Highly efficient synthesis of flavonol 5-O-glycosides with glycosyl ortho-alkynylbenzoates as donors

Article abstract of DOI:10.1039/c5ob02313k

With glycosyl ortho-alkynylbenzoates as donors, the highly efficient glycosylation of flavonoid 5-OH which are notorious for their low reactivity due to their involvement in the formation of strong intramolecular H-bonds was achieved under the catalysis of a Au(i) complex. Thus, a series of flavonoid 5-O-glycosides, including a kaempferol 5-O-disaccharide, were synthesized with good to excellent yields.

Full text of DOI:10.1039/c5ob02313k

Organic &
Biomolecular Chemistry
View Article Online
View Journal
COMMUNICATION
Highly efficient synthesis of flavonol
5-O-glycosides with glycosyl
Cite this: DOI: 10.1039/c5ob02313k
ortho-alkynylbenzoates as donors†
Received 10th November 2015,
Accepted 27th November 2015
Jin-Xi Liao, Nai-Li Fan, Hui Liu, Yuan-Hong Tu and Jian-Song Sun*
DOI: 10.1039/c5ob02313k
www.rsc.org/obc
With glycosyl ortho-alkynylbenzoates as donors, the highly promising pharmaceutical activities. For example, apigenin
efficient glycosylation of flavonoid 5-OH which are notorious for 5-O-glycoside(II), isolated from Cephalotaxus sinensis, was
their low reactivity due to their involvement in the formation of demonstrated to have a significant antihyperglycemic effect,
strong intramolecular H-bonds was achieved under the catalysis of although limited by accessibility the function mechanism was
a Au(I) complex. Thus, a series of flavonoid 5-O-glycosides, includ- not deciphered.3a Besides, as reference compounds, flavonol
ing a kaempferol 5-O-disaccharide, were synthesized with good to 5-O-glycosides are in high demand in the metabolic research
excellent yields.
of flavonol compounds in humans. Structurally, flavonol 5-O-
glycosides are distinguished from their cousins by the exist-
ence of flavonol 5-O-glycosidic linkages, which pose a con-
siderable challenge for synthetic chemists equipped with
modern synthetic arsenals. The difficulty associated with the
construction of 5-OH glycosidic linkages resides in the extra-
ordinary low reactivity of these phenolic OHs since they are
involved in the formation of intramolecular H-bonds with the
C-4 carbonyl group. This kind of intramolecular H-bond is so
strong that it makes simple protecting group manipulations
difficult, let alone sophisticated glycosylation reactions. Of
course, the electron-withdrawing properties of 4-carbonyl
groups can further diminish the nucleophilicity of 5-OHs. In
addition, because of the high tendency to form a stable intra-
molecular H-bond system, flavonol 5-O-glycosidic linkages are
so acid sensitive that even a mild acid can induce rapid glyco-
sidic linkage cleavage, which further increases the difficulty in
fashioning such special bonds.4
Flavonoid O-glycosides are widely found in the plant
kingdom,1 and they play a variety of important roles in the
growth and development of plants, e.g. as interspecies signal-
transferring molecules.2 In particular, they have also been
demonstrated to possess a wide spectrum of activities bene-
ficial to humans, such as antimicrobial, anticancer, and
radical-scavenging activities.1,2 Thus far, more than 1500 flavo-
noid O-glycosides have been characterized, and the number is
still increasing rapidly. As a special type of flavonoid O-glyco-
side, flavonol 5-O-glycosides are seldom found in nature, and
so far only around 10 of them including brachyside(I) and api-
genin 5-O-glycoside(II) have been isolated and characterized
(Fig. 1).3 In spite of the scarcity of flavonol 5-O-glycosides,
some of their known members have been proven to possess
Employing the powerful glycosyl trichloroacetimidate
donor,5 Schmidt et al. made the first attempt to overcome this
daunting problem.6 Under the catalysis of TMSOTf, a low yield
of the desired flavonol 5-O-glycoside was obtained. The ineffi-
ciency can probably be ascribed to the high acid-sensitivity of
the incipient formed flavonol 5-O-glycosidic linkage to
TMSOTf used as a glycosylation catalyst. To avoid the detri-
mental effect of a Lewis acid, an alternative PTC protocol with
glycosyl bromide as donors was introduced by the groups of
Dangles7a and Rolando,7b respectively. The intramolecular
H-bond prohibits the release of the 5-OH protons, therefore
the glycosylation yields recorded are far from satisfactory. We
also made some efforts in this field, and capitalizing on Yu
glycosylation8 the rhamnosyl residues were incorporated
onto the 5 and 4′-OHs of kaempferol at the same time.7c
Fig. 1 The chemical structures of representative flavonoid 5-O-
glycosides.
