Synthesis of ureido-linked glycosylated amino acids from N α-Fmoc-Asp/Glu-5-oxazolidinones and their application to neoglycopeptide synthesis

Article abstract of DOI:10.1080/00397910802213711

A simple route for the synthesis of ureido-linked glycosylated amino acids has been described. The key step involves the reaction of isocyanates derived from Nα-Fmoc-Asp/Glu-5-oxazolidinones 1 with glycosyl amines followed by hydrolysis. The resulting ureido-linked glycosylated amino acids have been incorporated into peptides. The overall procedure is simple, high-yielding, and involves fewer steps. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397910802213711

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Synthesis of Ureido-Linked
Glycosylated Amino Acids
from N α-Fmoc-Asp/Glu-5-
oxazolidinones and Their
Application to Neoglycopeptide
Synthesis
Vommina V. Sureshbabu a , Rao Venkataramanarao a
, Shankar A. Naik a & N. Narendra a
a Department of Studies in Chemistry, Central
College Campus , Bangalore University , Bangalore,
India
Published online: 14 Oct 2008.
To cite this article: Vommina V. Sureshbabu , Rao Venkataramanarao , Shankar
A. Naik & N. Narendra (2008) Synthesis of Ureido-Linked Glycosylated Amino
Acids from N α-Fmoc-Asp/Glu-5-oxazolidinones and Their Application to
Neoglycopeptide Synthesis, Synthetic Communications: An International Journal
for Rapid Communication of Synthetic Organic Chemistry, 38:21, 3640-3654, DOI:
10.1080/00397910802213711
To link to this article: http://dx.doi.org/10.1080/00397910802213711
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Synthetic Communications1, 38: 3640–3654, 2008
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910802213711
Synthesis of Ureido-Linked Glycosylated Amino
Acids from Na-Fmoc-Asp/Glu-5-oxazolidinones
and Their Application to Neoglycopeptide
Synthesis
Vommina V. Sureshbabu, Rao Venkataramanarao, Shankar A. Naik,
and N. Narendra
Department of Studies in Chemistry, Central College Campus, Bangalore
University, Bangalore, India
Abstract: A simple route for the synthesis of ureido-linked glycosylated amino
acids has been described. The key step involves the reaction of isocyanates derived
from Na-Fmoc-Asp=Glu-5-oxazolidinones 1 with glycosyl amines followed by
hydrolysis. The resulting ureido-linked glycosylated amino acids have been incor-
porated into peptides. The overall procedure is simple, high-yielding, and involves
fewer steps.
Keywords: Curtius rearrangement,
neoglycopeptides, ureido-linked glcosylated amino acids
Na-Fmoc-Asp=Glu-5-oxazolidinones,
INTRODUCTION
Glycopeptides play decisive roles in biomolecular functions such as cell–
cell adhesion, recognition and signaling.[1,2] The quantitative synthesis of
natural glycopeptides is challenging because of the complex nature of
native samples, extensive branching, exhaustive protection and deprotec-
tion steps, and difficulties involved in carbohydrate coupling reactions.[3]
Hence, synthesizing small molecules of glycopeptide mimics as substitutes
Received February 4, 2008.
Address correspondence to Vommina V. Sureshbabu, Department of Studies
in Chemistry, Central College Campus, Bangalore University, Dr. B. R.
Ambedkar Veedhi, Bangalore 560 001, India. E-mail: hariccb@rediffmail.com
3640

Ureido-Linked Glycosylated Amino Acids
3641
for the native compounds in biological studies has been emphasized.
Synthesis of urea-linked glycopeptides has gained importance because
of the biological applications of this class of neoglycopeptides. Therefore,
we turned our attention toward development of an efficient synthesis of
ureido-linked glycosylated amino acids.
Ichikawa et al. reported the synthesis of urea-linked neoglycopep-
tides by coupling 1-sugar isocyanates with protected 1,3-diamino propio-
nic acid[4] (Fig. 1a). Their method involves in situ generation of sugar
isocyanates through a multistep procedure and usage of diamino propio-
nic acid, which is expensive or prepared through another lengthy proto-
col.[5] On the other hand, Burger et al. synthesized these compounds
through the reaction of isocyanates of hexafluoro acetone (HFA)–
protected a-hydroxy acids with sugar amines.[6] This protocol is useful
particularly for Na-methyl amino acids,[7] a-hydroxy acids,[8] and a-
mercapto acids.[9] However, it cannot be extended to obtain isocyanates
of HFA-protected activated a-amino acids because of intramolecular
trapping of the generated x isocyanates by the ꢀNH of the oxazolidinone
ring.[10] Hence while following the approach as in Fig. 1 (b and c), a pro-
tocol devoid of internal urea formation is required. Thus, we envisaged
an alternate route of generating an isocyanate moiety from N-Fmoc
5-oxazolidinones of aspartic and glutamic acids and coupling it to a sugar
amine (Fig. 1, present approach). This approach is advantageous as it
Figure 1. Protocols for ureido-linked glycosylated amino acid synthesis:
(a) Ichikawa’s approach, (b) Burger’s approach, (c) present approach.

