Pivaloyl-protected glucosyl iodide as a glucosyl donor for the preparation of β-C-glucosides

Article abstract of DOI:10.1016/j.tetlet.2020.152173

A method for the selective synthesis of β-C-glucosides using α-D-tetra-O-pivaloylglucosyl iodide as a glucosyl donor is reported. Its diastereoselectivity differs from that of the respective acetyl-protected glucosyl bromide, as it reported in the literature under similar reaction conditions. The concentration of the catalyst, the solvent and the type of additive used are crucial factors that determine the reaction selectivity. This method has been applied in a short synthesis of dapagliflozin. The stability of α-D-tetra-O-pivaloylglucosyl iodide in CDCl₃ and THF at reflux was also studied. All side products in the coupling and decomposition reactions were isolated and characterized, and possible pathways for their formation are proposed.

Full text of DOI:10.1016/j.tetlet.2020.152173

Tetrahedron Letters xxx (xxxx) xxx
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Pivaloyl-protected glucosyl iodide as a glucosyl donor
for the preparation of b-C-glucosides
Virginia V. Triantakonstanti a, Alexandros Toskas a, Nicolaos S. Iordanidis a, Thanos Andreou b,
Theoharis V. Koftis b, John K. Gallos a,
⇑
a Department of Chemistry, Aristotle University of Thessaloniki, Thessaloniki 54124, Greece
b Pharmathen S.A. – R&D API Operations, 9th km Thermi-Thessaloniki, Thessaloniki 57001, Greece
a r t i c l e i n f o
a b s t r a c t
Article history:
A method for the selective synthesis of b-C-glucosides using a-D-tetra-O-pivaloylglucosyl iodide as a glu-
Received 13 May 2020
Revised 18 June 2020
Accepted 23 June 2020
Available online xxxx
cosyl donor is reported. Its diastereoselectivity differs from that of the respective acetyl-protected gluco-
syl bromide, as it reported in the literature under similar reaction conditions. The concentration of the
catalyst, the solvent and the type of additive used are crucial factors that determine the reaction selec-
tivity. This method has been applied in a short synthesis of dapagliflozin. The stability of a-D-tetra-O-
pivaloylglucosyl iodide in CDCl3 and THF at reflux was also studied. All side products in the coupling
and decomposition reactions were isolated and characterized, and possible pathways for their formation
are proposed.
Keywords:
C-glycosides
glucosyl iodide
dapagliflozin
Ó 2020 Elsevier Ltd. All rights reserved.
Grignard reagents
SGLT2 inhibitors
Introduction
glycosyl bromide often catalyzed by Ni, Co, Pd and Fe complexes
[5].
C-Glycosides are an important class of naturally occurring and
synthetic products with applications as potent pharmaceutical
compounds [1]. As they are less vulnerable to metabolic processes,
in contrast to O-glycosides, many C-glycosides have been proven to
be good drug candidates and inhibitors of carbohydrate processing
enzymes. Among them, a number of gliflozins, which act as selec-
tive sodium-glucose transporter-2 (SGLT2) inhibitors, have
attracted interest as alternative therapeutics for glycemic control
in a glucose-dependent but insulin independent manner [2]. Cur-
rently, several gliflozins, such as ertugliflozin (1), dapagliflozin
(2) and canagliflozin (3) are on the market for the treatment of type
2 diabetes (Fig. 1).
Glycosyl iodides, in contrast to the respective bromides, have
been seldom used as glycosyl donors in general, and much less
for the synthesis of C-glycosides. This is probably due to the long
held belief that glycosyl iodides are too difficult to prepare, and
too unstable, to be useful glycosyl donors [6]. Only in the 21st cen-
tury was it discovered that ‘‘disarmed” glycosyl iodides such as
glucose derivatives 4 and 5 (Fig. 1) are stable compounds while
the respective ‘‘armed” (6) and ‘‘super armed” (R = SiMe3) glycosyl
iodides can be prepared in situ [7]. Our interest in the development
of new methods to synthesize gliflozins [8] prompted us to inves-
tigate the possibility to prepare b-C-aryl glycosides, such as dapa-
gliflozin using glucosyl iodides 4–6 as glucosyl donors.
However, despite their importance, the stereoselective, synthe-
sis of C-glycosides remains a challenge and this topic has evoked
growing interest [3]. The construction of the key anomeric CAC
bond involves approaches that generate nucleophilic, electrophilic
or radical character on the C1 center, whereas glycals, glycosyl
halides, thioglycosides, anomeric trichloroacetimidates, glycosyl
acetates, glyconolactones and glycosyl stannanes have been used
as glucosyl donors [4]. Perhaps, the most common approach
involves nucleophilic addition of an organometallic reagent to a
Results and discussion
We commenced our work with the reaction of pivaloyl-pro-
tected glucosyl iodide 4 and excess phenyl magnesium bromide
7 (R = Ph; 2.5 equiv.). Without the presence of any catalyst, only
2-deoxyglucoside 11 and glucal 12 were isolated in low yields
(Table 1, Entry 1). Using catalytic CuI in THF in the presence of LiCl
(2 equiv.) and TMEDA (5 mol%), none of the desired C-glucoside
was formed and traces of the elimination product 10 were isolated
(Entry 2). Also, the reaction between 4 and 2.5 equivalents of 7
(R = Ph) in the presence of 10 mol% PEPPSI-iPr gave only trace
⇑
Corresponding author.
E-mail address: igallos@chem.auth.gr (J.K. Gallos).
https://doi.org/10.1016/j.tetlet.2020.152173
0040-4039/Ó 2020 Elsevier Ltd. All rights reserved.
Please cite this article as: V. V. Triantakonstanti, A. Toskas, N. S. Iordanidis et al., Pivaloyl-protected glucosyl iodide as a glucosyl donor for the preparation
of b-C-glucosides, Tetrahedron Letters, https://doi.org/10.1016/j.tetlet.2020.152173

