Synthesis and chiroptical properties of dendrimers elaborated from a chiral, nonracemic central core

Article abstract of DOI:10.1021/jo981508b

Three generations of ester-terminated dendrimers have been constructed from (1R,2S)-2-amino-1-phenyl-1,3-propanediol, 1, as the central core. The chiroptical properties of the dendrimers were measured revealing that the molar rotations [Φ] of the dendrimers decreased with increasing dendrimer generation. Comparison to a series of substituted benzoate derivatives of 1 suggested that the decrease in rotation was a consequence of a steric effect upon the conformational equilibrium of the central core that increased with increasing dendrimer generation. The optical rotations of dendrimers 7-9 were also observed to be solvent and temperature dependent.

Full text of DOI:10.1021/jo981508b

J . Org. Chem. 1998, 63, 9399-9405
9399
Syn th esis a n d Ch ir op tica l P r op er ties of Den d r im er s Ela bor a ted
fr om a Ch ir a l, Non r a cem ic Cen tr a l Cor e
J ean-Louis Chaumette, Matthew J . Laufersweiler, and J on R. Parquette*
Department of Chemistry, The Ohio State University, Columbus, Ohio 43210
Received J uly 29, 1998
Three generations of ester-terminated dendrimers have been constructed from (1R,2S)-2-amino-
1-phenyl-1,3-propanediol, 1, as the central core. The chiroptical properties of the dendrimers were
measured revealing that the molar rotations [Φ] of the dendrimers decreased with increasing
dendrimer generation. Comparison to a series of substituted benzoate derivatives of 1 suggested
that the decrease in rotation was a consequence of a steric effect upon the conformational equilibrium
of the central core that increased with increasing dendrimer generation. The optical rotations of
dendrimers 7-9 were also observed to be solvent and temperature dependent.
In tr od u ction
Recent studies suggest that dendrimers constructed
with Fre´chet-type polyether dendritic wedges are con-
formationally quite flexible.8 This program is directed at
studying the properties of internally hydrophobic chiral
dendrimers with polar groups on the periphery in aque-
ous media. Conformational order should be optimal in
polar solvents such as water that interact favorably with
the polar termini but poorly with the internal regions of
the polymer. This effect should lead to a compression of
the dendrimer that minimizes interactions of the polar
solvent with the nonpolar interior thereby enforcing a
more compact globular structure.9 Chiral, amphiphilic
dendrimers may be useful as enantioselective phase-
transfer catalysts in aqueous media especially if confor-
mational order can be induced throughout the dendritic
branches.
In this paper, we report the synthesis and chiroptical
properties of ester-terminated dendrimers constructed
with achiral polyether branch segments from (1R,2S)-2-
amino-1-phenyl-1,3-propanediol, 1 (Chart 1). The ob-
served chiroptical properties are rationalized by changes
occurring in the conformational equilibrium of the chiral,
nonracemic central core as a function of dendrimer
generation.
The stepwise synthesis and nanoscopic dimensions of
dendrimeric molecules places this new class of macro-
molecules at the interface of traditional organic and
polymer chemistry.1 The well-defined structure and
homogeneity of these materials provides an opportunity
to investigate whether the chirality of a particular
subunit can be transmitted to distal portions of the
molecule.2 Such conformational order,3 if present, would
be useful in developing asymmetric catalysts that derive
function through asymmetric three-dimensional order.
Several examples of dendrimers containing chiral
branches,4 or termini,5,6 have been reported to exhibit
rotations linearly proportionate to the number of chiral
subunits present in the molecule suggesting the absence
of conformational order. Seebach, however, has observed
anomalies in chiroptical data for dendrimers that were
either constructed from a chiral core with achiral branches
or with fully chiral branches suggesting possible confor-
mational order in the branches.7
(1) Newkome, G. R.; Moorefield, C. N.; Vo¨gtle, F. Dendritic Molecules-
Concepts-Synthesis-Perspectives; VCH: New York, 1996.
(2) For a recent discussion of the chirality of dendrimer systems,
see: (a) Peerlings, H. W. I.; Meijer, E. W. Chem. Eur. J . 1997, 3, 1563.
(b) Thomas, C. W.; Tor, Y. Chirality 1998, 10, 53. (c) Seebach, D.;
Rheiner, P. B.; Greiveldinger, G.; Butz, T.; Sellner, H. Top. Curr. Chem.
1998, 197, 125.
(3) We define “conformational order” as the presence of stable
secondary structure within the branch segments of the polymer.
(4) (a) Chow, H.-F.; Fok, L. F.; Mak, C. C. Tetrahedron Lett. 1994,
35, 3547. (b) Chow H.-F.; Mak, C. C. Tetrahedron Lett. 1996, 37, 5935.
(c) Chow, H.-F.; Mak, C. C. J . Chem. Soc., Perkin Trans. 1 1997, 91.
(d) McElhanon, J . R.; McGrath, D. V. J . Am. Chem. Soc. 1998, 120,
1647.
Resu lts a n d Discu ssion
Den d r im er Syn th esis a n d Ch a r a cter iza tion . An
initial objective was focused on delineating a modular
strategy to allow simple access to dendrimers containing
polar termini and chiral, nonracemic central cores.
Therefore, the convergent synthetic method of Fre´chet10
was exploited to prepare monodisperse dendrimers from
(5) (a) Newkome, G. R.; Lin, X.; Weis, C. D. Tetrahedron: Asymmetry
1991, 10, 957. (b) Lartigue, M.-L.; Caminade, A.-M.; Majoral, J .-P.
Tetrahedron: Asymmetry 1997, 8, 2697.
(7) (a) Seebach, D.; Lapierre, J .-M.; Greiveldinger, G.; Skobridis, K.
Helv. Chim. Acta 1994, 77, 1673. (b) Seebach, D.; Lapierre, J .-M.;
Skobridis, K.; Greiveldinger, G. Angew. Chem., Int. Ed. Engl. 1994,
33, 440. (c) Murer, P.; Seebach, D. Angew. Chem., Int. Ed. Engl. 1995,
34, 2116. (d) Murer, P. K.; Lapierre, J .-M.; Greiveldinger, G.; Seebach,
D. Helv. Chim. Acta 1997, 80, 1648.
(8) (a) Peerlings, H. W. I.; Struijk, M. P.; Meijer, E. W. Chirality
1998, 10, 46. (b) Kremers, J . A.; Meijer, E. W. J . Org. Chem. 1994, 59,
4262. (c) Peerlings, H. W. I.; Trimbach, D. C.; Meijer, E. W. J . Chem.