The National Engineering Research Center for Carbohydrate Synthesis, Jianxi
Normal University, Nanchang 330022, China. E-mail: jssun@jxnu.edu.cn
†Electronic supplementary information (ESI) available: Copies of the 1H NMR
and 13C spectra of all new compounds and the 2D spectra of 2, 4, 5, 10, and 28.
See DOI: 10.1039/c5ob02313k
This journal is © The Royal Society of Chemistry 2015
Org. Biomol. Chem.

View Article Online
Communication
Organic & Biomolecular Chemistry
Nevertheless, the method seriously suffered from a narrow correlations with one of the two signals assigned to H-6 and
substrate scope, only the rhamnosyl ortho-alkynylbenzoate H-8, which demonstrated that the free phenolic OH was 5-OH.
donor could offer a practically useful yield. Therefore, a highly If the free phenolic OH resided on C-7, then its correlations
efficient approach to forge flavonol-5-O-glycosidic linkages is with both H-6 and H-8 should be detected.11
in high demand.
Following known procedures, representative glycosyl ortho-
In line with our continuous interest in complex bioactive alkynylbenzoate donors including perbenzoylated glucosyl
flavonoid glycoside synthesis, we decided to make a systematic ortho-cyclopropylethynylbenzoate 6,9 6-OH selectively protected
investigation into the construction of the challenging flavonol glucosyl ortho-cyclopropylethynylbenzoate 7,9 galactosyl ortho-
5-O-glycosidic linkages. Although we have encountered some cyclopropylethynylbenzoate 8,9 and its rhamnosyl counterpart
unwanted problems in our preliminary studies with glycosyl 99 were obtained without any event (Fig. 2). In all the donors
ortho-alkynylbenzoate as a donor,7c we still believe that the 6–9, the 2-OHs were protected with benzyl groups which could
advent of this conceptually new glycosylation protocol can control the stereoselectivity via anchimeric participation in the
bring promise to solve this long-lasting problem, since the following glycosylation steps so as to afford 1,2-trans-glycosides
glycosylation protocol is powerful while the promotion con- solely.
ditions are extremely mild. Herein, we would like to report on
With both glycosyl donors and acceptors in hand, now the
an efficient approach to fashioning flavonol 5-O-glycosidic lin- stage was set for the pivotal glycosydic linkage construction
kages with glycosyl ortho-alkynylbenzoates as donors. Based (Table 1). Glycosylation between donor 6 and acceptor 2 was
on the established protocol, the first synthesis of the proposed selected as a model reaction to screen the optimal conditions.
apigenin 5-O-disaccharide was also accomplished.
The selection of TBS protected acceptor 2 was based on the
To facilitate the screening of the optimal conditions, a conjecture that the high electron-donating properties of TBS
series of flavonol 5-OH acceptors were synthesized (Scheme 1). protecting groups may increase the nucleophilicity of 5-OH,
The discrimination of 5-OHs became feasible with the help of therefore a reasonable glycosylation yield may be expected.
