Synthesis and gelation properties of a new class of α-amino acid-based sector block dendrons

Article abstract of DOI:10.1016/j.tet.2005.08.006

A new series of α-amino acid-based sector block dendrons containing alanine, phenylalanine, and valine dendritic sectors was prepared by a solution phase peptide coupling methodology. The structures of the dendrons were fully characterized by nuclear magn

Full text of DOI:10.1016/j.tet.2005.08.006

Tetrahedron 61 (2005) 11279–11290
Synthesis and gelation properties of a new class of a-amino
acid-based sector block dendrons
*
Hak-Fun Chow and Jie Zhang
Department of Chemistry, The Chinese University of Hong Kong, Shatin, NT, Hong Kong SAR, PR China
Received 7 June 2005; revised 1 August 2005; accepted 2 August 2005
Available online 10 October 2005
Abstract—A new series of a-amino acid-based sector block dendrons containing alanine, phenylalanine, and valine dendritic sectors was
prepared by a solution phase peptide coupling methodology. The structures of the dendrons were fully characterized by nuclear magnetic
resonance and mass spectroscopy and by optical polarimetry, and their purities were determined by size exclusion chromatography. Some of
the dendrons, especially those containing phenylalanine residues, were found to form strong physical gels with aromatic solvents. The
gelation mechanism was further investigated by infra-red and circular dichroism spectroscopy. It was found that both inter-molecular
hydrogen bonding and aromatic p–p stacking interactions were the main driving forces for gelation.
q 2005 Elsevier Ltd. All rights reserved.
1. Introduction
Reymond recently reported a combinatorial approach to the
synthesis of a library of catalytically active layer block
peptide-based dendrimers.7 One of the most intriguing
findings was that the catalytic reactivity was shown to be
dependent on the nature of the amino acid in the different
concentric layers. This suggested that the amino acid
arrangement within the dendrimers, similar to the amino
acid sequence of enzymes, has a profound effect on their
properties. Our group recently reported the synthesis of a
combinatorial series of amino acid-based layer block
dendrons using alanine, phenylalanine, and valine as the
ingredients.8 We also found that gelation properties of these
dendrons were strongly influenced by their layer block
amino acid sequence. Herein, we wish to describe the
synthesis of a new series of G1-G2 amino acid-based sector
block dendrons 1–4. In addition, we also show that such
amino acid-based dendrimers possess rich structural
diversities and exhibit amino acid dependent gelation
properties.
The use of a-amino acids as ingredients for the construction
of dendrimers has become a topic of current interest. Such
compounds can be used as biomimetic models toward the
property studies of artificial proteins and enzymes.1
Furthermore, such materials are also envisaged to possess
better bio-compatibility2 because of their compositions are
consisted of naturally occurring amino acids. Depending on
how the amino acids are linked together, such dendrimers
can be classified as peptide-based or amino acid-based
dendrimers. In the former category the amino acid units are
directly connected to each other, while in the latter they are
linked to each other through non-amino acid spacers.
Typical peptide-based dendrimers are the poly(lysine)
dendrimers,3 poly(ornithine) dendrimers,4 and poly(gluta-
mate) dendrimers,5 and some of their derivatives have
shown very interesting biological and gelation proper-
ties.3b–3e Interest in amino acid-based dendrimers and
dendrons has also appeared recently.6 However, most of
the peptide-based and amino acid-based dendrimers
reported to date are synthesized from only one type of
amino acid. Hence, if more than one kind of amino acids are
used in their constructions, such as those encountered in
natural proteins and peptides, dendrimers with diverse
structural variety can then be resulted. In this context
Keywords: Dendrimers; Amino acid-based dendrimers; Organogelators.
*
Corresponding author. Tel.: C852 26096341; fax: C852 26035057;
e-mail: hfchow@cuhk.edu.hk
0040–4020/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2005.08.006

11280
H.-F. Chow, J. Zhang / Tetrahedron 61 (2005) 11279–11290
protective group in compounds 7 was then removed by
catalytic hydrogenation to afford G1-(aa1)(NH2)-CO2Et 8 in
about 95% yields. The different products 8 were then
coupled to a second amino acid BocNH-aa2-CO2H in
the presence of EEDQ to give the three sector block
dendrons G1-(aa1)(aa2)-CO2Et 1 (aa1aa2ZAF, AV, and VF)
in 87–98% yield as white solids. Alkaline hydrolysis of the
ethyl ester group then gave the corresponding carboxylic
acid dendrons G1-(aa1)(aa2)-CO2H 2 (aa1aa2ZAF, AV, and
VF) in nearly quantitative yields.
The starting materials for the preparation of the three
different G2 sector block dendrons 3 were the known layer
block dendrons G1-(aa1)2-CO2H 9 reported earlier by us.8
Hence, coupling of compounds 9 (aa1ZA, F, or V) with
H3NC-(aa1)-CO2Me ClK bearing the same aa1 amino acid
residue in the presence of dicyclohexylcarbodiimide (DCC),
4-methylmorpholine, and 1-hydroxybenzotriazole (HOBt)
afforded the three G1-(aa1)2-CONH-(aa1)-CO2Me (aa1ZA,
F, or V) esters 10 in 62–80% yields (Scheme 2). The
methyl ester functionality was then removed by alkaline
hydrolysis to produce the corresponding acid dendrons
G1-(aa1)2-CONH-(aa1)-CO2H (aa1ZA, F, or V) 11 in
nearly quantitative yields. The acid dendrons 11 were then
anchored to the mono-protected branching unit 5 using
either DCC or 1-(3-dimethylaminopropyl)-3-ethylcarbo-
diimide methiodide (EDCI) as the coupling agent and
HOBt to give the hemi-substituted G2-(aa1)3(CbzNH)-
CO2Et dendrons 12 containing the aa1 amino acid dendritic
sector. The Cbz protecting group was then dismantled by
catalytic hydrogenolysis (10% Pd on C, MeOH) to release
the free amino group. The product 13 was then subsequently
coupled (DCC or EDCI, HOBt, THF) to an aa2 containing
acid dendron G1-(aa2)2-CONH-(aa2)-CO2H 12 to give the
target sector block dendrons G2-(aa1)3(aa2)3-CO2Et 3
(aa1aa2ZAF, AV, FV) in 35–46% yield as white solids.
Finally, base hydrolysis (1.0 M aqueous KOH in MeOH) of
the ethyl ester group produced the sector block acid
dendrons G2-(aa1)3(aa2)3-CO2H 4 (aa1aa2ZAF, AV, FV)
in 94–99% yields.
2. Results and discussion
2.1. Synthesis
We employed 3,5-diaminobenzoic acid as the branching
agent in this class of sector block dendrimers. In contrast to
the synthesis of layer block dendrimers, the synthesis of
sector block dendrimers or dendrons requires a high degree
of control over the number and placement of functional
groups within the same dendritic layer. Therefore, one needs
to differentiate the two amino groups in our branching agent.
Hence, one of the amino groups was selectively protected as
the Cbz derivative 5 in 71% yield by reacting ethyl 3,5-
diaminobenzoate with 1 equiv of benzyl chloroformate
(Scheme 1). The other amino group was subsequently
coupled to either BocNH-L-alanine 6 (aa1ZA) or BocNH-L-
valine 6 (aa1ZV) in the presence of 2-ethoxy-1-ethoxy-
carbonyl-1,2-dihydroquinoline (EEDQ) to furnish G1-
A(CbzNH)-CO2Et 7 (aa1ZA) or G1-V(CbzNH)-CO2Et 7
(aa1ZV), respectively, in excellent yields. The Cbz
Scheme 1. Reagents and conditions: (a) BnOCOCl, NaOH (1 M), THF, 5–25 8C, 3 h; (b) EEDQ, CH2Cl2, 0–25 8C, 12 h; (c) H2, 10% Pd on C, MeOH, 25 8C,
1 h; (d) EEDQ, THF/CH2Cl2, 0–25 8C, 12 h; (e) KOH (1.0 M), MeOH/H2O, 25 8C.

H.-F. Chow, J. Zhang / Tetrahedron 61 (2005) 11279–11290
11281
Scheme 2. Reagents and conditions: (a) ClK H3NC-(aa1)-CO2Me, DCC, HOBt, 4-methylmorpholine, THF, K10–25 8C, 22 h; (b) KOH (1.0 M), MeOH/H2O,
25 8C; (c) 5, DCC or EDCI, HOBt, THF, K10–25 8C, 22 h; (d) H2, 10% Pd on C, MeOH, 25 8C, 1 h; (e) G1-(aa2)2-CONH-(aa2)-CO2H, DCC or EDCI, HOBt,
THF, K10–25 8C, 50 h.
2.2. Characterization
phenylalanine side chain as a multiplet at d 2.72–3.05
(Fig. 1). The two anilide protons were chemically non-
equivalent and appeared as two separate singlets at d 10.18
and 10.29. The two Boc groups were also different and
showed up as two singlets at d 1.32 and 1.38. On the other
hand, the isopropyl side chain of the valine residue in the
dendron G1-AV-CO2Et appeared as a doublet at d 0.89 and
a multiplet at d 1.85–2.08. The chemical shift values of the
amino acid side chain protons remained essentially the same
on going from the G1 to the G2 dendrons when they were
located on the surface layer. Hence, in the 1H NMR
spectrum of G2-A3F3-CO2Et, the surface alanine methyl
groups appeared as a doublet at d 1.26 and the surface
2.2.1. NMR spectroscopy. The structural identities of all
the intermediates and target compounds were characterized
by 1H NMR and 13C NMR spectroscopy. In contrast to the
layer block dendrons reported earlier,8 the sector block
1
dendrons 1–4 are devoid of a pseudo-C2 axis and their H
NMR spectra are slightly more complex. Nonetheless, the
characteristics of the ‘finger print’ peaks due to the protons
on the amino acid side chains were readily diagnosed. For
1
example, the H NMR spectrum of G1-AF-CO2Et showed
the presence of the alanine methyl side chain as a doublet
at d 1.26 and the diastereotopic benzylic protons of the
Figure 1. 1H NMR spectrum of G1-AF-CO2Et in d6-DMSO.

