An efficient synthetic route to O-(2-O-benzyl-3,4-di-O-acetyl-α/β-L-fucopyranosyl)-trichloroacetimidate

Article abstract of DOI:10.1016/j.carres.2020.108221

An efficient synthetic route to prepare O-(2-O-benzyl-3,4-di-O-acetyl-α/β-L-fucopyranosyl)-trichloroacetimidate from L-fucose was developed by introducing the thiophenyl group at the anomeric center and the benzylidene functional group to protect the 3 an

Full text of DOI:10.1016/j.carres.2020.108221

Carbohydrate Research 499 (2021) 108221
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Carbohydrate Research
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An efficient synthetic route to O-(2-O-benzyl-3,4-di-O-acetyl-
α/
β-L-fucopyranosyl)-trichloroacetimidate
a
a
a
´
Blanca I. Tolon Murguía , Yuleidys de las Mercedes Iglesias Morales ,
a
a
´
´
Miriam Mesa Hernandez , Yaneisy Yu Perez , Claudia Labrada Regalado ,
a
b
a,*
´
´
Raine Garrido Arteaga , Françoise Paquet , Miguel A. Lopez Lopez
a Finlay Institute of Vaccines, Calle 200 y 21, Atabey Playa, 11600, La Habana, Cuba
b
´
´
Centre de Biophysique Moleculaire, CNRS UPR 4301, Rue Charles Sadron CS-80054, 45071 Orleans cedex 2, France
A R T I C L E I N F O
A B S T R A C T
An efficient synthetic route to prepare O-(2-O-benzyl-3,4-di-O-acetyl-
Keywords:
α
/β-L-fucopyranosyl)-trichloroacetimidate
Trichloroacetimidate
L-fucose
from L-fucose was developed by introducing the thiophenyl group at the anomeric center and the benzylidene
functional group to protect the 3 and 4 positions. Although three approaches were considered, the best result was
obtained when, after the 2-hydroxyl benzylation, both protective groups were simultaneously removed by using
acetic anhydride and perchloric acid supported on silica as catalyst. Selective deacetylation of the obtained tri-O-
acetate followed by the reaction of the resultant hemiacetal with trichloroacetonitrile and DBU afforded the
trichloroacetimidate with an overall yield of 56% from the L-fucose.
Benzylidene
Thiophenyl
1. Introduction
protection of the positions 3 and 4 with the isopropylidene group. As
additional drawbacks, long total reaction times are required (in both
Flowers was the first to observe that introducing ester groups at the
3- and 4-positions of a fucosyl donor could improve the stereoselectivity
compared to a perbenzylated compound [1]. Therefore, virtually all
modern donors of this type incorporate ester groups at either the 4-, or 3-
and 4-positions. Among these, the O-(2-O-benzyl-3,4-di-O--
cases higher than 80 h), as well as intensive chromatographic purifica-
tion (6 and 5 purification steps for Variants A and B, respectively).
2. Results and discussion
acetyl-
α/β-L-fucopyranosyl)-trichloroacetimidate (Fig. 1) is an efficient
To overcome all these problems, we developed an efficient procedure
(including three approaches) as shown in Scheme 2.
fucosyl donor that provides high
α
stereoselectivity and good yields
during fucosylation, and additionally is much more stable and easy to
use with respect to the corresponding extremely reactive totally ben-
zylated derivative.
To protect the anomeric position of L-fucose we select the thiophenyl
group due to its stability and tolerance to most reaction conditions used
in protecting group manipulations. At the same time, it can be easily
removed without the use of expensive reagents. In our first approach the
per-O-acetylated thioglycoside 1 was obtained with an excellent yield
following the one-pot method developed by Misra et al. [7] using acetic
anhydride and boron trifluoride diethyl etherate as catalyst for both, the
per-O-acetylation of L-fucose and the further glycosylation with thio-
phenol. The acetyl groups of 1 were removed by the Zemplen’s pro-
cedure to obtain a quantitative yield of compound 2. To protect the
positions 3 and 4 of L-fucose derivative 2, it was decided to use the
benzylidene group instead of the isopropylidene group, in order to
exclude the possibility of mixed acetal formation at the 2 position.
Although in the literature there are reports about mixed acetal formation
This trichloroacetimidate has been successfully used to obtain
several fucosylated protected derivatives of the lacto- and neolacto-series
corresponding to important epitopes, such as Lewisa, Lewisx, Lewisy
[2–4] and Sialyl Lewisx [5]. Also has been used to synthetize some
human milk oligosaccharides such as 2’-fucosyllactose and 3-fucosyllac-
tose [6]. However, despite the good qualities and proven usefulness of
this donor, only one low yielding procedure for preparing this tri-
chloroacetimidate is reported in the literature [5,6] (Scheme 1).
Despite the modifications introduced in the different reaction steps,
the overall yield from L-fucose is low (20–24%), mainly because large
amounts of the 2-O-mixed acetal (Fig. 2) are formed during the
* Corresponding author.
´
´
E-mail address: milopez@finlay.edu.cu (M.A. Lopez Lopez).
https://doi.org/10.1016/j.carres.2020.108221
Received 26 October 2020; Received in revised form 8 December 2020; Accepted 11 December 2020
Available online 18 December 2020
0008-6215/© 2020 Elsevier Ltd. All rights reserved.