2
V.V. Triantakonstanti et al. / Tetrahedron Letters xxx (xxxx) xxx
formed, but in lower yields together with products 10, 11, and
12 (Entries 7 and 8). Running the reaction in THF at reflux or dou-
bling the amount of the Co(acac)3 catalyst caused no change to the
results (Entries 9 and 10). However, the use of 10 mol% of TMEDA
suppressed the formation of desired phenyl b-glucoside 8a, while
at the same time increasing the yield of elimination product 10
(Entry 11). Once again, the role of TMEDA was proven crucial, since
its absence (Entry 12) led to the formation of 8a and 9a in very low
yields together with trace amounts of products 11 and 12. Finally,
reducing the equivalents of the Grignard reagent resulted in a dra-
matically different reaction pathway and only trace amounts of
elimination product 10 were detected (Entry 13).
Having established the optimum conditions leading to the
desired phenyl b-glucoside 8a (Entry 4), we considered that the
Fig. 1. Structure of selected gliflozins and protected glucosyl iodides.
moderate yields are due to the unreactive starting
a-glucosyl
iodide 4 and/or its partial decomposition to some of the side prod-
ucts during the work-up. Thus, it was proposed that the yields of
8a would be increased upon prolonged reaction times. Indeed,
after 4 days of reaction between 4 and 7 (R = Ph) at 25 °C
(1:2.5 M ratio) with 5 mol% Co(acac)3 and 5 mol% TMEDA in THF,
the desired phenyl b-glucoside 8a was isolated chromatographi-
cally in 63% yield (entry 14). Interestingly, this result was repro-
ducible on multi-gram scales.
High b-selectivity and better yields have been obtained from
analogous reactions of the respective tetra-O-pivaloyl-a-D-gluco-
syl bromide using organozinc reagents as nucleophiles [5c].
However, our method is much simpler with regard to the
reaction conditions, reagents availability and sensitivity, and
experimental and safety precautions.
amounts of 10 (Entry 3). Encouraging results were obtained when
we applied the catalytic conditions introduced by Reymond, Cossy
and co-workers [5b] and the reaction between 4 and 7 (R = Ph;
1:2.5 M ratio) with 5 mol% Co(acac)3 and 5 mol% TMEDA in THF
afforded the desired phenyl b-C-glucoside 8a in 42% yield
together with
a-C-glucoside 9a and compound 10 in 7% and 8%
yields, respectively (Table 1, Entry 4).
In order to optimize the yields of desired phenyl b-C-glucoside
8a, a number of experiments were carried out. It was found that
keeping the same reaction conditions and changing the solvent
