Silver Triflate. A Mild Alternative Catalyst for Glycosylation Conditions Using Trichloroacetimidates as Glycosyl Donors

Article abstract of DOI:10.1081/CAR-120025325

Although trimethylsilyl triflate (TMSOTf) has been widely used to promote glycosyl trichloroacetimidates in oligosaccharide synthesis, silver triflate (AgOTf) was proved to be a mild and in some cases more efficient catalyst in TMSOTf-sensitive glycosylations. Migration and degradation in some specific coupling reactions can be reduced significantly under this alternative glycosylation condition.

Full text of DOI:10.1081/CAR-120025325

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Silver Triflate. A Mild Alternative Catalyst for
Glycosylation Conditions Using Trichloroacetimidates as
Glycosyl Donors
Guohua Wei a , Guofeng Gu a & Yuguo Du a
a Research Center for Eco‐Environmental Sciences , Chinese Academy of Sciences , 100085,
Beijing, China
Published online: 23 Aug 2006.
To cite this article: Guohua Wei , Guofeng Gu & Yuguo Du (2003) Silver Triflate. A Mild Alternative Catalyst for Glycosylation
Conditions Using Trichloroacetimidates as Glycosyl Donors, Journal of Carbohydrate Chemistry, 22:6, 385-393, DOI: 10.1081/
CAR-120025325
To link to this article: http://dx.doi.org/10.1081/CAR-120025325
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JOURNAL OF CARBOHYDRATE CHEMISTRY
Vol. 22, No. 6, pp. 385–393, 2003
Silver Triflate. A Mild Alternative Catalyst
for Glycosylation Conditions Using
Trichloroacetimidates as
Glycosyl Donors
*
Guohua Wei, Guofeng Gu, and Yuguo Du
Research Center for Eco-Environmental Sciences,
Chinese Academy of Sciences, Beijing, China
ABSTRACT
Although trimethylsilyl triflate (TMSOTf) has been widely used to promote glycosyl
trichloroacetimidates in oligosaccharide synthesis, silver triflate (AgOTf) was proved
to be a mild and in some cases more efficient catalyst in TMSOTf-sensitive glyco-
sylations. Migration and degradation in some specific coupling reactions can be
reduced significantly under this alternative glycosylation condition.
INTRODUCTION
In synthetic carbohydrate chemistry, trichloroacetimidates have become the most
widely used glycosyl donors.[1] They can be easily prepared by a base-catalyzed
reaction of a lactol with trichloroacetonitrile. The standard glycosylation step
corresponding to this kind of donor is usually activated by a catalytic amount of
Lewis acid, with trimethylsilyl triflate (TMSOTf) and boron trifluoride etherate
(BF3 ꢀ Et2O) being the reagents most commonly used.[2–4]O-Trichloroacetimidates
*
Correspondence: Yuguo Du, Research Center for Eco-Environmental Sciences, Chinese Aca-
demy of Sciences, 100085 Beijing, China; E-mail: ygdu2001j@yahoo.com.
385
DOI: 10.1081/CAR-120025325
0732-8303 (Print); 1532-2327 (Online)
www.dekker.com
Copyright D 2003 by Marcel Dekker, Inc.

386
Wei, Gu, and Du
exhibit outstanding donor properties in terms of easy formation, stability, reactivity, and
applicability, and generally give high yields of the products. In addition to TMSOTf
and BF3 ꢀ Et2O, TsOH, TfOH, Sn(OTf)2 and ZnCl2 have been occasionally applied to
activate trichloroacetimidates.[5–8]
In our efforts to synthesize active avermectin B1a analogues, we need to couple a
monosaccharide to the avermectin macrolactone. To this end, glucopyranosyl tri-
chloroacetimdate 1[9] was selected to couple with a modified avermectin lactone 2[10]
with reaction promotion with TMSOTf in anhydrous methylene chloride. To our
surprise, sluggish glycosylation results were obtained although solvents and reaction
temperatures were considered seriously.a Further investigation found that the
macrolactone alone is highly unstable and decomposed quickly in the presence of
TMSOTf. However, when AgOTf, first introduced by Krepinsky and co-workers,[11]
was used to catalyze the glycosylation between imidate 1 and aglycone 2, a very clean
reaction was observed and a 91% yield of desired compound 3 was isolated (see
Scheme 1). Attracted by this result, we studied the AgOTf catalyzed glycosylation
carefully using trichloroacetimidates as donors.b Here, we wish to report more
examples of using silver triflate as a mild alternative catalyst for glycosylation con-
ditions using trichloroacetimidates as glycosyl donors.