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B.I. Tolon Murguía et al.
Carbohydrate Research 499 (2021) 108221
Fig. 1. Chemical structure of O-(2-O-benzyl-3,4-di-O-acetyl-α/β-L-fucopyr-
Fig. 2. Chemical structure of the 2-O-mixed acetal formed.
anosyl)-trichloroacetimidate.
In a second approach and to avoid the use of pyridine, it was decided
to perform the acetylation of the 5 free hydroxyls with acetic anhydride/
iodine. In this case, as reported in the literature for thioglycosides [12],
the acetolysis of thiophenyl group of 5 took place to obtain almost
during the introduction of the benzylidene group with benzaldehyde
dimethylacetal in acid media, this side reaction only occurs when is
performed under reduced pressure but not under atmospheric condi-
tions [8]. Thus, 2 was treated with benzaldehyde dimethylacetal in the
presence of p-toluenesulfonic acid as catalyst in N,N-dimethyl-forma-
mide to obtain the benzylidene derivative 3 as a mixture of endo and exo
isomers. Since the benzylidene protective group must further be
removed, both isomers were collected together during chromatographic
purification and the mixture used in the next step. The benzylation of the
3 free 2-hydroxyl group was performed with benzyl bromide and sodium
hydride in N,N-dimethyl-formamide to obtain compound 4 also as a
mixture of the endo and exo isomers. For the same reason explained
before, both isomers were collected together during chromatographic
purification and the mixture used in the next step. The benzylidene
group of 4 was removed in the usual way by treatment with aqueous
acetic acid at 80 ◦C to obtain the diol 5 in a 90% yield after chromato-
graphic purification. The acetylation of the 5 free hydroxyls with acetic
anhydride/pyridine afforded the 3,4-di-O-acetate 6 in a quantitative
yield. To cleave the anomeric thiophenyl group two methods were
explored: (a) the use of N-bromosuccinimide in wet acetone [9,10] and
(b) the treatment with N-iodosuccinimide/trifluoroacetic acid in a
dichloromethane/water mixture [11]. Although in both cases the
hemiacetal 8 was successfully obtained, a better yield was achieved
when the N-bromosuccinimide in wet acetone technique was used (92%)
as compared with the N-iodosuccinimide/trifluoroacetic acid method
(71%). As a consequence, the former procedure was selected to obtain
the compound 8. The final step was the synthesis of the tri-
chloroacetimidate 9 by reacting 8 with trichloroacetonitrile in the
presence of 1,8-diazabicyclo [5.4.0] undec-7-ene as catalyst and
dichloromethane as the solvent at room temperature. Under these con-
exclusively the
α tri-O-acetate 7 in 97% yield. The treatment of com-
pound 7 with hydrazine acetate in N,N-dimethyl-formamide afforded
the hemiacetal 8 also with an excellent yield (90%), which by further
reaction with trichloroacetonitrile and 1,8-diazabicyclo[5.4.0]unde-
c-7-ene (as previously described) gave the trichloroacetimidate 9.
Although by the two ways described before the overall yield ob-
tained from L-fucose was 50% (almost twice the yield reported and
performing the same number of synthetic steps as compared with the
method described in the literature), it was decided to explore the pos-
sibility to effect the simultaneous elimination of benzylidene function
and acetylation of the 3 and 4 hydroxyl groups from compound 4 by
using acetic anhydride and perchloric acid supported on silica gel as
catalyst, following the one pot method reported by Agnihotri et al. for
4,6-benzylidene derivatives [13]. The reaction took place in only 1 h at
room temperature and a clean substitution of the anomeric thiophenyl
group by an acetate group occurred to afford mainly the
α tri-O-acetate 7
(
α
/β relationship 77:13). This was an expected result, since it was pre-
viously described by Agnihotri et al. [14] during the per-O-acetylation of
thioalkyl or thioaryl glycosides with acetic anhydride and perchloric
acid supported on silica gel as catalyst. The compound 7 was selectively
1-O-deacetylated with hydrazine acetate in N,N-dimethyl-formamide as
described before to obtain de hemiacetal 8, and the trichloroacetimidate
9 was synthetized with a similar yield, but with the advantage that this
approach requires one less step and a total reaction time of only 26 h as
compared with the classical method reported in the literature.
However, despite the good results obtained before, this approach
required the purification of 5 intermediaries and the final product by
ditions 9 was obtained as a α/β (9:1) mixture in 89% yield.
