A highly efficient and practical new allylboronate tartramide for the asymmetric allylboration of achiral aldehydes

Article abstract of DOI:10.1002/ejoc.200400670

Chiral homoallylic alcohols can be prepared from aldehydes upon reaction with two optically pure allylboronate tartramides. The enantiomeric excess is 10-15% higher for the allylation of benzaldehyde when using N,N'-dibenzyl-tartramide auxiliary 5b than when using N,N'-diphenyl-tartramide (5a). 2-Allyl-N,N'-dibenzyl-1,3,2-dioxaborolane-4,5-dicarbamide (2b) affords homoallylic alcohols with 90-99% ee upon reaction with some representative aldehydes. The derivatised chiral auxiliaries can be recovered by simple recrystallization, in 85% yield, without any loss of specific rotation. Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005.

Full text of DOI:10.1002/ejoc.200400670

FULL PAPER
A Highly Efficient and Practical New Allylboronate Tartramide for the
Asymmetric Allylboration of Achiral Aldehydes
Wansuo Chen,*[a] Yanzhu Liu,[b] and Zhirong Chen[a]
Keywords: Asymmetric synthesis / Alcohols / Allylation / Allyboration
Chiral homoallylic alcohols can be prepared from aldehydes
upon reaction with two optically pure allylboronate
tartramides. The enantiomeric excess is 10–15% higher for
the allylation of benzaldehyde when using N,NЈ-dibenzyl-
tartramide auxiliary 5b than when using N,NЈ-diphenyl-
tartramide (5a). 2-Allyl-N,NЈ-dibenzyl-1,3,2-dioxaborolane-
4,5-dicarbamide (2b) affords homoallylic alcohols with 90–
99% ee upon reaction with some representative aldehydes.
The derivatised chiral auxiliaries can be recovered by simple
recrystallization, in 85% yield, without any loss of specific
rotation.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2005)
Introduction
Homoallylic alcohols have been incorporated, usually in
a racemic form, into various compounds with interesting
biological activity,[1] such as alkaloids,[2] micro antibiotics,[3]
(+)-discodermolide[4] and (+)-negamycin,[5] etc. Asymmet-
ric allylation of aldehydes to homoallylic alcohols has been
comprehensively reviewed,[6] and the method of stereoselec-
tive allylation using optically pure tartrates[7] and dialkylbo-
ranes[8] has been developed thoroughly by the groups of
Roush and Brown, respectively. The chiral tartramide auxil-
iaries used in this reaction have rarely been reported, with
the exception of N,NЈ-dibenzyl-N,NЈ-ethylenetartramide
(6), which was reported by the Roush group in 1988.[9a] In
this work, the synthesis of chiral homoallylic alcohols in
excellent enantioselectivity in the presence of two novel, chi-
ral tartramides auxiliaries (5a and 5b) is described, as well
as the comparison with 2-allyl-1,3,2-dioxaborolane-4,5-di-
carboxylic acid bis(1Ј-methylethyl) ester (9) and B-al-
lylbis(4-isocaranyl)borane (4-dIcr2Ball, 10) in allylboration.
The asymmetric allylation of aldehydes with allylboronates
(2Ј) gives homoallylic alcohols via a six-membered ring
chair-like transition state (TS)[10] (Scheme 1).
Scheme 1.
we rationally developed two novel, chiral auxiliaries in two
ways from arylamine with chiral tartrate or tartaric acid
(Scheme 2). Rotationally pure tartaric acid reacts with ani-
line directly in the presence of K2CO3 or BF3·OEt2 to give
5a in 90% yield; compound 5b was prepared by the amid-
ation of diethyl tartrate with benzylamine in 94% yield. The
two tartramides were purified by recrystallisation from
AcOH/H2O in high yield.
In general, aniline hardly reacts with organic acids be-
cause of the conjugation between the lone electron pair of
the nitrogen atom and the π electrons of the benzene ring.
Because of this we unexpectedly obtained imide 7 as the
main product when using benzylamine. Thus, optically pure
tartrates have to be chosen to avoid imide formation during
the preparation of 5b (Scheme 2).
