A new convergent approach to dendritic macromolecules

Article abstract of DOI:10.1081/SCC-120013726

The novel convergent growth approach to dendritic macromolecules is described. Polyether dendritic sectors [G-na]-OH from the first to the fourth generations, respectively, were prepared by this approach from readily available starting material.

Full text of DOI:10.1081/SCC-120013726

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A NEW CONVERGENT APPROACH TO DENDRITIC
MACROMOLECULES
Xiaoping Cao a , Feng Wang a & Shu Guo a
a National Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical
Engineering, Lanzhou University, Lanzhou, 730000, P.R. China
Available online: 16 Aug 2006
To cite this article: Xiaoping Cao, Feng Wang & Shu Guo (2002): A NEW CONVERGENT APPROACH TO DENDRITIC
MACROMOLECULES, Synthetic Communications: An International Journal for Rapid Communication of Synthetic Organic
Chemistry, 32:20, 3149-3158
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SYNTHETIC COMMUNICATIONS
Vol. 32, No. 20, pp. 3149–3158, 2002
A NEW CONVERGENT APPROACH TO
DENDRITIC MACROMOLECULES
*
Xiaoping Cao, Feng Wang, and Shu Guo
National Laboratory of Applied Organic Chemistry,
College of Chemistry and Chemical Engineering,
Lanzhou University, Lanzhou, 730000, P.R. China
ABSTRACT
The novel convergent growth approach to dendritic macro-
molecules is described. Polyether dendritic sectors [G-na]-OH
from the first to the fourth generations, respectively, were
prepared by this approach from readily available starting
material.
Dendrimersareperfectmonodispersemacromoleculeswitharegularand
highly branched three-dimensional architecture. The combination of discrete
number of functionalities in one molecular and high densities of active
groups, typical for dendritic molecules, has attracted a lot of attention in the
fileds of medicinal chemistry (e.g., in drug delivery system), host-guest chem-
istry and catalysis chemistry.[1] Dendrimers’ syntheses can be divided into two
mainapproaches:thedivergentandconvergentscheme.Thedivergentscheme,
originally developed by Vogtle,[2] Tomalia,[3] and Newkome,[4] involves the
addition of monomer units from the core to periphery. The alternative
*Corresponding author. E-mail: caoxplzu@163.com
3149
DOI: 10.1081/SCC-120013726
Copyright & 2002 by Marcel Dekker, Inc.
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3150
CAO, WANG, AND GUO
convergent scheme, first pioneered by Frechet,[5] and Miller,[6] involves the
preparation of the dendrimer propagation from the periphery to the core.
An apparent limitation of the ‘‘convergent’’ approach is in its
susceptibility to steric inhibition, as the macromolecules become larger, the
functional group at the focal point becomes ‘‘masked’’ by the growing macro-
molecule and its reactivity is lessened. This problem may be even less signifi-
cant with monomers that possess more flexible geometries. In the procedure
of our syntheses of dendrimers, we use Witting reactions to prolong branch
unit so we can synthesize higher generation dendrimers with less steric hin-
drance. Since speculation on the ultimate use of dendritic host-guest systems
in complex drug delivery agents have often been made, the dendrimers of
which we increase the branch unit can produce more spacious cavities and so
can trap larger molecules that may broaden the potential of these dendrimers
in medicine manufactory. We also adopt gallic acid as our starting material,
which has three-ramifying point. In this case, we can get same numbers of
periphery functionalites with less generation compared to two-ramifying
point such as 3,5-dihydroxybenzyl acid (a-resorcylic acid).
The monomer unit for the new ‘‘convergent-growth’’ approach was
3-(3,5-dihydroxyphenyl)-1-propanol (1a) or 3-(3,4,5-trihydroxyphenyl)-1-
propanol (1b). These were chosen in order to increase the flexibility of the
dendrimers. Benzyl ethers were selected as the protecting groups since
regionselective cleavage of thee ethers is possible by hydrogenolysis that
do not affect the aliphatic ether linkages within the dendrimers.[7,8] Thus
can afford a dendritic macromolecule with phenolic chain ends, further
modification of the chain ends will be readily accomplished. This work is
in progress. Monomer 1 was prepared as shown in the discussion below
(Sch. 1). Esterification of the 3,5-dihydroxybenzyl acid 2a or gallic acid 2b
with CH3OH/H2SO4 gave their methyl ester 3a in 90% yield and 3b in 81%
yield after recrystallization. Protection of the phenolic groups of 3a and 3b
as their benzyl ethers (BnBr, K2CO3, acetone, 50ꢀC, 5 h, 96% yield for 3a
and 95% yield for 3b) followed by reduction with LiAlH4 gave the protected
alcohol 4a in 91% yield and 4b in 92% yield. The resulting alcohol was
subjected to PCC oxidation (C5H5N ꢁ CrO3 ꢁ HCl, CH2Cl2, 0ꢀC, 8 h, 90%
yield for 4a and 87% yield for 4b), followed by Witting reaction
(Ph3P¼CHCO2C2H5, benzene, 25ꢀC, 24 h, 92% and 91% yield) and
reduction with LiAlH4 (77% and 75% yield), the first-generation alcohol
5a ([G-1a]-OH) in 50% overall yield and 5b ([G-1b]-OH) in 42% overall
yield were constructed after purification, from starting materials 2a and 2b
respectively. The deprotection reaction of the alcohol 5a or 5b by catalytic
transfer hydrogenation with Pd/C gave the desired monomer 1a in 80%
yield and 1b in 80% yield. On the other hands, activation of the hydroxyl
functional group of 5 with CBr4 and PPh3 in dry THF at r.t. gave the

