Nanometer-scale water-soluble macrocycles from nanometer-sized amino acids

Article abstract of DOI:10.1021/jo902268x

"Chemical Equation Presented" This paper introduces the unnatural amino acids m-Abc^2k and o-Abc^2k as nanometer-sized building blocks for the creation of water-soluble macrocycles with well-defined shapes. m-Abc^2k and o-Abc

Full text of DOI:10.1021/jo902268x

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Nanometer-Scale Water-Soluble Macrocycles from Nanometer-Sized
Amino Acids
Chris M. Gothard and James S. Nowick*
Department of Chemistry University of California, Irvine, California 92697-2025
jsnowick@uci.edu
Received October 21, 2009
This paper introduces the unnatural amino acids m-Abc2K and o-Abc2K as nanometer-sized building
blocks for the creation of water-soluble macrocycles with well-defined shapes. m-Abc2K and o-Abc2K are
homologues of the nanometer-sized amino acid Abc2K, which we recently introduced for the synthesis of
water-soluble molecular rods of precise length (J. Am. Chem. Soc. 2007, 129, 7272). Abc2K is linear (180°),
m-Abc2K creates a 120° angle, and o-Abc2K creates a 60° angle. m-Abc2K and o-Abc2K are derivatives of
30-amino-(1,10-biphenyl)-4-carboxylic acid and 20-amino-(1,10-biphenyl)-4-carboxylic acid, with two
propyloxyammonium side chains for water solubility. m-Abc2K and o-Abc2K are prepared as Fmoc-
protected derivatives Fmoc-m-Abc2K(Boc)-OH (1a) and Fmoc-o-Abc2K(Boc)-OH (1b). These derivatives
can be used alone or in conjunction with Fmoc-Abc2K(Boc)-OH (1c) as ordinary amino acids in Fmoc-
based solid-phase peptide synthesis. Building blocks 1a-cwere used to synthesize macrocyclic “triangles”
9a-c, “parallelograms” 10a,b, and hexagonal “rings” 11a-d. The macrocycles range from a trimer to a
dodecamer, with ring sizes from 24 to 114 atoms, and are 1-4 nm in size. Molecular modeling studies
suggest that all the macrocycles except 10b should have well-defined triangle, parallelogram, and ring
shapes if all of the amide linkages are trans and the o-alkoxy substituents are intramolecularly hydrogen
bonded to the amide NH groups. The macrocycles have good water solubility and are readily
characterized by standard analytical techniques, such as RP-HPLC, ESI-MS, and NMR spectroscopy.
1H and 13C NMR studies suggest that the macrocycles adopt conformations with all trans-amide linkages
in CD3OD, that the “triangles” and “parallelograms” maintain these conformations in D2O, and that the
“rings” collapse to form conformations with cis-amide linkages in D2O.
Introduction
creation of water-soluble macrocycles with well-defined
shapes. There is intense interest in nanometer-sized macro-
cycles with shapes such as hexagons,1 triangles,2 and paral-
lelograms1c,3 because “shape-persistent” molecules are
central to a bottom-up approach to functional well-defined
This paper introduces the unnatural amino acids m-Abc2K
and o-Abc2K as nanometer-sized building blocks for the
(1) (a) Mayor, M.; Lehn, J.-M. J. Am. Chem. Soc. 1999, 121, 11231–
€
€
11232. (b) Hoger, S.; Bonrad, K.; Mourran, A.; Beginn, U.; Moller, M.
J. Am. Chem. Soc. 2001, 123, 5651–5659. (c) Tobe, Y.; Utsumi, N.;
Kawabata, K.; Nagano, A.; Adachi, K.; Araki, S.; Sonoda, M.; Hirose,
(2) (a) Youngs, W. J.; Tessier, C. A.; Bradshaw, J. D. Chem. Rev. 1999, 99,
3153–3180. (b) Kryschenko, Y. K.; Seidel, S. R.; Arif, A. M.; Stang, P. J.
J. Am. Chem. Soc. 2003, 125, 5193–5198. (c) Martin-Redondo, M. P.; Scoles,
L.; Sterenberg, B. T.; Udachin, K. A.; Carty, A. J. J. Am. Chem. Soc. 2005,
127, 5038–5039. (d) Heo, J.; Jeon, Y.-M.; Mirkin, C. A. J. Am. Chem. Soc.
2007, 129, 7712–7713. (e) Ferguson, J. S.; Yamato, K.; Lui, R.; He, L.; Zeng,
X. C.; Gong, B. Angew. Chem., Int. Ed. 2009, 48, 3150–3154.
(3) (a) Kuehl, C. J.; Huang, S. D.; Stang, P. J. J. Am. Chem. Soc. 2001,
123, 9634–9641. (b) Li, F.; Gan, Q.; Xue, L.; Wang, Z.-m.; Jiang, H.
Tetrahedron Lett. 2009, 50, 2367–2369. (c) Zhu, Y.-Y.; Wang, G.-T.; Li,
Z.-T. Org. Biomol. Chem. 2009, 7, 3243–3250.
€
K.; Naemura, K. J. Am. Chem. Soc. 2002, 124, 5350–5364. (d) Hoger, S.;
Morrison, D. L.; Enkelmann, V. J. Am. Chem. Soc. 2002, 124, 6734–6736.
€
(e) Grave, C.; Lentz, D.; Schafer, A.; Samori, P.; Rabe, J. P.; Franke, P.;
Schluter, A. D. J. Am. Chem. Soc. 2003, 125, 6907–6918. (f) Huang, X.-C.;
€
Zhang, J.-P.; Chen, X.-M. J. Am. Chem. Soc. 2004, 126, 13218–13219.
(g) Yang, H.-B.; Das, N.; Huang, F.; Hawkridge, A. M.; Diaz, D. D.; Arif,
A. M.; Finn, M. G.; Muddiman, D. C.; Stang, P. J. J. Org. Chem. 2006, 71,
€
6644–6647. (h) Pan, G.-B.; Cheng, X.-H.; Hoger, S.; Freyland, W. J. Am.
Chem. Soc. 2006, 128, 4218–4219.
