Chloranil as an Efficient Oxidant of the Cyclohexadiene Intermediates Formed by a Cycloaddition of Substituted 2H-Pyran-2-ones and Styrenes En Route to the Boscalid Derivatives

Article abstract of DOI:10.1002/jhet.2603

We describe a three-step one-pot metal-free domino procedure yielding a set of boscalid derivatives, parent compound being a highly important agrochemical agent. The first step of the process consists of the Diels–Alder reaction between a substituted 2H-pyran-2-one and styrene derivatives, followed by the elimination of CO₂and aromatization (oxidation). The last step was found to be efficiently promoted by the application of chloranil as the oxidant. Alternatively, activated carbon Darco KB could also act as the dehydrogenation catalyst necessitating a larger excess of the styrene to act as the hydrogen acceptor.

Full text of DOI:10.1002/jhet.2603

Month 2016
Chloranil as an Efficient Oxidant of the Cyclohexadiene Intermediates
Formed by a Cycloaddition of Substituted 2H-Pyran-2-ones and Styrenes
En Route to the Boscalid Derivatives
Jasmin Suljagić,a,b Amadej Juranovič, Marko Krivec, Krištof Kranjc, and Marijan Kočevar
a
a
a
a
*
*
aFaculty of Chemistry and Chemical Technology, University of Ljubljana, Večna pot 113, SI-1000 Ljubljana, Slovenia
bOn leave from: Faculty of Technology, University of Tuzla, Univerzitetska 8, BA-75000 Tuzla, Bosnia and Herzegovina
*
E-mail: marijan.kocevar@fkkt.uni-lj.si; kristof.kranjc@fkkt.uni-lj.si
Received October 21, 2015
DOI 10.1002/jhet.2603
Published online 00 Month 2016 in Wiley Online Library (wileyonlinelibrary.com).
We describe a three-step one-pot metal-free domino procedure yielding a set of boscalid derivatives, parent
compound being a highly important agrochemical agent. The first step of the process consists of the Diels–Alder
reaction between a substituted 2H-pyran-2-one and styrene derivatives, followed by the elimination of CO2 and
aromatization (oxidation). The last step was found to be efficiently promoted by the application of chloranil as
the oxidant. Alternatively, activated carbon Darco KB could also act as the dehydrogenation catalyst necessitating
a larger excess of the styrene to act as the hydrogen acceptor.
J. Heterocyclic Chem., 00, 00 (2016).
INTRODUCTION
ones as dienes for the Diels–Alder reaction with pheny-
lacetylene, 4-chlorophenylacetylene, and some other ana-
logues (such as vinyl ethers acting as masked acetylenes)
[13,14] as means for the preparation of various biphenyl
systems additionally containing acylamino group. This
strategy was applicable also for the preparation of a methyl
derivative of boscalid, as well as for some other deriva-
tives. However, substituted acetylenes being rather expen-
sive and many of them commercially not available, we
decided to explore the other possible pathway: instead of
acetylenes to use styrenes as dienophiles (acting as syn-
thetic substitutes). Of course, such an approach needs an
additional synthetic step; the primary intermediate formed
upon the cycloaddition of 2H-pyran-2-one derivatives and
styrenes (i.e., oxabicyclo[2.2.2]octenes), followed by the
elimination of a molecule of CO2 (via a retro-hetero-Di-
els–Alder reaction) is of a cyclohexadiene structure, there-
fore, necessitating an oxidation step that leads to the final
aromatic product. It is obvious that this last step is not
The quest for the preparation of novel plant-protecting
compounds is an ongoing task, as the resistance of plant
pathogens to the established treatments is often evolving
rather rapidly. The case of boscalid [1–6], a well-known
fungicide widely applied against various ascomycete path-
ogens damaging commercially grown fruit and vegetable
plants is no exception in this regard, spurring the recent in-
troduction of its novel derivatives Xemium and Bixafen,
by BASF and Bayer, respectively. In this connection, we
would like to report our recent results from the field of syn-
thesis of various biphenyl–benzamide systems, including
such that closely resemble the parent boscalid compound.
2H-Pyran-2-ones represent an easily accessible and ver-
satile heterocyclic system that is a convenient starting point
for the preparation of plethora of important and useful
compounds [7,8]. Previously [9–12], we have reported
the application of substituted 3-acylamino-2H-pyran-2-
© 2016 Wiley Periodicals, Inc.

J. Suljagić, A. Juranovič, M. Krivec, K. Kranjc, and M. Kočevar
Vol 000
needed when acetylene derivative acts as the dienophile,
because in such case after the elimination of CO2, aromatic
system is formed directly.
active charcoal Darco KB (as the heterogeneous dehydro-
genation catalyst with the styrenes 2 acting as the sink for
hydrogen) or chloranil. In addition, we also investigated
the possibility of the use of some other co-oxidants (e.
g., KBrO3, V2O5, or SeO2) in conjunction with chloranil.
All reactions have been carried out under microwave irra-
diation in closed vessels. In the case of heterogeneous ca-
talysis, we have conducted the reactions in closed thick-
walled ACE glass tubes with the application of conven-
tional heating (oil bath).
For this oxidation step, we (and others) have already suc-
cessfully employed active charcoal Darco KB [9,15–17] as
a heterogeneous dehydrogenation catalyst (in such case,
the excess of the styrene was acting as the oxidant, pos-
sibly in conjunction with the acceptorless pathway of the
dehydrogenation taking place) [18–21]. Alternatively,
this dehydrogenation could also be achieved by the appli-
cation of chloranil (tetrachloro-1,4-benzoquinone) as
proved previously [9]. Herein, we report on the applica-
tion of chloranil for the preparation of an extended set
of boscalid derivatives via cycloaddition of styrenes as
dienophiles and a wider set of 3-aroylamino-2H-pyran-
2-ones as dienes in acetonitrile under microwave irradia-
tion. Our main emphasis was on the investigation of 2H-
pyran-2-one derivatives being unsubstituted at position 5
or, alternatively, containing an electron donating substi-
tuent at this position.