intramolecular H-bonds. Thus, treated with TBSCl and DBU, Thus, treated with 0.2 equivalents of PPh3AuNTf2, acceptor 2
apigenin, kaempferol, and quercetin were thoroughly silylated. was subjected to condensation with donor 6 (1.5 equiv.) at
After general work-up, the fully silylated kaempferol and quer- room temperature (10 °C). Disappointedly, no desired glycosy-
cetin were then subjected to regioselective desilylation of 5- lation product was detected, and the only detectable side-reac-
OTBS, which was facilitated by the propensity of the oxygen tion was the partial hydrolysis of donor 6 (entry 1). It is
atoms of 5-OTBS to coordinate a proton with the aid of the C-4
carbonyl group to form stable intramolecular H-bond systems,
to generate 29a and 3 in excellent yields (85% and 90%,
2 steps). For the synthesis of apigenin analogues 1,10 the 5-O-
desilylation step was not required because it was so labile that
the general work-up could remove it to afford 1 in a good 89%
yield. Replacing the strong DBU base with mild ones such as
K2CO3 and Et3N could lead to the removal of all phenolic
protons except for 5-OH protons restricted in intramolecular
Fig. 2 Chemical structures of glycosyl ortho-alkynylbenzoate donors.
H-bonds, thus in the presence of BnCl or hexanoyl chloride 49c
and 5 were generated conveniently. To confirm the structures
of all acceptors, the 1H NMR data of known compounds
including 1, 2, and 4 were compared with those reported in
Table 1 The optimizations of flavonol 5-OH glycosylation
the literature, and no evident discrepancy was detected. NOE
spectra of 3 and 5 were made, the proton signals of the free
phenolic OHs residing at 12.68 and 12.70 ppm only had cross-
Equivalents
of donor
Temperature
(°C)
Entries
Catalyst (equiv.)
Yielda
1
2
3
1.5
1.5
1.5
PPh3AuNTf2 (0.2)b
PPh3AuNTf2 (0.2)c
PPh3AuNTf2 (0.2)c
10
10
30
NP
35%
90%
Scheme 1 Synthesis of flavonol 5-OH acceptors.
a Isolated yields. b Ag(I) contaminated catalyst. c Pure Au(I) catalyst.
Org. Biomol. Chem.
This journal is © The Royal Society of Chemistry 2015

View Article Online
Organic & Biomolecular Chemistry
Communication
notable that the reaction mixture turned dark after being tor 3 afforded quercetin 5-O-glycosides 14–17 in yields ranging
stirred in the presence of PPh3AuNTf2 for 1 h. This phenom- from 65% to 99%. Surprisingly, condensation of 1, the sim-
enon likely resulted from the appearance of silver metal via plest one in the TBS protected acceptor series, and 6 under the
reduction of Ag(I) contained in PPh3AuNTf2 by reductive 2.12 identical conditions was found to proceed sluggishly, only a
The contamination of the Au(I) complex by Ag(I) salt is inevit- trace amount of the apigenin 5-O-glucoside product was
able since only filtration through a pad of Celite work-up is detected after stirring for 20 h at 30 °C.
adopted to obtain the PPh3AuNTf2 catalyst in the standard
The strikingly different performance of 1 in comparison
preparation procedure.13 In order to prevent the adverse effect with 2 and 3 in glycosylation reaction probably originates from
of excess Ag(I) in the desired glycosylation step, a slight modifi- the lack of the 3-OH. The presence of 3-OH in acceptors 2 and
cation of Au(I) catalyst preparation was made, that is, an 3 may probably weaken the strength of the intramolecular
additional filtration through a pad of silica gel was introduced. H-bond, in turn facilitating the glycosylation of the 5-OH.9c,14
Thus the obtained PPh3AuNTf2, as a greyish-white solid, was This hypothesis was confirmed by comparison of the chemical
then tried for glycosylation between 2 and 6. To our delight, shifts of the 5-OHs, indicative of intramolecular H-bond
35% yield of the desired glycoside 10 was isolated (entry 2). In strength.15 The 5-OH of 1 appeared at 12.72 ppm, while the
addition, no abnormal darkness was noticed any more in the corresponding protons of 2 and 3 had chemical shifts of
reaction mixture. Upon raising the reaction temperature to 12.67 ppm and 12.68 ppm, respectively. Evident up-field shifts
30 °C, we were rewarded by a dramatic enhancement in chemi- of 5-OH protons were observed for acceptors 2 and 3, indicat-
cal yield (90%, entry 3). The optimal reaction conditions were ing the strength decrease of the corresponding intramolecular
thus fixed as follows: under the effect of 0.2 equivalents of H-bonds. Actually, a hint of intramolecular H-bond strength
PPh3AuNTf2 generated by the slightly modified procedure, difference between 1 and 2, 3 was also provided during their
donor (1.5 equiv.) condensed with the acceptor in CH2Cl2 in synthesis. The stronger H-bond in 1 made the regioselective
the presence of activated 4 Å MS at 30 °C to afford the desired removal of 5-OTBS operation unnecessary, while in 2 and 3
glycoside. The chemical structure of 10 was ascertained by 1D cases, desilylation manipulation is required.