1822 J. Org. Chem. 2010, 75, 1822–1830
Published on Web 12/18/2009
DOI: 10.1021/jo902268x
r
2009 American Chemical Society

Gothard and Nowick
JOCArticle
nanometer-scale molecules and molecular assemblies (i.e.,
“nanotechnology”).1-5 Most of these structures have been
designed for solubility in organic solvents, and many
are assembled from building blocks through carbon-
carbon or metal-ligand bond formation. The creation of
large (e.g., g2 nm) water-soluble macrocycles with well-
defined shapes remains largely undeveloped. Such structures
offer the promise of bridging the gap between the aqueous
world of biology and the synthetic world of nanotechno-
logy. If the structures are sufficiently easy to assemble from
building blocks, they may even allow researchers from
other disciplines to create molecular architectures for their
own applications. Here, we describe our approach to large,
water-soluble macrocycles.
SCHEME 1. Synthesis of Building Blocks 1a and 1b
Grignard-based halogen-metal exchange reaction to gen-
erate 4-bromo-2,5-dimethoxybenzoic acid (3).8 This reac-
tion, which was reported recently by Yokozawa and co-
workers in a related system, proceeds in double the yield of
the conventional Grignard reaction.6,8
To evaluate the utility of building blocks 1a-c for the
preparation of macrocycles, we synthesized “triangles”
9a-c, “parallelograms” 10a,b, and hexagonal “rings”
11a-d (Figure 1). The triangles are composed of 3 o-Abc2K
units and 0, 3, or 6 Abc2K units; the parallelograms are
composed of 2 o-Abc2K units, 2 m-Abc2K units, and 0 or 4
Abc2K units; and the rings are composed of 6 m-Abc2K units
and 0, 2, 4, or 6 Abc2K units. The macrocycles range from a
trimer to a dodecamer, with ring sizes from 24 to 114 atoms.
The triangles are 1-3 nm on an edge, the parallelograms are
1-2 nm on an edge, and the rings are 2-4 nm across.
The macrocycles were synthesized by Fmoc-based solid-
phase synthesis of protected linear oligomers, followed by
macrocyclization, deprotection, and RP-HPLC purification
(Scheme 2). The linear oligomers were prepared on chloro-
trityl resin with ca. 2 equiv of protected amino acid, HCTU
coupling reagent, 2,4,6-collidine base, and 8-12 h coupling
times. After final Fmoc-deprotection, the protected linear
oligomers were cleaved from the resin and cyclized at tenth-
millimolar concentrations with HCTU, 2,4,6-collidine, and
24 h reaction times. Analytical HPLC studies established that
the purities of the unpurified cyclized peptides were generally
high, with the exception of 11d, which proved difficult to
cyclize. The cyclic peptides were generally easy to purify by
RP-HPLC. Lyophilization afforded the trifluoroacetate
salts as fluffy white solids. A typical synthesis starting with
0.05 mmol of chlorotrityl resin gave an initial resin loading
of ca. 0.03 mmol and afforded 0.002-0.008 mmol of analy-
tically pure macrocycle after a single purification.
We designed m-Abc2K and o-Abc2K as homologues of the
nanometer-sized amino acid Abc2K, which we recently in-
troduced for the synthesis of water-soluble molecular rods
of precise length.6 Abc2K is a derivative of the nanometer-
long amino acid 40-amino-(1,10-biphenyl)-4-carboxylic acid,
with two propyloxyammonium side chains for water solu-
bility. Abc2K is linear (180°), m-Abc2K creates a 120° angle,
and o-Abc2K creates a 60° angle.7 In this paper, we report
the syntheses of Fmoc-protected derivatives Fmoc-m-
Abc2K(Boc)-OH (1a) and Fmoc-o-Abc2K(Boc)-OH (1b) and
the application of these derivatives to the synthesis of
nanometer-scale water-soluble macrocycles.
Results and Discussion
The Fmoc-m-Abc2K(Boc)-OH (1a) and Fmoc-o-
Abc2K(Boc)-OH (1b) building blocks were prepared in a
fashion analogous to Fmoc-Abc2K(Boc)-OH (1c), as shown
in Scheme 1.6 Central to the synthesis of Abc2K building
blocks is the Suzuki cross-coupling reaction of 4-bromoben-
zoic acid derivative 6 with 3-aminophenylboronic acid or
2-aminophenylboronic acid. All of the reactions proceed in
good yield, and the synthesis permits the preparation of
gram-scale quantities of 1a and 1b. One noteworthy
improvement to the synthesis involves the use of an unusual
The macrocycles have good water solubility and are readily
characterized by standard analytical techniques, such as RP-
HPLC, ESI-MS, and NMR spectroscopy in D2O or CD3OD
solution. Figure 2 shows characterization data for cyclodo-
decamer 11d; the HPLC trace shows a single peak; the mass
spectrum shows a series of multiply-charged molecular ions
consistent with the molecular weight of the dodecamer; the
1H NMR spectrum is sharp and well resolved.
(4) Nakao, K.; Nishimura, M.; Tamachi, T.; Kuwatani, Y.; Miyasaka,
H.; Nishinaga, T.; Iyoda, M. J. Am. Chem. Soc. 2006, 128, 16740–16747.
1H and 13C NMR studies suggest that the macrocycles adopt
conformations with all trans-amide linkages in CD3OD, that the
“triangles” and “parallelograms” maintain these conformations
€
(5) (a) Hoger, S. Chem.;Eur. J. 2004, 10, 1320–1329. (b) Zhang, W.;
Moore, J. S. Angew. Chem., Int. Ed. 2006, 45, 4416–4439.
(6) Gothard, C. M.; Rao, N. A.; Nowick, J. S. J. Am. Chem. Soc. 2007,
129, 7272–7273.
(7) Neustadt, B. R.; Smith, E. M.; Lindo, N.; Nechuta, T.; Bronnenkant,
A.; Wu, A.; Armstrong, L.; Kumar, C. Bioorg. Med. Chem. Lett. 1998, 8,
2395–2398.
(8) Miyakoshi, R.; Shimono, K.; Yokoyama, A.; Yokozawa, T. J. Am.
Chem. Soc. 2006, 128, 16012–16013.