Scheme 1. Synthetic route reported in the literature. Variant A [lit. 5]. Reagents and conditions: (i) AllOH, H+, 98 ◦C, 8 h; (ii) acetone, TsOH, 56 ◦C, 4 h; (iii) BnBr,
NaH, DMF, rt, 2 h; (iv) TFA 50%, CH2Cl2, rt, 16 h; (v) Ac2O, Py, rt, 16 h; (vi) (Ph3P)3RhCl, DBU, EtOH/H2O, 80 ◦C, 3 h; (vii) 1 N HCl, rt, 36 h; (viii) tri-
chloroacetonitrile, DBU, CH2Cl2, rt, 90 min. Variant B [lit. 6]. Reagents and conditions: (i) AllOH, CSA, 90 ◦C, 2 h; (ii) 2,2-dimethoxypropane, acetone, CSA, rt, 24 h;
(iii) BnBr, NaH, TBAI, THF/DMF, rt, 16 h; (iv) AcOH 70%, 45 ◦C, 4 h; (v) Ac2O, Et3N, DMAP, rt, 16 h; (vi) [Ir(COD)(PMePh2)2]+ PF6─, H2, THF, rt, 30 min; (vii) I2,
H2O, THF, rt, 30 min; (viii) trichloroacetonitrile, DBU, CH2Cl2, rt, 20 h.
2

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Carbohydrate Research 499 (2021) 108221
column chromatography. Considering this handicap, we decided to
explore the possibility of reduce the number of chromatographic puri-
fication steps. Analyzing the synthetic sequence, we concluded that the
O-acetyl deprotection of compound 1, the introduction of benzylidene
group, the benzylation of the 2-hydroxyl and the reaction with
perchloric acid supported on silica gel as catalyst could be done
consecutively without the necessity of purification the intermediaries 3
and 4 by column chromatography. Thus the acetyl groups of 1 were
removed by the Zemplen’s procedure and after the usual work-up, the
obtained residue was dissolved in N,N-dimethyl-formamide followed for
the addition of benzaldehyde dimethylacetal and p-toluene sulfonic acid
at 50 ◦C. When the reaction is concluded, the mixture was neutralized
with triethylamine, cooled in an ice bath and treated with benzyl bro-
mide followed by sodium hydride. After the reaction is finished, meth-
anol was added to destroy the benzyl bromide excess, the mixture was
diluted with ethyl acetate and was washed several times with a saturated
aqueous solution of sodium chloride to remove the N,N-dimethyl-
formamide. Finally, the obtained residue after evaporation of the
organic layer was treated with acetic anhydride and perchloric acid
-silica gel catalyst. Following this sequence, and after chromatographic
purification, the tri-O-acetate 7 was obtained with an 80% overall yield
from 1. Selective 1-O-deacetylation of 7 and treatment of the hemiacetal
8 with trichloroacetonitrile and 1,8-diazabicyclo[5.4.0]undec-7-ene
provided the trichloroacetimidate 9 with a global yield of 56%
(Scheme 3).
benzylidene function and hydroxyls acetylation were performed by
using the acetic anhydride and perchloric acid supported on silica gel
method and when the tri-O-acetate 7 was obtained from compound 1
without chromatographic purification of the intermediaries. Under
these conditions, the overall yield obtained from L-fucose was 56%,
higher than the reported yield (20–24%) whereas one less synthetic step
and less chromatographic purifications were required as compared with
the classical procedure.
4. Experimental
4.1. General methods
Silica gel (grade 60, 230–400 mesh) was used for column chroma-
tography. Analytical HPTLC were performed on precoated plates of
Silica gel 60, and the chromatograms were visualized by spraying the
plates with 5% H2SO4/EtOH reagent followed by heating. Melting points
(mp) were determined in a Büchi Mꢀ 565 apparatus. Optical rotations
were measured with an ADP 220 Bellenghan + Stanley Ltd. polarimeter
◦
at 25 C. NMR analysis was performed on a Bruker 600 MHz Avance
spectrometer and a Bruker/Avance DPX 250 MHz spectrometer.
4.2. Synthetic methods
4.2.1. Phenyl 2,3,4-tri-O-acetyl-1-thio-β-L-fucopyranoside (1)
Although the yield of the trichloroacetimidate increased in only 6%,
the decision of performing the steps mentioned above without purify the
intermediaries allowed to eliminate two chromatographic purification
steps (only four are required now) improving even more the benefits of
the best developed procedure as compared with the method reported in
the literature.
A suspension of L-fucose (5 g, 30.5 mmol) in Ac2O (11.8 mL, 125
mmol) was placed in an ice bath with continuous stirring. To this cold
suspension was added BF3⋅OEt2 (5.8 mL, 45.8 mmol) in one portion. An
exothermic reaction started immediately and the mixture was allowed to
stir for 10 min. After completion of the reaction, thiophenol (4.9 mL;
47.6 mmol) was added and the reaction mixture was allowed to stir for
another 4 h. The reaction was quenched by addition of aqueous NaHCO3
and the mixture was extracted with CH2Cl2 (120 mL). The organic layer
was washed with water, dried over anhydrous Na2SO4 and concentrated
under reduced pressure. The residue was purified by silica gel column
chromatography (20:1 toluene/AcOEt) to obtain 1 (Yield: 9.82 g, 87%)
as a syrup: Rf: 0.30 (4:1 n-hexane/AcOEt); 1H NMR (CDCl3, 600 MHz): δ
7.57–7.52 (m, 2H, aromatics), 7.34–7.31 (m, 3H, aromatics), 5.28 (dd,
1H, J4,3 3.4, J4,5 1.1 Hz, H-4), 5.24 (dd, 1H, J2,1 = J2,3 10.0 Hz, H-2),
3. Conclusions
The introduction of thiophenyl group at the anomeric position of L-
fucose together with the protection of 3 and 4 hydroxyls with benzyli-
dene group provided a more efficient route to prepare the O-(2-O-
benzyl-3,4-di-O-acetyl-α/β-L-fucopyranosyl)-trichloroacetimidate (9).
