). FAB-MS: m/z 467 [M + H]+.
(5) Matsumoto, T.; Katsuki, M.; Suzuki, K. Tetrahedron Lett. 1988,
29, 6935.
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