RESULTS AND DISCUSSION
Glycosylation of 2,3,4,6-tetra-O-benzoyl-a-D-glucopyranosyl trichloroacetimdate
(1, Scheme 1) and avermectin derivative 2 in dry CH2Cl2 at – 42°C with AgOTf as
promoter afforded avermectin derivative 3 in a yield of 91%. Attempted preparation of
3 using TMSOTf, BF3 ꢀ Et2O, TfOH and ZnCl2 in CH2Cl2 as catalysts gave no desired
product at all. We thus explored more examples to see the scope and limitation of this
method. In our previous research project, we found that the condensation of
trichloroacetimidate 4 (Scheme 2) and 1,2:5,6-di-O-isopropylidene-a-D-glucofuranose
(5) gave the desired disaccharide 6 in 56% yield.[15] The by-product was proved to be
the 3,5-O-isopropylidenated product, presumably due to migration of the 5,6-O-
isopropylidene group of 5. We reinvestigated this glycosylation using AgOTf (0.5
equiv) in CH2Cl2 at – 42°C and found that the yield was significantly improved (88%).
The efficacy of this method was further demonstrated by the high yield reaction of
trisaccharide imidate 7[15] and furanosyl acceptor 5 using 0.5 equiv of AgOTf to give
schizophyllan tetrasacharide derivative 8 (85%). Doublets at d 5.64, 4.97, 5.10 and 5.16
1
ppm from H NMR spectra corresponding to H-1I (J 3.6 Hz), H-1II (J 9.5 Hz), H-1III (J
8.0 Hz) and H-1IV (J 8.4 Hz), respectively, confirming the structural assignment of 8.
Partially acetylated glycosyl imidate 9[16] was reacted with 1,6-hexanediol in CH2Cl2 in
the presence of TMSOTf and gave an acceptable yield of dimer 10 by consuming more
equiv of donor 9 (up to 4 equiv). The same reaction processed well using 1 equiv of
AgOTf and 2.2 equiv of 9, thus giving at 0°C a 69% yield of 10. When cholesterol 11
was glycosylated with 1 using 0.3 equiv of AgOTf, an orthoester intermediate was
aWe have tried to apply CH2Cl2, ether, toluene, acetonitrile, nitromethane, hexane and THF as
glycosylation solvents under temperatures from – 42°C to rt.
bExamples using AgOTf in glycosylation reactions: Refs. [12–14].

Silver Triflate
387
Scheme 1.
observed based on TLC and in situ transformation to the expected compound 12 (90%)
was carried out smoothly by adding more AgOTf (additional 0.7 equiv) into the
mixture. Silver triflate catalyzed regioselective glycosylation of furanosyl trichloro-
acetimidate 13 with triol 14 at 0°C gave desired (1 ! 5)-linked disaccharide 13 in 78%
yield, comparable to that of TMSOTf catalyzed regioselective glycosylation.[17] It is
noteworthy that the above AgOTf catalyzed reactions did not occur in THF or CH3CN
under similar glycosylation conditions (Scheme 3).
Scheme 2. Reaction conditions: AgOTf, CH2Cl2, ꢁ 42°C – 0°C; 88% for 6; 85% for 8; 69% for
10; 90% for 12; 78% for 15.

388
Wei, Gu, and Du
Scheme 3. Reaction conditions: AgOTf, CH2Cl2, ꢁ 42°C – 0°C; 95% for 18 (a:b = 1:9); 85%
for 20 (a:b = 7:10); 80% for 22 (a:b = 2:5); 73% for 25 (a:b = 6:1); 83% for 28.
