Unusual Acid- and Base-Catalyzed C-N Bond Formation Approach through Reaction of Chromonyl Meldrum's Acid and Nitrogen Binucleophiles

Article abstract of DOI:10.1055/s-0035-1560383

Reaction of chromonyl Meldrum's acid and N-substituted 2-aminobenzamides was studied in the presence of acidic and basic catalysts. The use of methanesulfonic acid as a catalyst in this reaction led to the synthesis of chromonyl quinazolinones through employing Meldrum's acid as a carbon leaving group. However, in the presence of basic catalyst, this reaction gave functionalized 2-pyridones.

Full text of DOI:10.1055/s-0035-1560383

SYNLETT
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© Georg Thieme Verlag Stuttgart · New York
2015, 26, A–G
letter
A
S. Balalaie et al.
Letter
Synlett
Unusual Acid- and Base-Catalyzed C–N Bond Formation Approach
through Reaction of Chromonyl Meldrum’s Acid and Nitrogen Bi-
nucleophiles
Saeed Balalaie*a,b
Hamid Reza Bijanzadeha
Saber Mehrparvara
Frank Romingerc
O
O
O
O
O
O
R
acid catalyst
base catalyst
X
N
H
H
+
H2
N
O
a Peptide Chemistry Research Center,
K. N. Toosi University of Technology,
P.O. Box 15875-4416 Tehran, Iran
balalaie@kntu.ac.ir
O
O
R
O
X
N
R
O
O
O
N
O
O
N
OH
O
X
X
NHR
O
N
H
HN
b Medical Biology Research Center,
Kermanshah University of Medical Sciences,
Kermanshah, Iran
4 examples
c Organisch-Chemisches Institut der
Universitaet Heidelberg,
14 examples
(66–74%)
(48–84%)
Im Neuenheimer Feld 270, 69120 Heidelberg,
Germany
Received: 22.08.2015
Accepted after revision: 20.10.2015
Published online: 30.11.2015
Fillion found that Meldrum’s acid could be used as a
highly effective and convenient leaving group in nucleo-
philic substitution.6 In the reported reaction, C–C σ bond
cleavage and C–C bond formation were achieved using Lew-
is acids such as AlCl3, Sc(OTf)3, and TMSOTf. The reactions
were carried out through direct modification of sp3-hybrid-
ized tertiary and quaternary carbon centers via cleavage of
a C–C σ bond.
DOI: 10.1055/s-0035-1560383; Art ID: st-2015-d0659-l
Abstract Reaction of chromonyl Meldrum’s acid and N-substituted
2-aminobenzamides was studied in the presence of acidic and basic
catalysts. The use of methanesulfonic acid as a catalyst in this reaction
led to the synthesis of chromonyl quinazolinones through employing
Meldrum’s acid as a carbon leaving group. However, in the presence of
basic catalyst, this reaction gave functionalized 2-pyridones.
Considering the versatile chemistry of the chromones,7
designing an efficient synthetic approach to compounds
that contain the chromone scaffold can profit from the ap-
plication of domino reactions. In this way, the Knoevenagel
reaction of 3-formylchromone and Meldrum’s acid can lead
to chromonyl Meldrum’s acid derivatives that are Michael
acceptors and could be used for nucleophilic addition.8 Re-
cently, we reported the three-component domino reaction
of 3-formylchromones, Meldrum’s acid, and primary
amines in the presence of a catalytic amount of diammoni-
um hydrogen phosphate in water which led to 2-pyridone
3-carboxylic acids.9 The products were formed through nu-
cleophilic addition of primary amines to chromonyl
Meldrum’s acid and led to the opening of the chromone
skeleton. The reaction of chromonyl Meldrum’s acid and
4-hydroxycoumarin in the presence of a base and alcohol
led to C–C bond formation. In this case, the Meldrum’s acid
was opened and acetone and carbon dioxide were eliminat-
ed as is often observed.10 However, based on the reagents
added and also the reaction conditions, the chromonyl moi-
ety could be opened and other heterocyclic skeletons could
be formed.