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DENDRITIC MACROMOLECULES
3151
Scheme 1. Synthesis of dendrimer [G-n]-OH.

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3152
CAO, WANG, AND GUO
first-generation bromide 6a ([G-1a-]-Br) in 98% yield and 6b ([G-1b]-Br) in
91% yield after purification. The bromide 6a (2.1 equiv.) was allowed to
react with the diphenolic monomer unit 1a (in K2CO3/acetone heated at
reflux 48 h) to afford the next generation alcohol 7a ([G-2a]-OH) in
89% yield. Alternatively, coupling of bromide 6b with monomer 1b afforded
7b ([G-2b]-OH) in 82% yield. The second-generation alcohol 7a was
converted to the second-generation bromide 8a ([G-2a]-Br) in 95% yield
with CBr4/PPh3. Reaction of 8a ([G-2a]-Br) with 1a, as above, gave
the third-generation alcohol 9a ([G-3a]-OH) in 85% yield, which was
brominated with CBr4/PPh3 and coupling reaction with monomer 1a to
give the fourth-generation alcohol 11a ([G-4a]-OH) in 73% yield after
purification.
1H and 13C NMR data, high resolution mass spectral data provide
unequivocal proof of the structural identities of these dendritic species.
1
Hence, H NMR data provide information regarding to the dendrimer gen-
1
eration. To illustrate this point, H NMR spectra [G-na]-OH (n ¼ 1–4) are
shown in Fig. 1. Thus, benzylic moiety in [G-na]-OH has characteristic
Figure 1. 1H NMR spectra of [G-na]-OH (n ¼ 1–4).

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DENDRITIC MACROMOLECULES
3153
signals at ꢀ 4.80–5.21 attributed to the methylene group and the aromatic
protons from ꢀ 7.30 to 7.74, while those of the branching phenyl unit reso-
nate at around ꢀ 6.41–6.70. The alkyl branching units have their proton
signals at four separate regions (ꢀ 1.82–2.20, 2.70–2.97, 3.65–3.72 and
3.92–4.30). As can be observed, the higher the generation, the smaller the
relative integral intensities of the hydroxymethyl group moiety proton sig-
nals (ꢀ 3.65–3.72) at the focal point to those of the signals resulting from the
benzylic and alkyl branching moieties. This is because the focal point/other
moieties ratio decrease with increase generation. Since all the signals were
non-overlapping, structure identification of all the dendritic fragments was
greatly facilitated.
In summary, we have demonstrated a novel convergent growth
approach using 3-(3,5-dihydroxyphenyl)-1-propanol (1a) and 3-(3,4,5-tri-
hydroxyphenyl)-1-propanol (1b) as the monomer units and benzyl ethers
as the protecting groups.
The two-step procedure, dialkylation of 1a or trialkylation of 1b,
followed by conversion of the hydroxyl groups to the corresponding
bromo groups, was optimized and proved to be highly efficient. In addition,
considerable flexibility can be introduced in the dendrimers, thereby
reducing the steric problems that may, under certain circumstances, limit
the applicability of the convergent growth approach.
EXPERIMENTAL
General Methods and Materials
Melting points were measured on the Reichert Microscope apparatus
and are uncorrected. IR spectra were recorded on a Nicolet 170 SX FT-IR
spectrophotometer and reported in wavenumbers (cmÀ1). NMR spectra
were recorded on Avance DRX-200 and Bruker AM-400 spectrometer,
and samples were measured in CDCl3 or Acetone-d6. Chemical shifts are
reported in ꢀ units (ppm) and coupling constants (J) are reported in Hz.
Mass spectra (MS) data were obtained on a V.G. ZAB-HS or Bruker
Daltonics APEXII mass spectrometer. All of materials were obtained
commercially and used as received without further purification.
3,5-Dihydroxybenzoic acid methyl ester (3a) and 3,4,5-trihydroxyben-
zoic acid methyl ester (3b) were prepared in 90% and 81% yield after recrys-
tallization from 2a and 2b by the method given in the literature[9] and have
been determined by NMR and mass spectroscopy.
3,5-Dibenzyloxy benzyl alcohol (4a) and 3,4,5-tribenzyloxy benzyl
alcohol (4b) were prepared in 87% and 87% yield from 3a and 3b by the