Application of Darco KB as the dehydrogenation
catalyst.
One of our aims was to investigate the
reactivity of electron-rich starting 2H-pyran-2-ones 1a,b
(i.e., those possessing a 3-(4-methoxyphenyl)- or 3-(3,4-
dimethoxyphenyl)- groups) for the cycloaddition of
styrenes 2 with the active charcoal Darco KB as the
dehydrogenation catalyst. In this regard, we first
conducted a set of reactions between 4-methoxyphenyl
derivative 1a (0.5mmol) and styrene (2a) (2.4mmol) in
toluene (4mL) with the addition of Darco KB (75mg).
The reactions were conducted in closed ACE glass tubes
with heating on an oil bath for 2h at various temperatures
(160°C, 180°C, and 200°C) (Table 1, Entries 1–3). With
RESULTS AND DISCUSSION
1
the increase of temperature, it was evident from the H
Besides the previously applied dienophiles, styrene (2a),
and 4-chlorostyrene (2b), here we additionally investigated
4-methylstyrene (2c) as the dienophile for the Diels–Alder
reaction with substituted 6-methyl-2H-pyran-2-ones 1
possessing at the position 5 electron-donating groups (i.
e., 4-MeO-C6H4- (1a) or 3,4-(MeO)2C6H3- (1b)), and for
comparison, we investigated also the reactivity of a deriv-
ative possessing an electron-withdrawing group (ÀCO2Et)
(1c) at the position 5 or, alternatively, having a hydrogen
(1d–h) at this position. Additionally, all 2H-pyran-2-ones
1 were substituted by various 3-aroylamino groups, with
aryl moieties including phenyl (1a,b,h), 3,4,5-trime-
thoxyphenyl (1c), various chloropyridyl groups (1d,e),
4-nitrophenyl (1f) or 4-dimethylaminophenyl group (1 g).
For the oxidation step (i.e., 4→ 5), we applied either
NMR spectra of crude reaction mixtures that the
conversion toward 5a [11] increases as well, and that at
the highest temperature it reaches 100% (Table 1, Entry 3).
On the other hand, when the amount of the dienophile 2a
was decreased from 2.4 to 1.8mmol under otherwise
identical conditions as in Entry 3 (Table 1), the reaction
was again not complete (1a:5a=0.2:1) (Table 1, Entry 4).
For easier isolation of the products, in these experiments
toluene was used instead of the decalin as applied
previously [9].
Similar to the results with styrene were also those ob-
tained for the cycloaddition of 4-chlorostyrene (2b); when
heating 1a (0.5 mmol) with 2b (2.4 mmol) in toluene with
the addition of Darco KB (75 mg) in closed ACE glass tube
at 200°C for 2 h, the conversion toward 5b also reached
Table 1
Effect of different reaction conditions on the formation of 5 from 1 (0.5 mmol) and 2 (2.4 mmol) in closed ACE glass tubes with Darco KB (75 mg) as the
dehydrogenation catalyst in toluene (4 mL).
Starting 2H-pyran-2-ones 1
Dienophiles 2
Temp.
(°C)
Time
(h)
Ratio 1:
Product (isolated
yield, %)
Entry
R1
Ar1
Ar2
5a
1
2
3
4
5
6
4-MeOC6H4
4-MeOC6H4
4-MeOC6H4
4-MeOC6H4
4-MeOC6H4
3,4-(MeO)2C6H3
Ph
Ph
Ph
Ph
Ph
Ph
1a
1a
1a
1a
1a
1b
Ph
Ph
Ph
2a
160
180
200
200
200
200
2
2
2
2
2
2
2.3: 1
0.4: 1
0: 1
0.2: 1
0: 1
5a
5a
5a (62)
5a
5b (70)
5d (53)
2a
2a
2ab
2b
2a
Ph
4-Cl-C6H4
Ph
0: 1
1
aData estimated from methyl signals in H NMR spectra of the crude reaction mixture.
bWith 1.8 mmol of 2a.
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

Month 2016
Chloranil as an Oxidant of the Cyclohexadienes En Route to the Boscalid Derivatives
100% (Table 1, Entry 5). Analogous were the results in the
case of starting 1b (0.5 mmol) and styrene (2a) (2.4 mmol)
as the dienophile, where 2 h of heating at 200°C (with the
addition of Darco KB (75 mg)) was necessary for the syn-
thesis of 5d to be complete (Table 1, Entry 6). In this way,
we prepared three boscalid derivatives 5a,b,d with satis-
factory isolated yields (53–70%). However, to shorten the
reaction times and to increase the conversions, a large ex-
cess of the dienophiles 2 was needed (4.8 eq.). This is un-
derstandable, as one has to take into account that styrenes
2 act not only as dienophiles but also as the sacrificial hy-
drogen acceptors after the dehydrogenation of 4 takes place
by the assistance of the heterogeneous catalyst Darco KB.
However, one has to keep in mind that at least a part of
the hydrogen liberated during this dehydrogenation might
take place via the acceptor-less mechanism and therefore
does not need the spare styrenes 2. It is also important to
stress that efficient isolation of the products from the reac-
tion mixtures containing Darco KB as the catalyst is only
possible by continuous extraction (with the Soxhlet appara-
tus), otherwise there are substantial losses of the product
due to its adsorption on the surface of the active carbon.
being increased to 1.2 mmol (for 0.5 mmol 1a and with
all other parameters being the same as previously
mentioned) (Table 2, Entry 3). In this way, it was
possible to prepare the product 5b in good isolated yield
(78%); here the result was better as that obtained with
Darco KB (70% yield of 5b) (Table 1, Entry 5).