Keywords chromonyl Meldrum’s acid, binucleophile, acid catalysis,
base catalysis, chromonyl quinazolinines, domino Michael–elimination–
cyclization reaction, functionalized 2-pyridones
Function-oriented synthesis (FOS) has an increasing and
important role in producing therapeutic compounds in a
step-economical fashion.1 In this way, domino reactions are
well-known to access compound collections with high
structural diversity and molecular complexity.2 Designed
novel reaction sequences to obtain functionalized small-to-
medium-sized molecules and active biologically molecules
through domino reactions lead to reliable and efficient
methodologies.3
Arylidene Meldrum’s acid is known as an electrophilic
Michael acceptor and has been used to generate extended
heterocyclic skeletons and biologically active compounds.
These compounds are termed ‘neutral organic acids’ and
they have been extensively used in studies on conjugate ad-
dition.4 In this regard, chromonyl Meldrum’s acid is a suit-
able substrate for the synthesis of various heterocyclic skel-
etons and a source of the chromone scaffold for its insertion
into the structure of biologically active compounds.5
© Georg Thieme Verlag Stuttgart · New York — Synlett 2015, 26, A–G

B
S. Balalaie et al.
Letter
Synlett
O
O
O
O
O
R
O
O
O
O
N
O
O
O
O
X
X
MeSO3H (20 mol%)
EtOH:H2O, 70 °C
X
H
H
N
H
O
O
+
O
X = H, Cl, Br
7a–n (48–84%)
1a–c
2
3a–c
O
O
O
K2CO3 (30 mol%)
EtOH–H2O, 70 °C
O
RNH2
+
RHN
H2N
OH
N
O
R = R1, R2NHNH2,
R3CONHNH2
N
H
O
NHR
O
5a–h
6a–h
4
8a–d (66–74%)
Scheme 1 Synthesis of chromonyl dihydroquinazolinones 7a–n and functionalized 2-pyridones 8a–d
In continuation of our research work to design multi-
component reactions based on 3-formylchromone, we re-
port herein the synthesis of chromonyl Meldrum’s acid 3a–
c and also N-substituted 2-aminobenzamides 6a–h and
their reaction in the presence of methanesulfonic acid in
ethanol to construct quinazolines 7a–n that contain the
chromone skeleton (Scheme 1). In contrast, carrying out the
reaction between chromonyl Meldrum’s acid and 2-amino-
benzamides in the presence of potassium carbonate (30
mol%) as basic catalyst gave functionalized 2-pyridones 8a–
d.
The synthesized compounds contain the chromone
skeleton in addition to quinazolinone scaffolds which are
known as privileged heterocycles and are found in many bi-
ologically active compounds and also in naturally occurring
alkaloids.11
action of chromonyl Meldrum’s acid 3a and N-benzyl 2-am-
inobenzamide 6b was selected as a model study. Stirring
the reaction mixture at room temperature and elevated
temperatures was investigated, but the reaction did not
proceed in the absence of an acid. Therefore, the model re-
action was carried out in the presence of a range of Brønst-
ed acids including methanesulfonic acid, trifluoroacetic ac-
id, phosphoric acid, p-toluenesulfonic acid, and fluoroboric
acid, as well as Lewis acids such as zinc chloride and boron
trifluoride. The product in all cases was 2-chromonyl dihy-
droquinazolinone 7b, and the best yield was obtained using
methanesulfonic acid in ethanol (84%)12 (Scheme 2). The
type and amount of acid and nature of the solvent have sig-
nificant impact on reaction times and yields. After finding
the most suitable acid catalyst, the model reaction was in-
vestigated in different solvents such as dichloromethane,
1,2-dichloroethane, N,N-dimethylformamide, and tetrahy-
drofuran but, in all cases, the yield of the desired product
was lower compared to ethanol as solvent. After choosing
Initially, the N-substituted 2-aminobenzamides 6a–h
were formed through the reaction of isatoic anhydride with
primary amines, hydrazines, and hydrazides 5a–h. The re-
O
R
O
O
N
O
O
O
O
O
O
O
R
X
acid catalyst
acid catalyst
N
H
H
N
H
+
H2N
O
R = Bn
O
O
O
R
O
:
R
O
O
N
H
N
O
O
O
N
H
N
H
O
7b
Scheme 2 Unexpected synthesis of 7b through elimination of Meldrum’s acid
© Georg Thieme Verlag Stuttgart · New York — Synlett 2015, 26, A–G

