Cyanide-Free Synthesis of Glycosyl Carboxylic Acids and Application for the Synthesis of Scleropentaside A

Article abstract of DOI:10.1021/acs.orglett.0c02949

We have developed a cyanide-free strategy for the synthesis of glycosyl carboxylic acids, which can provide 1,2-trans or 1,2-cis glycosyl carboxylic acids and is compatible with common protecting groups. The synthetic utility was demonstrated by the synth

Full text of DOI:10.1021/acs.orglett.0c02949

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Letter
Cyanide-Free Synthesis of Glycosyl Carboxylic Acids and Application
for the Synthesis of Scleropentaside A
Liang-Jing Zou,⊥ Qiang Pan,⊥ Cai-Yi Li, Ze-Ting Zhang, Xiao-Wei Zhang, and Xiang-Guo Hu*
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ABSTRACT: We have developed a cyanide-free strategy for the synthesis of
glycosyl carboxylic acids, which can provide 1,2-trans or 1,2-cis glycosyl
carboxylic acids and is compatible with common protecting groups. The
synthetic utility was demonstrated by the synthesis of 12 unreported glycosyl
acids and the total synthesis of scleropentaside A.
conditions are generally required for the transformation of the
cyanide group to a carboxylic acid group, rendering this
reaction incompatible with most of the protecting groups.2−5,7
Furthermore, because neighboring group participation deter-
mines the stereoselectivity in the cyanation step, this strategy is
only suitable for the synthesis of 1,2-trans glycosyl acids.2−5,7
Herein, we report a general cyanide-free strategy for the
synthesis of glycosyl carboxylic acids (Scheme 1b). Comple-
mentary to the cyanide-based strategy,2−5,7 this approach can
provide 1,2-trans or 1,2-cis glycosyl acids and is compatible
with common protecting groups. The key steps were recently
developed, i.e., stereoselective aryl C-glycosylation8−10 and the
classical Sharpless oxidation of arenes.11 Besides of its synthetic
potential, this work also highlights the importance of
developing new C-glycosylation methods as it can enable the
design of novel and useful strategies for target syntheses when
properly combined with a piece of old chemistry.
ugar acids are monosaccharides bearing at least one
1
Scarboxyl group in the molecule. Well-known examples,
such as neuraminic acid, ketodeoxyoctulosonic acid (Kdo),
and ascorbic acid (vitamin C), are all important building
blocks in both organic synthesis and biological chemistry.
Glycos-1-yl (glycosyl) carboxylic acids are a lesser-known yet
valuable subclass of sugar acids. For example, they can be used
as coupling reagents for biological probes2 and for the
synthesis of oligosaccharides via multicomponent reactions.3
They are also versatile intermediates used in C−H activation4
and radical reactions5,6 for the synthesis of C-glycosides.
Glycosyl carboxylic acids are mostly synthesized from glycosyl
acetates or halides via a sequence of cyanation and hydrolysis
(Scheme 1a).7 Being reliable and general on its own,
unfortunately this strategy inevitably requires superstoichio-
metric amounts of toxic cyanide-containing reagents (e.g.,
TMSCN and Hg(CN)2) and catalytic mercury salts (HgX2)
for the cyanation step.2−5,7 In addition to the toxicity, harsh
Our impetus to address this challenge arose from one
ongoing project in this laboratory, which aims to synthesize
scleropentaside A12 and its analogs, which are natural products
with unprecedented acyl β-C-glycosidic bonds.13 Recently,
Scheme 1. Cyanide-Based Synthesis of Glycosyl Carboxylic
Acids and This Work
Schutzenmeister et al. reported the first synthesis of
̈
scleropentaside A based on a β-selective acyl-glycosidation.14
Our retrosynthesis pointed to a strategy (Scheme 2) via the
coupling of a derivative of glucosyl carboxylic acid (e.g.,
thioester 2) and a furan species (e.g., boronic acid 3). Inspired
by the recent development in the stereoselective syntheses of
aryl C-glycosides8−10 as well as one of the authors’
experience15 in the synthesis of amino acids, we envisioned
Received: September 2, 2020
https://dx.doi.org/10.1021/acs.orglett.0c02949
© XXXX American Chemical Society
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A