NMR spectra and the NOE 2D spectrum (for the anomeric
proton: 5.71 ppm, J = 7.2 Hz; correlation of the anomeric terms of glycosyl accepters, compounds 4 and 5 carrying
proton with H-6 was observed in the NOE spectrum).11
benzyl and hexanoyl protecting groups were subsequently
To further extend the substrate scope of the method in
After the optimal reaction conditions were established, the studied. Although Bn is also regarded as an electron-donating
substrate generality and limitation were subsequently checked protecting group, its electron-donating propensity is much less
(Scheme 2). Besides 6, the more reactive glucosyl donor 7 than that of TBS. The hexanoyl group is a typical electron-with-
could offer an excellent yield of the product glycoside as well, drawing protecting group. Thus, by means of checking the
when coupled with 2 under the optimized glycosylation con- possibility of compounds 4 and 5 as acceptors in the estab-
ditions. Donor 8, the galactosyl counterpart of 6, was proved to lished protocol, the electronic effect exerted by protecting
be an even better substrate for the protocol, and an almost groups on acceptor activities could be deduced. Catalysed by
quantitative yield of 12 was recorded. As a representative of the the Au(I) complex, acceptor 4 reacted with donors 6–7 efficien-
L-series sugar donor, L-rhamnosyl donor 9 condensed with 2 tly, providing glycosides 18–21 in good to excellent yields
fluently, affording 13 in a good 72% yield. Not only enjoying (Scheme 3). In terms of the electron-deficient compound 5, it
quite broad substrate scope in terms of glycosyl donors, the was also proved to be a vital acceptor for the new glycosylation
glycosylation conditions could also be applicable to the quer- protocol, and above 80% yields of 22–25 were obtained. There-
cetin acceptor protected with TBS groups. When glycosylated fore, at this juncture, a conclusion that the electronic pro-
with donors 6–9 respectively under standard conditions, accep-
Scheme 2 Construction of TBS protected flavonoid 5-OH glycosidic
linkages with glycosyl ortho-alkynylbenzoates as donors.
Scheme 3 Glycosylation of flavonol 5-OH protected with benzyl and
hexanoyl groups.
This journal is © The Royal Society of Chemistry 2015
Org. Biomol. Chem.

View Article Online
Communication
Organic & Biomolecular Chemistry
perties of the protecting groups have little effect on the reactiv- between the 1H NMR of 28a and that reported in the litera-
ity of the acceptors could be drawn from these results.