C
S. Balalaie et al.
Letter
Synlett
O
O
O
O
O
O
O
N
O
O
O
OH
N
O
NH
O
N
H
H
acid catalyst (mol%)
solvent
base catalyst (mol%)
solvent
N
H
+
O
H2N
8b
3a
6b
7b
Scheme 3 Model reaction for the synthesis of 7b and 8b
ethanol as the optimal solvent, the model reaction was in-
vestigated using 10, 20, and 30 mol% methanesulfonic acid
and the obtained yield of product was 78, 84, and 84% re-
spectively. Subsequently, the influence of reaction tempera-
ture at 50, 60, 70, and 80 °C in the formation of 7b was in-
vestigated in ethanol using 20 mol% methanesulfonic acid
and the isolated yields were 73, 76, 78, and 78%, respective-
ly. Thus, the optimal reaction conditions for the synthesis of
7b involved conducting the reaction in ethanol with 20
mol% methanesulfonic acid at 70 °C. At the outset of this
study, the synthesis of the benzodiazapineone skeleton had
been our goal, and the formation of chromonyl quinazoli-
none 7b was an unexpected result which would appear to
proceed through elimination of Meldrum’s acid (Scheme 2).
A second study was carried out, this time using base as a
catalyst. The same model reaction was investigated using
different basic catalysts such as potassium carbonate,
piperidine, N,N-diisopropylethylamine, trimethylamine,
L-proline, and diammonium hydrogen phosphate. The
model reaction was also investigated in a range of solvents
including ethanol, tetrahydrofuran, acetonitrile, dichloro-
methane, 1,2-dichloroethane, and N,N-dimethylformamide.
The product obtained was 8b, and the best result was
formed using potassium carbonate (30 mol%) in ethanol at
70 °C (Scheme 3).
After finding suitable reaction conditions for the syn-
thesis of 7b, the scope of the reaction was explored using
different 3-formylchromones, primary amines, and phenyl
hydrazine derivatives, as well as phenyl hydrazide to access
2-chromonyl dihydroquinazolinones 7a–n in moderate to
good yields and with no side reactions. The results are sum-
marized in Table 1.
The structures of products 7a–n were confirmed using
NMR spectroscopy, and high-resolution mass spectrometry
(ESI). A characteristic resonance for all of these compounds
in the 1H NMR spectra appeared as a singlet at δ = 5.80–6.30
ppm for the aliphatic methine proton and also a distinctive
peak in the 13C NMR spectra for sp3 carbon at δ = 63.0–68.0
ppm. The 13C NMR spectra of 7a–n exhibited characteristic
signals in the δ = 160.0–192.0 ppm region associated with
the carbonyls of the amide and ketone moieties.12 The
structures of compounds 7c and 7k were further confirmed
by X-ray crystallographic analysis (Figure 1). The X-ray
crystallographic data showed clearly the orientation of the
chromonyl substituent and also the primary amine group.
There is a suitable disposition for hydrogen bonding be-
tween the carbonyl group and NH when benzhydrazide was
used. In support of this, a deshielded NH group was ob-
served at δ = 10.56–10.62 ppm in compounds 7l–n.
Figure 1 ORTEP structures of compound 7c and 7k
© Georg Thieme Verlag Stuttgart · New York — Synlett 2015, 26, A–G

D
S. Balalaie et al.
Letter
Synlett
Table 1 Acid-Catalyzed Synthesis of Chromonyl Dihydroquinazolinones 7a–n in Ethanol
O
O
O
O
O
O
O
O
O
O
X
X
H
H
O
O
O
R
+
O
N
O
O
X = H, Cl, Br
X
MeSO3H (20 mol%)
EtOH, 70 °C
N
H
2
3a–c
1a–c
O
O
O
7a–n
RNH2
+
RHN
H2N
14 examples (48–84%)
R = R1, R2NHNH2,
R3CONHNH2
N
H
O
5a–h
6a–h
4
Cl
O
N
O
O
N
O
N
O
O
N
O
O
O
O
O
N
H
N
H
N
H
N
H
O
7b (84%)
7c (72%)
7d (61%)
7a (78%)
O
O
O
N
O
O
N
O
O
N
O
O
Br
Br
Br
N
H
N
H
N
H
7g (74%)
7f (84%)
7e (80%)
OMe
O
O
O
HN
O
HN
N
O
O
HN
N
O
O
HN
N
O
O
N
O
O
Br
Cl
N
H
N
H
N
H
N
H
7j (71%)
7k (73%)
7h (67%)
7i (48%)
O
O
N
O
O
O
O
O
HN
HN
HN
O
O
N
O
O
N
O
O
Br
N
H
N
H
N
H
7l (66%)
7m (67%)
7n (71%)
In a further study, reaction of chromonyl Meldrum’s ac-
ids 3a,b with N-substituted-2-aminobenzamides 6a–c was
investigated in ethanol in the presence of 30 mol% potassi-
um carbonate and this led to 2-pyridones 8 (Scheme 4). To
extend the range of 2-pyridone derivatives, allylamine and
2-phenylethylamine were used as well as benzylamine, and
© Georg Thieme Verlag Stuttgart · New York — Synlett 2015, 26, A–G