11282
H.-F. Chow, J. Zhang / Tetrahedron 61 (2005) 11279–11290
benzylic protons of the phenylalanine side chains appeared
as a multiplet at d 2.70–3.08. On the other hand, the
chemical shift values of the interior amino acid side chain
protons were slightly downfield shifted due to a change of
the N-linking group from the Boc to the benzamide
functionality.
The 13C NMR spectra of the target sector block dendrons
share a common spectral feature; the chemical shift values
of the carbon nuclei due to the dendritic backbone and the
aromatic 3,5-diaminobenzamide branching unit are nearly
the same for the different dendrons of the same generation.
Hence, the primary and tertiary carbons of the Boc groups
resonated at d 29 and 79, respectively, while the peaks for
the ethyl ester focal point group were located at d 15 and 61.
Due to a broken down of the C2 symmetry, up to six
aromatic carbon signals originated from the interior
branching unit could be identified (d 115–140). Likewise,
the carbon signals due to the surface aromatic branching
units were also found to scatter between the same spectral
regions. The carbonyl 13C signals appeared at the most
downfield region of the spectra. The 13C signal(s) at wd 156
corresponded to the Boc carbamate group(s) while the
one(s) at wd 167 could be attributed to the focal point
carbonyl ester/acid or to the benzamide carbonyl moieties.
On the other hand, the signal(s) located at wd 172
corresponded to the anilide C]O(s). The chemical
identities of the amino acid side chains could be similarly
assigned by their respective ‘fingerprint’ signals as in the
Figure 2. SEC chromatograms of G2 ester dendrons (flow rate: 1 mL minK1
temperatureZ40 8C; solventZ5% HOAc/DMF). The negative absorption
peaks are due to signals of water and acetic acid.
;
often their homogeneities were assessed by size exclusion
chromatography (SEC). Hence, all the dendrons as well as
the intermediates were subjected to SEC analysis and they
were found to produce a symmetrical peak with a narrow
polydispersity (PDI%1.03) in their SEC chromatogram
(Table 1 and Fig. 2). Therefore, all the dendrons were
determined to be O95% pure by SEC analysis. More
interestingly, the G2 acid and ester dendrons all have nearly
the same retention volume (w16.1 min), suggesting that
they possess similar hydrodynamic radius, irrespective of
the amino acid composition and the focal point
functionality.
1
case of H NMR spectroscopic analysis.
2.2.2. Mass spectroscopic analysis. The structures of all
the G1 and G2 sector block dendrons were also character-
ized by FAB or ESI mass spectrometry. For both the G1 and
G2 series of compounds, molecular ions appeared in forms
of MC, (MCH)C, and/or (MCNa)C could be identified. It
was found that ESI was a superior ionization technique than
FAB in order to obtain the molecular mass data for the
higher molecular weight G2 series. The exact masses of the
molecular ion peak were measured and the results also
matched well with the theoretical values.
2.3. Properties
2.3.1. Chiroptical properties. The chiroptical properties of
the sector block dendrons were measured and tabulated
(Table 2). To avoid complications due to self aggregation,8,9
the data were acquired in 5% HOAc in 1,2-dichloroethane.
Previously, it was found that the sign of the specific
rotations of the layer block dendrons generally did not
change from the dendritic esters to their corresponding
carboxylic acids and that the molar rotation of the resulting
dendron was the simple sum of the molar rotations of all
the constituted amino acid chiral units resided within the
dendron.8 Such observations were consistent with the notion
that the focal point functionality had little effect on the
chiral conformation of the layer block dendrons and that
they adopted an open and conformationally flexible
2.2.3. Size exclusion chromatography. Due to the presence
of a large number of amide and carbamate functional
groups, the amino acid-based dendrons prepared here were
found to form hydrates.8,9 As a result, elemental analysis
data could not be used to assess their structural purity.
Furthermore, it has been noted that elemental analysis data
are not reliable in assessing dendrimer purities due to the
extremely small variation of the analysis data across a series
of dendrimers bearing the same architectural elements. Most
Table 1. SEC data of sector block dendrons
Dendron
Retention time (min)
PDI
Dendron
Retention time (min)
PDI
G1-AF-CO2Et
G1-AV-CO2Et
G1-FV-CO2Et
G1-AF-CO2H
G1-AV-CO2H
G1-FV-CO2H
G1-A2-CONH-A-CO2Me
G1-F2-CONH-F-CO2Me
G1-V2-CONH-V-CO2Me
17.92
18.00
17.61
17.36
17.42
17.52
17.01
17.23
17.40
1.02
1.02
1.01
1.01
1.01
1.02
1.02
1.03
1.02
G1-A2-CONH-A-CO2H
G1-F2-CONH-F-CO2H
G1-V2-CONH-V-CO2H
G2-A3F3-CO2Et
G2-A3V3-CO2Et
G2-F3V3-CO2Et
G2-A3F3-CO2H
G2-A3V3-CO2H
G2-F3V3-CO2H
16.81
16.79
16.98
16.10
16.18
16.09
15.92
15.98
16.03
1.02
1.01
1.02
1.01
1.01
1.02
1.02
1.02
1.02

H.-F. Chow, J. Zhang / Tetrahedron 61 (2005) 11279–11290
Table 2. Chiroptical data of sector block dendrons
11283
a
b
a
b
Dendrons
[a]D
[F]D
Dendrons
[a]D
[F]D
G1-AF-CO2Et
G1-AV-CO2Et
G1-FV-CO2Et
G1-AF-CO2H
G1-AV-CO2H
G1-FV-CO2H
G1-A2-CONH-A-CO2Me
G1-F2-CONH-F-CO2Me
G1-V2-CONH-V-CO2Me
K12.1
K50.8
C8.6
K14.1
K57.7
K7.3
K50.8
C46.6
K15.3
K72
G1-A2-CONH-A-CO2H
G1-F2-CONH-F-CO2H
G1-V2-CONH-V-CO2H
G2-A3F3-CO2Et
G2-A3V3-CO2Et
G2-F3V3-CO2Et
G2-A3F3-CO2H
G2-A3V3-CO2H
G2-F3V3-CO2H
K43.9
C42.0
K8.9
C25.8
C38.5
C78.3
C22.9
C27.9
C63.8
K248
C333
K58
C388
C523
C1242
C338
C371
C995
K279
C54
K80
K301
K44
K294
C376
K101
a Specific rotation (10K1 degrees cm2 gK1).
b Molar rotation (10 degrees cm2 molK1).
architecture in such a solvent system. However, for the
sector block dendrons reported here, some anomalies to
these trends were found. Hence, the absolute signs of G1-
FV-CO2H and G1-FV-CO2Et were different. In addition,
while the specific rotations of G1-A2-CONH-A-CO2R (RZ
H or Me) and G2-V2-CONH-V-CO2R (RZH or Me) are
negative, the values of G2-A3V3-CO2R (RZH or Et) are
positive. We speculated that these abnormalities were
probably due to a change of the chiral conformations on
going from G1 to G2 dendrons, but were unable to offer an
explanation at the present moment.
The gel was prepared by heating a weighted amount of an
organogelator in a specified solvent inside a septum-capped
vial until dissolution, and the sample was allowed to stand
at room temperature for 1 day and the state of the sample
was then examined. A stable gel was formed when a
homogeneous phase exhibited no gravitation flow by
inverting the sample. The gel state could be further
classified as transparent gel (CG) or opaque gel (OG)
according to its transparency. If part of the sample formed a
gel and part of it remained in solution, it was considered as a
partial gel (PG). The sample was regarded as soluble (S)
when the solution remained clear and no gelation occurred.
For the transparent gels described here, they were stable up
to several months when stored in screw-capped vials.
2.3.2. Gelation properties. Similar to the strong gelation
property of the layer block dendrons8 reported by us, the
sector block dendrons described here also exhibited very
excellent gelation behavior in aromatic solvents (Fig. 3).
Generally, the G1 acid dendrons are better organogelators
than the G1 ester dendrons. This observation was similar to
that observed with the G1 layer block dendrons.8 It was also
noted that the phenylalanine containing dendron G1-AF-
CO2H was the best organogelator with minimum gelation
concentration (mgc) below 10 mg mLK1 in nearly all
aromatic solvent tested (Table 3). The other phenylalanine
containing dendron, G1-FV-CO2H, was also a good
organogelator with slightly inferior mgc values
(!100 mg mLK1). Again, these observations were in line
with the fact that G1-F2-CO2H was found to be the best
gelating agent among the layer block dendrons.8 All these
findings strongly suggested that the aromatic phenylalanine
side chain was responsible for stabilizing the gel via p–p
stacking interaction with the aromatic solvents.
Figure 3. Gels (concentrationZ10 mg mLK1) formed from G1-AF-CO2H
in (2) toluene, (3) nitrobenzene, (4) o-xylene, (5) anisole, (6) G1-FV-CO2H
in nitrobenzene, G2-A3V3-CO2Et in (7) o-xylene and (8) toluene. The first
one on the left is KMnO4 solution.
Similar to the G2 layer block dendrons, the G2 sector block
dendrons generally possess weaker gelation ability. Another
Table 3. Gelation behavior of sector block dendrons in aromatic solventsa
Dendrons
Toluene
o-Xylene
Anisole
Nitrobenzene
o-Dichlorobenzene
G1-AF-CO2Et
G1-AV-CO2Et
G1-FV-CO2Et
G1-AF-CO2H
G1-AV-CO2H
G1-FV-CO2H
G2-A3F3-CO2Et
G2-A3V3-CO2Et
G2-F3V3-CO2Et
G2-A3F3-CO2H
G2-A3V3-CO2H
G2-F3V3-CO2H
S
PG
S
S
PG
S
S
PG
S
CG (10)
OG (50)
OG (100)
S
PG
PG
S
PG
S
CG (4)
CG (100)
CG (5)
S
PG
PG
S
PG
S
CG (40)
PG
OG (100)
PG
CG (100)
PG
PG
OG (100)
CG (100)
CG (3)
OG (40)
OG (100)
OG (50)
CG (30)
PG
OG (50)
PG
OG (50)
OG (4)
OG (50)
OG (100)
OG (50)
OG (40)
PG
OG (50)
PG
OG (50)
S
S
PG
PG
CG (100)
PG
a CG, transparent gel; OG, opaque gel; PG, partial gel; S, soluble (O100 mg mLK1). The values given in parentheses are the minimum concentration
(mg mLK1) to achieve gelation at 25 8C.

