Dual stereoselectivity of 1-(2′-carboxy)benzyl 2-deoxyglycosides as glycosyl donors in the direct construction of 2-deoxyglycosyl linkages

Article abstract of DOI:10.1002/anie.200390139

Different protecting groups on the glycosyl donor changes the stereoselectivity of the new glycosylation reaction shown in the scheme. Glycosylation of CB 4,6-Obenzylidene-2-deoxyglycoside 1 with secondary alcohol acceptors gave predominantly β-disacchari

Full text of DOI:10.1002/anie.200390139

Angewandte
Chemie
treatment) at an incident wavelength of 1.907 mm. Dipole moments m
were determined in anhydrous CHCl3 by the Guggenheim method.[4]
All solutions for EFISH, conductivity, and capacitance measurements
were used immediately after preparation.
Stereoselective Glycosylations
Dual Stereoselectivity of 1-(2’-Carboxy)benzyl 2-
Deoxyglycosides as Glycosyl Donors in the Direct
Construction of 2-Deoxyglycosyl Linkages**
Received: August 2, 2002 [Z19874]
Kwan Soo Kim,* Jin Park, Yong Joo Lee, and
Yong Sung Seo
[1] a) N. J. Long, Angew. Chem. 1995, 107, 37 55; Angew. Chem. Int.
Ed. Engl. 1995, 34, 21 38; b) Optoelectronic Properties of
Inorganic Compounds (Eds.: D. M. Roundhill, J. P. Fackler, Jr.),
Plenum, New York, 1999; c) J. Heck, S. Dabek, T. Meyer-
Friedrichsen, H. Wong, Coord. Chem. Rev. 1999, 190 192, 1217
1254; d) H. Le Bozec, T. Renouard, Eur. J. Inorg. Chem. 2000,
229 239; e) P. G. Lacroix, Eur. J. Inorg. Chem. 2001, 339 348;
f) S. Di Bella, Chem. Soc. Rev. 2001, 30, 355 366.
[2] a) B. F. Levine, C. G. Bethea, Appl. Phys. Lett. 1974, 24, 445 447;
b) J. L. Oudar, J. Chem. Phys. 1977, 67, 446 457; c) K. D. Singer,
A. F. Garito, J. Chem. Phys. 1981, 75, 3572 3580; d) I. Ledoux, J.
Zyss, Chem. Phys. 1982, 73, 203 213.
[3] a) D. W. Bruce, A. Thornton, Mol. Cryst. Liq. Cryst. 1993, 231,
253 256; b) D. R. Kanis, P. G. Lacroix, M. A. Ratner, T. J. Marks,
J. Am. Chem. Soc. 1994, 116, 10089 10102; c) M. J. G. Lesley, A.
Woodward, N. J. Taylor, T. B. Marder, I. Cazenobe, I. Ledoux, J.
Zyss, A. Thornton, D. W. Bruce, A. K. Kakkar, Chem. Mater.
1998, 10, 1355 1365; d) D. Roberto, R. Ugo, S. Bruni, E. Cariati,
F. Cariati, P. C. Fantucci, I. Invernizzi, S. Quici, I. Ledoux, J. Zyss,
Organometallics 2000, 19, 1775 1788; e) D. Roberto, R. Ugo, F.
Tessore, E. Lucenti, S. Quici, S. Vezza, P. C. Fantucci, I. Invernizzi,
S. Bruni, I. Ledoux-Rak, J. Zyss, Organometallics 2002, 21, 161
170.
2-Deoxyglycosides are found as the integral part of many
important natural products.[1] The stereocontrolled construc-
tion of 2-deoxyglycosyl linkages is more challenging than that
of usual glycosyl linkages as the 2-deoxyglycosyl donors lack
the stereodirecting substituent at C2 and the resulting glyco-
sides are more acid-labile. Although the synthesis of 2-deoxy-
a-glycopyranosides is relatively easier, the construction of 2-
deoxy-b-glycopyranosyl linkages is a far more challenging
problem[2] as the axial addition of a glycosyl acceptor to the
oxocarbenium ion intermediate is a favorable process owing
to the anomeric effect.[3] Therefore, the frequently used
method for the synthesis of 2-deoxy-b-glycopyranosides
employs the glycosyl donors with equatorial heteroatom
substituents at C2 that can participate during glycosylation to
direct the incoming acceptor to the b-face and are removed at
a later stage.[4] Nevertheless, a direct and efficient method for
the construction of the 2-deoxy-b-glycopyranosyl linkage by
using a 2-deoxyglycopyranosyl donor would be more efficient
and practical than the indirect methods. Although a few
methods (e.g. glycosylation mediated by silver silicate[5] or
silver zeolite,[6] the 2-deoxyglycosyl phosphite/TMSOTf
(TMSOTf ¼ trimethylsilyl trifluoromethanesulfonate) meth-
od,[7] and the glycosylation of 2-deoxyglycosyl fluoride with
TiF4[8] or SnCl2,[9]) have been devised, the direct synthesis of 2-
deoxy-b-glycopyranosides from 2-deoxyglycosyl donors still
remains a difficult task. We have previously introduced (2’-
carboxy)benzyl (CB) glycosides as glycosyl donors for the
efficient b-mannopyranosylation.[10] We applied this CB
glycoside methodology to the direct synthesis of 2-deoxygly-
copyranosides and herein report the preparation of CB 2-
deoxyglycosides from glycals and their a- or b-stereoselec-
tivity (dual stereoselectivity) as glycosyl donors in the
glycosylation reaction, depending on their protecting groups.
CB 2-deoxyglucoside 4 was readily obtained from the
tribenzylglucal 1 as shown in Scheme 1. CB 4,6-O-benzyli-
dene-2-deoxyglucoside 7, on the other hand, was prepared
from a 3,4,6-tri-O-acetyl-d-glucal via 2-deoxyglycosyl bro-
mide 5.[13] Glycosylations of the CB 2-deoxyglycosyl donors 4
and 7 were performed with primary alcohol acceptors 8 12
and with secondary alcohol acceptors 13 17. Glycosylation of
the benzyl-protected donor 4 with primary alcohol acceptors
[4] a) E. A. Guggenheim, Trans. Faraday Soc. 1949, 45, 714 720;
b) H. B. Thompson, J. Chem. Educ. 1966, 43, 66 73.
[5] a) M. Bourgault, C. Mountassir, H. Le Bozec, I. Ledoux, G.
Pucetti, J. Zyss, J. Chem. Soc. Chem. Commun. 1993, 1623 1624;
b) M. Bourgault, K. Baum, H. Le Bozec, G. Pucetti, I. Ledoux, J.
Zyss, New J. Chem. 1998, 517 522; c) A. Hilton, T. Renouard, O.
Maury, H. Le Bozec, I. Ledoux, J. Zyss, Chem. Commun. 1999,
2521 2522.
[6] V. Alain, M. Blanchard-Desce, I. Ledoux-Rak, J. Zyss, Chem.
Commun. 2000, 353 354.
[*] Prof. Dr. K. S. Kim, J. Park, Y. J. Lee, Y. S. Seo
Department of Chemistry, Yonsei University
Seoul 120-749 (Korea)
Fax: (þ82)2-364-7050
E-mail: kwan@yonsei.ac.kr
[**] This work was supported by a grant from KOSEF-CMDS (Center for
Molecular Design and Synthesis).
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Table 1: Glycosylation of the benzyl-protected CB 2-deoxyglycoside 4.
HO
O
BnO
Tf2O, DTBMP
78 oC, CH2Cl2
BnO
O
OBn
4
ROH
BnO
BnO
BnO
BnO
BnO
BnO
OR
2
BnO
O
BnO
O
18–26
O
b
a
BnO
BnO
Entry
1
ROH
Product
Yield [%]
98
Ratio [a/b]
OCB
OBCB
1
4
3
18
19
20
21
1:1
AcO
AcO
AcO
AcO
AcO
Ph
O
O
O
O
d
g
c
O
AcO
BnO
OBCB
Br
OCB
5
6
7
2
3
4
93
94
78
1:1
O
O
CH2
CH2
BCB =
CB =
OBn
OH
1:1.2
1:1.5
Scheme 1. Synthesis of CB 2-deoxyglycosides 4 and 7: a) 2 (2.5 equiv),
Ph3P¥HBr[11] (0.05 equiv), 0!408C, 1h, 89%; b) H 2, Pd/C (10%),
NH4OAc[12] (3.5 equiv), MeOH, 1h, 90%; c) 2 (1.2 equiv), Hg(CN)2,
HgBr2, CH3CN, molecular sieves (4 ä), 08C, 20 min, 90%; d) NaOMe,
MeOH, 20 min; e) PhCH(OMe)2, CSA, DMF, 508C, 1h, 90% over two
steps; f) BnBr, NaH, DMF, 1h, 84%; g) same as b), 92%. CSA ¼(ꢀ)-
camphorsulfonic acid, DMF¼N,N-dimethylformamide, Bn¼benzyl.
5
22
93
1:1.7
8 12 exhibited no significant stereoselectivity (Table 1, en-
tries 1 5). For example, the dropwise addition of Tf2O
(1.2 equiv) to a stirred solution of 4 (1.0 equiv), the acceptor
6
7
23
24
91
91
a only
a only
8
(1.5 equiv),
and
2,6-di-tert-butyl-4-methylpyridine
(DTBMP, 2.4 equiv) in CH2Cl2 at À788C for 1 h led to the
isolation of equal amounts of a- and b-disaccharides 18 in
98% yield (Table 1, entry 1). Some b-selectivity was observed
when primary alcohols 10 (a/b 1:1.2), 11 (a/b 1:1.5), and 12
(a/b 1:1.7) were employed as the glycosyl acceptors (Table 1,
entries 3 5). On the other hand, glycosylation of 4 with
secondary alcohol donors 13 15 afforded the a-disaccharides
exclusively (Table 1, entries 6 8) and with the secondary
alcohol 16 afforded predominantly the a-disaccharide 26
(a/b 9.4:1; Table 1, entry 9).
8
9
25
26
88
94
a only
9.4:1
Under the same reaction conditions, glycosylation of the
benzylidene-protected donor 7 with primary alcohol accep-
tors 8 and 9 provided a mixture of almost equal amounts of a-
and b-anomers of disaccharides 27 (a/b 1:1) and 28 (a/b
1:1.2), respectively, together with a minor amount of the self-
condensed ester 35, which must be derived from two
Table 2: Glycosylation of the benzylidene-protected CB 2-deoxyglycoside
7.
Ph
O
Tf2O, DTBMP
78 oC, CH2Cl2
O
O
7
ROH
BnO
27–34
OR
Entry ROH
Product Yield [%] Ratio [a/b] Ester 35 [%]
1
2
3
4
5
6
7
8
9
27
28
29
30
31
32
33
92
80
83
85
76
72
78
1:1
1:1.2
1:2
1:5.1
1:10
b only
1:10
8
16
14
12
14
16
16
11
12
13
14
15
molecules of the glycosyl donor 7 (Table 2, entries 1 and 2).
When primary alcohols 11 and 12 were used as glycosyl
acceptors, the ratio of the resulting disaccharides increased
substantially in favor of the b-anomer; a/b 1:2 for 29 and 1:5.1
for 30 (Table 2, entries 3 and 4). The highly b-selective
glycosylation of 7 could be carried out with secondary alcohol
acceptors. Thus, the b/a ratio in the reactions of 7 with
acceptors 13 and 15 was 10:1 and that with 17 was 8:1
8
34 :8 7711
1
460
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side 7 with secondary alcohols, the b-disaccharides were
Ph
O
Ph
O
O
O
O
O
formed by SN2-like displacement of the a-triflate D (or a tight
ion pair) by the acceptor alcohol ROH (Scheme 2). In this
case, the equilibrium might highly favor the a-triflate D over
the oxocarbenium ion E owing to the 4,6-O-benzylidene
protective group. Previously, highly b-selective mannopyra-
nosylations with 4,6-O-benzylidene-protected mannosyl do-
nors have been reported[10,14] and the SN2-like displacement of
the a-triflate by the acceptor has been suggested for the b-
selectivity by Crich and Sun.[15] No or poor b-selectivity in the
reaction of 7 with primary alcohols might be interpreted by
assuming that more reactive primary alcohols reacted with
both the a-triflate D and the oxocarbenium ion C (Scheme 2)
before the equilibrium was established. In this respect, it is
also noteworthy that the more hindered primary alcohol
donor 13 showed a substantial b-selectivity (a/b 1:5.1). The a-
selectivity in the glycosylation of the benzyl-protected CB 2-
deoxyglycoside 4 with secondary alcohols might be explained
if the b-triflate exists in equilibrium. It has been suggested
that the a-glucoside was formed by the SN2-like displacement
of the b-halide[16] found in equilibrium and was less stable but
more reactive than the corresponding a-anomer.
