Regioselective Synthesis of Difluorinated C-Furanosides Involving a Debenzylative Cycloetherification

Article abstract of DOI:10.1021/acs.orglett.9b01878

A highly regioselective synthesis of valuable gem-difluorinated C-furanosides from unprotected aldoses via a debenzylative cycloetherification (DBCE) reaction induced by diethylaminosulfur trifluoride is descibed. The scope and limitations of this DBCE reaction are described using a series of commercially available pentoses and hexoses to afford, without selective protection/deprotection sequences, the corresponding gem-difluorinated C-furanosides in moderate to good yields.

Full text of DOI:10.1021/acs.orglett.9b01878

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Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Regioselective Synthesis of Difluorinated C‑Furanosides Involving a
Debenzylative Cycloetherification
‡
,†
Julien A. Delbrouck,†,§ Valentin N. Bochatay,†,§ Abdellatif Tikad, and Stephane P. Vincent*
́
†
́
University of Namur, Departement de Chimie, Laboratoire de Chimie Bio-Organique, rue de Bruxelles 61, B-5000 Namur, Belgium
‡
́
́
́
Laboratoire de Chimie Moleculaire et Substances Naturelles, Faculte des Sciences, Universite Moulay Ismail, B.P. 11201, Zitoune,
̀
Meknes 50050, Morocco
S
* Supporting Information
ABSTRACT: A highly regioselective synthesis of valuable gem-difluorinated C-furanosides from unprotected aldoses via a
debenzylative cycloetherification (DBCE) reaction induced by diethylaminosulfur trifluoride is descibed. The scope and
limitations of this DBCE reaction are described using a series of commercially available pentoses and hexoses to afford, without
selective protection/deprotection sequences, the corresponding gem-difluorinated C-furanosides in moderate to good yields.
arbohydrates and their conjugates play an important role
Cin various biological processes such as protein structure−
activity modulation or cell−cell recognition.1 These biomole-
cules have thus received a great deal of attention in drug
research and chemical biology.2 However, the low metabolic
stability of the anomeric bond, which is easily cleaved in vivo
by glycosidases, remains a major limitation for their develop-
ment as drug candidates. Because of their stability toward
chemical and enzymatic hydrolysis, carbohydrate analogues
such as C-glycosides have shown improved pharmacokinetic
properties compared to those of the natural saccharides.3,4 In
recent years, the fluorination of bioactive molecules has also
been successfully used to improve the bioavailability and the
activity of drugs or drug candidates.5,6 As a consequence, the
hydrolytic stability of C-glycosidic structures and the
interesting properties of the fluorine atom have been combined
to give rise to a new class of carbohydrate mimetics.7,8 In
particular, the introduction of CF2 and CHF groups, as
Figure 1. Known procedures for the synthesis of exo-CF2-furanosides
(a and b) and our retrosynthetic pathway (c).
isosteres of oxygen, at the pseudoanomeric center of a C-
glycoside can modify the pKa of the neighboring groups and
increase the lipophilicity of the entire molecule9 compared to
that of the native O-glycoside.10,11
between difluoroenoxysilanes and a glycosyl donor (Figure
1b).23 Importantly, this C-glycosylation works with only 2-
Synthetically, although several approaches have been
deoxy furanosides.
developed for the synthesis of gem-difluoromethylated pyrano-
sides,9,12−20 the incorporation of a CF2 group at the anomeric
position of a furanosyl ring has been described much less often.
The synthesis of difluoro-C-furanosides was first reported by
Motherwell et al. in low yields starting from difluoro-exo-
glycals through an addition of nucleophilic or electrophilic
radicals (Figure 1a).21,22 It is noteworthy that the stereo-
selectivity of the radical addition is generally controlled by the
configuration at C2. The second way involves a C-glycosylation
The main synthetic approaches giving access to tetrahy-
drofuran derivatives through a debenzylative cycloetherifica-
tion reaction (DBCE) have been reviewed recently.24−27
Compared to classical C-glycosylation reactions, this cycliza-
tion is usually more efficient for the preparation of C-
Received: May 31, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b01878
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Organic Letters
Letter
Figure 2. Proposed mechanistic pathway for DAST-induced formation of (α)-5a.
