Palladium-catalyzed borylation of ortho-substituted phenyl halides and application to the one-pot synthesis of 2,2′-disubstituted biphenyls

Full text of DOI:10.1021/jo005663d

9268
J . Org. Chem. 2000, 65, 9268-9271
Sch em e 1
P a lla d iu m -Ca ta lyzed Bor yla tion of
Or th o-Su bstitu ted P h en yl Ha lid es a n d
Ap p lica tion to th e On e-P ot Syn th esis of
2,2′-Disu bstitu ted Bip h en yls
Olivier Baudoin,* Daniel Gue´nard, and
Franc¸oise Gue´ritte
Institut de Chimie des Substances Naturelles, CNRS,
91198 Gif-sur-Yvette, France
baudoin@icsn.cnrs-gif.fr
Received September 29, 2000
Ortho-disubstituted biaryls are key structural elements
of numerous pharmacologically active natural or syn-
thetic products, for instance, the antibiotic vancomycin,1
the antimitotics colchicine and steganacin,2 the CDK
inhibitors paullones,3 and the antihypertensive losartan.4
The Suzuki-Miyaura coupling is a well-established and
powerful tool for the construction of the biaryl bond of
such molecules due to both its efficiency and the innocu-
ous character of the boron derivatives produced.5 Nev-
ertheless improvements of the existing methods are still
necessary especially when sterically hindered or elec-
tronically deactivated coupling partners are involved.
In the course of our research program directed toward
the total synthesis of the antimitotic (-)-rhazinilam 16,7
and biaryl analogues such as 28 (Scheme 1), we envi-
sioned to build the ortho-disubstituted biaryl framework
(precursor 3) via a Suzuki coupling. We expected that
the reaction conditions used in this key step would be
versatile enough to allow a rapid and efficient construc-
tion of a library of analogues for biological screening
purposes.
Sch em e 2
the more recently developed PdCl2(dppf)-catalyzed bory-
lation of aryl halides or triflates 4 with a tetra(alkoxy)-
diboron10 or with a dialkoxyborane such as pinacolborane
7 (Scheme 2).11 We turned our attention toward the last
method due to the better availability of pinacolborane and
its apparent greater reactivity compared to the diboron
species.11
The pinacolboronates 5 formed in this process are
attractive synthons since they are air-, moisture-, tem-
perature-, and generally chromatography-stable. How-
ever there are rather few examples of the Pd-catalyzed
borylation of ortho-substituted aryl halides, especially of
bromides that are yet more readily available, compared
to their para and meta counterparts.10,11 To synthesize
ortho-substituted phenylboronates 5, leading potentially
to rhazinilam analogues 2, we began with the study of
the borylation of 2-bromoaniline 4a (Table 1).
Under catalysis with PdCl2(dppf), we were able to
obtain the corresponding boronate 5a in 48% yield (entry
1) after 4 h at 100 °C. With other ortho-substituted
phenyl bromides, the boronate was obtained in very low
to moderate yields (vide infra). We ascribed these results
mostly to the steric hindrance created by the ortho
substituent. We then turned our attention toward the
very interesting results obtained by the Buchwald and
the Fu groups concerning the Suzuki couplings of steri-
cally hindered substrates under mild conditions, using
sterically hindered phosphine ligands, respectively, 2-(di-
alkylphosphino)biphenyls such as 8 (Table 1)12 and
The ortho-substituted arylboronic acid or arylboronate
component of the coupling can be synthesized by two
principal methods, namely: (1) the transmetalation be-
tween an arylmetal and a boron halide or alkoxide,9 (2)
(1) (a) Williams, D. H.; Bardsley, B. Angew. Chem., Int. Ed. 1999,
38, 1172. (b) Nicolaou, K. C.; Boddy, C. N. C.; Bra¨se, S.; Winssinger,
N. Angew. Chem., Int. Ed. 1999, 38, 2096.
(2) (a) Hamel, E. Med. Res. Rev. 1996, 16, 207. (b) Shi, Q.; Chen,
K.; Morris-Natschke, S. L.; Lee, K.-H. Curr. Pharm. Des. 1998, 4, 219
and refs therein.
(3) Gussio, R.; Zaharevitz, D. W.; McGrath, C. F.; Pattabiraman,
N.; Kellogg, G. E.; Schultz, C.; Link, A.; Kunick, C.; Leost, M.; Meijer,
L.; Sausville, E. A. Anti-Cancer Drug Des. 2000, 15, 53 and refs therein.
