Regioselective syntheses of β-N-linked glycoaminoacids and glycopeptides

Article abstract of DOI:10.1021/jo7018637

(Chemical Equation Presented) Acylation of tetra-O-pivaloyl-β-D- galactopyranosylamine 2 by readily available N- (Cbz or Fmoc-α-aminoacyl) benzotriazoles under microwave irradiation proceeded diastereoselectively to give β-N-glycoaminoacids 3a-d, (3c+3c′)

Full text of DOI:10.1021/jo7018637

Regioselective Syntheses of â-N-Linked Glycoaminoacids and
Glycopeptides
Alan R. Katritzky,* Tamari Narindoshvili, Bogdan Draghici, and Parul Angrish
Center for Heterocyclic Compounds, Department of Chemistry, UniVersity of Florida,
GainesVille, Florida 32611-7200
katritzky@chem.ufl.edu
ReceiVed August 24, 2007
Acylation of tetra-O-pivaloyl-â-D-galactopyranosylamine 2 by readily available N- (Cbz or Fmoc-R-
aminoacyl) benzotriazoles under microwave irradiation proceeded diastereoselectively to give â-N-
glycoaminoacids 3a-d, (3c+3c′) (compound numbers written within brackets represent a racemate or a
diastereomeric mixture; compound numbers without brackets represent a single enantiomer) (83-92%),
and glycosylated asparagine building block 9 (65%). N-Cbz-Protected peptidoylbenzotriazoles 4a-c
similarly afforded â-N-glycodipeptides 5a-c (76-81%). Regiospecific â-N-linkage formation was
established by 1D and 2D NMR techniques for 3b.
Introduction
The development of novel building blocks opens new
possibilities for organic synthesis. Synthetic N- and O-linked
glycopeptides are analogous to those found in naturally occur-
ring carbohydrates linked glycosidically to an R-amino acid of
a peptide or a protein. In natural N-linked glycoproteins,
oligosaccharides are often covalently attached to asparagine side
chain via a â-N-glycosidic linkage,1-3 while O-glycopeptides
and proteins are usually attached anomerically to a serine or
threonine side chain with an R-O-glycosidic linkage1,3,4 (Figure
1).
FIGURE 1. R-O-Glycosidic and â-N-glycosidic linkages between
carbohydrate and peptides.
The isomer diversity of carbohydrate chains and the roles of
nucleic acids and proteins in recording biological information
have made glycopeptides and glycoconjugates therapeutic targets
and models for biologically relevant systems.5-7 Glycopeptides
are important in biochemical processes ranging from cell growth
regulation, immune response, binding of pathogens to intercel-
lular communication, intercellular targeting, cancer cell me-
tastasis, and inflammation.8-11 Carbohydrate residues are crucial
in receptor recognition; antigens are recognized by the immune
system in normal tissue,12 and the isolation and careful structural
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10.1021/jo7018637 CCC: $40.75 © 2008 American Chemical Society
Published on Web 12/19/2007
J. Org. Chem. 2008, 73, 511-516
511

Katritzky et al.
identification of specific carbohydrate antigens overexpressed
in cancer cells has inspired carbohydrate-based tumor immu-
notherapy.13
the presence of N-ethylmorpholine gave a glycosylated aspar-
agine building block in 76% yield.27 However, in solution, NR-
Fmoc-Asp(Cl)-OPft is suitable only for coupling with protected
glycosylamines,27,28 which possess sufficient solubility in dry
organic solvents. In solid phase methodology, NR-Fmoc-Asp-
(Cl)-OPft was utilized for the preparation of unprotected
N-glycoaminoacid building blocks.20 Additional routes utilized
were DCC/HOBt,14 isobutyl chloroformate (mixed anhydride
method),14 and EEDQ.22 Recently, one-bead-one-compound
(OBOC) technology pioneered by Lam et al.30 was applied to
the synthesis of glycopeptide libraries.31-33
However, the above methodologies can result in anomeriza-
tion leading to partial, or even predominant, formation of the
R-anomeric structures.14,22,23 The coupling of protected glycosyl
amines to Asp-containing peptides may be complicated by
competing intramolecular succinimide formation,25,26 resulting
in low yields (25-53%),14,25 and sometimes requiring reaction
times of 7-24 h.22,29 This has encouraged the search for better
methodologies for the syntheses of â-N-glycoaminoacids and
â-N-glycopeptides.
