Asymmetric Syn-selective Henry reaction catalyzed by the sulfonyldiamine-CuCl-pyridine system

Article abstract of DOI:10.1021/jo800412x

(Chemical Equation Presented) A catalytic asymmetric Henry reaction has been developed with use of a sulfonyldiamine-CuCl complex as a catalyst. A series of new binaphthyl-containing sulfonyldiamine ligands (2a-h) were readily synthesized in two steps starting from commercially available chiral 1,2-diamines. The (R,R)-diamine-(R)-binaphthyl ligand (2d)-CuCl complex smoothly catalyzed the enantioselective Henry reaction with the assistance of pyridine to give the corresponding adduct with high enantiomeric excess (up to 93%). Moreover, the 2d-CuCl-pyridine system promotes the diastereoselective Henry reaction in syn-selective manner to give the adduct in up to 99% yield with 92:8 synlanti selectivity. The enantiomeric excess of the syn-adduct was 84% ee.

Full text of DOI:10.1021/jo800412x

Asymmetric Syn-Selective Henry Reaction Catalyzed by the
Sulfonyldiamine-CuCl-Pyridine System
Takayoshi Arai,* Ryuta Takashita, Yoko Endo, Masahiko Watanabe, and Akira Yanagisawa
Department of Chemistry, Graduate School of Science, Chiba UniVersity, Inage, Chiba 263-8522, Japan
tarai@faculty.chiba-u.jp
ReceiVed February 20, 2008
A catalytic asymmetric Henry reaction has been developed with use of a sulfonyldiamine-CuCl complex
as a catalyst. A series of new binaphthyl-containing sulfonyldiamine ligands (2a-h) were readily
synthesized in two steps starting from commercially available chiral 1,2-diamines. The (R,R)-diamine-
(R)-binaphthyl ligand (2d)-CuCl complex smoothly catalyzed the enantioselective Henry reaction with
the assistance of pyridine to give the corresponding adduct with high enantiomeric excess (up to 93%).
Moreover, the 2d-CuCl-pyridine system promotes the diastereoselective Henry reaction in syn-selective
manner to give the adduct in up to 99% yield with 92:8 syn/anti selectivity. The enantiomeric excess of
the syn-adduct was 84% ee.
Introduction
such as Zn,4 Co,5 Cu,6 Mg,7 and Cr8) and organocatalysts9 have
been developed. Among these, a Cu-based asymmetric catalyst,
examined at room temperature, is particularly promising due
to its high catalytic activity and excellent enantioselectivity.
Following on from the ground-breaking work of Jørgensen and
Evans,6a,b we succeeded in developing a C2-symmetric diamine
(1)-CuCl-catalyzed Henry reaction.10 However, little research
has been carried out into Cu-catalyzed diastereoselective Henry
reactions.11 We report herein the development of a syn-selective
asymmetric Henry reaction using newly developed sulfonyl-
The Henry (nitroaldol) reaction is a powerful and atom-
economical carbon-carbon bond-forming reaction that can be
used to create a new stereogenic center at the ꢀ-position of a
nitro functionality.1 Because the resulting ꢀ-nitro alcohol adducts
can be transformed to various biologically important building
blocks such as ꢀ-amino alcohols and R-hydroxy ketones, recent
research has focused on the catalytic asymmetric version of the
Henry reaction.2 Since the original pioneering work on hetero-
bimetallic multifunctional catalysts containing lanthanoid ele-
ments,3 various types of chiral metal catalysts (containing metals
(3) (a) Sasai, H.; Suzuki, T.; Arai, S.; Arai, T.; Shibasaki, M. J. Am. Chem.
Soc. 1992, 114, 4418–4420. (b) Arai, T.; Yamada, Y. M. A.; Yamamoto, N.;
Sasai, H.; Shibasaki, M. Chem. Eur. J. 1996, 2, 1368–1372. And also see for a
lanthanum-consisting catalyst: (c) Tur, F.; Saa´, J. M. Org. Lett. 2007, 9, 5079–
5082. (d) Nitabaru, T.; Kumagai, N.; Shibasaki, M. Tetrahedron Lett. 2008, 49,
272–276.
(1) (a) Rosini, G. In ComprehensiVe Organic Synthesis; Trost, B. M., Fleming,
I., Eds.; Pergamon: Oxford, UK, 1999; Vol. 2, pp 321-340. (b) Pinnick, H. W.
In Organic Reactions; Paquette, L. A., Ed.; Wiley: New York, 1990; Vol. 38,
Chapter 3.
(2) (a) Shibasaki, M.; Gro¨ger, H. In ComprehensiVe Asymmetric Catalysis
I-III; Jacobsen, E. N., Pfaltz, A., Yamamoto, H., Eds.; Springer-Verlag: Berlin-
Heidelberg, Germany, 1999; Chapter 29.3. (b) Palomo, C.; Oiarbide, M.; Mielgo,
A. Angew. Chem., Int. Ed. 2004, 43, 5442–5444. (c) Palomo, C.; Oiarbide, M.;
Laso, A. Eur. J. Org. Chem. 2007, 2561–2574.
(4) (a) Trost, B. M.; Yeh, V. S. C. Angew. Chem., Int. Ed. 2002, 41, 861–
863. (b) Palomo, C.; Oiarbide, M.; Laso, A. Angew. Chem., Int. Ed. 2005, 44,
3881–3884. (c) Liu, S.; Wolf, C. Org. Lett. 2008, 10, 1831–1834.
(5) Kogami, Y.; Nakajima, T.; Ashizawa, T.; Kezuka, S.; Ikeno, T.; Yamada,
T. Chem. Lett. 2004, 33, 614–615.
10.1021/jo800412x CCC: $40.75
Published on Web 05/31/2008
2008 American Chemical Society
J. Org. Chem. 2008, 73, 4903–4906 4903

Arai et al.
SCHEME 1. Synthesis of Sulfonyldiamine Ligands (2)
FIGURE 1. Sulfonyldiamine ligands.
diamine (2)-CuCl catalysts.12 Remarkable acceleration effects
due to pyridine are also presented.
Results and Discussion
As proposed by Evans et al., the Lewis acidity of the Cu
atom is important for activating the aldehyde.6b This realization
led us to design a new asymmetric ligand in which one of the
nitrogen atoms of the 1,2-diamine group was sulfonylated to
increase the acidity of the metal complex (Figure 1).
TABLE 1. Effects of Basic Additives on the 2d-CuCl-Catalyzed
Enantioselective Henry Reaction
The binaphthyl-containing sulfonyldiamines were readily
synthesized in two steps starting from commercially available
1,2-diamines.13 Sulfonylation of the appropriate 1,2-diamine,
followed by alkylation with chiral 2,2′-dibromomethyl-1,1′-
binaphthalene, provided a series of new sulfonyldiamine ligands,
as shown in Scheme 1. (See details in the Experimental Section.)
Before examining the diastereoselective Henry reaction, the
potential of the newly developed sulfonyldiamine ligands 2 was
examined in an enantioselective reaction by using o-nitroben-
zaldehyde (3a), which have been utilized as a test substrate.10
However, it was soon revealed that the reaction of 3a with
nitromethane was not smoothly promoted by 2-CuCl. When
entry
base
DBU
DIPEA
Et3N
2,6-lutidine
DMAP
Py
amount of basea time (h) yield (%) ee (%)
1
2
3
4
5
6
7
8
9
23
21
21
23
23
17
18
16
18
16
68
96
92
84
90
84
83
85
97
94
31
<1
27
15
31
<1
57
63
67
55
1
1
1
10
50
10
20
25
30
Py
Py
Py
10
(6) (a) Christensen, C.; Juhl, K.; Jørgensen, K. A. Chem. Commun. 2001,
2222–2223. (b) Evans, D. A.; Seidel, D.; Rueping, M.; Lam, H. W.; Shaw, J. T.;
Downey, C. W. J. Am. Chem. Soc. 2003, 125, 12692–12693. (c) Gan, C.; Lai,
G.; Zhang, Z.; Wang, Z.; Zhou, M.-M. Tetrahedron: Asymmetry 2006, 17, 725–
728. (d) Maheswaran, H.; Prasanth, K. L.; Krishna, G. G.; Ravikumar, K.; Sridhar,
B.; Kantam, M. L. Chem. Commun. 2006, 4066–4068. (e) Ma, K.; You, J. Chem.
