Stereodivergent synthesis of C-glycosamino acids via Pd/Cu dual catalysis

Article abstract of DOI:10.1007/s11426-020-9930-7

Herein, we reported the stereodivergent synthesis of C-glycosamino acids via Pd/Cu dual catalysis and found a suitable system to resolve many challenges, such as the tolerance towards the density of functional groups, the variability of the anomeric position, the compatibility of appropriate catalyst combinations, the regioselectivity of nucleophiles, and the match/mismatch problems between chiral substrates and chiral ligand-metal complexes. The method enables the efficient preparation of a series of unnatural C-glycosamino acid skeletons bearing two contiguous stereogenic centers in good yields with excellent diastereos-electivity. From this crucial precursor, various C-glycosamino acid derivatives have been achieved diversely. The readily prepared C-glycosamino acid hybrids will meet the growing demands for the development of new molecular entities for discovering new drugs and materials. This stereodivergent synthesis of C-glycosamino acids will further accelerate the study of their structural features, mode of action, and potential biological applications in the near future. [Figure not available: see fulltext.]

Full text of DOI:10.1007/s11426-020-9930-7

SCIENCE CHINA
Chemistry
•ARTICLES•
April 2021 Vol.64 No.4: 552–557
https://doi.org/10.1007/s11426-020-9930-7
Stereodivergent synthesis of C-glycosamino acids via Pd/Cu dual
catalysis
Xiaoxiao Yan1†, Feng Feng1†, Lin Zhou1, Linrong Chen1, Shouchu Tang1, Jian Liu1*,
Feng Cai2 & Xiaolei Wang1*
1State Key Laboratory of Applied Organic Chemistry, Department of Chemistry and School of Pharmacy, Lanzhou University,
Lanzhou 730000, China;
2The National Glycoengineering Research Center and Shandong Provincial Key Laboratory of Carbohydrate Chemistry and Glycobiology,
Shandong University, Qingdao 266237, China
Received September 25, 2020; accepted December 29, 2020; published online February 9, 2021
Herein, we reported the stereodivergent synthesis of C-glycosamino acids via Pd/Cu dual catalysis and found a suitable system to
resolve many challenges, such as the tolerance towards the density of functional groups, the variability of the anomeric position,
the compatibility of appropriate catalyst combinations, the regioselectivity of nucleophiles, and the match/mismatch problems
between chiral substrates and chiral ligand-metal complexes. The method enables the efficient preparation of a series of
unnatural C-glycosamino acid skeletons bearing two contiguous stereogenic centers in good yields with excellent diastereos-
electivity. From this crucial precursor, various C-glycosamino acid derivatives have been achieved diversely. The readily
prepared C-glycosamino acid hybrids will meet the growing demands for the development of new molecular entities for
discovering new drugs and materials. This stereodivergent synthesis of C-glycosamino acids will further accelerate the study of
their structural features, mode of action, and potential biological applications in the near future.
C-glycosamino acid, glycomimetics, peptidomimetics, stereodivergent, dual catalysis
Citation:
Yan X, Feng F, Zhou L, Chen L, Tang S, Liu J, Cai F, Wang X. Stereodivergent synthesis of C-glycosamino acids via Pd/Cu dual catalysis. Sci China
Chem, 2021, 64: 552–557, https://doi.org/10.1007/s11426-020-9930-7
1 Introduction
cesses, including cell-cell interactions, tumor metastasis,
infection, inflammation, and cancers [3]. However, O/N-
glycans are unstable under glycosidases or acidic physiolo-
gical environments. Thus, C-glycosamino acids, in which the
α-position of amino acids is connected to the anomeric car-
bon atom of a sugar motif through carbon-carbon bond lin-
kages, were developed to address these issues [4,5]. Related
investigations have shown that many C-glycosamino acids
and C-glycopeptides have interesting solution conforma-
tions, and their biological activities are similar to those of
naturally occurring analogs [6]. In particular, some C-
glycosamino acids depict significantly improved biological
prosperity and have been successfully employed in specific
therapeutics [7], such as the anti-influenza drug oseltamivir.
Glycosamino acids are hybrid structures of carbohydrates
and amino acids that can be utilized to generate potent gly-
comimetics and peptidomimetics (Figure 1) [1]. In-
corporating glycosamino acids into peptides could allow the
engineering of carbohydrate-binding sites into synthetic
polypeptides, which can further modify the pharmacoki-
netics and dynamic properties of the peptides in a drug-like
manner [2]. There are many different types of naturally oc-
curring glycosamino acids, such as sialic acid and O/N-
glycans, which play essential roles in many biological pro-
†These authors contributed equally to this work.
*Corresponding authors (email: jianliu@lzu.edu.cn; wangxiaolei@lzu.edu.cn)
© Science China Press and Springer-Verlag GmbH Germany, part of Springer Nature 2021
chem.scichina.com link.springer.com