3642
V. V. Sureshbabu et al.
Scheme 1. Synthesis of Na-Fmoc-Asp=Glu-5-oxazolidinone isocyanate: (i) Fmoc-
Asp=Glu-oxazolidinone, paraformaldehyde, PTSA, MW; (ii) N-methylmorpho-
line (NMM), isobutylchloroformate, aq. NaN3; and (iii) MW, 2 min.
involves isocyanates of Fmoc-Asp=Glu-derived 5-oxazolidinones and
sugar amines whose preparation is simple, inexpensive, and high-yielding
and involves fewer steps.
RESULTS AND DISCUSSION
Na-Fmoc-Asp=Glu-5-oxazolidinones were prepared in high yields follow-
ing the reported protocols.[11,12] The yields of these oxazolidinones
obtained after column purification were higher by 20% than the corre-
sponding Boc counterparts, thus adding to the advantage of the Fmoc
protection strategy. They were then converted to the corresponding acyl
azides by the reaction of mixed anhydride with sodium azide and subse-
quently to isocyanates via Curtius rearrangement upon heating or expo-
sure to microwave irradiation for 1–2 min (Scheme 1). The resulting
isocyanates 1 were isolated as solid powders and fully characterized. 1-
Amino-2,3,4,6-O-protected-b-D-sugars
2
were prepared following
reported protocols.[13] In all these cases, the b anomers were obtained in
an anomeric excess of 98:2, which was evident from their NMR spectra.
The isocyanates 1 were then reacted with 2 in presence of
diisopropylethyl amine (DIEA) to obtain the ureido-linked glycosylated
amino acid-5-oxazolidinones 3 in good yields. Finally, ureido bonds con-
taining glycosylated amino acids (Scheme 2) were obtained through
LiOH-mediated hydrolysis. The compounds 3 were treated with aqueous
1N LiOH solution in THF for 20 min to give corresponding Na-Fmoc-
protected glycosylated amino acids 4 in 70–90% yields (Table 1).
It is noteworthy that the conditions employed selectively hydrolyzed
the oxazolidinone ring without any deprotection of either the Fmoc
group of amino acid residue or the acetyl or the benzoyl group of the
sugar component.
Although the isocyanates 1 made are stable enough at 0 ꢁC for a rea-
sonable period of time, it is desirous to have shelf-stable compounds
as key intermediates. Thus the isocyanate 1 was converted to active

Ureido-Linked Glycosylated Amino Acids
3643
Scheme 2. Synthesis of ureido-linked glycosylated amino acids: (i) reagents 1, 2,
N-methylmorpholine, THF; (ii) 1N LiOH, THF. n ¼ 1 or 2; R ¼ Ac, Bz.
pentafluorophenyl carbamates 5 by reacting with pentafluorophenol in
the presence of NMM (Scheme 3). These precursoric carbamates retain
the reactivity of isocyanates but show greater stability. The carbamates
were found to be stable even up to several months or a year and hence
could be stored with extended shelf life. On reaction of 5 with 2, the
ureido-linked glycopeptides were readily obtained. The side product pen-
tafluorophenol can be removed completely during crystallization using
ethyl acetate–hexane (2:8). Further, the insertion of ureido-linked glyco-
sylated amino acids into the peptide sequences was done in a straightfor-
ward way. Direct coupling of the compounds 4 with amino acid methyl
esters using N,N-dicyclohexyl carbodiimide-N-hydroxy benzotriazole
(DCC-HOBt) as peptide coupling agent yielded dipeptidic neoglycope-
tides 6 (Scheme 4). The yields in all cases were satisfactory.
For completeness, we undertook the synthesis of the model neoglyco-
peptide 8 by the solid-phase method. The synthesis was accomplished
using commercial Fmoc-Val-Wang resin, and stepwise assembly of the
peptide was carried out. Thus, the Fmoc-protected urea bond containing
glycosylated aspartic acid was incorporated using HBTU=HOBt in the
presence of DIEA, and the peptide was cleaved from the acyl resin using
TFA containing phenol=water=thioanisole=1,2-ethanedithiol. The final
peptide 8 (Fig. 2) was obtained in 98% purity with 75% yield and was
1
characterized using H-NMR, 13C NMR, and MALDI mass spectra.
CONCLUSION
In conclusion, we have developed a simple and rapid approach for the
synthesis of ureido-linked glycosylated amino acids employing isocya-
nates of Na-Fmoc-Asp=Glu-5-oxazolidinones as key and stable
intermediates. The sugar amines were prepared using a common
literature procedure and coupled to the isocyanates to obtain the target
ureas. In parallel, the isocyanates were converted into stable active