Eur. J. 2007, 13, 1863–1871. (f) Bandini, M.; Piccinelli, F.; Tommasi, S.; Umani-
Ronchi, A.; Ventrici, C. Chem. Commun. 2007, 616–618. (g) Qin, B.; Xiao, X.;
Liu, X.; Huang, J.; Wen, Y.; Feng, X. J. Org. Chem. 2007, 72, 9323–9328. (h)
Bandini, M.; Benaglia, M.; Sinisi, R.; Tommasi, S.; Umani-Ronchi, A. Org.
Lett. 2007, 9, 2151–2153. (i) Xiong, Y.; Wang, F.; Huang, X.; Wen, Y.; Feng,
X. Chem. Eur. J. 2007, 13, 829–833. (j) Colak, M.; Aral, T.; Hosgo¨ren, H.;
Demirel, N. Tetrahedron: Asymmetry 2007, 18, 1129–1133. (k) Jiang, J.-J.; Shi,
M. Tetrahedron: Asymmetry 2007, 18, 1376–1382. (l) Blay, G.; Climent, E.;
Fera´ndez, I.; Herna´ndaz-Olmos, V.; Pedro, J. R. Tetrahedron: Asymmetry 2007,
18, 1603–1612. (m) Arai, T.; Yokoyama, N.; Yanagisawa, A. Chem. Eur. J.
2008, 14, 2052–2059.
a Equivalent based on ligand.
we examined straightforward analogues of 1 (2a or 2b),
containing an (R,R)-diamine group and an (S)-binaphthyl
skeleton, the reaction with 2a provided only trace amounts of
the adduct (rt, 45 h); 2b-CuCl gave the adduct in 22% yield
with only 1% ee (rt, 40 h). However, the use of the (R,R)-
diamine-(R)-binaphthyl ligand 2d, an epimer of 2b, unexpectedly
gave the (R)-enriched product in 68% yield with 31% ee (rt,
23 h); 2c-CuCl gave only trace amounts of the adduct.
Because the Henry reaction is thought to employ basicity to
generate the nitronate, the effects of various basic additives in
promoting the reaction were examined.
As expected, the addition of basic amines enhanced the 2d-
catalyzed Henry reaction, although the enantiomeric excesses
were diminished (entries 2-6, Table 1). Surprisingly, the
addition of pyridine, a weak base, improved the chemical yield
while increasing the enantiomeric excess to 57% (entry 7). When
the amount of pyridine was increased to 25 equiv to ligand, the
adduct was obtained in 97% yield with 67% ee (entry 9); the
use of 30 equiv of pyridine resulted in a reduction in enanti-
oselectivity to 55% ee (entry 10). These remarkable acceleration
effects with improvement of enantioselectivity were also
examined in the Henry reaction by using the other sulfonyl-
diamine ligands (2); the results are summarized in Table 2. The
use of the cyclohexyldiamine analogue 2c gave the adduct
almost racemically (entry 3, Table 2). Ligand 2b, the diastere-
omer of 2d, was also unsuitable for the enantioselective Henry
reaction (entry 2).
(7) Choudary, B. M.; Ranganath, K. V. S.; Pal, U.; Kantam, M. L.; Sreedhar,
B. J. Am. Chem. Soc. 2005, 127, 13167–13171.
(8) Kowalczyk, R.; Sidorowicz, L.; Skarzewski, J. Tetrahedron: Asymmetry
2007, 18, 2581–2586.
(9) (a) Corey, E. J.; Zhang, F.-Y. Angew. Chem., Int. Ed. 1999, 38, 1931–
1934. (b) Ooi, T.; Doda, K.; Maruoka, K. J. Am. Chem. Soc. 2003, 125, 2054–
2055. (c) Marcelli, T.; van der Haas, R. N. S.; van Maarseveen, J. H.; Hiemstra,
H. Angew. Chem., Int. Ed. 2006, 45, 929–931. (d) Sohtome, Y.; Hashimoto, Y.;
Nagasawa, K. Eur. J. Org. Chem. 2006, 2894–2897. (e) Mandal, T.; Samanta,
S.; Zhao, C.-G. Org. Lett. 2007, 9, 943–945. (f) Uraguchi, D.; Sakaki, S.; Ooi,
T. J. Am. Chem. Soc. 2007, 129, 12392–12393.
(10) Arai, T.; Watanabe, M.; Fujiwara, A.; Yokoyama, N.; Yanagisawa, A.
