Stereoselective direct glycosylation with anomeric hydroxy sugars by activation with phthalic anhydride and trifluoromethanesulfonic anhydride involving glycosyl phthalate intermediates

Article abstract of DOI:10.1021/ja710935z

An efficient direct one-pot glycosylation method with anomeric hydroxy sugars as glycosyl donors employing phthalic anhydride and triflic anhydride as activating agents has been developed. Thus, highly stereoselective β-mannopyranosylations were achieved

Full text of DOI:10.1021/ja710935z

Published on Web 06/04/2008
Stereoselective Direct Glycosylation with Anomeric Hydroxy
Sugars by Activation with Phthalic Anhydride and
Trifluoromethanesulfonic Anhydride Involving Glycosyl
Phthalate Intermediates
Kwan Soo Kim,*,† Dinanath Baburao Fulse,† Ju Yuel Baek,† Bo-Young Lee,† and
Heung Bae Jeon‡
Center for BioactiVe Molecular Hybrids and the Department of Chemistry, Yonsei UniVersity,
Seoul 120-749, Korea, and the Department of Chemistry, Kwangwoon UniVersity, Seoul
139-701, Korea
Received December 8, 2007; E-mail: kwan@yonsei.ac.kr
Abstract: An efficient direct one-pot glycosylation method with anomeric hydroxy sugars as glycosyl donors
employing phthalic anhydride and triflic anhydride as activating agents has been developed. Thus, highly
stereoselective ꢀ-mannopyranosylations were achieved by the reaction of 2,3-di-O-benzyl-4,6-O-ben-
zylidene-D-mannopyranose (2) with phthalic anhydride in the presence of DBU at room temperature followed
by sequential addition of DTBMP and Tf2O and glycosyl acceptors to the reaction mixture at -78 °C in
one-pot. Stereoselective R-glucopyranosylations with 2,3-di-O-benzyl-4,6-O-benzylidene-D-glucopyranose
(25) and other glycosylations with glucopyranoses and mannopyranoses having tetra-O-benzyl- and tetra-
O-benzoyl protecting groups were also possible by utilizing the present one-pot glycosylation protocol.
The possible mechanism for the ꢀ-mannosylation with 2 was proposed based on the NMR study, in which
R-mannosyl phthalate 55R and R-mannosyl triflate 59 were detected as intermediates. The versatility and
efficiency of the present glycosylation methodology, especially those of the ꢀ-mannopyranosylation protocol,
were readily demonstrated by the efficient synthesis of protected ꢀ-(1f4)-D-mannotriose 62 and ꢀ-(1f4)-
D-mannotetraose 67 with perfect ꢀ-stereoselectivity.
phosphates,9 and glycosyl phosphites10 have been successfully
used for the synthesis of various important oligosaccharides and
Introduction
The development of efficient and stereoselective glycosylation
methodologies1 has been a major concern in synthetic organic
chemistry over the past decade due to important roles of
complex oligosaccharides in many fundamental life-sustaining
processes.2 The selection of an appropriate glycosyl donor is
one of the key processes for the successful glycosylation in terms
of efficiency and stereoselectivity and thus, the bulk of the
efforts in this area have focused on devising new efficient
glycosyl donors. Several glycosyl donors such as glycosyl
trichloroacetimidates,3 thioglycosides,4 glycosyl sulfoxides,5
glycals,6 n-pentenyl glycosides,7 glycosyl fluorides,8 glycosyl
glycoconjugates. We have also previously reported 2′-carboxy-
benzyl (CB) glycosides as a new type of glycosyl donors for
efficient stereoselective glycosylations,11 and their application
to the synthesis of complex oligosaccharides and galactosph-
ingolipids.12 Nevertheless, the stereoselective construction of
certain glycosyl linkages such as ꢀ-mannopyranosyl,13 R-glu-
copyranosyl,14 ꢀ-arabinofuranosyl,15 and R-sialyl linkages16 still
poses a great challenge. In addition, application of the glyco-
sylation methods with known glycosyl donors to the automated
(7) Fraser-Reid, B.; Madsen, R. In PreparatiVe Carbohydrate Chemistry;
Hanessian, S., Ed.; Marcel Dekker, Inc.: New York, 1997; pp 339-
356.
† Yonsei University.
‡ Kwangwoon University.
(8) Shimizu, M.; Togo, H.; Yokoyama, M. Synthesis 1998, 799–822.
(9) Plante, O. J.; Andrade, R. B.; Seeberger, P. H. Org. Lett. 1999, 1,
211–214.
(1) For reviews on the glycosylation, see (a) Toshima, K.; Tatsuta, K.
Chem. ReV. 1993, 93, 1503–1531. (b) Ernst. B., Hart, G. W., Sinay¨,
P., Eds. Carbohydrates in Chemistry and Biology; Wiley-VCH:
Weinheim, Germany, 2000; Vol. 1, pp 5-237.
(10) Zhang, Z. Y.; Wong, C.-H. In Carbohydrates in Chemistry and
Biology; Ernst. B., Hart, G. W., Sinay¨, P., Eds.; Wiley-VCH:
Weinheim, Germany, 2000; Vol. 1, pp 117-134.
(2) (a) Varki, A. Glycobiology 1993, 3, 97–130. (b) Dwek, R. A. Chem.
ReV. 1996, 96, 683–720. (c) Bertozzi, C. R.; Kiessling, L. L. Science
2001, 291, 2357–2364.
(11) (a) Kim, K. S.; Kim, J. H.; Lee, Y. J.; Lee, Y. J.; Park, J. J. Am.
Chem. Soc. 2001, 123, 8477–8481. (b) Kim, K. S.; Park, J.; Lee, Y. J.;
Seo, Y. S. Angew. Chem., Int. Ed. 2003, 42, 459–462. (c) Lee, Y. J.;
Lee, K.; Jung, E. H.; Jeon, H. B.; Kim, K. S. Org. Lett. 2005, 7, 3263–
3266.
(3) (a) Schmidt, R. R.; Kinzy, W. AdV. Carbohydr. Chem. Biochem. 1994,
50, 21–123. (b) Schmidt, R. R. In Modern Methods in Carbohydrate
Synthesis; Khan, S. H., O’Neil, R. A., Eds.; Harwood Academic:
Amsterdam, The Netherlands, 1996; pp 20-54.
(12) (a) Kwon, Y. T.; Lee, Y. J.; Lee, K.; Kim, K. S. Org. Lett. 2004, 6,
3901–3904. (b) Lee, Y. J.; Lee, B.-Y.; Jeon, H. B.; Kim, K. S. Org.
Lett. 2006, 8, 3971–3974. (c) Lee, B. R.; Jeon, J. M.; Jung, J. H.;
Jeon, H. B.; Kim, K. S. Can. J. Chem. 2006, 84, 506–515. (d) Lee,
B.-Y.; Baek, J. Y.; Jeon, H. B.; Kim, K. S. Bull. Korean Chem. Soc.
2007, 28, 257–262.
(4) Garegg, P. J. AdV. Carbohydr. Chem. Biochem. 1997, 52, 179–205.
(5) Kahne, D.; Walker, S.; Cheng, Y.; Engen, D. V. J. Am. Chem. Soc.
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2008 American Chemical Society
J. AM. CHEM. SOC. 2008, 130, 8537–8547 8537

A R T I C L E S
Kim et al.
solid-phase synthesis17 or in the one-pot solution-phase syn-
thesis18 of oligosaccharides remains a difficult task. In this
regard, there is still a need for the development of efficient and
stereoselective glycosylation methods by devising new glycosyl
donors or modifying known ones. The majority of known
glycosylation methodologies consist of the preparation of a
glycosyl donor by conversion of an anomeric substituent into a
latent leaving group in the first step and activation of the isolated
glycosyl donor by a promoter followed by formation of a
glycosyl bond by the reaction between the activated donor and
a nucleophilic glycosyl acceptor in the second step. On the other
hand, a direct glycosylation with anomeric hydroxy glycosyl
donors, in which all the operations of anomeric derivatization,
activation, and glycosyl bond formation are combined into a
one-pot procedure, would offer some advantages in oligosac-
charide synthesis over the stepwise glycosylation methods.
Although there have been several reports on the direct glyco-
sylation with C1-hydroxy glycosyl donors,19 they have not
attracted much attention. Recently, Gin and co-workers have
developed a new method for the glycosylation with anomeric
hydroxy sugars involving oxosulfonium intermediates20 and
reported its application to the synthesis of complex oligosac-
charides.21
Scheme 1
C followed by cyclization of the resulting triflate mixed
anhydride C would afford oxocarbenium ion D by extrusion of
stable nonnucleophilic 1. Subsequent reaction of D with a
nucleophilic glycosyl acceptor (SugarOH) would provide gly-
coside E. Herein we report a new stereoselective direct
glycosylation with anomeric hydroxy sugars, in which to the
anomeric hydroxy donor are sequentially added phthalic anhy-
dride, triflic anhydride, and an acceptor alcohol in the presence
of an appropriate base in one-pot without isolation of intermedi-
ates as shown in Scheme 1. We also report the NMR study for
detection of glycosylation intermediates and the application of
the present method to the synthesis of ꢀ-(1f4)-mannotetraose
of ꢀ-(1f4)-mannan.