Similar were also results with the starting 2H-pyran-2-one
1b (0.5 mmol) upon the cycloaddition of styrene (2a)
(0.6 mmol or 1.2 mmol, Table 2) conducted under micro-
wave irradiation in a closed vessel with chloranil
(0.5 mmol) as the oxidant and acetonitrile (1.5 mL) as the
solvent. After 2 h of irradiation at 140°C with 0.6 mmol of
2a being applied, the conversion toward 5d was only
around 50% (1b:5d =1:1) (Table 2, Entry 4). Upon the in-
crease of the reaction time to 3 h and increase of the amount
of 2a to 1.2 mmol, the reaction could be completed (Table 2,
Entry 5) and not a trace of the starting 1b could be detected
by 1H NMR spectrum of the crude reaction mixture. In this
case, the isolated yield of 5d reached by the application of
chloranil was also higher than the yield obtained with Darco
KB (74% vs 53%).
In both of the examples described earlier (i.e., the prep-
aration of 5b,d), it is of interest to note that in the case of
chloranil acting as the oxidant the amount of dienophiles
2 necessary for the complete conversion of 1 to 5 could
be halved (from 4.8 eq. to 2.4 eq. of 2), the reaction temper-
ature could be decreased (from 200°C oil bath to the micro-
wave irradiation at the temperature set to 140°C), albeit
with an increase of reaction time (from 2 to 3 h) (Table 1,
Entries 5 and 6 vs Table 2, Entries 3 and 5). All these
changes of reaction conditions are presumably a direct con-
sequence of the change in the dehydrogenation step
(4 → 5); in the cases of the chloranil acting as the oxidant,
the dehydrogenation takes places under milder conditions
than is the case of Darco KB, which is acting merely as
the dehydrogenation catalyst, whereas the actual oxidant
is represented by the excess of the dienophiles (styrenes
2) (Scheme 1).
Application of chloranil as the oxidant.
The other
option for the dehydrogenation of 4 to yield the final
boscalid-like products 5 is an application of a suitable
oxidant. Previously, we have shown that chloranil is one
of the possible choices; therefore, we decided to re-
examine the reactions between 1a and 4-chlorostyrene
(2b). In a closed vessel, a mixture of the starting 1a
(0.5 mmol), 4-chlorostyrene (2b) (0.6 mmol), chloranil
(0.5 mmol), and acetonitrile (1.5 mL) was irradiated in a
focused microwave reactor at 140°C for 2 h. 1H NMR
analysis of the crude reaction mixture showed that some
of the starting 1a remained unreacted (1a:5b =0.25:1)
(Table 2, Entry 1). When the reaction was repeated with
a longer reaction time (3 h) (Table 2, Entry 2) and other
reaction parameters being identical as previously
mentioned, the conversion was increased, but the reaction
was still not completed (1a:5b= 0.2:1). The complete
conversion toward 5b was achieved only after 3 h of
microwave irradiation and with the amount of styrene 2b
To the reaction mixtures, a small amount of BHT (2,6-
di-tert-butyl-4-methylphenol) (1.8 mol%) was also added
[9], acting as a radical scavanger to diminish the extent
Table 2
Effect of different reaction conditions on the formation of 5 from 1 (0.5 mmol) and 2 in closed vessels under microwave irradiation at 140°C with chloranil
(0.5 mmol) as the oxidant, BHT (1.8 mol%) and in acetonitrile (1.5 mL) as the solvent.
Starting 2H-pyran-2-ones 1
Dienophiles 2
Amount
of 2
(mmol)
Time
(h)
Ratio 1:
Product (isolated
yield, %)
Entry
R1
Ar1
Ar2
Oxidant
5a
1
2
3
4
5
4-MeOC6H4
4-MeOC6H4
4-MeOC6H4
3,4-(MeO)2C6H3
3,4-(MeO)2C6H3
Ph
Ph
Ph
Ph
Ph
1a
1a
1a
1b
1b
4-Cl-C6H4
2b
2b
2b
2a
2a
0.6
0.6
1.2
0.6
1.2
chloranil
chloranil
chloranil
chloranil
chloranil
2
3
3
2
3
0.25: 1
0.2: 1
0: 1
1: 1
0: 1
5b
5b
5b (78)
5d
5d (74)
4-Cl-C6H4
4-Cl-C6H4
Ph
Ph
1
aData estimated from methyl signals in H NMR spectra of the crude reaction mixture.
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

J. Suljagić, A. Juranovič, M. Krivec, K. Kranjc, and M. Kočevar
Vol 000
Scheme 1. Reaction sequence for the preparation of boscalid derivatives 5 starting from 2H-pyran-2-ones 1 and either styrenes 2 or acetylenes 6 acting as
the dienophiles.
of unwanted radical-initiated polymerizations of styrenes
2.
of the multiple bond (such as N,N-diethylpropynamine
and ethyl but-2-ynoat) yield both regioisomers. In some
cases, the regioselectivity was also corroborated by HMBC
NMR studies showing the correlation between 5-Me and 6-
H groups (that would not appear if the regioselectivity was
reversed) [11].
When the reaction mixture is heated by the microwave
irradiation, it is also important to note that its actual tem-
perature could be higher than the measured one, as the
measurement is conducted on the outer wall of the reaction
vessel, whereas the reaction sample is heated in situ by the
direct interaction of the microwave irradiation and polar
and/or ionic species [24].
Besides the products 5b,d, prepared as described earlier
(Table 2, Entries 3 and 5), the conditions elucidated earlier
have been found to be appropriate for successful prepara-
tion of further cycloadducts 5c,e–h,j–o. For comparison,
the product 5i was prepared under neat microwave condi-
tions (with the liquid additive n-BuOH) [25] by the
In all cases described herein, the cycloaddition step is
regiospecific yielding after the complete domino reaction
only the products 5 having the 1,4-arrangement of the hy-
drogen atoms 3-H and 6-H on the aromatic ring. None of
the other regioisomers, that is, structures with 1,3-
arrangement of these two hydrogen atoms, have been ob-
1
served. This is clearly evident from the H NMR spectra
of products 5a–g showing singlet signals for 3-H and 6-
H, consistent with the smallest coupling constants existing
between the 1,4 arranged protons on the benzene rings
(J =0–1 Hz) [11], whereas in the other case two doublets
should appear. Furthermore, these findings are also in
agreement with our previous results [10,11,22,23] that cy-
cloadditions of unsymmetrically substituted dienophiles
with a hydrogen substituent on one side of the multiple
bond generally proceed in a regioselective manner,
whereas dienophiles containing substituents on both sides
Table 3
Reaction conditions and yields for the synthesis of 5a.