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3154
CAO, WANG, AND GUO
method given in the literature[10,11] and have been determined by NMR and
mass spectroscopy.
3-(3,5-Dibenzyloxyphenyl)-1-propanol (5a):Compound4awassubjected
to PCC oxidation (C5H5N ꢁ CrO3 ꢁ HCl, 1.1 mol equiv., CH2Cl2, 0ꢀC, 8 h,
90% yield), followed by Wittig reaction (Ph3P¼CHCO2C2H5, 1.2 mol
equiv., benzene, 25ꢀC, 24 h, 92%) and reduction with LiAlH4 (1.1 mol
equiv., THF, 0ꢀC, 2 h, 77% yield), 5a was obtained as a solid. M.p.
54–56ꢀC. Rf ¼ 0.46 (EtOAc/hexane 3/2). IR (KBr) ꢁmax: 3465, 3003, 1595,
1361, 1154, 890, 681; 1H NMR (CDCl3): 1.74 (s, 1H, OH), 1.87–2.10 (m, 2H,
CH2), 2.67–2.87 (s, 2H, PhCH2), 3.65 (t, 2H, J ¼ 7.0 CH2OH), 5.08 (s, 4H,
2 Â OCH2Ph), 6.56 (s, 3H, 3 Â ArH), 7.36–7.54 (m, 10H, 10 Â PhH);
13C NMR (CDCl3): 32.29, 33.60, 62.06, 69.98, 99.58, 107.73, 127.47,
127.68, 128.50, 136.96, 144.22, 159.94; MS (FAB) m/z: 348 (Mþ, 8%);
HRMS (ESI) Calcd. for C23H25O3 [M þ H]þ 349.1798. Found 349.1793.
3-(3,4,5-Tribenzyloxyphenyl)-1-propanol (5b) was prepared from 4b by
the similar method for the preparation of 5a. IR (KBr) ꢁmax: 3445, 2926,
1555, 1241, 1115, 736, 698; 1H NMR (CDCl3): 1.70–2.05 (m, 2H, CH2), 2.48
(t, J ¼ 7.5, 2H, PhCH2), 3.63 (t, J ¼ 7.5, 2H, CH2OH), 5.17 (s, 6H,
3 Â OCH2Ph), 6.59 (s, 2H, 2 Â ArH), 7.31–7.47 (m, 15H, 15 Â PhH); MS
(EI) m/z: 453([M À H]þ, 8%); HRMS (ESI) Calcd. for C30H31O4
[M þ H]þ 455.2217. Found 455.2221.
3-(3,5-Dihydroxyphenyl)-1-propanol (1a): A suspension of 3-(3,5-dibenzyl-
oxybenzyloxyphenyl)-1-propanol 5a (1.68 g, 100 mmol) and 10% palladium
on charcoal (0.1 g) in ethanol/EtOAc (3/1 v/v, 100 mL) was stirred under
hydrogen at 20ꢀC. The reaction progress was monitored by thin layer
chromatography until all starting material disappeared. The mixture was
filtered through celite and the filtrate concentrated under reduced pressure.
The crude product was purified by flash chromatography on silica gel
(EtOAc/hexane 1/3) to afford the product as a white solid (1.34 g, 80%).
M.p., NMR and mass spectroscopy are the same as in the literature.[12]
3-(3,4,5-Trihydroxyphenyl)-1-propanol (1b) was prepared in 80% yield
from 5b by the similar method for the preparation of 1a. IR (KBr) ꢁmax
:
1
3314, 2927, 1543, 1021, 978, 680; H NMR (CDCl3): 1.56–1.96 (m, 2H,
CH2), 2.46 (t, J ¼ 7.6, 2H, PhCH2), 3.56 (t, J ¼ 7.6, 2H, CH2OH), 6.00
(s, 3H, 3 Â ArOH), 6.22 (s, 2H, 2 Â ArH); 13C NMR 30.57, 33.43, 60.43,
106.41, 129.31, 132.57, 144.26; HRMS (ESI) Calcd. for C9H12O4Na
[M þ Na]þ 391.1363. Found 391.1351.
General procedure for the synthesis of dendritic bromides [G-n]-Br
(n 5 1–3) 6a, 8a, 10a, 6b: A mixture of the dendritic alcohol [G-n]-OH,
carbon tetrabromide (1.2 mol equiv.) and triphenylphosphine (1.2 mol equiv.)
was stirred in dry THF at 20ꢀC for 9–12 h. The reaction progress was
monitored by thin layer chromatography until all starting material

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DENDRITIC MACROMOLECULES
3155
disappeared. The reaction mixture was filtered and the filtered cake washed
with dry ether. The combined filtrate was evaporated in vacuo to leave a
residue, which was then purified as described in the following text.
[G-1a]-Br (6a): This compound was prepared as an oil (98% yield)
from [G-1a]-OH (5a) and purified by flash chromatography on silica gel
(EtOAc/hexane 1/10), Rf ¼ 0.42 (EtOAc/hexane 1/6). IR (KBr) ꢁmax: 3 43 1,
3031, 1594, 1154, 860, 678; 1H NMR (CDCl3): 2.19–2.02 (m, 2H, CH2), 2.78
(t, J ¼ 7.0, 2H, PhCH2), 3.42 (t, J ¼ 7.0, 2H, CH2Br), 5.08 (s, 4H,
2 Â OCH2Ph), 6.54–6.88 (m, 3H, 3 Â ArH), 7.39–7.54 (m, 10H, 10 Â PhH);
13C NMR (CDCl3): 32.93, 33.71, 34.05, 69.64, 99.71, 107.70, 127.38, 127.79,
128.40, 136.78, 142.74, 159.69; MS (FAB) m/z: 410 (Mþ, 36%), 412
(Mþ þ 2, 36%); HRMS (ESI) Calcd. for C23H23BrO2Na [M þ Na]þ
433.0774. Found 433.0773.
[G-2a]-Br (8a): This compound was prepared as an oil (95% yield)
from [G-2a]-OH (7a) and purified by flash chromatography on silica gel
(EtOAc/hexane 1/10), Rf ¼ 0.42 (EtOAc/hexane 1/6). IR (KBr) ꢁmax: 3399,
1
2925, 1595, 1378, 878, 658; H NMR (CDCl3): 2.15–2.20 (m, 6H, 3 Â CH2),
2.79–2.81 (m, 6H, 3 Â PhCH2), 3.42 (t, J ¼ 7.1, 2H, CH2Br), 3.98–4.20 (m,
4H, 2 Â OCH2), 5.08 (s, 8H, 4 Â OCH2Ph), 6.47–6.53(m, 9H, 9 Â ArH),
7.35–7.51 (m, 20H, 4 Â PhH); 13C NMR (CDCl3): 30.53, 32.43, 33.13,
33.93, 34.26, 66.62, 70.03, 99.15, 99.72, 107.26, 107.62, 127.54, 127.94,
128.55, 136.98, 142.84, 143.91, 160.02, 160.31; MS (FAB) m/z: 890 (Mþ,
2%), 892 (Mþ þ 2, 2%); HRMS (ESI) Calcd. for C55H55BrO6Na [M þ Na]þ
913.3074. Found 913.3057.
[G-3a]-Br (10a): This compound was prepared as an oil (89% yield)
from [G-3a]-OH (9a) and purified by flash chromatography on silica gel
(EtOAc/hexane 1/10), Rf ¼ 0.42 (EtOAc/hexane 1/8). IR (KBr) ꢁmax: 3429,
2925, 1595, 876, 690; 1H NMR (CDCl3): 2.11–2.20 (m, 14H, 7 Â CH2),
2.77–2.89 (m, 14H, 7 Â PhCH2), 3.40 (t, J ¼ 7.0, 2H, CH2Br), 3.92–4.23
(m, 12H, 6 Â OCH2), 5.01–5.03(m, 16H, 8 Â OCH2Ph), 6.28–6.36 (m, 7H,
7 Â ArH), 6.40–6.51 (m, 14H, 14 Â ArH), 7.35–7.46 (m, 40H, 40 Â PhH);
13C NMR (CDCl3): 30.50, 32.37, 33.13, 33.87, 34.18 (overlapping signals),
66.70, 68.35, 69.91, 70.01, 99.59, 105.81, 106.37, 107.12, 107.67, 127.49,
127.68, 128.49, 136.75, 136.69, 142.76, 143.87, 159.72, 159.92, 160.02,
160.21; HRMS (ESI) Calcd. for C119H120BrO14 [M þ H]þ 1851.7856.
Found 1851.7815.
[G-1b]-Br (6b): This compound was prepared as a colorless oil
(91% yield) from [G-1b]-OH (5b). IR (KBr) ꢁmax: 2933, 1535, 1245, 1113,
1
736, 698; H NMR (CDCl3): 2.02–2.34 (m, 2H, CH2), 2.79–2.83(m, 2H,
PhCH2), 3.14 (t, J ¼ 6.8, 2H, CH2Br), 5.19–5.21 (m, 6H, 3 Â OCH2Ph),
6.62 (s, 2H, 2 Â ArH), 7.23–7.80 (m, 15H, 15 Â PhH); 13C NMR (CDCl3):
32.92, 33.83, 33.87, 71.05, 75.04, 100.25, 127.29, 127.57, 127.66, 127.82,