Soc., Chem. Commun. 1998, 497 (d) Karakaya, B.; Claussen, W.;
Gessler, K.; Saenger, W.; Schlu¨ter, A. D. J . Am. Chem. Soc. 1997, 119,
3296.
(6) The examples in ref 5 exhibit rotations that increase with
increasing numbers of chiral termini. However, in the following system,
due to dense packing on the surface occurring due to hydrogen bonding,
optical activity decreased as the number of amino acid termini present
on poly(propylene imine) dendrimers was increased: J ansen, J . F. G.
A.; Peerlings, J . H. W. I.; de Brabander-Van den Berg, E. M. M.; Meijer,
E. W. Angew. Chem., Int. Ed. Engl. 1995, 34, 1206. Dyes encapsulated
in the poly(propylene imine) system with an optical rotation of zero
exhibited a small induced circular dichroism which could be the result
of the achiral pockets adopting chiral, nonracemic conformations:
J ansen, J . F. G. A.; de Brabander-van den Berg, E. M. M.; Meijer. E.
W. Recl. Trav. Chim. 1995, 114, 225.
(9) Tanford, C. The Hydrophobic Effect: Formation of Micelles and
Biological Membranes, 2nd ed.; J ohn Wiley and Sons: New York, 1980.
10.1021/jo981508b CCC: $15.00 © 1998 American Chemical Society
Published on Web 11/24/1998

9400 J . Org. Chem., Vol. 63, No. 25, 1998
Chaumette et al.
Ch a r t 1
(1R,2S)-1-phenyl-2-amino-1,3-propanediol, 1. We found
that attaching a phenolic linker to 1 simplified alkylation
with the ester-terminated dendron monobromides by
preventing polyalkylation of the amine and transesteri-
fication of the ester termini with alkoxides generated
from 1. This strategy should afford a general method to
introduce a series of chiral, nonracemic cores with diverse
functional groups using a similar protocol for attachment
of the dendron branch segments.
Accordingly, aminodiol 1 was acylated with 4-(tert-
butyldimethylsiloxy)benzoyl chloride and then depro-
tected with (n-C4H9)4NF affording 2 in 56% overall yield.
Alternatively, acylation with 4-(benzyloxy)benzoyl chlo-
ride followed by hydrogenation over Raney-Ni provided
a more convenient, albeit lower yielding,11 procedure to
prepare 2. Convergent trialkylation of the central core,
2, with the first ((MeO2C)2[G-1]-Br), second ((MeO2C)4-
[G-2]-Br), and third ((MeO2C)8[G-3]-Br)12 generation
polyether dendritic monobromides was achieved using
K2CO3/18-C-6 in THF-DMF13 (3:1) at 70 °C. Purification
over silica gel afforded monodisperse dendrimers with
molecular weights of 1781 (7, 74%), 3402 (8, 80%), and
6643 (9, 70%) (Chart 2) with 6, 12, and 24 terminal
esters, respectively.14
Dendrimers 7-9 all showed gel permeation chroma-
tography (GPC) traces with low polydispersities and the
expected increase in Mw with increasing generation. The
structure of the first generation dendrimer 7 was readily
assigned by the 13C and 1H NMR spectra and electrospray
mass spectrometry. However, the core protons of the
second and third generation molecules were almost
invisible due to the large number of resonances arising
from the branch segments. Consequently, MALDI-TOF
spectrometry15 was performed for the larger dendrimers.
Second generation dendrimer 8 exhibited a signal in the
MALDI-TOF spectrum at m/z ) 3425 (M + 23 (Na)),
which verified the structure, and at m/z ) 2306 resulting
from a fragmentation at the benzylic position of the
core.16 The signal/noise in the MALDI-TOF spectrum of
the third generation dendrimer 9 was low due to inef-
ficient ionization but exhibited a signal at m/z ) 6683
(M + 39 (K)). GPC analysis of 9 revealed a molecular
weight, by comparison to polystyrene standards, also
consistent with the expected structure (Mcalc ) 6643; Mw
) 7178, Mn ) 7156, polydispersity ) 1.0032). Gel
permeation analysis was definitive due to the large
molecular weight changes that occurred upon going from
the starting core (Mw ) 527) to mono- (Mw ) 2567), di-
(Mw ) 4608) and trialkylated (Mw ) 6643) products.
Ch ir op tica l P r op er ties. The specific and molar opti-
cal activities of 1-9 are presented in Table 1. Both the
molar and specific rotations of the dendrimers decrease
going from the first to the third generation dendrimers
(7-9). Several reports suggest that significant backfold-
ing of the terminal groups occurs in dendrimers leading
to a density maximum that is located near the central
core region rather than at the periphery.8d,17 Moreover,
the sign and magnitude of optical rotation is extremely
sensitive to shifts in conformational equilibrium that are
brought about by changes in structure, solvent, or tem-
perature.18 Consequently, the decrease in molar rotation
with increasing generation could be a manifestation of a
steric effect on the population of certain conformations
of the core that increases with increasing dendron size.19
As the dendrimers increase in size, it is conceivable that
the conformational mobility of the central core region
should decrease. Support for this supposition was ob-
tained by preparing a series of tribenzoate derivatives
of 1 designed to impart increasing steric congestion
(16) Fragmentation at this benzylic position was also observed in
the mass spectrum of central core 2.
(17) (a) 2H NMR T1 relaxation time measurements of PAMAM
dendrimers have revealed that internal segment density increases with
increasing generation: Meltzer, A. D.; Tirrell, D. A.; J ones, A. A.;
Inglefield, P. T. Macromolecules 1992, 25, 4549. Further studies: (b)
Wooley, K. L.; Klug, C. A.; Tasaki, K.; Schaefer, J . J . Am. Chem. Soc.
1997, 119, 53. (c) Scherrenberg, R.; Coussens, B.; van Vliet, P.;
Edouard, G.; Brackman, J .; de Brabander, E. Macromolecules 1998,
31, 456.
(10) (a) Hawker, C. J .; Fre´chet, J . M. J . J . Am. Chem. Soc. 1990,
112, 7638. (b) Fre´chet, J . M. J .; Hawker, C. J .; Wooley, K. L J .
Macromol. Sci., Pure Appl. Chem. 1994, A31, 1627.
(11) Hydrogenolysis of the secondary benzyloxy group competes with
deprotection of the benzyl groups and is minimized by using Raney-
Ni.
(12) The dendron monobromides were prepared using a slightly
modified procedure developed by Fre´chet: Hawker, C. J .; Wooley, K.
L.; Fre´chet, J . M. J . J . Chem. Soc., Perkin Trans. 1 1993, 1287.
(13) In the absence of DMF, the alkylation efficiency decreases
significantly.
(18) (a) Djerassi, C.; Geller, L. E.; Eisenbraun, E. J . J . Org. Chem.
1960, 25, 1. (b) Kumata, Y.; Furukawa, J .; Fueno, T. Bull. Chem. Soc.
J pn. 1970, 43, 3920. (c) Menger, F. M.; Boyer, B. J . Org. Chem. 1984,
49, 1826. (d) Suga, T.; Ohta, S.; Aoki, T.; Hirata, T. Chem. Lett. 1985,
1331. (e) Lauricella, R.; Ke´chayan, J .; Bodot, H. J . Org. Chem. 1987,
52, 1577. (f) Polavarapu, P. L. Tetrahedron: Asymmetry 1997, 8, 3397.
(g) Galisteo, D.; Gordaliza Ramos, G.; Lo´pez Sastre, J . A.; Martinez
Garcia, M. H.; Nu´n˜ez Miguel, R. J . Mol. Struct. 1998, 442, 135.
(19) Peerlings, H. W. I.; Meijer, E. W. Eur. J . Org. Chem. 1998, 4,
573.
(14) All dendrimers gave satisfactory combustion analysis and
monodisperse GPC traces.
(15) Indoleacrylic acid was used as the matrix: Leon, J . W.; Fre´chet,
J . M. J . Polym. Bull. 1995, 35, 449.

Chiroptical Properties of Dendrimers
J . Org. Chem., Vol. 63, No. 25, 1998 9401
Ch a r t 2
Ta ble 1. Sp ecific a n d Mola r Rota tion s of 1-9 a n d Ma ss Sp ectr a l Da ta
b,c
d
compd
desc
fwa
[R]22
[Φ]22
MSe
365
365
1
2
3
4
5
6
7
8
9
AD
4-OH
167.09
527.16
797.30
659.23
1115.42
659.23
1781.58
3402.57
6643.19
138
218
350
78.3
50.4
-213
229
1148
2792
516
390.1354 (M - C7H5O3)
798.3089 (MH)
660.2619 (MH)
1116.4323 (MH)
660.2600 (MH)
1782.8 (MH)
4-OBn
3,5-diOMe
3,5-diOBn
2,6-diOMe
G1
563
-1402
132 (10.0)f
41.7 (9.5)f
18.2 (4.4)f
2352 (178)f
1418 (323)f
1209 (292)f
G2
G3
3425.9 (M + Na)
6683.4 (M + K)
a
b
d
Exact mass. Specific rotation in 10-1 deg cm2 g-1
.
c 1 and 2 measured in 1:10 MeOH/CHCl3, 3-9 in CHCl3. Molar rotation in 10-1
deg cm2 mol-1
CH3CN.
.
e 2-6 by FAB MS, 7 by electrospray MS, 8 and 9 by MALDI-TOF MS. f Values in parentheses measured in 1:4 THF/
around the core going from 3 to 6. As shown in Table 1,
the 4-benzyloxy derivative (3) exhibited a rotation slightly
more positive than the G1 dendrimer (7). However, both
the 3,5-dimethoxy (4) and 3,5-dibenzyloxy (5) benzoates
exhibited molar rotations less positive than 7-9 and the
most congested 2,6-dimethoxy derivative (6) displayed a
negative rotation.20 Interestingly, whereas 6 is the most
sterically congested benzoate derivative, the absolute
value of the rotation is more than twice that of 4 or 5
suggesting a significant shift in conformational equilib-
rium favoring a conformation displaying a large negative
rotation.18 The reversal in the sign of rotation is a
consequence of the 2,6-disubstitution of the benzoate
which forces the carbonyl to rotate out of the plane of
the phenyl group. This conformation also imparts an
electronic perturbation to the central core resulting in a
slight downfield shift of H3′ in the proton NMR spectrum
of 6 relative to 3-5. These electronic and conformational
changes are also reflected in the J 1,2 and J 2,3 coupling
constants in the 1H NMR spectrum which are signifi-
cantly different in 6 than in 3-5 and 7 (Table 2).
Therefore, we conclude that the observed decrease in
rotatory power with increasing dendrimer generation is
due, in part, to the increase in steric congestion develop-
ing around the central core as the dendritic wedges grow
in size.

9402 J . Org. Chem., Vol. 63, No. 25, 1998
Ta ble 2. 1H NMRa Cou p lin g Con sta n ts for 3-7
Chaumette et al.
b
c
compd
δH3 (J 2.3
)
δ
H3′(J 2.3′
)
δH1 (J 1.2)
3
4
5
6
7
4.25 (4.7)
4.31 (4.8)
4.39 (5.3)
4.32 (3.9)
4.23 (4.7)
4.46 (3.7)
4.47 (3.0)
4.50 (3.6)
4.64 (9.0)
4.49 (3.2)
6.29 (8.4)
6.29 (7.7)
6.28 (8.3)
6.22 (9.1)
6.33 (8.6)
a
b
Spectra recorded in CHCl3 at 22 °C. ppm. c Hz.
F igu r e 3. Minimum energy conformer of 3 (4-methoxy
derivative) found using molecular mechanics.
F igu r e 1. Molar ellipticity of 3 and 7-9 in CHCl3 at 22 °C.
F igu r e 4. Molar ellipticity of 3, 7, and 8 in 1:4 THF-CH3CN.
9 (not shown for clarity) exhibits no detectable CE.
be rationalized. However, the central tribenzoyl core
contains three potentially interacting chromophores and
it is well established that the amplitude of the bisignate
CE can be approximated by the addition of the ampli-
tudes of each pairwise interaction.22 Inspection of the
lowest energy conformation23 of the 4-methoxy derivative
of 3 suggests that the 1,2- and 2,3-dibenzoyl chro-
mophores are of opposing chirality which would lead to
a CE of reduced amplitude as observed for 3, 8, and 9
(Figure 3). A conformation of 7 in which the relative
orientation of one of these dibenzoate interactions changes
could account for the augmented CE.
Prompted by a report that meta-linked phenylacetylene
oligomers organize into helices in acetonitrile but not in
chloroform due to solvophobic effects,24 we investigated
the optical rotations (Table 1) and ellipticities of 7-9 in
a 1:4 THF-acetonitrile solvent mixture (Figure 4). THF
was necessary because 7-9 exhibited poor solubility in
pure acetonitrile. The optical rotations and ellipticities
of 7-9 were significantly diminished in this solvent
system, and dendrimer 9 exhibited no detectable CE in
the CD spectrum (Figure 4). Interestingly, the molar
rotations in this solvent system increased slightly going
from 7 to 8 but remained constant from 8 to 9. The
methyl benzoate termini of 7-9 interact more favorably
with the polar solvent than the internal regions of the
dendrimers. Therefore, in this poor solvent system, the
F igu r e 2. Molar ellipticity of 4-6 in CHCl3 at 22 °C.
The circular dichroism spectra of 3 and 7-9 exhibit
comparable excitonic couplings at 255-260 nm indicating
that the tribenzoyl central core is primarily responsible
for this coupling rather than interactions among the
chromophores present in the branches or on the periph-
ery (Figure 1).21 The shape and amplitude of the Cotton
effects (CE) for 3, 8, and 9 are similar; however, 7 exhibits
a much more intense CE. The CD spectra of 4-6 exhibit
CEs of significantly reduced intensity relative to 3 and
7-9 and without any observable excitonic coupling
(Figure 2). Similar to the optical rotation, a reversal in
the sign of the CE occurs in the spectrum of 6. The origin
of the anomalously large ellipticity for 7 cannot easily
(20) 1-6 exhibit their most intense UV absorptions bands at 250-
260 nm; however, 4-6 also exhibit a less intense absorption band at
308, 308, and 282 nm, respectively. Dendrimers 7-9 exhibit the most
intense bands at 246 nm with a weaker absorption at 260-270 nm.
These electronic differences are not solely responsible for the difference
in optical rotatory power that is observed because the additional bands
at longer wavelength in 4-6 should lead to a slight increase in rotation
which is not observed.
(22) Nakanishi, K.; Liu, H.-W. J . Am. Chem. Soc. 1982, 104, 1178.
(23) The lowest energy conformation was generated by employing
a Monte Carlo conformational search using the MM2* force field and
the GB/SA model for CHCl3 implemented in Macromodel 5.0.
(24) Nelson, J . C.; Saven, J . G.; Moore, J . S.; Wolynes, P. G. Science
1997, 277, 1793.
(21) Since the terminal esters are electronically different than the
tribenzoyl core, an interaction of these chromophores would be expected
to generate a more complex Cotton effect than is observed.

Chiroptical Properties of Dendrimers
J . Org. Chem., Vol. 63, No. 25, 1998 9403
at 200 or 300 MHz and 13C NMR spectra at 62.5 or 75 MHz.
EI or FAB mass spectra were recorded at The Ohio State
University Chemical Instrument Center. MALDI-TOF spec-
trometry was performed using indoleacrylic acid as the matrix
in tetrahydrofuran (THF). Electrospray mass spectrometry
was performed on a triple quadrupole mass spectrometer, with
positive ion electrospray. Optical rotations were measured at
a concentration of 10 mg/mL (c ) 1). Gel permeation chroma-
tography (GPC) was performed using three divinylbenzene
columns arranged in series (500, 103, and 104 Å) with RI
detection. All runs were carried out using THF as the carrier
solvent at 40 °C. All reactions were performed under an argon
or nitrogen atmosphere. Dimethylformamide (DMF) was dried
by distillation from barium oxide or magnesium sulfate, THF
was distilled from sodium/benzophenone ketyl, and acetone
was distilled from CaSO4 and dichloromethane was distilled
from calcium hydride. Chromatographic separations were
performed on silica gel 60 (230-400 mesh, 60 Å) using the
indicated solvents.
F igu r e 5. Molar rotation at 365 nm versus temperature (7-9
in CHCl3; 1 in 1:10 CH3OH/CHCl3).
nonpolar dendritic wedges could be expected to collapse
upon the central core in an effort to minimize potentially
unfavorable interactions with the solvent resulting in a
corresponding conformational change of the central core.25
Due to the apparent conformational mobility of the
central core and the resultant variation in optical activity,
it is not possible at this point to determine the extent, if
at all, that asymmetric conformational order contributes
to the chiroptical properties; however, a few observations
are noteworthy. Upon heating of the samples from 10 to
50 °C, 7 and 8 experience a reversible 15% decrease in
optical rotation and the third generation dendrimer, 9,
shows a 41% decrease (Figure 5). While the rate of
decrease is similar for 7-9 between 20 and 50 °C, the
rotation of 9 decreases much more rapidly between 10
and 20 °C. If conformationally chiral substructures were
present, an increase in temperature would lead to a
decrease in the stability of these structures and a
corresponding decrease in optical rotation. However,
variation of optical activity of 1-2%/°C is not uncommon
and could also reflect a perturbation in solvent-solute
equilibria or in the degree of aggregation.26 DSC analysis
of 9 reveals a Tg at 45 °C, but no phase transitions were
observed between 10 and 20 °C. Therefore, the steep
decrease in rotation between 10 and 20 °C observed for
9 may be indicative of a change in conformational
equilibrium that occurs in 9 at low temperatures.
Acylation of (1R,2S)-2-Am in o-1-ph en yl-1,3-pr opan ediol.
Met h od 1. A. ter t-Bu t yld im et h ylsilyl 4-((ter t-Bu t yld i-
m eth ylsilyl)oxy)ben zoa te. To a solution of 4-hydroxybenzoic
acid (6.9 g, 50 mmol) in dry dimethylformamide (40 mL) was
added a solution of imidazole (13.6 g, 200 mmol) in dimeth-
ylformamide (40 mL) followed by a solution of tert-butyldi-
methylsilyl chloride (15.8 g, 105 mmol) in dimethylformamide
(40 mL). The mixture was stirred at 60 °C under argon for 16
h and then poured over ice water (ca. 200 g), and the aqueous
phase was extracted with diethyl ether (5 × 50 mL). The
combined extracts were washed with water (50 mL) and brine
(50 mL) and dried over magnesium sulfate. The solvent was
removed in vacuo affording 18.2 g (99%) of tert-butyldimeth-
ylsilyl 4-((tert-butyldimethylsilyl)oxy)benzoate as an oil which
was used without further purification in part B.1H NMR (200
MHz, CDCl3): δ 0.23 (s, 6H), 0.36 (s, 6H), 0.99 (s, 9H), 1.02 (s,
9H), 6.85 (d, J ) 8.7 Hz, 2H), 7.93 (d, J ) 8.7 Hz, 2H).