11284
H.-F. Chow, J. Zhang / Tetrahedron 61 (2005) 11279–11290
interesting observation was that none of the phenylalanine
containing G2 dendrons showed strong gelating ability. In
fact, the most efficient organogelator among the G2
dendrons was G2-A3V3-CO2Et, highlighting the intriguing
influence of the amino acid side chain on the gelation
ability. Hence, the amino acid side chains have a subtle yet
determining role on the gelation property, and only those
possessing favorable hydrophobic and steric parameters can
stabilize the physical gel.
temperature, the solution sample exhibited a positive Cotton
effect at 338 nm. As the temperature decreases below the
melting temperature, the molar ellicipity of the gel sample
increases, indicating the emergence of secondary chiral
architecture in the gel state. Furthermore, a gradual shift
(w2 nm) of the lmax to the blue region was also noted on
cooling of the sample, suggesting the gradual transition
from an isotropic solution to an anisotropic environment in
the gel state.
2.4. Gelation mechanism
3. Conclusions
The gelation mechanism of the organogelators was
investigated by infra-red (IR) spectroscopy. The FT-IR
spectra of a 10 mg mLK1 CHCl3 solution of G1-AF-CO2H
and a 10 mg mLK1 clear transparent gel in o-xylene were
recorded and the data were tabulated (Table 4). The IR
spectrum in CHCl3 exhibited two peaks at 3439 and
3329 cmK1 in the nN–H region, which could be attributed
to the stretching bands of the carbamate and anilide N–H,
A new series of G1 and G2 a-amino acid-based sector block
dendrons based on alanine, phenylalanine, and valine was
synthesized by a convergent synthesis strategy involving
selective functionalization and protection of a branching
agent. Such dendrons were found to exhibit very strong
gelation property (mgc down to 3 mg mLK1) with aromatic
solvents through aromatic p–p stacking, intermolecular
hydrogen bonding, and more interestingly, hydrophobic
interactions among the amino acid side chains. As a result,
only those dendrons with the optimal amino acid consti-
tution and focal point functionality can form strong physical
gels. This new library of sector block dendrons, together
with the layer block dendrons reported earlier, provide a
new repertoire of biomimetic dendrimers with rich
structural diversity and interesting physico–chemical prop-
erties that can be used as new bio-compatible materials.
respectively. In addition, one broad peak at 1691 cmK1
,
assignable to C]O stretching frequency was noted. On the
other hand, the IR spectrum of the o-xylene gel showed a
broad N–H absorption peak at 3313 cmK1 and another
broad C]O absorption at peak 1668 cmK1, both of which
were significantly red-shifted. Hence, in addition to
aromatic p–p stacking, intermolecular hydrogen bonding
is also an important driving force for gelation.
Table 4. FT-IR data of G1-AF-CO2H (concentrationZ10 mg mLK1) in the
solution and gel statesa
4. Experimental
4.1. General
Conditions
Absorption frequency (cmK1
)
n (carbamate/anilide
N–H)
n (acid/anilide/carba-
mate C]O)
CHCl3 solution
o-Xylene gel
3429, 3329
3313
1691
1668
THF was distilled from sodium benzophenone ketyl and
CH2Cl2 from P2O5 prior to use. Silica gel for flash
chromatography is Macherey Nagel 60 M (230–400 mesh)
silica gel. N-Boc-protected L-amino acids [BocNH-(aa1)-
CO2H], L-amino acid methyl ester hydrochlorides [H3NC-
(aa1)-CO2Me ClK] and other reagents were used as supplied
from Aldrich or Sigma. All reactions were conducted under
dry N2 unless otherwise stated. All NMR spectra were
a All spectra were recorded at 25 8C.
To further investigate the chirality of the gel aggregates,
circular dichroism (CD) spectra of G1-AF-CO2H in
o-xylene (10 mg mLK1) were recorded at different tem-
peratures (Fig. 4). At a temperature above the melting
˚
recorded in d6-DMSO (dried over molecular sieve 4 A) on a
Bru¨ker DPX spectrometer at 300 MHz for 1H and 75.5 MHz
for 13C nucleus at 25 8C. The residual proton or carbon
signals of d6-DMSO (dHZ2.50; dCZ40.5) were used as
internal references. All chemical shifts are reported in ppm
(d) and coupling constants in Hz. Positive ion ESI and FAB
spectra were carried out on a Thermo Finnigan MAT 95XL
mass spectrometer. Melting points were measured on an
Electrothermal IA9100 Digital Melting Point Apparatus and
were uncorrected. Melting temperatures (Tm) were recorded
on a Perkin-Elmer DCS6 differential scanning meter and are
referred to the onset of the transition. IR spectra were
recorded on a Nicolet 420 FT-IR spectrophotometer.
Optical rotations were taken on a Perkin Elmer 341
Polarimeter at 589 nm and at 20 8C, in a solvent mixture
of 1,2-dichloroethane/HOAc (v/v 95:5). CD spectra were
recorded on a JASCO J-715 spectropolarimeter connected
to a NESLAB RTE-211 temperature controller. Size
exclusion chromatography (SEC) analyses were performed
on Watersw Styragel columns (HR 1 and HR 3 in serial) at
Figure 4. Temperature dependent CD spectra of G1-AF-CO2H in o-xylene
(10 mg mLK1).

H.-F. Chow, J. Zhang / Tetrahedron 61 (2005) 11279–11290
11285
40 8C in 5% HOAc/DMF as eluent (flow rate Z
1.0 mL minK1) on a Waters HPLC 515 pump equipped
with a Waters 486 tunable UV absorbance detector.
Molecular weights obtained from SEC measurements
were based on a calibration curve derived from polystyrene
standards. Elemental analyses were performed at MEDAC
LTD, Brunel Science Centre, Cooper’s Hill Lane, Engle-
field Green, Egham, Surrey TW20 0JZ, UK.
140.7, 154.3, 156.1, 166.4, 173.1. MS (FAB): 486 [(MC
H)C, 21%]. HRMS (FAB): calcd for C25H31N3O7CHC,
486.2235; found, 486.2234.
4.2.2. G1-V(CbzNH)-CO2Et (7, aa1ZV). Starting from
BocNH-V-CO2H (1.46 g, 6.7 mmol), ethyl 3-amino-5-(N-
benzyloxycarbonylamino)benzoate 5 (2.10 g, 6.7 mmol)
and EEDQ (1.66 g, 6.7 mmol), the target compound was
obtained as a white solid (2.90 g, 85%) after flash
chromatography (eluent: EtOAc/hexane 1:4 gradient to
1:3). Rf 0.20 (EtOAc/hexane 1:4). Mp 154–156 8C. [a]2D0K
29.4 (c 0.97). 1H NMR: 0.89 (6H, d, JZ6.5 Hz, CH(CH3)2),
1.31 (3H, t, JZ7.1 Hz, CH2CH3), 1.39 (9H, s, C(CH3)3),
1.85–2.08 (1H, m, CHMe2), 3.91 (1H, t, JZ7.7 Hz, NCH),
4.30 (2H, q, JZ7.1 Hz, CH2Me), 5.17 (2H, s, PhCH2), 6.92
(1H, d, JZ8.5 Hz, NHCH), 7.28–7.49 (5H, m, PhH), 7.81
(1H, s, ArH), 7.97 (1H, s, ArH), 8.06 (1H, s, ArH), 10.02
(1H, s, CONHAr), 10.20 (1H, s, CONHAr). 13C NMR: 15.1,
19.4, 20.1, 29.1, 31.2, 61.6, 61.8, 66.8, 79.0, 114.1, 114.7,
114.9, 129.0, 129.4, 131.6, 137.5, 140.5, 140.8, 154.3,
156.5, 166.4, 172.0. MS (FAB): 514 [(MCH)C, 25%].
HRMS (FAB): calcd for C27H35N3O7CHC, 514.2548;
found, 514.2557.
4.1.1. Ethyl 3-amino-5-(N-benzyloxycarbonylamino)
benzoate 5. A mixture of benzyl chloroformate (14.4 mL,
100 mmol) and aqueous NaOH (100 mL, 1.0 M) in THF
(200 mL) was added dropwise to a stirred solution of ethyl
3,5-diaminobenzoate9 (18.0 g, 100 mmol) in THF (500 mL)
at 5 8C. After the mixture had been kept at 25 8C for 3 h, the
solvent was evaporated in vacuo. The residue was dissolved
in CHCl3 (300 mL) and washed with water (2!75 mL).
The organic layer was dried (MgSO4), filtered and
evaporated in vacuo. The residue was chromatographed
on silica gel (eluent: EtOAc/hexane 1:2) to afford the title
compound as a white solid (22.3 g, 71%). Rf 0.53 (EtOAc/
1
hexane 2:3). Mp 160–161 8C. H NMR: 1.28 (3H, t, JZ
7.1 Hz, CH2CH3), 4.24 (2H, q, JZ7.1 Hz, CH2Me), 5.13
(2H, s, PhCH2), 5.39 (2H, s, NH2), 6.85 (1H, s, ArH), 6.98
(1H, s, ArH), 7.26 (1H, s, ArH), 7.28–7.49 (5H, m, PhH),
9.67 (1H, s, CONHAr). 13C NMR: 15.2, 61.3, 66.6, 107.8,
108.6, 110.0, 129.0, 129.2, 129.4, 131.7, 137.7, 140.9,
150.3, 154.2, 167.1. MS (FAB): 315 [(MCH)C, 58%].
HRMS (FAB): calcd for C17H18N2O4CHC, 315.1339;
found, 315.1340. Anal. Calcd for C17H18N2O4: C, 64.96; H,
5.77; N, 8.91; found: C, 65.20; H, 5.88; N, 8.89.
4.3. General procedure for the preparation of G1-(aa1)
(NH2)-CO2Et 8
A suspension of G1-(aa1)(CbzNH)-CO2Et 7 and 10%
palladium on charcoal in MeOH was stirred under H2
for 1 h. The catalyst was removed by filtration and the
solvent evaporated in vacuo to give the target compound
G1-(aa1)(NH2)-CO2Et 8.
4.2. General procedure for the preparation of G1-(aa1)
(CbzNH)-CO2Et 7
4.3.1. G1-A(NH2)-CO2Et (8, aa1ZA). Starting from G1-
A(CbzNH)-CO2Et (2.00 g, 4.1 mmol), the target compound
was obtained as a white solid (1.38 g, 95%). Mp 142–
EEDQ (1.0 equiv) was added to a stirred mixture of BocNH-
(aa1)-CO2H 6 (1 equiv) and ethyl 3-amino-5-(N-benzyl-
oxycarbonylamino)benzoate 5 (1 equiv) in dry CH2Cl2 at
0 8C. The mixture was kept at 25 8C for 12 h after which the
solvent was evaporated in vacuo. The residue was
redissolved in EtOAc (200 mL), and washed successively
with saturated NaHCO3 solution (50 mL), 10% citric acid
solution (2!50 mL), saturated NaHCO3 solution (2!
50 mL), and water (2!50 mL). The organic layer was
dried (MgSO4), filtered and evaporated in vacuo to give the
target compound, which was further purified by flash
chromatography on silica gel.
144 8C. [a]2D0K49.5 (cZ0.77). H NMR: 1.22 (3H, d, JZ
1
7.1 Hz, CHCH3), 1.29 (3H, t, JZ7.0 Hz, CH2CH3), 1.38
(9H, s, C(CH3)3), 3.90–4.12 (1H, m, NCHMe), 4.25 (2H, q,
JZ7.1 Hz, CH2Me), 5.42 (2H, s, NH2), 6.89 (1H, s, ArH),
7.03 (1H, d, JZ7.1 Hz, OCONH), 7.18 (1H, s, ArH), 7.31
(1H, s, ArH), 9.80 (1H, s, CONHAr). 13C NMR: 15.2, 18.9,
29.1, 51.4, 61.3, 78.9, 108.7, 109.6, 110.5, 131.6, 140.8,
150.3, 156.1, 167.1, 172.8. MS (FAB): 351 (MC, 26%).
HRMS (FAB): calcd for C17H25N3OC5 , 351.1789; found,
351.1795.
4.2.1. G1-A(CbzNH)-CO2Et (7, aa1ZA). Starting from
BocNH-A-CO2H (1.02 g, 5.4 mmol), ethyl 3-amino-5-(N-
benzyloxycarbonylamino)benzoate 5 (1.70 g, 5.4 mmol)
and EEDQ (1.34 g, 5.4 mmol), the titled compound was
obtained as a white solid (2.37 g, 90%) after flash
chromatography (eluent: EtOAc/hexane 2:3). Rf 0.29
(EtOAc/hexane 2:3). Mp 153–155 8C. [a]2D0K42.5 (c
4.3.2. G1-V(NH2)-CO2Et (8, aa1ZV). Starting from
G1-V(CbzNH)-CO2Et (1.62 g, 3.2 mmol), the titled product
was obtained as a white solid (1.13 g, 94%). Mp 88–90 8C.
1
[a]2D0K34.1 (c 0.63). H NMR: 0.88 (6H, d, JZ6.3 Hz,
CH(CH3)2), 1.28 (3H, t, JZ7.3 Hz, CH2CH3), 1.38 (9H, s,
C(CH3)3), 1.85–2.08 (1H, m, CHMe2), 3.88 (1H, t, JZ
7.8 Hz, NCH), 4.25 (2H, q, JZ6.9 Hz, CH2Me), 5.42 (2H, s,
NH2), 6.84 (1H, d, JZ8.1 Hz, OCONH), 6.89 (1H, s, ArH),
7.19 (1H, s, ArH), 7.32 (1H, s, ArH), 9.85 (1H, s, CONHAr).
13C NMR: 15.2, 19.4, 20.1, 29.1, 31.3, 61.3, 61.6, 79.0,
108.7, 109.7, 110.6, 131.6, 140.6, 150.3, 156.5, 167.0,
171.6. MS (FAB): 379 (MC, 100%). HRMS (FAB): calcd
for C19H29N3OC5 , 379.2102; found, 379.2112.
1
1.16). H NMR: 1.25 (3H, d, JZ7.1 Hz, CHCH3), 1.31
(3H, t, JZ7.1 Hz, CH2CH3), 1.38 (9H, s, C(CH3)3), 4.02–
4.18 (1H, m, NCHMe), 4.30 (2H, q, JZ7.1 Hz, CH2Me),
5.17 (2H, s, PhCH2), 7.10 (1H, d, JZ7.1 Hz, NHCHMe),
7.28–7.49 (5H, m, PhH), 7.81 (1H, s, ArH), 7.95 (1H, s,
ArH), 8.05 (1H, s, ArH), 10.03 (1H, s, CONHAr), 10.15
(1H, s, CONHAr). 13C NMR: 15.1, 18.8, 29.2, 51.4, 61.8,
66.8, 79.0, 114.1, 114.6, 115.0, 129.0, 129.4, 131.6, 137.5,