3644
V. V. Sureshbabu et al.
Table 1. Synthesis of urea-linked glycosylated amino acids
Mp Yield Mass [M þ Na]
Entry
1
Product
(ꢁC) (%)
123 82
(calcd.)
722.2173a
(722.2120)
2
115 83
722.2173a
(722.2132)
3
127 86
914.4b
(914.1)
4
113 90
914.4b
(914.2)
5
105 71
1175.3b
(1175.8)
(Continued )

Ureido-Linked Glycosylated Amino Acids
3645
Table 1. Continued
Mp Yield Mass [M þ Na]
Entry
6
Product
(ꢁC) (%)
(calcd.)
99
79
736.2336a
(736.2308)
7
8
135 84
736.2336a
(713.2310)
102 87
928.4b
(928.1)
9
129 89
928.4b
(928.2)
10
107 72
1189.3b
(1189.1)
aHRMS.
bES-MS.

3646
V. V. Sureshbabu et al.
Scheme 3. Synthesis of Na-Fmoc-Asp=Glu-5-oxazolidinone pentafluorophenyl
carbamate 5: (i) 1, pentafluorophenol, N-methylmorpholine, THF. n ¼ 1 or 2.
pentafluorophenyl carbamates and were employed to generate the same
ureido-linked glycosylated amino acids. The glycosylated oxazolidinones
were then hydrolyzed to free acids. The overall procedure is efficient as it
involves fewer steps, is simple to execute, and results in good yields of
products. The utility of such ureido-linked glycosylated amino acids to
synthesize neoglycopeptides both by solid phase and in solution has also
been demonstrated.
EXPERIMENTAL
All solvents were freshly distilled before use. Amino acids were used as
received from Sigma-Aldrich Company. Melting points were determined
on a Buchi model 150 melting-point apparatus in open capillaries and are
uncorrected. IR spectra were recorded on a Nicolet model impact 400D
1
FT-IR spectrometer (KBr pellets, 3 cmꢀ1 resolution). H NMR spectra
were recorded on a Bruker AMX 400-MHz spectrometer. Mass spectra
were recorded on MALDI-TOF (Kratos) mass spectrometer. Unless or
otherwise mentioned, all amino acids used have the L-configuration.
Thin-layer chromatography (TLC) was carried out using the precoated
silica-gel G254 plates.
Synthesis of Na-Fmoc-Asp/Glu-oxazolidinone Isocyanate 1:
Typical Procedure
To a solution of Na-Fmoc-Asp=Glu-oxazolidinone (1 mmol) in 10 mL of
dry THF, N-methylmorpholine (1.1 mmol) and isobutylchloroformate
(1.1 mmol) were added and stirred for 5 min at 0 ꢁC in an ice–salt bath.

Ureido-Linked Glycosylated Amino Acids
3647
Scheme 4. Synthesis of neoglycopeptides using ureido-linked glycosylated amino
acids: (i) reagent 4, DCC, HOBt, amino acid ester, HCl, N-methylmorpholine,
DCM. 6a) n ¼ 1 R1 ¼ CH(CH3)2; 6b) n ¼ 1, R1 ¼ CH3; 6c) n ¼ 1, R1 ¼ CH3; 6d)
n ¼ 2, R1 ¼ CH(CH3)2; 6e) n ¼ 2 R1 ¼ CH3); 6f) n ¼ 2, R1 ¼ CH3: R ¼ Ac or Bz.
Sodium azide (1.5 mmol) in water (1 mL) was added to the solution and
stirred at the same temperature for about 15 min. After the reaction was
complete, the THF was evaporated and extracted with ethyl acetate
(15 mL). The organic layer was washed with dilute citric acid, 10%
Na2CO3, and brine; dried over anhydrous Na2SO4; and evaporated in
vacuo. The resulting azide was dissolved in toluene (10 mL) and exposed
to microwave irradiation for 2 min. After the reaction was complete,
solvent was removed in vacuo to get the isocyanate.
Urea-Linked Glycosylated Fmoc-Asp/Glu-oxazolidinone Derivatives:
Procedure for the Synthesis of 3
To the solution containing Na-Fmoc-Asp=Glu-oxazolidinone isocyanate
(1 mmol) in dry DCM (10 mL), glycosyl amine and N-methylmorpholine
(1.1 mmol) were added at 0 ꢁC and stirred overnight. After the comple-
tion of the reaction, the residue was washed with citric acid and brine
Figure 2. A model ureido-linked glycopeptide synthesized by the solid-phase
approach.

3648
V. V. Sureshbabu et al.
and dried over anhydrous Na2SO4. The solvent was evaporated under
reduced pressure to get the desired compound.
Ureido-Linked Amino Acids: Procedure for the Synthesis of 4 (Table 1)
Solution of LiOH (1 N, 2 mmol) was added in one portion to a solution
of 3 (1 mmol) in THF (10 mL) and stirred for 1 h. The resulting solution
was acidified with 10% HCl (10 mL) and extracted with EtOAc
(2ꢂ10 mL). The combined organic extract was washed with brine and
dried (Na2SO4). The solvent was removed in vacuo to leave the
crude product as a colorless solid, which was column purified (CHCl3–
methanol–acetic acid; 40:2:1).
Data
Fmoc-Asp-b-[NH-CO-NH-(2,3,4,6-tetra-O-acetyl-b-D-
glucopyranosyl)]-OH (Entry 1)
1
IR (KBr) 1251, 1707, 1740 cmꢀ1; H NMR (400 MHz, CDCl3): d 1.99,
2.02, 2.04, 2.06 (4 s, 12H), 3.72 (m, 2H), 3.81 (m, 1H), 4.12 (dd,
J ¼ 7.1 Hz, 1H), 4.19 (t, J ¼ 6.9 Hz, 1H), 4.21 (t, J ¼ 7.1 Hz, 1H), 4.26
(dd, J ¼ 8.4 Hz, 1H), 4.55 (d, J ¼ 6.7 Hz, 2H), 4.65 (d, J ¼ 7.1 Hz, 1H),
4.93 (dd, J ¼ 8.2 Hz, 1H), 5.06 (t, J ¼ 9.6 Hz, 1H), 5.17 (t, J ¼ 9.8 Hz,
1H), 5.57 (d, J ¼ 6.2 Hz, 1H), 6.58 (br, 1H), 6.72 (br, 1H), 7.25–7.75 (m, 8H).
Fmoc-Asp-b-[NH-CO-NH-(2,3,4,6-tetra-O-acetyl-b-D-
galactopyranosyl)]-OH (Entry 2)
1
IR (KBr): 1250, 1707, 1740 cmꢀ1; H NMR (400 MHz, CDCl3): d 2.00,
2.02, 2.05, 2.07 (4 s, 12H), 3.74 (m, 2H), 3.80 (m, 1H), 4.14 (dd, J ¼ 7.1 Hz,
Hz, 1H), 4.22 (t, J ¼ 6.9 Hz, 1H), 4.26 (t, J ¼ 7.1 Hz, 1H), 4.28 (dd,
J ¼ 8.4 Hz, 1H), 4.56 (d, J ¼ 6.7 Hz, 2H), 4.65 (d, J ¼ 7.1 Hz, 1H), 4.90
(dd, J ¼ 8.1 Hz, 1H), 5.08 (t, J ¼ 9.5 Hz, 1H), 5.15 (t, J ¼ 9.8 Hz, 1H),
5.60 (d, J ¼ 6.2 Hz, 1H), 6.56 (br, 1H), 6.70 (br, 1H), 7.20–7.75 (m, 8H).
Fmoc-Asp-b-[NH-CO-NH-(2,3,4,6-tetra-O-benzoyl-b-D-
glucopyranosyl)]-OH (Entry 3)
1
IR (KBr): 1251, 1703, 1740 cmꢀ1; H NMR (400 MHz, CDCl3): d 1.99,
2.01, 2.03, 2.06 (4 s, 12H), 3.73 (m, 2H), 3.82 (m, 1H), 4.13 (dd, J ¼ 7.1 Hz,

Ureido-Linked Glycosylated Amino Acids
3649
Hz, 1H), 4.19 (t, J ¼ 6.8 Hz, 1H), 4.21 (t, J ¼ 7.2 Hz, 1H), 4.30 (dd,
J ¼ 8.4 Hz, 1H), 4.56 (d, J ¼ 6.5 Hz, 2H), 4.65 (d, J ¼ 7.3 Hz, 1H), 4.90
(dd, J ¼ 8.2 Hz, 1H), 5.08 (t, J ¼ 9.6 Hz, 1H), 5.15 (t, J ¼ 9.8 Hz, 1H),
5.60 (d, J ¼ 6.1 Hz, 1H), 6.59 (br, 1H), 6.71 (br, 1H), 7.10–7.70 (m, 28H).
Fmoc-Asp-b-[NH-CO-NH-(2,3,4,6-tetra-O-benzoyl-b-D-
galactopyranosyl)]-OH (Entry 4)
1
IR (KBr): 1251, 1702, 1745 cmꢀ1; H NMR (400 MHz, CDCl3): d 1.99,
2.01, 2.03, 2.05 (4 s, 12H), 3.73 (m, 2H), 3.82 (m, 1H), 4.13 (dd, J ¼ 7.1 Hz,
Hz, 1H), 4.19 (t, J ¼ 6.8 Hz, 1H), 4.21 (t, J ¼ 7.2 Hz, 1H), 4.30 (dd,
J ¼ 8.4 Hz, 1H), 4.56 (d, J ¼ 6.5 Hz, 2H), 4.65 (d, J ¼ 7.3 Hz, 1H), 4.90
(dd, J ¼ 8.2 Hz, 1H), 5.08 (t, J ¼ 9.6 Hz, 1H), 5.15 (t, J ¼ 9.8 Hz, 1H),
5.60 (d, J ¼ 6.1 Hz, 1H), 6.59 (br, 1H), 6.71 (br, 1H), 7.15–7.70 (m, 28H).
Fmoc-Asp-b-[NH-CO-NH-(2,20,3,30,40,6,60-hepta-O-acetyl-b-D-
maltosyl)]-OH (Entry 5)
1
IR (KBr): 1257, 1715, 1735 cmꢀ1
;
H NMR (400 MHz, CDCl3): d 2.03–
2.13 (s, 21H), 2.61 (d, J ¼ 4.5 Hz, 1H), 3.30 (d, J ¼ 9.0 Hz, 1H), 4.13 (t,
J ¼ 7.1 Hz, 1H), 4.24 (d, J ¼ 7.0 Hz, 2H), 4.28 (m, 1H), 4.48 (dd,
J ¼ 7.1 Hz, 1H), 4.63 (dd, J ¼ 6.9 Hz, 1H), 4.66 (d, J ¼ 7.0 Hz, 1H), 5.50
(t, J ¼ 9.6 Hz, 1H), 5.71 (t, J ¼ 9.1 Hz, 1H), 5.92 (t, J ¼ 7.1 Hz, 1H),
6.58 (br, 1H), 6.72 (br, 1H), 7.20–7.80 (m, 8H).
Fmoc-Glu-c-[NH-CO-NH-(2,3,4,6-tetra-O-acetyl-b-D-
glucopyranosyl)]-OH (Entry 6)
1
IR (KBr): 1222, 1698, 1749 cmꢀ1; H NMR (400 MHz, CDCl3): d 1.65–
1.75 (m, 2H), 2.00–2.07 (4 s, 12H), 2.95–3.12 (br d, 2H), 3.15–3.25
(m, 2H), 4.12 (m, 1H), 4.20 (t, J ¼ 7.0 Hz, 1H), 4.29 (m, 1H), 4.43 (m,
2H), 4.45 (q, J ¼ 7.1 Hz, 1H), 4.62 (q, J ¼ 7.0 Hz, 1H), 4.99 (d, J ¼ 7.1 Hz,
Hz, 1H), 5.50 (t, J ¼ 9.5 Hz, 1H), 5.71 (t, J ¼ 9.0 Hz 1H), 5.93 (t,
J ¼ 7.2 Hz, 1H), 6.55 (br, 1H), 6.69 (br, 1H), 7.33–7.77 (m, 8H).
Fmoc-Glu-c-[NH-CO-NH-(2,3,4,6-tetra-O-acetyl-b-D-
galactopyranosyl)]-OH (Entry 7)
1
IR (KBr): 1222, 1701, 1750 cmꢀ1; H NMR (400 MHz, CDCl3): d 1.70–
1.78 (br d, 2H), 1.99–2.06 (4 s, 12H), 2.92–3.10 (br d, 2H), 3.12–3.20

3650
V. V. Sureshbabu et al.
(m, 2H), 4.13 (m, 1H), 4.21 (t, J ¼ 7.1 Hz, 1H), 4.30 (m, 1H), 4.43 (m,
2H), 4.45 (q, J ¼ 6.9 Hz, 1H), 4.60 (q, J ¼ 7.0 Hz, 1H), 4.97 (d, J ¼ 7.2 Hz,
Hz, 1H), 5.51 (t, J ¼ 9.6 Hz, 1H), 5.70 (t, J ¼ 9.0 Hz, 1H), 5.93 (t,
J ¼ 7.1 Hz, 1H), 6.62 (br, 1H), 7.30–7.78 (m, 8H).
Fmoc-Glu-c-[NH-CO-NH-(2,3,4,6-tetra-O-benzoyl-b-D-glucopyrano-
syl)]-OH (Entry 8)
1
IR (KBr): 1222, 1698, 1749, 1800 cmꢀ1; H NMR (400 MHz, CDCl3): d
1.72–1.78 (br d, 2H), 1.99–2.08 (4 s, 12H), 2.92 (m, 2H), 3.82 (m, 1H),
4.12 (dd, J ¼ 7.1 Hz, 1H), 4.19 (t, J ¼ 6.8 Hz, 1H), 4.21 (t, J ¼ 7.2Hz, 1H),
4.30 (dd, J¼ 8.4 Hz, 1H), 4.56 (d, J¼ 6.5 Hz, 2H), 4.65 (d, J ¼ 7.3 Hz,
1H), 4.90 (dd, J¼ 8.2 Hz, 1H), 5.08 (t, J ¼ 9.6 Hz, 1H), 5.15 (t, J ¼ 9.8 Hz,
1H), 5.60 (d, J ¼ 6.1Hz, 1H), 6.59 (br, 1H), 6.71 (br, 1H), 7.10–7.70 (m, 28H).
Fmoc-Glu-c-[NH-CO-NH-(2,3,4,6-tetra-O-benzoyl-b-D-
galactopyranosyl)]-OH (Entry 9)
IR (KBr): 1222, 1700, 1745 cmꢀ1; 1H NMR (400 MHz, CDCl3): d 1.72–1.76
(br d, 2H), 1.98–2.05 (4 s, 12H), 3.73 (m, 2H), 3.82 (m, 1H), 4.13 (dd,
J ¼ 7.1 Hz, 1H), 4.19 (t, J ¼ 6.8 Hz, 1H), 4.20 (t, J ¼ 7.1 Hz, 1H), 4.31 (dd,
J ¼ 8.5 Hz, 1H), 4.56 (d, J ¼ 6.2 Hz, 2H), 4.66 (d, J ¼ 7.3 Hz, 1H), 4.91
(dd, J ¼ 8.1 Hz, 1H), 5.09 (t, J ¼ 9.5 Hz, 1H), 5.14 (t, J ¼ 9.8 Hz, 1H), 5.62
(d, J ¼ 6.1 Hz, 1H), 6.58 (br, 1H), 6.70 (br, 1H), 7.15–7.70 (m, 28H).
Fmoc-Glu-c-[NH-CO-NH-(2,20,3,30,40,6,60-hepta-O-acetyl-b-D-
maltosyl)]-OH (Entry 10)
1
IR (KBr): 1254, 1698, 1742 cmꢀ1; H NMR (400 MHz, CDCl3): d 1.70–
1.76 (br d, 2H), 2.03–2.13 (s, 21H), 2.61 (d, J ¼ 4.5 Hz, 1H), 3.30
(d, J ¼ 9.0 Hz, 1H), 4.12 (t, J ¼ 7.1 Hz, 1H), 4.20 (t, J ¼ 7.1 Hz,1H),
4.24 (d, J ¼ 7.0 Hz, 2H), 4.28 (m, 1H), 4.48 (dd, J ¼ 7.1 Hz, 1H), 4.63
(dd, J ¼ 6.9 Hz, 1H), 4.66 (d, J ¼ 7.0 Hz, 1H), 5.50 (t, J ¼ 9.6 Hz, 1H),
5.71 (t, J ¼ 9.1 Hz, 1H), 5.92 (t, J ¼ 7.1 Hz, 1H), 6.58 (br, 1H), 6.72 (br,
1H), 7.20–7.80 (m, 8H).
Synthesis of Neoglycopeptides: General Procedure for Synthesis of 6
(Scheme 4)
To a stirred solution of Na-Fmoc-urea-linked glycosyl amino acid
(1 mmol) in dry DCM (50 mL), DCC (1.2 mmol) and HOBt (1.2 mmol)