Starting 2H-pyran-2-ones 1
Ar1
Dienophiles 2 (or 6)
Time
(h)
Isolated yield
(%)b
Entry
R1
Ar2
Product 5
1
2
3
4
5
6
7
8
4-MeOC6H4
3,4-(MeO)2C6H3
3,4-(MeO)2C6H3
CO2Et
H
H
H
H
H
Ph
Ph
Ph
Ph
1a
1b
1b
1c
1d
1e
1f
1f
1 g
1 g
1 h
1 h
4-Me-C6H4
4-Cl-C6H4
4-Me-C6H4
4-Me-C6H4
4-Cl-C6H4
Ph
2c
2b
2c
2c
2b
6a
2a
2b
2a
2b
2a
2b
5c
5e
5f
5 g
5 h
5i
5j
5 k
5 l
5 m
5n
5o
3
3
3
3
3
1c
3
3
3
3
3
3
75
71
72
82
60
90
75
68
74
76
70
76
3,4,5-(MeO)3C6H2
2-Cl-nicotinoyl
6-Cl-nicotinoyl
6-Cl-nicotinoyl
4-O2NC6H4
4-O2NC6H4
4-(NMe2)C6H4
4-(NMe2)C6H4
Ph
4-Cl-C6H4
Ph
4-Cl-C6H4
Ph
9
10
11
12
H
H
H
4-Cl-C6H4
aReaction conditions: 2H-pyran-2-ones 1 (1 mmol), dienophiles 2 (2.4 mmol) in acetonitrile (1.5 mL) with the addition of chloranil (1 mmol) and BHT
(1.8 mol%) under microwave irradiation at 140°C (140 W).
bIsolated yield.
cDienophile 6a (1.2 mmol) and n-BuOH (100 mg) as the additive under microwave irradiation at 180°C (150 W).
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

Month 2016
Chloranil as an Oxidant of the Cyclohexadienes En Route to the Boscalid Derivatives
cycloaddition of phenylacetylene (6a) instead of styrene
(2a), therefore eliminating the need of applying chloranil.
All of the products 5b–o thus prepared represent novel de-
rivatives of boscalid and were isolated in good yields (60–
90%) (Table 3).
against the central line of the solvent signal (CDCl3 triplet
at 77.0ppm or DMSO-d6 septet at 39.5 ppm). The coupling
constants (J) are given in Hertz. IR spectra were obtained
with a BioRad (Hercules, CA, USA) FTS 3000MX spec-
trometer as KBr pellets or with a Bruker (Ettlingen, Ger-
many) Alpha Platinum ATR spectrometer on a solid
support. MS spectra were recorded with a VG Analytical
(Wythenshawe, UK) AutoSpec Q and Waters-Micromass
(Milford, MA, USA) Q-TOF Premier spectrometers. Ele-
mental analyses (C, H, N) were conducted using a
Perkin–Elmer (Waltham, MA, USA) 2400 series II
CHNS/O analyzer. The starting compounds 1a–h were
prepared according to the published procedures: the syn-
thesis of 1a–c starts from appropriate compounds contain-
ing activated CH2 groups (i.e., 4-methoxyphenylacetone,
3,4-dimethoxyphenylacetone or ethyl acetoacetate), C1-
synthon (DMFDMA) and hippuric acid as described previ-
ously [26–28]. Starting 1d–h are obtained from the parent
3-benzoylamino compounds (prepared as described ear-
lier), followed by the removal of the benzoyl group and
re-protecting the amine functionality with another aroyl
group, as described [29,30]. All other reagents and solvents
were used as received from commercial suppliers.
The application of other mixed oxidation systems, where
an appropriate co-oxidant would decrease the amount of
the necessary chloranil was unfortunately not successful.
We investigated the cycloaddition between the starting
2H-pyran-2-one 1a (1mmol) and styrene (2) (2.4 mmol)
in acetonitrile solvent (1.5 mL) with the addition of
chloranil (0.1 mmol) and a suitable co-oxidant (KBrO3,
V2O5 or SeO2) (0.9 mmol). The reaction mixtures were
heated under reflux condition for 3 h, and thereafter, the
crude reaction mixtures were analyzed by 1H NMR analy-
sis. In all three cases, no conversion toward 5a was found,
in the complex reaction mixture only the starting 1a was
detected (together with the compounds stemming from ox-
idative degradation and/or polymerization side reactions).
CONCLUSION
The application of suitable styrenes 2 (economically
more viable than their corresponding substituted
phenylacetylenes 6) acting as dienophiles for the Diels–
Alder reaction with easily obtainable substituted 2H-py-
ran-2-ones 1 was found to be the most appropriate way to
prepare derivatives of boscalid, that is, a set of compounds
5a–o, obtained in good yields (60–90%) via a three-step
one-pot metal-free domino procedure. When the
substituted aromatic products 5 are desired, the importance
of the last reaction step is crucial; the boscalid derivatives
can only be obtained after the oxidation (dehydrogenation)
of the intermediary-formed cyclohexadienes 4. The most
efficient reagent for this transformation was found to be
chloranil, closely followed by Darco KB as a heteroge-
neous dehydrogenation catalyst (with the excessive
amount of the styrenes acting as the actual hydrogen scav-
engers). Other oxidative systems (chloranil together with
an inorganic co-oxidant) were not found to be appropriate
under the applied conditions.
Microwave reactions were conducted in air using a fo-
cused microwave unit (Discover by CEM Corporation,
Matthews, NC). The machine consists of a continuous, fo-
cused microwave power-delivery system with an operator-
selectable power output from 0 to 300W. Reactions were
performed in darkness in glass vessels (capacity 10mL)
sealed with the septum. The temperature of the contents
of the vessel was monitored using a calibrated infrared
temperature controller mounted under the reaction vessel
and measuring the temperature of the outer surface of the
reaction vessel. The mixtures were stirred with a Teflon-
coated magnetic stirring bar in the vessel. Temperature,
pressure, and power profiles were recorded using commer-
cially available software provided by the manufacturer of
the microwave unit.