Results and Discussion
Roush has designed the chiral tartramide auxiliary 6,[9a]
which reacts with aldehydes with high enantioselectivity but
just 40–43% yield. Derived from this restricted auxiliary,
[a] Department of Chemical Engineering and the Unilab Research
Center of Chemical Reaction Engineering, Zhejiang University,
Hangzhou 310027, P. R. China
[b] Department of Chemistry, Nanchang University,
Nanchang 330047, P.R. China
Allylboronate tartramides 2 were synthesized by two
routes: the reaction of triallylborane with aryl tartramide 5
or monoallylboron difluoride with 8 (Scheme 3).
Fax: +86-571-8795-1227
E-mail: wansuochen@yahoo.com.cn
Eur. J. Org. Chem. 2005, 1665–1668
DOI: 10.1002/ejoc.200400670
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1665

W. Chen, Y. Liu, Z. Chen
Table 1. Asymmetric allylation of benzaldehyde with 2.
FULL PAPER
Entry
Reac-
tants
T [°C] t [h] Solvent
Yield[a]
[%]
ee[b] [%]
1
(S,S)-2a
23
18
diethyl
ether
62.5
6.4 (R)
2
3
4
5
6
7
8
(R,R)-2b
(R,R)-2b
(S,S)-2b
(S,S)-2b
(S,S)-2a
(S,S)-2a
(R,R)-2b
0
14
14
16
16
14
18
18
toluene
toluene
THF
toluene
THF
toluene
diethyl
ether
77.7
80.5
79.6
83.5
76.2
82.3
87.0
34 (S)
46 (S)
61 (R)
85 (R)
72 (R)
80 (R)
83 (S)
–20
–40
–60
–78
–78
–78
Scheme 2.
9
(R,R)-2b
–78
14
toluene
87.8
90 (S)
[a] Yield of isolated product. [b] Determined by chiral HPLC analy-
sis on a Daicel chiralcel OD-H column (C18, 250×4.6 mm, 5 μm)
with 2% IPA/hexane (v/v) as mobile phase with UV/Vis detection
(λ = 220 nm, SPD-M10A). The single-enantiomer structures were
assigned by comparison of HPLC traces and optical rotation values
with known compounds and based on the assumption of a single
reaction pathway for all substrates.
As shown, the yields and optical purities are up to 87.8%
and 90%, respectively (Table 1, entry 9), and the aprotic sol-
vent toluene is better than diethyl ether because protic sol-
vents break the bond between the boron atom and carbonyl
group in the tartramide auxiliaries. The enantioselectivity
benefits from a lower temperature, whereas the time
scarcely affects yield and enantioselectivity because the re-
action is fast in the first two hours, but then slows down
according to HPLC monitoring. More importantly, the
enantioselectivity is better when using benzyl amide auxil-
iary 2b than aniline auxiliary 2a in the allylboration
(Table 1, entries 7 and 9).
Some other representative achiral aldehydes were also se-
lected to react with 2b at –78 °C. The experimental data are
summarized in Table 2 and compared with the reagents
used by the Roush (reagent 9) and Brown (reagent 10)
groups.
Scheme 3.
As can be seen from Scheme 3, we prefer the second one
because of the high yield for the preparation of 2. However,
it must be noted that the sodium hydride used to prepare 8
is very sensitive to moisture, so the reagents and solvents
used must be dried prior to use. Fortunately, the unreacted
sodium hydride does not affect the following allylboration
reaction. For analytical purposes, benzaldehyde was se-
As shown, we prefer 2b to the widely used DIPT auxil-
lected to react with 2 under various conditions and the re- iary 9 and 4-dIcr2Ball because of its high enantioselectivity
sults are listed in Table 1.
(Table 2, entries 2–4), and the derivatised chiral auxiliary 5b
Table 2. Comparison of the enantioselectivities achieved in the allylboration of representative aldehydes with 2b (–78 °C in toluene), 9
(Roush reagent, –78 °C) and 4-dIcr2Ball (10, Brown reagent, –100 °C).
Aldehydes
Products
ee [%]
9[a]
10[b]
2b
1
2
3
4
benzaldehyde (1a)
n-butyraldehyde (1b)
(R)-3a
(S)-3b
(R)-3c
(R)-3d
71
79[c]
82
98
98
Ն99
90
95[d]
98[d]
99[d]
pivaldehyde (1c)
cyclehexanecarboxaldehyde (1d)
87
[a] See ref.[9a] [b] At –100 °C, essentially instantaneous.[7c] [c] Value for 1-decanal.[9b] [d] At –78 °C [16 h, determined by the same chiral
column with a Refractive Index detector (RID-10A)].