As continuation of our search for new glycosyl donors and
stereoselective glycosylation methods, we envisaged that treat-
ment of anomeric hydroxy sugar A with phthalic anhydride (1)
in the presence of a base would generate glycosyl phthalate
anion B (Scheme 1). Then, the addition of Tf2O to the phthalate
Results and Discussion
Stereoselective ꢀ-Mannopyranosylation. The stereoselective
construction of 1,2-cis-ꢀ-mannopyranosyl linkages still remains
one of major challenges in oligosaccharide synthesis.13 In our
initial study, 2,3-di-O-benzyl-4,6-O-benzylidene-D-mannopy-
ranose (2) was chosen as a glycosyl donor for the mannopy-
ranosylation. The reason for choosing compound 2 as an
mannosyl donor was based on the important discovery by Crich
and co-workers22 that the 4,6-O-benzylidene protective group
facilitates the high stereoselectivity in the ꢀ-mannopyranosy-
lation. In fact, Seeberger and co-workers,23 utilized compound
2 for the ꢀ-mannopyranosylation employing Gin’s dehydrative
glycosylation method.20 The present mannosylation with com-
pound 2 requires a sequence of three steps in one-pot: (i) reaction
of 2 and 1 in the presence of a base, then (ii) addition of Tf2O,
and finally, (iii) addition of a glycosyl acceptor. Selection of
an appropriate base or a combination of bases in the first step
was found to be crucial for the success of the present one-pot
glycosylation. When triethylamine (3.3 equiv) was used in
refluxing CH2Cl2, the reaction between 2 (R/ꢀ ) 2.1:1) and 1
in the first step completed in 5 h to afford an intermediate based
on TLC but further addition of Tf2O and acceptor 3 sequentially
to the reaction mixture at -78 °C failed to provide the desired
disaccharide 5 (entry 1 in Table 1). We speculated that
triethylamine interfered with the second step of the glycosylation
by deactivation of Tf2O. With a weaker and hindered base, di-
tert-butylmethylpyridine (DTBMP) in refluxing CH2Cl2, the
glycosylation failed again and compound 2 appeared to be
decomposed in the first step based on TLC (entry 2). With a
combination of Et3N (1.1 equiv) and DTBMP (2.2 equiv) in
refluxing CH2Cl2, the first step proceeded smoothly to give the
(13) For reviews on theꢀ-mannopyranosylation, see: (a) Barresi, F.;
Hindsgaul, O. In Modern Methods in Carbohydrate Synthesis; Khan,
S. H., O’Neil, R. A., Eds.; Harwood Academic Publishers: Amsterdam,
The Netherlands, 1996; pp 251-276. (b) Gridley, J. J.; Osborn,
H. M. I. J. Chem. Soc., Perkin Trans. 1 2000, 1471–1491. (c)
Demchenko, A. V. Curr. Org. Chem. 2003, 7, 35–79.
(14) For recent references on the R-glucopyranosylation, see: (a) Kim, J.-
H.; Yang, H.; Boons, G.-J. Angew. Chem., Int. Ed. 2005, 44, 947–
949. (b) Kim, J.-H.; Yang, H.; Park, J.; Boons, G.-J. J. Am. Chem.
Soc. 2005, 127, 12090–12097. (c) Crich, D.; Vinogradova, O. J. Org.
Chem. 2006, 71, 8473–8480. (d) Crich, D.; Li, L. J. Org. Chem. 2007,
72, 1681–1690.
(15) For recent references on theꢀ-D-arabinofuranosylation, see: (a) D’Souza,
F. W.; Lowary, T. L. Org. Lett 2000, 2, 1493-1495. (b) Yin, H.;
Lowary, T. L. Tetrahedron Lett. 2001, 42, 5829–5832. (c) Yin, H.;
D’Souza, F. W.; Lowary, T. L. J. Org. Chem. 2002, 67, 892–903. (d)
Zhu, X.; Kawatkar, S.; Rao, Y.; Boons, G.-J. J. Am. Chem. Soc. 2006,
128, 11948–11957.
(16) (a) Boons, G.-J.; Demchenko, A. V. Chem. ReV. 2000, 100, 4539–
4565. (b) Halcomb, R. L.; Chappell, M. D. J. Carbohydr. Chem. 2002,
7-9, 723–768.
(17) (a) Plante, O. J.; Palmacci, E. R.; Seeberger, P. H. AdV. Carbohydr.
Chem. Biochem. 2003, 58, 35–54. (b) Plante, O. J.; Palmacci, E. R.;
Seeberger, P. H. Science 2001, 291, 1523–1527. (c) Danishefsky, S. J.;
McClure, K. F.; Randolph, J. T.; Ruggeri, R. B. Science 1993, 260,
1307–1309.
(18) (a) Sears, P.; Wong, C.-H. Science 2001, 291, 2344–2350. (b) Koeller,
K. M.; Wong, C.-H. Chem. ReV. 2000, 100, 4465–4493. (c) Zhang,
Z.; Ollmann, I. R.; Ye, X.-S.; Wischnat, R.; Baasov, T.; Wong, C.-H.
J. Am. Chem. Soc. 1999, 121, 534–753. (d) Wang, C.-C.; Lee, J.-C.;
Luo, S.-Y.; Kulkarni, S. S.; Huang, Y.-W.; Lee, C.-C.; Chang, K.-L.;
Hung, S.-C. Nature 2007, 446, 896–899.
(19) For a review for glycosylations with 1-hydroxy donors, see: Gin, D. Y.
J. Carbohydr. Chem. 2002, 21, 645-665.
(20) (a) Garcia, B. A.; Poole, J. L.; Gin, D. Y. J. Am. Chem. Soc. 1997,
119, 7597–7598. (b) Garcia, B. A.; Gin, D. Y. J. Am. Chem. Soc.
2000, 122, 4269–4279. (c) Nguyen, H. M.; Chen, Y.; Duron, S, G.;
Gin, D. Y. J. Am. Chem. Soc. 2001, 123, 8766–8772. (d) Boebel, T. A.;
Gin, D. Y. Angew. Chem., Int. Ed. 2003, 42, 5874–5877.
(21) (a) Wang, P.; Kim, Y.-J.; Navarro-villalobos, M.; Rohde, B. D.; Gin,
D. Y. J. Am. Chem. Soc. 2005, 127, 3256–3257. (b) Bodine, K. D.;
Gin, D. Y.; Din, M. S. Org. Lett. 2005, 7, 4479–4482. (c) Kim, Y.-J.;
Wang, P.; Navarro-villalobos, M.; Rohde, B. D.; Derryberry, J.; Gin,
D. Y. J. Am. Chem. Soc. 2006, 128, 11906–11915.
(22) (a) Crich, D.; Sun, S. J. Org. Chem. 1996, 61, 4506–4507. (b) Crich,
D.; Sun, S. J. Org. Chem. 1997, 62, 1198–1199. (c) Crich, D.; Sun,
S. J. Am. Chem. Soc. 1998, 120, 435–436. (d) Crich, D.; Sun, S.
Tetrahedron 1998, 54, 8321–8348.
(23) Code´e, J. D. C.; Hosaine, L. H.; Seeberger, P. H. Org. Lett. 2005, 7,
3251–3254.
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Stereoselective Direct Glycosylation
A R T I C L E S
Table 1. Screening of Bases for One-Pot ꢀ-Mannopyranosylation with 2 in CH2Cl2
the 1st step
entry
base
reaction temp
reaction time
acceptor ROH
product
yield, %a (ratio, ꢀ/R)a
1
2
3
4
5
6
Et3N (3.3 equiv)
DTBMP (3.3 equiv)
Et3N (1.1 equiv) + DTBMP (2.2 equiv)
Et3N (1.1 equiv) + DTBMP (2.2 equiv)
DBU (1.1 equiv) + DTBMP (2.2 equiv)
DBU (1.1 equiv) + DTBMP (2.2 equiv)
reflux
reflux
reflux
reflux
room temp
room temp
5 h
5 h
2.5 h
2.5 h
15 min
15 min
3
3
3
4
3
4
no reaction
no reaction
5
6
5
6
79 (ꢀ only)
73 (ꢀ only)
89 (ꢀ only)
88 (ꢀ only)
a Determined after isolation.
intermediate in 2.5 h based on TLC, and then sequential addition
of Tf2O and acceptor 3 to the reaction mixture at -78 °C
provided ꢀ-mannoside 5 exclusively in 79% yield (entry 3).
Similarly, the reaction of 2 with acceptor 4 employing Et3N
(1.1 equiv) and DTBMP (2.2 equiv) afforded ꢀ-mannoside 6
exclusively 73% yield (entry 4). The best result was obtained
with a combination of 1,8-diazobicyclo[5,4,0]undec-7-ene (DBU)
and DTBMP as bases. Thus, reaction of 2 and 1 in the first
step proceeded rapidly with DBU (1.1 equiv) at room temper-
ature in 15 min and then sequential addition of DTBMP (2.2
equiv), Tf2O, and acceptor 3 to the reaction mixture at -78 °C
afforded ꢀ-mannoside 5 exclusively in 89% yield (entry 5).
Similarly, reaction of the mannose 2 with the acceptor 4
employing DBU/DTBMP afforded ꢀ-mannoside 6 exclusively
in 88% yield (entry 6).