To further investigate the scope and the limitation of this reaction, more examples
were subjected to the AgOTf-imidate glycosylation system. When fully benzylated a-
trichloroacetimidate 16,[18] without a C-2 neighboring participation group, was coupled
with hexanediol derivative 17[19] in the presence of 0.3 equiv of AgOTf in anhydrous
CH2Cl2 at – 42°C, b product 18 was formed predominantly (b:a > 9:1 based on NMR
spectra) in 95% isolated yield. However the same donor, in parallel reactions, gave
inseparable a,b mixtures when employing acceptors as glucuronate lactone 19
(! inseparable 20, 85%, a:b = 7:10) and macrolactone 21 (! inseparable 22, 80%,
a:b = 2:5).c The reactions gave generally better total yields in CH2Cl2 (>80%) than
in THF and toluene (< 30%) for benzylated imidate 16. Condensation of acetylated
cFor inseparable mixture of 20, MALDITOF-MS Calcd for C43H46O11: 738.30; Found 761
(M + Na). Selected 1H NMR (CDCl3): b isomer, d 4.51 (d, 1 H, J 8.1 Hz, H-1’), 6.13 (d, 1 H, J 1.7
Hz, H-1); a isomer, d 4.92 (d, 1 H, J 3.5 Hz, H-1’), 6.05 (d, 1 H, J 1.2 Hz, H-1). Inseparable mixture
of 22 gave highly overlapped 1H NMR spectra. MALDITOF-MS Calcd for C74H96O13Si: 1220.66;
Found 1243.6 (M + Na).

Silver Triflate
389
2-deoxy-a-D-glucopyranosyl imidate 23[20] with saponin derivative 24[21] under the
same reaction conditions gave 25 as a mixture in favor of a product (6:1, 73% of
total yield). More impressively, when fucosyl imidate 26[22] was reacted with fucosyl
thioglycoside 27 in the presence of TMSOTf, only a 50% yield of the disaccharide
28 was isolated. The major side reaction was the formation of the thioglycoside of
the donor, i.e., ethyl 3,4-di-O-acetyl-2-O-benzyl-1-thio-L-fucopyranoside.[23] When the
above reaction was catalyzed with 0.1 equiv of AgOTf in CH2Cl2 at –42°C, a-L-
disaccharide 28 was afforded in an excellent yield (83%) with no detectable thio-
migration product. This advantage may be applicable to the one-pot sequential
synthesis of fucosyl oligosaccharides.[24]
In summary, we have demonstrated that the silver triflate promoted glycosylation
reactions of various trichloroacetimidate donors are highly efficient. The stereochemical
outcome of C-2 benzoylated imidates gave exclusively 1,2-trans linked products but
differed case by case in C-2 benzylated counterparts. Side reactions such as migration
and degradation in coupling reactions could be reduced significantly under this mild
glycosylation condition.
EXPERIMENTAL
General methods. Optical rotations were determined at 20°C with a Perkin–
1
1
Elmer Model 241-Mc automatic polarimeter. H NMR, 13C NMR, H–1H COSY and
HMQC spectra were recorded with ARX 400 spectrometers for solutions in CDCl3.
Chemical shifts are given in ppm downfield from internal Me4Si. Mass spectra were
measured using MALDI-TOF-MS with a-cyano-4-hydroxycinnamic acid (CCA) as
matrix. Thin-layer chromatography (TLC) was performed on silica gel HF254 with
detection by charring with 30% (v/v) H2SO4 in MeOH or in some cases by a UV
detector. Column chromatography was conducted by elution of a column (10 ꢂ 250
mm, 18 ꢂ 300 mm, 35 ꢂ 400 mm) of silica gel (100–200 mesh) with EtOAc–
petroleum ether (bp 60–90°C) as the eluent. Solutions were concentrated at < 60°C
under diminished pressure.
Typical procedure for AgOTf catalyzed glycosylation: To a solution of imidate
donor (1.05 mmol) and alcohol acceptor (1 mmol) in dry CH2Cl2 (5–8 mL) was
˚
added 4A molecular sieves. The mixture was stirred at rt for about 15 min under an
N2 atmosphere, then cooled down to ꢁ 42°C. AgOTf (0.1–1.0 equiv) was then
added avoiding light. The mixture was stirred under these conditions for 30 to 60
min, then neutralized with triethylamine, diluted with CH2Cl2 (20 mL) and washed
with water (2 ꢂ 10 mL). The organic phase was dried over MgSO4, concentrated and
purified on a silica gel column using EtOAc-petroleum ether as the eluent to give the
desired compound.
20
1
Avermectin B1a derivative 3. [a]D + 15 (c 1, CHCl3); H NMR (CDCl3) d
0.12–0.13 (m, 6 H), 0.80–0.92 (m, 19 H), 1.11 (d, 3 H), 1.23–1.25 (m, 3 H), 1.45–
1.60 (m, 6 H), 1.70–1.79 (m, 4 H), 2.0–2.05 (m, 4 H), 2.13–2.27 (m, 5 H), 2.47–2.51
(m, 1 H), 3.31–3.46 (m, 4 H), 3.76–3.87 (m, 4 H), 4.10–4.14 (m, 3 H), 4.43–4.50 (m,
2 H), 4.59 (dd, 1 H), 4.62–4.67 (m, 2 H), 4.72 (d, 1 H, J 3.7 Hz, H-1’), 4.97 (br t, 1 H),

390
Wei, Gu, and Du
5.25 (d, 1 H, J 8.0 Hz, H-1@), 5.31–5.36 (m, 2 H), 5.50 (dd, 1 H, H-2@), 5.54 (dd, 1 H),
5.62-5.68 (m, 2 H), 5.70–5.75 (m, 2 H), 5.80–5.86 (m, 1 H, H-9), 5.93 (t, 1 H, H-3@),
7.15–7.35 (m, 20 H). MALDITOF-MS Calcd for C81H100O20Si: 1420.66; Found
1443.6 (M + Na).
Anal. Calcd for C81H100O20Si: C, 68.43; H, 7.09. Found: C, 68.18; H, 7.23.
2,3-Di-O-benzoyl-4,6-O-benzylidene- -D-(1
3)-1,2:5,6-di-O-isopyopylidene-a-
ꢁ!ꢁ
20
D-glucopyranose (6). [a]D ꢁ 29 (c 2.1, CHCl3); 1H NMR (CDCl3) d 1.17, 1.24,
1.26, 1.38 (4 s, 12 H, CH3), 3.70 ꢁ 3.74 (m, 1 H, H-5’), 3.87–3.91 (m, 2 H, H-3, H-4),
3.96 (t, 1 H, J 9.6 Hz, H-4’), 4.04 (d, 1 H, J 1.8 Hz, H-6a), 4.35 (br s, 1 H, H-5), 4.40–
4.46 (m, 2 H, 2 H-6’), 4.52 (d, 1 H, J 1.8 Hz, H-6b), 4.55 (d, 1 H, J 3.6 Hz, H-2), 4.83
(d, 1 H, J 7.9 Hz, H-1’), 5.54 (s, 1 H, PhCH), 4.56 (dd, 1 H, J 7.9, 9.6 Hz, H-2’), 5.81
(t, 1 H, J 9.6 Hz, H-3’), 5.95 (d, 1 H, J 3.6 Hz, H-1), 7.31–8.00 (m, 10 H, Ph).
MALDITOF-MS Calcd for C39H42O13: 718; Found 741.15 (M + Na).
2,3,4,6-Tetra-O-benzoyl- -D-glucopyranosyl-(1
3)-[2,3,4,6-tetra-O-benzoyl-
ꢁ!ꢁ
-D-glucopyranosyl-(1
6)]-2,4-di-O-acetyl- -D-glucopyranosyl-(1
3)-1,2:5,6-
ꢁ!ꢁ
ꢁ!ꢁ
20
di-O-isopropylidene-a-D-glucofuranose (8). [a]D ꢁ 53 (c 1, CHCl3); 1H NMR
(CDCl3) d 1.27, 1.43, 1.45, 1.50 (4 s, 12 H), 1.60, 1.73 (2 s, 6 H, Ac), 3.55 (dd, 1 H),
3.65 (t, 1 H), 3.74–3.76 (m, 2 H), 3.82–3.86 (m, 2 H), 3.91–3.96 (m, 2 H), 4.06–4.08
(m, 2 H), 4.11–4.28 (m, 2 H), 4.36–4.39 (m, 2 H), 4.55–4.64 (m, 4 H), 4.97 (d, 1 H, J
9.5 Hz, H-1II), 5.10 (d, 1 H, J 8.0 Hz, H-1III), 5.16 (d, 1 H, J 8.4 Hz, H-1IV), 5.43–5.54
(m, 3 H), 5.64 (d, 1 H, J 3.6 Hz, H-1I), 5.72 (t, 1 H), 5.84 (t, 1 H), 5.92 (t, 1 H), 7.30–
8.0 (m, 40 H). MALDITOF-MS Calcd for C90H86O31: 1662.53; Found 1685.3
(M + Na).