General procedure for the synthesis of 5a–o with the
application of chloranil as the oxidant. A mixture of the
starting 2H-pyran-2-one 1a–h (1 mmol), the corresponding
substituted styrene 2a–c (2.4 mmol), chloranil (tetrachloro-
1,4-benzoquinone) (1 mmol, 246mg), and BHT (4mg,
1.8 mol%) in acetonitrile (1.5 mL) was irradiated in the
focused monomode microwave equipment for 3 h
(Table 2, Entries 3 and 5 and Table 3). The power was
set to 140W, the final temperature to 140°C, and ramp
time to 5 min. Thereafter, the reaction mixture was
cooled, diluted with CH2Cl2 (100mL), and washed with
5% NaOH solution (2× 30mL). The organic phase was
dried over Na2SO4, and the volatile components were
removed in vacuo yielding crude products 5a–o.
Products 5a,h–o were crystallized from the appropriate
solvents, whereas 5b–g were purified by column
EXPERIMENTAL
Melting points were determined on a micro hot stage ap-
1
paratus. H NMR spectra were recorded at 29°C with a
Bruker (Rheinstein, Germany) Avance DPX 300 spectrom-
eter at 300MHz or Bruker Avance III spectrometer at
500MHz instruments using TMS as an internal standard.
13C NMR spectra were recorded at 29°C with the Bruker
Avance DPX 300 spectrometer at 75.5MHz or Bruker
Avance III spectrometer at 125.5 MHz and were referenced
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

J. Suljagić, A. Juranovič, M. Krivec, K. Kranjc, and M. Kočevar
Vol 000
chromatography (SiO2, elution with petroleum
ether/EtOAc, 3:1). Fractions containing products were
combined and evaporated under vacuo; the residue was
treated with a mixture of petroleum ether and EtOAc
(3:1) (0.5 mL), and the remaining solid was collected by
filtration under reduced pressure to yield pure products
5b–g. For the analysis, all of the products were
crystallized from the appropriate solvent.
132.6, 134.2, 134.9, 138.1, 142.0, 148.2, 148.6,
165.1 ppm (1 signal hidden); ms (ESI+, TOF): m/z 424
([M + H]+). Anal. Calcd for C28H25NO3: C, 79.41; H,
5.95; N, 3.31. Found: C, 79.15; H, 6.05; N, 3.20.
N-(4-Chloro-3’’,4’’-dimethoxy-5’-methyl-[1,1’:4’,1’’-terphenyl]-
2’-yl)benzamide (5e).
Pale yellow solid, yield 71%,
325 mg, purification by column chromatography (elution
with petroleum ether: EtOAc 3:1), mp 214–217°C; ir:
N-(4’’-Methoxy-5’-methyl-[1,1’:4’,1’’-terphenyl]-2’-yl)benzamide
3228, 1735, 1634, 1554, 1509, 1452 cmÀ1 1H nmr
;
(5a) [11].
Off-white solid, yield 62%, 122 mg,
(500 MHz, CDCl3): δ 2.32 (s, 3H, Me), 3.92 (s, 3H,
MeO), 3.94 (s, 3H, MeO), 6.95 (m, 3H, Ar), 7.18 (s, 1H,
6-H), 7.42 (m, 4H, Ar), 7.50 (m, 3H, Ar), 7.64 (m, 2H,
Ar), 7.80 (br s, 1H, NH), 8.30 (s, 1H, 3-H) ppm; 13C nmr
(125 MHz, CDCl3): δ 20.3, 56.10, 56.11, 111.0, 112.7,
121.7, 123.8, 126.9, 129.0, 129.5, 130.7, 130.8, 131.97,
132.01, 132.3, 132.4, 134.0, 134.3, 134.8, 136.6, 142.4,
148.3, 148.6, 165.3 ppm; ms (ESI+, TOF): m/z 458 ([M
+ H]+). Anal. Calcd for C28H24ClNO3 × 0.25 H2O: C,
72.72; H, 5.34; N, 3.03. Found: C, 72.81; H, 5.06; N, 2.95.
N-(3’’,4’’-Dimethoxy-4,5’-dimethyl-[1,1’:4’,1’’-terphenyl]-2’-yl)
recrystallization from EtOH, mp 129–130°C (EtOH), lit.
[11] mp 129–130°C (EtOH); ir: 1671, 1609, 1579, 1561,
1525, 1504, 1490 cmÀ1
.
N-(4-Chloro-4’’-methoxy-5’-methyl-[1,1’:4’,1’’-terphenyl]-2’-yl)
benzamide (5b). Pale yellow solid, yield 78%, 166 mg,
purification by column chromatography (elution with
petroleum ether: EtOAc 3:1), mp 198–201°C; ir: 3438,
1666, 1607, 1579, 1569, 1273 cmÀ1; H nmr (300MHz,
1
CDCl3): δ 2.31 (s, 3H, Me), 3.87 (s, 3H, MeO), 6.97 and
7.35 (AA’XX’, J= 8.5 Hz, 2H each, C6H4), 7.17 (s, 1H,
6-H), 7.43 (m, 4H, C6H4), 7.49 (m, 3H, Ph), 7.63 (m,
2H, Ph), 7.79 (br s, 1H, NH), 8.29 (s, 1H, 3-H) ppm; 13C
nmr (125 MHz, CDCl3): δ 20.3, 55.5, 113.7, 123.9,
126.9, 129.0, 129.5, 130.49, 130.55, 130.8, 131.9, 132.0,
132.3, 132.4, 133.6, 134.2, 134.8, 136.7, 142.2, 158.9,
165.2 ppm; ms (ESI+, TOF): m/z 428 ([M+ H]+). Anal.
Calcd for C27H22ClNO2 × 0.5 H2O: C, 74.22; H, 5.31; N,
3.21. Found: C, 74.35; H, 5.26; N, 3.10.
benzamide (5f).