furanosides like C-vinylfuranosides especially because it does
not require selective protection/deprotection steps, which can
often be cumbersome.28 This cyclization occurs when an
activated alcohol is placed in the δ position of a benzyloxy
group and gives regio- and stereoselectively tetrahydrofurans at
the expense of a pyran form. This regioselectivity comes from a
remarkable conformation of an acyclic substrate properly
positioning the δ-benzyloxy group for an SN reaction (Figure
2). Inspired by the unique properties of this reaction, we
recently developed an efficient and regioselective strategy for
the preparation of γ-lactone glycosides involving an original
debenzylative lactonization (DBL).25 To the best of our
knowledge, the DBCE reaction has never previously been used
to produce fluorinated C-glycosides. However, it is worth
mentioning that the O’Hagan group described a deoxofluor-
mediated DBCE reaction on an α,β,δ-trifluorinated substrate
that was a undesired but high-yield (94%) transformation.29
Continuing our research in the field of sugar mimics,27,30−33
we report herein an efficient method for synthesizing
regioselectively gem-difluorinated C-furanosides in moderate
to good yields through debenzylative cycloetherifications.
Our retrosynthetic analysis of gem-difluorinated C-furano-
sides I is illustrated in Figure 1c. Alcohol II, the key
intermediate for performing the debenzylative cycloetherifica-
tion reaction (DBCE), would be generated by a Reformatsky
reaction between ethyl bromodifluoroacetate and the aldehyde
obtained from the deprotection of dithioacetal III. The latter
can be easily accessed in two steps from commercially available
sugars.
To explore this transformation, aldehydes 3a−h were
prepared in three steps [thioacetalation, benzylation, and
thioacetal hydrolysis (see the Supporting Information)] in
good yields from commercially available carbohydrates.
The Reformatsky reaction between ethyl bromodifluoroace-
tate and D-arabino-aldehyde 2a in the presence of activated
zinc dust with TMSCl, under Barbier conditions,34,35 afforded
the desired product 3a in 53% yield with low diastereose-
lectivity. The configuration of the new stereogenic center in 3a
was not assigned prior to the cyclization because the DBCE
reaction proceeds through an SN2 mechanism. Thus, the (R,S)
configurations of the secondary alcohol could be deduced after
the assignment of the (α,β) anomeric configuration of the
resulting exo-gem-difluorinated C-furanosides (α)-5a and (β)-
5a, thanks to NOE NMR experiments (Figure 2). The yields of
the Reformatsky couplings with the seven other aldehydes
were also moderate, but they compare well with values from
the literature.36 In most cases, diastereoisomers could be
separated through silica gel chromatography except for D-
xylose and D-lyxose, which were used as a mixture of epimers
(Table 1). We wish to outline that we did not try to optimize
the diastereoselectivities of the Reformatsky couplings as our
intention was to study the DBCE reactions of the two epimeric
alcohols II, thus giving access to both α- and β-fluorinated C-
furanosides. Indeed, an important feature of the DBCE
reaction is its stereospecific (SN2) mechanism, as will be
Table 1. Reformatsky Reactions with Aldehydes 2a−h
sugar
aldehyde
product, yield (%)
dr (%)
D-arabinose
D-ribose
2a
2b
2c
2d
2e
2f
3a, 53
3b, 47
3c, 45
3d, 50
3e, 43
3f, 56
3g, 60
3h, 52
70:30
75:25
65:35
a
D-xylose
a
D-lyxose
64:36
L-fucose
43:57
55:45
69:31
55:45
D-galactose
D-glucose
D-mannose
2g
2h
a
Mixture that cannot be separated by silica gel chromatography.
further demonstrated below. In principle, the α/β stereo-
selectivity will depend on only the stereoselectivity of the
Reformatsky coupling, as long as the two epimers are reactive
under these conditions. Mechanistically, this approach is
strikingly different from the two other methods (Figure 1a,b)
that strongly depend on classical stereoelectronic effects at play
in carbohydrate chemistry.
Then, we optimized the conditions of the DBCE reaction
with alcohol 3a. First, the activation of the hydroxy group with
triflic anhydride37 gave only partial conversion, and the analysis
of the 1H NMR spectrum of the crude reaction mixture
showed 7% cyclized product (α)-5a along with 41% triflate
intermediate 4a (Table 2, entry 1). Classical Mitsunobu
conditions also did not efficiently promote the cyclization
(entry 2), whereas the use of mesyl chloride as an activating
agent gave the corresponding mesylate intermediate 4c in 71%
yield but without the formation of the expected cyclized
product (entry 3). DAST26,38 was eventually identified to be
the optimal promotor of the DBCE reaction yielding the
difluorinated C-furanoside (α)-5a in 62% isolated yield (entry
4). Interestingly, no traces of C-pyranoside isomers were
observed, and only the α-furanoside could be isolated, thus
showing the regioselective and stereospecific character of this
cycloetherification. It is important to mention that no
competitive fluorination of alcohol (R)-3a was observed with
DAST.