The best result was obtained when the simultaneous elimination of
Scheme 2. Synthetic routes developed. Reagents and conditions: (i) Ac2O, BF3⋅OEt2, rt, 10 min, then PhSH, rt, 4 h; (ii) NaOMe, MeOH, rt, 2 h; (iii) benzaldehyde
dimethylacetal, p-TsOH, DMF, 50 ◦C, 1 h; (iv) BnBr, NaH, DMF, rt, 1 h; (v) aq. AcOH 80%, 80 ◦C, 1 h; (vi) Ac2O, Py, rt, 6 h; (vii) NBS, acetone, H2O, rt, 1 h; (viii)
trichloroacetonitrile, DBU, CH2Cl2, rt, 16 h; (ix) Ac2O, I2, rt, 15 min; (x) hydrazine acetate, DMF, 50 ◦C, 30 min; (xi) Ac2O, HClO4–SiO2, rt, 1 h.
3

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B.I. Tolon Murguía et al.
Carbohydrate Research 499 (2021) 108221
Scheme 3. Best synthetic approach developed without chromatographic purification of the intermediaries 3 and 4. Reagents and conditions: (i) Ac2O, BF3⋅OEt2, rt,
10 min, then PhSH, rt, 4 h; (ii) NaOMe, MeOH, rt, 2 h; (iii) benzaldehyde dimethylacetal, p-TsOH, DMF, 50 ◦C, 1 h; (iv) BnBr, NaH, DMF, rt, 1 h; (v) Ac2O,
HClO4–SiO2, rt, 1 h; (vi) hydrazine acetate, DMF, 50 ◦C, 30 min; (vii) trichloroacetonitrile, DBU, CH2Cl2, rt, 16 h.
5.07 (dd, 1H, J3,2 9.9, J3,4 3.4 Hz, H-3), 4.72 (d, 1H, J1,2 9.9 Hz, H-1),
3.85 (qd, 1H, J5,Me 6.4, J5,4 1.1 Hz, H-5), 2.16, 2.10, 1.99 [(s, 3H,
COCH3) × 3], 1.26 (d, 3H, JMe,5 6.4 Hz, 5-CH3); 13C NMR (CDCl3, 151
MHz): δ 170.7, 170.2, 169.6, 133.0, 132.4 (2C), 129.0 (2C), 128.1, 86.6,
73.3, 72.5, 70.4, 67.5, 21.0, 20.8, 20.7, 16.5. (NMR data in agreement
with reported literature [15]).
104.7, 87.5, 78.9, 78.7, 72.7, 71.7, 17.1.
Exo isomer: a white solid: mp: 115–116 ◦C; Rf: 0.42 (4:1 n-hexane/
1
AcOEt). H NMR (CDCl3, 600 MHz): δ 7.61–7.56 (m, 2H, aromatics),
7.47–7.42 (m, 2H, aromatics), 7.40–7.30 (m, 6H, aromatics), 6.10 (s,
1H, CHPh), 4.50 (d, 1H, J1,2 10.1 Hz, H-1), 4.39 (dd, 1H, J3,2 7.0, J3,4 5.3
Hz, H-3), 4.02 (dd, 1H, J4,3 5.3, J4,5 2.0 Hz, H-4), 3.86 (qd, 1H, J5,Me 6.6,
J
5,4 2.0 Hz, H-5), 3.73 (ddd, 1H, J2,1 9.8, J2,3 7.1, J2,2-OH 2.4 Hz, H-2),
4.2.2. Phenyl 1-thio-β-L-fucopyranoside (2)
2.68 (d, 1H, J2-OH,2 2.4 Hz, 2-OH), 1.47 (d, 3H, JMe,5 6.5 Hz, 5-CH3); 13
C
To a solution of 1 (7.6 g, 19.9 mmol) in MeOH (50 mL), a 0.3 mol/L
solution of NaOMe in MeOH (18 mL) was added and the obtained
mixture was stirred for 2 h at room temperature. The reaction mixture
was neutralized with Amberlite IRA 120 (H+), the resin was removed by
filtration, was washed with MeOH, and the combined filtrates were
evaporated to dryness under reduced pressure to obtain 2 (Yield: 5.1 g,
NMR (CDCl3, 151 MHz): δ 138.9, 132.8 (2C), 132.2, 129.2 (2C), 129.1,
128.5 (2C), 128.2, 126.2 (2C), 103.31, 88.1, 80.1, 76.3, 73.1, 69.2, 17.2.
4.2.4. Phenyl 2-O-benzyl-3,4-O-benzylidene-1-thio-β-L-fucopyranoside (4)
To a solution of 3 (4 g, 11.6 mmol, the mixture of endo and exo
isomers) in DMF (43 mL), benzyl bromide (1.76 mL, 14.8 mmol) was
added, the solution was cooled to 0–5 ◦C, a 60% dispersion of NaH in
mineral oil (0.61 g, 15.3 mmol) was then added and the mixture was
stirred for 1 h at room temperature. After completion of the reaction,
MeOH (4 mL) was added; the mixture was stirred for 15 min, was diluted
with AcOEt (40 mL) and washed with saturated NaCl solution (10 × 15
mL). The organic layer was dried over anhydrous Na2SO4, the solvents
were evaporated under reduced pressure and the obtained residue was
purified by silica gel column chromatography (8:1 hexane/AcOEt) to
obtain 4 as a mixture of endo and exo isomers (Yield: 4.64 g, 92%).