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Yan et al. Sci China Chem April (2021) Vol.64 No.4
Scheme 1 Planned synthesis of C-glycosamino acid hybrids (color on-
line).
2 Experimental
General procedure for the synthesis of C-glycosamino acid
3a–3m. To a well-dried 10-mL seal tube with Cu(MeCN)4-
BF6 (1.5 mg, 0.005 mmol, 0.05 equiv.) and L4 (2.8 mg,
0.005 mmol, 0.05 equiv.), dichloromethane (DCM) (0.5 mL)
was then added under Ar and stirred for 30 min at room
temperature (rt). Then the above mixture was transferred to
another well-dried 10-mL seal tube containing tert-butyl
carbonate glucenose 1 (33.0 mg, 0.1 mmol, 1.0 equiv.), al-
dimine ester 2 (0.15 mmol, 1.5 equiv.), Pd(PPh3)4 (5.5 mg,
0.005 mmol, 0.05 equiv.) and DCM (0.5 mL) under Ar at rt.
The reaction was monitored by thin layer chromatography
(TLC). About 5 h, the solvent dichloromethane was con-
centrated in vacuo. The residue was then diluted with tetra-
hydrofuran (1 mL) and 10% citric acid solution (4 mL).
After 2 h at room temperature, saturated sodium bicarbonate
solution was added to adjust the pH to neutral and extracted
with EtOAc. The combined organic layers were dried over
anhydrous Na2SO4, filtered, and concentrated under vacuum.
The residue was purified by flash column chromatography
(PE/EtOAc) to afford the desire products.
Figure 1 General structures of glycosamino acids (color online).
Given their tremendous structural and functional diversity,
unfortunately, C-glycosamino acid hybrids do not yet to be
fully explored due to the limited availability of these struc-
turally complex compounds [4].
The limited availability of C-glycosamino acid blocks has
significantly restricted the research of their structural fea-
tures and mode of action. To date, only a few reports have
addressed the synthesis of C-glycosamino acids. Most of
these synthetic strategies have significant drawbacks, such as
low diastereoselectivity [8a], multistep synthesis [8a–8f],
unspecified stereochemistry [8d], and lack of stereo-
divergence [1,8]. However, efficient and diverse preparation
of C-glycosamino acid building blocks remains quite chal-
lenging.
With the development of dual catalysis in recent years [9],
we speculated that the C-glycosamino acid skeleton might be
constructed via a dual catalytic system in a diverse manner.
As shown in Scheme 1, the stereocenter of the anomeric
position can be controlled via the substrate-palladium com-
plex, and the other chiral center close to the α-amino acid
motif can be elegantly controlled via the metal-chiral ligand
complex. The applications of transition metal catalysis for
constructing C-glycosyl linkages have significantly in-
creased in recent years [10,11]. However, the dual catalysis
strategy has never been successfully applied in the synthesis
of C-glycosamino acids due to the difficulty in controlling
the remote formation of glycosidic bonds with high dia-
stereoselectivity and regioselectivity. In this area, one must
address the density of functional groups of carbohydrates,
the variability of the anomeric position, the judicious selec-
tion of appropriate catalyst combinations, the regioselec-
tivity of nucleophiles, and the match/mismatch problems
between chiral substrates and chiral ligand-metal complexes.
Herein, we describe the regio- and diastereodivergent
synthesis of C-glycosamino acids via bimetallic catalyzed
Heck-type glycosylation [12,13], a protocol that is highly
desired from a synthetic efficiency perspective (Scheme 1).
3 Results and discussion
After extensive exploration of the reaction conditions (Ta-
bles S1–S3, Supporting Information online), glycal donor 1a
[14] was found to react with α-substituted aldimine esters 2a
under Pd(PPh3)4 and Cu(MeCN)4BF4/phosferrox (L4) [15]
conditions to afford 3a in 85% yield with >20:1 dr (Scheme
2, entry 1). The detailed reaction conditions were as follows:
the coupling reaction was performed with 1a (1.0 equiv.), 2a
(1.5 equiv.), Et3N (1.5 equiv.), Pd(PPh3)4 (0.05 equiv.),
Cu(MeCN)4BF4 (0.05 equiv.) and phosferrox-L4 (0.05
equiv.) in dichloromethane (1.0 mL) at room temperature for
5 h. No reaction was observed when using glycal donors 1b
and 1c (entries 2 and 3). The product was generated with
only 50% yield using glycal donor 1d (entry 4). The reaction
yield was considerably diminished using other phosferrox
ligands (L1–L3, entries 5–7) or different palladium catalysts
(entries 8–12). Reducing the reaction temperature to 0 °C for
a prolonged time did not further improve the diastereos-
electivity while maintaining the yield (entry 13). The control