E
S. Balalaie et al.
Letter
Synlett
O
O
O
O
O
O
O
O
O
O
X
H
H
O
O
+
O
O
X = H
1a
K2CO3 (30 mol%)
EtOH, 70 °C
3a
2
OH
N
O
NHR
O
O
O
O
RHN
RNH2
+
N
H
O
H2N
8a–d (66–74%)
5a–c
4
6a–c
Scheme 4 Synthesis of functionalized 2-pyridones 8a–d using potassium carbonate (30 mol%)
Table 2 Base-Catalyzed Synthesis of Functionalized 2-Pyridones 8a–d
O
O
O
O
Br
N
OH
O
NH
O
N
OH
N
OH
O
O
N
OH
O
NH
O
NH
O
NH
O
8a (70%)
8b (74%)
8c (66%)
8d (70%)
the products 8a–d were obtained (Table 2). A distinctive
feature in the structure of these products compared to our
last report9 is the absence of the carboxylic acid moiety.
In basic medium, the nucleophilic addition of amine bi-
nucleophile to the unsaturated carbonyl system leads to the
opening of the chromone skeleton. After nucleophilic addi-
tion of the amine to the carbonyl group, elimination of ace-
tone and carbon dioxide leads to the formation of function-
alized 2-pyridines 8a–d. A characteristic signal for the func-
tionalized 2-pyridones 8a–d was a doublet at δ = 7.80–7.90
ppm for the olefinic (H-6) pyridone proton. Meanwhile the
phenolic hydrogen resonated at δ = 10.30 ppm and, in all
cases, the NH group was observed as a triplet at δ = 8.80–
9.00 ppm. The 13C NMR spectra of 8a–d all exhibited char-
acteristic signals in the δ = 160–192 ppm region associated
with the carbonyl groups of the amide and ketone moi-
eties.12
ate led to 2-pyridones 8a–d. The ease of workup and
compatibility with a range of active functional groups are
features of this methodology.
Acknowledgment
We gratefully acknowledge the Iran National Science Foundation
(INSF) for financial support.
Supporting Information
Supporting information for this article is available online at
http://dx.doi.org/10.1055/s-0035-1560383. Copies of 1H NMR,
13C NMR for compound 3a and 1H NMR, 13C NMR, IR, HRMS spectra
for compounds 7a–n and 8a–d, and X-ray crystal data for compounds
7c and 7k are included.
S
u
p
p
o
nrtogI
f
rmoaitn
S
u
p
p
ortiInfogrmoaitn
In conclusion, we have reported a novel approach to ac-
cess 2-chromonyl quinazolinone derivatives 7a–n in good
yields through reaction of chromonyl Meldrum’s acid 3a–c
and N-substituted 2-aminobenzamides 6a–h in the pres-
ence of methanesulfonic acid in ethanol. Meanwhile, the re-
action of N-substituted 2-aminobenzamides with chrom-
onyl Meldrum’s acid in the presence of potassium carbon-
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Synlett
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Typical Procedure for 3
To a solution of 3-formylchromone (1; 1 mmol, 174 mg) in
EtOH–H2O (4 mL; 1:1) was added Meldrum’s acid (2; 1 mmol,
144 mg), and the mixture was stirred for 3 h at ambient tem-
perature. The precipitate formed was isolated by filtration.
2,2-Dimethyl-5-[(4-oxo-4H-chromen-3-yl)methylene]-1,3-
dioxane-4,6-dione (3a)
Yellow powder. 1H NMR (300 MHz, DMSO-d6): δ = 1.78 (s, 6 H, 2
CH3), 7.56 (t, 1 H, J = 7.6 Hz, HAr), 7.73 (d, 1 H, J = 8.4 Hz, HAr),
7.87 (t, 1 H, J = 7.5 Hz, HAr), 8.10 (d, 1 H, J = 7.5 Hz, HAr), 8.26 (s,
1 H, =CH), 9.31 [s, 1 H, (CO2)2C=CH]. 13C NMR (75 MHz, DMSO-
d6): δ = 27.0, 104.9, 117.9, 118.0, 118.7, 123.1, 125.7, 126.7,
135.1, 145.5, 155.1, 159.8, 162.0, 162.2, 173.3.
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General Procedure for 7
To a solution of isatoic anhydride (4; 1 mmol, 163 mg) in EtOH
(4 mL) was added the primary amine, aryl hydrazine, or aryl
hydrazide 5a–h (1 mmol), and the mixture was heated for 1 h at
70 °C. This mixture was used for the synthesis of 7a–n or 8a–d
without purification.
To a solution of product 6a–h (1 mmol) in EtOH (10 mL) was
added product 3 (1 mmol), and methanesulfonic acid (20 mol%,
20 mg), and the mixture was heated for 12 h at 70 °C. The reac-
tion reached completion as indicated by TLC (EtOAc–n-hexane,
1:3), and the precipitate was filtered. The precipitate was
washed with MeOH, and the resulting powder was pure
product 7a–n (yields 48–84%).
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2-(4-Oxo-4H-chromen-3-yl)-3-phenethyl-2,3-dihydro-
quinazolin-4(1H)-one (7c)
Yellow powder, 285 mg (72%), mp 228–230 °C. IR (KBr): ν =
3327, 1724, 1632 cm–1. 1H NMR (300 MHz, DMSO-d6): δ = 2.88
(dt, 2 H, J = 15.0, 6.6 Hz, CH2Ph), 3.10–3.14 (m, 1 H, CHPh), 4.05–
4.09 (m, 1 H, CHN), 5.97 (s, 1 H, CH), 6.68 (t, 1 H, J = 7.1 Hz, HAr),
6.75 (d, 1 H, J = 7.9 Hz, HAr), 6.92 (s, 1 H, NH), 7.16 (d, 1 H, J = 6.6
Hz, HAr), 7.19–7.23 (m, 5 H, HAr), 7.45 (t, 1 H, J = 7.2 Hz, HAr),
7.53 (d, 1 H, J = 6.6 Hz, HAr), 7.68–7.77 (m, 2 H, HAr), 7.97 (s, 1
H, =CH), 8.04 (d, 1 H, J = 7.6 Hz, HAr). 13C NMR (75 MHz, DMSO-
d6): δ = 33.8, 46.2, 64.3, 114.8, 114.9, 117.5, 118.4, 121.4, 123.1,
124.9, 125.7, 126.1, 127.4, 128.3, 128.7, 133.1, 134.5, 138.9,
146.0, 153.2, 155.7, 162.1, 176.1. ESI-HRMS: m/z calcd for
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C
25H21N2O3 [M + H]+: 397.1545; found: 397.1545.
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Ghabraie, E. Org. Biomol. Chem. 2014, 12, 5757.
Colorless crystal (polyhedron), dimensions 0.270 × 0.150 ×
0.130 mm3, crystal system monoclinic, space group C2/c, Z = 8,
a = 21.6199(9) Å, b = 15.8126(7) Å, c = 12.0816(5) Å, α = 90°, β =
95.9595(18)°, γ = 90°, V = 4108.0(3) Å3, ρ = 1.282 g cm–3, T =
200(2) K, θmax = 25.661°, radiation Mo Kα, λ = 0.71073 Å, 0.5° ω
scans with CCD area detector, covering the asymmetric unit in
reciprocal space with a mean redundancy of 4.29 and a com-
pleteness of 98.6% to a resolution of 0.83 Å, 17018 reflections
measured, 3847 unique [R(int) = 0.0280], 2992 observed [I >
2σ(I)], intensities were corrected for Lorentz and polarization
effects, an empirical absorption correction was applied using
SADABS based on the Laue symmetry of the reciprocal space, μ
= 0.09 mm–1, Tmin = 0.89, Tmax = 0.96, structure refined against F2
with a full-matrix least-squares algorithm using the SHELXL
(Version 2014-3) software, 275 parameters refined, hydrogen
atoms were treated using appropriate riding models, except H6
(11) (a) Mhaske, S. B.; Argade, N. P. Tetrahedron 2006, 62, 9787.