Ureido-Linked Glycosylated Amino Acids
3651
were added at 0 ꢁC and stirred for 10 min. A solution of amino acid methyl
ester (1.5 mmol) neutralized with NMM (2 mmol) in CH2Cl2 was added to
the reaction mixture and stirred for 4 h. Precipitated DCU was filtered
out, and the filtrate was acidified using dil. HCl (20 mL). The resulting
organic layer was washed with 10% Na2CO3 and brine, then dried over
anhydrous Na2SO4. After removal of the solvent under reduced pressure,
the crude product was purified by column chromatography using EtOAc–
hexane (3:7) as eluant to afford the final product as a white solid.
Data
Fmoc-Asp-b-[NH-CO-NH-(2,3,4,6-tetra-O-acetyl-b-D-
glucopyranosyl)]-val-OMe (6a)
Mp 126–128 ꢁC; Yield: 82%; IR (KBr): 1700, 1740 cmꢀ1 1H NMR
;
(400 MHz, CDCl3): d 0.93 (d, J ¼ 6.6 Hz, 6H), 1.84 (m, 1H), 2.00–2.06
(s, 12H), 2.70 (br s, 1H), 3.29 (br s, 1H), 3.70 (s, 3H), 4.11 (m, 1H),
4.20 (t, J ¼ 6.7 Hz, 1H), 4.40 (d, J ¼ 7.0 Hz, 2H), 4.49 (dd, J ¼ 6.9 Hz,
1H), 4.61 (dd, J ¼ 7.1 Hz, 1H), 5.01 (d, J ¼ 9.5 Hz, 1H), 5.51
(t, J ¼ 9.6 Hz, 1H), 5.71 (t, J ¼ 9.0 Hz, 1H), 5.93 (t, J ¼ 4.3 Hz, 1H),
7.26–7.95 (m, 8H); HRMS (ES): m=z calcd. for C39H48N4NaO15,
[M þ Na]þ: 835.8162; found: 835.7024.
Fmoc-Asp-b-[NH-CO-NH-(2,3,4,6-tetra-O-benzoyl-b-D-
glucopyranosyl)]-Ala-OMe (6b)
1
Mp 126–128 ꢁC; yield: 82%; IR (KBr): 1251, 1702, 1745 cmꢀ1; H NMR
(400 MHz, CDCl3): d 1.16 (d, J ¼ 7.9 Hz, 3H), 2.70 (br s, 1H), 3.70 (s, 3H),
4.12 (m, 1H), 4.20 (t, J ¼ 7.1 Hz, 1H), 4.40 (d, J ¼ 7.0 Hz, 2H), 4.50 (d,
J ¼ 6.9 Hz, 1H), 4.61 (d, J ¼ 7.1 Hz, 1H), 5.00 (d, J ¼ 9.0 Hz, 1H), 5.50
(t, J ¼ 9.6 Hz, 1H), 5.70 (t, J ¼ 9.0 Hz, 1H), 5.93 (t, J ¼ 4.3 Hz, 1H),
7.23–8.07 (m, 28H); ESI MS: m=z calcd. for C57H52N4NaO15, [M þ Na]þ:
: 1055.3; found: 1055.5.
Fmoc-Asp-b-[NH-CO-NH-(2,3,4,6-tetra-O-benzoyl-b-D-
galactopyranosyl)]-Ala-OMe (6c)
1
Mp 122–124 ꢁC; yield: 82%; IR (KBr): 1251, 1700, 1740 cmꢀ1; H NMR
(400 MHz, CDCl3): d 1.16 (d, J ¼ 7.9 Hz, 3H), 2.70 (br s, 1H), 3.70 (s, 3H),
4.12 (m, 1H), 4.20 (t, J ¼ 6.7 Hz, 1H), 4.40 (d, J ¼ 7.0 Hz, 2H), 4.50 (d,
J ¼ 6.9 Hz, 1H), 4.61 (d, J ¼ 7.1 Hz, 1H), 5.00 (d, J ¼ 9.0 Hz, 1H), 5.50