(4) (a) Goa, K. L.; Wagstaff, A. J . Drugs 1996, 51, 820. (b) Birken-
hager, W. H.; de Leeuw, P. W. J . Hypertens. 1999, 17, 873.
(5) (a) Miyaura, N.; Suzuki, A. Chem. Rev. 1995, 95, 2457. (b)
Stanforth, S. P. Tetrahedron 1998, 54, 263. (c) Suzuki, A. J . Organomet.
Chem. 1999, 576, 147.
(6) David, B.; Se´venet, T.; Morgat, M.; Gue´nard, D.; Moisand, A.;
Tollon, Y.; Thoison, O.; Wright, M. Cell Motil. Cytoskeleton 1994, 28,
317.
(7) For total syntheses, see: (a) Ratcliffe, A. H.; Smith, G. F.; Smith,
G. N. Tetrahedron Lett. 1973, 52, 5179. (b) J ohnson, J . A.; Sames, D.
J . Am. Chem. Soc. 2000, 122, 6321.
(8) (a) Pascal, C.; Dubois, J .; Gue´nard, D.; Gue´ritte, F. J . Org. Chem.
1998, 63, 6414. (b) Pascal, C.; Dubois, J .; Gue´nard, D.; Tchertanov,
L.; Thoret, S.; Gue´ritte, F. Tetrahedron 1998, 54, 14737. (c) Dupont,
C.; Gue´nard, D.; Thal, C.; Thoret, S.; Gue´ritte, F. Tetrahedron Lett.
2000, 41, 5853 and refs therein.
(9) (a) Vaultier, M.; Carboni, B. In Comprehensive Organometallic
Chemistry II; Abel, E. W., Stone, F. G. A., Wilkinson, G., Eds.;
Pergamon: Oxford, 1995; Vol. 11, p 191. (b) Miyaura, N.; Maruoka,
K. In Synthesis of Organometallic Compounds; Komiya, S., Ed.;
Wiley: New York, 1997.
(10) (a) Ishiyama, T.; Murata, M.; Miyaura, N. J . Org. Chem. 1995,
60, 7508. (b) Ishiyama, T.; Itoh, Y.; Kitano, T.; Miyaura, N. Tetrahedron
Lett. 1997, 38, 3447.
(11) (a) Murata, M.; Watanabe, S.; Masuda, Y. J . Org. Chem. 1997,
62, 6458. (b) Murata, M.; Oyama, T.; Watanabe, S.; Masuda, Y. J . Org.
Chem. 2000, 65, 164.
(12) (a) Wolfe, J . P.; Buchwald, S. L. Angew. Chem., Int. Ed. 1999,
38, 2413. (b) Wolfe, J . P.; Singer, R. A.; Yang, B. H.; Buchwald, S. L.
J . Am. Chem. Soc. 1999, 121, 9550.
10.1021/jo005663d CCC: $19.00 © 2000 American Chemical Society
Published on Web 12/05/2000

Notes
J . Org. Chem., Vol. 65, No. 26, 2000 9269
Ta ble 1. Bor yla tion of 2-Br om oa n ilin e 4a a
Ta ble 2. Syn th esis of Or th o-Su bstitu ted
P h en ylbor on a tes 5 fr om Ha lid es 4 (a ccor d in g to
Sch em e 2)a
entry
Pd catalyst
5% PdCl2(dppf)
5% Pd(OAc)2/10% 8
5% Pd(OAc)2/20% 8
5% Pd(OAc)2/20% P(t-Bu)3
2.5% Pd2dba3/6% P(t-Bu)3
5% Pd(PPh3)4
time (h) temp (°C) 5a (%)b
1
2
3
4
5
6
7
8
4
1
1
100
80
80
48
49
81
0
1
80
2
80
0
14
14
14
100
100
100
0
0
0
5% PdBnCl(PPh3)2
2.5% Pd2dba3
a
Conditions: Et3N (4.0 equiv), 7 (3.0 equiv), dioxane. bIsolated
yields after flash chromatography (aniline was the only isolated
byproduct).
trialkylphosphines such as P(t-Bu)3.13 By analogy, we
thought these ligands might improve the borylation of
ortho-substituted aryl halides. As shown in Table 1, the
use of biphenylphosphine ligand 8 improved significantly
the borylation process (entries 2, 3). With a ratio of four
ligands to the catalyst (entry 3), a high isolated yield of
81% was obtained, with reduced time and temperature
compared to the original conditions (entry 1).14 With P(t-
Bu)3 in different ratios to the palladium catalyst (entries
4, 5), no boronate was observed, the major isolated
compound being the dehalogenated product, i.e., aniline.
The use of other Pd catalysts (entries 6-8) failed to give
the borylation product, variable mixtures of starting
material and aniline being recovered after a prolonged
period of heating.
To investigate the scope of this method, other ortho-
subsituted phenyl halides were subjected to the same
reaction conditions (Table 2). Whereas the borylation of
2-chloroaniline was inefficient (entry 1), reasonable yields
could be obtained from 2-bromoanisole (entry 2) and
2-bromophenylacetonitrile (entry 3) using Pd(OAc)2/8. In
the latter example, employing PdCl2(dppf) gave, like in
the 2-bromoaniline case (Table 1), a much lower yield
despite harsher conditions (entry 4). With bromides
bearing electron-withdrawing groups, such as 2-bromoni-
trobenzene (entry 7) and 2′-bromoacetophenone (entry 9),
little or no pinacolboronate was obtained, in accordance
with the literature observations.11 The borylation of
sterically hindered bromides using Pd(OAc)2/8 (entries
10, 13) furnished the pinacolboronate in moderate yields.
Yet the borylation using this catalytic system was again
much more efficient and milder than with PdCl2(dppf)
(entry 14). It should be noted that changing the solvent
from dioxane to toluene or DME in the borylation of Boc-
protected 2-bromoaniline (entry 10) gave similar or lower
yields of pinacolboronate15 (43% 5 + 43% 6 for toluene,
31% 5 + 57% 6 for DME), as reported for PdCl2(dppf)-
mediated borylations.10,11 Similarly, among the bases
tested which do not promote undesired Suzuki coupling
a
Conditions: see note a in Table 1. bA ) 5 mol % Pd(OAc)2/20
mol % 8, B ) 5 mol % PdCl2(dppf). cIsolated yields after flash
chromatography. dCompound described in ref 10b. eComplex
f
mixtures were obtained. Compound described in ref 15. Boc )
tert-butyloxycarbonyl, TES ) triethylsilyl.
of the produced boronate 5 with the starting halide 4
during the borylation process (KOAc and tertiary
amines),10,11 Et3N seemed the best candidate. Compared
to bromides, the borylation of iodides with Pd(OAc)2/8
was higher yielding (entries 5, 8, 11), with identically
mild reaction conditions. The difference in the borylation
of these iodides using Pd(OAc)2/8 (entries 5, 11) or PdCl2-
(dppf) (entries 6, 12) seemed smaller than in the case of
bromides, yet better yields under milder conditions were
again obtained with the former catalyst. The negative
effect of strong electron-withdrawing substituents on the
borylation was again observed with 2-iodonitrobenzene
(entry 8), as expected from earlier works.11 In the case
of sterically hindered 2-iodobenzenes (entries 15-17, both
compounds being advanced intermediates in the synthe-
sis of rhazinilam analogues),8a-b the borylation with Pd-
(OAc)2/8 was very high-yielding whereas PdCl2(dppf) was
rather inefficient (entry 16). Thus, increasing the steric
hindrance in the ortho position to the halogen atom
(entries 5 f 15 f 17) did not substantially affect the
efficiency of the borylation with Pd(OAc)2/8, similarly to
what was observed in the case of Suzuki couplings
mediated by this catalyst.12 In conclusion, the use of
ligand 8 in the borylation of ortho-substituted phenyl
bromides and sterically hindered phenyl iodides afforded
higher yields of pinacolboronate in milder conditions than
with PdCl2(dppf).
(13) (a) Littke, A. F.; Fu, G. Angew. Chem., Int. Ed. 1998, 37, 3387.
(b) Littke, A. F.; Dai, C.; Fu, G. J . Am. Chem. Soc. 2000, 122, 4020.
(14) Using excess 7 (3 equiv) was necessary, the reagent being partly
consumed by the deprotonation of the amino group.
(15) This compound was previously described: Lamba, J . J . S.; Tour,
J . M. J . Am. Chem. Soc. 1994, 116, 11723.
With these results in hand, we thought that this
methodology could be extended to the synthesis of 2,2′-
disubstituted biphenyls via a one-pot Suzuki cross-
coupling reaction. This was indeed achieved by adding,

9270 J . Org. Chem., Vol. 65, No. 26, 2000
Notes
Ta ble 3. On e-P ot Syn th esis of Bip h en yls 3a
Exp er im en ta l Section
All compounds were commercially available or synthesized
according to the literature procedures.8b,15 Dioxane was distilled
from Na/benzophenone and triethylamine from KOH prior to
use.
Gen er a l P r oced u r e for th e Syn th esis of P h en ylbor -
on a tes 5 (Ta bles 1, 2). To a solution of phenyl halide (0.41
mmol) in 1 mL of dioxane were added, under argon, triethyl-
amine (229 µL, 1.65 mmol), palladium(II) acetate (4.6 mg, 0.021
mmol), 2-(dicyclohexylphosphino)biphenyl 8 (28.8 mg, 0.08
mmol), and pinacolborane (179 µL, 1.23 mmol) dropwise. The
reaction mixture was heated at 80 °C for 30 min. After cooling
to room temperature, the reaction was quenched by adding a
sat. solution of NH4Cl, and the aqueous phase was extracted
with ether. After drying over MgSO4, the solution was filtered
and evaporated under vacuum. The resulting oil was purified
by flash chromatography (SiO2 deactivated with 5% Et3N).
P in a col (2-a m in op h en yl)bor on a te: 1H NMR (250 MHz,
CDCl3) δ ) 7.63 (dd, J ) 7.5, 1.6 Hz, 1H), 7.23 (td, J ) 8.0, 1.8
Hz, 1H), 6.69 (td, J ) 7.3, 1.5 Hz, 1H), 6.61 (d, J ) 8.2 Hz, 1H),
4.75 (br s, 2H), 1.36 (s, 12H) ppm; 13C NMR (62.5 MHz, CDCl3)
δ ) 153.6, 136.8, 132.7, 129.2, 116.8, 114.8, 83.5, 24.9 ppm; FTIR
(film) ν ) 3487, 3384, 2977, 1606, 1354 cm-1 ; HRMS (IE) calcd
for C12H18BNO2 [M+•]: 219.1431; found: 219.1437.
entry
iodide
base
time (h)
product
yield (%)
1
2
3
4
9a
9a
9b
9b
K3PO4
Ba(OH)2
K3PO4
12
1
12
1
3a
3a
3b
3b
65
73
40
66
Ba(OH)2
a
Conditions: (1) 4a , Et3N (4.0 equiv), 7 (3.0 equiv), Pd(OAc)2
(5 mol %), 8 (20 mol %), dioxane, 80 °C, 1 h; (2) iodide 9 (1.0 equiv),
base (3.0 equiv), H2O, 100 °C.
after completion of the borylation step, the needed
ingredients in the same reaction vessel, i.e., a second aryl
halide and a suitable base (Table 3).
Thus, the borylation of 2-bromoaniline 4a in the above
conditions (80 °C, 1 h) was followed by addition of H2O
(which both hydrolyzes excess 7 and favors the coupling),
1 equiv of 2-iodophenylacetonitrile 9a , excess K3PO4 (3
equiv), and heating overnight to 100 °C (entry 1). Under
these conditions, the cross-coupling product 3a could be
obtained in 65% isolated yield (together with dehaloge-
nated products). Adding fresh catalyst for the coupling
step did not improve the yield of the reaction, on the
contrary to previously reported examples.16 With diethyl-
substituted phenylacetonitrile 9b (entry 3), the coupling
product 3b was obtained in 40% yield under the same
conditions, showing the negative effect of the steric
hindrance on the Suzuki coupling step. Significant
improvements were accomplished by using the stronger
base Ba(OH)2 instead of K3PO4, the former being known
to favor the cross-coupling of sterically hindered sub-
strates.5 Thus, 3a and 3b were, respectively, obtained
in 73% and 66% yields from 4a after a very short heating
time of 1 h (entries 2, 4).17 Reversing the order of
reagents, i.e., starting with the borylation of 9b (Table
2, entry 15) and performing the Suzuki coupling with 4a
failed to give any coupled material 3b, 4a and borylated
9b being recovered after 24 h at 100 °C.18 Next, 3a and
3b can be further elaborated to give rhazinilam biphenyl
analogues 2 (Scheme 1).8
P in a col (2-m eth oxyp h en yl)bor on a te: 1H NMR (300 MHz,
CDCl3) δ ) 7.68 (dd, J ) 7.8, 1.8 Hz, H), 7.39 (td, J ) 8.0, 1.8
Hz, 1H), 6.93 (td, J ) 7.4, 1.4 Hz, 1H), 6.85 (d, J ) 8.7 Hz, 1H),
3.82 (s, 3H), 1.35 (s, 12H) ppm; 13C NMR (75 MHz, CDCl3) δ )
164.1, 136.6, 132.4, 120.1, 110.4, 83.3, 55.7, 24.7 ppm; FTIR
(film) ν ) 2977, 1600, 1354 cm-1 ; HRMS (IE) calcd for C13H19
BO3 [M+•]: 234.1427; found: 234.1439.