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3156
CAO, WANG, AND GUO
127.96, 128.29, 128.34, 135.94, 137.01, 137.63, 152.56; HRMS (ESI) Calcd.
for C30H29BrO3Na [M þ Na]þ 539.1192. Found 539.1188.
General procedure for the synthesis of dendritic alcohols [G-n]-OH
(n 5 2–4) 7a, 9a, 11a, 7b: A mixture of the dendritic bromide [G-(n-1)a]-Br
(2.1 mol equiv.), 3-(3,5-dihydroxyphenyl)-1-propanol (1a, 1.0 mol equiv.),
potassium carbonate (6.0 mol equiv.), 18-crown-6 (0.1 mol equiv.) was
refluxed in acetone. The reaction time required was 2,3and 4 days for
n ¼ 2 to 4 respectively. The reaction mixture was filtered through a pad of
silica gel to remove the inorganic materials. After concentration of the
filtrate on a rotary evaporator, the crude product was purified as described
in the following text.
[G-2a]-OH (7a): This compound was prepared as an oil (89%) from
[G-1a]-Br (6a) and purified by flash chromatography on silica gel
(EtOAc/hexane 1/4); Rf ¼ 0.32 (EtOAc/hexane 1/2). IR (KBr) ꢁmax: 3403,
1
2924, 1595, 1155, 896, 675; H NMR (CDCl3): 1.97–2.20 (m, 6H, 3 Â CH2),
2.74–2.85 (m, 6H, 3 Â PhCH2), 3.72 (t, J ¼ 7.0, 2H, CH2OH), 4.04–4.30
(m, 4H, 2 Â OCH2), 5.08 (s, 8H, 4 Â OCH2Ph), 6.51 (s, 3H, 3 Â ArH), 6.64
(s, 6H, 6 Â ArH), 7.38–7.57 (m, 20H, 20 Â PhH); 13C NMR (CDCl3): 30.53,
32.41 (overlapping signals), 33.96, 62.25, 66.78, 70.02, 99.01, 99.72, 107.15,
107.61, 127.53, 127.93, 128.54, 136.98, 143.92, 144.15, 160.00, 160.24; MS
(FAB) m/z: 828 (Mþ, 2%); HRMS (ESI) Calcd. for C55H56O7Na [M þ Na]þ
851.3918. Found 851.3935.
[G-3a]-OH (9a): This compound was prepared as an oil (85%) from
[G-2a]-Br (8a) and purified by flash chromatography on silica gel
(acetone/CH2Cl2/hexane 1/2/4); Rf ¼ 0.33 (acetone/CH2Cl2/hexane 1/2/2).
1
IR (KBr) ꢁmax: 3437, 2932, 1595, 1452, 1156, 890, 685; H NMR (CDCl3):
1.91–2.09 (m, 14H, 7 Â CH2), 2.70–2.81 (m, 14H, 7 Â PhCH2), 3.68 (t, 2H,
CH2OH), 3.92–4.10 (m, 12H, 6 Â OCH2), 4.80–5.21 (m, 16H, 8 Â OCH2Ph),
6.41–6.70 (m, 21H, 21 Â ArH), 7.30–7.58 (m, 40H, 40 Â PhH); 13C NMR
(CDCl3): 29.81, 30.49, 30.98, 32.14, 32.36 (overlapping signals), 63.60,
66.74, 69.99 (overlapping signals), 99.10, 99.67, 107.20, 107.37, 127.11,
127.51, 127.91, 128.12, 128.51, 136.91, 143.75, 143.91, 156.75, 159.94,
160.19, 160.33; HRMS (ESI) Calcd. for C119H120O15Na [M þ Na]þ
1811.8519. Found 1811.8554.
[G-4a]-OH (11a): This compound was prepared as an oil (73%) from
[G-3a]-Br (10a) and purified by flash chromatography on silica gel (acetone/
1
CH2Cl2/hexane 1/2/4); Rf ¼ 0.29 (acetone/CH2Cl2/hexane 1/2/2). H NMR
(CDCl3): 1.82–2.10 (m, 30H, 15 Â CH2), 2.80–2.97 (m, 30H, 15 Â PhCH2),
3.65 (t, J ¼ 7.0, 2H, CH2OH), 3.94–4.01 (m, 28H, 14 Â OCH2), 4.85–5.18
(m, 32H, 16 Â OCH2Ph), 6.42–6.64 (m, 45H, 45 Â ArH), 7.46–7.72 (m, 80H,
80 Â PhH); 13C NMR (CDCl3): 29.20, 30.48 (overlapping signals), 32.35
(overlapping signals), 33.90, 53.69, 62.10 (overlapping signals), 66.70, 69.89