Therefore, the one-pot ꢀ-mannopyranosylation of various
glycosyl acceptors with the mannopyranose 2 (R/ꢀ ) 2.1:1) were
carried out by the following sequence as a standard reaction
condition: (i) stirring a solution of 2 (1.0 equiv), phthalic
anhydride (1, 1.1 equiv), and DBU (1.2 equiv) in the presence
of 4 Å molecular sieves for 15 min at room temperature in
CH2Cl2, (ii) sequential addition of DTBMP (2.2 equiv) and Tf2O
(1.5 equiv) to this solution at -78 °C and stirring the resulting
solution for 15 min, and then (iii) slow addition of the glycosyl
acceptor (1.2 equiv) to the above solution at -78 °C and stirring
briefly the reaction mixture at -78 °C and allowing to warm
over 1 h to 0 °C. Mannosylations of primary alcohol acceptors
3 and 4 having benzoyl-protective groups with 2 afforded
exclusively ꢀ-disaccharides 5 and 6 in 89% and 88% yield,
respectively, after chromatographic separation (entries 1 and 2
in Table 2), while the same mannosylations of primary alcohol
acceptors 7 and 8 having benzyl-protective groups gave
predominantly corresponding ꢀ-mannosyl disaccharides 16 (ꢀ/R
) 29:1) and 17 (ꢀ/R ) 12:1) (entries 3 and 4 in Table 2).
Completely ꢀ-selective mannosylations of secondary alcohol
acceptors 9-11 with 2 were also achieved in one-pot to provide
corresponding ꢀ-mannosyl disaccharides 18-20, respectively,
in high yields (entries 5-7). The mannosylation of diacetone
galactose 12, azido sugar 13, and 1-octanol (15) with 2 afforded
predominantly ꢀ-disaccharides 21 (ꢀ/R ) 11:1), 22 (ꢀ/R ) 16:
1), and 24 (ꢀ/R ) 10:1), respectively in high yields (entries 8,
9, and 11). On the other hand, the mannosylation of N-
phthalimido sugar 14 with 2 gave a mixture of R- and
ꢀ-disaccharides 23 (ꢀ/R ) 5.6:1) in 91% yield (entry 10). The
stereochemistries of the newly generated anomeric centers of
unknown mannosides 19 and 23 were determined unequivocally
on the basis of their 1H and 13C NMR spectral data, in particular
one-bond C1-H1 coupling constants: JC1′-H1′ ) 161 Hz in the
ꢀ-disaccharide 19 and JC1′-H1′ ) 170 Hz in the R anomer and
JC1′-H1′ ) 164 Hz in the ꢀ anomer of compound 23.24 The results
indicate that the present one-pot direct mannosylation employing
4,6-O-benzylidenemannose 2 with phthalic anhydride and Tf2O
is highly efficient and ꢀ-stereoselective and thus it appears to
be superior to the previously reported method employing the
same mannose 2 with diphenyl sulfoxide and Tf2O.23
r-Glucopyranosylation and Other Glycosylations. We then
applied the present one-pot mannosylation protocol to the
glucosylation with 2,3-di-O-benzyl-4,6-O-benzylidene-D-glu-
copyranose (25). It has been previously reported that the 4,6-
O-benzylidene protective group promoted the stereoselective
R-glucopyranosylation by Crich and co-workers25 and by us.26
Reaction of 25 with the glycosyl acceptor 3 by the same
procedure used for the ꢀ-mannosylation gave not only desired
R-disaccharide 26 in 38% yield but also unexpectedly self-
condensed ester 27 (Figure 1) in 52% yield. The undesired 27
probably resulted from the coupling between the oxocarbenium
ion D and the carboxylate anion B in Scheme 1. Unlike in the
mannopyranosylation discussed above, the conversion of the
carboxylate B into the triflate C in the glucopyranosylation might
be slower than the conversion of the triflate C into the
oxocarbenium ion D (Scheme 1) so that a substantial amount
of B might remain even after generation of the oxocarbenium
ion D. We envisaged that if the carboxylate anion B is
protonated and if the resulting protonated carboxylic acid is still
readily triflated by Tf2O in the presence of a weaker base than
DBU, the oxocarbenium ion D would preferentially react with
the glycosyl acceptor alcohol over the less nucleophilic car-
boxylic acid. We therefore ran the glucosylation reaction under
the modified condition, in which TfOH was added as the proton
(24) Bock, K.; Pedersen, C. J. Chem. Soc., Perkin Trans. 2 1974, 293–
297.
(25) Crich, D.; Cai, W. J. Org. Chem. 1999, 64, 4926–4930.
(26) Kim, K. S.; Kang, S. S.; Seo, Y. S.; Kim, H. J.; Lee, Y. J.; Jeong,
K.-S. Synlett 2003, 1311–1314.
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A R T I C L E S
Kim et al.
Table 2. Mannosylations of Various Acceptors with
Benzylidene-Protected Donor 2
addition of DTBMP (3.3 equiv) and TfOH (1.1 equiv) to this
solution at -78 °C and stirring the resulting solution for 15
min, (iii) addition of Tf2O (1.5 equiv) to the above solution at
-78 °C and stirring the resulting solution for 15 min, and (iv)
slow addition of a glycosyl acceptor (1.2 equiv) at -78 °C and
stirring the reaction mixture at -78 °C for 15 min and allowing
to warm over 1 h to 0 °C. Indeed, the reaction of 25 with the
primary alcohol acceptor 3 having benzoyl protective groups
under the modified condition afforded exclusively R-disaccha-
ride 26 in 74% yield along with the self-condensed ester 27
(R,ꢀ/ꢀ,ꢀ ) 4:1) in much reduced yield, 20% (entry 1 in Table
3). With this modified protocol, we examined the glucosylation
of various other glycosyl acceptors with 25. Glucosylation of
another primary alcohol acceptor 4 having benzoyl protective
groups with 25 afforded predominantly R-disaccharide 28 (R/ꢀ
) 18:1) in 87% yield along with a small amount (5%) of the
self-condensed ester 27 (entry 2). However, glucosylations of
primary alcohol acceptors 7 and 8 having benzyl protective
groups with 25 provided mixtures of R- and ꢀ-disaccharides
29 (R/ꢀ ) 1.6:1) and 30 (R/ꢀ ) 1.5:1), respectively, in high
yields without generation of the self-condensed ester (entries 3
and 4). On the other hand, glucosylations of secondary alcohol
acceptors 9 and 11 with 25 provided exclusively R-disaccharides
31 and 32 in 80% and 85% yields, respectively (entries 5 and 6).
To explore the scope of the present one-pot glycosylation,
we further examined glycosylation reactions with 1-hydroxy
sugars having other protective groups rather than the benzylidene
group. Glycosylations of the acceptor 3 with benzyl-protected
glucose 33 and benzyl-protected mannose 34 under the same
reaction condition as that employed for the ꢀ-mannosylation
were, however, not satisfactory so that the original condition
was modified by changing the solvent, the reaction temperature,
and the order of addition of reagents. The solvent was changed
to CH2Cl2/CH3CN (10:1) from CH2Cl2, the reaction temperature
was raised to 0 °C from -78 °C in the second and the third
steps, and the acceptor was added in the second step before the
slow addition of Tf2O in the final third step. Thus, the
glycosylations with the tetrabenzylglucose 33 or tetrabenzyl-
mannose 34 were carried out by the following sequence: (i)
stirring the solution of 33 or 34 (1.0 equiv), phthalic anhydride
(1, 1.1 equiv), and DBU (1.2 equiv) in the presence of 4 Å
molecular sieves for 15 min at room temperature in CH2Cl2/
CH3CN (10:1), (ii) sequential addition of DTBMP (2.2 equiv)
and the glycosyl acceptor (1.2 equiv) to this solution at 0 °C
and stirring the resulting solution for 15 min, and (iii) slow
addition of Tf2O (1.5 equiv) to the above solution at 0 °C,
stirring the reaction mixture for 15 min at 0 °C, and allowing
it to warm over 30 min to room temperature. Under this new
standard reaction condition, the reaction of the glucosyl donor
33 and the acceptor 3 afforded disaccharide 35 (R/ꢀ ) 1.6:1)
in 73% yield and self-condensed ester 36 (R,ꢀ/R,R ) 2:1) in
10% yield (entry 1 in Table 4).
Glucosylations of other primary alcohol acceptors 7 and 12
with 33 also gave mixtures of R- and ꢀ-disaccharides 37 (R/ꢀ
) 1.2:1) and 40 (R/ꢀ ) 1:1.3), respectively (entries 2 and 5)
along with the self-condensed ester 36, which was generated
about in 10% yield in all glucosylation reactions with 33.
Glucosylations of secondary alcohol acceptors 10 and 11 with
33 also afforded mixtures of R- and ꢀ-disaccharides 38 (R/ꢀ )
1:1.5) and 39 (R/ꢀ ) 1:7.5), respectively (entries 3 and 4). On
the other hand, reactions of tetrabenzylmannose 34 as the
mannosyl donor with various acceptors provided only desired
mannosides without formation of the undesired self-condensed
a Isolated yields. bDetermined after isolation. Number in parentheses
is the ratio from the crude product determined by LC-Mass. cAfter
isolation of most of the ꢀ-anomer, the ratio of the remaining R/ꢀ
mixture was determined by 1H NMR.
source together with DTBMP just before addition of Tf2O. Thus,
the glucosylation with compound 25 was carried out by a
sequence of four steps in one-pot in CH2Cl2: (i) stirring the
solution of 25 (1.0 equiv), phthalic anhydride (1, 1.1 equiv),
and DBU (1.2 equiv) in the presence of 4 Å molecular sieves
for 15 min at room temperature in CH2Cl2, (ii) sequential
9
8540 J. AM. CHEM. SOC. VOL. 130, NO. 26, 2008

Stereoselective Direct Glycosylation
A R T I C L E S
Figure 1. Self-condensed esters 27 and 36.