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Letter
Scheme 2. Retrosynthesis of Scleropentaside A
Scheme 4. Synthesis of other Pyranosyl Carboxylic Acids
a b
,
from GLYCOSYL Bromides
that the development of a cyanide-free strategy was possible
(Scheme 1b).
At the outset of the project, we first tested the feasibility of
this strategy with D-glucose-derived aryl C-glycosides 5a and
5b, which could be synthesized readily from glucosyl bromide
4 according to the procedure reported by Lemaire et al.
(Scheme 3).10b The oxidation of 5a (Ar = Ph) under the
Scheme 3. Feasibility of This Strategy with D-Glucose-
Derived Substrates
a b
,
a
b
Method A. Method B.
glycosyl acids, thus complementing the cyanide-based
strategy.2−5,7 The synthesis of 22, the C1 diastereomer of 9,
from aryl C-glycoside 21 further strengthened the above
statement. Furhermore, the reaction is also suitable for C6-
deoxygenated substrates, and 24 was obtained from rhamnose-
derived 23 in an 81% yield (for the syntheses of 21 and 23, see
the Supporting Information).10a
Finally, we turned our attention to the synthesis of furanosyl
carboxylic acids. As shown in Scheme 5, peracylated ribosyl
a
Conditions for method A: RuCl3 (cat.), NaIO4 (18 equiv), and
b
Scheme 5. Synthesis of Furanosyl Carboxylic Acids and the
Effect of the Protecting Group on the Oxidation
CCl4/CH3CN/H2O (v/v/v 1:1:1.25). Conditions for method B:
RuCl3 (cat.), NaIO4 (18 equiv), and EtOAc/CH3CN/pH 7 buffer (v/
v/v 1:1:8).
standard conditions developed by Sharpless et al. (method A)
proceeded very slowly.11,16 The replacement of the phenyl ring
with the more electron-rich 4-MeOPh accelerated the reaction
significantly, which is consistent with the observation by
Spitzer et al.17 After some optimization, it was found that the
oxidation can be best achieved with the following solvent
system (method B): EtOAc/CH3CN/pH 7 buffer (v:v:v =
1:1:8); the yield was as high as 80%, even on the gram scale,
from 5b. Consistent with previous reports, this oxidation
proceeds with the retention of the C1 stereochemistry, and no
epimerization was observed.15 In principle, 7 and 8 could be
synthesized from their corresponding bromides.9a,d However,
we prepared these two compounds via a protecting group
exchange from 5b for convenience. Next, 9 and 10 were
synthesized from 7 and 8, respectively, in 73% and 62% yields.
Interestingly, we found that method A was only suitable for
acetyl protected compounds, whereas method B was suitable
for other substrates throughout this work (Scheme 3 and vide
infra).
Next, we tested this strategy with representative aryl C-
glycosides of D-galactose and mannose 13 and 14 (Scheme 4),
respectively, which were synthesized readily from glycosyl
bromides 11 and 12 according to the procedure reported by
Cossy and Reymond et al.10a The oxidation of 13 and 14 with
method A yielded 15 and 16, respectively, in 56% and 62%
yields. Similarly, the oxidation of benzoyl-protected 17 and 18
with method B, synthesized in the same sequence as that for 8,
afforded 19 and 20, respectively, in 76% and 61% yields.
Importantly, the success in the synthesis of 16 and 20 showed
that our strategy could be used for the synthesis of 1,2-cis
carboxylic acid 26a could be obtained from its precursor 25a
(for the synthesis of 25, see the Supporting Information)10c in
a 61% yield. Previously, it was reported that the Sharpless
oxidation of arenes can oxidize electron-rich heterocycles,16
which would exclude the possibility of using acetal-, ketal-, and
ether-type protecting groups as these groups make the sugar
ring more electron rich. Interestingly, however, we found the
acetonide group could be used in this work by tuning the
electron property of the protecting group on O5. Whereas the
TBS protection yielded no desired product but instead
complete decomposition,16 the benzoyl protection afforded
the desired product 26c in a low yield (45%). Further
increasing the electron-withdrawing ability of the protecting
group with p-nitrobenzoyl (PNB) enhanced the yield of 26d to
75%. This protecting group effect could be useful in related
oxidation reactions in the future. Furthermore, the compati-
bility of the tosyl (26e) and azido groups (26f) is of
B
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Scheme 6. Synthesis of Scleropentaside A
importance as they leave a reactive handle at C5 and provide a
viable means to synthesize sugar amino acids.18
synthetic potential of this strategy was further demonstrated by
the gram-scale synthesis of 6 and the total synthesis of
scleropentaside A, during which a base-free Liebeskind-Srogl
coupling reaction and the judicious decrease of the acidity of