3652
V. V. Sureshbabu et al.
(t, J ¼ 9.6 Hz, 1H), 5.70 (t, J ¼ 9.0 Hz, 1H), 5.93 (t, J ¼ 4.3 Hz, 1H), 7.23–
8.07 (m, 28H); ESI MS: m=z calcd. for C57H52N4NaO15, [M þ Na]þ:
1055.3; found: 1055.8.
Fmoc-Glu-c-[NH-CO-NH-(2,3,4,6-tetra-O-acetyl-b-D-
glucopyranosyl)] Val-Ome (6d)
1
Mp 152–154ꢁC; yield: 85%; IR (KBr): 1222, 1700, 1749cmꢀ1; H NMR
(400MHz, CDCl3): d 0.93 (t, J ¼ 6.6 Hz, 6H), 1.84 (m, 1H), 2.00–2.06
(s, 12H), 1.90–2.01 (br d, 2H), 2.95–3.12 (br d, 2H), 3.70 (s, 3H), 4.12 (m,
1H), 4.48 (m, 1H), 4.62 (m, 1H), 4.99 (d, J ¼ 9.0 Hz, 1H), 5.50 (t, J ¼ 9.6 Hz,
Hz, 1H), 5.71 (t, J ¼ 9.0 Hz, 1H), 5.93 (t, J¼ 4.3 Hz, 1H), 7.28–7.77 (m, 8H);
ESI MS: m=z calcd. for C40H50N4NaO15, [M þ Na]þ: 849.84; found: 849.4.
Fmoc-Glu-c-[NH-CO-NH-(2,3,4,6-tetra-O-benzoyl-b-D-
glucopyransyl)]-Ala-OMe (6e)
1
Mp 110–112 ꢁC; yield: 85%; IR (KBr): 1222, 1698, 1750 cmꢀ1; H NMR
(400 MHz, CDCl3): d 1.17 (d, J ¼ 7.9 Hz, 3H), 1.90–2.01 (br d, 2H), 2.95–
3.12 (br d, 2H), 3.69 (s, 3H), 4.11 (m, 1H), 4.47 (m, 1H), 4.62 (m, 1H),
5.00 (d, J ¼ 9.0 Hz, 1H), 5.50 (t, J ¼ 9.6 Hz, 1H), 5.71 (t, J ¼ 9.0 Hz,
1H), 5.93 (t, J ¼ 4.3 Hz, 1H), 7.23–8.07 (m, 28H); ESI MS: m=z calcd.
for C58H54N4NaO15, [M þ Na]þ: 1069.3; found: 1069.7.
Fmoc-Glu-c-[NH-CO-NH-(2,3,4,6-tetra-O-benzoyl-b-D-
galactopyranosyl)]-Ala-OMe (6f)
1
Mp 164–166 ꢁC; yield: 85%; IR (KBr): 1222, 1700, 1750 cmꢀ1; H NMR
(400 MHz, CDCl3): d 1.17 (d, J ¼ 7.9 Hz, 3H), 1.90–2.01 (br d, 2H), 2.95–
3.12 (br d, 2H), 3.69 (s, 3H), 4.11 (m, 1H), 4.47 (m, 1H), 4.62 (m, 1H),
5.00 (d, J ¼ 9.0 Hz, 1H), 5.50 (t, J ¼ 9.5 Hz, 1H), 5.71 (t, J ¼ 9.0 Hz,
1H), 5.93 (t, J ¼ 4.3 Hz, 1H), 7.23–8.07 (m, 28H); ESI MS: m=z calcd.
for C58H54N4NaO15, [M þ Na]þ: 1069.3, found: 1069.3.
Peptide Data
H-Leu-Asp[w(NH-CO-NH)]-Gly-Val-OH
1H NMR (400 MHz, CDCl3) d 0.93 (m, 12H), 1.3–1.35 (m, 2H), 1.63–1.72
(m, 2H), 2.02–2.10 (s, 12H), 3.67 (m, 2H), 4.12 (m, 2H), 4.34 (m, 2H),

Ureido-Linked Glycosylated Amino Acids
3653
4.51–4.78 (m, 3H); 13C NMR (100 MHz, CDCl3) d 17.2, 20.1, 20.2, 24.3,
30.8, 38.7, 39.2, 43.5, 53.8, 54.0, 63.0, 64.2, 67.0, 68.6, 70.8, 91.2, 152.3,
166.3, 166.7, 167.1, 170.2, 171.0 MALDI-TOF: anal. calcd. for
C31H50N6O15 [M þ 1]þ: 747. 2.
ACKNOWLEDGMENTS
Authors are grateful to the Department of Science and Technology, Gov-
ernment of India, for financial support. R. V. is grateful to the CSIR for
the award of a fellowship.
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