White solid, yield 72%, 315 mg,
purification by column chromatography (elution with
petroleum ether: EtOAc 3:1), mp 146–149°C; ir: 3228,
1
1634, 1603, 1572, 1554, 1243 cmÀ1; H nmr (500 MHz,
CDCl3): δ 2.32 (s, 3H, Me), 2.44 (s, 3H, Me), 3.92 (s,
3H, MeO), 3.94 (s, 3H, MeO), 6.96 (m, 3H, Ar), 7.20 (s,
1H, 6-H), 7.32 (m, 2H, Ar), 7.39 (m, 4H, Ar), 7.48 (m,
1H, Ar), 7.63 (m, 2H, Ar), 8.00 (br s, 1H, NH), 8.42 (s,
1H, 3-H) ppm; 13C nmr (125MHz, CDCl3): δ 20.3, 21.4,
56.1, 111.0, 112.8, 121.7, 122.8, 127.0, 128.9, 129.3,
130.0, 131.4, 131.7, 131.8, 132.1, 132.7, 134.3, 135.0,
138.1, 141.8, 148.2, 148.6, 165.1 ppm (2 signals hidden);
ms (ESI+, TOF): m/z 438 ([M + H]+). Anal. Calcd for
C29H27NO3 × 0.33 H2O: C, 78.54; H, 6.29; N, 3.16.
Found: C, 78.44; H, 6.03; N, 3.09.
N-(4’’-Methoxy-4,5’-dimethyl-[1,1’:4’,1’’-terphenyl]-2’-yl)
benzamide (5c). Pale yellow solid, yield 75%, 306 mg,
purification by column chromatography (elution with
petroleum ether: EtOAc 3:1), mp 147–150°C; ir: 3436,
1668, 1608, 1561, 1526, 1274 cmÀ1; H nmr (500MHz,
1
CDCl3): δ 2.31 (s, 3H, Me), 2.44 (s, 3H, Me), 3.86 (s,
3H, MeO), 6.96 (m, 2H, Ar), 7.19 (s, 1H, 6-H), 7.32 (m,
2H, Ar), 7.38 (m, 6H, Ar), 7.47 (m, 1H, Ar), 7.63 (m,
2H, Ar), 8.00 (br s, 1H, NH), 8.40 (s, 1H, 3-H) ppm; 13C
nmr (125 MHz, CDCl3): δ 20.3, 21.4, 55.4, 113.6, 123.0,
127.0, 128.8, 129.3, 130.0, 130.5, 131.2, 131.70, 131.73,
132.1, 132.7, 133.9, 135.0, 135.1, 138.0, 141.6, 158.8,
165.0 ppm; ms (ESI+, TOF): m/z 408 ([M+ H]+). Anal.
Calcd for C28H25NO2 × 0.6 H2O: C, 80.39; H, 6.31; N,
3.35. Found: C, 80.44; H, 6.02; N, 3.29.
Ethyl 2-benzamido-4’,5-dimethyl-[1,1’-biphenyl]-4-carboxylate
(5 g). White solid, yield 82%, 305 mg, purification by
column chromatography (elution with petroleum ether:
EtOAc 3:1), mp 175–178°C; ir: 3220, 1666, 1643, 1567,
1
1540, 1220cmÀ1; H nmr (500 MHz, CDCl3): δ 1.43 (t,
J = 7.0 Hz, 3H, CH3CH2), 2.43 (s, 3H, Me), 2.61 (s, 3H,
Me), 4.39 (q, J = 7.0 Hz, 2H, CH3CH2), 7.17 (s, 1H, 6-H),
7.32 (m, 4H, Ar), 7.41 (m, 2H, Ar), 7.50 (m, 1H, Ar),
7.64 (m, 2H, Ar), 7.97 (br s, 1H, NH), 9.01 (s, 1H, 3-H)
ppm; 13C nmr (125 MHz, CDCl3): δ 14.5, 21.3, 21.4,
61.0, 123.5, 127.0, 128.9, 129.0, 129.7, 130.1, 131.9,
132.8, 133.4, 134.3, 134.8, 136.0, 136.2, 138.6, 165.2,
167.4 ppm; ms (ESI+, TOF): m/z 374 ([M + H]+). Anal.
Calcd for C24H23NO3: C, 77.19; H, 6.21; N, 3.75.
Found: C, 77.25; H, 6.10; N, 3.71.
N-(3’’,4’’-Dimethoxy-5’-methyl-[1,1’:4’,1’’-terphenyl]-2’-yl)
benzamide (5d).
White solid, yield 74%, 156 mg,
purification by column chromatography (elution with
petroleum ether: EtOAc 3:1), mp 218–221°C; ir: 3217,
1635, 1601, 1577, 1551, 1290 cmÀ1; H nmr (300MHz,
1
CDCl3): δ 2.32 (s, 3H, Me), 3.92 (s, 3H, MeO), 3.94 (s,
3H, MeO), 6.96 (m, 3H, Ar), 7.21 (s, 1H, 6-H), 7.38 (m,
2H, Ar), 7.50 (m, 6H, Ar), 7.60 (m, 2H, Ar), 7.94 (br s,
1H, NH), 8.42 (s, 1H, 3-H) ppm; 13C nmr (125MHz,
CDCl3): δ 20.3, 56.09, 56.10, 111.0, 112.8, 121.7, 122.9,
126.9, 128.2, 128.9, 129.3, 129.5, 131.5, 131.8, 132.0,
N-(4’-Chloro-5-methyl-[1,1’-biphenyl]-2-yl)-3,4,5-trime-
thoxybenzamide (5 h).