C1−H through protection were pivotal.
With a general strategy for the synthesis of glycosyl
carboxylic acids established, we proceeded to the synthesis of
scleropentaside A. As shown in Scheme 6, the reaction of 6
with thiophenol furnished thioester 2 in a 96% yield in the
presence of peptide-coupling reagents. The base-free Pd-
catalyzed Liebeskind-Srogl reaction19 was very effective,
delivering 27 in a nearly quantitative yield; note that the
Liebeskind-Srogl reaction remains underexplored in the area of
carbohydrate chemistry,13b despite its wide application in other
areas.19 However, we encountered great difficulties in the
deprotection of the pivaloyl (Piv) group of 27. For example,
the Piv group remained intact after a prolonged reaction time
under acidic conditions (Supporting Information). Although
the Piv groups were removed effectively under basic
conditions, glycal 28, instead of 1, was isolated even when
using potassium carbonate as a base (Scheme 6 and the
Supporting Information). This side reaction thus highlighted
the instability of 27 toward bases13b as well as the importance
of using the Liebeskind-Srogl19 reaction to form it.
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c02949.
Experimental procedures, spectroscopic data, and NMR
spectra of new compounds (PDF)
Accession Codes
CCDC 1996345−1996346 and 2024898 contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge via www.ccdc.cam.ac.uk/data_request/
cif, or by emailing data_request@ccdc.cam.ac.uk, or by
contacting The Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
Presumably, 28 was formed from the elimination of the
OPiv group after the deprotonation of C1−H via an E1cB
pathway. Therefore, we reasoned that reducing the acidity of
the C1−H could solve the problem. Indeed, after switching the
ketone group to a ketal group, the deprotection of 29 was
successful under basic conditions. After the neutralization of
the reaction mixture with an acidic resin, natural scleropenta-
side A 1 was obtained in a 75% yield from 10.
In summary, we have developed a cyanide-free strategy for
the synthesis of glycosyl carboxylic acids, utilizing the
stereoselective aryl C-glycosylation and Sharpless oxidation of
aryl C-glycosides as key steps. Complementary to the cyanide-
based strategy, this approach can provide 1,2-trans or 1,2-cis
glycosyl acids and is compatible with various protecting groups
(e.g., acetonide, acyl, and benzoyl) and reactive functional
groups (e.g., azido and tosyl). This is evidenced by the fact that
12 out of 14 of the carboxylic acids synthesized are unreported.
Because a great variety of aryl C-glycosides could be obtained
in a stereoselective and easy manner utilizing the ever-
expanding toolbox of aryl C-glycosylation,8−10 more glycosyl
acids could be prepared using this strategy if desired. The
AUTHOR INFORMATION
■
Corresponding Author
Xiang-Guo Hu − National Engineering Research Center for
Carbohydrate Synthesis and Key Laboratory of Small
Functional Organic Molecule, Ministry of Education, Jiangxi
Normal University, Nanchang, Jiangxi 330022, China;
orcid.org/0000-0002-5909-2582; Email: Xiangguo.hu@
jxnu.edu.cn
Authors
Liang-Jing Zou − National Engineering Research Center for
Carbohydrate Synthesis, Jiangxi Normal University, Nanchang,
Jiangxi 330022, China
Qiang Pan − National Engineering Research Center for
Carbohydrate Synthesis, Jiangxi Normal University, Nanchang,
Jiangxi 330022, China
Cai-Yi Li − National Engineering Research Center for
Carbohydrate Synthesis, Jiangxi Normal University, Nanchang,
Jiangxi 330022, China
C
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Letter
Some Anomeric Pairs of per-O-Acetylated Aldohexopyranosyl
Cyanides (per-O-acetylated 2,6-anhydroheptononitriles). On The
Reaction of per-O-Acetylaldohexopyranosyl Bromides with Mercuric
Cyanide in Nitromethane. Carbohydr. Res. 1984, 132, 61−82.
(g) Phiasivongsa, P.; Gallagher, J.; Chen, C.-N.; Jones, P. R.;
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4587−4590.
Ze-Ting Zhang − National Engineering Research Center for
Carbohydrate Synthesis, Jiangxi Normal University, Nanchang,
Jiangxi 330022, China
Xiao-Wei Zhang − National Engineering Research Center for
Carbohydrate Synthesis, Jiangxi Normal University, Nanchang,
Jiangxi 330022, China; orcid.org/0000-0002-2262-4143
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c02949
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⊥These authors contributed equally.
Notes
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We thank the National Natural Science Foundation of China
(no. 22067009).
■
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E
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