to toluene, 1,4-dioxane or ethyl ether did not give 8a but small
amounts of the elimination product 10 (Entries 5 and 6). Interest-
ingly, using THF as the solvent and replacing the TMEDA additive
with Et3N (5 mol%) or DABCO (5 mol%), resulted in a reversal of
To our surprise, peracetylated glucosyl iodide 5 (Fig. 1) under
the established reaction conditions did not give any phenyl
a- or
b-C-glucoside, but 2,3,4,6-tetraacetyl-O-glucopyranose (37%)
the reaction stereoselectivity and phenyl
a-C-glucoside 9a was
Table 1
Optimization study and reactions of pivaloyl-protected glucosyl iodide with Grignard reagents
.
Entry
R
Reaction Conditions
Products (%)
1
Ph
Et2O, 25 °C, 12 h
11 (2), 12 (3)
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
CuI (5 mol%), LiCl (2 equiv.), TMEDA (5 mol%), THF, 25 °C, 12 h
PEPPSI-IPr (10 mol%), TMEDA (5 mol%), THF, 25 °C, 12 h
Co(acac)3 (5 mol%), TMEDA (5 mol%), THF, 25 °C, 12 h
Co(acac)3 (5 mol%), TMEDA (5 mol%), Toluene, 25 °C, 48 h
Co(acac)3 (5 mol%), TMEDA (5 mol%), Dioxane, 25 °C, 48 h
Co(acac)3 (5 mol%), Et3N (5 equiv.), THF, 25 °C, 12 h
Co(acac)3 (5 mol%), DABCO (5 eqiv.), THF, 25 °C, 12 h
Co(acac)3 (5 mol%), TMEDA (5 mol%), THF, 60 °C, 12 h
Co(acac)3 (10 mol%), TMEDA (5 mol%), THF, 25 °C, 12 h
Co(acac)3 (5 mol%), TMEDA (10 mol%), THF, 25 °C, 12 h
Co(acac)3 (5 mol%), no additive, THF, 25 °C, 12 h
Co(acac)3 (5 mol%), TMEDA (5 mol%), THF, 25 °C, 12 h
Co(acac)3 (5 mol%), TMEDA (5 mol%), THF, 25 °C, 4 d
Co(acac)3 (5 mol%), TMEDA (5 mol%), THF, 25 °C, 4 d
Co(acac)3 (5 mol%), TMEDA (5 mol%), THF, 25 °C, 4 d
Co(acac)3 (5 mol%), TMEDA (5 mol%), THF, 25 °C, 4 d
Co(acac)3 (5 mol%), TMEDA (5 mol%), THF, 25 °C, 4 d
Co(acac)3 (5 mol%), TMEDA (5 mol%), THF, 25 °C, 4 d
Co(acac)3 (5 mol%), TMEDA (5 mol%), THF, 25 °C, 2 d
Co(acac)3 (5 mol%), TMEDA (5 mol%), THF, 25 °C, 2 d
10 (5)
10 (3)
8a (42), 9a (7), 10 (8)
10 (10%)
10 (4)
9a (24), 10 (10), 11 (7), 12 (5)
9a (27), 10 (10), 11 (7), 12 (6)
8a (42), 9a (7), 10 (8)
8a (42), 9a (7), 10 (8)
8a (27), 9a (15), 10 (17)
8a (2), 9a (3), 11 (1), 12 (5)
10 (3)
8a (63), 9a (9), 10 (10)
10 (41)
11 (8), 12 (48)
Ph
Ph
Ph
Pha
Ph
allyl
n-butyl
cyclohexyl
2-tolyl
4-tolyl
4-anisyl
vinyl
11 (7), 12 (33)
10 (64), 13 (28)
8b (38), 9b (19), 10 (4), 12 (5)
8c (35), 10 (1), 12 (3)
10 (19), 12 (13), 13 (2)
a
1.5 equiv. of PhMgBr.
Please cite this article as: V. V. Triantakonstanti, A. Toskas, N. S. Iordanidis et al., Pivaloyl-protected glucosyl iodide as a glucosyl donor for the preparation
of b-C-glucosides, Tetrahedron Letters, https://doi.org/10.1016/j.tetlet.2020.152173