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Yan et al. Sci China Chem April (2021) Vol.64 No.4
Scheme 2 a) Optimization of reaction conditions. Unless otherwise no-
ted, the reaction of 1 (0.10 mmol) with 2a (1.5 equiv.) was carried out using
a catalytic system consisting of a palladium catalyst (0.05 equiv.),
Cu(MeCN)4BF4 (0.05 equiv.), and (R,Rp)-L (0.05 equiv.) under an Ar at-
mosphere in the presence of triethylamine (1.5 equiv.) in CH2Cl2 (1.0 mL)
at room temperature for 5 h. b) Isolated yields. c) The dr value was de-
Scheme 3 Substrate scope of aldimine esters. a) 1a (0.10 mmol), 2a (1.5
equiv.), Pd(PPh3)4 (0.05 equiv.), Cu(MeCN)4BF4 (0.05 equiv.), (R,Rp)-L4
(0.05 equiv.), and triethylamine (1.5 equiv.) in CH2Cl2 (1.0 mL) at room
temperature for 5 h; then, the reaction system was treated with 10% citric
acid. b) Lactam was formed when treated with 10% citric acid (color
online).
1
termined by crude H NMR. d) the related product 3w in scheme 4. e) At
0 °C for 14 h (color online).
experiments suggested that both copper and palladium cat-
alysts were indispensable for this chemical transformation
(entries 14 and 15).
However, the dr value decreased to 7:1 due to the bulky
protecting group of the tyrosine-type aldimine ester (2l). The
bulky tert-butyldimethylsilyl (TBS) protecting group might
come from the less approachable of two metal-chelated
coupling partners, which further decreases the diastereos-
electivity [15f–15j]. To further clarify the decrease of dr, we
prepared a less hinderance substrate containing OAc group,
and the related product’s dr restored to 10:1.
Subsequently, the scope of the glycone part was explored
(Scheme 4). First, we assessed aldimine esters 2a with dif-
ferent glycosyl donors [16]. The reaction system shows good
tolerance toward both substituted (3n–3p) and deoxygenated
substrates (3s, 3t) with different protecting groups. Even
changing the absolute configuration of the anomeric position
and epimerization of the C5 position had no influence on the
diastereoselectivity and yield (3q–3s). For the galactose-type
carbohydrate donor, we also obtained the corresponding
product in good yield and excellent diastereoselectivity (3u).
With optimized reaction conditions in hand, the scope of
the α-substituted aldimine esters was evaluated. As shown in
Scheme 3, we were pleased to find that both natural and
unnatural α-amino acid-derived aldimine esters were suitable
in this reaction system, yielding the precursors of C-
glycosamino acids. Diverse substituents, including alkyl (3b,
3c), thioether (3d), (substituted) phenyl (3e, 3f), ester (3g),
and protected amine (3h) groups, are all well tolerated in this
reaction system, giving desired C-glycosamino acid deriva-
tives in good yields with excellent diastereoselectivity. In
addition, other aldimine esters (2i, 2j) also gave the desired
products (3i, 3j) without losing diastereoselectivity. Sig-
nificantly, the tert-butyl masked ester will provide an op-
portunity for further peptide ligation. The phenylalanine-
type aldimine ester also provided the desired product (3k).