11286
H.-F. Chow, J. Zhang / Tetrahedron 61 (2005) 11279–11290
4.4. General procedure for the preparation of G1-(aa1)
(aa2)-CO2Et 1
8.4 Hz, OCONH), 7.09–7.43 (6H, m, ArH and OCONH),
7.93 (1H, s, ArH), 7.96 (1H, s, ArH), 8.26 (1H, s, ArH),
10.23 (1H, s, CONHAr), 10.30 (1H, s, CONHAr). 13C
NMR: 15.1, 19.5, 20.1, 29.1, 31.2, 38.2, 57.6, 61.7, 61.9,
79.1, 115.1, 115.7, 127.3, 129.0, 130.2, 131.5, 138.9, 140.4,
140.5, 156.4, 156.5, 166.4, 172.1. MS (FAB): 626 (MC,
26%). HRMS (FAB): calcd for C33H46N4OC8 , 626.3310;
found, 626.3302.
EEDQ (1 equiv) was added to a stirred mixture of BocNH-
(aa2)-CO2H (1 equiv) and G1-(aa1)(NH2)-CO2Et (1 equiv)
in dry THF/CH2Cl2 (v/v 1:1) at 0 8C. The mixture was kept
at 25 8C overnight and the solvent was evaporated in vacuo.
The residue was redissolved in EtOAc (200 mL), and
washed successively with saturated NaHCO3 solution
(50 mL), 10% citric acid solution (2!50 mL), saturated
NaHCO3 solution (2!50 mL), and water (2!50 mL). The
organic layer was dried (MgSO4), filtered, and evaporated in
vacuo to give the target compound that was purified by flash
chromatography on silica gel.
4.5. General procedure for the preparation of G1-(aa1)
(aa2)-CO2H 2
An aqueous KOH solution (1.0 M) was added to a solution
of G1-(aa1)(aa2)-CO2Et 1 in MeOH. The reaction mixture
was stirred at 25 8C until completion of the reaction as
monitored by TLC analysis. The solvent was evaporated in
vacuo and the residue was poured into large amount of
water. The precipitate formed was collected by filtration,
washed with water and dried in vacuo.
4.4.1. G1-AF-CO2Et (1, aa1aa2ZAF). Starting from
BocNH-F-CO2H (0.75 g, 2.9 mmol), G1-A(NH2)-CO2Et
(1.00 g, 2.9 mmol) and EEDQ (0.70 g, 2.9 mmol), the target
compound was obtained as a white solid (1.48 g, 87%) after
flash chromatography (eluent: EtOAc/hexane 1:2). Rf 0.23
(EtOAc/hexane 1:2). Mp 167–169 8C. [a]2D0K12.1 (c 1.04).
1H NMR: 1.26 (3H, d, JZ7.1 Hz, CHCH3), 1.25–1.32 (12H,
m, CH2CH3 and C(CH3)3), 1.38 (9H, s, C(CH3)3), 2.72–2.91
(1H, m, CHHPh), 2.91–3.05 (1H, m, CHHPh), 4.00–4.15
(1H, m, NCHMe), 4.26–4.37 (3H, m, CH2Me and NCHBn),
7.03–7.39 (7H, m, ArH and OCONH), 7.93 (2H, s, ArH),
8.26 (1H, s, ArH), 10.18 (1H, s, CONHAr), 10.29 (1H, s,
CONHAr). 13C NMR: 15.1, 18.8, 29.1, 29.2, 38.2, 51.4,
57.6, 61.8, 79.0, 79.1, 115.1, 115.6, 127.3, 129.0, 130.2,
131.5, 138.9, 140.5, 140.6, 156.1, 156.3, 166.4, 172.1,
173.2. MS (FAB): 599 [(MCH)C, 31%]. HRMS (FAB):
calcd for C31H42N4O8CHC, 599.3075; found, 599.3067.
4.5.1. G1-AF-CO2H (2, aa1aa2ZAF). Starting from G1-
AF-CO2Et (0.50 g, 0.84 mmol) in MeOH (10 mL) and
aqueous KOH solution (2 mL, 1.0 M), the target compound
was obtained as a white solid (0.47 g, 98%). Tm 160 8C.
[a]2D0K14.1 (c 1.07). H NMR: 1.26 (3H, d, JZ7.2 Hz,
1
CHCH3), 1.32 (9H, s, C(CH3)3), 1.38 (9H, s, C(CH3)3),
2.72–2.91 (1H, m, CHHPh), 2.91–3.05 (1H, m, CHHPh),
3.90–4.15 (1H, m, NCHMe), 4.22–4.36 (1H, m, NCHBn),
7.03–7.39 (7H, m, ArH and OCONH), 7.92 (2H, s, ArH),
8.20 (1H, s, ArH), 10.13 (1H, s, CONHAr), 10.24 (1H, s,
CONHAr), 12.95 (1H, br s, COOH). 13C NMR: 18.8, 29.1,
29.2, 38.3, 51.5, 57.7, 79.0, 79.1, 114.8, 116.0, 127.3, 129.0,
130.2, 132.6, 138.9, 140.3, 140.5, 156.1, 156.4, 168.0,
172.0, 173.1. MS (FAB): 571 [(MCH)C, 100%]. HRMS
(FAB): calcd for C29H38N4O8CHC, 571.2762; found,
571.2770.
4.4.2. G1-AV-CO2Et (1, aa1aa2ZAV). Starting from
BocNH-V-CO2H (0.41 g, 1.9 mmol), G1-A(NH2)-CO2Et
(0.67 g, 1.9 mmol) and EEDQ (0.47 g, 1.9 mmol), the target
compound was obtained as a white solid (1.0 g, 98%) after
flash chromatography (eluent: EtOAc/hexane 1:2). Rf 0.17
(EtOAc/hexane 1:2). Mp 150–151 8C. [a]2D0K50.8 (c 1.05).
1H NMR: 0.89 (6H, d, JZ6.5 Hz, CH(CH3)2), 1.25 (3H, d,
JZ7.1 Hz, CHCH3), 1.32 (3H, t, JZ7.1 Hz, CH2CH3), 1.38
(18H, s, C(CH3)3), 1.85–2.08 (1H, m, CHMe2), 3.90 (1H, t,
JZ7.8 Hz, NCHCH), 4.05–4.16 (1H, m, NCHMe), 4.31
(2H, q, JZ7.0 Hz, CH2Me), 6.93 (1H, d, JZ8.3 Hz,
OCONH), 7.11 (1H, d, JZ7.0 Hz, OCONH), 7.94 (2H, s,
ArH), 8.25 (1H, s, ArH), 10.17 (1H, s, CONHAr), 10.21
(1H, s, CONHAr). 13C NMR: 15.1, 18.8, 19.4, 20.1, 29.2,
31.2, 51.4, 61.6, 61.8, 79.0, 115.1, 115.5, 115.6, 131.5,
140.4, 140.7, 156.1, 156.5, 166.4, 172.0, 173.2. MS (FAB):
551 [(MCH)C, 30%]. HRMS (FAB): calcd for
C27H42N4O8CHC, 551.3075; found, 551.3073.
4.5.2. G1-AV-CO2H (2, aa1aa2ZAV). Starting from G1-
AV-CO2Et (0.79 g, 1.4 mmol) in MeOH (10 mL) and
aqueous KOH solution (3.0 mL, 1.0 M), the titled com-
pound was obtained as a white solid (0.72 g, 96%). Tm
1
202 8C. [a]2D0K57.7 (c 0.22). H NMR: 0.89 (6H, d, JZ
6.6 Hz, CH(CH3)2), 1.25 (3H, d, JZ7.1 Hz, CHCH3), 1.38
(18H, s, C(CH3)3), 1.85–2.08 (1H, m, CHMe2), 3.91 (1H, t,
JZ7.7 Hz, NCHCH), 4.00–4.19 (1H, m, NCHMe), 6.91
(1H, d, JZ8.4 Hz, OCONH), 7.10 (1H, d, JZ7.1 Hz,
OCONH), 7.92 (2H, s, ArH), 8.19 (1H, s, ArH), 10.12 (1H,
s, CONHAr), 10.16 (1H, s, CONHAr), 12.98 (1H, br s
COOH). 13C NMR: 18.8, 19.5, 20.1, 29.2, 31.2, 51.4, 61.6,
78.98, 79.02, 114.9, 115.9, 116.0, 132.5, 140.2, 140.5,
156.1, 156.5, 167.9, 172.0, 173.1. MS (FAB): 523 [(MC
H)C, 7%]. HRMS (FAB): calcd for C25H38N4O8CHC,
523.2762; found, 523.2752.
4.4.3. G1-FV-CO2Et (1, aa1aa2ZFV). Starting from
BocNH-F-CO2H (0.70 g, 2.6 mmol), G1-V(NH2)-CO2Et
(1.00 g, 2.6 mmol) and EEDQ (0.65 g, 2.6 mmol), the
product was obtained as a white solid (1.43 g, 87%) after
flash chromatography (eluent: EtOAc/hexane 1:3). Rf 0.27
(EtOAc/hexane 1:2). Mp 114–116 8C. [a]2D0C8.6 (c 1.09).
1H NMR: 0.90 (6H, d, JZ6.6 Hz, CH(CH3)2), 1.17–1.35
(12H, m, CH2CH3 and C(CH3)3), 1.39 (9H, s, C(CH3)3),
1.85–2.08 (1H, m, CHMe2), 2.72–2.91 (1H, m, CHHPh),
2.91–3.05 (1H, m, CHHPh), 3.91 (1H, t, JZ7.9 Hz, NCH),
4.26–4.37 (3H, m, NCHBn and CH2Me), 6.94 (1H, d, JZ
4.5.3. G1-FV-CO2H (2, aa1aa2ZFV). Starting from G1-
FV-CO2Et (0.66 g, 1.1 mmol) in MeOH (10 mL) and
aqueous KOH solution (2.1 mL, 1.0 M), the target product
was obtained as a white solid (0.63 g, 100%). Tm 167 8C.
[a]2D0K7.3 (c 0.39). 1H NMR: 0.90 (6H, d, JZ6.5 Hz,
CH(CH3)2), 1.32 (9H, s, C(CH3)3), 1.39 (9H, s, C(CH3)3),
1.85–2.08 (1H, m, CHMe2), 2.72–2.91 (1H, m, CHHPh),
2.91–3.05 (1H, m, CHHPh), 3.91 (1H, t, JZ7.8 Hz,
NCHCH), 4.23–4.36 (1H, m, NCHBn), 6.92 (1H, d, JZ