ture.16 To eliminate the difficulties possibly brought about by
To test the synthetic potential of the glycosylation protocol H-bonds in spectrum discrimination21 and to facilitate the
in natural product synthesis, a synthetic study toward kaemp- final structural determination, 28a was thus acetylated to
ferol 5-O-disaccharide 28a, isolated from Nerium oleander,16 afford 28. All 1D and 2D spectra supported that the obtained
was undertaken. As depicted in Scheme 4, the synthesis com- compound had the correct structure of 28, in turn the struc-
menced with 23. Selective removal of the TBDPS group on ture of 28a was also verified. In the HMBC spectrum, corre-
sugar residue was achieved under buffered TBAF conditions to lation between C-5 and H″-1 was observed, which
yield 26 in a moderate 57% yield.17 The main by-product of demonstrated that the sugar chain was attached to 5-OH. C″-5,
this conversion was the 7,6″-OH free glycoside, derived from discriminated from the DEPT spectrum, correlated with H′′′-1
simultaneous removal of basic labile 7-O-hexanol and 6′-O- in the HMBC spectrum, which proved that the rhamnosyl
TBDPS groups. Given the acidic labile nature of the glucosydic residue was attached to the 6-OH of the glucosyl residue. The
linkage in 26, donor 9 was applied for the installation of rham- NOE spectrum could offer further evidence to support the
nosyl residue. Under standard conditions, disaccharide 27 was attaching position of the sugar chain, and correlations
isolated in a good 82% yield. What remained to complete the between H-6 and H″-1 and H′′′-1 were detected.11 In fact, in the
synthesis of 28a was the global deprotection of all acyl groups isolation literature, a quite similar compound to 28 was also
in 27, a process that proved to be quite problematic. Saponifi- made except for the free 3-OH. Evident spectroscopic discre-
cation with a generally applied NaOMe/MeOH system only led pancies of 28 and its 3-OH analogue were also noticed. In 28,
to a complex mixture due to the instability of the electron-rich all protons attached to carbons substituted with acetylated
free flavonol nuclei under basic conditions.18 Changing the hydroxyl groups appeared in a reasonable area (protons of H″-
19
20
base from NaOMe to mild K2CO3 and Mg(OMe)2 didn’t 2,3,4 and H′′′-2,3,4 in 28 appeared at above 5.0 ppm; while in
bring about considerable improvement, therefore, this route the original document, protons of H″-2,3,4 and H′′′-4 were
was finally abandoned.
To avoid the exposure of free flavonol subunits to basic con- 28 are easily discriminated at 3.93 and 3.85 ppm in multiple
ditions, two-step deprotection procedure was adopted forms (these two protons were assigned to 5.1 and 5.24 ppm,
assigned to resonate at below 5.0 ppm). The H″-5 and H′′′-5 of
a
(Scheme 5). Compound 19 was subjected to TBAF mediated respectively in the isolation paper). Additional support could
desilylation to furnish 29 which was ready for sugar chain be obtained from quercetin 5-O-2″,3″,4″,6″-tetra-O-acetyl-gluco-
elongation (99%). The condensation between 29 and 9 pro- side synthesized by Rolando et al.,7b whose H″-2, 3 and 4
ceeded fluently under standard glycosylation conditions, yield- resided between 5.11–5.49 ppm, while H″-5 appeared at
ing kaempferol disaccharide 30 (67%). Saponification was 3.76 ppm. The diagnostic 13C chemical shifts for the two
followed by hydrogenation to generate 28a successfully (70%, 2 anomeric carbons C″-1 and C′′′-1 in 28 were 106.2 and
steps). Unfortunately, an evident discrepancy was observed 100.5 ppm, respectively, in contrast to the reported 104.5 and
102.4 ppm for these two carbons.
In summary, capitalizing on Yu glycosylation, an efficient
protocol to construct the challenging flavonol 5-OH glycosidic
linkages was established. The protocol enjoys a broad sub-
strate scope in terms of both donors and acceptors. Employing
this protocol the synthesis of kaempferol 5-O-disaccharide was
achieved for the first time. To the best of our knowledge, this
is the first time that flavonol 5-O-disaccharide was obtained by
the synthetic method.
We gratefully acknowledge the National Natural Science
Foundation of China (21202166 and 21372252), and the finan-
Scheme 4 Synthetic attempt toward the proposed naturally occurring
cial support from Jiangxi Normal University is also highly
appreciated.
kaempferol 5-O-glycoside 28a.
Notes and references
1 (a) J. B. Harborne and C. A. Williams, Nat. Prod. Rep., 1995,
12, 639; (b) J. B. Harborne and C. A. Williams, Nat. Prod.
Rep., 1998, 15, 631; (c) J. B. Harborne and C. A. Williams,
Nat. Prod. Rep., 2001, 18, 310; (d) N. C. Veitch and
R. J. Grayer, Nat. Prod. Rep., 2008, 25, 555.
2 J. B. Harborne and H. Baxter, The Handbook of Natural
Flavonoids, John Wiley & Sons, Chichester, UK, 1999, vol. 1.
Scheme 5 Synthesis of kaempferol 5-O-glycoside 28.
Org. Biomol. Chem.