Angew. Chem., Int. Ed. 2006, 45, 5978–5981.
(11) A study on Cu-catalyzed anti-selective Henry reaction: Risgaard, T.;
Gothelf, K. V.; Jørgensen, K. A. Org. Biomol. Chem 2003, 1, 153–156.
(12) Though we recently report a diastereoselective Henry reaction using
newly developed diamine-Cu(OAc)2 catalyst, it was 80:20 syn/antiselectivity: Arai,
T.; Watanabe, M.; Yanagisawa, A. Org. Lett. 2007, 9, 3595–3597.
(13) Representative reports on chiral sulfonyl-1,2-diamines in asymmetric
catalysis: (a) Hashiguchi, S.; Fujii, A.; Takehara, J.; Ikariya, T.; Noyori, R. J. Am.
Chem. Soc. 1995, 117, 7562–7563. (b) Watanabe, M.; Murata, K.; Ikariya, T.
J. Am. Chem. Soc. 2003, 125, 7508–7509. (c) Hayes, A. M.; Morris, D. J.;
Clarkson, G. J.; Wills, M. J. Am. Chem. Soc. 2005, 127, 7318–7319.
4904 J. Org. Chem. Vol. 73, No. 13, 2008

Asymmetric Syn-SelectiVe Henry Reaction
TABLE 2. Enantioselective Henry Reaction Catalyzed by
2-CuCl-Pyridine
TABLE 3. 2d-CuCl-Catalyzed Enantioselective Henry Reaction
entry
sulfonyldiamine
time (h)
Yield (%)
ee (%)
1
2
3
4
5
6
7
8
9
2a
2b
2c
2d
2e
2f
2g
2h
2i
40
16
40
18
23
17
20
20
27
53
86
98
97
86
92
99
90
73
<1
3
1
67
27
<1
35
63
46
a 25 equiv of py to ligand.
entry
aldehyde
time (h)
yield (%)
ee (%)
1
2
3
4
5
6
7
8
3a
3b
3c
3d
3e
3f
3g
3h
3i
18
23
23
18
15
22
18
17
18
21
20
16
97
89
94
94
99
72
96
97
95
98
79
90
67
70
83
60
59
83
87
80
85
92
93
93
Regarding the efficient reaction sphere of 2d produced by
the combination of (R,R)-diphenylethylenediamine and (R)-
binaphthyl moiety, an analogue of tosyl-(R,R)-diphenyl-
ethylenediamine-biphenyl ligand (2i) was prepared, and applied
to the enantioselective Henry reaction of 3a. Under similar
conditions with 25 equiv of py to 2i, the reaction was smoothly
catalyzed to give the adduct in 73% yield with 46% ee, which
is superior to the results with 2b (86% yield, 3% ee in entry 2,
Table 2). This suggests the 2i-CuCl-catalyzed reaction would
have a similar transition state to that of 2d-CuCl catalysis, and
the appropriate transition state of 2d-CuCl catalysis would be
more stable than that of 2b-CuCl. The formation of the
2d-CuCl complex was strongly suggested by ESI mass
spectrometry by the observation of an ion peak [2d + CuCl +
H]+ at m/z 742.
9
10
11
12
3j
3k
3l
a 25 equiv of py to ligand.
TABLE 4. 2d-CuCl-Catalyzed Diastereoselective Henry Reactiona
The nature of the R2 group substituted at the sulfonyl group
had a strong effect on the asymmetric induction ability, and
the 2d-CuCl-pyridine system gave the best overall result, with
high chemical yield and enantiomeric excess. The scope and
limitations of the 2d-CuCl-pyridine catalyst system were
examined, and the results are summarized in Table 3.