H.-F. Chow, J. Zhang / Tetrahedron 61 (2005) 11279–11290
11287
8.3 Hz, OCONH), 7.09–7.43 (6H, m, ArH and OCONH),
7.93 (2H, s, ArH), 8.20 (1H, s, ArH), 10.18 (1H, s,
CONHAr), 10.25 (1H, s, CONHAr), 12.98 (1H, br s,
COOH). 13C NMR: 19.5, 20.1, 29.1, 31.2, 38.3, 57.7, 61.7,
79.1, 114.9, 116.1, 127.3, 129.0, 130.2, 132.6, 138.9, 140.2,
140.4, 156.4, 156.5, 168.0, 172.0. MS (FAB): 599 [(MC
H)C, 100%]. HRMS (FAB): calcd for C31H42N4O8CHC,
599.3075; found, 599.3079.
4.6.3. G1-V2-CONH-V-CO2Me (10, aa1ZV). Starting
from L-valine methyl ester hydrochloride (1.68 g,
10.0 mmol), G1-V2-CO2H8 (5.50 g, 10.0 mmol), 4-methyl-
morpholine (1.01 g, 10.0 mmol), DCC (2.06 g, 10.0 mmol)
and HOBt (1.35 g, 10.0 mmol), the titled compound was
obtained as a white solid (5.34 g, 80%) after flash
chromatography (eluent: EtOAc/hexane 3:8 gradient to
2:5). Rf 0.46 (EtOAc/hexane 1:1). Tm 196 8C. [a]2D0K15.3 (c
0.79). 1H NMR: 0.87–0.99 (18H, m, (CH3)2CH), 1.38 (18H,
s, C(CH3)3), 1.85–2.08 (2H, m, CHMe2), 2.08–2.28 (1H, m,
CHMe2), 3.65 (3H, s, CO2CH3), 3.93 (2H, t, JZ7.9 Hz,
NCHCON), 4.26 (1H, t, JZ7.5 Hz, NCHCO2Me), 6.91
(2H, d, JZ8.5 Hz, OCONH), 7.66 (2H, s, ArH), 8.19 (1H, s,
ArH), 8.63 (1H, d, JZ7.5 Hz, ArCONH), 10.12 (2H, s,
CONHAr). 13C NMR: 19.5, 19.9, 20.1, 20.2, 29.2, 30.4,
31.2, 52.6, 59.5, 61.6, 79.0, 113.7, 114.6, 136.6, 139.9,
156.5, 168.4, 171.9, 173.1. MS (FAB): 663 (MC, 11%).
HRMS (FAB): calcd for C33H53N5OC9 , 663.3838; found,
663.3832.
4.6. General procedure for the synthesis of G1-(aa1)2-
CONH-(aa1)-CO2Me 10
DCC (1 equiv) was added to a stirred mixture of ClK
H3NC-(aa1)-CO2Me (1 equiv), G1-(aa1)2-CO2H8
9
(1 equiv), 4-methylmorpholine (1 equiv) and HOBt
(1 equiv) in dry THF at K10 8C. After stirring at K10 8C
for 2 h and then at 25 8C for 20 h, the insoluble DCU was
removed by filtration and the solvent evaporated in vacuo.
The residue was dissolved in EtOAc, and washed
successively with saturated NaHCO3 solution, 10% citric
acid solution, saturated NaHCO3 solution, and water. The
organic layer was dried (MgSO4), filtered and evaporated in
vacuo to give the target compound 10 that was purified by
flash chromatography on silica gel.
4.7. General procedure for the synthesis of G1-(aa1)2-
CONH-(aa1)-CO2H 11
A mixture of aqueous KOH solution (1.0 M) and G1-(aa1)2-
CONH-(aa1)-CO2Me 10 in MeOH was stirred at 25 8C until
completion of the reaction as monitored by TLC analysis.
The solvent was evaporated in vacuo and the residue poured
into large amount of water. The precipitate formed was
collected by filtration, washed with water and dried in
vacuo.
4.6.1. G1-A2-CONH-A-CO2Me (10, aa1ZA). Starting
from L-alanine methyl ester hydrochloride (1.40 g,
10.0 mmol), G1-A2-CO2H8 (4.94 g, 10.0 mmol), 4-methyl-
morpholine (1.01 g, 10.0 mmol), DCC (2.06 g, 10.0 mmol)
and HOBt (1.35 g, 10.0 mmol), the target compound was
obtained as a white solid (3.60 g, 62%) after flash
chromatography (eluent: EtOAc/hexane 1:1 gradient to
2:1). Rf 0.20 (EtOAc/hexane 2:1). Tm 182 8C. [a]2D0K50.8 (c
4.7.1. G1-A2-CONH-A-CO2H (11, aa1ZA). Starting from
G1-A2-CONH-A-CO2Me (1.30 g, 2.3 mmol) in MeOH
(20 mL) and aqueous KOH solution (5.0 mL, 1.0 M), the
titled compound was obtained as a white solid (1.16 g,
91%). Tm 182 8C. [a]2D0K43.9 (c 0.12). 1H NMR: 1.26 (6H,
d, JZ7.0 Hz, CH3CHCON), 1.29–1.49 (21H, m, C(CH3)3
and CH3CHCO2H), 3.92–4.21 (2H, m, NCHCON), 4.30–
4.45 (1H, m, NCHCO2H), 7.09 (2H, d, JZ7.1 Hz,
OCONH), 7.67 (2H, s, ArH), 8.14 (1H, s, ArH), 8.64 (1H,
d, JZ6.8 Hz, ArCONH), 10.07 (2H, s, CONHAr), 12.52
(1H, br s, CO2H). 13C NMR: 17.8, 18.9, 29.2, 49.1, 51.4,
79.0, 113.8, 114.4, 136.5, 140.1, 156.1, 167.5, 173.0, 175.0.
MS (FAB): 565 (MC, 8%). HRMS (FAB): calcd for
C26H39N5O9CHC, 566.2821; found, 566.2809.
1
1.10). H NMR: 1.25 (6H, d, JZ7.1 Hz, CH3CH), 1.33–
1.40 (21H, m, C(CH3)3 and CH3CH), 3.64 (3H, s, CO2CH3),
3.92–4.21 (2H, m, NCHCON), 4.35–4.51 (1H, m,
NCHCO2Me), 7.09 (2H, d, JZ7.1 Hz, OCONH), 7.68
(2H, s, ArH), 8.13 (1H, s, ArH), 8.81 (1H, d, JZ6.8 Hz,
ArCONH), 10.07 (2H, s, CONHAr). 13C NMR: 17.7, 18.9,
29.2, 49.2, 51.4, 52.8, 79.0, 113.9, 114.4, 136.3, 140.2,
156.1, 167.7, 173.0, 174.0. MS (FAB): 579 (MC, 27%).
HRMS (FAB): calcd for C27H41N5OC9 , 579.2899; found,
579.2890.
4.6.2. G1-F2-CONH-F-CO2Me (10, aa1ZF). Starting
from L-phenylalanine methyl ester hydrochloride (2.16 g,
10.0 mmol), G1-F2-CO2H8 (6.46 g, 10.0 mmol), 4-methyl-
morpholine (1.01 g, 10.0 mmol), DCC (2.06 g, 10.0 mmol)
and HOBt (1.35 g, 10.0 mmol), the product was obtained as
a white solid (5.12 g, 63%) after flash chromatography
(eluent: EtOAc/hexane 2:3). Rf 0.46 (EtOAc/hexane 1:1).
4.7.2. G1-F2-CONH-F-CO2H (11, aa1ZF). Starting from
G1-F2-CONH-F-CO2Me (2.05 g, 2.5 mmol) in MeOH
(40 mL) and aqueous KOH solution (6.0 mL, 1.0 M), the
target compound was obtained as a white solid (1.95 g,
97%). Tm 159 8C. [a]2D0C42.0 (c 1.30). 1H NMR: 1.32 (18H,
m, C(CH3)3), 2.72–2.91 (2H, m, PhCHH), 2.91–3.08 (2H,
m, PhCHH), 3.08–3.25 (2H, m, PhCH2CHCO2H), 4.18–
4.42 (2H, m, NCHCON), 4.52–4.65 (1H, m, NCHCO2H),
7.06–7.42 (17H, m, ArH and OCONH), 7.65 (2H, s, ArH),
8.13 (1H, s, ArH), 8.65 (1H, d, JZ7.4 Hz, ArCONH), 10.21
(2H, s, CONHAr), 12.80 (1H, br s, CO2H). 13C NMR: 29.1,
37.2, 38.3, 39.6, 55.0, 57.6, 79.0, 114.0, 114.5, 127.3, 129.0,
129.1, 130.0, 130.2, 136.5, 138.9, 139.1, 140.0, 156.3,
167.4, 172.0, 174.0. MS (FAB): 794 [(MCH)C, 7%].
HRMS (FAB): calcd for C44H51N5O9CHC, 794.3760;
found, 794.3772.
1
Tm 195 8C. [a]2D0C46.6 (c 0.85). H NMR: 1.33 (18H, s,
C(CH3)3), 2.72–2.91 (2H, m, PhCHH), 2.91–3.08 (2H, m,
PhCHH), 3.08–3.25 (2H, m, PhCH2CHCO2Me), 3.65 (3H,
s, CO2CH3), 4.18–4.42 (2H, m, NCHCON), 4.58–4.72 (1H,
m, NCHCO2Me), 7.06–7.42 (17H, m, ArH and OCONH),
7.68 (2H, s, ArH), 8.13 (1H, s, ArH), 8.90 (1H, d, JZ7.2 Hz,
ArCONH), 10.23 (2H, s, CONHAr). 13C NMR: 29.1, 37.1,
38.3, 52.9, 55.1, 57.6, 79.1, 114.1, 114.5, 127.3, 127.4,
129.0, 129.2, 130.0, 130.2, 136.2, 138.6, 138.9, 140.1,
156.4, 167.7, 172.0, 173.0. MS (FAB): 808 [(MCH)C, 7%].
HRMS (FAB): calcd for C45H53N5O9CHC, 808.3916;
found, 808.3940.