White solid, yield 60%, 248 mg,
recrystallization from i-Pr2O, mp 148.5–151°C; ir: 3252,
1
1639, 1585, 1526, 1504, 1493 cmÀ1; H nmr (300 MHz,
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

Month 2016
Chloranil as an Oxidant of the Cyclohexadienes En Route to the Boscalid Derivatives
CDCl3): δ 2.38 (s, 3H, Me), 3.80 (s, 6H, 3-MeO, 5-
MeO), 3.86 (s, 3H, 4-MeO), 6.77 (s, 2H, C6H2), 7.08
(d, J = 2.1 Hz, 1H, 6-H), 7.26 (dd, J1 =8.4Hz, J2 =2.1Hz,
1H, 4-H), 7.39 and 7.48 (AA’XX’, J=8.6Hz, 2H each,
C6H4), 7.68 (br s, 1H, NH), 8.33 (s, 1H, 3-H) ppm; 13C
nmr (75.5MHz, CDCl3): δ 20.8, 56.0, 60.8, 103.9, 121.5,
129.2, 129.4, 129.9, 130.1, 130.8, 131.4, 132.3, 134.0,
134.2, 136.9, 140.9, 153.1, 164.3ppm; ms (ESI–, TOF):
m/z 410 ([M–H]À). Anal. Calcd for C23H22ClNO4: C,
67.07; H, 5.38; N, 3.40. Found: C, 67.06; H, 5.27; N, 3.46.
2-Chloro-N-(5-methyl-[1,1’-biphenyl]-2-yl)nicotinamide (5i).
Prepared from 1e (1 mmol) and 6a (1.2 mmol), with n-
BuOH (100 mg), under microwave irradiation at 180°C
(150 W), without chloranil. Pale yellow solid, yield 90%,
290mg, recrystallization from i-Pr2O, mp 117–120°C; ir:
147.6, 154.5, 162.3ppm; ms (ESI+, TOF): m/z 357 ([M
+H]+). Anal. Calcd for C19H14Cl2N2O: C, 63.88; H,
3.95; N, 7.84. Found: C, 64.06; H, 4.18; N, 7.69.
N-(5-Methyl-[1,1’-biphenyl]-2-yl)-4-nitrobenzamide
(5 l).
Pale brown solid, yield 74%, 245mg, recrystallization
from EtOH, mp 149–152°C; ir: 3252, 1647, 1601, 1528,
1
1487 cmÀ1; H nmr (300MHz, CDCl3): δ 2.40 (s, 3H,
Me), 7.14 (d, J = 2.1 Hz, 1H, 6-H), 7.26 (dd, J1 =8.1Hz,
J2 = 2.1 Hz, 1H, 4-H), 7.40 À 7.54 (m, 5H, Ar), 7.74 and
8.23 (AA’XX’, J =8.7 Hz, 2H each, C6H4), 7.90 (br s,
1H, NH), 8.32 (d, J = 8.1 Hz, 1H, 3-H) ppm; 13C nmr
(75.5 MHz, CDCl3): δ 20.9, 121.7, 123.8, 127.9, 128.2,
129.08, 129.13, 130.62, 131.6, 133.1, 135.0, 137.9,
140.3, 149.5, 162.8ppm (1 signal hidden); ms (ESI–,
TOF): m/z 331 ([M–H]À). hrms calcd for C20H15N2O3
(M–H) 331.1083. Found 331.1088.
1
3242, 1672, 1656, 1587, 1580cmÀ1; H nmr (300MHz,
CDCl3): δ 2.39 (s, 3H, Me), 7.12 (d, J= 1.4 Hz, 1H, 6-
H), 7.25 (dd, J1 = 8.4 Hz, J2 = 1.4 Hz, 1H, 4-H), 7.32
(dd, J1 =7.7Hz, J2 =4.7 Hz, 1H, 5’-H), 7.40 (m, 5H,
Ar), 8.13 (dd, J1 = 7.7 Hz, J2 = 2.0 Hz, 1H, 4’-H or 6’-
H), 8.15 (br s, 1H, NH), 8.32 (d, J =8.4 Hz, 1H, 3-H),
8.42 (dd, J1 =4.7 Hz, J2 = 2.0 Hz, 1H, 4’-H or 6’-H)
ppm; 13C nmr (75.5 MHz, CDCl3): δ 20.9, 121.8, 122.7,
128.05, 128.95, 129.3, 130.7, 131.3, 131.8, 133.4,
134.8, 138.0, 139.9, 146.7, 151.0, 162.3ppm (1 signal
hidden); ms (ESI+, TOF): m/z 323 ([M+ H]+). Anal.
Calcd for C19H15ClN2O: C, 70.70; H, 4.68; N, 8.68.
Found: C, 70.92; H, 4.60; N, 8.57.
N-(4’-Chloro-5-methyl-[1,1’-biphenyl]-2-yl)-4-nitrobenzamide
(5 m).
Pale brown solid, yield 76%, 279 mg,
recrystallization from EtOH, mp 194–197°C; ir: 3233,
1639, 1599, 1523, 1485 cmÀ1 1H nmr (300 MHz,
;
CDCl3): δ 2.39 (s, 3H, Me), 7.10 (d, J = 2.0 Hz, 1H, 6-H),
7.26 (dd, J1 = 8.1 Hz, J2 = 2.0 Hz, 1H, 4-H), 7.35 and 7.47
(AA’XX’, J = 8.7 Hz, 2H each, C6H4), 7.76 (br s, 1H,
NH), 7.77 and 8.26 (AA’XX’, J= 8.7 Hz, 2H each,
C6H4), 8.20 (d, J = 8.1 Hz, 1H, 3-H) ppm; 13C nmr
(75.5 MHz, CDCl3): δ 20.9, 122.5, 124.0, 128.0, 129.4,
129.5, 130.5, 130.7, 131.4, 132.2, 134.4, 135.4, 136.5,
140.1, 149.7, 163.2 ppm; ms (ESI–, TOF): m/z 365
([M–H]À). Anal. Calcd for C20H15ClN2O3: C, 65.49; H,
4.12; N, 7.64. Found: C, 65.75; H, 4.11; N, 7.37.
6-Chloro-N-(5-methyl-[1,1’-biphenyl]-2-yl)nicotinamide (5j).
Pink-brown solid, yield 75%, 240mg, recrystallization
from i-Pr2O, mp 119–122°C; ir: 3275, 1643, 1589,
4-Dimethylamino-N-(5-methyl-[1,1’-biphenyl]-2-yl)benzamide
1
1524 cmÀ1; H nmr (300 MHz, CDCl3): δ 2.39 (s, 3H,
(5n).