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DENDRITIC MACROMOLECULES
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(overlapping signals), 69.99, 98.95, 99.59 (overlapping signals), 105.80,
106.36, 107.10, 107.68, 127.46, 127.85, 128.47, 136.73, 136.88, 139.34,
139.86, 159.70 (overlapping signals), 159.90, 160.00, 160.05, 160.16; HRMS
(ESI) Calcd. for C247H248O31Na [M þ Na]þ 3732.7722. Found 3732.7686.
[G-2b]-OH (7b): This compound was prepared as an oil (82% yield)
from [G-1b]-Br (6b) and purified by flash chromatography on silica gel
(EtOAc/hexane 1/4), Rf ¼ 0.30 (EtOAc/hexane 1/2). IR (KBr) ꢁmax: 3524,
2924, 1433, 1232, 1110, 733, 695; 1H NMR (CDCl3): 1.62 (broad, 1H, OH),
1.78–1.85 (m, 2H, CH2), 2.06–2.09 (m, 6H, 3 Â CH2), 2.61 (t, J ¼ 7.4, 2H,
PhCH2), 2.79 (t, J ¼ 7.1, 4H, 2 Â PhCH2), 2.93(t, 2H, J ¼ 7.4, PhCH2), 3.61
(t, 2H, J ¼ 6.8, CH2OH), 3.96 (t, J ¼ 6.8, 4H, 2 Â OCH2), 4.12 (t, 2H,
J ¼ 6.8, OCH2), 5.00–5.05 (m, 18H, 9 Â OCH2Ph), 6.39 (s, 2H, 2 Â ArH),
6.55 (s, 4H, 4 Â ArH), 6.61 (s, 2H, 2 Â ArH), 7.29–7.47 (m, 45H, 45 Â PhH);
HRMS (ESI) Calcd. for C99H97O13 [M þ H]þ 1493.6924. Found 1493.6922.
ACKNOWLEDGMENTS
We are grateful for financial support from National Natural Science
Foundation of China (QT program) and Foundation for University Key
Teacher by the Ministry of Education.
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Received in Japan May 28, 2001