Table 3. Glucosylations of Various Acceptors with
Benzylidene-Protected Donor 25
Table 5. Glycosylations of Various Acceptors with
Benzoyl-Protected Donors 45 and 46
entry
acceptor ROH
product
yield, %a
ratio R/ꢀb
entry
glycosyl donor
glycosyl acceptor (ROH)
product
yield, %a (ratio, R/ꢀ)a
1
2
3
4
5
6
3
4
7
8
9
11
26 + 27
28 + 27
29
30
31
74 + 20
87 + 5
87
85
80
R only (R only)
(18:1)
1
2
3
4
5
6
7
8
45
45
45
45
46
46
46
46
3
4
10
11
3
9
10
12
47
48
49
50
51
52
53
54
83 (ꢀ only)
82 (ꢀ only)
80 (ꢀ only)
81 (ꢀ only)
82 (R only)
83 (R only)
81 (R only)
82 (R only)
1.6:1c
1.5:1c
R only (R only)
R only (R only)
32
85
a Isolated yields. b Determined after isolation. Number in parentheses
is the ratio from the crude product determined by LC–MS. c The ratio
was determined by 1H NMR.
a Determined after isolation.
Table 4. Glycosylations of Various Acceptors with
Benzyl-Protected Donors 33 and 34
Glucosylations of all primary and secondary alcohol acceptors
3, 4, 10, and 11 with the benzoyl-protected glucose 45 afforded
exclusively corresponding ꢀ-disaccharides 47-50, respectively,
in high yields (entries 1-4 in Table 5), and mannosylations of
all primary and secondary alcohol acceptors 3, 9, 10, and 12
with the benzoyl-protected mannose 46 gave exclusively cor-
responding R-disaccharides 51-54, respectively, in high yields
(entries 5-8). This result indicates that less reactive disarmed
benzoyl-protected glycosyl donors are also very effective and
the neighboring group participation by the benzoate at the C-2
position is operative in the present one-pot glycosylation method.
Investigation of Intermediates and the Mechanism of the
One-Pot ꢀ-Mannosylation. To identify intermediates generated
during the present glycosylation process, we have attempted to
isolate them and performed a NMR study to detect them in the
ꢀ-mannosylation with 4,6-O-benzylidenemannose 2. On the
basis of our working hypothesis, the intermediates in the reaction
of 2 with 1 in the first step of the present mannosylation would
be mannosyl phthalate anions 55R and 55ꢀ as shown in Scheme
2. Reaction of 2 (R/ꢀ ) 2.1:1) with 1 in the presence of DBU
in CH2Cl2 at room temperature provided exclusively the
R-mannosyl phthalate 55R within a few minutes. Although we
were able to isolate mannosyl hydrogen phthalate 56, the
protonated form of 55R, and obtain its NMR spectra, it slowly
decomposed back to the starting materials, 2 and 1, during
isolation and sampling for NMR.28 Reaction of 2 and 1 in the
presence of DBU was so fast that tracking the progress of the
reaction was difficult by NMR either at 25 °C or at -60 °C.
Triethylamine, instead of DBU, was found to be the proper base
for the purpose of tracking the progress of the reaction of 2
entry glycosyl donor glycosyl acceptor (ROH)
product
yield, %a (ratio, R/ꢀ)b
1
2
3
4
5
6
7
8
9
33
33
33
33
33
34
34
34
34
3
7
10
11
12
3
9
11
12
35 + 36
37 + 36
38 + 36
39 + 36
40 + 36
41
42
43
44
73 (1.6:1) + 10
78 (1.2:1) + 10
82 (1:1.5) + 10
70 (1:7.5) + 10
78 (1:1.3) + 10
86 (1:1.6)
85 (R only)
81 (R only)
82 (1.8:1)c
a Isolated yields. b The ratio determined by 1H NMR. c The ratio after
isolation.
ester at all. Glycosylations of primary alcohol acceptors 3 and
12 with the mannosyl donor 34 gave mixtures of R- and
ꢀ-disaccharides 41 (R/ꢀ ) 1:1.6) and 44 (R/ꢀ ) 1.8:1),
respectively, in good yields (entries 6 and 9). Interestingly,
complete R-selective mannosylations of secondary alcohol
acceptors 9 and 11 with 34 were achieved to afford correspond-
ing R-disaccharides 42 and 43, respectively, in good yields
(entries 7 and 8).
Glycosylations of various glycosyl acceptors with disarmed27
tetrabenzoylglucose 45 and tetrabenzoylmannose 46 as glycosyl
donors were also examined under the same reaction condition
as that described above for the tetrabenzylglucose 33 and the
tetrabenzylmannose 34 except using CH2Cl2 solvent (Table 5).
(28) (a) Other glycosyl hydrogen phthalates were found to be more labile
than 56. We have previously observed the instability of glycosyl
hydrogen phthalates. See : Kim, K. S.; Lee, Y. J.; Kim, H. Y.; Kang,
S. S; Kwon, S. Y. Org. Biomol. Chem. 2004, 2, 2408–2410. (b) Kwon,
S. Y.; Lee, B.-Y.; Jeon, H. B.; Kim, K. S. Bull. Korean Chem. Soc.
2005, 26, 815–818.
(27) Mootoo, D. R.; Konradsson, P.; Udodong, U.; Fraser-Reid, B. J. Am.
Chem. Soc. 1988, 110, 5583–5584.
9
J. AM. CHEM. SOC. VOL. 130, NO. 26, 2008 8541

A R T I C L E S
Kim et al.
Scheme 2. Proposed Mechanism for the Formation of R-Mannopyranosyl Phthalate Anion 55R from 2 and Phthalic Anhydride (1)
and 1. When a mixture of 2 (R/ꢀ ) 2.1:1) (1.0 equiv) and 1
(1.1 equiv) in CD2Cl2 at 25 °C in the NMR tube was treated
with triethylamine (4.0 equiv), H NMR spectrum after 5 min
with complete disappearance of the 55ꢀ signal. NMR analysis
and the isolation of the R-mannosyl hydrogen phthalate 56
indicate that ꢀ-hemiacetal 2ꢀ reacts preferentially with 1 over
2R in the presence of a base to produce initially 55ꢀ as observed
in the trichloroacetimidate formation by Schmidt et al.29 Then,
decomposition of the kinetic product 55ꢀ back to 2ꢀ and 1
would make the reaction reversible and, consequently, the
equilibrium could be established not only between 55ꢀ and 2ꢀ
but also between55ꢀ and 55R through 2ꢀ and 2R so that the
thermodynamic product 55R is produced exclusively under the
present mannosylation condition as shown in Scheme 2. On the
other hand, NMR study showed that the intermediates in the
first step of the glycosylation with other anomeric hydroxy
sugars were predominantly R-glycosyl phthalates for 4,6-
benzylideneglucose 25 and tetrabenzylglucose 33 along with a
small amount of corresponding ꢀ-anomers.
1
showed the anomeric proton resonance at δ 5.90 for ꢀ-mannosyl
phthalate 55ꢀ along with a small peak at δ 6.29 for the
corresponding R-anomer 55R (Figure 2a). 1H NMR spectra after
1 h at room temperature (Figure 2b) and then at 10 min after
raising temperature of the same sample to 35 °C (Figure 2c)
clearly exhibited slow increase of the anomeric signal of 55R
over 55ꢀ. After 3 h at 35 °C, 1H NMR indicated that almost all
55ꢀ were converted into 55R (Figure 2d). Finally, prolonged
reaction time at 35 °C, the anomeric signal of 55R remained
Possible intermediates in the second step of the present one-
pot mannosylation would be triflate mixed anhydride 57,
mannosyl oxocarbenium ion 58, and/or mannosyl triflate 59 as
shown in Scheme 3. To detect those intermediates, we first ran
the NMR tube-scale reaction of mannose 2 (1.0 equiv) with
phthalic anhydride (1, 1.1 equiv) in CD2Cl2 in the presence of
DBU (1.2 equiv) and DTBMP (2.2 equiv) at room temperature.
Within a few min, the 1H NMR spectrum of the reaction mixture
showed the anomeric proton peak at δ 6.31 for R-mannosyl
phthalate anion 55R (Figure 3b). Then, the reaction mixture in
the NMR tube was cooled down to -78 °C and Tf2O (1.5 equiv)
1
was added. At 15 min after addition of Tf2O, the H NMR
spectrum at -60 °C showed the anomeric proton peak at δ 6.03
for R-mannopyranosyl triflate 59 (Figure 3c), which turned out
to be the same species as that produced from a thioglycoside
by Crich and Sun.30 The 13C NMR spectrum at -60 °C also
indicated the formation of 59 with an anomeric carbon peak at
δ 105.4. Then, upon addition of isopropanol to the reaction
mixture, 1H and 13C NMR spectra at -60 °C showed immediate
consumption of the triflate 59 and appearance of the anomeric
carbon peak at δ 100.2 for isopropyl 2,3-di-O-benzyl-4,6-O-
benzylidene-ꢀ-D-mannopyranoside.