Pale orange solid, yield 70%, 230 mg,
Me), 7.13 (d, J =2.0 Hz, 1H, 6-H), 7.24 (dd, J1 = 8.4 Hz,
J2 =2.0 Hz, 1H, 4-H), 7.45 (m, 6H, Ph, 5’-H), 7.82 (br s,
1H, NH), 7.98 (dd, J1 = 8.4 Hz, J2 = 2.4 Hz, 1H, 4’-H),
8.31 (d, J =8.4 Hz, 1H, 3-H), 8.46 (d, J= 2.4 Hz, 1H, 2’-
H) ppm; 13C nmr (75.5 MHz, CDCl3): δ 20.9, 121.7,
124.4, 128.2, 129.04, 129.06, 129.15, 129.4, 130.6,
131.5, 133.1, 134.9, 137.84, 137.88, 147.5, 154.2,
162.0 ppm; ms (ESI+, TOF): m/z 323 ([M + H]+). Anal.
Calcd for C19H15ClN2O: C, 70.70; H, 4.68; N, 8.68.
Found: C, 70.85; H, 4.94; N, 8.39.
recrystallization from i-Pr2O, mp 118–120°C; ir: 3427,
1
3317, 1665, 1636, 1607, 1511 cmÀ1; H nmr (300 MHz,
CDCl3): δ 2.36 (s, 3H, Me), 2.99 (s, 6H, NMe2), 6.60
(m, 2H, Ar), 7.09 (d, J =2.0 Hz, 1H, 6-H), 7.21 (dd,
J1 = 8.4 Hz, J2 = 2.0 Hz, 1H, 4-H), 7.46 (m, 7H, Ar),
7.81 (br s, 1H, NH), 8.40 (d, J= 8.4 Hz, 1H, 3-H)
ppm; 13C nmr (75.5 MHz, CDCl3): δ 20.8, 39.9,
111.0, 121.1, 121.3, 127.8, 128.3, 128.9, 129.0, 129.3,
130.3, 131.9, 132.9, 133.1, 138.4, 152.4, 164.8 ppm;
ms (ESI+, TOF): m/z 331 ([M + H]+). Anal. Calcd for
C22H22N2O: C, 79.97; H, 6.71; N, 8.48. Found: C,
80.17; H, 6.78; N, 8.18.
6-Chloro-N-(4’-chloro-5-methyl-[1,1’-biphenyl]-2-yl)
nicotinamide (5 k).
Off-white solid, yield 68%, 242 mg,
recrystallization from i-Pr2O/aceton, mp 86–89°C; ir:
3294, 1646, 1617, 1586, 1556, 1522 cmÀ1 1H nmr
N-(4’-Chloro-5-methyl-[1,1’-biphenyl]-2-yl)-4-(dimethylamino)
;
benzamide (5o).
Off-white solid, yield 76%, 275 mg,
recrystallization from i-Pr2O, mp 133–136°C; ir: 3426,
3229, 1634, 1609, 1515 cmÀ1 1H nmr (300 MHz,
(300 MHz, CDCl3): δ 2.39 (s, 3H, Me), 7.10 (d, J= 2.1 Hz,
1H, 6-H), 7.26 (m, 1H, 4-H), 7.35 and 7.46 (AA’XX’,
J=8.4Hz, 2H each, C6H4), 7.41 (dd, J1 =8.4Hz, J2 =0.6Hz,
1H, 5’-H), 7.67 (br s, 1H, NH), 8.00 (dd, J1 =8.4Hz,
J2 =2.5Hz, 1H, 4’-H), 8.21 (d, J=8.4Hz, 1H, 3-H), 8.53
(d, J=2.5Hz, 1H, 2’-H) ppm; 13C nmr (75.5MHz,
CDCl3): δ 20.9, 122.4, 124.5, 129.3, 129.4, 129.5,
130.4, 130.7, 131.3, 132.2, 134.4, 135.3, 136.4, 137.9,
;
CDCl3): δ 2.36 (s, 3H, Me), 3.01 (s, 6H, NMe2), 6.63
and 7.51 (AA’XX’, J = 9.0 Hz, 2H each, C6H4), 7.05 (d,
J = 1.8 Hz, 1H, 6-H), 7.22 (dd, J1 = 8.4 Hz, J2 = 1.8Hz,
1H, 4-H), 7.37 and 7.45 (AA’XX’, J= 8.7 Hz, 2H each,
C6H4), 7.66 (br s, 1H, NH), 8.31 (d, J= 8.4 Hz, 1H, 3-H)
ppm; 13C nmr (75.5 MHz, CDCl3): δ 20.8, 40.0, 111.1,
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

J. Suljagić, A. Juranovič, M. Krivec, K. Kranjc, and M. Kočevar
Vol 000
[9] Juranovič, A.; Kranjc, K.; Polanc, S.; Kočevar, M. Synthesis
2014, 46, 909.
[10] Kranjc, K.; Štefane, B.; Polanc, S.; Kočevar, M. J Org Chem
2004, 69, 3190.
121.1, 122.0, 128.3, 129.1, 129.3, 130.3, 130.6, 131.2,
132.7, 133.6, 133.8, 137.0, 152.5, 165.0 ppm; ms (ESI+,
TOF): m/z 365 ([M + H]+). Anal. Calcd for
C22H21ClN2O× 0.2 H2O: C, 71.71; H, 5.85; N, 7.60.
Found: C, 71.75; H, 5.59; N, 7.35.
[11] Kranjc, K.; Kočevar, M. New J Chem 2005, 29, 1027.
[12] Kranjc, K.; Kočevar, M. Tetrahedron 2008, 64, 45.
[13] Juranovič, A.; Kranjc, K.; Polanc, S.; Perdih, F.; Kočevar, M.
Monatsh Chem 2012, 143, 771.
[14] Kranjc, K.; Kočevar, M. Synlett 2008, 2613.
[15] Krivec, M.; