Synthesis ofC-acyl furanosidesviathe cross-coupling of glycosyl esters with carboxylic acids

Article abstract of DOI:10.1039/d1sc03596g

C-Acyl furanosides are versatile synthetic precursors to a variety of natural products, nucleoside analogues, and pharmaceutical molecules. This report addresses the unmet challenge in preparingC-acyl furanosides by developing a cross-coupling reaction between glycosyl esters and carboxylic acids. A key step is the photoredox activation of the glycosyl ester, which promotes the homolysis of the strong anomeric C-O bond through CO₂evolution to afford glycosyl radicals. This method embraces a large scope of furanoses, pyranoses, and carboxylic acids, and is readily applicable to the synthesis of a thymidine analogue and diplobifuranylone B, as well as the late-stage modification of (+)-sclareolide. The convenient preparation of the redox active glycosyl ester from native sugars and the compatibility with common furanoses exemplifies the potential of this method in medicinal chemistry.

Full text of DOI:10.1039/d1sc03596g

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Synthesis of C-acyl furanosides via the cross-
coupling of glycosyl esters with carboxylic acids†
Cite this: DOI: 10.1039/d1sc03596g
*
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have been paid for by the Royal Society
of Chemistry
Yongliang Wei, Jenny Lam and Tianning Diao
C-Acyl furanosides are versatile synthetic precursors to a variety of natural products, nucleoside analogues,
and pharmaceutical molecules. This report addresses the unmet challenge in preparing C-acyl furanosides
by developing a cross-coupling reaction between glycosyl esters and carboxylic acids. A key step is the
photoredox activation of the glycosyl ester, which promotes the homolysis of the strong anomeric C–O
bond through CO2 evolution to afford glycosyl radicals. This method embraces a large scope of
furanoses, pyranoses, and carboxylic acids, and is readily applicable to the synthesis of a thymidine
analogue and diplobifuranylone B, as well as the late-stage modification of (+)-sclareolide. The
convenient preparation of the redox active glycosyl ester from native sugars and the compatibility with
common furanoses exemplifies the potential of this method in medicinal chemistry.
Received 1st July 2021
Accepted 22nd July 2021
DOI: 10.1039/d1sc03596g
rsc.li/chemical-science
Introduction
C-Acyl glycosides are versatile synthetic intermediates to natural
products, nucleoside analogues, and pharmaceutical molecules
(Scheme 1A).1 Downstream derivatives of C-acyl glycosides,
including C-alkyl glycosides,2 diazo derivatives,3 alcohols,4
nucleoside analogues,5 and cyclopentitols,6 display signicant
potentials in drug discovery and utilities in chemical biology
and biochemistry studies. A C-glycoside linkage confers in vivo
stability towards hydrolysis and enzymatic degradation.7
Therefore, C-furanosides, including nucleoside analogues in
particular, have been extensively explored as antiviral drug
candidates. In addition, unnatural nucleosides are essential
tools for studying the origins of mutagenicity and the mecha-
nism of replication and evolution.8 C-acyl furanosides serve as
a convenient intermediate to nucleoside analogues through
formation of heterocycles at the anomeric position via
condensation and cyclization.5
Synthetic efforts towards C-acyl glycosides have been
primarily focused on pyranosides. Conventional methods
include the nucleophilic addition of glycosyl lithium9 or
samarium10 reagents to carbonyl compounds and the func-
tionalization of C-nitriles,4a,11 C-allyls,4b C-allenes3,12 and ben-
zothiazoles (Scheme 1A).13 Recent cross-coupling approaches
employ stannane reagents14 and glycosyl bromides15 to deliver
a broad range of C-acyl pyranoside products. In contrast to the
widespread means to prepare C-acyl pyranosides, access to C-
acyl furanosides is limited and lacks a robust scope.16 For
Department of Chemistry, New York University, 100 Washington Square East, New
York, NY 10003, USA. E-mail: diao@nyu.edu
† Electronic supplementary information (ESI) available: Optimisation studies,
experimental procedures, spectroscopic data, NMR spectra. See DOI:
10.1039/d1sc03596g
Scheme 1 Strategies in C-acyl furanoside synthesis.
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Table 1 Scope of C-acyl glycosylation with DHP-derived glycosyl estersaa
a
Isolated yields. The major anomer was isolated as a pure product, unless specied. Stereochemistry of the products was assigned based on NOESY
b
and COSY experiments. Reaction conditions: carboxylic acid (0.20 mmol), glycosyl ester (0.26 mmol, 1.3 equiv.), 1,4-dioxane (4.8 mL). 2,20-Bi-2-
c
oxazoline (biOx) as the ligand, NiBr2$diglyme (5 mol%), 4CzIPN (0.25 mol%), glycosyl ester (1.2 equiv.), LiBr (2.5 mol%), 86 ꢀC. The
corresponding anhydride was used as the electrophile without DEDC, 1,4-dioxane (3.2 mL).
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example, furanosyl stannane reagents have not yet been re- high yield of 16 suggests that this method shows promise for
ported. Furanosyl bromides, especially 2-deoxy derivatives, are glyco-peptide synthesis by allowing conjugation of glutamic
oen unstable and have not been applied to C-acyl furanoside acid with a variety of carboxylic acids. The coupling of D-man-
synthesis. Currently, the best way to prepare C-acyl furanosides nofuranosyl DHP ester to benzoic acids utilizes a variant of the
is through the nucleophilic addition of metal reagents to standard conditions that employs unsubstituted 2,20-bi-2-
glycosyl nitriles,17 but this method requires multiple steps and oxazoline (biOx) as the ligand to afford products 20–25.
precludes base-sensitive functional groups.
Electron-decient benzoic acids gave lower yields than electron-
The dearth of furanosylation methods may be associated rich derivatives. In all reactions, the a-anomer was isolated as
with the lack of furanosyl radical precursors that can be used for the predominant product due to the approach of the incoming
cross-coupling. Pyranosyl radicals have been generated from catalyst from the a-face, as the b-face was hindered by the C2-
various unnatural glycosyl derivatives, including bromides,18 substituent. Alternatively, it is possible that the capture of
xanthates,19 glycals,20 stannanes,21 thiol ethers,22 acyl tellu- glycosyl radical by Ni is reversible.31 Under the Curtin–Hammett
rides,23 and triuoroborates.24 Precursors to furanosyl radicals, condition, the stereoselectivity is determined by a faster
however, are limited to unnatural tetrahydrofuran derivatives reductive elimination of the a-product relative to the b-product.
with a carboxylic acid at the C1 position, restricting the scope of
We then investigated the scope with respect to furanoses and
carbohydrate substrates.25 Furthermore, redox auxiliaries based the compatibility of protecting groups using 4-piper-
on C–C bond homolysis can only substitute at the C5 of idinecarboxylic acid (PPR–CO2H) as the coupling partner (Table
furanosides and the C6 of pyranosides, not allowing for 1B). D-Ribofuranoses bearing common protecting groups, such
anomeric functionalization.26 The most ideal glycosyl radical as benzoyl 26, tert-butyldiphenylsilyl (TBDPS) 27, and benzyl 28,
precursor is the native carbohydrate, but the BDE of the afforded the corresponding b-C-acyl ribofuranosides in good
anomeric C–O bond (99 kcal molꢁ1) is even higher than that of yields. Other furanosides also underwent smooth C-acylation to
a typical alcohol (96 kcal molꢁ1).27 Thus, formation of glycosyl generate D-glucofuranoside 29, D-galactofuranoside 30, D-xylo-
radicals via anomeric C–O bond homolysis represents a signi- furanoside 31, and D-arabinofuranoside 32. The stereo-
cant fundamental challenge.
selectivity appears to be governed by the stereochemistry at C2,
Herein, we report a solution to the unmet challenge of C-acyl favoring the entering group from the opposite face of the C2-
furanoside synthesis by utilizing redox auxiliary 1 to generate substituent. While the b-anomer was favored by most prod-
furanosyl radicals from furanosyl esters and cross-coupling ucts, D-arabinofuranoside 32 was formed as the a-anomer.
them to carboxylic acids (Scheme 1B). Inspired by the success Benzyl-protected 2-deoxy-D-ribose generated a mixture of a and
of alkyl dihydropyridine (DHP) as a radical precursor28 and its b anomers of 33 in a ratio of 1.2 : 1, presumably due to the lack
utility in C5- and C6-functionalization of glycosides,26a,b we of steric hindrance at C2 to distinguish the a from the b face.
developed DHP-based redox auxiliary 1 by incorporating
A variety of pyranoses were transformed to C-acyl glycosides
a carbonyl group between DHP and the glycoside, which can 34–41 under the standard conditions (Table 1C). a-Anomers
induce the homolysis of the strong anomeric C–O bond through were favored for D-mannopyranoses 34–36, D-mannosamine 37,
CO2 evolution.29 The favorable condensation between 1 and L-rhamnopyranose 38, and 2-deoxy-D-glucopyranosides 40 and
furanoses and pyranoses readily furnishes glycosyl esters that 41, as determined by the kinetic anomeric effect, namely the
are bench stable and compatible with all common native stabilization of the transition state by the donation of the lone
carbohydrates.