1666
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Eur. J. Org. Chem. 2005, 1665–1668

Asymmetric Allylboration of Achiral Aldehydes
FULL PAPER
DMF (2.5 mL, 33 mmol) was added and refluxed for an additional
3 h. The mixture was then cooled to room temperature to afford a
white solid, which was isolated by filtration, washed with water and
95% EtOH in water, and recrystallised from 90% acetic acid in
water to obtain 5a as white crystals. Yield: 4.1 g (90%). M.p. 261.1–
263.0 °C. [α]2D0 = +139 (c = 1.0, DMF). 1H NMR ([D6]DMSO,
500 MHz): δ = 9.62 (s, 2 H), 7.76–7.74 (d, 4 H), 7.34–7.30 (t, 4 H),
7.10–7.05 (t, 2 H), 6.05–6.00 (d, 2 H), 4.51–4.48 (d, 2 H) ppm. 13C
NMR ([D6]DMSO, 125.7 MHz): δ = 172.00, 138.54, 129.00,
can easily be recovered by recrystallisation, in 85% yield,
without any loss of specific rotation.
We also explored the allylboration of 2b [or N,NЈ-di-
benzyl-2-methylallyl-1,3,2-dioxaborolane-4,5-dicarbamide
(2bЈ)] with several other achiral aldehydes. To the best of
our knowledge, this is the first time that 2-methylallylborol-
ane has been found to react with this novel auxiliary with
satisfactory yields and enantioselectivities (Table 3, entry 2).
124.42, 121.63, 73.41 ppm. IR:
ν =
˜
3410, 3300, 1670 cm–1.
Table 3. Reactions of (S,S)-2b and (S,S)-2bЈ with some other achi-
ral aldehydes.
C16H16N2O4 (300.3): calcd. C 63.99, H 5.37, N 9.33; found C 63.03,
H 5.14, N 10.52.
Amidation of Chiral Diethyl Tartrate to N,NЈ-Dibenzyltartramide
[(S,S)-5b]: A mixture of methanol (20 mL), benzylamine (6.0 g,
56 mmol), K2CO3 (0.1 g, 0.72 mmol) and d-(–)-diethyl tartrate
(5.2 g, 25 mmol) was refluxed for 8 h. It was then cooled to room
temperature to afford a white solid, which was filtered, washed with
water, and recrystallised from 50% ethanol in water to obtain 5b
as white crystals. Yield: 7.8 g (94%). M.p. 202.0–204.0 °C. [α]2D0
=
–75.08 (c = 2.0, DMF). 1H NMR ([D6]DMSO, 500 MHz): δ =
7.22–7.35 (m, 10 H), 5.74–5.75 (d, 2 H), 4.30–4.42 (m, 4 H) ppm.
13C NMR ([D6]DMSO, 125.7 MHz): δ = 173.15, 141.70, 128.62,
[a] Yield of isolated product after purification by column
chromatography). [b] Determined on a Chiralcel OD-H Column
(C18, 250×4.6 mm, 5 μm) with 2% IPA/hexane (v/v) as mobile
phase with a UV/Vis Detector (SPD-M10A) or a Refractive Index
Detector (RID-10A).
127.00, 126.85, 73.81, 44.48 ppm. IR: ν = 3360, 3300, 1650 cm–1.
˜
C18H20N2O4 (328.4): calcd. C 65.84, H 6.14, N 8.53; found C 65.62,
H 6.48, N 8.41.
In summary, we have described a highly convenient pro-
cedure for the asymmetric allylboration of a variety of rep-
resentative aldehydes controlled by an efficient and practi-
cal chiral auxiliary 5b, which affords homoallylic alcohols
in 90–99% ee. This novel chiral auxiliary can be recovered
by recrystallisation, in high yield, without any decrease in
enantioselectivity. Consequently, we believe that this novel
auxiliary will become even more valuable for stereoselective
synthesis of natural products and new chiral drugs.
Preparation of Triallylborane:[11] Mg turnings (3.0 g, 125 mmol), a
crystal of iodine, BF3·Et2O (4.7 g, 33 mmol) and dry diethyl ether
(80 mL) were placed in the reaction flask, and the reaction was
initiated by a dropwise addition of 1.0 mL of neat allyl bromide,
while vigorously stirring the reaction mixture. A further portion of
allyl bromide (7.36 mL, 100 mmol) dissolved in anhydrous diethyl
ether (50 mL) was added slowly over a period of 1 h, and the sol-
vent was allowed to reflux smoothly. The reaction mixture was
stirred for an additional 2 h. The clear ethereal layer was transfer-
red into a distillation flask and the solvent was removed at atmo-
spheric pressure. Distillation under vacuum in a short-path distil-
lation assembly afforded triallylborane (3.7 g, 84%), b.p. 65 °C at
20 Torr. 1H NMR (CDCl3, 500 MHz): δ = 5.96–5.83 (m, 2 H), 5.05
(d, 2 H), 1.92 (d, 2 H) ppm. C9H15B (134.0): calcd. C 80.65, H
11.28, B 8.07; found C 80.12, H 11.07, B 8.81.
Experimental Section
General: 1H and 13C NMR spectra were measured at 500 MHz and
125 MHz, respectively, on an Avance DMX500 instrument. Chemi-
cal shifts are reported in δ units relative to internal CHCl3 (δ = 7.24
or 77.0 ppm) or DMSO (δ = 2.43 ppm) as the internal standard.
Preparation of Monoallylboron Difluoride:[11] The procedure is sim-
ilar to the synthesis of triallylaborane except that the ratio of BF3
to allyl bromide used in the reaction is 1:1. C3H5BF2 (89.9): calcd.
C 40.09, H 5.61, B 12.03, F 42.27; found C 40.16, H 5.07, B 12.85,
F 41.92.
All reactions were conducted in oven-dried (120 °C) glassware un-
der an atmosphere of nitrogen. All solvents were freshly distilled
before use. Et2O and THF were distilled from sodium benzophe-
none ketyl and toluene was distilled from sodium metal.
Preparation of 8: Optically pure N,NЈ-diaryltartramide (20 mmol,
5a or 5b) was added to a solution of 80% sodium hydride (1.3 g,
40 mmol) in 50 mL of dry diethyl ether in three portions under an
atmosphere of nitrogen, and then stirred for 1 h. The mixture was
not purified and was used directly in the subsequent reaction with
monoallylboron difluoride.
Analytical TLC was performed with 2.5×10 cm plates coated with
a 0.25 mm layer of silica gel containing PF254 indicator. Analytical
HPLC was performed with a UV/Vis or refractive index detector
connected to a reversed phase HPLC system (C18 column,
250×4.6 mm, 5 μm) on a Shimadzu instrument. Preparative HPLC
was conducted with a C18 column (250×40 mm, 20 μm) on a Lab-
Alliance instrument. All chromatograph solvents were distilled
prior to use.
Preparation
of
2-Allyl-N,NЈ-dibenzyl-1,3,2-dioxaborolane-4,5-
dicarbamide [(S,S)-2] from 5: A suspension of 5 (20 mmol) in 60 mL
of dry diethyl ether was treated with triallylborane (2.68 g,
20 mmol) at 23 °C. The suspension became a clear solution within
a few minutes and was stirred for 3 h before being concentrated in
vacuo with exclusion of moisture. The resulting white foam was
stripped overnight at 0.1 mmHg to give reagent 2 in 86% yield,
which was used directly in the next experiments.
The enantiomeric excess was determined with a Daicel chiralcel
OD-H column (C18, 250×4.6 mm, 5 μm) with 2% IPA/hexane
(v/v) as mobile phase and a UV/Vis detector (λ = 220nm, SPD-
M10A) or s Refractive Index Detector (RID-10A).