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FIGURE 1. Nanometer-scale macrocyclic peptides prepared from m-Abc2K, o-Abc2K, and Abc2K (R = CH2CH2CH2NH3þ CF3CO2-).
in D2O, and that the “rings” collapse to form conformations
with cis-amide linkages in D2O. The 1H and 13C NMR spectra
of ring 11a in CD3OD are sharp and well resolved and clearly
show 6-fold symmetry (Figure 3a,b). The 1H NMR spectrum
shows six aromatic resonances and two sets of aliphatic reso-
nances for each of the three methylene groups, consistent with a
single type of m-Abc2K unit. The 13C NMR spectrum shows
one carbonyl resonance, 12 aromatic resonances, and six ali-
phatic resonances, also consistent with one type of m-Abc2K unit
and 6-fold symmetry of ring 11a. All of the aromatic 1H
resonances are downfield (7-8 ppm), suggesting that all of
the amide linkages are trans and that the ring adopts an open
structure without significant interactions between the aromatic
rings.
1
In D2O, the H and 13C NMR spectra of 11a are more
FIGURE 2. Characterization data for cyclododecamer ring 11d.
1824 J. Org. Chem. Vol. 75, No. 6, 2010
complex (Figure 3c,d). Some of the aromatic 1H resonances

Gothard and Nowick
SCHEME 2. Synthesis of Cyclohexamer Triangle 9b from Building Blocks Fmoc-o-Abc2K(Boc)-OH (1b) and Fmoc-Abc2K(Boc)-OH (1c)
JOCArticle
are shifted upfield (6-7 ppm), suggesting the presence of
cis-amide linkages.9 The 13C NMR spectrum shows ca. four
times as many resonances; while the 13C NMR spectrum
in CD3OD clearly shows two sets of propyloxyammonium
groups, the 13C NMR spectrum in D2O appears to show
eight. Collectively, these data suggest the presence of two
conformers in D2O: a collapsed 2-fold-symmetrical cttctt-
conformer, with two cis-amide linkages, in addition to
the open ringlike tttttt-conformer, with all trans-amide link-
ages. We have previously documented the formation of
both open (tttt) and collapsed (ctct) conformers in a related
4-fold-symmetrical macrocycle and have prepared and
studiedcontrolcompoundsthatadoptcis-amide conformers.9
These related systems have also exhibited upfield shifting of
1
the H aromatic resonances, reflecting the proximity of the
aromatic rings that is bought on by the cis-amide geometry.
Rings 11b-d show similar behavior. The 1H NMR
spectrum of 11d in CD3OD shows one set of resonances
associated with 6-fold symmetry, and the aromatic reso-
1
nances are downfield (7-8 ppm). The H NMR spectrum
in D2O is more complex, and some of the aromatic
resonances are shifted upfield (6-7 ppm), suggesting a
collapsed conformer with cis-amide linkages. The 1H NMR
spectra of 11b and 11c in CD3OD show downfield aromatic
resonances and appear to be consistent with all trans-amide
conformers with 2-fold symmetry. The spectra of 11b and 11c
in D2O are more complex, and some of the aromatic reso-
nances are shifted upfield, suggesting cis-amide conformers.
In contrast to cyclohexamer ring 11a, cyclohexamer tri-
angle 9b appears to adopt an open conformation with all
trans-amide linkages in both D2O and CD3OD. The 13C
NMR spectrum clearly shows 3-fold symmetry; the 1H NMR
spectrum, although slightly broadened, suggests 3-fold
symmetry and shows downfield aromatic resonances
FIGURE 3. NMR spectra of cyclohexamer ring 11a in CD3OD
and D2O.
(9) Kang, S. W.; Gothard, C. M.; Maitra, S.; Atia-tul-Wahab; Nowick,
J. S. J. Am. Chem. Soc. 2007, 129, 1486–1487.
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(Figure 4). The cyclononamer triangle 9c also shows 3-fold
symmetry and appears to adopt an open conformation with
all trans-amide linkages in D2O.
The cyclotrimer triangle 9a exhibits behavior atypical of
the other macrocycles. It elutes from RP-HPLC as two peaks
1
in a 2:1 ratio that re-equilibrates upon standing. The H
and 13C NMR spectra of 9a in D2O and in CD3OD show
two conformers in a 2:1 ratio: a 3-fold-symmetrical confor-
mer and an unsymmetrical conformer. The symmetrical
conformer likely has all trans-amide linkages and must have
3-fold symmetry. The unsymmetrical conformer could
either contain a cis-amide linkage or be an unsymmetrical
atropisomer associated with slow rotation about the
ortho-disubstituted biphenyl groups of the o-Abc2K units
FIGURE 4. NMR spectra of cyclohexamer triangle 9b in D2O.
FIGURE 5. Molecular models of low-energy conformers of macrocycles 9a-c, 10a,b, and 11a-d. The macrocycles were modeled as simplified
homologues 9a0-c0, 10a0,b0, and 11a0-d0 in which the propyloxyammonium side chains (R = CH2CH2CH2NH3þ) were replaced with methoxy
groups (R = Me). Overlays represent conformers identified within the lowest 5.00 kJ/mol from 1000 Monte Carlo conformational search steps.
1826 J. Org. Chem. Vol. 75, No. 6, 2010

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(shown below). The small size of the macrocycle likely raises
the energy barrier to conformational interconversion and
slows equilibration of the two conformers.
Parallelogram 10b appears to adopt conformations with all
trans-amide linkages in both D2O and CD3OD. The 13C NMR
spectrum in D2O shows a single 2-fold-symmetrical confor-
mer. The 1H NMR spectra show no significant upfield shifting
of the aromatic resonances in either D2O or CD3OD, suggest-
ing that the 2-fold-symmetrical conformer has all trans-amide
linkages. Parallelogram 10a shows similar behavior at slightly
elevated temperatures (e.g., 330 K). Some decoalescence of the
1H NMR spectra in either D2O or CD3OD occurs at lower
temperatures (e.g., 298 K), suggesting the occurrence of
atropisomers or, less likely, cis-amide linkages.
Molecular modeling studies suggest that all the macro-
cycles except 10b should have well-defined triangle, paral-
lelogram, and ring shapes if all the amide linkages are trans
and the o-alkoxy substituents are intramolecularly hydrogen
bonded to the amide NH groups. The macrocycles were
modeled as simplified homologues in which the propyl-
oxyammonium side chains (R = CH2CH2CH2NH3þ) were
replaced with methoxy groups (R = Me, Figure 5). Each
molecule was modeled using Maestro/MacroModel v8.5
with the MMFFs implementation of the MMFF force field
and MCMM conformational searching. The Ar-Ar and
Ar-N bonds were rotated during the search procedure.
Rotations about the Ar-CO and Ar-O bonds were not
performed. Amide linkages were assumed to adopt trans
conformations and were not allowed to adopt cis conforma-
tions (except as noted below). One thousand Monte Carlo
search steps were performed for each structure, and no effort
was made to ensure that all of the lowest energy conformers
or the global minimum were identified.10,11
FIGURE 6. Molecular models of cttctt-and tttttt-conformers of
macrocycle 11a. The macrocycle was modeled as simplified homo-
logue 11a0 in which the propyloxyammonium side chains (R =
CH2CH2CH2NH3þ) were replaced with methoxy groups (R = Me).
Structures represent the lowest energy cttctt- and tttttt-conformers
identified from 1000 Monte Carlo conformational search steps.
tttttt-conformer in Figure 6. The conformer represents
the global minimum among the cttctt-conformers and was
identified from 1000 Monte Carlo search steps starting with
a cttctt-conformer.12,13 In this conformer, the cis-amide
linkages and adjacent aromatic rings create tight turn struc-
tures and the other aromatic rings of the m-Abc2K units
stack.
Molecular modeling studies also suggest that introduction
of cis-amide linkages permits the formation of collapsed
structures. A simplified model of the cttctt-conformer of
11a (11a0, R = Me) was generated and is shown with the
Conclusion
(10) Thorough identification of all low-energy conformers is not practical
for the larger structures and is only marginally practical for the smaller
structures.
In summary, we have now developed a family of nano-
meter-sized amino acids, Abc2K, m-Abc2K, and o-Abc2K
,
(11) The modeling should be interpreted with several caveats: (1) The
MMFF and MMFFs force fields lack good parameters for some of the
stretches, bends, and torsions associated with the structures. (2) The MMFFs
force field is designed to enforce planarity of the amide nitrogen atoms and
may therefore overemphasize the conformational regularity of the structures.
(3) The absence of H2O solvation in the modeling should decrease the effect
of hydrophobic interactions within the structures. (4) The conformational
search procedure excluded conformers with cis-amide linkages and did not
explicitly seek those lacking intramolecular hydrogen bonds between the
o-methoxy group and the amide NH group or with alternative rotations
about the Ar-OMe bonds.
that can easily be combined to create water-soluble nano-
meter-scale macrocycles with well-defined shapes. The
(12) Other low energy cttctt-conformers found in the search are virtually
identical in structure to the global-minimum structure.
(13) A meaningful comparison of the relative energies of the cttctt- and
tttttt-conformers is not possible because of the absence of charges and
solvation in the modeling studies and because of the limitations of the force
field.
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Gothard and Nowick
JOCArticle
macrocycles are easy to synthesize, purify, and characterize
because they are water-soluble cationic peptides. The macro-
cycles can adopt conformations with all trans-amide linkages
and cavities as large as 4 nm or undergo hydrophobic
collapse depending on their composition and on the solvent.
The size, water solubility, and ease of synthesis of these
compounds should help bridge the gap between biology and
nanotechnology.
acetate-hexanes, 2/1, v/v) provided H-m-Abc2K(Boc)-OCH2-
COPh (7a) as an off-white solid (3.60 g, 63%): mp 118-
120 °C; IR (KBr) 3430, 3361, 1729, 1681 cm-1 1H NMR,
;
(500 MHz, CD3SOCD3, 298 K) δ 8.03 (d, J = 7.2 Hz, 2 H),
7.72 (t, J = 7.4 Hz, 1 H), 7.59 (t, J = 7.8 Hz, 2 H), 7.45 (s, 1 H),
7.08 (t, J = 7.8 Hz, 1 H), 7.03 (s, 1 H), 6.90 (t, J = 5.4 Hz, 1 H),
6.85 (t, J = 5.5 Hz, 1 H), 6.80 (s, 1 H), 6.72 (d, J = 7.7 Hz, 1 H),
6.59 (d, J = 7.9 Hz, 1 H), 5.72 (s, 2 H), 5.15 (br s, 2 H), 4.05
(t, J = 6.1 Hz, 2 H), 3.93 (t, J = 6.2 Hz, 2 H), 3.12 (q, J = 6.5 Hz,
2 H), 3.06 (q, J = 6.4 Hz, 2 H), 1.81 (quintet, J = 6.4 Hz, 2 H),
1.77 (quintet, J = 6.5 Hz, 2 H), 1.37 (s, 9 H), 1.32 (s, 9 H); 13
C
Experimental Section
NMR (125 MHz, CD3SOCD3, 298 K) δ 192.8, 164.3, 155.6,
155.5, 152.6, 148.8, 148.16, 137.5, 136.7, 133.9, 133.9, 128.8,
128.4, 127.7, 117.7, 116.9, 116.6, 115.2, 114.8, 113.3, 77.4, 77.3,
67.2, 66.8, 66.2, 37.0, 36.7, 29.3, 29.0, 28.14, 28.09; HRMS
(ESIMS) m/z for C37H47N3O9Na [M þ Na]þ calcd 700.3210,
found 700.3194.