â-Anomers of the glycoaminoacids are sought since the â-N-
glycosyl linkage is biochemically more common than the R-N-
linkage;33 thus N-asparagine glycoproteins and glycopeptides
are â-N-linked to an N-acetyl glucosamine segment of a
chitobiose, as part of a high mannose-containing antennary
structure.2
Carbohydrates are valuable as enantiomerically pure starting
materials in chiral pool syntheses of many chiral natural products
and drugs.34 The polyfunctionality of carbohydrates is useful
for binding or coordinating a substrate. Carbohydrate derivatives
are efficient auxiliaries for stereodifferentiation in many ste-
reoselective chiral syntheses.35-42
The sugar moieties of glycopeptides define their biological
activity, and thus the potential of carbohydrates for the develop-
ment of new drugs is being explored. Linking peptides to
carbohydrates can improve activity and selectivity.6,14 Protein
glycosylation affects pharmacological parameters including the
rate of circulation,15 solubility,16 proteolytic stability,17 and
immunogenicity.18 Surfactants combining optically pure, readily
available carbohydrates with proteins of agricultural origin are
environmentally advantageous. Hydrophilic sugar amino acids
linked to lipophilic long chains comprise novel trimodular
surfactants.19 Such potential of glycoaminoacids and glycopep-
tides has led to the development of chemoenzymatic, convergent,
and building block synthetic approaches.2,14,20-33
Considerable efforts have been devoted to the synthesis of
N-glycoaminoacids and N-glycopeptides utilizing solution and
solid phase20-22 methodologies by forming N-glycosidic linkages
between protected or unprotected glycosylamine and suitably
protected and activated amino acid (often aspartic acid) units.
Methodologies applicable for both protected and unprotected
glycosylamines include (i) coupling with the 3,4-dihydro-4-oxo-
1,2,3-benzotriazin-3-yl (Dhbt)sside chain activated aspartic acid
in the presence of DCCI21 or DIPEA,21,23 and (ii) utilization of
uronium or phosphonium salts: HBTU/DIEA,22 HBTU/
HOBt,24,25 BOP/HOBt,25 BOP/DIPEA;29 however, complications
in coupling may arise due to competitive ring closures to form
succinimides.25,26 To minimize undesired side reactions, fre-
quently encountered in the use of in situ coupling reagents, the
dually activated asparagine derivative NR-Fmoc-Asp(Cl)-OPft
was employed:27,28 the coupling step in dry solvent (THF) in
We have used N-acylbenzotriazoles extensively for N-
acylation of amines43 and amides,43,44 C-acylation,45 and O-
acylation.46 Chiral di-, tri-, and tetrapeptides were prepared using
(R-Boc-, -Cbz-, and -Fmoc-aminoacyl)benzotriazoles in aqueous
acetonitrile in good to high yields from unprotected amino acids,
both without (L-Ala, L-Phe, L-Val, L-Leu) and with unprotected
(L-Trp, L-Tyr, L-Gln, L-Ser, L-Cys, L-Asn) and protected (Nω-
Cbz-L-Lys) side chain functionality.47 The original chirality was
preserved in all cases (>95% as evidenced by NMR and >99%
by HPLC).
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(13) Hakomori, S.-I.; Zhang, Y. Chem. Biol. 1997, 4, 97-104.
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(21) Meinjohanns, E.; Meldal, M.; Paulsen, H.; Dwek, R. A.; Bock, K.
J. Chem. Soc., Perkin Trans. 1 1998, 549-560.
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8210-8213.
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Trans. 1 2002, 1042-1049.
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2002, xi, 39-44.
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W. M.; Knapp, R. J. Nature 1991, 354, 82-84.
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3682. (b) Katritzky, A. R.; Abdel-Fattah, A. A. A.; Wang, M. Y. J. Org.
Chem. 2003, 68, 1443-1446. (c) Katritzky, A. R.; Suzuki, K.; Singh, S.