Amidation of Optically Pure Tartaric Acid to N,NЈ-Diphenyltartra-
mide [(S,S)-5a]: A mixture of xylene (25 mL), aniline (5 g, 70 mmol)
2a: 1H NMR ([D6]DMSO, 500 MHz): δ = 8.26–8.24 (t, 2 H), 7.75–
and l-(+)-tartaric acid (2.25 g, 15 mmol) was refluxed for 2 h. 7.07 (m, 10 H), 5.93–5.87 (m, 2 H), 5.22 (d, 2 H), 4.86 (d, 2 H),
Eur. J. Org. Chem. 2005, 1665–1668
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W. Chen, Y. Liu, Z. Chen
FULL PAPER
3.93 (d, 2 H) ppm. 13C NMR ([D6]DMSO, 125.7 MHz): δ = 173.11, H), 5.10–4.95 (d, 2 H), 4.65 (m, 1 H), 2.65–2.30 (m, 2 H), 1.76–
141.72, 135.54, 128.60, 126.80, 117.26, 78.72, 29.30 ppm.
1.72 (m, 3 H) ppm. 13C NMR (CDCl3, 125.7 MHz): δ = 142.62,
2b: 1H NMR ([D6]DMSO, 500 MHz): δ = 8.26–8.24 (t, 2 H), 7.75– 137.44, 132.57, 129.13, 128.66, 127.11, 111.33, 64.22, 47.84,
7.07 (m, 10 H), 5.93–5.87 (m, 2 H), 5.20 (d, 2 H), 4.85 (d, 2 H),
23.95 ppm. MS: m/z (%) = 197 [MH+]. C11H13ClO (196.7): calcd.
4.40–4.29 (m, 4 H), 3.93 (d, 2 H) ppm. 13C NMR ([D6]DMSO, C 67.18, H 6.66, Cl 18.03; found C 67.06, H 6.52, Cl 18.29.
125.7 MHz): δ = 173.12, 141.71, 135.54, 128.60, 126.80, 117.26,
(R)-1-Pyridin-3-ylbut-3-en-1-ol (3g): [α]2D0 = +28.0 (c = 1.0, EtOH).
78.74, 44.42, 29.31 ppm.
1H NMR (CDCl3, 500 MHz): δ = 8.70–8.64 (d, 1 H), 8.60 (d, 1
Preparation of 2 from 8: A solution of monoallylboron difluoride
in anhydrous diethyl ether was added dropwise to the mixture of 8
obtained above at room temperature under an atmosphere of nitro-
gen. The solution was stirred for 3 h before being concentrated in
vacuo with exclusion of moisture. The resulting white foam was
stripped at 0.1 mmHg to give 2a or 2b in 98% yield.
H), 7.98 (d, 1 H), 7.53 (m, 1 H), 5.83–5.63 (m, 1 H), 5.10–5.01 (m,
2 H), 4.87 (t, 1 H), 3.51 (s, 1 H), 2.47 (m, 2 H) ppm. 13C NMR
(CDCl3, 125.7 MHz): δ = 147.12, 146.30, 141.82, 137.93, 132.84,
125.22, 119.20, 70.00, 43.16 ppm. MS: m/z (%) = 150 [MH+].
C9H11NO (149.2): calcd. C 72.46, H 7.43, N 9.39; found C 71.92,
H 7.32, N 10.04.
General Preparation of Chiral Homoallyl Alcohols 3a–g: A solution
of freshly distilled aldehyde in dry toluene was added dropwise to
a solution of 2a or 2b in toluene at –78 °C containing 4-Å molecu-
lar sieves. The resulting mixture was kept at –78 °C until the reac-
tion was judged complete by TLC analysis (about 16 h), and was
then terminated by addition of an excess of NaHCO3 in H2O. This
two-phase mixture was stirred for 2 h, and then separated. The or-
ganic phase was extracted with Et2O. The upper phase was dried
with anhydrous Na2SO4, and then concentrated in vacuo to obtain
the product homoallylic alcohols, which were purified by prepara-
tive HPLC with methanol as the mobile phase.
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(R)-Phenylbut-3-en-1-ol (3a): [α]2D0 = +30.9 (c = 2.0, benzene). IR
1
(NaCl): ν = 3383, 3075, 1641 cm–1. H NMR (CDCl , 500 MHz):
˜
3
δ = 7.65–7.29 (m, 5 H), 5.70–5.65 (m, 1 H), 5.11–5.00 (m, 2 H), 4.74
(t, 1 H), 2.90–2.40 (m, 2 H) ppm. 13C NMR (CDCl3, 125.7 MHz): δ
= 143.99, 134.55, 128.30, 127.41, 125.92, 117.94, 73.40, 43.64 ppm.