BnO
BnO
DTBMP
O
O
O
O
7
A
O
OH
Tf2O
Ph
O
O
O
OTf
O
BnO
Ph
O
Ph
O
O
O
O
O
O
BnO
BnO
OTf
O
O
B
OTf
ROH
C
D
A
35
Ph
O
O
O
BnO
OR
E
Scheme 2. Plausible mechanism for the formation of disaccharides
and the self-condensed ester 35.
(Table 2, entries 5, 7, and 8). Glycosylation of 7 with 14, on the
other hand, afforded the b-anomer of disaccharide 32
exclusively (Table 2, entry 6) in 72% yield along with the
self-condensed ester 35 in 16% yield.
In conclusion, we have developed a highly a- and b-
stereoselective (dual stereoselective) method for the syn-
thesis of 2-deoxyglycosides by employing CB 2-deoxyglyco-
sides as glycosyl donors. Glycosylation of the 4,6-O-benzyl-
idene-protected glycosyl donor 7 with secondary alcohols
afforded predominantly b-glycosides whereas glycosylation of
the benzyl-protected glycosyl donor 4 with secondary alcohols
afforded a-glycosides.
Formation of the self-condensed ester 35 probably
resulted from the coupling between the carboxlyate anion
A, which was generated by deprotonation of 7 by DTBMP,
and the oxocarbenium ion C or the C-1 triflate intermediate
D as shown in Scheme 2. Generation of a substantial amount
of ester 35 led us to postulate that the conversion of the
carboxylate A into the triflate B was slower than the
formation of C or D by the lactonization of the mixed
anhydride B. We envisaged that if the conversion rate of A
into B was enhanced or the concentration of A was kept at a
minimum during the glycosylation, the formation of the ester
35 would be suppressed. We therefore ran several glycosyla-
tion reactions under modified conditions. Other bases such as
triethylamine, pyridine, and 4-dimethylaminopyridine were
rather detrimental to the glycosylation reaction, whereas a
three- to fivefold excess of the glycosyl acceptor did not
decrease the amount of the ester 35. On the other hand, slow
addition of either the glycosyl donor 7 or DTBMP to the
mixture of all other reactants greatly affected the course of
the reaction to give only the disaccharide without the
formation of the ester 35, although the a/b selectivity was
lost. For example, slow addition of the donor 7 to a solution of
the acceptor 13, DTBMP, and Tf2O in CH2Cl2 at À788C
afforded only the disaccharide 31 (a/b 1:1.5) in 92% yield,
whereas slow addition of DTBMP to a solution of 7, 13, and
Tf2O in CH2Cl2 at À788C gave exclusively 31 (a/b 1:1) in 90%
yield.