General Procedures. Commercial solvents and reagents were
used without further purification, unless otherwise stated. All
solution-phase reactions were carried out under an atmosphere
of nitrogen and were monitored by thin-layer chromatography
(TLC) and carried out on 250 μm silica gel polyester plates on
fluorescent silica gel with UV visualization. Column chroma-
tography was performed on 40-63 μm silica gel (EMD Science)
using flash chromatography. Solvents were removed by rotary
evaporation, followed by further drying of residual solvents
under vacuum (<0.01 mmHg) or by air suction through a filter
funnel. High-resolution mass spectra were obtained by electro-
spray ionization (ESI) on a Waters Micromass LCT Premier
(instrument variation σ < 5 ppm). NMR spectra were recorded
using a 500 MHz Bruker AVANCE spectrometer. Chemical
shifts are reported in parts per million (ppm) on the δ scale. 1H
NMR spectra in CD3SOCD3 were referenced with TMS
Fmoc-m-Abc2K(Boc)-OCH2COPh (8a). A solution of H-m-
Abc2K(Boc)-OCH2COPh (7a) (3.60 g, 5.31 mmol), pyridine
(0.52 mL, 6.4 mmol), and CH2Cl2 (100 mL) was cooled to
0 °C in an ice bath. Fmoc-Cl (1.51 g, 5.84 mmol) in CH2Cl2
(50 mL) was added in drops over 5 min. After 30 min, the
solution was allowed to warm to 25 °C and stirred for an
additional 30 min. The mixture was then washed with water
(ca. 100 mL), dried (MgSO4), and filtered, and the filtrate was
concentrated by rotary evaporation to give a yellow oil. The
yellow oil was dissolved in CH2Cl2 (ca. 15 mL), precipitated
using ethyl acetate (ca. 50 mL) and hexanes (ca. 100 mL), and
filtered to provide Fmoc-m-Abc2K(Boc)-OCH2COPh (8a) as a
white solid (4.36 g, 91%): mp 118-120 °C; IR (KBr) 3369, 1733,
1
(δ = 0.00 ppm); H NMR spectra in D2O were referenced to
HDO (δ = 4.80 ppm); 1H NMR spectra in CD3OD were
referenced to CHD2OD (δ = 3.34). IR spectra were obtained
using a Galaxy Series FTIR 5000. HPLC analysis was per-
formed on an analytical RP-HPLC instrument, using a C18
column (Alltech, Platinum Rocket, 3 μm packing, 7 mm ꢀ
53 mm). Preparative RP-HPLC was performed using an Agilent
1697 cm-1; H NMR, (500 MHz, CD3SOCD3, 298 K) δ 9.63
1
(s, 1 H), 8.01 (d, J = 8.5 Hz, 2 H), 7.89 (d, J = 7.5 Hz, 2 H), 7.75
(d, J = 7.5 Hz, 2 H), 7.70 (t, J = 7.4 Hz, 1 H), 7.65 (br s, 1 H),
7.58 (t, J = 7.7 Hz, 2 H), 7.51-7.44 (m, 2 H), 7.42 (t, J = 7.5 Hz,
2 H), 7.37-7.30 (m, 3 H), 7.21 (d, J = 7.7 Hz, 1 H), 7.05 (s, 1 H),
6.66 (br s, 2 H), 5.68 (s, 2 H), 4.49 (d, J = 6.7 Hz, 2 H), 4.31
(t, J = 6.7 Hz, 2 H), 4.05 (t, J = 6.1 Hz, 2 H), 3.94 (t, J = 6.3 Hz,
2 H), 3.12 (q, J = 6.6 Hz, 2 H), 3.02 (q, J = 6.2 Hz, 2 H), 1.82
(quintet, J = 6.5 Hz, 2 H), 1.76 (quintet, J = 6.5 Hz, 2 H), 1.34
(s, 9 H), 1.32 (s, 9 H); 13C NMR (125 MHz, CD3SOCD3, 298 K)
δ 192.7, 164.3, 155.54, 155.50, 153.4, 152.5, 148.8, 143.7, 140.7,
138.8, 137.4, 135.6, 133.9, 128.8, 128.3, 127.7, 127.6, 127.0,
125.0, 123.5, 120.1, 119.2, 118.3, 117.6, 116.8, 115.2, 77.4,
77.3, 67.3, 66.8, 66.3, 65.5, 46.5, 37.0, 36.6, 29.3, 29.1, 28.10,
28.08; HRMS (ESIMS) m/z for C52H57N3O11Na [M þ Na]þ
calcd 922.3891, found 922.3863.
C
18 column (1 in. diameter).
4-Bromo-2,5-dimethoxybenzoic Acid (3).8
A flame-dried
3-necked 100-mL round bottomed flask equipped with a mag-
netic stirring bar, rubber septum, ground-glass stopper, and a
nitrogen inlet adapter was charged with 1,4-dibromo-2,5-
dimethoxybenzene (2) (5.0 g, 16.9 mmol). THF (ca. 25 mL)
was added by syringe until a clear solution was formed. A 2.0 M
solution of i-PrMgCl in THF (8.7 mL) was then added by
syringe in a single portion at 25 °C to a stirring solution of 2
and was then stirred under an atmosphere of nitrogen for 20 h.
Dry ice (ca. 20 g) was added in small portions resulting in a color
change of the reaction mixture from light yellow to bluish-green.
The solution returned to its original light yellow color after
being allowed to mix for an additional 90 min. The reaction
mixture was acidified with concd HCl (aq) (ca. 5 mL) and then
extracted into ether (3 ꢀ 100 mL) with water (75 mL). The
organic layer was washed with water (ca. 100 mL) and then
extracted into aq 1 M KOH (3 ꢀ ca. 30 mL). The alkaline
solution was acidified with concd HCl (aq), and the resultant
white precipitate was washed with water to provide 4-bromo-
2,5-dimethoxybenzoic acid (3) as a white powder (3.7 g, 85%).15
H-m-Abc2K(Boc)-OCH2COPh (7a). Diether 66 (5.63 g, 8.86
mmol) was dissolved in THF (150 mL) by heating to ca. 60 °C in
an oil bath. To the warmed solution were added 3-aminophe-
Fmoc-m-Abc2K(Boc)-OH (1a). Zn dust (8.0 g, 122 mmol) was
added to a solution of Fmoc-m-Abc2K(Boc)-OCH2COPh (8a)
(4.36 g, 4.84 mmol), AcOH (90 mL), and H2O (10 mL) and was
stirred at 25 °C for ca. 18 h. The suspension was then diluted
with CH2Cl2 (75 mL) and stirred for an additional ca. 1 h,
concentrated to approximately one-half of its original volume,
and partitioned between CH2Cl2 (75 mL) and aq 0.2 N HCl
(ca. 75 mL). The aqueous layer was extracted with CH2Cl2 (2 ꢀ
75 mL), and the combined organic layers were washed with H2O
(ca. 100 mL) and saturated aq sodium chloride (ca. 100 mL)
and dried (MgSO4). Filtration and concentration by rotary
evaporation afforded a yellow oil. The oil was dissolved in
ether (ca. 20 mL) and CH2Cl2 (ca. 10 mL), and the resulting
solution was added in drops to hexanes (ca. 150 mL) over ca. 2
min. The resulting white suspension was filtered to afford
Fmoc-m-Abc2K(Boc)-OH (1a) as a white solid (3.78 g, 94%):
mp 98-100 °C; IR (KBr) 3361, 1702 cm-1; 1H NMR,(500 MHz,
CD3SOCD3, 298 K) δ 12.58 (br s, 1 H), 9.77 (br s, 1 H), 7.91
(d, J = 7.5 Hz, 2 H), 7.76 (d, J = 7.4 Hz, 2 H), 7.64 (br s, 1 H),
7.49 (br s, 1 H), 7.43 (t, J = 7.4 Hz, 2 H), 7.39-7.26 (m, 4 H),
7.18 (d, J = 7.6 Hz, 1 H), 6.99 (s, 1 H), 6.87 (t, J = 5.6 Hz, 1 H),
6.84 (t, J = 5.7 Hz, 1 H), 4.49 (d, J = 6.5 Hz, 2 H), 4.31
nylboronic acid (1.16 g, 8.86 mmol), PdCl2(dppf) CH2Cl2
3
(0.345 g, 0.423 mmol), K2CO3 (5.86 g, 42.3 mmol), water
(30 mL), and the mixture was stirred at 60 °C for 24 h under
an atmosphere of nitrogen. The solution was then concentrated
by rotary evaporation and the resultant oil was dissolved in
CH2Cl2 (ca. 100 mL). The organic layer was washed with water
(ca. 100 mL), dried (MgSO4), and filtered. The filtrate was
concentrated by rotary evaporation to afford a brown oil.
Purification using column chromatography (silica gel, ethyl
1828 J. Org. Chem. Vol. 75, No. 6, 2010

Gothard and Nowick
JOCArticle
(t, J = 6.7 Hz, 1 H), 4.01 (t, J = 6.0 Hz, 2 H), 3.92 (t, J = 6.2 Hz,
2 H), 3.10 (q, J = 6.5 Hz, 2 H), 3.01 (q, J = 6.2 Hz, 2 H), 1.81
(quintet, J = 6.4 Hz, 2 H), 1.74 (quintet, J = 6.4 Hz, 2 H), 1.34
(s, 18 H); 13C NMR (125 MHz, CD3SOCD3, 298 K) δ 166.8,
155.5, 153.3, 151.7, 148.8, 143.7, 140.7, 138.7, 137.6, 134.3,
128.3, 127.6, 127.0, 125.0, 123.5, 120.9, 120.1, 119.1, 117.4,
116.7, 115.0, 77.38, 77.34, 67.2, 66.2, 65.5, 46.5, 36.9, 36.6,
29.2, 29.1, 28.1; HRMS (ESIMS) m/z for C44H51N3O10 [M þ
Na]þ calcd 804.3472, found 804.3470.
(1.85 g, 2.06 mmol), AcOH (90 mL), and H2O (10 mL) and
was stirred at 25 °C for 18 h. The suspension was then diluted
with CH2Cl2 (100 mL), stirred for an additional 3 h, and washed
with 0.2 N HCl (100 mL), H2O (100 mL), and saturated aqueous
sodium chloride (100 mL). The organic layer was dried over
MgSO4 and concentrated by rotary evaporation to give a yellow
oil. The oil was dissolved in ether (ca. 20 mL), and the resultant
solution was added in drops to hexanes (ca. 80 mL) over ca.
2 min. The resultant white suspension was filtered to provide
Fmoc-o-Abc2K(Boc)-OH (1b) as a white solid (1.54 g, 96%): mp
98-100 °C. An analytical sample of Fmoc-o-Abc2K(Boc)-OH
(1b) was prepared by purification of 50 mg by RP-HPLC
(water-CH3CN buffers with 0.1% TFA), concentrating, dis-
solving the resultant oil in CH2Cl2, and passing the solution
through a plug of silica gel (eluted with ethyl acetate): mp
H-o-Abc2K(Boc)-OCH2COPh (7b). Diether 66 (2.31 g, 3.47
mmol) was dissolved in THF (80 mL) by heating to ca. 60 °C
in an oil bath. To the warmed solution were added 2-amino-
phenylboronic acid (0.874 g, 3.99 mmol), K2CO3 (2.40 g,
17.4 mmol), water (20 mL), and PdCl2(dppf) CH2Cl2 (0.142 g,
3
0.174 mmol), and the mixture was stirred at 60 °C for 24 h under
an atmosphere of nitrogen. The solution was then concentrated
by rotary evaporation to an oily slurry and was partitioned
between water (150 mL) and CH2Cl2 (75 mL). The aqueous
layer was extracted with CH2Cl2 (2 ꢀ 75 mL), and the combined
organic layers were washed with saturated aqueous sodium
chloride (ca. 100 mL) and dried (MgSO4). The suspension was
then filtered and concentrated by rotary evaporation to afford a
brown oil. The oil was purified using column chromatography
(silica gel, ethyl acetate-hexanes, 2/1, v/v) to provide H2N-o-
Abc2K(Boc)-OCH2COPh (7a) as an off-white solid (1.61 g, 69%):
mp 54-56 °C; IR (KBr) 3367, 1701 cm-1; 1H NMR, (500 MHz,
CD3SOCD3, 298 K) δ 8.03 (d, J = 7.0 Hz, 2 H), 7.72 (t, J = 7.5
Hz, 1 H), 7.59 (t, J =7.7 Hz, 2H), 7.46(s, 1 H), 7.06 (t, J= 7.0 Hz,
1 H), 6.98 (J = 7.5 Hz, 1 H), 6.95 (s, 1 H), 6.82 (t, J = 5.5 Hz, 2 H),
6.61 (t, J = 7.3 Hz, 1 H), 5.72 (s, 2 H), 4.72 (br s, 2 H), 4.02 (t, J =
6.0 Hz, 2 H), 3.93 (t, J = 6.5 Hz, 2 H), 3.11 (q, J = 6.5 Hz, 2 H),
3.00 (q, J = 6.0 Hz, 2 H), 1.80 (quintet, J = 6.5 Hz, 2 H), 1.68
(quintet, J = 6.25 Hz, 2 H), 1.37 (s, 9 H), 1.32 (s, 9 H); 13C NMR
(125 M, Hz, CD3SOCD3, 298 K) δ 192.8, 164.5, 155.50, 155.48,
152.5, 149.2, 145.6, 134.6, 133.89, 133.85, 130.5, 128.8, 128.43,
127.7, 122.1, 118.1, 117.6, 115.9, 114.9, 114.8, 77.34, 77.29, 67.1,
66.8, 65.8, 37.0, 36.3, 29.3, 29.2, 28.13, 28.09; HRMS (ESIMS) m/z
for C37H48N3O9 [M þ H]þ calcd 678.3391, found 678.3391.