-
P in a col (2-cya n om eth ylp h en yl)bor on a te: 1H NMR (250
MHz, CDCl3) δ ) 7.88 (d, J ) 6.8 Hz, 1H), 7.46 (m, 2H), 7.33
(m, 1H), 4.11 (s, 2H), 1.37 (s, 12H) ppm; 13C NMR (62.5 MHz,
CDCl3) δ ) 136.8, 136.6, 131.8, 128.5, 127.2, 118.9, 84.1, 24.9,
23.5 ppm; FTIR (film) ν ) 2980, 2248, 1602, 1350 cm-1 ; HRMS
(IE) calcd for C14H18BNO2 [M+•]: 243.1431; found: 243.1445.
P in a col (2-n itr op h en yl)bor on a te: 1H NMR (300 MHz,
CDCl3) δ ) 8.16 (d, J ) 8.1 Hz, 1H), 7.66 (t, J ) 8.4 Hz, 1H),
7.55 (m, 2H), 1.43 (s, 12H) ppm; 13C NMR (75 MHz, CDCl3) δ )
133.7, 132.8, 130.0, 122.9, 84.6, 24.7 ppm; FTIR (film) ν ) 2979,
1526, 1352 cm-1 ; HRMS (FAB) calcd for C12H16BNO4 [(M + H)+]:
250.1251; found: 250.1258.
P in a col [N-(ter t-bu toxyca r bon yl)-2-a m in op h en yl]bor -
on a te: identical spectroscopic data to those previously de-
scribed.15
P in a col (2,6-d im eth ylp h en yl)bor on a te: 1H NMR (250
MHz, CDCl3) δ ) 7.13 (t, J ) 7.6 Hz, 1H), 6.95 (d, J ) 7.5 Hz,
2H), 2.40 (s, 6H), 1.39 (s, 12H) ppm; 13C NMR (75 MHz, CDCl3)
δ ) 141.7, 129.1, 126.4, 83.6, 24.9, 22.2 ppm; FTIR (film) ν )
-1
2978, 1597, 1333 cm
; HRMS (IE) calcd for C14H21BO2 [M+•]:
232.1635; found: 232.1648.
P in a col [2-(1-cya n o-1-eth ylp r op -1-yl)p h en yl]bor on a te:
1H NMR (300 MHz, CDCl3) δ ) 7.65 (dd, J ) 7.5, 1.5 Hz, 1H),
7.60 (d, J ) 7.5 Hz, 1H), 7.39 (td, J ) 7.7, 1.5 Hz, 1H), 7.27 (td,
J ) 7.5, 1.2 Hz, 1H), 2.31 (dq, J ) 14.7, 7.5 Hz, 2H), 2.06 (dq, J
) 14.7, 7.8 Hz, 2H), 1.37 (s, 12H), 0.92 (t, J ) 7.8 Hz, 6H) ppm;
13C NMR (75 MHz, CDCl3) δ ) 141.9, 135.8, 129.8, 128.3, 126.5,
123.5, 84.2, 51.6, 32.5, 24.8, 9.7 ppm; FTIR (film) ν ) 2976, 2231,
1596, 1342 cm-1 ; HRMS (IE) calcd for C18H26BNO2 [M+•]:
299.2057; found: 299.2084.
In conclusion, we reported the borylation of ortho-
substituted phenyl halides, in particular bromides and
sterically hindered iodides, using the previously reported
phosphine ligand 8. These findings were extended to one-
pot Suzuki-Miyaura reactions with ortho-substituted
phenyl iodides, yielding sterically hindered 2,2′-biphenyls
in an unprecedented simple, rapid, and efficient manner.
Further developments of the methodology will include the
solution and solid-phase synthesis of analogues of the
antimitotic rhazinilam 1, as well as other biologically
active compounds having ortho-disubstituted biaryl frame-
works.
P in a col [2-(1-t r iet h ylsila n yloxym et h yl-1-et h ylp r op -1-
1
yl)p h en yl]bor on a te: H NMR (250 MHz, CDCl3) δ ) 7.43 (d,
J ) 6.8 Hz, 1H), 7.26 (m, 1H), 7.13 (m; 2H), 3.86 (s, 2H), 1.84
(dq, J ) 7.0, 2.8 Hz, 4H), 1.37 (s, 12H), 0.93 (t, J ) 7.9 Hz, 9H),
0.65 (t, J ) 6.8 Hz, 6H), 0.55 (q, J ) 7.8 Hz, 6H) ppm; 13C NMR
(75 MHz, CDCl3) δ ) 148.8, 133.7, 128.5, 127.3, 124.5, 83.7, 64.9,
47.3, 28.1, 24.7, 8.3, 6.8, 4.4 ppm; FTIR (film) ν ) 2959, 1594,
1339 cm-1 ; HRMS (FAB) calcd for C24H42BO3Si [(M - H)+]
(nonclassical fragmentation): 417.2996; found: 417.2974; calcd
for C22H38BO3Si [(M - Et)+]: 389.2683; found: 389.2680.
Gen er a l P r oced u r e for th e Syn th esis of Bip h en yls 3
(Ta ble 3). To a solution of 2-bromoaniline 4a (80 mg, 0.47 mmol)
in dioxane (1 mL) were added, under argon, triethylamine (259
µL, 1.86 mmol), palladium(II) acetate (5.2 mg, 0.023 mmol),
2-(dicyclohexylphosphino)biphenyl (33 mg, 0.093 mmol), and
(16) One pot syntheses of biaryls having meta and para substituents
were reported in the case of diboron-mediated borylations: see ref 10b
and Giroux, A.; Han, Y.; Prasit, P. Tetrahedron Lett. 1997, 38, 3841.
(17) Changing the solvent to toluene lead to decreased yields of
coupled materials 3a ,b (data not shown).
(18) Further experiments are underway to propose a rationale
accounting for this behavior.

Notes
J . Org. Chem., Vol. 65, No. 26, 2000 9271
pinacolborane (202 µL, 1.40 mmol) dropwise. The mixture was
stirred at 80 °C for 1 h and then cooled to room temperature,
and water (200 µL), barium hydroxide octahydrate (440 mg, 1.40
mmol), and iodide 9a or 9b (0.47 mmol) dissolved in 0.1 mL of
dioxane were successively added. The mixture was heated to 100
°C under stirring for 1 h and then cooled to room temperature
and filtered through Celite. Brine was added, and the aqueous
phase was extracted with CH2Cl2. After drying over MgSO4, the
solvents were removed under vacuum, and the residue was
purified by flash chromatography (SiO2).
129.4, 129.2, 128.5, 124.8, 118.6, 118.2, 115.3, 21.3 ppm; FTIR
(film) ν ) 3460, 3368, 2248, 1616, 1482 cm-1 ; HRMS (FAB) calcd
for C13H10N2 [(M + H)+]: 209.1079; found: 209.1066.
2-(2′-Am in obip h en -2-yl)-2-eth ylbu tyr on itr ile (3b): identi-
cal spectroscopic data to those previously described.8b
Ack n ow led gm en t. We gratefully thank Dr. Claude
Thal for several helpful discussions.
2′-Am in obip h en -2-yla ceton itr ile (3a ): 1H NMR (250 MHz,
CDCl3) δ ) 7.59 (m, 1H), 7.41 (m, 2H), 7.27 (m, 1H), 7.19 (td,
7.8, 1.5 Hz, 1H), 6.99 (d, J ) 7.8, 1.5 Hz), 6.79 (m, 2H), 3.72 (d,
J ) 19.0 Hz, 1H), 3.52 (d, J ) 18.8 Hz, 1H), 3.45 (br s, 2H) ppm;
13C NMR (62.5 MHz, CDCl3) δ ) 143.4, 138.2, 130.6, 129.9,
Su p p or tin g In for m a tion Ava ila ble: Copies of 1H and 13C
NMR spectra for all new compounds. This material is available
free of charge via the Internet at http://pubs.acs.org.
J O005663D