11288
H.-F. Chow, J. Zhang / Tetrahedron 61 (2005) 11279–11290
4.7.3. G1-V2-CONH-V-CO2H (11, aa1ZV). Starting from
G1-V2-CONH-V-CO2Me (0.39 g, 0.6 mmol) in MeOH
(10 mL) and aqueous KOH solution (1.2 mL, 1.0 M), the
titled compound was obtained as a white solid (0.36 g,
94%). Tm 194 8C. [a]2D0K8.9 (c 0.61). 1H NMR: 0.90 (12H,
d, JZ6.0 Hz, (CH3)2CH), 0.96 (6H, d, JZ4.7 Hz,
(CH3)2CH), 1.38 (18H, s, C(CH3)3), 1.85–2.08 (2H, m,
Me2CHCHCONH), 2.08–2.28 (1H, m, Me2CHCHCO2H),
3.93 (2H, t, JZ7.5 Hz, NCHCONH), 4.25 (1H, t, JZ
7.1 Hz, NCHCO2H), 6.90 (2H, d, JZ8.2 Hz, OCONH),
7.66 (2H, s, ArH), 8.20 (1H, s, ArH), 8.38 (1H, d, JZ7.3 Hz,
ArCONH), 10.12 (2H, s, CONHAr), 12.64 (1H, br s,
CO2H). 13C NMR: 19.4, 19.6, 20.17, 20.20, 29.1, 30.4, 31.2,
59.2, 61.6, 79.0, 113.6, 114.5, 136.8, 139.9, 156.5, 168.2,
171.9, 173.9. MS (ESI): 672 [(MC Na)C, 100%]. HRMS
(ESI): calcd for C32H51N5O9C NaC, 672.3579; found,
672.3579.
DCC (0.36 g, 1.8 mmol), and HOBt (0.24 g, 1.8 mmol), the
target product was obtained as a white solid (1.23 g, 64%)
after precipitation from H2O/MeOH. Tm 195 8C. [a]2D0C
1
54.9 (c 1.01). H NMR: 1.18–1.42 (21H, m, CH2CH3 and
C(CH3)3), 2.72–2.91 (2H, m, PhCHH), 2.91–3.08 (2H, m,
PhCHH), 3.08–3.25 (2H, m, interior PhCH2C), 4.20–4.42
(4H, m, surface NCH and CH2Me), 4.78–4.92 (1H, m,
interior NCH), 5.18 (2H, s, PhCH2O), 7.06–7.42 (22H, m,
ArH and OCONHCH), 7.69 (2H, s, surface ArH), 7.84 (1H,
s, interior ArH), 7.98 (1H, s, interior ArH), 8.11 (1H, s,
interior ArH), 8.14 (1H, s, surface ArH), 8.71 (1H, br s,
ArCONH), 10.05 (1H, s, CONHAr), 10.22 (2H, s, surface
CONHAr), 10.43 (1H, s, interior CONHAr). 13C NMR:
15.1, 29.1, 38.1, 38.3, 56.7, 57.6, 61.8, 66.8, 79.0, 114.0,
114.2, 114.6, 114.8, 115.1, 127.3, 129.0, 129.1, 129.4,
130.2, 131.6, 136.5, 137.5, 138.9, 140.0, 140.6, 140.8,
154.3, 156.4, 166.4, 167.7, 171.4, 172.0. MS (FAB): 1090
[(MCH)C, 100%]. HRMS (FAB): calcd for
C61H67N7O12CHC, 1090.4920; found, 1090.4942.
4.8. General procedure for the synthesis of
G2-(aa1)3(CbzNH)-CO2Et 12
4.9. General procedure for the synthesis of
G2-(aa1)3(NH2)-CO2Et 13
DCC or EDCI (1 equiv) was added to a stirred mixture of
G1-(aa1)2-CONH-(aa1)-CO2H (1 equiv), ethyl 3-amino-5-
(N-benzyloxycarbonylamino)benzoate 5 (1 equiv) and
HOBt (1 equiv) in dry THF at K10 8C. The mixture was
kept at K10 8C for 2 h and then at 25 8C for 20 h. When
DCC was used as coupling reagent, the insoluble DCU
produced was first removed by filtration. The filtrate or the
reaction solvent (in the case of EDCI as the coupling agent)
was then evaporated in vacuo. The residue was redissolved
in EtOAc, and washed successively with saturated NaHCO3
solution, 10% citric acid solution, saturated NaHCO3
solution, and water. The organic layer was dried (MgSO4),
filtered, and evaporated in vacuo to give the target
compound that was purified by precipitation or flash
chromatography on silica gel.
A suspension of G2-(aa1)3(CbzNH)-CO2Et 12 and 10%
palladium on charcoal in MeOH was stirred under H2 for
1 h. After the reaction, the catalyst was removed by filtration
and the solvent evaporated in vacuo to give the target
compound.
4.9.1. G2-A3(NH2)-CO2Et (13, aa1ZA). Starting from
G2-A3(CbzNH)-CO2Et (1.30 g, 1.5 mmol), the titled com-
pound was obtained as a white solid (1.08 g, 98%). Tm
1
189 8C. [a]2D0K27.9 (c 0.69). H NMR: 1.26 (6H, d, JZ
6.2 Hz, surface NCHCH3), 1.29 (3H, t, JZ7.1 Hz,
CH2CH3), 1.32–1.42 (21H, m, C(CH3)3 and interior
NCHCH3), 4.02–4.18 (2H, m, surface NCH), 4.25 (2H, q,
JZ7.1 Hz, CH2Me), 4.42–4.60 (1H, m, interior NCH), 5.42
(2H, s, NH2), 6.90 (1H, s, interior ArH), 7.09 (2H, d, JZ
7.2 Hz, OCONH), 7.20 (1H, s, interior ArH), 7.35 (1H, s,
interior ArH), 7.70 (2H, s, surface ArH), 8.15 (1H, s, surface
ArH), 8.52 (1H, br s, ArCONH), 9.95 (1H, s, CONHAr),
10.07 (2H, s, CONHAr). 13C NMR: 15.2, 18.8, 18.9, 29.2,
50.8, 51.4, 61.3, 79.0, 108.7, 109.7, 110.6, 113.8, 114.4,
131.6, 136.5, 140.2, 140.8, 150.3, 156.1, 167.1, 167.5,
172.2, 173.0. MS (FAB): 727 (MC, 100%). HRMS (FAB):
calcd for C35H49N7OC10, 727.3535; found, 727.3542.
4.8.1. G2-A3(CbzNH)-CO2Et (12, aa1ZA). Starting from
G1-A2-CONH-A-CO2H (1.57 g, 2.8 mmol), ethyl 3-amino-
5-(N-benzyloxycarbonylamino)benzoate (0.87 g, 2.8 mmol),
EDCI (0.85 g, 2.8 mmol) and HOBt (0.38 g, 2.8 mmol), the
desired compound was obtained as a white solid (1.66 g,
69%) after flash chromatography (eluent: EtOAc/hexane
2:1). Rf 0.24 (EtOAc/hexane 2:1). Tm 202 8C. [a]2D0K21.3 (c
1
1.01). H NMR: 1.26 (6H, d, JZ7.1 Hz, surface CHCH3),
1.30 (3H, t, JZ7.1 Hz, CH2CH3), 1.32–1.48 (21H, m,
C(CH3)3 and interior CHCH3), 4.02–4.18 (2H, m, surface
NCH), 4.30 (2H, q, JZ7.1 Hz, CH2Me), 4.48–4.64 (1H, m,
interior NCH), 5.17 (2H, s, PhCH2), 7.09 (2H, d, JZ7.1 Hz,
OCONHCH), 7.28–7.49 (5H, m, ArH), 7.70 (2H, s, surface
ArH), 7.81 (1H, interior ArH), 7.97 (1H, s, interior ArH),
8.08 (1H, s, interior ArH), 8.16 (1H, s, surface ArH), 8.58
(1H, br s, ArCONH), 10.02 (1H, s, CONHAr), 10.07 (2H, s,
CONHAr) 10.29 (1H, s, CONHAr). 13C NMR: 15.1, 18.7,
18.9, 29.2, 50.8, 51.4, 61.8, 66.8, 79.0, 113.8, 114.1, 114.5,
114.7, 115.0, 129.0, 129.4, 131.6, 136.4, 137.5, 140.1,
140.7, 154.3, 156.1, 166.4, 167.6, 172.5, 173.0. MS (FAB):
862 [(MCH)C, 100%]. HRMS (FAB): calcd for
C43H55N7O12CHC, 862.3981; found, 862.3991.
4.9.2. G2-F3(NH2)-CO2Et (13, aa1ZF). Starting from
G2-F3(CbzNH)-CO2Et (1.22 g, 1.1 mmol), the titled com-
pound was obtained as a white solid (1.02 g, 95%). Tm
191 8C. [a]2D0C53.0 (c 0.70). 1H NMR: 1.18–1.42 (21H, m,
CH2CH3 and C(CH3)3), 2.72–2.91 (2H, m, PhCHH), 2.91–
3.08 (2H, m, PhCHH), 3.08–3.25 (2H, m, PhCH2), 4.26 (2H,
q, JZ7.1 Hz, CH2Me), 4.20–4.42 (2H, m, surface NCH),
4.78–4.92 (1H, m, interior NCH), 5.45 (2H, s, NH2), 6.92 (s,
1H, interior ArH), 7.06–7.42 (19H, m, ArH, OCONH and
interior ArH), 7.69 (2H, s, surface ArH), 8.13 (1H, s, surface
ArH), 8.67 (1H, br s, ArCONH), 10.10 (1H, s, interior
CONHAr), 10.23 (2H, s, surface CONHAr). 13C NMR:
15.2, 29.1, 38.2, 38.3, 56.8, 57.6, 61.4, 79.0, 108.8, 109.8,
110.7, 113.9, 114.6, 127.3, 129.0, 129.1, 130.2, 131.6,
136.5, 138.9, 140.0, 140.6, 150.3, 156.4, 167.1, 167.6,
4.8.2. G2-F3(CbzNH)-CO2Et (12, aa1ZF). Starting from
G1-F2-CONH-F-CO2H (1.40 g, 1.8 mmol), ethyl 3-amino-
5-(N-benzyloxycarbonylamino)benzoate (0.55 g, 1.8 mmol),