Analytical samples of the exo and endo isomers were obtained by silica
gel column chromatography using the same solvent system.
15
◦
◦
quantitative) as a white solid: mp: 90–92 C, lit. 91–92 C; Rf: 0.38
(15:1 CH2Cl2/MeOH); 1H NMR (DMSO‑d6, 600 MHz): δ 7.44–7.40 (m,
2H), 7.33–7.28 (m, 2H), 7.22–7.18 (m, 1H), 5.09 (d, 1H, J4-OH,4= 5.7 Hz,
4-OH), 4.82 (d, 1H, J2-OH,2= 5.5 Hz, 2-OH), 4.58 (d, 1H, J1,2 9.3 Hz, H-
1), 4.51 (d, 1H, J3-OH,3 = 4.6 Hz, 3-OH), 3.63 (qd, 1H, J5,Me 6.5, J5,4 1.2
Hz, H-5), 3.49 (m, 1H, H-3), 3.40 (td, 1H, J2,1 9.2, J2,3 5.4 Hz, H-2), 3.36
(m, 1H, H-4), 1.12 (d, 3H, JMe,5 6.5 Hz); 13C NMR (DMSO‑d6, 151 MHz):
δ 135.4, 129.3 (2C), 128.8 (2C), 126.1, 87.3, 74.8, 73.9, 71.2, 68.9,
16.9. (NMR data in agreement with reported literature [15]).
4.2.3. Phenyl 3,4-O-benzylidene-1-thio-β-L-fucopyranoside (3)
To a solution of 2 (4.8 g, 18.7 mmol) in DMF (24 mL), benzaldehyde
dimethylacetal (4.3 mL, 28.6 mmol) was added, followed by p-tolue-
nesulfonic acid monohydrate (0.36 g, 1.9 mmol) and the obtained
mixture was stirred at 50 ◦C (bath temperature) for 1 h. After completion
of the reaction, the solution was neutralized with Et3N, was diluted with
AcOEt (300 mL) and was washed with saturated NaCl solution (10 × 35
mL). The organic layer was dried over anhydrous Na2SO4, the solvent
was evaporated under reduced pressure and the residue was purified by
silica gel column chromatography (6:1 toluene/AcOEt), to obtain 3 as a
mixture of endo and exo isomers (Yield: 5.48 g, 85%). Analytical samples
of the exo and endo isomers were obtained by silica gel column chro-
matography using the same solvent system.
Endo isomer: a syrup: Rf: 0.62 (6:1 n-hexane/AcOEt); 1H NMR
(CDCl3, 600 MHz): δ 7.58–7.54 (m, 2H, aromatics), 7.43–7.32 (m, 8H,
aromatics), 7.32–7.26 (m, 5H, aromatics), 5.92 (s, 1H, CHPh), 4.73 (d,
1H, JH,H’ 11.3 Hz, CH2Ph), 4.65 (d, 1H, J1,2 9.7 Hz, H-1), 4.57 (d, 1H, JH,
H’ 11.3 Hz, CH2Ph), 4.39 (dd, 1H, J3,2 = J3,4 6.2 Hz, H-3), 4.14 (dd, 1H,
J
4,3, 6.2, J4,5 2.2 Hz, H-4), 3.94 (qd, 1H, J5,Me 6.6, J5,4 2.2 Hz, H-5), 3.55
(dd, 1H, J2,1 9.7, J2,3 6.1 Hz, H-2), 1.50 (d, 3H, JMe,5 6.6 Hz, 5-CH3); 13
C
NMR (CDCl3, 151 MHz): δ 138.0, 137.4, 133.3, 132.7 (2C), 129.5, 128.9
(2C), 128.5 (2C), 128.4 (2C), 128.3 (2C), 127.8, 127.6, 127.1 (2C),
104.6, 86.0, 79.6, 78.9, 78.5, 73.3, 72.5, 17.0.