(b) Zhou, J.; Fang, J. J. Org. Chem. 2011, 76, 7730. (c) Granger, B.
A.; Kaneda, K.; Martin, S. F. Org. Lett. 2011, 13, 4542. (d) Sigel, E.
Med. Chem. Rev. 2005, 2, 251. (e) Hester, J. B. Jr. US 3,987,052,
1969. (f) Keller, O.; Steiger, N.; Sternbach, L. H. US 3,442,946,
1969. (g) Sharpless, K. B.; Manetsch, R. Expert Opin. Drug Discov-
ery 2006, 1, 525. (h) Mohapatra, D. K.; Maity, P. K.; Shabab, M.;
Khan, M. I. Bioorg. Med. Chem. Lett. 2009, 19, 5241. (i) Michael, J.
P. Nat. Prod. Rep. 2003, 20, 476. (j) Bandekar, P. P.; Roopnarine,
K. A.; Parekh, V. J.; Mitchell, T. R.; Novak, M. J.; Sinden, R. R.
J. Med. Chem. 2010, 53, 3558. (k) Bhattacharjee, A. K.; Skanchy,
D. J.; Jennings, B.; Hudson, T. H.; Brendle, J. J.; Werbovetz, K. A.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2015, 26, A–G

G
S. Balalaie et al.
Letter
Synlett
at N6, which was refined isotropically, goodness of fit 1.03 for
observed reflections, final residual values R1(F) = 0.038, wR(F2)
= 0.090 for observed reflections, residual electron density –0.17
to 0.18 eÅ–3. CCDC 1057685 contains the supplementary crys-
tallographic data for this paper. These data can be obtained free
of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
N-Benzyl-2-[5-(2-hydroxybenzoyl)-2-oxopyridin-1(2H)-
yl]benzamide (8b)
Yellow oil; 314 mg (74%). IR (KBr): ν = 3263, 1677, 1658 cm–1
.
1H NMR (300 MHz, DMSO-d6): δ = 4.31 (dd, 1 H, J = 15.0, 8.0 Hz,
CH2), 4.35 (dd, 1 H, J = 15.0, 6.0 Hz, CH2), 6.53 (d, 1 H, J = 9.5 Hz,
HAr), 6.88 (t, 1 H, J = 9.5 Hz, HAr), 6.94 (d, 1 H, J = 7.8 Hz, HAr),
7.18–7.30 (m, 5 H, HAr), 7.35 (t, 1 H, J = 7.4 Hz, HAr), 7.44 (d, 1
H, J = 7.4 Hz, HAr), 7.55–7.67 (m, 3 H, HAr), 7.85 (d, 1 H, J = 2.5
Hz, HAr), 7.89 (dd, 1 H, J = 8.7, 2.5 Hz, HAr), 9.00 (t, 1 H, J = 5.9
Hz, NH), 10.29 (s, 1 H, OH). 13C NMR (75 MHz, DMSO-d6): δ =
54.9, 116.3, 116.6, 116.7, 119.2, 119.4, 124.4, 124.5, 126.7,
127.0, 128.2, 128.4, 129.3, 129.9, 131.2, 132.8, 134.2, 138.1,
139.1, 139.2, 146.4, 155.8, 156.0, 161.3, 165.9, 191.6. ESI-MS:
m/z = 425.0 [M + H]+. Anal. Calcd. for C26H20N2O4: C, 73.57; H,
4.75; N, 6.60. Found: C, 73.49; H, 4.79; N, 6.64.
General Procedure for the Synthesis of 8
To a solution of chromonyl Meldrum’s acid (3, 1 mmol) in EtOH
(10 mL) was added product 6 (1 mmol) and K2CO3 (30 mol%, 50
mg), and the mixture was heated at 70 °C for 12 h, monitoring
progress of reaction by TLC (eluent: EtOAc–n-hexane, 1:3). The
EtOH was removed under vacuum, and further purification was
carried out using preparative TLC (n-hexane–EtOAc, 2:1). The
products were obtained as yellow oils.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2015, 26, A–G