B. 4-((ter t-bu tyld im eth ylsilyl)oxy)ben zoyl Ch lor id e.
tert-Butyldimethylsilyl 4-((tert-butyldimethylsilyl)oxy)benzoate
(18.2 g, 50 mmol, crude from part A) in dichloromethane (50
mL) containing 10 drops of DMF was treated with oxalyl
chloride (1.5 equiv, 9.52 g, 6.5 mL, 75 mmol). After 40 h at
room temperature, the solvents were evaporated in vacuo and
the crude acyl chloride was used in part C without further
1
purification. H NMR (200 MHz, CDCl3): δ 0.26 (s, 6H), 0.99
(s, 9H), 6.90 (d, J ) 9.0 Hz, 2H), 8.03 (d, J ) 9.0 Hz, 2H).
C. (1R,2S)-(2-(4-((ter t-b u t yld im et h ylsilyl)oxy)b en z-
a m id o)-1-p h e n yl-1,3-p r op a n e d iyl)b is(4-((t er t -b u t yld i-
m eth ylsilyl)oxy)ben zoa te). 4-((tert-Butyldimethylsilyl)oxy)-
benzoyl chloride (50 mmol, crude from part B) in dichloro-
methane (10 mL) was added dropwise to a cooled (0 °C)
solution of (1R,2S)-2-amino-1-phenyl-1,3-propanediol (2.09 g,
12.5 mmol), (dimethylamino)pyridine (DMAP) (0.15 g, 1.25
mmol), and pyridine (10 mL) in dichloromethane (50 mL). After
the addition was complete, the mixture was allowed to warm
to room temperature overnight (14 h). Diethyl ether (100 mL)
was added to the mixture, and the pyridinium hydrochloride
that precipitated was removed by filtration. The filtrate
solution was washed with 50 mL portions of 1 N hydrochloric
acid (aqueous) until the aqueous phase remained acidic, then
with 50 mL portions of 5% aqueous sodium bicarbonate until
the aqueous solution remained basic, and finally with water
(50 mL). The organic layer was then dried over magnesium
sulfate and evaporated to dryness. 1H NMR (200 MHz,
CDCl3): δ 0.19 (s, 6H), 0.21 (s, 6H), 0.25 (s, 6H), 0.96 (s, 9H),
0.97 (s, 9H), 1.00 (s, 9H), 4.21 (dd, J ) 11.6, 4.6, Hz, 1H), 4.46
(dd, J ) 11.6, 3.7 Hz, 1H), 5.09 (m, 1H), 6.34 (d, J ) 8.5 Hz,
1H), 6.65 (d, J ) 9.1 Hz, 1H), 6.71-6.91 (m, 6H), 7.26-7.60
(m, 7H), 7.90-7.97 (m, 4H).
Su m m a r y
A series of dendrimers with a chiral central core and
methyl benzoate termini have been prepared. On the
basis of the investigation of a series of model compounds,
the observed decrease in optical activity with increasing
generation has been attributed to an increasing steric
effect that perturbs the conformational preference of the
chiral core. CD measurements also suggest that the
observed Cotton effects are due to changes in the popula-
tion of certain conformations of the central core. We are
currently preparing water-soluble derivatives of these
and other chiral dendrimers so that the induced CD of
achiral dyes can be used as a measure of conformational
chirality in the branch segments of the dendrimers.
Exp er im en ta l Section
Gen er a l Meth od s. Melting points were determined in open
D. (1R,2S)-(2-(4-h yd r oxyben za m id o)-1-p h en yl-1,3-p r o-
p a n ed iylbis(4-h yd r oxyben zoa te (2). The crude protected
core from part C (12.5 mmol) in dry tetrahydrofuran (THF)
(50 mL) was treated at 0 °C with a 1 M solution of tetrabu-
tylammonium fluoride in THF (4 equiv, 50 mL, 50 mmol). After
the addition was complete, the mixture was allowed to warm
to room temperature for 1 h. The mixture was then quenched
capillaries and are uncorrected. 1H NMR spectra were recorded
(25) Such variation of rotation values with solvent is not unusual.
For an example of a solvent-induced reorientation of dipoles that
reverses the sign of an ORD curve, see ref 18c.
(26) Eliel, E. L.; Wilen, S. H. Stereochemistry of Organic Compounds;
Wiley: New York, 1994; p 1076.

9404 J . Org. Chem., Vol. 63, No. 25, 1998
Chaumette et al.
by adding 1 N aqueous hydrochloric acid solution (100 mL)
and was stirred for an additional 5 min. The mixture was
extracted into dichloromethane (4 × 50 mL), and the combined
organic layers were dried over magnesium sulfate and evapo-
rated to dryness. Purification by flash chromatography (SiO2)
with (1-5%) methanol/dichloromethane afforded (1R,2S)-(2-
(4-hydroxybenzamido)-1-phenyl-1,3-propanediylbis(4-hydroxy-
benzoate) (2) (3.7 g, 56%) as a white solid: mp ) 160-163 °C,
1H NMR (300 MHz, MeOD4) δ 4.25 (dd, J ) 11.3, 7.3 Hz, 1H),
4.36 (dd, J ) 11.3, 4.6 Hz, 1H), 5.09 (m, 1H), 6.20 (d, J ) 7.9
Hz, 1H), 6.74-6.82 (m, 7H), 7.27-7.62 (m, 3H), 7.80-8.03 (m,
3H); 13C NMR (75 MHz, MeOH-d4) 54.6, 64.1, 76.9, 116.18,
116.32, 116.40, 121.98, 122.07, 126.73, 128.39, 129.87, 129.96,
130.01, 130.55, 133.11, 133.17, 133.29, 139.25, 162.21, 163.90,
167.42, 167.93, 170.97, 173.10 ppm; IR (KBr) 3302 (OH), 1687
(CO) cm-1. HRMS for C23H20NO5 (EI) (M+ - C7H5O3): calcd,
m/e 390.1342; obsd, m/e 390.1354.
combined organic layers were washed sequentially with 1.2
N aqueous hydrochloric acid, 5% aqueous sodium bicarbonate,
and saturated aqueous sodium chloride and then dried over
Mg2SO4 and evaporated. The crude material was purified via
flash chromatography on silica gel.
(1R,2S)-(2-(3,5-dim eth oxyben zam ido)-1-ph en yl-1,3-pr o-
p a n ed iylbis(3,5-d im eth oxyben zoa te) (4). This compound
was prepared from 3,5-dimethoxybenzoyl chloride and purified
via flash chromatography on silica gel (elution with 25% ethyl
acetate/ hexanes) to yield 4 as a white solid (100 mg, 0.15
mmol, 51%): mp ) 120-124 °C; 1H NMR (300 MHz, CDCl3) δ
3.75 (s, 6H), 3.79 (s, 6H), 3.81 (s, 6H), 4.31 (dd, J ) 11.5, 4.8
Hz, 1H), 4.47 (dd, J ) 11.5, 3.0 Hz, 1H), 5.15 (m, 1H), 6.29 (d,
J ) 7.7 Hz, 1H), 6.52 (t, J ) 1.9 Hz, 1H), 6.62 (t, J ) 2.2 Hz,
1H), 6.64 (t, J ) 2.3 Hz, 1H), 6.78 (d, J ) 2.0 Hz, 2H), 7.16 (d,
J ) 2.2 Hz, 2H), 7.18 (d, J ) 2.2 Hz, 2H), 7.38 (m, 3H), 7.52
(m, 2H); 13C NMR (75 MHz, CDCl3) 53.1, 55.3, 55.4, 55.5, 63.9,
75.4, 103.9, 104.5, 104.6, 105.9, 106.0, 107.2, 107.3, 127.0,
128.9, 128.97, 131.16, 131.23, 133.35, 136.32, 136.38, 160.57,
160.62, 160.78, 165.85, 166.0, 167.24 ppm; IR (neat) 1722, 1669
cm-1; UV (CHCl3) λmax ) 256 nm (ꢀ 12 104), 308 (ꢀ 4288) nm.
HRMS for C36H38NO11 (FAB) (MH): calcd, m/e 660.2444; obsd,
m/e 660.2619.
Meth od 2. (1R,2S)-(2-(4-(ben zyloxy)ben zam ido)-1-ph en -
yl-1,3-p r op a n ed iylbis(4-(ben zyloxy)ben zoa te) (3). Oxalyl
chloride (4.5 g, 3.1 mL, 35.54 mmol) was added dropwise to a
suspension of 4-(benzyloxy)benzoic acid (4.1 g, 18 mmol) in
dichloromethane (20 mL) and DMF (3 drops), at 25 °C. After
3 h, the solvent was evaporated in vacuo and the yellow oil
was dissolved in dichloromethane (25 mL) and added dropwise
over 5 min to a solution of (1R, 2S)-(+)-2-amino-1-phenyl-1,3-
propanediol (0.5 g, 3.0 mmol) and DMAP (0.04 g, 0.33 mmol)
in pyridine (15 mL) at 0 °C. The reaction mixture was allowed
to warm to 25 °C and was stirred for an additional 18 h. The
reaction mixture was transferred to a separatory funnel and
washed with 1.2 N aqueous hydrochloric acid (100 mL). The
aqueous layer was extracted with dichloromethane (3 × 100
mL), and the combined organic layers were washed sequen-
tially with 1.2 N hydrochloric acid (100 mL), 5% aqueous
sodium bicarbonate (100 mL), and saturated aqueous sodium
chloride (100 mL) and then dried over Mg2SO4 and evaporated
to yield 3.97 g of a yellow oil The oil was purified via flash
chromatography on silica gel (elution with 100% CH2Cl2 to 5%
diethyl ether/ CH2Cl2) to yield 3 as a white solid (1.52 g, 1.91
mmol, 64%): Rf ) 0.58 (5% ether/CH2Cl2), mp ) 69-74 °C;
1H NMR (300 MHz, CDCl3) δ 4.25 (dd, J ) 11.6, 4.7 Hz, 1H),
4.46 (dd, J ) 11.6, 3.7 Hz, 1H), 5.06 (s, 2H), 5.08 (s, 2H), 5.13
(s, 2H), 6.29 (d, J ) 8.4 Hz, 1H), 6.69 (d, J ) 9 Hz, 1H), 6.9-
7.0 (m, 6H), 7.26-7.55 (m, 20H), 7.64 (d, J ) 8.8 Hz, 2H),
7.95-8.03 (m, 4H); 13C NMR (62.5 MHz, CDCl3) 53.3, 63.5,
70.02, 70.05, 70.11, 75.14, 114.4, 114.5, 114.6, 122.02, 126.47-
128.8 (mult. overlapping carbons), 131.7, 131.8, 136.04, 136.24,
136.74, 161.28, 162.7, 165.9, 166.0, 167.03 ppm; IR (KBr) 1715,
1652 cm-1; UV (CHCl3) λmax ) 258 nm (ꢀ 56 906). HRMS for
(1R,2S)-(2-(3,5-d ib en zyloxyb en za m id o)-1-p h en yl-1,3-
p r op a n ed iylbis(3,5-d iben zyoxyben zoa te) (5). This com-
pound was prepared from 3,5-dibenzyloxybenzoic acid,27 and
the resultant crude oil was purified via flash chromatography
on silica gel (elution with 100% CH2Cl2 to 5% diethyl ether/
CH2Cl2) to yield 5 as a white solid (0.21 g, 0.19 mmol, 32%):
1
mp ) 56-60 °C; H NMR (300 MHz, CDCl3) δ 4.39 (dd, J )
11.7, 5.31 Hz, 1 H) 4.50 (dd, J ) 11.7, 3.6 Hz, 1H), 4.95 (s,
4H), 5.01 (s, 4H), 5.10 (s, 4 H), 5.26 (m, 1H), 6.28 (d, J ) 8.28
Hz, 1H), 6.57 (d, J ) 7.8 Hz, 1H), 6.75 (t, J ) 2.17 Hz, 1H),
6.86 (t, J ) 2.24 Hz, 1H), 6.89 (t, J ) 2.32 Hz, 1H), 7.04 (d, J
) 1.92 Hz, 2H), 7.24-7.58 (m, 40 H); 13C NMR (75 MHz,
CDCl3) 53.3, 64.1, 70.13, 70.25, 75.67, 105.6, 105.9, 107.7,
107.8, 108.4, 108.6, 127.2-129.1 (multiple overlapping car-
bons), 131.3, 131.4, 136.3, 136.4, 136.4, 136.5, 137.8, 159.8,
159.9, 160.0, 165.9, 166.1, 167.5 ppm; IR (cm-1) 1720, 1664;
UV (CHCl3) λmax ) 256 nm (ꢀ 17 330), 308 (ꢀ 7882) nm. HRMS
for C72H62NO11 (FAB) (MH): calcd, m/e 1116.4323; obsd, m/e
1116.4323.