electron pair on the ring oxygen to the anti-bonding orbital of
the newly-formed s bond (s*‡) in the axial position.32 Both
anomers were observed for 2-deoxy-D-ribopyranoside 39, with
the b-anomer favored due to the steric hindrance remotely
Results and discussion
We performed C-acylation of glycosides by cross-coupling imparted by C3 and C4 substituents. C-Acylation of D-gal-
glycosyl DHP carboxylate esters with carboxylic acids under actopyranose and D-glucopyranose was less effective and affor-
modied photoredox-nickel dual catalytic conditions, with ded a mixture of a- and b-anomers 42 and 43 in low yields. We
diethyl dicarbonate (DEDC) as an activator for the carboxylic attribute this limitation to the contradictory preferences of the
acids (Table 1).30 The elevated temperature of 90 ꢀC was steric and stereoelectronic effects. In the preferred boat
necessary to facilitate DHP fragmentation and the subsequent conformation 44,32 b-attack of the glycosyl radical intermediate
decarboxylation. Lower temperature led to incomplete decar- is favored due to the steric hindrance at C2, whereas the tran-
boxylation and the formation of glycosyl ester as a byproduct. sition state of a-attack is stabilized by the kinetic anomeric
We rst evaluated the scope with respect to carboxylic acids effect.
coupling to diacetonide-protected D-mannofuranosyl DHP
carboxylate ester (Table 1A). Aliphatic carboxylic acids con- explore its application to derivatize natural products and
taining various functional groups, including amines, alkenes, synthesize biologically relevant molecules (Scheme 2).
The generality of this C-acylation method prompted us to
A
and ketones, underwent efficient coupling to afford C-acyl D- (+)-sclareolide derivative 45 can be readily coupled with 4-piper-
mannosides 2–13. The scope of the carboxylic acids extends to idinecarboxylic acid to afford 46 in 60% yield with a diastereo-
biologically active substrates, including fenbufen 14, urso- meric ratio (d.r.) higher than 10 : 1 (Scheme 2A). This late-stage
deoxycholic acid 15, L-glutamic acid (Glu) 16, a-D-galactopyr- modication can be extended to natural and synthetic molecules
anuronic acid 17, D-biotin 18, and mycophenolic acid 19. The containing hemiacetal functionality, in light of a large variety of
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Scheme 3 Proposed catalytic cycle.
but a discrepancy between the spectroscopic data of 55 and that
of diplobifuranylone A suggests that the original structural
assignment requires reconsideration.
Based on literature precedents and a previous radical trap-
ping experiment,29 we propose that the reaction follows the
photoredox-nickel dual catalytic cycles shown in Scheme 3. The
key step involves the subsequent fragmentation of the DHP
ester to afford the glycosyl radical upon release of Hantzsch
pyridine and CO2. The carboxylic acid coupling partner is acti-
vated by DEDC by forming the corresponding anhydride.
Conclusions
We developed a cross-coupling reaction to prepare C-acyl furan-
osides and pyranosides from glycosyl esters and carboxylic acids.
Upon photoredox activation, the glycosyl ester can fragment to
generate a glycosyl radical through CO2 evolution. This method
tolerates a broad scope of carboxylic acids, furanoses, and pyra-
noses to deliver products with excellent diastereoselectivity, gov-
erned by the stereochemistry at C2. The reaction is particularly
useful for the synthesis of unnatural nucleosides and the late-
stage modication of natural products. The convenient prepara-
tion of the redox active glycosyl esters, their stability, and their
compatibility with common carbohydrates exemplies the
potential of this method in medicinal chemistry.
Scheme 2 Synthetic applications of C-acylation of furanosides.
commercially available carboxylic acids.
A thymidine (T)
analogue dPzo 49 has attracted considerable attention,5a as the
corresponding nucleotide can be incorporated into DNA by Kle-
now fragments and forms a Watson–Crick base pair with adenine
(A).5b C-Acylation of D-2-deoxyribosyl ester 47 accomplishes
a more efficient formal synthesis of 48 compared to a previous
Data availability
route from 50 (Scheme 2B).5a Alternatively, a better overall yield All experimental procedures and spectroscopic data can be
may be achieved by conducting the C-acylation of D-ribose (74% found in the ESI.†
yield, a only, cf. compound S7 in the ESI†), followed by reduction
at C2.5b Finally, we demonstrate that acyl mannofuranose 51,
obtained in 68% yield, can lead to natural product diplobifur-
anylone B 54 33 in a concise synthetic route (Scheme 2C).34 The
Author contributions
Y. W. and T. D. conceived the idea. Y. W. optimized the
structure of 55 was assigned to diplobifuranylone A in the orig-
inal report of these molecules, produced by fungus pathogens,33
conditions. Y. W. and J. L. explored the scope. T. D. supervised
the project and wrote the manuscript.
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12 M. Kolympadi, M. Fontanella, C. Venturi, S. Andre,
Conflicts of interest
´
H.-J. Gabius, J. Jimenez-Barbero and P. Vogel, Chem.–Eur.
J., 2009, 15, 2861–2873.
There are no conicts to declare.
13 A. Dondoni, N. Catozzi and A. Marra, J. Org. Chem., 2005, 70,
9257–9268.
Acknowledgements
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This work was supported by the National Institute of Health
(R01 GM-127778). T. D. thanks the Alfred P. Sloan Foundation
(FG-2018-10354) and the Camille and Henry Dreyfus Founda-
tion (TC-19-019) for providing fellowships to partially support
this work.
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