H.-F. Chow, J. Zhang / Tetrahedron 61 (2005) 11279–11290
11289
171.1, 172.0. MS (FAB): 955 (MC, 100%). HRMS (FAB):
calcd for C53H61N7OC10, 955.4474; found, 955.4492.
7.9 Hz, surface NCHCHMe2), 4.05–4.19 (2H, m, surface
NCHMe), 4.31 (2H, q, JZ7.1 Hz, CH2Me), 4.35 (1H, t, JZ
7.0 Hz, interior NCHCHMe2), 4.48–4.64 (1H, m, interior
NCHMe), 6.90 (2H, d, JZ8.4 Hz, OCONH), 7.08 (2H, d,
JZ7.1 Hz, OCONH), 7.70 (4H, s, surface ArH), 7.98 (1H, s,
ArH), 8.00 (1H, s, ArH), 8.16 (1H, s, ArH), 8.20 (1H, s,
ArH), 8.31 (1H, s, ArH), 8.43 (1H, br s, ArCONH), 8.59
(1H, br s, ArCONH), 10.07 (2H, s, surface CONHAr), 10.13
(2H, s, surface CONHAr), 10.32 (1H, s, CONHAr), 10.41
(1H, s, CONHAr). 13C NMR: 15.1, 18.6, 18.9, 19.4, 20.0,
20.2, 29.1, 31.1, 31.2, 50.9, 51.4, 61.0, 61.6, 61.8, 79.0,
113.7, 114.5, 115.2, 115.7, 115.8, 131.5, 136.4, 136.7,
139.9, 140.1, 140.3, 140.6, 156.1, 156.5, 166.4, 167.6,
168.1, 171.6, 171.9, 172.6, 173.0. MS (FAB): 1382 [(MC
Na)C, 12%]. HRMS (FAB): calcd for C67H98N12O18C
NaC, 1381.7014; found, 1381.7036.
4.10. General procedure for the preparation of
G2-(aa1)3(aa2)3-CO2Et 3
DCC or EDCI (1 equiv) was added to a stirred mixture of
G2-(aa1)3(NH2)-CO2Et 13 (1 equiv), G1-(aa2)2-CONH-
(aa2)-CO2H (1 equiv) and HOBt (1 equiv) in dry THF at
K10 8C. The mixture was kept at K10 8C for 2 h and then
at 25 8C for 48 h. When DCC was used as the coupling
agent, the insoluble DCU produced was removed by
filtration. The filtrate or the reaction solvent (in the case
of EDCI as the coupling agent) was evaporated in vacuo.
The residue was redissolved in CHCl3, and washed
successively with saturated NaHCO3 solution, 10% citric
acid solution, saturated NaHCO3 solution and water. The
organic layer was dried (MgSO4), filtered and evaporated in
vacuo to give the target compound that was purified by flash
chromatography on silica gel.
4.10.3. G2-F3V3-CO2Et (3, aa1aa2ZFV). Starting from
G2-F3(NH2)-CO2Et (0.41 g, 0.43 mmol), G1-V2-CONH-V-
CO2H (0.28 g, 0.43 mmol), EDCI (0.13 g, 0.43 mmol) and
HOBt (0.06 g, 0.43 mmol), the target compound was
obtained as a white solid (0.31 g, 46%) after flash
chromatography (eluent: CHCl3/MeOH 80:1 gradient to
60:1). Rf 0.18 (CHCl3/MeOH 50:1). Tm 192 8C. [a]2D0C78.3
(c 0.79). 1H NMR: 0.90 (12H, d, JZ6.5 Hz, surface
CH(CH3)2), 0.99 (3H, d, JZ6.8 Hz, interior CH(CH3)2),
1.01 (3H, d, JZ6.8 Hz, interior CH(CH3)2), 1.14–1.35
(21H, m, CH2CH3 and C(CH3)3), 1.38 (18H, s, C(CH3)3),
1.85–2.08 (2H, m, surface CHMe2), 2.10–2.29 (1H, m,
interior CHMe2), 2.70–2.91 (2H, m, surface PhCHH), 2.91–
3.08 (2H, m, surface PhCHH), 3.08–3.25 (2H, m, interior
PhCH2), 3.93 (2H, t, JZ8.0 Hz, surface NCHCHMe2),
4.17–4.49 (5H, CH2Me, interior NCHCHMe2 and surface
CHBn), 4.70–4.92 (1H, m, interior CHBn), 6.90 (2H, d, JZ
8.6 Hz, OCONH), 7.03–7.45 (17H, m, ArH and OCONH),
7.69 (2H, s, surface ArH), 7.71 (2H, s, surface ArH), 8.00
(2H, s, ArH), 8.13 (1H, s, ArH), 8.19 (1H, s, ArH), 8.31 (1H,
s, ArH), 8.42 (1H, br s, ArCONH), 8.73 (1H, br s,
ArCONH), 10.13 (2H, s, surface CONHAr), 10.21 (2H, s,
surface CONHAr), 10.43 (1H, s, interior CONHAr), 10.44
(1H, s, interior CONHAr). 13C NMR: 15.2, 19.5, 20.0, 20.2,
29.1, 31.1, 31.2, 38.0, 38.3, 56.9, 57.6, 61.1, 61.6, 61.9,
79.0, 113.7, 114.0, 114.6, 115.3, 116.0, 127.3, 129.0, 129.1,
130.2, 131.6, 136.5, 136.7, 138.9, 140.0, 140.3, 140.5,
156.4, 156.5, 166.4, 167.8, 168.1, 171.5, 171.6, 171.9,
172.0. MS (ESI): 1610 [(MCNa)C, 100%]. HRMS (ESI):
calcd for C85H110N12O18C NaC, 1609.7953; found,
1609.7946.
4.10.1. G2-A3F3-CO2Et (3, aa1aa2ZAF). Starting from
G2-F3(NH2)-CO2Et (0.75 g, 0.78 mmol), G1-A2-CONH-A-
CO2H (0.44 g, 0.78 mmol), DCC (0.16 g, 0.78 mmol) and
HOBt (0.11 g, 0.81 mmol), the target compound was
obtained as a white solid (0.51 g, 43%) after flash
chromatography (eluent: CHCl3/MeOH 60:1). Rf 0.14
(CHCl3/MeOH 50:1). Tm 191 8C. [a]2D0C25.8 (c 0.89). H
1
NMR: 1.26 (6H, d, JZ7.1 Hz, surface CHCH3), 1.27–1.51
(42H, m, C(CH3)3, interior CHCH3 and CH2CH3), 2.70–
2.91 (2H, m, surface PhCHH), 2.91–3.08 (2H, m, surface
PhCHH), 3.08–3.25 (2H, m, interior PhCH2), 3.98–4.19
(2H, m, surface NCHMe), 4.19–4.44 (4H, m, surface
NCHBn and CH2Me), 4.48–4.64 (1H, m, interior
NCHMe), 4.75–4.92 (1H, m, interior NCHBn), 7.08 (2H,
d, JZ7.1 Hz, OCONHMe), 7.11–7.45 (17H, m, PhH and
OCONHBn), 7.69 (2H, s, surface ArH), 7.71 (2H, s, surface
ArH), 7.99 (2H, s, interior ArH), 8.14 (1H, s, surface ArH),
8.16 (1H, s, surface ArH), 8.31 (1H, s, interior ArH),
8.60 (1H, br s, ArCONHCHMe), 8.73 (1H, br s,
ArCONHCHBn), 10.08 (2H, s, surface CONHAr), 10.22
(2H, s, surface CONHAr), 10.34 (1H, s, interior CONHAr),
10.44 (1H, s, interior CONHAr). 13C NMR: 15.1, 18.6, 18.9,
29.1, 38.1, 38.3, 50.9, 51.4, 56.8, 57.6, 61.8, 78.96, 79.04,
113.8, 114.5, 114.6, 115.3, 115.8, 127.3, 129.0, 129.1,
130.2, 131.5, 136.5, 138.9, 140.0, 140.1, 140.5, 140.6,
156.1, 156.4, 166.4, 167.7, 171.4, 172.0, 172.6, 173.0. MS
(FAB): 1526 [(MC Na)C, 100%]. HRMS (ESI): calcd for
C79H98N12O18C NaC, 1525.7014; found, 1525.7036.
4.11. General procedure for the synthesis of
G2-(aa1)3(aa2)3-CO2H 4
4.10.2. G2-A3V3-CO2Et (3, aa1aa2ZAV). Starting from
G2-A3(NH2)-CO2Et (0.42 g, 0.58 mmol), G1-V2-CONH-V-
CO2H (0.37 g, 0.58 mmol), DCC (0.12 g, 0.58 mmol) and
HOBt (0.08 g, 0.58 mmol), the target product was obtained
as a white solid (0.27 g, 35%) after flash chromatography
(eluent: CHCl3/MeOH 70:1 gradient to 30:1). Rf 0.55
A mixture of aqueous KOH solution (1.0 M) and
G2-(aa1)3(aa2)3-CO2Et 3 in MeOH was stirred at 25 8C
until completion of the reaction as monitored by TLC. The
solvent was evaporated in vacuo and the residue was poured
into large amount of water. The precipitate formed was
collected by filtration, washed with water, and dried in
vacuo.
(CHCl3/MeOH 50:1). Tm 194 8C. [a]2D0C38.5 (c 0.86). H
1
NMR: 0.90 (12H, d, JZ6.5 Hz, surface CH(CH3)2), 0.97
(3H, d, JZ6.9 Hz, interior CH(CH3)2), 1.00 (3H, d, JZ
6.9 Hz, interior CH(CH3)2), 1.26 (6H, d, JZ7.1 Hz, surface
CHCH3), 1.32–1.55 (42H, m, C(CH3)3, CH2CH3 and
interior CHCH3), 1.85–2.08 (2H, m, surface CHMe2),
2.10–2.29 (1H, m, interior CHMe2), 3.92 (2H, t, JZ
4.11.1. G2-A3F3-CO2H (4, aa1aa2ZAF). Starting from
G2-A3F3-CO2Et (0.32 g, 0.21 mmol) in MeOH (10 mL) and
aqueous KOH solution (0.5 mL, 1.0 M), the target