Starting from epimeric alcohol (S)-3a, under the same
conditions, the cyclization occurred efficiently, and C-furano-
side (β)-5a was isolated in 74% yield (Table 2, entry 5), as
expected from an inverting substitution mechanism. The
anomeric configurations of both C-furanosides (α)-5a and (β)-
5a were determined on the basis of one-dimensional NOE
NMR experiments, in which H1−H3 and H1−H4 correlations
were observed for the α and β isomers, respectively. These
results for D-arabinoside 3a confirmed the concerted SN2
mechanism of the DBCE reaction because for each stereo-
B
DOI: 10.1021/acs.orglett.9b01878
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Letter
Table 2. Optimization of DBCE Reaction Conditions on Alcohol (R)-3a
entry
reaction conditions
intermediate
conversion (%)
product, yield (%)
a
a
1
2
3
4
Tf2O (1.5 equiv), 2,6-lutidine (3.0 equiv), DCM, −78 °C to rt, 12 h
DIAD (1.5 equiv), PPh3 (1.5 equiv), Et3N (1.5 equiv), THF, rt, 17 h
MsCl (1.5 equiv), Et3N (1.5 equiv), DCM, rt, 17 h
DAST (2 equiv), DCM, 0 °C to rt, 24 h
DAST (2 equiv), DCM, 0 °C to rt, 24 h
(R)-4a
(R)-4b
(R)-4c
(R)-4d
(S)-4d
48
12
(α)-5a, 7
−
−
b
100
100
100
c
(α)-5a, 62
d
c
5
(β)-5a, 74
a
b
c
1
The ratio [41:7 (R)-4a:(α)-5a] was calculated from the crude H NMR spectrum. Mesylate (R)-4c was isolated in 71% yield. Isolated yield.
(S)-3a was used as the starting material instead of (R)-3a.
d
isomeric alcohol (R)- or (S)-3a a single C-furanoside was
isolated (Figure 2).
The scope of this transformation was then examined with
various aldoses to study the influence of the substrate on the
DBCE process. First, a series of pentose derivatives (Scheme
1) were subjected to the optimal DBCE reaction conditions. D-
explained by the steric repulsions between the two benzyloxy
groups at positions C-2 and C-3, and the CF2CO2Et group,
which are all-cis in (α)-5b and trans in the transition state
generating (β)-5b.
With regard to the D-lyxo substrates (R,S)-3d, engaged as a
mixture in DBCE reaction, the respective C-furanosides (α)-5d
and (β)-5d were isolated in 17% and 24% yields (i.e., 41%
overall), respectively. Here again, only a partial conversion of
diastereoisomer (S)-3d was observed while its epimer (R)-3d
was fully consumed. The steric repulsions existing at the
transition state can also be invoked to explain the difference in
reactivity. Indeed, to access molecule (β)-5d, all of the
substituents at positions C-1−C-4 of the furanose ring are in a
cis relationship that can strongly increase the energy of the
transition state.
Scheme 1. Synthesis of Various Fluorinated C-Furanosides
from Pentoses
The scope of the DBCE reaction was then expanded to a
series of hexoses (Scheme 2). D-Galactose and L-fucose
derivatives showed similar reactivities under the optimized
DBCE conditions. Indeed, alcohols (R)-3f and (S)-3f derived
from D-galactose were successfully converted to the corre-
sponding C-furanosides (α)-5f and (β)-5f in 58% and 54%
yields, respectively. When fucoside (R)-3e was activated by
DAST, the cyclization occurred in 53% yield, whereas its
epimer, (S)-3e, gave the desired product (β)-5e in only 25%
yield accompanied by (β)-6e as a byproduct, increasing the
yield of cyclization to 61% (Scheme 2). The formation of (β)-
6e could be explained by a selective debenzylation at C-5
under slightly acidic conditions in the presence of the excess of
DAST (Figure 2).