K.; He, H. Y. J. Org. Chem. 2003, 68, 5720-5723.
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2004, 77, 175-178. (b) Katritzky, A. R.; Angrish, P.; Narindoshvili, T.
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512 J. Org. Chem., Vol. 73, No. 2, 2008

â-Linked Glycoaminoacids and Glycopeptides
SCHEME 1
TABLE 1. Preparation of â-N-Linked Glycoaminoacids 3a-d,
(3b+3b′), from Tetra-O-pivaloyl-â-D-galactopyranosylamine 2 and
N-(Cbz-r-aminoacyl)benzotriazoles 1a-d, (1a+1b′)
Results and Discussion
Synthesis of â-N-Linked Glycoaminoacids 3a-d, (3b+3b′).
â-N-Linked glycoaminoacids 3a-d, (3b+3b′), were prepared
by coupling readily available47 N-(Z-R-aminoacyl)benzotriazoles
1a-d, (1b+1b′), with 2,3,4,6-tetra-O-pivaloyl-â-D-galactopy-
ranosylamine35 2, a versatile chiral auxiliary utilized for regio-
and stereoselective syntheses.35-42
Optimally for stereoselective glycosylation, the starting
materials 1 and 2 were irradiated at 100 W at 60 °C for 75 min
in dry dichloromethane in the presence of 1 equiv of DMAP
(Scheme 1 and Table 1). Slow reaction is attributed to the bulky
pivaloyl protecting groups. After base work up, column chro-
matography with ethyl acetate/hexane (1:2) gave glycoami-
noacids 3a-d, (3b+3b′) (82-92%). Pivaloyl groups can be
removed without epimerization.48
amino
acid
product
yielda
(%)
R.T.b
(min)
Gly
1a
L-Phe
1b
L-Trp
1c
L-Met
1d
DL-Phe
(1b+1b′)
Cbz-Gly-N-galactopyranose
3a
Cbz-L-Phe-N-galactopyranose
3b
Cbz-L-Trp-N-galactopyranose
3c
Cbz-L-Met-N-galactopyranose
3d
87
92
82
83
87
2.81
2.78
2.87
2.81
Cbz-DL-Phe-N-galactopyranose
(3b+3b′)
2.80, 3.25
a Isolated yield. b For HPLC conditions, refer to the Experimental Section.
Diastereospecific formation of â-N-linked building blocks and
absence of anomerization was investigated by 1D and 2D NMR
analysis (Figures 2-5). Conventional room temperature 1H
NMR spectra (CDCl3) for 3a-d, (3b+3b′), indicated the
existence of two species. For example, the proton at amino acid
Herein, we demonstrate the efficient and exclusive formation
of â-N-linked glycoaminoacids and glycodipeptides (65-92%)
by linking a protected amino sugar moiety to an amino acid or
peptide under microwave irradiation.
FIGURE 2. Variable temperature experiment for 3b in DMSO-d6.
J. Org. Chem, Vol. 73, No. 2, 2008 513

Katritzky et al.
FIGURE 3. Selective decoupling experiment for 3b in CDCl3 at 20 °C.
FIGURE 4. 1H-1H COSY experiment of 3b in CDCl3 at 20 °C.
assumption that the complex peaks obtained at 20 °C were due
to restricted rotation at the amide bonds. The two rotameric
forms underwent coalescence at 55-65 °C; thus the multiplet
(47) (a) Katritzky, A. R.; Suzuki, K.; Singh, S. K. Synthesis 2004, 2645-
2652. (b) Katritzky, A. R.; Angrish, P.; Hur, D.; Suzuki, K. Synthesis 2005,
397-402. (c) Katritzky, A. R.; Angrish, P.; Suzuki, K. Synthesis 2006,
411-424. (d) Katritzky, A. R.; Todadze, E.; Cusido, J.; Angrish, P.;
Shestopalov, A. A. Chem. Biol. Drug Des. 2006, 68, 37-41. (e) Katritzky,
A. R.; Meher, G.; Angrish, P. Chem. Biol. Drug Des. 2006, 68, 326-333.
(f) Katritzky, A. R.; Mohapatra, P.; Fedoseyenko, D.; Duncton, M.; Steel,
P. J. J. Org. Chem. 2007, 71, 4268-4271.
FIGURE 5. Chemical shifts of 3b for major (no brackets) and minor
rotamers (in brackets).