This journal is © The Royal Society of Chemistry 2015

View Article Online
Organic & Biomolecular Chemistry
Communication
3 (a) S. Perveen, A. M. Al-Taweel, N. Al-Musayeib, G. A. Fawzy, 12 M. C. Akilonu, S. L. Bonnet and van der J. H. Westhuizen,
A. Khan, R. Mehmood and A. Malik, Nat. Prod. Res., 2015, Org. Lett., 2008, 10, 3865.
29, 615; (b) R.-J. Dai, Y.-H. Yu, L. Li, C.-M. Wu, W.-W. Luan, 13 (a) N. Mezailles, L. Ricard and F. Gagosz, Org. Lett., 2005, 7,
W.-W. Meng, X.-S. Zhang and Y.-L. Deng, Biol. Pharm. Bull.,
2007, 30, 1123.
4133; (b) Y. Li, X. Yang, Y. Liu, C. Zhu, Y. Yang and B. Yu,
Chem. – Eur. J., 2010, 16, 1871.
4 Y. Du, G. Wei and R. J. Linhardt, Tetrahedron Lett., 2003, 14 (a) M. B. Cid, F. Alfonso, I. Alonso and M. Martin-Lomas,
44, 6887.
Org. Biomol. Chem., 2009, 7, 1471; (b) M. I. Colombo,
E. A. Ruveda and C. A. Storttz, Org. Biomol. Chem., 2011, 9,
3020.
5 R. R. Schmidt and J. Michel, Angew. Chem., Int. Ed. Engl.,
1980, 19, 731.
6 J. A. Mahling and R. R. Schmidt, Synthesis, 1993, 325.
15 B. Bernet and A. Vassella, Helv. Chim. Acta, 2000, 83, 995.
7 (a) B. Alluis and O. Dangles, Helv. Chim. Acta, 2001, 84, 16 B. S. Siddiqui, N. Khatoon, S. Begum, A. D. Farooq,
1133; (b) M. Kajjout and C. Rolando, Tetrahedron, 2011, 67,
4731; (c) W. Yang, R. Li, W. Han, W.-D. Zhang and J. Sun,
Chin. J. Org. Chem., 2012, 32, 1067.
K. Qamar, H. A. Bhatti and S. K. Ali, Phytochemistry, 2012,
77, 238.
17 S. Kim, A. Tsuruyama, A. Ohmori and K. Chiba, Chem.
Commun., 2008, 1816.
8 (a) Y. Li, Y. Yang and B. Yu, Tetrahedron Lett., 2008, 49, 3604;
(b) Y. Zhu and B. Yu, Angew. Chem., Int. Ed., 2011, 50, 8329; 18 (a) M. Li, X. Han and B. Yu, J. Org. Chem., 2003, 68, 6842;
(c) Y. Yang, X. Zhang and B. Yu, Nat. Prod. Rep., 2015, 32, 1331. (b) J. Sun, S. Laval and B. Yu, Synthesis, 2014, 1030.
9 (a) W. Yang, J. Sun, W. Lu, Y. Li, L. Shan, W. Han, 19 (a) N. Al-Maharik and N. P. Botting, Tetrahedron Lett., 2006,
W.-D. Zhang and B. Yu, J. Org. Chem., 2010, 75, 6879;
(b) Y. Li, W. Yang, Y. Ma, J. Sun, L. Shan, W.-D. Zhang and
47, 8703; (b) N. Al-Maharik and N. P. Botting, Eur. J. Org.
Chem., 2008, 5622.
B. Yu, Synlett, 2011, 915; (c) J. Yu, J. Sun, Y. Niu, R. Li, 20 (a) Y.-C. Xu, A. Bizuneh and C. Walker, Tetrahedron Lett.,
F. Zhang and B. Yu, Chem. Sci., 2013, 4, 3899.
10 K.-i. Oyama and T. Kondo, Tetrahedron, 2004, 60, 2025.
11 See ESI.†
1996, 37, 455; (b) Y.-F. Li, B. Yu, J.-S. Sun and R.-X. Wang,
Tetrahedron Lett., 2015, 56, 3816.
21 C. Nawrat and C. J. Moody, Org. Lett., 2012, 14, 1484.
This journal is © The Royal Society of Chemistry 2015
Org. Biomol. Chem.