Experimental Section
Typical procedure (34): A mixture of 7 (40 mg, 0.084 mmol, 1 equiv),
17 (59 mg, 0.127 mmol, 1.5 equiv), DTBMP (41 mg, 0.202 mmol,
2.4 equiv), and activated molecular sieves (4 ä) in dichloromethane
(8 mL) was stirred at room temperature for 30 min. Tf2O (17.0 mL,
0.101 mmol, 1.2 equiv) was added to this mixture at À788C. The
resulting mixture was stirred at À788C for 1 h before the addition of
saturated aqueous NaHCO3 (10 mL). The reaction mixture was
filtered and the organic layer was separated, washed with saturated
aqueous sodium chloride solution (10 mL), dried, filtered, and
concentrated. The residue was purified by means of silica-gel flash-
column chromatography to afford the separable disaccharides b-34
(42 mg, 63%), a-34 (5 mg, 8%), and the self-condensed ester 35
(6 mg, 17%). b-34: Rf ¼ 0.70 (hexane/ethyl acetate 2:1); [a]D ¼ þ 0.74
1
(c ¼ 0.20 in CHCl3); H NMR (250 MHz, CDCl3): d ¼ 1.68 1.81 (m,
1H), 2.25 (ddd, J ¼ 2.1, 4.5, 13.6 Hz, 1H), 3.24 3.33 (m, 1H), 3.30 (s,
3H), 3.58 3.90 (m, 8H), 4.12 (d, J ¼ 9.8 Hz, 1H), 4.29 (dd, J ¼ 4.9,
10.4 Hz, 1H), 4.66 (dd, J ¼ 2.0, 9.8 Hz, 1H), 4.56 and 4.81 (q, J ¼
12.1 Hz, 2H), 4.59 (s, 2H), 4.69 and 4.93 (q, J ¼ 11.3 Hz, 2H), 4.71 (s,
2H), 4.74 (d, J ¼ 1.6 Hz, 1H), 5.58 (s, 1H), 7.21 7.57 ppm (m, 25H);
13C NMR (63 MHz, CDCl3): d ¼ 37.7, 54.8, 66.7, 68.9, 69.1, 71.5, 72.2,
72.6, 72.7, 74,6, 75.1, 77.4, 80.4, 83.2, 99.0, 101.0, 101.4, 126.2, 127.8,
128.0, 128.4, 128.5, 129.0, 137.7, 138.4, 138.6, 138.7 ppm; elemental
analysis (%): calcd for C48H52O10 (788.9): C 73.08, H 6.64; found: C
73.10, H 6.70. a-34: Rf ¼ 0.60 (hexane/ethyl acetate 2:1); [a]D ¼ þ 0.94
Complete reversal of the stereoselectivity from a to b
simply by changing the protecting group of the 2-deoxygly-
cosyl donor from a benzyl to a benzylidene in the present
glycosylation reaction is unprecedented. The precise mecha-
nistic details underlying this reversal of selectivity and the
lack of selectivity with primary alcohol acceptors are not yet
clear. Nevertheless, it is quite reasonable to assume that in the
glycosylation of the benzylidene-protected CB 2-deoxyglyco-
1
(c ¼ 0.20 in CHCl3); H NMR (250 MHz, CDCl3): d ¼ 1.76 (ddd, J ¼
3.6, 11.3, 13.2 Hz, 1H), 2.33 (dd, J ¼ 5.1, 13.2 Hz, 1H), 3.27 (s, 3H),
3.64 3.93 (m, 10H), 4.21 (dd, J ¼ 4.5, 9.8 Hz, 1H), 4.57 (d, J ¼ 10.9 Hz,
1H), 4.62 (s, 2H), 4.63 (d, J ¼ 11.6 Hz, 1H), 4.67 and 4.77 (q, J ¼
11.9 Hz, 2H), 4.72 4.76 (m, 2H), 4.94 (d, J ¼ 11.0 Hz, 1H), 5.06 (d, J ¼
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3.0 Hz, 1H), 5.61 (s, 1H), 7.21 7.51 ppm (m, 25H); 13C NMR
Spin Coupling between Metal Ions
(63 MHz, CDCl3): d ¼ 36.5, 54.8, 63.2, 69.3, 71.5, 72.2, 72.8, 74.8,
74.9, 75.2, 77.2, 80.4, 84.1, 98.2, 98.9, 101.5, 124.3, 126.3, 127.6, 127.7,
127.8, 127.9, 128.0, 128.3, 128.4, 128.5, 129.0, 138.4, 138.6, 138.8 ppm;
elemental analysis (%): calcd for C48H52O10 (788.9): C 73.08, H 6.64;