On the basis of these results, we propose the mechanism for
the present ꢀ-mannosylation employing 2 as the mannosyl donor
(Scheme 3). Treatment of 2 with 1 in the presence of DBU and
DTBMP provides the R-mannosyl phthalate anion 55R, which
then reacts with Tf2O to afford triflate mixed anhydride 57. The
instantaneous lactonization of 57 by extrusion of stable 1 would
generate oxocarbenium ion 58, which might be in equilibrium
with R-mannosyl triflate 59. Subsequent reaction of 58 or 59
Figure 2. Reaction of 2 (R/ꢀ ) 2.1:1) and phthalic anhydride monitored
by 1H NMR in CD2Cl2: (a) mannosyl phthalates 55R and 55ꢀ, room
temperature, 5 min after addition of Et3N; (b) room temperature, 1 h after
addition of Et3N; (c) 35 °C, 10 min after raising temperature; (d) 35 °C,
3 h after raising temperature.
(29) Schmidt, R. R.; Michel, J. Tetrahedron Lett. 1984, 25, 821–824.
(30) Crich, D.; Smith, M. J. Am. Chem. Soc. 2001, 123, 9015–9020.
9
8542 J. AM. CHEM. SOC. VOL. 130, NO. 26, 2008

Stereoselective Direct Glycosylation
A R T I C L E S
Scheme 3. Proposed Mechanism of the ꢀ-Mannopyranosylation with 2 Employing Phthalic Anhydride (1) and Tf2O as Activating Agents
with a glycosyl acceptor (ROH) would provide the desired
ꢀ-mannopyranoside.31 We also confirmed by NMR study that
the intermediate in the first step of this one-pot mannosylation
is the carboxylate anion 55R but not carboxylic acid 56.32
Synthesis of ꢀ-(1f4)-Linked D-Manno-oligosaccahrides. We
applied this new one-pot glycosylation method to the synthesis
of ꢀ-(1f4)-D-mannotriose 62 and ꢀ-(1f4)-D-mannotetraose 67
to show its effectiveness for the stereoselective synthesis of
oligosaccharides containing ꢀ-mannopyranosyl linkages. ꢀ-(1f4)-
linked D-manno-oligosaccharides are integral parts of ꢀ-(1f4)-
mannans, which are polysaccharides found in wood and plant
seeds and have both structure and energy-reserve functions.33
Our synthesis as shown in Scheme 4 commenced with acid
hydrolysis of the benzylidene group of the protected ꢀ-(1f4)-
D-mannobiose 19, which was prepared from the reaction of the
donor 2 with the acceptor 10 (entry 6 in Table 2). Selective
benzoylation of resulting disaccharide diol 60 affords 6-O-
benzoate 61. The mannosylation of the mannobiose acceptor
Figure 3. 1H NMR spectra in CD2Cl2 for (a) mannose 2, (b) R-mannosyl phthalate 55R (δ 6.31) generated after addition of 1 and DBU to 2 at room
temperature, and (c) R-mannosyl triflate 59 (δ 6.03) generated after addition of Tf2O to 55R at -60 °C.
9
J. AM. CHEM. SOC. VOL. 130, NO. 26, 2008 8543

A R T I C L E S
Kim et al.
Scheme 4. Synthesis of ꢀ-(1f4)-D-Mannotetraose 67
61 with the mannosyl donor 2 under the standard ꢀ-mannosy-
lation condition provided protected ꢀ-(1f4)-mannotriose 62 in
92% yield with complete ꢀ-stereoselectivity. Hydrolysis of the
benzylidene group of the mannotriose 62 followed by selective
benzoylation of resulting diol 63 gave trisaccharide acceptor
64 without any problem. Then, the repetitive glycosylation of
the acceptor 64 with the donor 2 gave ꢀ-tetrasaccharide 65
exclusively in 91% yield. Saponification of the benzoyl ester
functionality of 65 with sodium methoxide and subsequent
hydrogenolysis of a benzylidene and nine benzyl protective
groups of resulting tetrasaccharide 66 afforded fully deprotected
ꢀ-(1f4)-mannotetraose 67 as a methyl glycoside in high yield.
One-bond C1-H1 coupling constants of the tetrasaccharide 65,
158.4, 155.0, and 154.7, clearly indicated that its three newly
generated glycosyl linkages are all in ꢀ-configurations. Although
syntheses of ꢀ-(1f4)-manno-oligosaccharides have been re-
ported before,34 our syntheses appear to be comparable to and
maybe even better than earlier syntheses in terms of the
efficiency and the stereoselectivity; all three mannosylation steps
produced desired ꢀ-mannosyl linkages in higher than 90% yields
with perfect stereoselectivities.
Conclusion
We have described a new efficient one-pot direct glycosy-
lation method with anomeric hydroxy sugars as glycosyl donors.
Highly stereoselective ꢀ-mannopyranosylation of various gly-
cosyl acceptors with 4,6-O-benzylidene-protected mannopyra-
nose 2 has been achieved by a sequence of three steps in one-
pot without isolation of intermediates: (i) reaction of 2 and
phthalic anhydride in the presence of DBU at room temperature,
(ii) addition of DTBMP and Tf2O to this solution at -78 °C,
and (iii) addition of the glycosyl acceptor at -78 °C. The
R-glucopyranosylation with 4,6-O-benzylidene-protected glu-
copyranose 25 and glycosylations with other types of glycosyl
donors having tetra-O-benzyl and tetra-O-benzoyl protective
groups were also possible by utilizing the present one-pot
glycosylation protocol. Glycosyl phthalates including R-man-
nosyl phthalate 55R were detected by NMR as the intermediates
in the first step of the glycosylation, and unstable R-mannosyl
hydrogen phthalate 56, the protonated form of 55R, was isolated.
We have also detected R-mannosyl triflate 59 as the intermediate
in the second step of the ꢀ-mannosylation by low temperature
NMR. On the basis of the investigation of the intermediates,
(31) For discussions on the mechanism of the 4,6-O-benzylidene directedꢀ-
mannosylations, see: (a) Crich, D.; Chandrasekera, N. S. Angew.
Chem., Int. Ed. 2004, 43, 5386-5389. (b) Weingart, R.; Schmidt, R. R.
Tetrahedron Lett. 2000, 41, 8753–8758. (c) Nukuda, T.; Berces, A.;
Whitfield, D. M. Carbohydr. Res. 2002, 337, 765–774. For discussions
on the mechanism of the 4,6-O-benzylidene directed R-glucosylations,
see references 14c, 25, and 31c.
(32) On the basis of HMBC NMR data of the carboxylic acid 56 in the
presence of DTBMP in CD2Cl2, a peak at δ 170.5 was assigned as
the carbon-13 resonance in the 2′-carboxy group of 56. And, upon
addition of DBU to this solution, the carboxy carbon peak moved
toward a little down field to appear at δ 172.2, which was assigned as
the 2′-carboxy carbon peak for the carboxylate anion 55R. The 13C
NMR spectrum of the product mixture directly obtained from the
reaction of the hydroxy sugar 2 and phthalic anhydride (1) in the
presence of DTBMP, and DBU in CD2Cl2 also showed the 2′-carboxy
carbon peak at δ 172.3 for the carboxylate anion 55R. 1H NMR spectra
also indicated that DTBMP was not protonated when it was mixed
with 56 but showed protonated DBU peaks when DBU was added to
the mixture of 56 and DTBMP. It is known that the carboxy carbon
of the carboxylate anion resonates at lower field than that of the
corresponding carboxylic acid. See: Hagen, R.; Roberts, J. D. J. Am.
Chem. Soc. 1969, 91, 4504–4506.
(33) Aspinall, G. O. The Polysaccharides; Academic Press: London, 1982;
Vol. 1.
(34) (a) Twaddle, G. W. J.; Yashunsky, D. V.; Nikolaev, A. V. Org. Biomol.
Chem. 2003, 1, 623–628. (b) Crich, D.; Banerjee, A.; Yao, Q. J. Am.
Chem. Soc. 2004, 126, 14930–14934.
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Stereoselective Direct Glycosylation
A R T I C L E S
saturated aqueous NaHCO3, and extracted with CH2Cl2. The
combined organic phase was washed with saturated aqueous
NaHCO3 and brine, dried over MgSO4, and concentrated in vacuo.
The residue was purified by flash column chromatography (hexane/
EtOAc, 1:1) to afford the compound 60 (393 mg, 87%): colorless
the possible mechanism of the ꢀ-mannopyranosylation with
2 has been proposed. The present glycosylation methodology,
especially the ꢀ-mannopyranosylation protocol, was success-
fully applied to the efficient synthesis of ꢀ-(1f4)-D-
mannotriose 62 and ꢀ-(1f4)-D-mannotetraose 67 with com-
plete ꢀ-stereoselectivity.
oil, Rf ) 0.38 (hexane/EtOAc, 1:1, v/v); [R]20 -23.6 (c 0.6,
D
CHCl3). 1H NMR (400 MHz, CDCl3) δ 1.97 (brs, 2H), 3.05-3.12
(m, 2H), 3.37 (s, 3H), 3.45 (dd, J ) 11.6, 6.8 Hz, 1H), 3.68-3.86
(m, 8H), 4.24 (t, J ) 8.8 Hz, 1H), 4.25 (d, J ) 12.0 Hz, 1H), 4.42
(d, J ) 12.0 Hz, 1H), 4.44 (d, J ) 6.0 Hz, 1H), 4.48 (d, J ) 12.4
Hz, 1H), 4.56 (d, J ) 11.6 Hz, 1H), 4.68 (d, J ) 12.0 Hz, 1H),
4.70-4.78 (m, 3H), 4.71 (d, J ) 12.4 Hz, 1H), 4.80 (d, J ) 12.0
Hz, 1H), 4.82 (d, J ) 11.6 Hz, 1H), 7.20-7.40 (m, 25H). 13C NMR
(100 MHz, CDCl3) δ 55.1, 62.9, 67.5, 69.5, 71.2, 71.7, 72.9, 73.2,
73.7, 74.2, 74.4, 75.6, 75.7, 75.9, 77.8, 82.1, 99.7, 101.2, 127.5,
127.59, 127.60, 127.75, 127.80, 127.9, 128.00, 128.04, 128.08,
128.10, 128.3, 128.4, 128.5, 128.6, 128.7, 137.9, 138.3, 138.5,
138.8, 139.0. Anal. Calcd for C48H54O11: C, 71.44; H, 6.75. Found:
C, 71.48; H, 6.81.