entry aldehyde R′ time (h) yield (%) syn/anti ee [%] syn/anti
1
2
3
4
5
6
7
8
3f
3i
3j
3l
Me
Me
Me
Me
Et
Me
Et
Me
16
43
94
41
42
66
96
14
55
70
89
99
63
90
92
81
80/20
73/27
85/15
92/8
66/22
64/27
82/30
84/43
81/55
80/22
68/23
81/32
Various aldehydes were smoothly converted to Henry adducts
at room temperature, and in all cases, the (R)-enriched products
were obtained by using the tosyl-(R,R)-diphenylethylenedi-
amine-(R)-binaphthyl ligand 2d. The simplest aromatic alde-
hyde, benzaldehyde (3c), was converted to the Henry adduct in
94% yield with 83% ee. Typically, the use of aliphatic aldehydes
provided the corresponding adducts with higher enantioselec-
tivities than those obtained with aromatic aldehydes. In par-
ticular, R-branched aliphatic aldehydes such as pivalaldehyde
(3k) and cyclohexanecarboxaldehyde (3l) gave the adducts with
excellent enantioselectivitysup to 93% eeswithout a significant
decrease in reaction rate. Although the role of pyridine is unclear
at present, its effect in accelerating the Henry reaction would
suggest that it enforces nucleophilic addition of nitronate to the
Cu Lewis acid-activated aldehyde in a cooperative manner.3b
Finally, the optimized catalyst system was applied to the
diastereoselective Henry reaction; the results are shown in
Table 4.
3l
91/9
3m
3m
3n
76/24
74/26
90/10
a 25 equiv of py to ligand.
when R-branched aliphatic aldehydes were used; for example,
the reaction of 3l with nitroethane gave the product in 99%
yield with 92:8 syn/anti selectivity. The enantiomeric excess
of the syn-adduct was 84% (entry 4).14
To explain the syn-selectivity of the reaction, a plausible
transition state is shown in Scheme 2. The formation of this
Cu-containing cyclic transition state species would result in syn-
selectivity due to steric hindrance.15 In the Newman projections,
the structure S1, which leads to the syn-adduct, is the most
favorable.
Although the reaction with nitroethane was slow, the reaction
of 3f without the use of ethanol as a solvent provided the adduct
with moderate syn-selectivity. This syn-selectivity was in
contrast with the results of Jørgensen’s bis(oxazoline)-
Cu(OTf)2-catalyzed anti-selective Henry reaction with silyl
nitronate.11 Diastereoselectivity was also dramatically improved
(14) 2i-CuCl-catalyzed diastereoselective Henry reaction of 3l with nitro-
ethane gave the adduct in 58% yield with 82:18 syn/anti selectivity. The
enantiomeric excess of the syn-adduct was 60%.
(15) Sasai, H.; Tokunaga, T.; Watanabe, S.; Suzuki, T.; Itoh, N.; Shibasaki,
M. J. Org. Chem. 1995, 60, 7388–7389.
J. Org. Chem. Vol. 73, No. 13, 2008 4905

Arai et al.
SCHEME 2. Plausible Transition Structure for the
Syn-Selective Henry Reaction
(m, 2H), 7.31-7.43 (m, 8H), 7.73 (d, J ) 8.5 Hz, 2H), 7.84 (d, J
) 8.2 Hz, 2H). 13C NMR (100 MHz, CDCl3) δ 21.3, 52.1, 58.4,
74.9, 125.3, 125.5, 126.8, 126.9, 127.2, 127.3, 127.48, 127.53,
127.6, 127.7, 127.8, 128.0, 128.15, 128.20, 128.4, 128.8, 129.2,
130.9, 132.8, 133.2, 134.6, 135.4, 137.7, 137.8, 142.6. HRMS
(FAB+) calcd for C43H37N2O2S (M+ + H) 645.2576, found
645.2527.
General Procedure of the 2d-CuCl-Catalyzed Enantioselec-
tive Henry Reaction. The catalyst was prepared by a complex
formation of ligand 2d (12.9 mg, 0.02 mmol) with CuCl (1.8 mg,
0.018 mmol) in anhydrous dichloromethane (1.0 mL) under Ar.
After the solution was stirred overnight at room temperature, solvent
was removed under reduced pressure. Then, the residue was
dissolved in EtOH (0.72 mL). To the resulting clear green solution
were added nitromethane (196 µL, 3.64 mmol), pyridine (40 µL,
0.5 mmol), and cyclohexanecarboxaldehyde (44 µL, 0.364 mmol)
under Ar. After the reaction was stirred for 16 h at room
temperature, the volatile components were removed under reduced
pressure and the residue was purified by column chromatography
(using neutral silica gel, n-hexane/ethyl acetate ) 4:1) to afford
the adduct (56.6 mg, 90% yield). The enantiomeric excess was
determined by HPLC analysis.
