Catalytic and chemoselective glycosylation between "armed" and "disarmed" glycosyl p-trifluoromethylbenzylthio-p-trifluoromethylphenyl formimidates

Article abstract of DOI:10.1246/cl.2002.746

Catalytic and chemoselective glycosylation between novel "armed" and "disarmed" glycosyl p-trifluoromethylbenzylthio-p-trifluoromethylphenyl formimidates was effectively performed in the presence of a catalytic amount of TfOH and MS 4A in CH₂Cl

Full text of DOI:10.1246/cl.2002.746

746
Chemistry Letters 2002
Catalytic and Chemoselective Glycosylation between ‘‘Armed’’ and ‘‘Disarmed’’
Glycosyl p-Trifluoromethylbenzylthio-p-trifluoromethylphenyl Formimidates
Hiroyuki Chiba, Setsuo Funasaka, Ko¯ichi Kiyota, and Teruaki Mukaiyamaꢀ
The Kitasato Institute, Center for Basic Research, TCI, 6-15-5 Toshima, Kita-ku, Tokyo 114-0003
(Received April 22, 2002; CL-020353)
Catalytic and chemoselective glycosylation between novel
stereoselectivity proceeded when CH3CN or EtCN was used as a
solvent (entries 1,2). On the other hand, 1,2-cis glycoside was
stereoselectively formed in quantitative yield when the same
reaction was carried out in tBuOMe or Et2O (entries 8,9).
Also, it was observed that the glycosylation with this glycosyl
donor 1 proceeded smoothly even at lower temperatures (Table 2).
As a result, 1,2-trans glycoside was formed in an almost perfectly
controlled manner when the glycosylation was carried out in EtCN
at ꢁ78 ꢂC (entry 4). This perfect 1,2-trans stereoselectivity may be
‘‘armed’’ and ‘‘disarmed’’ glycosyl p-trifluoromethylbenzylthio-p-
trifluoromethylphenyl formimidates was effectively performed in
the presence of a catalytic amount of TfOH and MS 4A in CH2Cl2 at
ꢁ78 ꢂC. In addition, it was noted that highly 1,2-cis or 1,2-trans
stereoselective synthesis was established when the above glyco-
sylation was carried out at ꢁ78 ꢂC in tBuOMe or EtCN,
respectively.
Table 1. Effect of solvent
Study on establishing effective and stereoselective glycosyla-
tion is one of the most important topics in carbohydrate chemistry.1
Recent interesting contribution to this field is the verification of the
‘‘armed-disarmed’’ effect proposed by B. Fraser-Reid.2 This effect
is related to the rate of formation of oxocarbonium ion at an
anomeric position of a carbohydrate, which is ruled by a protecting
group at the neighboring position. The oxocarbonium ion is less
favorably formed when an adjacent electron withdrawing group is
located in place of an adjacent alkoxy group. Pioneering reports
about this effect describe ‘‘armed’’ derivatives of n-pentenyl
glycoside are selectively activated by iodonium dicollidine
perchlorate (IDCP) in the coexistence of ‘‘disarmed’’ derivatives.3
Concerning other glycosyl donors, e.g. glycals,4 thioglycosides,5
glycosyl fluorides,6 similar phenomena have also been reported:
however, ‘‘armed-disarmed’’ glycosylation using the two corre-
sponding glycosyl trichloroacetimidates catalyzed by TfOH has not
been reported.
Very recently, a newly devised efficient ‘‘disarmed’’ glycosyl
donor, glycosyl p-trifluoromethylbenzylthio-p-trifluoromethylphe-
nyl formimidate, was reported from our laboratory.7 This glycosyl
donor was easily prepared in a crystalline form in good yield with
high ꢀ-stereoselectivity. Catalytic glycosylation of several glycosyl
acceptors with this glycosyl donor was efficiently performed in the
presence of 5 mol% TfOH and MS 5A in CH2Cl2 at 0 ꢂC and 1,2-
trans stereoselectivity of the glycosides was achieved by the
assistance of neighboring effect from C(2)-position. In this
communication, we would like to report on catalytic, stereoselec-
tive and chemoselective glycosylation between novel ‘‘armed’’ and
‘‘disarmed’’ glycosyl p-trifluoromethylbenzylthio-p-trifluoro-
methylphenyl formimidates.
Table 2. Effect of reaction temperature
2,3,4,6-Tetra–O–benzyl-ꢀ-D-glucopyranosyl
p-trifluoro-
methylbenzylthio-p-trifluoromethylphenyl formimidate (1) was
also prepared easily in a crystalline form in good yields with high
ꢀ-stereoselectivity. In the first place, stereoselective glycosylation
of methyl 2,3,4-tri-O-benzyl-ꢀ-D-glucopyranoside (2) with the
glycosyl donor 1 was tried (Table 1). The glycosylations were
carried out in various solvents by using 5 mol% of TfOH for 1 h at
0 ꢂC. As a result, the reactions proceeded smoothly in almost all
solvents except DMF. It should be noted that the glycosylation
which afforded the glycoside in quantitative yield with 1,2-trans
Copyright Ó 2002 The Chemical Society of Japan

Chemistry Letters 2002
747
Table 4. Catalytic and stereoselective ‘‘armed-disarmed’’ glycosyla-
tion
controlled by the nature of solvent under kinetic condition.
Next, catalytic and chemoselective glycosylation between the
‘‘armed’’ glycosyl donor 1 and the ‘‘disarmed’’ glycosyl acceptor 48
was examined in the presence of various additives (Table 3). As a
result, the ‘‘armed-disarmed’’ chemoselective glycosylation pro-
ceeded smoothly and afforded the desired disaccharide 5 in high
yield in the coexistence of 5–10 mol% of TfOH and MS 4A in
CH2Cl2 at ꢁ78 ꢂC without giving any damage to a reducing end of
an acceptor (entries 7,8). The reason why the yields of the desired
disaccharide 5 shown in entries 2and 4 were low is ascribed to the
hydrolysis that took place in usual work-upprocedure of the initially
formed 5 whose reactive leaving group still remained in.
Table 3. Catalytic ‘‘armed-disarmed’’ glycosylation
phenyl formimidates was effectively performed in the presence of
t
a catalytic amount of TfOH and MS 4A at ꢁ78 ꢂC in BuOMe or
EtCN, respectively. It is expected that these glycosylations using
‘‘armed’’ and ‘‘disarmed’’ glycosyl p-trifluoromethylbenzylthio-p-
trifluoromethylphenyl formimidates in appropriate solvents would
be applicable to the efficient one-pot glycosylation.
The present research is partially supported by Grant-in-Aids for
Scientific Research from Ministry of Education, Science, Sports and
Culture.
References and Notes
1
Reviews: K. Toshima and K. Tatsuta, Chem. Rev., 93, 1503 (1993);
K. Suzuki and T. Nagasawa, J. Synth. Org. Chem. Jpn., 50, 378
(1992).
Finally, the above glycosylation was tried in EtCN at ꢁ78 ꢂC
and the glycoside was formed in good yield with high 1,2-trans
stereoselectivity, as expected (Table 4, entry 3). It is surprising to
note that the glycoside was also formed in good yield with
2B. Fraser-Reid, Acc. Chem. Res., 29, 57 (1996).
3
4
5
6
D. R. Mootoo, P. Konradsson, U. Udodong, and B. Fraser-Reid, J.
Am. Chem. Soc., 110, 5583 (1988); B. Fraser-Reid, Z. Wu, U. E.
Udodong, and H. Ottosson, J. Org. Chem., 55, 6068 (1990).
R. W. Friesen and S. J. Danishefsky, J. Am. Chem. Soc., 111, 6656
(1989); K. Suzuki, G. A. Sulikowski, R. W. Friesen, and S. J.
Danishefsky, J. Am. Chem. Soc., 112, 8895 (1990).
t
extremely high 1,2-cis stereoselectivity at ꢁ78 ꢂC when BuOMe
was used as solvent (entry 6). It is explained by considering the
bulkiness of acceptor 4 which is making it more difficult to approach
oxocarbonium ion of the donor from ꢁ-side than acceptor 2.
The typical experimental procedure is as follows: to a stirred
suspension of MS 4A (88 mg), 1 (29.1 mg, 0.032 mmol) and 4
G. H. Veeneman and J. H. van Boom, Tetrahedron Lett., 31, 2 75
(1990); G. H. Veeneman, S. H. van Leeuwen, and J. H. van Boom,
Tetrahedron Lett., 31, 1331 (1990).
t
(25.0 mg, 0.029 mmol) in BuOMe (2.0 mL) was added a toluene
a
´
M . I. Barrena, R. Echarri, and S. Castillon, Synlett, 1996, 675; L.
solution (ca. 0.1 mL) of TfOH (0.48 mg, 3.2 ꢂmmol) at ꢁ78 ꢂC.
The reaction mixture was stirred for 1 h at the same temperature and
was quenched by adding saturated aqueous NaHCO3. The mixture
was filtered through the pad of celite, and aqueous layer was
extracted with CH2Cl2. The combined organic layer was washed
with brine, and dried over Na2SO4. After filtration and evaporation,
the resulted residue was purified by preparative TLC (hexane/
EtOAc 4 : 1) to give the desired product 5 (37.4 mg, 93%,
ꢀ=ꢁ ¼ 95 : 5).
Green, B. Hinzen, S. J. Ince, P. Langer, S. V. Ley, and S. L.
Warriner, Synlett, 1998, 440; D. K. Baeschlin, L. G. Green, M. G.
Hahn, B. Hinzen, S. J. Ince, and S. V. Ley, Tetrahedron:
Asymmetry, 11, 173 (2000); T. Mukaiyama, K. Takeuchi, H. Jona,
H. Maeshima, and T. Saitoh, Helv. Chim. Acta, 83, 1901 (2000); H.
Jona, H. Mandai, W. Chavasiri, K. Takeuchi, and T. Mukaiyama,
Bull. Chem. Soc. Jpn., 75, 291 (2002).
T. Mukaiyama, H. Chiba, and S. Funasaka, Chem. Lett., 2002, 392.
‘‘Disarmed’’ glycosyl acceptor 4 was synthesized by 4 steps
procedure from ethyl 2,3,4-tri-O-benzoyl-1-thio-ꢁ-D-glucopyrano-
side as follows; 1) TBDPSCl, imidazole/DMF, 2) 70% TfOH aq., n-
Bu4NIO4/MeCN, 0 ꢂC, 3) KHMDS, p-CF3PhNCS/THF, ꢁ78 ꢂC,
then p-CF3BnBr, 0 ꢂC, 4) TBAF, AcOH/THF.
7
8
Thus, catalytic, highly 1,2-cis or 1,2-trans stereoselective and
chemoselective glycosylation between novel ‘‘armed’’ and ‘‘dis-
armed’’ glycosyl p-trifluoromethylbenzylthio-p-trifluoromethyl-