Experimental Section
General Procedure for the ꢀ-Mannopyranosylation with 2,3-
Di-O-benzyl-4,6-O-benzylidene-D-mannopyranose (2) (Table 2). A
solution of 2 (0.15 mmol, 1.0 equiv, R/ꢀ ) 2.1:1), phthalic
anhydride (1, 1.1 equiv), and DBU (1.2 equiv) in CH2Cl2 (15 mL)
in the presence of 4 Å molecular sieves was stirred for 15 min at
room temperature and cooled down to -78 °C. Then DTBMP (2.2
equiv) and Tf2O (1.5 equiv) were added sequentially at -78 °C
and the resulting solution was stirred for further 15 min at -78
°C. After dropwise addition of a solution of a glycosyl acceptor
(1.2 equiv) in CH2Cl2 (2 mL) to the above solution via cannula,
the reaction mixture was stirred at -78 °C for 15 min, allowed to
warm up over 1 h to 0 °C, quenched with saturated aqueous
NaHCO3, and then extracted with CH2Cl2. The combined organic
phase was washed with brine, dried over MgSO4, and concentrated
in vacuo. The residue was purified by silica gel flash column
chromatography.
General Procedure for the r-Glucopyranosylation with 2,3-Di-
O-benzyl-4,6-O-benzylidene-D-glucopyranose (25) (Table 3). A
solution of 25 (0.15 mmol, 1.0 equiv, R/ꢀ ) 1.3:1), phthalic
anhydride (1, 1.1 equiv), and DBU (1.2 equiv) in CH2Cl2 (15 mL)
in the presence of 4 Å molecular sieves was stirred for 15 min at
room temperature and cooled down to -78 °C. DTBMP (3.3 equiv)
and TfOH (1.1 equiv) were added sequentially and the resulting
solution was stirred at -78 °C for 15 min. Then Tf2O (1.5 equiv)
was added and the resulting solution was stirred at -78 °C for 15
min. After dropwise addition of a solution of a glycosyl acceptor
(1.2 equiv) in CH2Cl2 (2 mL) to the above solution via cannula,
the reaction mixture was stirred at -78 °C for 15 min, allowed to
warm up over 1 h to 0 °C, quenched with saturated aqueous
NaHCO3, and then extracted with CH2Cl2. The combined organic
phase was washed with brine, dried over MgSO4, and concentrated
in vacuo. The residue was purified by silica gel flash column
chromatography.
General Procedure for the Glycosylation with 2,3,4,6-Tetra-O-
benzyl-D-glucopyranose (33) and with 2,3,4,6-Tetra-O-benzyl-D-
mannopyranose (34) (Table 4). A solution of 33 or 34 (0.1 mmol,
1.0 equiv), phthalic anhydride (1, 1.1 equiv), and DBU (1.2 equiv)
in CH2Cl2/CH3CN (10:1, 3 mL) in the presence of 4 Å molecular
sieves was stirred for 15 min at room temperature and cooled down
to 0 °C. Then DTBMP (2.2 equiv) and a glycosyl acceptor (1.2
equiv) were added sequentially and the resulting solution was stirred
at 0 °C for 15 min. After dropwise addition of a solution of Tf2O
(1.5 equiv) in CH2Cl2 (2 mL) to the above solution via cannula,
the reaction mixture was stirred at 0 °C for 15 min, allowed to
warm up over 30 min to room temperature, quenched with saturated
aqueous NaHCO3, and then extracted with CH2Cl2. The combined
organic phase was washed with brine, dried over MgSO4, and
concentrated in vacuo. The residue was purified by silica gel flash
column chromatography.
Methyl (6-O-Benzoyl-2,3-di-O-benzyl-ꢀ-D-mannopyranosyl)-
(1f4)-2,3,6-tri-O-benzyl-r-D-mannopyranoside (61). A solution of
60 (766 mg, 0.95 mmol), pyridine (0.15 mL, 1.90 mmol), and
benzoyl chloride (0.11 mL, 0.95 mmol) in CH2Cl2 (15 mL) was
stirred at 0 °C for 10 min and at room temperature for 4 h. The
reaction mixture was washed with saturated aqueous NH4Cl and
brine and extracted with CH2Cl2. The combined organic layer was
dried with MgSO4 and concentrated in vacuo. The residue was
purified by flash column chromatography (hexane/EtOAc, 2:1) to
afford compound 61 (780 mg, 90%): colorless oil, Rf ) 0.20
1
(hexane/EtOAc, 2:1, v/v); [R]20 -18.6 (c 0.7, CHCl3). H NMR
D
(400 MHz, CDCl3) δ 2.49 (brs, 1H), 3.17 (dd, J ) 9.6, 2.8 Hz,
1H), 3.25-3.31 (m, 1H), 3.31 (s, 3H), 3.64-3.70 (m, 1H),
3.71-3.80 (m, 4H), 3.91 (dd, J ) 8.4, 3.2 Hz, 1H), 3.99 (t, J )
9.6 Hz, 1H), 4.25 (t, J ) 8.8 Hz, 1H), 4.35 (d, J ) 11.6 Hz, 1H),
4.41-4.52 (m, 4H), 4.57 (d, J ) 12.0 Hz, 1H), 4.61 (d, J ) 15.2
Hz, 1H), 4.64-4.71 (m, 4H), 4.73 (d, J ) 2.0 Hz, 1H), 4.80 (d, J
) 11.6 Hz, 1H), 4.83 (d, J ) 12.0 Hz, 1H), 7.16-7.37 (m, 28H),
7.92-7.97 (m, 2H). 13C NMR (100 MHz, CDCl3) δ 55.0, 64.0,
66.4, 69.6, 71.3, 71.5, 72.7, 72.8, 73.6, 74.2, 74.5, 74.6, 75.7, 75.8,
78.0, 81.8, 99.6, 101.7, 127.35, 127.40, 127.6, 127.7, 127.76,
127.80, 127.9, 128.1, 128.2, 128.3, 128.4, 128.5, 128.7, 129.9,
133.0, 137.9, 138.5, 138.6, 139.1, 139.2, 167.0. Anal. Calcd for
C55H58O12: C, 72.51; H, 6.42. Found: C, 72.55; H, 6.43.
Methyl (2,3-Di-O-benzyl-4,6-O-benzylidene-ꢀ-D-mannopyrano-
syl)-(1f4)-(6-O-benzoyl-2,3-di-O-benzyl-ꢀ-D-mannopyranosyl)-
(1f4)-2,3,6-tri-O-benzyl-r-D-mannopyranoside (62). A solution of
2 (94 mg, 0.21 mmol), phthalic anhydride (1, 37 mg, 0.25 mmol),
and DBU (38 µL, 0.25 mmol) in CH2Cl2 (3 mL) was stirred in the
presence of 4 Å molecular sieves for 15 min at room temperature
and cooled down to -78 °C. Then DTBMP (95 mg, 0.46 mmol)
and Tf2O (53 µL, 0.31 mmol) were added sequentially and the
resulting solution was stirred at -78 °C for a further 15 min. After
dropwise addition of a solution of glycosyl acceptor 61 (286 mg,
0.31 mmol) in CH2Cl2 (5 mL) to the above solution via cannula,
the reaction mixture was stirred at -78 °C for 15 min, allowed to
warm up over 1 h to 0 °C, quenched with saturated aqueous
NaHCO3, and then extracted with CH2Cl2. The combined organic
phase was washed with brine, dried over MgSO4, and concentrated
in vacuo. The residue was purified by flash column chromatography
(hexane/EtOAc, 2:1) to afford compound 62 (259 mg, 92%):
colorless oil, Rf ) 0.25 (hexane/EtOAc, 2:1, v/v); [R]20D -18.0 (c
General Procedure for the Glycosylation with 2,3,4,6-Tetra-O-
benzoyl-D-glucopyranose (45) and with 2,3,4,6-Tetra-O-benzoyl-
D-mannopyranose (46) (Table 5). The exactly same procedure was
employed as that for the glycosylation with 33 and with 34 except
the solvent. Only CH2Cl2 was used as the solvent instead of CH2Cl2/
CH3CN (10:1).