(48) (a) Plante, O. J.; Palmacci, E. R.; Seeberger, P. H. Science 2001,
291, 1523-1527. (b) Danishefsky, S. J.; McClure, K. F.; Randolph, J. T.;
Ruggeri, R. B. Science 1993, 260, 1307-1309. (c) Denmark, S. E.; Regens,
C. S.; Kobayashi, T. J. Am. Chem. Soc. 2007, 129, 2774-2776. (d) Ora,
M.; Maki, E.; Poijarvi, P.; Neuvonen, K.; Oivanen, M.; Lonnberg, H. J.
Chem. Soc., Perkin Trans. 2 2001, 881-885. (e) Kinugawa, Y.; Kawashima,
E. Nucleic Acids Res. 2002, 2, 19-20. (f) Boeckel, C. A. A.; Boom, J. H.
Tetrahedron Lett. 1979, 37, 3561-3564.
chiral center C*H is displayed for each compound as two sets
of quartets with different intensity, which we believe are due
to two rotamer species with one conformation preferred.
Variable temperature proton spectra (Figure 2) for 3b (inves-
tigated in DMSO-d6 for better resolution of signals) over the
range of 25-115 °C (using 10 °C increments) supported our
514 J. Org. Chem., Vol. 73, No. 2, 2008

â-Linked Glycoaminoacids and Glycopeptides
TABLE 2. Preparation of â-N-Linked Glycodipeptides 5a-c from
Tetra-O-pivaloyl-â-D-galactopyranosylamine 2 and
N-Cbz-dipeptidoylbenzotriazoles 4a-c
4
product
yielda
(%)
R1
R2
L-Phe
4a
L-Met
4b
L-Met
4c
L-Met
4a
L-Ala
4b
L-Trp
4c
Cbz-L-Phe-L-Met-N-galactopyranose
5a
Cbz-L-Met-L-Ala-N-galactopyranose
5b
Cbz-L-Met-L-Trp-N-galactopyranose
5c
76
81
77
a Isolated yield.
FIGURE 6. Fragment of natural â-N-linked asparagine glycoproteins
and glycopeptides.
SCHEME 2
SCHEME 3. Preparation of â-N-Linked Piv-Protected
Galactopyranosylamine Asparagine Building Block 9
at 2.64-2.89 ppm coalesced at 65 °C, and the saccharide signals
at 3.90-4.42 and 5.26-5.46 ppm coalesced at 55 °C; these
changes were reversible in heating-cooling sequences. The high
temperature spectra (Figure 2) thus demonstrated the absence
of anomerization.
1
Room temperature H NMR spectra of 3a-d, (3b+3b′),
showed that each H1 (anomeric proton) signal from the sugar
residue is overlapped by other proton signals resulting in a
multiplet at 5.10-5.40 ppm. Therefore, the stereochemistry of
the anomeric proton and thus the orientation of the C1-NH
glycosidic bond was investigated by selective decoupling
experiment and 2D homonuclear correlation COSY for 3b
(Figures 3 and 4).
Selective decoupling of 3b (in CDCl3) was achieved by
irradiation at the appropriate frequency to eliminate coupling
of the anomeric proton with the C1NH proton (displayed
as doublet at 6.48 ppm; Figure 3). The NH proton in 3b
was selectively decoupled to reveal the coupling constant
H1-H2 to be 9.7 Hz, which indicates that H1 and H2 are both
axial.
The â-N-linked stereochemistry of glycoaminoacid 3b was
confirmed by the homonuclear correlations, COSY (Figure 4).
The R-anomeric proton of sugar displays a triplet with J ) 9.5
Hz, interpreted as equal coupling with the C1NH proton and
the adjacent H2 axial proton.