V.V. Triantakonstanti et al. / Tetrahedron Letters xxx (xxxx) xxx
3
together with the elimination product 10 (9%) were isolated. Sim-
ilarly from the perbenzylated glucosyl iodide 6 (prepared in situ),
2,3,4,6-tetrabenzyl-O-glucopyranose was isolated (15%) from a
complex mixture of products.
4-anisyl magnesium bromide led to exclusive formation of the b-
C-glucoside in moderate yield, together with the known side prod-
ucts, but the vinyl magnesium bromide gave none of the C-glu-
coside (Entries 20, 21).
Comparing our results to those reported by Reymond, Cossy and
co-workers [5b] in analogous reactions where the respective
peracetylated glucosyl bromide was used as a glucosyl donor, it
is interesting to note the reverse diastereoselectivity obtained.
Furthermore, we studied the stability of glucosyl iodide 4. It is a
solid (m.p. 140–142 °C) and can be stored in the refrigerator for
months without decomposition as determined by NMR. Also, it is
similarly stable in the air at room temperature as a solid under
light protection. A solution of 4 in CDCl3 at room temperature
was followed by 1H NMR and after 22 days, it was completely
decomposed to products 13 and 14 in roughly equally amounts.
In addition, heating a solution of 4 in THF at reflux for 12 h
(Scheme 1) resulted in appreciable decomposition and the already
mentioned compounds 13 and 14 were formed in 5% and 2% yields,
respectively, together with compound 15 (58%) and unchanged
staring material (12%).
Reymond and Cossy reported an
a/b 3:1 diastereoselectivity with
preference for the -anomer whereas we obtained an
a
a/b 1:6.3
ratio with preference for the b-anomer. This might be due to
either different protecting groups or replacement of the bromine
atom by iodine. Additional mechanistic studies are needed to
shed light on these results. Another interesting point in our
results is the inversion of diastereoselectivity when we switched
the additive from TMEDA to Et3N or DABCO. No b-anomer was
isolated in these cases and only
significant amounts of side products were formed.
To develop further the reaction, a number of Grignard reagents
were used instead of PhMgBr. Although it has been reported that
the ‘‘armed” per-O-benzylated galactopyranosyl iodide analogous
to 6 undergoes an SN2 displacement of iodide by allyl magnesium
a
-anomer together with
All compounds 8, 9, 10, 12 and 13 [5c,5f,9–11] are known and
their structure was assigned on the basis of their 1H and 13C
NMR spectra, which were in good agreement with those reported
in the literature. Both b- and
a-anomers 8 and 9, are easily
discerned from their 1-H and aromatic chemical shifts (500 MHz,
CDCl3). Exemplifying for 8a, the 1-H signal appeared as a doublet
at d 4.41 (J = 9.3 Hz, axial disposition) whereas in 9a it appeared
at d 5.37 (d, J = 5.3 Hz, equatorial disposition). Also interestingly,
the aromatic protons in 8a are almost equivalent and appeared
as a narrow multiple centered at d 7.31, whereas in 9a the signal
of deshielded ortho-protons (d 7.61) is distinct from those meta-
(d 7.38) and para-protons (d 7.32). Glucosides 11, 14 and 15 are
new compounds and their structure assignment was easily made
by comparison of their NMR spectra with those of the respective
bromide [7b], affording the respective C-glycoside, no
a- or b-C-
glucoside was obtained when we attempted the reactions of 4
with allyl, n-butyl or cyclohexyl magnesium bromides under the
optimum conditions established for PhMgBr (Entries 15–17).
o-Tolyl magnesium bromide gave none of the C-glucosylation
product and the elimination product 10 was obtained in 64% yield
together with 2,3,4,6-tetrapivaloyl-O-glucopyranose (13, 28%),
possibly due to steric reasons (Entry 18). However, the reaction
of p-tolyl magnesium bromide led to the formation of both
a-
known acetates, which are very similar [12–14]. Their a- (com-
and b-C-glucosides but in lower diastereoselectivity ( /b, 1:2),
together with the known side products (Entry 19). Interestingly,
a
pounds 11 and 14) and b-anomeric structure (compound 15) was
deduced from the 1-H signal in the 1H NMR spectra, which
appeared at d 6.23, d, J = 2.8 Hz for 11, at d 6.21, d, J = 3.8 Hz for
14 (both indicating equatorial disposition) and
J = 8.0 Hz for 15 (axial disposition).
d 4.48, d,
Side product 13 formed in the coupling reactions is apparently a
decomposition product of iodide 4 by water, whereas the forma-
tion of compounds 10, 11 and 12 is caused by side-reactions of
Grignard reagents or the additive with iodide 4, as shown in
Scheme 2. By the action of a base (Grignard reagent or additive)
an elimination reaction in 4 occurs, affording compound 10. Iodide
4 is evidently in equilibrium with intermediate 16 as previously
Scheme 1. Decomposition products of 4 in THF at reflux.
Scheme 2. Possible reaction pathways leading to side and decomposition products.
Please cite this article as: V. V. Triantakonstanti, A. Toskas, N. S. Iordanidis et al., Pivaloyl-protected glucosyl iodide as a glucosyl donor for the preparation
of b-C-glucosides, Tetrahedron Letters, https://doi.org/10.1016/j.tetlet.2020.152173

4
V.V. Triantakonstanti et al. / Tetrahedron Letters xxx (xxxx) xxx
Scheme 3. Preparation of dapagliflozin.
reported for acylated glucosyl halides [5c]. Further opening of the
dioxolane ring in 16 by attack of the iodide anion to C-2 gives
intermediate iodide 18, which is subsequently converted to the
respective Grignard reagent 19 by metallation with magnesium
or transmetallation with RMgBr. This intermediate can be
transformed into glucal 12 by an elimination reaction or
decomposed by water during the work-up leading to the deoxy
derivative 11.
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.tetlet.2020.152173.
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Acknowledgments
This research has been co-financed by the European Union and
Greek national funds through the Operational Program Competi-
tiveness, Entrepreneurship and Innovation, under the call
RESEARCH – CREATE – INNOVATE (project code:T1EDK-01161).
Please cite this article as: V. V. Triantakonstanti, A. Toskas, N. S. Iordanidis et al., Pivaloyl-protected glucosyl iodide as a glucosyl donor for the preparation
of b-C-glucosides, Tetrahedron Letters, https://doi.org/10.1016/j.tetlet.2020.152173