Methyl (2,3-Di-O-benzyl-ꢀ-D-mannopyranosyl)-(1f4)-2,3,6-tri-
O-benzyl-r-D-mannopyranoside (60). A solution of the compound
19 (500 mg, 0.56 mmol) and trifluoroacetic acid (0.22 mL, 2.79
mmol) in CH2Cl2 (10 mL) was stirred at 0 °C for 10 min and at
room temperature for 2 h. The reaction mixture was quenched with
1
0.7, CHCl3). H NMR (400 MHz, CDCl3) δ 2.96-3.03 (m, 1H),
3.31 (s, 3H), 3.33-3.37 (m, 1H), 3.39 (dd, J ) 9.2, 2.8 Hz, 1H),
3.44 (dd, J ) 10.0, 3.2 Hz, 1H), 3.58 (t, J ) 10.0 Hz, 1H),
3.66-3.75 (m, 5H), 3.86-3.93 (m, 3H), 4.03 (t, J ) 9.6 Hz, 1H),
4.15 (t, J ) 9.2 Hz, 1H), 4.22 (t, J ) 9.2 Hz, 1H), 4.31 (dd, J )
12.0, 2.0 Hz, 1H), 4.36 (dd, J ) 12.0, 3.6 Hz, 1H), 4.43 (d, J )
12.0 Hz, 1H), 4.45-4.60 (m, 3H), 4.61-4.75 (m, 10H), 4.79-4.88
(m, 3H), 5.48 (s, 1H), 7.13-7.42 (m, 43H), 7.91-7.96 (m, 2H).
9
J. AM. CHEM. SOC. VOL. 130, NO. 26, 2008 8545

A R T I C L E S
Kim et al.
13C NMR (100 MHz, CDCl3) δ 55.0, 63.6, 67.5, 68.6, 69.5, 71.3,
72.2, 72.4, 72.7, 72.9, 73.5, 73.6, 74.3, 75.3, 75.7, 75.9, 76.06,
76.10, 77.3, 78.4, 78.46, 78.50, 80.5, 99.5 (JC-H ) 170.4 Hz), 101.4
(JC-H ) 161.2 Hz), 101.7 (JC-H ) 155.2 Hz), 102.4, 126.2, 127.17,
127.20, 127.27, 127.30, 127.55, 127.60, 127.8, 127.9, 128.16,
128.20, 128.27, 128.30, 128.4, 128.5, 133.1, 137.7, 138.5, 138.6,
138.8, 138.9, 139.3, 166.4. HRMS: Calcd for C82H84O17Na [M +
Na]+, 1363.5606; found, 1363.5602.
addition of a solution of glycosyl acceptor 64 (182 mg, 0.13 mmol)
in CH2Cl2 (4 mL) via cannula, the reaction mixture was stirred at
-78 °C for 15 min, allowed to warm up over 1 h to 0 °C, quenched
with saturated aqueous NaHCO3, and extracted with CH2Cl2. The
combined organic phase was washed with brine, dried over MgSO4,
and concentrated in vacuo. The residue was purified by flash column
chromatography (hexane/EtOAc, 2:1) to afford compound 65 (145
mg, 91%): colorless oil, Rf ) 0.45 (hexane/EtOAc, 2:1, v/v); [R]20
D
1
-21.8 (c 0.9, CHCl3). H NMR (400 MHz, CDCl3) δ 2.90-2.98
Methyl (2,3-Di-O-benzyl-ꢀ-D-mannopyranosyl)-(1f4)-(6-O-ben-
zoyl-2,3-di-O-benzyl-ꢀ-D-mannopyranosyl)-(1f4)-2,3,6-tri-O-ben-
zyl-r-D-mannopyranoside (63). Compound 62 (430 mg, 0.32 mmol)
was hydrolyzed under the same reaction condition as that for the
preparation of 60 from 19. The reaction mixture was purified by
flash column chromatography (hexane/EtOAc, 1:1) to give title
compound 63 (342 mg, 85%): colorless oil, Rf ) 0.25 (hexane/
EtOAc, 1:1, v/v); [R]20D -21.0 (c 0.4, CHCl3). 1H NMR (400 MHz,
CDCl3) δ 2.95-3.01 (m, 1H), 3.12 (dd, J ) 9.2, 2.4 Hz, 1H), 3.32
(s, 3H), 3.33 (t, J ) 2.8 Hz, 1H), 3.34-3.39 (m, 1H), 3.41 (t, J )
6.0 Hz, 1H), 3.62 (t, J ) 3.2 Hz, 1H), 3.64 (t, J ) 2.8 Hz, 1H),
3.67-3.84 (m, 4H), 3.86-3.91 (m, 2H), 4.14-4.27 (m, 3H), 4.33
(dd, J ) 12.0, 4.4 Hz, 1H), 4.38-4.52 (m, 6H), 4.58-4.78 (m,
10H), 4.82 (d, J ) 4.0 Hz, 1H), 4.85 (d, J ) 4.4 Hz, 1H), 7.15-7.40
(m, 38H), 7.92-7.98 (m, 2H). 13C NMR (100 MHz, CDCl3) δ
55.0, 62.8, 63.8, 67.2, 69.5, 71.2, 71.3, 72.6, 72.7, 72.9, 73.60,
73.63, 74.2, 74.5, 75.6, 75.7, 75.8, 76.0, 76.5, 78.3, 80.3, 82.0, 99.5,
101.6, 101.8, 127.85, 127.88, 127.90, 128.1, 128.2, 128.27, 128.33,
128.4, 128.5, 128.6, 128.7, 129.8, 130.0, 133.3, 137.7, 138.6, 138.7,
139.2, 139.3, 166.5. HRMS: Calcd for C75H80O17Na [M + Na]+,
1275.5293; found, 1275.5289.
(m, 1H), 3.25-3.34 (m, 2H), 3.31 (s, 3H), 3.35 (d, J ) 2.4 Hz,
1H), 3.38 (t, J ) 3.2 Hz, 1H), 3.41 (t, J ) 1.2 Hz, 1H), 3.43 (d, J
) 2.0 Hz, 1H), 3.56 (t, J ) 10.4 Hz, 1H), 3.63 (d, J ) 2.8 Hz,
1H), 3.64 (d, J ) 4.8 Hz, 1H), 3.67-3.73 (m, 4H), 3.83-3.89 (m,
4H), 4.03 (t, J ) 10.0 Hz, 1H), 4.11-4.21 (m, 4H), 4.24-4.36
(m, 2H), 4.38 (d, J ) 12.4 Hz, 1H), 4.46 (d, J ) 6.0 Hz, 1H), 4.48
(d, J ) 13.6 Hz, 1H), 4.50 (d, J ) 6.0 Hz, 1H), 4.53-4.65 (m,
7H), 4.68 (d, J ) 10.0 Hz, 1H), 4.69-4.79 (m, 8H), 4.83 (d, J )
12.0 Hz, 1H), 5.47 (s, 1H), 7.13-7.48 (m, 56H), 7.90-7.99 (m,
4H). 13C NMR (100 MHz, CDCl3) δ 55.0, 63.5, 63.7, 67.5, 68.6,
69.5, 71.3, 72.0, 72.3, 72.4, 72.7, 72.9, 73.45, 73.53, 74.3, 74.5,
75.27, 75.33, 75.7, 75.8, 75.97, 76.00, 76.1, 78.2, 78.4, 78.5, 80.1,
80.6, 99.5 (JC-H ) 169.4 Hz), 101.4 (JC-H ) 158.4 Hz), 101.6
(JC-H ) 155.0 Hz), 101.97 (JC-H ) 154.7 Hz), 102.01, 126.2,
127.17, 127.24, 127.30, 127.34, 127.4, 127.5, 127.6, 127.8, 128.9,
129.8, 129.87, 129.92, 130.0, 133.2, 137.8, 138.4, 138.5, 138.6,
138.7, 138.9, 139.2, 139.3, 166.37, 166.41. MALDI-TOF Calcd
for C109H110O23Na [M + Na]+, 1809.7336; found, 1809.7303.
Methyl (2,3-Di-O-benzyl-4,6-O-benzylidene-ꢀ-D-mannopyrano-
syl)-(1f4)-(2,3-di-O-benzyl-ꢀ-D-mannopyranosyl)-(1f4)-(2,3-di-O-
benzyl-ꢀ-D-mannopyranosyl)-(1f4)-2,3,6-tri-O-benzyl-r-D-man-
nopyranoside (66). A solution of compound 65 (60 mg, 0.034
mmol) and sodium methoxide (2.0 mg, 0.034 mmol) in CH2Cl2/
MeOH (1:4, v/v, 5 mL) was stirred for overnight at room
temperature. The reaction mixture was neutralized with DOWEX
CCR-3 (H+ mode) resin, filtered through Celite, and concentrated
in vacuo. The residue was purified by flash column chromatography
(hexane/EtOAc, 1:1) to afford compound 66 (48 mg, 90%):
amorphous solid, Rf ) 0.36 (hexane/EtOAc, 1:1, v/v); [R]20D -29.3
(c 0.4, CHCl3); 1H NMR (400 MHz, CDCl3) δ 2.99-3.05 (m, 1H),
3.10-3.18 (m, 2H), 3.22 (dd, J ) 2.4, 9.4 Hz, 1H), 3.29-3.36 (m,
2H), 3.37 (s, 3H), 3.46 (dd, J ) 2.8, 9.2 Hz, 1H), 3.51-3.63 (m,
4H), 3.65-3.73 (m, 4H), 3.78-3.86 (m, 4H), 3.96-4.05 (m, 3H),
4.09 (t, J ) 9.6 Hz, 1H), 4.23 (t, J ) 8.8 Hz, 1H), 4.40-4.63 (m,
7H), 4.68-4.86 (m, 14H), 4.89 (d, J ) 12.0 Hz, 1H), 5.51 (s, 1H),
7.23-7.46 (m, 50H). 13C NMR (100 MHz, CDCl3) δ 55.1, 61.9,
62.1, 67.5, 68.7, 69.2, 71.6, 72.4, 72.7, 72.8, 73.2, 73.7, 74.6, 74.8,
75.17, 75.20, 75.4, 75.7, 75.9, 76.0, 76.6, 77.3, 77.4, 78.0, 78.5,
78.8, 80.4, 80.6, 99.6, 101.1, 101.4, 101.5, 102.2, 126.3, 126.9,
127.0, 127.1, 127.3, 127.45, 127.47, 127.52, 127.58, 127.62, 127.67,
127.72, 127.88, 127.94, 128.1, 128.2, 128.26, 128.28, 128.35,
128.44, 128.46, 128.54, 128.9, 137.8, 138.3, 138.56, 138.59, 138.7,
138.9, 138.99, 139.01, 139.3. HRMS Calcd for C95H102O21Na [M
+ Na]+, 1601.6811; found, 1601.6814.