The DAST-induced DBCE reaction of the D-gluco
derivatives 3g proceeded efficiently starting from the S-epimer
as C-furanoside (β)-5g was isolated in 87% yield. However, the
cyclization of epimer (R)-3g failed, thus showing again that the
relative configuration of the CF2CO2Et and the benzyl group at
C-2 strongly influences the reactivity of the starting alcohol. As
expected,25 both diastereoisomers of D-mannose (R)-3h and
(S)-3h did not undergo DBCE reaction. Indeed, this lack of
reactivity has been explained by an unfavorable “cis” relation-
ship between all substituents of the furan ring, in the oxonium
intermediates. The specific behavior of D-mannose derivatives
toward debenzylative cyclizations has already been observed by
Nicotra and our group.25,39 The structures of 5a−g have been
a
Two equivalents of DAST was used.
xylo derivative 3c used as a 35:65 R/S epimeric mixture was
converted into fluorinated C-furanosides 5c in 83% yield with
the same ratio (35:65 α:β), as anticipated from an SN2
mechanism. Interestingly, the two diastereoisomers derived
from D-ribose, (R)-3b and (S)-3b, reacted differently in the
presence of DAST. While the conversion of the DBCE reaction
with epimer (R)-3b was only 6% to yield α-configured product
(α)-5b, its diastereoisomer, (S)-3b, afforded C-furanoside (β)-
5b in 62% yield (Scheme 1). Increasing the reaction time and
the amount of reagent did not improve the isolated yield of
(α)-5b. This significant difference in reactivity can be
C
DOI: 10.1021/acs.orglett.9b01878
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Letter
starting from perbenzylated acyclic aldoses bearing a gem-
difluoromethyl ester moiety, giving access to various
difluorinated C-furanosides. The scope and limitations of this
reaction have been described on various substrates derived
from widely used commercially available pentoses (D-ribose, D-
arabinose, D-xylose, and D-lyxose) and hexoses (D-glucose, D-
galactose, and L-fucose) and to give a series of difluorinated C-
furanosides in moderate to good yields without the require-
ment of a selective protection/deprotection strategy of the
starting polyol.
Scheme 2. Synthesis of Various Fluorinated C-Furanosides
from Hexoses
Moreover, because of its stereoinverting SN2 mechanism,
this DBCE reaction could be an important alternative for the
future design of natural or artificial bioactive molecules
containing a furanoside ring. Indeed, the α/β stereoselectivity
that is often difficult to control in most of the known
approaches is not problematic in this case because the
cyclization is stereospecific. Thus, as a direct perspective, it
can be envisioned to exploit novel methodologies for the
introduction of the CF2CO2R moiety such as Honda’s Rh-
catalyzed Reformatsky reaction.41 Additionally, the ester
moiety present in each final molecule can be easily modified
and transformed in many other functional groups or
heterocyclic structures that are biologically relevant.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b01878.
Experimental procedures and NMR spectra (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: stephane.vincent@unamur.be.
ORCID
a
A 15% conversion was calculated, and no trace of (α)-5g was
b
observed. A complex mixture was obtained starting from both
isomers of 3h.
́
Stephane P. Vincent: 0000-0001-6258-9058
Author Contributions
§J.A.D. and V.N.B. contributed equally to this work.
fully demonstrated using literature data analysis and NMR
spectroscopy (1H, 13C, 19F, COSY, HSQC, and HMBC). The
pyran structure has been ruled out by HMBC NMR
experiments in which a spatial coupling could be observed
between H-5 and a benzylic carbon. On the other hand, the
absence of coupling between the benzylic carbons and H-4 is
in good accordance with the furanoside ring. In all cases,
regardless of the starting acyclic sugar, only C-furanosides were
formed, showing the highly regioselective character of the
DBCE process. Additionally, all reactions described in this
study are stereospecific because each diastereoisomer provided
a distinct C-furanoside.
To summarize, the stereochemical pattern of the sugar
moiety showed an important effect on the efficiency of the
DBCE process. Similar observations had been reported by
Barker and co-workers during their pioneering work on the
anchimeric assistance of the benzyloxy group.40 While D-
arabinose, D-xylose, L-fucose, and D-galactose showed no
difference in reactivity between the two isomers with respect
to cyclization, D-ribose, D-lyxose, D-glucose, and D-mannose
have difficulty undergoing cyclization for at least one of the
two diastereoisomers.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank the Fonds National de la Recherche
Scientifique (FNRS; Mandat FRIA for J.A.D., PDR “Ni-Pincer”
for V.N.B.).
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