◦
Exo isomer: a white solid: mp: 97–99 C; Rf: 0.80 (6:1 n-hexane/
1
AcOEt); H NMR (CDCl3, 600 MHz): δ 7.60–7.57 (m, 2H, aromatics),
Endo isomer: a white solid: mp: 109–110 ◦C; Rf: 0.12 (4:1 n-hexane/
7.47–7.44 (m, 3H, aromatics), 7.43–7.27 (m, 10H, aromatics), 6.01 (s,
1H, CHPh), 4.91 (d, 1H, JH,H’ 11.3 Hz, CH2Ph), 4.81 (d, 1H, JH,H’ 11.3
Hz, CH2Ph), 4.70 (d, 1H, J1,2 9.4 Hz, H-1), 4.57 (dd, 1H, J3,2 = J3,4 6.0
Hz, H-3), 4.11 (dd, 1H, J4,3 5.6, J4,5 1.9 Hz, H-4), 3.81 (qd, 1H, J5,Me 6.6,
1
AcOEt); H NMR (CDCl3, 600 MHz): δ 7.61–7.55 (m, 2H, aromatics),
7.40–7.28 (m, 8H, aromatics), 5.92 (s, 1H, CHPh), 4.46 (d, 1H, J1,2 10.2
Hz, H-1), 4.20 (dd, 1H, J3,2 6.8, J3,4 5.8 Hz, H-3), 4.10 (dd, 1H, J4,3 5.9,
J
J
4,5 2.3 Hz, H-4), 3.97 (qd, 1H, J5,Me 6.6, J5,4 2.3 Hz, H-5), 3.55 (ddd, 1H,
2,1 9.7, J2,3 6.8, J2,2-OH 2.5 Hz, H-2), 2.49 (d, 1H, J2-OH,2 2.5 Hz, 2-OH),
J
J
5,4 1.9 Hz, H-5), 3.67 (dd, 1H, J2,1 9.4, J2,3 6.3 Hz, H-2), 1.44 (d, 3H,
Me,5 6.5 Hz, 5-CH3); 13C NMR (CDCl3, 151 MHz): δ 138.8, 137.8, 133.9,
1.52 (d, 3H, JMe,5 6.6 Hz, 5-CH3); 13C NMR (CDCl3, 151 MHz): δ 137.4,
132.2 (2C), 129.4, 128.9 (2C), 128.6 (2C), 128.5 (4C), 128.0, 127.6,
126.4 (2C), 103.5, 86.3, 80.6, 76.6, 75.8, 73.6, 72.9, 17.1. (NMR data in
133.3 (2C), 131.6, 129.5, 129.1 (2C), 128.5 (2C), 128.3, 126.8 (2C),
4

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Carbohydrate Research 499 (2021) 108221
agreement with reported literature [16]).
reaction mixture was neutralized with Amberlite IRA 120 (H+), the resin
was removed by filtration, was washed with MeOH, and the combined
filtrates were evaporated to dryness under reduced pressure. The residue
was dissolved in DMF (10 mL), benzaldehyde dimethylacetal (1.8 mL,
12 mmol) was added, followed by p-toluenesulfonic acid monohydrate
(0.15 g, 0.79 mmol) and the obtained mixture was stirred at 50 ◦C for 1 h
and was neutralized with Et3N. Then the solution was cooled to 0–5 ◦C,
benzyl bromide (1.2 mL, 10.1 mmol) was added followed by a 60%
dispersion of NaH in mineral oil (0.4 g, 10.3 mmol) and the mixture was
stirred for 1 h at room temperature. After completion of the reaction,
MeOH (2 mL) was added, the mixture was diluted with AcOEt (30 mL)
and washed with saturated NaCl solution (10 × 10 mL). The organic
layer was dried over anhydrous Na2SO4 and the solvent was evaporated
under reduced pressure. Thus, the obtained crude 6 was dissolved in
Ac2O (6.8 mL, 72 mmol), HClO4–SiO2 (400 mg) was added and the re-
action mixture was stirred at room temperature for 1 h. After comple-
tion, the reaction mixture was diluted with AcOEt (35 mL), filtered
through a Celite bed and the filtrate was evaporated to dryness under
reduced pressure. The residue was purified by silica gel column chro-
matography (6:1 n-hexane/AcOEt), to obtain 7 (Yield: 3 g, 80%).
In all the cases 7 was obtained as a syrup: Rf: 0.3 (6:1 n-hexane/
4.2.5. Phenyl 2-O-benzyl-1-thio-β-L-fucopyranoside (5)
A suspension of 4 (4.3 g, 9.9 mmol, the mixture of endo and exo
isomers) in aqueous 80% AcOH solution (88 mL) was stirred for 1 h at
80 ◦C (bath temperature). After completion of the reaction, the solvents
were removed by evaporation under reduced pressure followed by co-
evaporation with toluene and the obtained residue was purified by sil-
ica gel column chromatography (2:1 n-hexane/AcOEt), to obtain 5
◦
(Yield: 3.1 g, 90%) as a white solid: mp: 106–108 C; Rf: 0.05 (2:1 n-
hexane/AcOEt); 1H NMR (CDCl3, 600 MHz): δ 7.59–7.55 (m, 2H, aro-
matics), 7.42–7.26 (m, 8H, aromatics), 4.95 (d, 1H, JH,H’ 11.0 Hz,
CH2Ph), 4.69 (d, 1H, JH,H’ 11.0 Hz, CH2Ph), 4.61 (d, 1H, J1,2 9.7 Hz, H-
1), 3.72 (m, 1H, H-3), 3.65 (1H, m, H-4), 3.61 (qd, 1H, J5,Me 6.4, J5,4 1.0
Hz, H-5), 3.55 (dd, 1H, J2,1 = J2,3 9.3 Hz, H-2), 2.64 (d, 1H, J4-OH,4 5.3
Hz, 4-OH), 2.31 (d, 1H, J3-OH,3 5.1 Hz, 3-OH), 1.35 (d, 3H, JMe,5 6.5 Hz,
5-CH3); 13C NMR (CDCl3, 151 MHz): δ 138.2, 134.1, 131.8 (2C), 129.1
(2C), 128.7 (2C), 128.4 (2C), 128.2, 127.6, 87.5, 78.2, 75.4, 75.3, 74.6,
71.8, 16.8. (NMR data in agreement with reported literature [16]).