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Yan et al. Sci China Chem April (2021) Vol.64 No.4
While using (S,Sp)-L4, we can still obtain the target molecule
(3v) without losing yield and diastereoselectivity. As men-
tioned in Scheme 2, we obtained a free alcohol product (3w)
while using glycosyl donor 1d in 50% yield. Finally, the
absolute configuration of compound 3o was determined by
X-ray analysis (CCDC 2025082).
To show the utility of this methodology, a gram-scale re-
action was tested, as shown in Scheme 6. When the reaction
was scaled up to gram-scale, this reaction proceeded
smoothly without a notable change in terms of the isolated
yield and diastereoselectivity (see Supporting Information
online for details). With gram-scale 3x in hand, further
chemical transformations were conducted. For the glycone
motif, the double bond was then either reduced to saturated
alkane (4) or oxidized to mannose-type amino acid building
blocks (6) under OsO4 [8e,17]. For the transformation of the
aldimine ester part, we can obtain amide (5) after treatment
with acetic anhydride or benzylamine (7) with NaBH3CN.
To further confirm the absolute configuration, we obtained
a crystal for 3o as shown in Scheme 4. After extension ef-
forts, we could not obtain any other crystal of products and
their derivatives as shown in Schemes 3 and 4. As is known
to us, the circular dichroism (CD) spectra have emerged as a
promising tool for the determination of absolute configura-
tions and predominant conformations of chiral molecules in
academic laboratories [18]. We can use this spectroscopy to
deduce the absolute configuration via comparing the ex-
perimental with calculated CD spectra. Guided by this
technology, we prepared 8, 3f and 10c (see Supporting In-
formation online) containing a UV sensitive protecting
group, the experimental CD spectra totally agree with cal-
culated via Gaussian. Thus the absolute configuration of
these products can be confirmed through comparing the
After examining the substrate scope of both aldimine es-
ters and glycosyl donors, we further explored the stereo-
divergence of the current method with aldimine esters 2a
under standard reaction conditions. As shown in Scheme 5,
the configuration of the anomeric position might have stereo-
match or mismatch issues with the chiral Cu(I)/L4/2a com-
plex. Thus, changing the absolute configuration of ligand L4
might influence the reactivity, diastereoselectivity and yield.
Under the optimized conditions, the reaction of α-D-
glycoside 1a and (S,Sp)-L4 proceeded smoothly, affording 3y
in good yields with 10:1 dr. The reaction of β-D-glycoside
1a′ with (R,Rp)-L4 and (S,Sp)-L4 also went smoothly, giving
3z and 3aa in good yields with acceptable dr values. These
results indicated that the two distinct metal-substrate com-
plexes play an essential role in the formation of the corre-
sponding two new continuous stereogenic centers.
Scheme 5 Stereodivergence study. 1a or 1a′ (0.10 mmol), 2a (1.5 equiv.),
Pd(PPh3)4 (0.05 equiv.), Cu(MeCN)4BF4 (0.05 equiv.), L4 (0.05 equiv.),
and triethylamine (1.5 equiv.) in CH2Cl2 (1.0 mL) at room temperature for
5 h (color online).
Scheme 6 Gram-scale synthesis and chemical transformation study of 3a.
(a) Pd/C (cat.) under H2 (balloon) in methanol; (b) 10% citric acid in THF;
(c) Ac2O (1.5 equiv.) in pyridine; (d) Boc2O (1.5 equiv.), NaHCO3 (s, 1.2
equiv.) in DCM; (e) OsO4 (5% cat.) with NMO (3.0 equiv.) in acetone/H2O
= 4/1 (color online).
Scheme 4 Substrate scope of glycal donors. a) The reaction mixture was
treated with 10% citric acid. b) (S,Sp)-L4 was used. c) Glycal donor 1d was
used (color online).

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Yan et al. Sci China Chem April (2021) Vol.64 No.4
experimental with calculated CD spectra.
We then obtained 10a–10c via three steps chemical
transformation. From the key NOESY signal, we can also
confirm the absolute configuration of 10a–10c. After this,
we then compared the experimental with calculated CD
spectra of 11, prepared from (S,Sp)-L4 ligand. As is shown in
Scheme 7, the experimental CD spectra is also consistent
with the calculated CD spectra. And also the analytical and
spectral data of 11 was in good agreement with previous
report [8e].
4 Conclusions
In conclusion, we have developed the stereodivergent Pd/Cu
dual catalyzed α-glycosylation of aldimine esters via the
combination of two chiral metal complexes, we can access
all possible stereoisomers under identical conditions. Ac-
cording to the results, the reaction between D-glycal donors
and (R,Rp)-L4-Cu complex is more favorable, giving C-
glycosamino acid skeleton with excellent diastereoselec-
tivity. The diverse preparation of unnatural C-glycosamino
acid derivatives will further facilitate the biological study of
these glycomimetics and peptidomimetics. Ongoing studies
are focused on the synthesis and biological evaluation of C-
mannose-amino acids as novel fimH antagonists towards
uropathogenic Escherichia coli for the treatment of urinary
tract infections [19].
Acknowledgements This work was supported by the National Natural
Science Foundation of China (051170001, 21772084, 22071087), the
Fundamental Research Funds for the Central Universities (lzujbky-2017-
k06) and the Open Projects Funds of Shandong Key Laboratory of Carbo-
hydrate Chemistry and Glycobiology, Shandong University (2019CCG05).
Xiaolei Wang thanks the Thousand Young Talents Program for financial
support. We also thank Sumit O. Bajaj (Corden Pharma Boulder) for lan-
guage polishing and Prof. Quanxiang Wu & Ya Li for giving suggestion
towards the absolute configuration determining via CD spectra.
Conflict of interest The authors declare no conflict of interest.
Supporting information The supporting information is available online at
http://chem.scichina.com and http://link.springer.com/journal/11426. The
supporting materials are published as submitted, without typesetting or
editing. The responsibility for scientific accuracy and content remains en-
tirely with the authors.
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