This experiment also confirmed the existence of the rotamers
postulated above for 3b. For the major rotamer, coupling occurs
between the proton CH(a) at 4.30 ppm, the NH(b) at 7.47 ppm,
and the prochiral protons (c,c′) at 2.89 and 2.68 ppm. For the
minor rotamer, the corresponding proton at 4.32 ppm coupled
with the NH at 7.55 ppm and the prochiral protons at 1.87 and
1.84 ppm (Figure 5).
to afford â-N-linked glycodipeptides 5a-c (Table 2) under
conditions similar to those adopted for the preparation of
glycoaminoacids 3a-d, except that complete reaction needed
3.5 h. Base work up (sat. sol. of Na2CO3) followed by silica
gel chromatography using ethyl acetate/hexane (1:1) as eluent
gave glycodipeptides 5a-c (76-81%) (Scheme 2).
Synthesis of â-N-Linked Piv-Protected Galactopyranosy-
lamine Asparagine Building Block 9 Using Side Chain
Functionality in Asparagines. The N-glycoproteins character-
ized by a â-N-glycosidic linkage between N-acetylglucosamine
and asparagine are especially important in cell biology. Previous
syntheses of conjugates of type 6 (Figure 6) have been
complicated by isomer formation and side reactions.42
The construction of the â-N-glycosidic linkage for asparagine
glycopeptides by the present method proceeds through coupling
of amino sugar 2 to the benzotriazole (Bt)-activated side chain
of Fmoc-Asp (Bt)-OtBu 8. Our approach to 9 is selective for
the â-anomer without side reactions. Fmoc-L-Asp(OH)-R-O-
tert-Bu ester 7 was converted into 8 in 87% yield. The anomeric
amino sugar tetra-O-pivaloyl-â-D-galactopyranosylamine 2 was
acylated by 8 under the conditions described above (in
microwave, 1 h) to afford the â-N-linked, fully protected,
asparagine building block 9, obtained in 65% yield after
chromatographic purification (Scheme 3).
The detailed NMR study performed for Z-L-Phe-â-N-galac-
topyranose 3b establishes complete retention of original ster-
eochemistry of the anomeric (R) proton from 2 and therefore
exclusive formation of â-N-linked glycoaminoacids under the
conditions utilized in this method.
Conclusion
Synthesis of â-N-Linked Glycodipeptides 5a-c. N-Cbz-
Dipeptidoylbenzotriazoles 4a-c (obtained as reported previ-
ously47) acylated tetra-O-pivaloyl-â-D-galactopyranosylamine 2
The present coupling of N-(R-protected aminoacyl)benzot-
riazoles and N-Cbz-protected dipeptidoylbenzotriazoles with
J. Org. Chem, Vol. 73, No. 2, 2008 515

Katritzky et al.
galactopyranose, 5a: Colorless solid (76%), mp 106-107 °C,
2,3,4,6-tetra-O-pivaloyl-â-D-galactopyranosylamine 2 provides
selectively the â-anomers of â-N-linked glycoaminoacids and
glycopeptides, with the purity and yield significantly increased
over previous approaches. Stereochemistry was confirmed by
NMR study including 1H-1H COSY, selective decoupling, and
variable temperature experiments. The protocol described should
be suitable for the synthesis of other â-N-linked glycopeptides.
1
[R]23 ) +11.0 (c 1.90, CH2Cl2); H NMR (CDCl3) δ 1.05-1.12
D
(m, 11H), 1.12-1.19 (m, 15H), 1.22 (s, 6H), 1.26 (s, 4H), 1.60-
1.90 (m, 3H), 2.00 (s, 2H), 2.12-2.24 (m, 1H), 2.32-2.44 (m,
1H), 3.00-3.19 (m, 2H), 3.96 (dt, J ) 9.9, 6.3 Hz, 1H), 4.01-
4.21 (m, 2H), 4.32 (q, J ) 7.2 Hz, 0.6H), 4.44 (q, J ) 6.4 Hz,
0.4H), 4.49-4.60 (m, 1H), 5.01-5.10 (m, 1H), 5.10-5.20 (m, 2H),
5.20-5.31 (m, 2H), 5.37-5.48 (m, 2H), 6.45 (d, J ) 8.5 Hz, 0.6H),
6.70 (d, J ) 7.4 Hz, 0.4H), 7.10 (d, J ) 9.1 Hz, 0.4H), 7.14-7.38
(m, 10.6H); 13C NMR (CDCl3) δ 15.1, 15.1, 26.9, 27.0, 27.1, 29.8,
29.9, 30.8, 31.0, 37.9, 38.7, 38.8, 39.0, 39.0, 52.1, 57.1, 60.6, 60.6,
66.6, 67.3, 67.9, 70.9, 71.1, 72.6, 72.7, 78.4, 78.5, 127.2, 128.1,
128.2, 128.2, 128.3, 128.5, 128.5, 128.8, 129.1, 129.2, 135.8, 135.9,
136.2, 156.2, 170.9, 171.0, 171.3, 176.7, 177.0, 177.8, 178.2. Anal.