4.2.6. Phenyl 2-O-benzyl-3,4-di-O-acetyl-1-thio-β-L-fucopyranoside (6)
To a solution of 5 (3 g, 8.7 mmol) in pyridine (11 mL), Ac2O (11 mL,
116 mmol) was added and the mixture was stirred for 6 h at room
temperature. After completion of the reaction, the obtained solution was
cooled to 0–5 ◦C, MeOH (4 mL) was added, the mixture was stirred for
15 min and the solvents were evaporated under reduced pressure fol-
lowed by co-evaporation with toluene to obtain 6 (Yield: 2.74 g, quan-
titative) as a white solid: mp: 123–124 ◦C; lit.17 124 ◦C; Rf: 0.47 (4:1 n-
hexane/AcOEt); 1H NMR (CDCl3, 600 MHz): δ 7.62–7.58 (m, 2H, aro-
matics), 7.36–7.26 (m, 8H, aromatics), 5.26 (dd, 1H, J4,3 3.3, J4,5 1.0 Hz,
H-4), 5.03 (dd, 1H, J3,2 9.6, J3,4 3.3 Hz, H-3), 4.85 (d, 1H, JH,H’ 10.9 Hz,
CH2Ph), 4.71 (d, 1H, J1,2 9.7 Hz, H-1), 4.58 (d, 1H, JH,H’ 10.9 Hz,
AcOEt);
α
anomer: 1H NMR (CDCl3, 600 MHz): δ 7.40–7.22 (m, 5H,
aromatics), 6.38 (d, 1H, J1,2 3.7 Hz, H-1), 5.29 (m, 2H, H-3 and H-4),
4.66 (d, 1H, JH,H’ 11.9 Hz, CH2Ph), 4.56 (d 1H, JH,H’ 11.9 Hz, CH2Ph),
4.23 (dd, 1H, J5,Me 6.4, J5,4 1.2 Hz, H-5), 3.92 (1H, dd, J2,3 10.0, J2,1 3.7
Hz, H-2), 2.14, 2.13, 2.00 [(s, 3H, COCH3) × 3], 1.13 (d, 3H, JMe,5 6.5
Hz; 5-CH3); 13C NMR (CDCl3, 151 MHz): δ 170.5, 170.2, 169.5, 137.8,
128.5 (2C), 128.1, 127.9 (2C), 90.4, 73.1, 72.4, 71.2, 70.0, 67.2, 21.2,
20.9, 20.7, 16.1.
4.2.8. 2-O-benzyl-3,4-di-O-acetyl-α/β-L-fucopyranose (8)
CH2Ph), 3.80 (qd, 1H, J5,Me 6.4, J5,4 1.0 Hz, H-5), 3.73 (dd, 1H, J2,1
=
4.2.8.1. From compound 6. To a solution of 6 (2.6 g, 6 mmol) in acetone
(30 mL) and H2O (3.4 mL), NBS (3.2 g, 18 mmol) was added and the
mixture was stirred for 1 h at room temperature. After completion of the
reaction, the obtained solution was diluted with CH2Cl2 (130 mL) and
successively washed with H2O (25 mL), aqueous solution of Na2S2O3
(15 mL) and H2O (25 mL). The organic layer was dried over anhydrous
Na2SO4, the solvents were evaporated under reduced pressure and the
obtained residue was purified by silica gel column chromatography (2:1
n-hexane/AcOEt) to obtain 8 (Yield: 1.87 g, 92%).
J
2,3 9.7 Hz, H-2), 2.15, 1.93 [(s, 3H, COCH3) × 2], 1.23 (d, 3H, JMe,5 6.5
Hz, 5-CH3); 13C NMR (CDCl3, 151 MHz): δ 170.6, 170.1, 138.1, 133.7,
132.1 (2C), 129.0 (2C), 128.5 (2C), 128.0 (2C), 127.9, 127.7, 87.8, 75.5,
75.2, 74.7, 73.0, 71.0, 20.8 (2C), 16.7. (NMR data in agreement with
reported literature [17]).
4.2.7. 2-O-benzyl-1,3,4-tri-O-acetyl-α-L-fucopyranose (7)
4.2.7.1. From compound 5. To a suspension of 5 (2 g, 5.77 mmol) in
Ac2O (10 mL, 106 mmol) was added I2 (0.1 g, 0,4 mmol) and the mixture
was stirred for 15 min at room temperature. The obtained solution was
diluted with CH2Cl2 (50 mL) and washed with an aqueous Na2S2O3 so-
lution. The organic layer was separated, the solvent was removed under
reduced pressure, the residue was poured into cool water (0–5 ◦C) and
the mixture was stirred for 2 h. After being neutralized to pH 7 with
saturated Na2CO3 solution, the mixture was extracted with CH2Cl2 (3 ×
50 mL), the organic extracts were combined, dried over anhydrous
Na2SO4, the solvent was evaporated under reduced pressure and the
obtained residue was purified by silica gel column chromatography (6:1
n-hexane/AcOEt), to obtain 7 (Yield: 2.15 g, 98%).