found: C 73.06, H 6.69. 35: Rf ¼ 0.55 (hexane/ethyl acetate 2:1);
Porphyrazines as Molecular Scaffolds: Periphery
Core Spin Coupling between Metal Ions of a
Schiff Base Porphyrazine**
1
[a]D ¼ þ 0.15 (c ¼ 0.20 in CHCl3); H NMR (250 MHz, CDCl3): d ¼
1.86 (ddd, J ¼ 3.8, 11.3, 13.2 Hz, 1H), 1.99 (m, 1H), 2.36 (dd, J ¼ 5.3,
13.2 Hz, 1H), 2.45 (ddd, J ¼ 2.4, 4.8, 13.2 Hz, 1H), 3.50 3.60 (m, 1H),
3.69 3.94 (m, 6H), 4.04 4.16 (m, 1H), 4.25 (dd, J ¼ 4.2, 9.5 Hz, 1H),
4.37 (dd, J ¼ 4.8, 9.8 Hz, 1H), 4.70 and 4.87 (q, J ¼ 11.8 Hz, 2H), 4.72
and 4.85 (q, J ¼ 12.1 Hz, 2H), 4.91 and 5.03 (q, J ¼ 14.5 Hz, 2H), 5.05
(d, J ¼ 2.7 Hz, 1H), 5.61 (s, 1H), 5.63 (s, 1H), 6.00 (dd, J ¼ 2.2, 9.9 Hz,
1H), 7.24 7.64 (m, 23H), 8.03 ppm (d, J ¼ 7.8 Hz, 1H); 13C NMR
(63 MHz, CDCl3): d ¼ 36.4, 36.7, 63.5, 67.3, 67.5, 68.8, 69.3, 72.9,
73.2(2), 74.3, 82.8, 84.1, 92.6, 98.0, 101.5, 101.6, 126.2, 127.1, 127.5,
127.7, 127.8, 127.9, 128.0, 128.4(2), 128.5, 128.6, 129.0, 129.1, 131.2,
133,3, 137.5, 137.7, 138.3, 138.9, 140.8, 164.8 ppm; calcd for C48H48O11
(800.9): C 71.98, H 6.04; found: C 71.78, H 6.10.
Min Zhao, Charlotte Stern, Anthony G. M. Barrett,*
and Brian M. Hoffman*
The periphery of a metalloporphyrazine (pz) can be readily
functionalized with heteroatoms that coordinate additional
metal ions, and this opens the way to high-spin molecules and
molecular arrays based on exchange coupling between the
metal ions.[1] We previously reported evidence for core per-
iphery spin coupling when a metal ion is bound to a peripheral
dithiolene.[1 3] We show here that the pz molecule can be used
as a scaffold on which to build complex peripheral chelating
ligands, and that the resulting dimetallic complex shows some
remarkable core periphery spin coupling. We have synthe-
sized the Schiff base appended porphyrazine (1) where the
core metal ion is MnIII with spin S ¼ 2, and the peripheral ion
is CuII with S ¼ 1/2; have obtained its crystal structure, and
characterized the S ¼ 3/2 and 5/2 total-spin manifolds created
by strong core periphery M1 M2 spin coupling; and have
Received: August 30, 2002 [Z50069]
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[*] Prof. A. G. M. Barrett
Department of Chemistry
Imperial College of Science, Technology, and Medicine
London SW7 2AY (UK)
Fax: (þ44)207-594-5805
E-mail: agmb@ic.ac.uk
Prof. B. M. Hoffman, M. Zhao, Dr. C. Stern
Department of Chemistry
Northwestern University
2145 Sheridan Road
Evanston, IL 60208 (USA)
Fax: (þ1)847-491-7713
E-mail: bmh@northwestern.edu
[**] This work was supported by the National Science Foundation (CHE-
0091364, B.M.H.), the EPSRC (A.G.M.B.), and through the NU
Materials Research Center (DMR-0076097).
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¹ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1433-7851/03/4204-0462 $ 20.00+.50/0
Angew. Chem. Int. Ed. 2003, 42, No. 4