Anal. Calcd for C90H86O31: C, 64.98; H, 5.21. Found: C, 64.72; H, 5.25.
1,6-Di-O-(2,4,6-tri-O-acetyl-3-O-benzyl-b-D-glucopyranosyl)hexane (10).
20
1
[a]D + 22 (c 1, CHCl3); H NMR (CDCl3) d 1.24–1.32 (m, 4 H), 1.50–1.60 (m, 6
H), 1.97, 2.00, 2.08 (3 s, 18 H, 6 Ac), 3.43, 3.84 (2 dt, 4 H, J 6.6, 9.5 Hz, 2 OCH2),
3.57 (ddd, 2 H, 2 H-5), 3.69 (t, 2 H, J 9.4 Hz, 2 H-3), 4.10 (dd, 2 H, J 2.5, 12.0 Hz, 2
H-6a), 4.20 (dd, 2 H, J 6.0, 12.0 Hz, 2 H-6b), 4.39 (d, 2 H, J 8.0 Hz, 2 H-1), 4.57, 4.61
(2 d, 4 H, J 12.0 Hz, 2 PhCH2), 5.03 (dd, 2 H, J 8.0, 9.6 Hz, 2 H-2), 5.12 (t, 2 H, J 9.6
Hz, 2 H-4), 7.21–7.35 (m, 10 H, Ph).
Anal. Calcd for C44H58O18: C, 60.40; H, 6.68. Found: C, 60.23; H, 6.80.
Cholest-5-en-3b-yl 2,3,4,6-tetra-O-benzoyl-b-D-glucopyranoside (12).
20
1
[a]D + 41 (c 1, CHCl3); H NMR (CDCl3) d 0.64 (s, 3 H), 0.80–0.90 (m, 14 H),
0.95–1.0 (m, 3 H), 1.05–1.13 (m, 6 H), 1.24–1.41 (m, 8 H), 1.48–1.57 (m, 3 H),
1.68–1.72 (m, 2 H), 1.73–1.86 (m, 1 H), 1.88–1.92 (m, 2 H), 1.97–2.00 (m, 1 H),
2.12–2.16 (m, 2 H), 3.47–3.54 (m, 1 H), 4.11–4.16 (m, 1 H, H-5), 4.80–4.53 (dd, 1
H, J5,6a 6.0, J6a,6b 12.0 Hz, H-6a), 4.57–4.61 (dd, 1 H, J5,6b 3.4, J6a,6b 12.0 Hz, H-6b),
4.92 (d, 1 H, J1,2 7.8 Hz, H-1), 5.21 (br d, 1 H), 5.46–5.50 (dd, 1 H, J2,3 9.7, J1,2 7.8
Hz, H-2), 5.58–5.64 (dd, 1 H, J3,4 9.3, J4,5 9.7 Hz, H-4), 5.86–5.90 (dd, 1 H, J2,3 9.7,
J4,3 9.3 Hz, H-3), 7.24–8.0 (m, 20 H).
Anal. Calcd for C61H72O10: C, 75.91; H, 7.52. Found: C, 76.26; H, 7.69.

Silver Triflate
Allyl 2,3,5-tri-O-benzoyl- -L-arabinofuranosyl-(1
391
5)- -L-arabinofuranoside
ꢁ!ꢁ
20
(15). [a]D + 105 (c 1, CHCl3); 13C NMR (100 MHz, CDCl3) d 54.9, 63.5, 67.0,
68.0, 77.5, 78.1, 79.5, 81.8, 82.0, 85.6, 106.1, 109.9, 117.0, 128.3, 128.5, 128.6, 128.7,
129.6, 129.8, 129.9, 130.1, 133.1, 133.5, 133.6, 165.2, 165.9, 166.0.
Anal. Calcd for C34H34O12: C, 64.35; H, 5.36. Found: C, 64.11; H, 5.45.