(1R,2S)-(2-(2,6-dim eth oxyben zam ido)-1-ph en yl-1,3-pr o-
p a n ed iylbis(2,6-d im eth oxyben zoa te) (6). This compound
was prepared from 2,6-dimethoxybenzoyl chloride, and the
resultant crude oil was purified via flash chromatography on
silica gel (elution with 50% ethyl acetate/ hexanes) to yield 6
as a white solid (0.09 g, 0.14 mmol, 46%): mp ) 105-108 °C;
1H NMR (300 MHz, CDCl3) δ 3.59 (s, 6H), 3.71 (s, 6H), 3.80
(s, 6H), 4.32 (dd, J ) 10.6, 3.9 Hz, 1H), 4.64 (dd, J ) 10.6, 9.0
Hz, 1H), 4.99 (m, 1H), 6.22 (d, J ) 9.1 Hz, 1H), 6.46 (d, J )
8.41 Hz, 2H), 6.56 (d, J ) 8.45 Hz, 2H), 6.58 (d, J ) 8.56 Hz,
2H), 7.23 (t, J ) 8.37 Hz, 1H), 7.26-7.39 (m, 6H), 7.51 (d, J )
8.41 Hz, 2H); 13C NMR (75 MHz, CDCl3) 52.1, 55.6, 55.8, 56.0,
62.8, 73.6, 103.7, 126.3, 126.6, 127.6, 128.0, 130.5, 131.2, 137.2,
157.3, 157.5, 157.7, 164.9, 165.3, 165.9 ppm; IR (cm-1) 3054,
2968, 1736, 1671, 1597, 1473; UV (CHCl3) λmax ) 250 nm (ꢀ
7999), 282 nm (ꢀ 5731). HRMS for C36H38NO11 (FAB) (MH):
calcd, m/e 660.2444; obsd, m/e 660.2600.
Den d r on Mon obr om id e Syn th esis. (MeO2C)2[G-1]-Br,
(MeO2C)4[G-2]-Br, and (MeO2C)8[G-3]-Br were prepared by the
method of Fre´chet;12 however, the starting methyl 4-(bromo-
methyl)benzoate was prepared as follows.
Meth yl 4-(Br om om eth yl)ben zoa te.12 Methyl p-toluate
(212 g, 1.4 mol) and 2 L of benzene were added to a 5-L three-
necked flask fitted with a condenser, a mechanical stirrer, and
a 1-L addition funnel. The mixture was heated to reflux, and
a 250-W sunlamp was focused on the reaction. Bromine (72.6
mL, 1.4 mol) in 900 mL of benzene was then added dropwise
via addition funnel over a period of 2 h while the solution was
at reflux. After the addition of bromine was complete, the lamp
C
51H44NO8 (FAB) (MH): calcd, m/e 798.3007; obsd, m/e
798.3089.
(1R ,2S )-(2-(4-h yd r oxyb e n za m id o)-1-p h e n yl-1,3-p r o-
p a n ed iylbis(4-h yd r oxyben zoa te) (2). A solution of 3 (290
mg, 0.36 mmol) in 5 mL of methanol/ethyl acetate (1:1) was
degassed by bubbling nitrogen through the solution for 5 min.
W-2 Raney Ni (60 mg) was added, and the vessel was flushed
with H2 and fitted with a H2 balloon at 1 atm. After 18 h, the
Ni was removed by filtration through a pad of Celite and the
filtrate was evaporated in vacuo affording a white solid. The
crude solid was purified via flash chromatography on silica
gel (elution with 1% methanol/CH2Cl2-4% methanol/CH2Cl2)
to yield 2 (100 mg, 52%) as a white solid.
P r ep a r a tion of Mod el Com p ou n d s 4-6. The acid chlo-
ride was either obtained commercially or prepared by the
following procedure: The appropriate carboxylic acid (6 equiv)
was treated with oxalyl chloride (12 equiv) and DMF (2 drops)
in methylene chloride. After 2 h, the solvent was evaporated
in vacuo and the acid chloride was dissolved in dichlo-
romethane and added dropwise over 5 min to a solution of
(1R,2S)-2-amino-1-phenyl-1,3-propanediol, 1 (1 equiv), and
DMAP (10 mol %) in pyridine at 0 °C to obtain a final
concentration of 0.25 M in 1. The reaction mixture was allowed
to warm to room temperature and stirred an additional 18 h.
The mixture was transferred to a separatory funnel and
washed with 1.2 N aqueous hydrochloric acid. The aqueous
layer was extracted into dichloromethane (3×), and the
(27) Haddleton, D. M.; Sahota, H.; Taylor, P. C.; Yeates, S. G. J .
Chem. Soc., Perkin Trans. 1 1996, 649.

Chiroptical Properties of Dendrimers
J . Org. Chem., Vol. 63, No. 25, 1998 9405
remained focused on the flask for an additional 10 minutes
until the dark red color of the solution dissipated and the
solution became pale yellow. The reaction mixture was then
cooled to ambient temperature, and the solvent was evaporated
in vacuo. The remaining oily residue crystallized at 4 °C
overnight affording light yellow/orange crystals. Recrystalli-
zation from heptane afforded methyl 4-(bromomethyl)benzoate
(157 g, 0.69 mol, 49%) as light yellow/white crystals.
Gen er a l P r oced u r e for Syn th esis of Den d r im er s 7-9.
The dendron benzyl bromide (3.33 equiv), (1R,2S)-(2-(4-hy-
droxybenzamido)-1-phenyl-1,3-propanediylbis(4-hydroxyben-
zoate) (2) (1 equiv), finely powdered potassium carbonate (5
equiv), and 18-crown-6 (0.2 equiv) were mixed in a sealed
vessel. The vessel was charged with THF (5 mL), and dimeth-
ylformamide (1.5 mL) was added to homogenize the suspen-
sion. The sealed vessel was heated to 70 °C for 12 h. After
being cooled to room temperature, the pressure tube was
opened and water was added. The mixture was then extracted
with dichloromethane (3×), and the combined organic layers
were washed with brine, dried over magnesium sulfate, and
evaporated to dryness. Purification was accomplished by
column chromatography over silica gel.