4.2.8.2. From compound 7. To a solution of 7 (2 g, 5.26 mmol) in dry
DMF (15 mL), hydrazine acetate (0.63 g, 6.84 mmol) was added and the
mixture was heated for 30 min at 50 ◦C (bath temperature). After
completion of the reaction, the obtained solution was diluted with
AcOEt (30 mL) and was washed with an aqueous saturated solution of
NaCl (5 × 10 mL). The organic layer was dried over anhydrous Na2SO4,
was evaporated under reduced pressure and the obtained residue was
purified by silica gel column chromatography (2:1 n-hexane/AcOEt) to
obtain 8 (Yield: 1.6 g, 90%).
6
◦
In both cases 8 was obtained as a white solid: mp: 80–82 C, lit.
80–83 ◦C; [
α
]
25 –51.8 (c 1.0, CHCl3), lit.6 –52.1; Rf: 0.33 (2:1 n-hexane/
AcOEt); 1H NDMR (CDCl3, 600 MHz): δ 7.38–7.23 (m, 10H, aromatics),
4.2.7.2. From compound 4. To a solution of 4 (2 g, 4.6 mmol) in Ac2O (4
mL, 42 mmol) was added HClO4–SiO2 (230 mg) and the reaction
mixture was stirred at room temperature for 1 h. After completion, the
reaction mixture was diluted with AcOEt (20 mL), filtered through a
Celite bed and the filtrate was evaporated to dryness under reduced
pressure. The residue was purified by silica gel column chromatography
(6:1 n-hexane/AcOEt), to obtain 7 (Yield: 1.72 g, 98%).
5.31 (dd, 1H, J3,2 10.3, J3,4 3.4 Hz, H-3α), 5.28 (d, 1H, J1,2 3.6 Hz, H-1α),
5.26 (dd, 1H, J4,3 3.4, J4,5 1.3 Hz, H-4α), 5.19 (dd, 1H J4,3 3.5, J4,5 1.1
Hz, H-4β), 4.97 (dd, 1H, J3,2 10.2, J3,4 3.5 Hz, H-3β), 4.88 (d, 1H, JH,H”
11.6 Hz, CH2Ph), 4.74 (d, 1H, J1,2 7.6 Hz, H-1β), 4.66 (m, 3H, CH2Ph),
4.35 (qd, 1H, J5,Me 6.4, J5,4 1.3 Hz, H-5α), 3.82 (dd, 1H, J2,3 10.3, J2,1
3.6 Hz, H-2α), 3.79 (qd, 1H, J5,Me 6.4, J5,4 1.1 Hz, H-5β), 3.57 (dd, 1H,
J2,3 10.2, J2,1 7.6 Hz, H-2β), 2.13 (s, 6H, COCH3), 1.99, 1.96 [(s, 3H,
COCH3) × 2], 1.18 (d, 3H, JMe,5 6.4 Hz, 5β-CH3), 1.11 (d, 3H, JMe,5 6.5
Hz, 5
α
-CH3); 13C NMR (CDCl3, 151 MHz): δ 170.8, 170.7, 170.3 (2C),
4.2.7.3. From compound 1. To a solution of 1 (3 g, 7.84 mmol) in MeOH
(20 mL), a 0.3 mol/L solution of NaOMe in MeOH (7 mL) was added and
the obtained mixture was stirred for 2 h at room temperature. The
138.4, 137.8, 128.6 (2C), 128.4 (2C), 128.2, 127.8 (5C), 97.5, 91.7,
77.7, 74.8, 74.0, 73.4, 72.9, 71.6, 70.8, 70.1, 69.2, 64.7, 20.9, 20.8, 20.7
5

´
B.I. Tolon Murguía et al.
Carbohydrate Research 499 (2021) 108221
(2C), 16.3, 16.1. (NMR data in agreement with reported literature [6]).
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D
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6.47 (d, 1H, J1,2 3.4 Hz, H-1), 5.32 (m, 2H, H-3 and H-4), 4.66 (d, 1H, JH,
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170.1, 161.4, 137.9, 128.5 (2C), 127.9, 127.5 (2C), 94.7, 91.2, 72.9,
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11.4 Hz, CH2Ph), 3.88 (m, 2H, H-2 and H-5), 2.12, 1.90 [(s, 3H, COCH3)
× 2], 1.18 (d, 3H, JMe,5 6.4, 5-CH3); 13C NMR (CDCl3, 62.9 MHz): δ
170.6, 169.9, 161.3, 138.0, 128.4 (2C), 127.8 (3C), 98.5, 91.0, 75.6,
75.0, 72.8, 70.6, 70.2, 20.7, 20.6, 16.1. (NMR data in agreement with
reported literature [6]).
Declaration of competing interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
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Appendix A. Supplementary data
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Supplementary data to this article can be found online at https://doi.
org/10.1016/j.carres.2020.108221.
´
´
´
´
[17] G. Dekany, I. Bajza, J. Boutet, I. Perez, M. Hederos, F. Horvath, P. Kovacs-Penzes,
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6