Methyl (6-O-Benzoyl-2,3-di-O-benzyl-ꢀ-D-mannopyranosyl)-
(1f4)-(6-O-benzoyl-2,3-di-O-benzyl-ꢀ-D-mannopyranosyl)-(1f4)-
2,3,6-tri-O-benzyl-r-D-mannopyranoside (64). A solution of 63 (292
mg, 0.23 mmol), pyridine (47 µL, 0.58 mmol), and benzoyl chloride
(33 µL, 0.28 mmol) in the presence of a catalytic amount of DMAP
in CH2Cl2 (15 mL) was stirred at 0 °C for 10 min and at room
temperature for further 3 h. The reaction mixture was washed with
saturated aqueous NH4Cl and brine, and then extracted with CH2Cl2.
The combined organic layer was dried over MgSO4 and concen-
trated in vacuo. The residue was purified by flash column
chromatography (hexane/EtOAc, 2:1) to afford compound 64 (281
mg, 89%): colorless oil, Rf ) 0.15 (hexane/EtOAc, 2:1, v/v); [R]20
D
1
-24.4 (c 0.7, CHCl3). H NMR (400 MHz, CDCl3) δ 3.15-3.21
(m, 2H), 3.31 (s, 3H), 3.36-3.43 (m, 2H), 3.61-3.73 (m, 5H),
3.87 (dd, J ) 8.8, 3.2 Hz, 1H), 3.90 (d, J ) 2.0 Hz, 1H), 4.00 (t,
J ) 9.2 Hz, 1H), 4.18 (t, J ) 8.8 Hz, 1H), 4.19 (t, J ) 8.8 Hz,
1H), 4.32-4.37 (m, 3H), 4.38 (d, J ) 12.8 Hz, 1H), 4.41-4.53
(m, 6H), 4.55 (d, J ) 8.8 Hz, 1H), 4.58 (d, J ) 5.6 Hz, 1H), 4.63
(d, J ) 12.0 Hz, 1H), 4.68 (d, J ) 12.8 Hz, 1H), 4.69-4.77 (m,
5H), 4.82 (d, J ) 6.8 Hz, 1H), 4.85 (d, J ) 7.2 Hz, 1H), 7.12-7.49
(m, 41H), 7.90-7.99 (m, 4H). 13C NMR (100 MHz, CDCl3) δ
55.0, 63.7, 66.2, 69.5, 71.3, 71.4, 72.3, 72.7, 72.9, 73.49, 73.54,
74.3, 74.4, 74.5, 74.7, 75.67, 75.73, 76.07, 76.12, 78.3, 80.3, 81.6,
99.5, 101.6, 102.2, 127.2, 127.3, 127.4, 127.55, 127.63, 127.7,
127.75, 127.79, 127.9, 128.0, 128.1, 128.2, 128.3, 128.36, 128.39,
128.42, 128.5, 128.55, 128.60, 128.7, 129.86, 129.91, 130.0, 130.3,
133.0, 133.2, 133.7, 137.8, 138.4, 138.6, 138.8, 139.0, 139.3, 166.5,
166.9. HRMS: Calcd for C82H84O18Na [M + Na]+, 1379.5555;
found, 1379.5557.
Methyl (ꢀ-D-Mannopyranosyl)-(1f4)-(ꢀ-D-mannopyranosyl)-
(1f4)-(ꢀ-D-mannopyranosyl)-(1f4)-r-D-mannopyranoside (67). A
mixture of compound 66 (30 mg, 0.019 mmol) and Pd/C (10%, 20
mg) in MeOH/CH2Cl2/AcOH (5:3:2, v/v/v, 5 mL) was stirred under
hydrogen atmosphere using a balloon at room temperature for
overnight. The reaction mixture was filtered through Celite and
concentrated in vacuo. The residue was purified by flash column
chromatography on Iatrobeads (CHCl3/MeOH, 1:5, v/v) to afford
compound 67 (11 mg, 85%): colorless oil, Rf ) 0.08 (CHCl3/
MeOH, 1:5, v/v); [R]D20 -7.1 (c 0.4, MeOH); 1H NMR (400 MHz,
D2O) δ 3.14-3.32 (m, 2H), 3.26 (s, 3H), 3.37-3.52 (m, 4H),
3.55-3.86 (m, 15H), 3.90 (brs, 1H), 3.97 (brs, 2H), 4.55-4.65
(m, 4H); 13C NMR (100 MHz, CD3OD) δ 55.7, 61.4, 61.5, 62.0,
67.7, 70.2, 70.5, 70.95, 71.00, 71.5, 72.0, 72.5, 73.8, 76.1, 77.48,
77.52, 77.6, 101.1, 101.15, 101.20, 101.7; HRMS Calcd for
C25H44O21Na [M + Na]+: 703.2273, found: 703.2270.
Methyl (2,3-Di-O-benzyl-4,6-O-benzylidene-ꢀ-D-mannopyrano-
syl)-(1f4)-(6-O-benzoyl-2,3-di-O-benzyl-ꢀ-D-mannopyranosyl)-
(1f4)-(6-O-benzoyl-2,3-di-O-benzyl-ꢀ-D-mannopyranosyl)-(1f4)-
2,3,6-tri-O-benzyl-r-D-mannopyranoside (65). A solution of 2 (40
mg, 0.089 mmol), phthalic anhydride (1, 16 mg, 0.11 mmol), and
DBU (16 µL, 0.11 mmol) in CH2Cl2 (2 mL) was stirred in the
presence of 4 Å molecular sieves for 15 min at room temperature
and cooled down to -78 °C. Then DTBMP (40 mg, 0.20 mmol)
and Tf2O (23 µL, 0.13 mmol) were added sequentially and the
resulting solution was stirred at -78 °C for 15 min. After dropwise
9
8546 J. AM. CHEM. SOC. VOL. 130, NO. 26, 2008

Stereoselective Direct Glycosylation
A R T I C L E S
Procedure for the Detection of Intermediates 55r and 59
in the ꢀ-Mannopyranosylation with 2 in CD2Cl2 by NMR. To a
5 mm NMR tube containing 2 (4.5 mg, 0.010 mmol), phthalic
anhydride (1, 1.6 mg, 0.011 mmol), and DTBMP (4.5 mg, 0.022
mmol) in CD2Cl2 (750 µL) was added DBU (1.7 µL, 0.012 mmol)
at room temperature. After being briefly agitated, the tube was
placed in the NMR probe at room temperature. The 1H NMR
spectrum indicated the conversion of mannose 2 to mannosyl
phthalate 55R was complete within a few minutes and showed the
R-anomeric proton peak of 55R at δ 6.31 (Figure 3b). The reaction
mixture in the NMR tube was cooled down to -78 °C and Tf2O
(2.5 µL, 0.015 mmol) was added to this solution. After being briefly
agitated, the NMR tube was placed in the precooled NMR probe
at -60 °C and the conversion of 55R to R-mannosyl triflate 59
indicated immediate consumption of 59 with formation of isopropyl
2,3-di-O-benzyl-4,6-O-benzylidene-ꢀ-D-mannopyranoside, which
was confirmed by the comparison of its spectral data with those of
the authentic sample and by mass spectrometry without isolation.
Acknowledgment. This paper is dedicated to Professor E. J.
Corey on the occasion of his 80th birthday. This work was
supported by a grant from the Korea Science and Engineering
Foundation through the Center for Bioactive Molecular Hybrids
(CBMH). D.B.F., J.Y.B., and B.Y.L. thank the fellowship of the
BK 21 program from the Ministry of Education and Human
Resources Development. We thank Dr. Hye-Seo Park for her help
with NMR studies.
1
was almost instantaneous. The H and 13C NMR spectra showed
Supporting Information Available: Experimental procedure,
characterization data, and NMR spectra. This material is
available free of charge via the Internet at http://pubs.acs.org.
the R-anomeric proton peak at δ 6.03 (Figure 3c) and the anomeric
carbon peak at δ 105.4 of 59. Then, 2-propanol (0.9 µL, 0.015
mmol) was added to the reaction mixture in the tube at -78 °C.
After being briefly agitated, the NMR tube was placed in the
1
precooled NMR probe at -60 °C. The H and 13C NMR spectra
JA710935Z
9
J. AM. CHEM. SOC. VOL. 130, NO. 26, 2008 8547