G1 Den d r im er (7). 6-Ca sca d e: (1R,2S)-1-P h en ylp r o-
p a n e [3-1,2,3]:2-[(4-(1-oxo-2-a za eth yl)-1-p h en ylen e):(5-
(2-oxaethyl)-1,3-phenylene):(4-(2-oxaethyl)-1-(carbomethoxy)-
b en zen e)]:1,3-b is[4-(1-oxo-2-oxa et h yl)-1-p h en ylen e):(5-
(2-oxa e t h yl)-1,3-p h e n yle n e ):(4-(2-oxa e t h yl)-1-(ca r b o-
m et h oxy)b en zen e). This compound was prepared from
(MeO2C)2[G-1]-Br and 2. Purification by flash chromatography
(5-10% ether/dichloromethane) afforded the G1-(COOMe)6 (7)
dendrimer as a white solid in 74% yield (250 mg): 1H NMR
(200 MHz, CDCl3) δ 3.90 (s, 18H), 4.23 (dd, J ) 11.3, 4.7 Hz,
1H), 4.49 (dd, J ) 11.3, 3.2 Hz, 1H), 5.06-5.09 (m, 19H), 6.33
(d, J ) 8.6 Hz, 1H), 6.52-6.60 (m, 3H), 6.8-6.7 (m, 6H), 6.81-
6.95 (m, 7H), 7.34-7.53 (m, 17H), 7.64 (d, J ) 8.8 Hz, 2H),
7.94-8.06 (m, 16H); 13C NMR (62.5 MHz, CDCl3) 52.07, 53.26,
63.83, 69.43, 69.74, 75.21, 101.32, 101.79, 105.87, 106.32,
106.37, 106.45, 114.57, 121.91, 122.29, 126.71, 126.89, 126.97,
127.19, 127.60, 128.03, 128.41, 128.78, 128.87, 129.73, 129.83,
131.73, 131.86, 132.31, 136.92, 138.86, 139.04, 141.79, 159.90,
159.93, 161.09, 162.51, 165.88, 165.92, 166.71, 166.99 ppm;
chromatography (5-10% ether/dichloromethane) affording the
G2-(COOMe)12 (8) dendrimer as a white solid in 80% yield (520
mg): 1H NMR (200 MHz, CDCl3) δ 3.87 (s, 36H), 4.25 (br dd,
1H) 4.50 (br s, 1H), 4.93-5.03 (m, 43H), 6.33 (d, J ) 8.3 Hz,
1H), 6.4-6.6 (m, 27H), 6.83-6.98 (m, 7H), 7.32-7.49 (m, 27H),
7.64 (d, J ) 8.4 Hz, 2H), 7.97-8.02 (m, 30H); 13C NMR (62.5
MHz, CDCl3) 52 (ArCO2CH3’s), 53 (1CH-NH, core), 63 (1CH2-
O, core), 69.3-69.7 (CH2O’s), 75 (1PHCHO, core), 101, 107,
114, 121, 121-133 (multiple carbons), 137-142 (multiple
carbons) 159-162 (multiple carbons) ppm; IR (CDCl3) 1719,
1598 cm-1; MS (MALDI-TOF) m/z 3425 (3425 calcd for
C201H175NO50Na (M + Na)); UV (CHCl3) λmax ) 246 nm (ꢀ
148 216), 265 nm (ꢀ 54 291). Anal. Calcd for C201H175NO50: C,
70.91; H, 5.18; N, 0.41. Found: C, 70.65; H, 5.19; N, 0.43.
G3 Den d r im er (9). 24-Ca sca d e: (1R,2S)-1-P h en ylp r o-
p a n e [3-1,2,3]:2-[(4-(1-oxo-2-a za eth yl)-1-p h en ylen e):(5-
( 2 -o x a e t h y l ) -1 ,3 -p h e n y l e n e ) 3 :( 4 -( 2 -o x a e t h y l ) -1 -
ca r b om et h oxyb en zen e)]:1,3-b is[4-(1-oxo-2-oxa et h yl)-1-
phenylene):(5-(2-oxaethyl)-1,3-phenylene)3:(4-(2-oxaethyl)-1-
(ca r bom eth oxy)ben zen e). This compound was prepared
from (MeO2C)8[G-3]-Br and 2. Purification was performed by
flash chromatography (5% to 15% ether/dichloromethane)
affording the G3-(COOMe)24 (9) dendrimer as a white solid in
70% yield (880 mg): 1H NMR (200 MHz, CDCl3) δ 3.86 (s,
72H), 4.23 (br s, ca. 1H), 4.48 (br s, ca. 1H), 4.87-5.1 (m, 91H),
6.31 (d, J ) 7.9 Hz, 1H), 6.49-6.62 (m, 63H), 6.80-6.92 (m,
7H), 7.38-7.43 (m, 52H), 7.62 (d, J ) 8.8 Hz, 2H), 7.97-8.01
(m, 54H); 13C NMR (62.5 MHz, CDCl3) (multiple overlapping
carbons) 52 (ArCO2CH3’s), 69.3 (CH2O’s), 69.8 (CH2O’s), 101,
107, 114, 126, 129, 138-139, 141, 159-160, 162.4, 162.5, 165.9,
166.6 ppm; IR (CDCl3) 1719, 1598 cm-1; UV (CHCl3) λmax
244 nm (ꢀ 313 096), 265 nm (ꢀ 102 622); GPC analysis Mcalc
)
)
6648, Mw ) 7178, Mn ) 7156, polydispersity index ) 1.0032;
MS (MALDI-TOF) m/z 6683 (6682 calcd for C393H343NO98K (M
+ K)) Anal. Calcd for C393H343NO98: C, 71.00; H, 5.20; N, 0.21.
Found: C, 70.78; H, 5.21; N, 0.31.
Ack n ow led gm en t. This work was supported by The
Ohio State University, and acknowledgment is made to
the donors of the Petroleum Research Fund, adminis-
tered by the American Chemical Society, for partial
support of this work.
MS (electrospray) m/z 1783.8 (1782.87 calcd for C105H92NO26
,
MH); IR (CDCl3) 1719, 1605 cm-1; UV (CHCl3) λmax ) 246 nm
(ꢀ 75 166), 265 nm (ꢀ 33 061). Anal. Calcd for C105H91NO26: C,
70.74; H, 5.14; N, 0.79. Found: C, 70.49; H, 5.19; N, 0.79.
G2 Den d r im er (8). 12-Ca sca d e: (1R,2S)-1-p h en ylp r o-
p a n e [3-1,2,3]:2-[(4-(1-oxo-2-a za eth yl)-1-p h en ylen e):(5-
(2-oxaethyl)-1,3-phenylene)2:(4-(2-oxaethyl)-1-(carbomethoxy)-
b e n ze n e )]:1,3-b is[4-(1-oxo-2-oxa e t h yl)-1-p h e n yle n e ):
(5-(2-oxa et h yl)-1,3-p h en ylen e)2:(4-(2-oxa et h yl)-1-(ca r -
bom eth oxy)ben zen e). This compound was prepared from
(MeO2C)4[G-2]-Br and 2. Purification was performed by flash
1
Su p p or tin g In for m a tion Ava ila ble: Copies of H NMR
spectra for compounds 2-6 (5 pages). This material is con-
tained in libraries on microfiche, immediately follows this
article in the microfilm version of the journal, and can be
ordered from the ACS; see any current masthead page for
ordering information.
J O981508B