Calcd for C48H69N3O13S: C, 62.11; H, 7.51; N, 4.53. Found: C,
62.20; H, 7.64; N, 4.43.
Experimental Section
General Procedure for the Preparation of Glycoaminoacids
3a-d, (3b+3b′), under Microwave Irradiation. A dried heavy-
walled Pyrex tube containing a small stir bar was charged with
N-(Z-R-aminoacyl)benzotriazole 1 (1.0 mmol), 2,3,4,6-tetra-O-
pivaloyl-â-D-galactopyranosylamine 2 (1 mmol), DMAP (1 mmol),
and CH2Cl2 (1 mL). The reaction mixture was exposed to
microwave irradiation (100 W) for 75 min at a temperature of
60 °C. After the irradiation, the reaction mixture was allowed to
cool through an inbuilt system in the instrument until the temper-
ature had fallen below 30 °C. The reaction mixture obtained was
diluted with EtOAc (20 mL), and the organic layer was washed
with sat. Na2CO3 solution (3 × 15 mL), sat. NaCl solution (20
mL), and dried over MgSO4. After evaporation of the solvent, the
residue was subjected to silica gel column chromatography using
EtOAc/hexanes (1:2) as an eluent, affording the desired glycoami-
noacids 3a-d, (3b+3b′).
NR-(Fluoren-9-ylmethoxycarbonyl)-Nδ-(2,3,4,6-tetra-O-pivaloyl-
â-D-galactopyranosyl)-L-asparagine, 9: A dried heavy-walled
Pyrex tube containing a small stir bar was charged with (S)-4-
benzotriazol-1-yl-2-(9H-fluoren-9-ylmethoxycarbonylamino)-4-ox-
obutyric acid tert-butyl ester (0.3 mmol) 8, 2,3,4,6-tetra-O-pivaloyl-
â-D-galactopyranosylamine 2 (0.2 mmol), DMAP (0.05 mmol), and
CH2Cl2 (1 mL). The reaction mixture was exposed to microwave
irradiation (100 W) for 1 h at a temperature of 60 °C. After
irradiation, the reaction mixture was allowed to cool down through
an inbuilt system in the instrument until the temperature had fallen
below 30 °C. The reaction mixture was diluted with EtOAc (20
mL), and the solution was washed with sat. Na2CO3 solution (3 ×
15 mL), sat. NaCl solution (1 × 20 mL), and dried over MgSO4.
After evaporation of solvent, the residue was subjected to silica
gel column chromatography using EtOAc/hexanes (1:6) as an eluent
NR-Carbobenzyloxy-Nδ-(2,3,4,6-tetra-O-pivaloyl-â-D-galacto-
pyranosyl)glycine, Cbz-Gly-N-galactopyranose, 3a: Colorless
1
solid (87%), [R]23 ) +13.5 (c 2.03, CH2Cl2); mp 76-77 °C; H
D
NMR (CDCl3) δ 1.08-1.1.14 (m, 17H), 1.17 (s, 9H), 1.27 (s, 10H),
3.82 (dd, J ) 17.2, 5.7 Hz, 1H), 3.91 (d, J ) 5.8 Hz, 1H), 3.94-
4.05 (m, 1H), 4.06-4.17 (m, 2H), 5.06-5.23 (m, 1H), 5.15 (s,
2H), 5.23-5.36 (m, 3H), 5.47 (d, J ) 3.2 Hz, 1H), 6.78 (d, J )