2d-CuCl-Catalyzed Diastereoselective Henry Reaction (En-
try 4, Table 4). The catalyst was prepared by a complex formation
of ligand 2d (12.9 mg, 0.02 mmol) with CuCl (1.8 mg, 0.018 mmol)
in anhydrous dichloromethane (1.0 mL) under Ar. After the reaction
was stirred an overnight at room temperature, solvent was removed
under reduced pressure. To the residue were added nitroethane (261
µL, 3.64 mmol), pyridine (40 µL, 0.5 mmol), and cyclohexanecar-
boxaldehyde (44 µL, 0.364 mmol) under Ar. After the reaction was
stirred for 41 h at room temperature, the volatile components were
removed under reduced pressure and the residue was purified by
column chromatography (using neutral silica gel, n-hexane/ethyl
acetate ) 6:1) to afford the adduct (67.8 mg, 99% yield).
Diastereoselectivity was determined by 1H NMR spectroscopy and
the enantiomeric excess was determined by HPLC analysis.
Conclusion
In conclusion, we have succeeded in developing a new chiral
sulfonyldiamine ligand as part of a catalyst system for diastereo-
and enantioselective Henry reactions. With the assistance of
pyridine, the reactions proceeded smoothly to provide the
corresponding adducts with high syn-selectivity and high
enantiomeric excess. Further detailed elucidation of the reaction
mechanism is currently in progress.
Experimental Section
Preparation of (1R,2R)-N-(R)-Binaphtyl-N′-tosyl-1,2-diphe-
nylethanediamine (2d). A solution of (1R,2R)-1,2-diphenyl-
ethanediamine (212 mg, 1.0 mmol), p-toluenesulfonyl chloride (191
mg, 1.0 mmol), and triethylamine (154 µl, 1.1 mmol) in dichlo-
romethane (5.0 mL) was stirred at room temperature for 28 h under
Ar. The reaction was quenched by the addition of distilled water,
and the aqueous layer was extracted with dichloromethane. The
organic phase was washed with brine, and then dried over sodium
sulfate. After removal of the solvent under reduced pressure, the
residue was purified by column chromatography (using neutral silica
gel, n-hexane/ethyl acetate ) 1:1) to give (1R,2R)- N-tosyl-1,2-
diphenylethanediamine (265 mg, 72% yield, as a white solid).
A solution of (1R,2R)-N-tosyl-1,2-diphenylethanediamine (160
mg, 0.437 mmol), (R)-2,2′-dibromomethyl-1,1′-binaphthalene (211
mg, 0.481 mmol), and triethylamine (134 µL, 0.960 mmol) in
dichloromethane (2.2 mL) was stirred at room temperature. After
being stirred for 67 h under Ar, the reaction was quenched by the
addition of distilled water, and the aqueous layer was extracted
with dichloromethane. The organic phase was washed with brine,
and dried over sodium sulfate. After removal of the solvent under
reduced pressure, the residue was purified by column chromatog-
raphy (using neutral silica gel, n-hexane/ethyl acetate ) 5:1) to
give 2d (280 mg, 99% yield, as a pale yellow solid).
Acknowledgment. This work was supported by the Industrial
Technology Research Grant Program (2006) from the New
Energy and Industrial Technology Development Organization
(NEDO) of Japan, a Grant-in Aid for Scientific Research from
the Ministry of Education, Culture, Sports, Science. We thank
Prof. K. Nagasawa at the Tokyo University of Agriculture and
Technology for providing the analytical data for the diastereo-
selective Henry reactions. We thank Mr. Y. Taneda and Ms.
K. Suzuki for technical support.
Supporting Information Available: HPLC conditions for
analyzing Henry adducts and 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.
Spectral data for 2d:. [R]20D +28.6 (c 0.95, CHCl3). IR (ATR)
3263, 3035, 2339, 1508, 1456, 1317, 1157, 1066, 928, 812, 752,
696, 667 cm-1. H NMR (400 MHz, CDCl3) δ 2.29 (s, 3H), 3.58
1
(d, J ) 12.1 Hz, 2H), 3.85 (d, J ) 12.3 Hz, 2H), 3.96 (d, J ) 10.6
Hz, 1H), 4.92 (d, J ) 10.6 Hz, 1H), 6.80-7.03 (m, 12H), 7.16-7.22
JO800412X
4906 J. Org. Chem. Vol. 73, No. 13, 2008