Fmoc-o-Abc2K(Boc)-OCH2COPh (8b). A solution of H-o-
Abc2K(Boc)-OCH2COPh (7b) (1.51 g, 2.23 mmol), pyridine
(0.220 mL, 2.68 mmol), and CH2Cl2 (75 mL) was cooled to
0 °C in an ice bath. Fmoc-Cl (1.51 g, 3.70 mmol) in CH2Cl2
(20 mL) was added in drops over 10 min. After 20 min, the
solution was allowed to warm to 25 °C and stirred for an
additional 30 min. The mixture was then washed with water
(150 mL), dried (MgSO4), and filtered, and the filtrate was
concentrated by rotary evaporation to afford a yellow oil.
Purification using column chromatography (silica gel, ethyl
acetate-hexanes, 2/1, 5% CH2Cl2, v/v) provided Fmoc-o-
Abc2K(Boc)-OCH2COPh (8b) as a white solid (2.0 g, 100%):
mp 68-70 °C; IR (KBr) 3373, 1732, 1695 cm-1; 1H NMR, (500
MHz, CD3SOCD3, 298 K) δ 8.51 (br s, 1 H), 8.05 (d, J = 8.1 Hz,
2 H), 7.88 (d, J = 7.9 Hz, 2 H), 7.72 (t, J = 7.7 Hz, 1 H),
7.65-7.55 (m, 4 H), 7.48 (s, 2 H), 7.44-7.34 (m, 3 H), 7.34-7.27
(m, 3 H), 7.24 (t, J = 7.7 Hz, 1 H), 6.96 (s, 1 H), 6.80 (t, J = 5.5
Hz, 1 H), 6.73 (t, J = 5.5 Hz, 1 H), 5.75 (s, 2 H), 4.30 (d, J = 7.0
Hz, 2 H), 4.22 (t, J = 7.0 Hz, 1 H), 4.01-3.92 (m, 2 H), 3.85
(t, J = 6.3 Hz, 2 H), 3.08 (q, J = 6.4 Hz, 2 H), 2.86 (q, J = 6.2
Hz, 2 H), 1.78 (quintet, J = 6.5 Hz, 2 H), 1.60 (quintet, J = 6.5
Hz, 2 H), 1.32 (s, 9 H), 1.31 (s, 9 H); 13C NMR (125 MHz,
CD3SOCD3, 298 K) δ 192.7, 164.5, 155.5, 155.4, 154.1, 152.3,
149.0, 143.6, 140.6, 135.5, 133.89, 133.87, 133.5, 131.6, 130.6,
128.8, 128.2, 127.8, 127.6, 127.0, 125.1, 124.6, 120.0, 118.7,
117.7, 114.9, 77.4, 77.3, 67.1, 66.9, 66.3, 65.9, 46.4, 37.1, 36.3,
29.2, 29.0, 28.1; HRMS (ESIMS) m/z for C52H57N3O11Na
[M þ Na]þ calcd 922.3891, found 922.3895.
1
98-100 °C; IR (KBr) 3500-3100, 1706 cm-1; H NMR (500
MHz, CD3SOCD3, 320 K) δ 12.50 (br s, 1 H), 8.11 (br s, 1H),
7.85 (d, J = 7.6 Hz, 2 H), 7.56 (d, J = 7.4 Hz, 2 H), 7.49 (br d,
J = 7.6 Hz, 1 H), 7.39 (t, J = 7.4 Hz, 2 H), 7.33 (td, J = 7.7, 1.2
Hz, 1 H), 7.31-7.23 (m, 4 H), 7.20 (td, J = 7.3, 0.9 Hz, 1 H), 6.84
(s, 1 H), 6.77 (br s, 1 H), 6.55 (br s, 1 H), 4.29 (d, J = 7.1 Hz, 2 H),
4.21 (t, J = 7.0 Hz, 1 H), 3.95 (t, J = 6.1 Hz, 2 H), 3.84 (t, J = 6.4
Hz, 2 H), 3.09 (q, J = 6.4 Hz, 2 H), 2.86 (q, J = 6.3 Hz, 2 H), 1.77
(quintet, J = 6.4 Hz, 2 H), 1.61 (quintet, J = 6.6 Hz, 2 H), 1.34
(s, 9 H), 1.32 (s, 9 H); 13C NMR (125 MHz, CD3SOCD3, 298 K)
δ 166.9, 155.5, 155.4, 154.0, 151.5, 149.0, 143.6, 140.6, 135.4,
132.2, 131.8, 130.6, 128.0, 127.5, 126.9, 125.1, 124.9, 124.6,
121.2, 120.0, 117.6, 114.7, 67.1, 66.3, 65.9, 46.4, 37.0, 36.4,
29.1, 29.0, 28.11, 28.08; HRMS (ESIMS) m/z for C44H51N3O10-
Na [M þ Na]þ calcd 804.3472, found 804.3463.
Representative Procedure for Macrocycle Synthesis: Synthesis
of Cyclohexamer Triangle 9b. A Bio-Rad Poly-Prep column was
charged with 2-chlorotrityl resin (73 mg, nominally 1.4 mmol/g,
0.10 mmol, Novabiochem). The resin was derivatized by
gently agitating with a solution of Fmoc-o-Abc2K(Boc)-OH (1b)
(102 mg, 0.13 mmol) in 20% 2,4,6-collidine-CH2Cl2 (ca. 1 mL)
for ca. 12 h. The solution was then drained using nitrogen
pressure, and the resin was washed with CH2Cl2 (5 ꢀ ca.
5 mL, 1 min each). After the loading step, unreacted sites
on the resin were capped using a solution of CH2Cl2-
MeOH-DIPEA (17/2/1) for ca. 30 min. (Measurement of the
weight gain in related experiments demonstrated the efficiency
of the resin loading step to be ca. 50%.) The column was then
drained, and the resin was washed with DMF (6 ꢀ ca. 5 mL,
1 min each) and then CH2Cl2 (6 ꢀ ca. 5 mL, 1 min each). The
Fmoc group was removed by adding a solution of 20% piper-
idine-DMF (1 ꢀ ca. 5 mL for 1 min, 1 ꢀ ca. 5 mL for 20 min) to
the resin followed by gentle agitation of the resultant suspen-
sion. The piperidine solution was drained, and the resin was
washed with DMF (6 ꢀ ca. 5 mL, 1 min each) followed by
CH2Cl2 (6 ꢀ ca. 5 mL, 1 min each).