MS: m/z (%) = 148 [M+]. C10H12O (148.2): calcd. C 81.04, H 10.81;
found C 80.78, H 11.07.
(S)-Hept-4-en-1-ol (3b): [α]2D0= –12.5 (c = 10.2, benzene). H NMR
1
(CDCl3, 500 MHz): δ = 5.75–5.65 (m, 1 H), 5.05–4.95 (d, 2 H),
3.30–3.20 (m, 1 H), 2.25–2.20 (m, 1 H), 2.02–1.95 (d, 1 H), 1.50–
1.42 (d, 2 H), 1.33 (m, 2 H), 0.96 (m, 3 H) ppm. 13C NMR (CDCl3,
125.7 MHz): δ = 137.71, 115.75, 72.18, 42.34, 40.23, 16.72,
14.45 ppm. MS: m/z (%) = 115 [MH+]. C7H14O (114.2): calcd. C
73.63, H 12.36; found C 73.22, H 12.77.
(R)-2,2-Dimethylhex-5-en-3-ol (3c): [α]2D0 = +3.9 (c = 1.5, benzene).
1H NMR (CDCl3, 500 MHz): δ = 5.74 (m, 1 H), 5.20–5.10 (m, 2
H), 3.25 (d, J = 11.1 Hz 1 H), 2.40–2.32 (m, 1 H), 2.05–1.95 (m, 1
H), 0.88 (s, 9 H) ppm. 13C NMR (CDCl3, 125.7 MHz): δ = 137.72,
115.76, 80.38, 38.41, 37.05, 23.73 ppm. MS: m/z (%) = 138 [M+].
C8H16O (128.2): calcd. C 74.94, H 12.58; found C 75.11, H 12.41.
(S)-1-Cyclohexylbut-3-en-1-ol (3d): [α]2D0 = –8.9 (c = 0.54, EtOH).
1H NMR (CDCl3, 500 MHz): δ = 5.90–5.72 (m, 1 H), 5.18–5.11
(m, 2 H), 3.35 (m, 1 H), 2.40–2.24 (m, 1 H), 2.20–2.05 (m, 1 H),
1.92–1.50 (m, 6 H), 1.45–0.90 (m, 6 H) ppm. 13C NMR (CDCl3,
125.7 MHz): δ = 137.72, 115.76, 73.34, 44.93, 40.12, 28.31, 27.54,
26.15 ppm. MS: m/z (%) = 154 [M+]. C10H18O (154.3): calcd. C
77.87, H 11.76; found C 77.26, H 12.37.
(R)-1-Bromohex-5-en-3-ol (3e): [α]2D0 = +7.4 (c = 1.0, EtOH). 1H
NMR (CDCl3, 500 MHz): δ = 5.70–5.65 (m, 1 H), 5.10–4.96 (d, 2
H), 3.30–3.25 (m, 3 H), 2.25–2.20 (m, 1 H), 2.05–1.90 (m, 3 H)
ppm. 13C NMR (CDCl3, 125.7 MHz): δ = 137.71, 115.75, 70.82,
41.34, 39.76, 27.65 ppm. MS: m/z (%) = 194 [MH+]. C7H13BrO
(193.1): calcd. C 43.54, H 6.79, Br 41.38; found C 43.68, H 6.82,
Br 41.21.
[10] a) Y. Li, K. N. Houk, J. Am. Chem. Soc. 1989, 111, 1236; b)
B. W. Gung, X. W. Xue, Tetrahedron: Asymmetry 2001, 12,
2955; c) B. W. Gung, X. W. Xue, W. R. Roush, J. Am. Chem.
Soc. 2002, 124, 10 692; d) W. S. Chen, Z. R. Chen, J. Zhejiang
Univ. Sci. 2004, in press.
[11] a) B. M. Mikhailov, Organomet. Chem. Rev. Sect. A 1972, 8, 1;
b) H. C. Brown, U. S. Racherla, J. Org. Chem. 1986, 51, 427.
Received: September 22, 2004
(R)-1-(2-Chlorobenzyl)-3-methylbut-3-en-1-ol (3f): [α]2D0 = +18.5 (c
1
= 1.0, EtOH). H NMR (CDCl3, 500 MHz): δ = 7.45–7.00 (m, 4
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Eur. J. Org. Chem. 2005, 1665–1668