8.5 Hz, 1H), 7.30-7.42 (m, 5H); 13C NMR (CDCl3) δ 26.9, 27.0,
27.0, 27.2, 38.7, 38.7, 39.0, 39.0, 44.4, 60.7, 66.6, 67.4, 68.1, 70.7,
72.8, 78.5, 128.2, 128.3, 128.6, 135.9, 156.2, 169.2, 176.7, 176.9,
177.8, 178.7. Anal. Calcd for C36H54N2O12: C, 61.17; H, 7.72; N,
3.96. Found: C, 61.03; H, 7.87; N, 4.06.
to afford the 9 in 65% yield: colorless solid, mp 86-87 °C, [R]23
D
) +5.4 (c 1.88, CH2Cl2); 1H NMR (CDCl3) δ 1.09-1.15 (m, 9H),
1.15-1.21 (m, 18H), 1.23-1.29 (m, 9H), 1.42-1.48 (m, 9H),
2.48-2.64 (m, 1H), 2.90-3.05 (m, 1H), 3.92-4.22 (m, 3H), 4.23-
4.40 (m, 1H), 4.40-4.47 (m, 1H), 4.48-4.60 (m, 1H), 5.00-5.32
(m, 3H), 5.43 (d, J ) 3.0 Hz, 0.1H), 5.46 (d, J ) 3.0 Hz, 0.9H),
5.94 (t, J ) 9.1 Hz, 1H), 7.28-7.46 (m, 5H), 7.59 (d, J ) 7.3 Hz,
1H), 7.62-7.70 (m, 1H), 7.77 (d, J ) 7.6 Hz, 2H); 13C NMR
(CDCl3) δ 26.8, 26.9, 27.0, 27.1, 27.9, 36.5, 38.6, 38.7, 38.7, 38.9,
39.0, 47.0, 47.1, 51.2, 60.3, 60.6, 61.1, 66.6, 67.0, 67.4, 67.5, 67.9,
68.3, 69.7, 70.8, 71.2, 71.5, 72.4, 72.6, 78.6, 78.8, 81.8, 82.1, 85.4,
120.0, 125.0, 125.1, 127.0, 127.7, 141.2, 143.6, 143.7, 156.1, 171.0,
171.1, 176.6, 176.7, 176.8, 176.9, 177.2, 177.7, 177.8, 178.5. Anal.
Calcd for C49H68N2O14: C, 64.74; H, 7.54; N, 3.08. Found: C,
64.37; H, 7.82; N, 2.92.
General Procedure for the Preparation of Glycodipeptides
5a-c under Microwave Irradiation. A dried heavy-walled Pyrex
tube containing a small stir bar was charged with NR-Cbz-protected
dipeptidoylbenzotriazoles 4 (1.0 mmol), 2,3,4,6-tetra-O-pivaloyl-
â-D-galactopyranosylamine 2 (1 mmol), DMAP (1 mmol), and CH2-
Cl2 (0.5 mL). The reaction mixture was exposed to microwave
irradiation (100 W) for 3 h and 30 min at a temperature of 60 °C.
After irradiation, the reaction mixture was allowed to cool down
through an inbuilt system in the instrument until the temperature
had fallen below 30 °C. The reaction mixture was diluted with
EtOAc (20 mL), and the solution was washed with sat. Na2CO3
solution (3 × 15 mL), sat. NaCl solution (20 mL), and dried over
MgSO4. After the evaporation of the solvent, the residue was
subjected to silica gel column chromatography using EtOAc/
hexanes (1:1) as eluent, affording the desired glycodipeptides
5a-c.
Acknowledgment. We thank Dr. C. Denis Hall and Dr. Ion
Ghiviriga for helpful suggestions.
Supporting Information Available: Compound characterization
data for 3a-d, (3b+3b′), 4c, 5a-c, 8, and 9 are available. This
material is available free of charge via the Internet at http:
//pubs.acs.org.
NR-Carbobenzyloxy-Nδ-(2,3,4,6-tetra-O-pivaloyl-â-D-galacto-
pyranosyl)-L-phenylalanine-L-methionine, Cbz-L-Phe-L-Met-N-
JO7018637
516 J. Org. Chem., Vol. 73, No. 2, 2008