Elongation of the protected linear Abc2K oligomer was
accomplished by preactivating the appropriate Fmoc-protected
Abc2K building block (Abc2K or o-Abc2K for 9b; 78 mg,
0.10 mmol) with HCTU (0.10 mmol, 41 mg) in a ca. 1 mL solu-
tion of 20% 2,4,6-collidine-DMF. (These amounts correspond
to ca. 2 equiv, based on 50% loading of the resin.) The coupling
solution was then added to the resin and gently agitated for ca.
12 h. To determine if Abc2K couplings were complete, a small
amount of resin was treated in a new Bio-Rad column with a
solution of CF3COOH/water (9/1, v/v) and the column was
vigorously agitated for ca. 2 h. The solution was then drained,
concentrated by rotary evaporation, and the residue was dis-
solved in a mixture of water and CH3CN and then injected
into an analytical RP-HPLC instrument to determine if any
unreacted amine was present. In all cases, the amount of
uncoupled amine was not significant enough to warrant a
second coupling. The coupling procedure was repeated until
Fmoc-o-Abc2K(Boc)-OH (1b). Zn dust (3.2 g, 49 mmol) was
added to a solution of Fmoc-o-Abc2K(Boc)-OCH2COPh (8b)
J. Org. Chem. Vol. 75, No. 6, 2010 1829

Gothard and Nowick
JOCArticle
the desired length of the linear oligomer was obtained [H-(o-
Abc2K(Boc)-Abc2K(Boc))3-OH].
It should be noted that the percentage yields based on resin
loading are significantly higher than the above percentages
would indicate, because the actual resin loading was found to
be roughly half (0.7-1.0 mmol/g) of the nominal resin loading
(1.4 mmol/g). Treatment of 36 mg of 2-chlorotrityl resin
(nominally 1.4 mmol/g, 0.050 mmol, Novabiochem,) with
Fmoc-o-Abc2K(Boc)-OH (1b) (51 mg, 0.065 mmol) in 20%
2,4,6-collidine-CH2Cl2 resulted in a weight gain of 19 mg, thus
indicating 0.7 mmol/g actual loading. Treatment of 36 mg of
2-chlorotrityl resin (nominally 1.4 mmol/g, 0.050 mmol,
Novabiochem,) with Fmoc-m-Abc2K(Boc)-OH (1a) (51 mg,
0.065 mmol) in 20% 2,4,6-collidine-CH2Cl2 resulted in a
weight gain of 27 mg, thus indicating 1.0 mmol/g actual loading.
A control study in which 36 mg of 2-chlorotrityl resin
(nominally 1.4 mmol/g, 0.050 mmol, Novabiochem) was
treated with MeOH in 20% 2,4,6-collidine-CH2Cl2 resulted
in no weight gain, thereby validating the use of gravimetric
analysis to assess resin loading. Gravimetric analysis after
elongation to the full-length linear peptides indicated a final
resin loading of 0.3-0.6 mmol/g, suggesting that some loss of
peptide from the labile 2-chlorotrityl resin occurs during the
synthesis.
The protected linear Abc2K(Boc) oligomer was cleaved from
the resin using a solution of AcOH-TFE-CH2Cl2 (1/1/4) for
ca. 12 h. The peptide solution was drained from the resin and
concentrated to form a gel. From the gel, remaining acetic acid
was azeotropically removed by rotary evaporating with hexanes
(3ꢀ) and CH2Cl2 (3ꢀ) to provide a white a solid. (This step is
particularly important to avoid acetylation of the aniline group
by trace amounts of acetic acid during cyclization.) The pro-
tected linear oligomer was split into two equal portions, and the
cyclization step was carried out using half the material. (The
remaining half of the material was reserved for studies of the
uncyclized linear peptide.)
Cyclization was accomplished by charging a 250-mL round
bottomed flask with H-(o-Abc2K(Boc)-Abc2K(Boc))3-OH (half of
the material from above step) and 100 mL of 2% collidine-
CH2Cl2. HCTU (0.1 mmol, dissolved in 1 mL of DMF) was
added in drops over 2 min, and the mixture was stirred for
ca. 24 h. The reaction solution was then washed with ca. 100 mL
of 0.2 N HCl (aq) to remove excess base, and the organic layer
was concentrated to an oil. Global deprotection of the Boc
groups was carried out in a small round-bottomed flask
equipped with a magnetic stirring bar and mixed with
10-20 mL of CF3COOH/CH2Cl2 (1/1, v/v) for ca. 4 h. The
solution was then concentrated by rotary evaporation, and the
resultant oil was purified by preparative RP-HPLC (water-
CH3CN with 0.1% TFA). Pure fractions of the product (ca.
98% purity) were concentrated by rotary evaporation to remove
most of the CH3CN, frozen, and then lyophilized to afford
a white powder (16 mg, 0.005 mmol, 9% based on the nominal
1.4 mmol/g loading of the resin and use of half of the linear
oligomer in the cyclization step). The remaining less pure HPLC
fractions (<98% purity) were also lyophilized (10 mg, 0.003
mmol, 6%) and stored for future use. Macrocycles 9a,c, 10a,b,
and 11a-d were prepared in a similar fashion.14
Acknowledgment. We thank the ACS-PRF (38986-AC1)
and NSF (CHE-0750523) for grant support. C.M.G. thanks
the UCI Institute for Brain Aging and Dementia for
training grant support (NIA-5T32AG00096). We also thank
Christoph Haase for donating his photograph depicting the
construction near Siam Square.
Supporting Information Available: NMR, ESI-MS, HPLC
data, and additional details of molecular modeling studies. This
material is available free of charge via the Internet at http://
pubs.acs.org/.
(14) Using similar procedures, yields of 4-16% of pure (ca. 98% purity)
product were recorded for 9a (11%), 9c (13%), 10a (16%), 10b (10%), 11a
(5%), 11c (4%), and 11d (1%) based on the nominal 1.4 mmol/g loading of
the resin. Only 11d proved difficult to prepare, requiring three separate
attempts on the synthesis and multiple HPLC purifications and giving a low
yield of product.
(15) We had previously6 generated intermediate 3 by way of a conven-
tional Grignard reaction, in which the Grignard reagent was formed by
treatment of 1,4-dibromo-2,5-dimethoxybenzene with magnesium. Generat-
ing the Grignard reagent through halogen-metal exchange8 of 1,4-dibromo-
2,5-dimethoxybenzene with isopropylmagnesium chloride doubles the yield
of 3.
1830 J. Org. Chem. Vol. 75, No. 6, 2010