11290
H.-F. Chow, J. Zhang / Tetrahedron 61 (2005) 11279–11290
compound was obtained as a white solid (0.30 g, 95%). Tm
169 8C. [a]2D0C22.9 (c 0.52). H NMR: 1.26 (6H, d, JZ
4.70–4.92 (1H, m, interior NCHBn), 6.90 (2H, d, JZ8.3 Hz,
OCONH), 7.03–7.45 (17H, m, ArH and OCONH), 7.68
(2H, s, surface ArH), 7.71 (2H, s, surface ArH), 7.98 (2H, s,
ArH), 8.14 (1H, s, ArH), 8.20 (1H, s, ArH), 8.25 (1H, s,
ArH), 8.42 (1H, br s, ArCONH), 8.73 (1H, br s, ArCONH),
10.13 (2H, s, surface CONHAr), 10.22 (2H, s, surface
CONHAr), 10.39 (1H, s, interior CONHAr), 10.40 (1H, s,
interior CONHAr), 12.91 (1H, br s, COOH). 13C NMR:
19.5, 20.0, 20.2, 29.1, 31.2, 38.0, 38.3, 56.9, 57.6, 61.0,
61.6, 78.99, 79.04, 113.6, 113.9, 114.5, 115.1, 116.2, 127.3,
129.0, 129.1, 130.2, 132.5, 136.5, 136.7, 138.9, 139.96,
140.01, 140.2, 140.3, 156.4, 156.5, 167.8, 168.0, 168.1,
171.4, 171.5, 171.9, 172.0, 172.2. MS (FAB): 1582 [(MC
Na)C, 100%]. HRMS (ESI): calcd for C83H106N12O18C
NaC, 1581.7640; found, 1581.7656.
1
7.2 Hz, surface CHCH3), 1.27–1.51 (39H, m, C(CH3)3,
interior CHCH3), 2.70–2.91 (2H, m, surface PhCHH), 2.91–
3.08 (2H, m, surface PhCHH), 3.08–3.25 (2H, m, interior
PhCH2), 3.98–4.19 (2H, m, surface NCHMe), 4.19–4.44
(2H, m, surface NCHBn), 4.48–4.64 (1H, m, interior
NCHMe), 4.75–4.92 (1H, m, interior NCHBn), 7.08 (2H,
d, JZ7.1 Hz, OCONH), 7.11–7.45 (17H, m, ArH and
OCONH), 7.69 (2H, s, surface ArH), 7.71 (2H, s, surface
ArH), 7.96 (1H, s, ArH), 7.98 (1H, s, ArH), 8.14 (1H, s,
ArH), 8.16 (1H, s, ArH), 8.26 (1H, s, ArH), 8.60 (1H, br s,
ArCONH), 8.72 (1H, br s, ArCONH), 10.08 (2H, s, surface
CONHAr), 10.22 (2H, s, surface CONHAr), 10.29 (1H, s,
interior CONHAr), 10.40 (1H, s, interior CONHAr), 12.93
(1H, br s, COOH). 13C NMR: 18.7, 18.9, 29.2, 38.1, 38.3,
50.9, 51.4, 56.8, 57.6, 79.0, 79.1, 113.8, 114.5, 114.6, 115.1,
116.1, 127.3, 129.0, 129.1, 130.2, 132.5, 136.5, 138.9,
140.0, 140.2, 140.3, 140.5, 156.1, 156.4, 167.7, 168.0,
171.4, 172.0, 172.6, 173.0. MS (FAB): 1476 [(MCH)C,
10%]. HRMS (FAB): calcd for C77H94N12O18CHC,
1475.6882; found, 1475.6873.
Acknowledgements
The work described in this paper was fully supported by a
grant from the Research Grants Council, HKSAR (Project
no. CUHK4273/00P).
4.11.2. G2-A3V3-CO2H (4, aa1aa2ZAV). Starting from
G2-A3V3-CO2Et (0.34 g, 0.25 mmol) in MeOH (7.0 mL)
and aqueous KOH solution (1.0 mL, 1.0 M), the titled
compound was obtained as a white solid (0.33 g, 99%). Tm
181 8C. [a]2D0C27.9 (c 0.89). H NMR: 0.90 (12H, d, JZ
1
References and notes
6.4 Hz, surface CH(CH3)2), 0.97 (3H, d, JZ7.0 Hz, interior
CH(CH3)2), 1.00 (3H, d, JZ7.0 Hz, interior CH(CH3)2),
1.26 (6H, d, JZ6.9 Hz, surface CHCH3), 1.31–1.55 (39H,
m, C(CH3)3 and interior CHCH3), 1.95–2.10 (2H, m,
surface CHMe2), 2.10–2.29 (1H, m, interior CHMe2), 3.93
(2H, t, JZ7.7 Hz, surface NCHCMe2), 4.07–4.21 (2H, m,
surface NCHMe), 4.39 (1H, t, JZ7.0 Hz, interior
NCHCMe2), 4.48–4.64 (1H, m, interior NCHMe), 6.90
(2H, d, JZ7.5 Hz, OCONH), 7.08 (2H, d, JZ7.1 Hz,
OCONH), 7.70 (4H, s, surface ArH), 7.96 (2H, s, ArH), 8.17
(1H, s, ArH), 8.20 (2H, s, ArH), 8.25 (1H, s, ArH), 8.41 (1H,
br s, ArCONH), 8.59 (1H, br s, ArCONH), 10.07 (2H, s,
surface CONHAr), 10.13 (2H, s, surface CONHAr), 10.27
(1H, s, interior CONHAr), 10.36 (1H, s, interior CONHAr).
13C NMR: 18.7, 18.9, 19.5, 20.0, 20.2, 29.2, 31.3, 50.9,
51.4, 61.0, 61.6, 78.97, 79.01, 113.7, 113.8, 114.5, 115.0,
116.1, 116.2, 132.7, 136.4, 136.7, 140.0, 140.1, 140.5,
156.1, 156.5, 167.7, 168.0, 168.1, 171.5, 171.9, 172.5,
173.0. MS (FAB): 1353 [(MCNa)C, 15%]. HRMS (FAB):
calcd for C65H94N12O18C NaC, 1353.6701; found,
1353.6727.
1. Smith, D. K.; Diederich, F. Chem. Eur. J. 1998, 4, 1353–1361.
2. For reviews on dendrimers with biomedical applications,
see (a) Stiriba, S.-E.; Frey, H.; Haag, R. Angew. Chem., Int. Ed.
2002, 41, 1329–1334. (b) Grinstaff, M. W. Chem. Eur. J. 2002,
8, 2838–2846.
3. (a) Denkewalter, R. G.; Kolc, J. F.; Lukasavage, W. J. U.S.
Patent 4,410,688, 1983.(b) Partridge, K. S.; Smith, D. K.;
Dykes, G. M.; McGrail, P. T. Chem. Commun. 2001, 319–320.
(c) Hirst, A. R.; Smith, D. K.; Feiters, M. C.; Geurts, H. P. M.;
Wright, A. C. J. Am. Chem. Soc. 2003, 125, 9010–9011.
(d) Love, C. S.; Hirst, A. R.; Chechik, V.; Smith, D. K.;
Ashworth, I.; Brennan, C. Langmuir 2004, 20, 6580–6585.
(e) Hirst, A. R.; Smith, D. K.; Feiters, M. C.; Geurts, H. P. M.
Langmuir 2004, 20, 7070–7077.
4. Baigude, H.; Katsuraya, K.; Okuyama, K.; Hatanaka, K.; Ikeda,
E.; Shibata, N.; Uryu, T. J. Polym. Sci., Part A: Polym. Chem.
2004, 42, 1400–1414.
5. (a) Twyman, L. J.; Beezer, A. E.; John C. Mitchell, J. C.
Tetrahedron Lett. 1994, 35, 4423–4424. (b) Bellis, E.; Markidis,
T.; Kokotos, G. Synthesis 2002, 1359–1364.
´ ˆ ´
6. (a) Prevote, D.; Le Roy-Gourvennec, S.; Caminade, A.-M.;
4.11.3. G2-F3V3-CO2H (4, aa1aa2ZFV). Starting from
G2-F3V3-CO2Et (0.26 g, 0.16 mmol) in MeOH (7 mL) and
aqueous KOH solution (0.6 mL, 1.0 M), the titled com-
pound was obtained as a white solid (0.24 g, 94%). Tm
Masson, S.; Majoral, J.-P. Synthesis 1997, 1199–1207.
(b) Brouwer, A. J.; Mulders, S. J. E.; Liskamp, R. M. J. Eur.
J. Org. Chem. 2001, 1903–1915.
7. (a) Esposito, A.; Delort, E.; Lagnoux, D.; Djojo, F.; Reymond,
J.-L. Angew. Chem., Int. Ed. 2003, 42, 1381–1383. (b) Lagnoux,
D.; Delort, E.; Douat-Casassus, C.; Esposito, A.; Reymond,
J.-L. Chem. Eur. J. 2004, 10, 1215–1226. (c) Clouet, A.; Darbre,
T.; Reymond, J.-L. Adv. Synth. Catal. 2004, 346, 1195–1204.
(d) Clouet, A.; Darbre, T.; Reymond, J.-L. Angew. Chem., Int.
Ed. 2004, 43, 4612–4615.
175 8C. [a]2D0C63.8 (c 0.90). H NMR: 0.90 (12H, d, JZ
1
6.5 Hz, surface CH(CH3)2), 0.98 (3H, d, JZ6.8 Hz, interior
CH(CH3)2), 1.00 (3H, d, JZ6.8 Hz, interior CH(CH3)2),
1.32 (18H, s, C(CH3)3), 1.38 (18H, s, C(CH3)3), 1.85–2.08
(2H, m, surface CHMe2), 2.10–2.29 (1H, m, interior
CHMe2), 2.70–2.91 (2H, m, surface PhCHH), 2.91–3.08
(2H, m, surface PhCHH), 3.08–3.25 (2H, m, interior
PhCH2), 3.93 (2H, t, JZ8.0 Hz, surface NCHCHMe2),
4.17–4.49 (3H, interior NCHCHMe2 and surface NCHBn),
8. Chow, H.-F.; Zhang, J. Chem. Eur. J. 2005, 11, 5817–5831.
9. Mong, T. K.-K.; Niu, A.; Chow, H.-F.; Wu, C.; Li, L.; Chen, R.
Chem. Eur. J. 2001, 7, 686–699.