6-Tert-butyldimethylsilyloxyhexyl 2,3,4,6-tetra-O-benzyl-b-D-glucopyranoside
20
1
(18). [a]D + 5 (c 1, CHCl3); H NMR (CDCl3) d ꢁ 0.01 (s, 6 H, CH3), 0.89 (m,
9 H, (CH3)3C), 1.26–1.43 (m, 5 H), 1.47–1.57 (m, 3 H), 1.63–1.69 (m, 2 H), 3.42–
3.47 (m, 2 H, H-2, H-5), 3.50–3.54 (m, 1 H, one proton of O(CH)2), 3.56–3.73 (m, 3
H, H-6a, H-3, H-4), 3.75 (dd, 1 H, J 1.8, 10.8 Hz, H-6b), 3.93–3.99 (m, 1 H, one
proton of O(CH)2), 4.38 (d, 1 H, J1,2 7.8 Hz, H-1), 4.51, 4.54, 4.63, 4.72, 4.78, 4.83,
4.92, 4.95 (8 d, 8 H, 4 PhCH2), 7.15–7.35 (m, 20 H, Ph).
Anal. Calcd for C46H62O7Si: C, 73.17; H, 8.28. Found: C, 73.40; H, 8.31.
Cholest-5-en-3b-yl 2-deoxy-3,4,6-tri-O-acetyl-a-D-glucopyranosyl-(1
2)-
ꢁ!ꢁ
20
3,4,6-tri-O-benzoyl-b-D-glucopyranoside (25). [a]D + 18 (c 1.7, CHCl3); 1H
NMR (CDCl3) d 0.67 (s, 3 H), 0.83–0.95 (m, 14 H), 1.00–1.03 (m, 1 H), 1.08–1.14
(m, 6 H), 1.24–1.40 (m, 8 H), 1.43–1.62 (m, 9 H), 1.75 (s, 3 H), 1.77 (s, 3 H), 1.97–
2.02 (m, 5 H), 2.25–2.30 (m, 2 H), 3.47–3.62 (m, 4 H, 2 H-6’, H-5’, H-3 of cholesterol),
3.90 (dd, 1 H, J 7.8, 9.7 Hz, H-2), 4.01–4.06 (m, 1 H, J 6.2, 3.7, 9.6 Hz, H-5), 4.47–
4.54 (m, 2 H, 2 H-6), 4.74 (d, 1 H, J 7.8 Hz, H-1), 4.83 (t, 1 H, J 9.8 Hz, H-4’), 5.01–
5.05 (m, 1 H, H-3’), 5.35 (br d, 1 H, J 5.2 Hz), 5.50 (dd, 1 H, J 9.7, 9.3 Hz, H-4), 5.56
(d, 1 H, J1,2a 3.2 Hz, H-1’), 5.70–5.75 (dd, 1 H, J 9.3, 9.7 Hz, H-3), 7.24–8.0 (m, 15 H).
Anal. Calcd for C66H84O16: C, 69.94; H, 7.47. Found: C, 70.29; H, 7.51.
Ethyl 3,4-di-O-acetyl-2-O-benzyl-a-L-fucopyranosyl-(1
2)-3,4-O-isopropyle-
ꢁ!ꢁ
20
1
dene-1-thio-b-L-fucopyranoside (28). [a]D ꢁ 63 (c 1, CHCl3); H NMR (CDCl3)
d 1.10 (d, 3 H), 1.23 (t, 3 H), 1.35 (s, 3 H), 1.39 (d, 3 H), 1.52 (s, 3 H), 1.99, 2.11 (2 s,
6 H, Ac), 2.70–2.74 (m, 2 H), 3.69 (dd, 1 H), 3.80–3.87 (m, 2 H), 4.05 (dd, 1 H), 4.18
(t, 1 H), 4.43 (d, 1 H, J 10.3 Hz, H-1), 4.53 (q, 1 H), 4.55, 4.88 (2 d, 2 H), 5.28–5.34
(m, 2 H), 5.52 (d, 1 H, J 3.6 Hz, H-1’), 7.25–7.36 (m, 5 H). MALDITOF-MS Calcd for
C28H40O10S: 568.23; Found 581.1 (M + Na).
Anal. Calcd for C28H40O10S: C, 59.14; H, 7.09. Found: C, 58.91; H, 6.94.
ACKNOWLEDGMENTS
We thank the NNSF of China (Project 29972053), RCEES of CAS and student
innovation fellowship (G.W.) for supporting this research.
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Received April 10, 2003
Accepted June 17, 2003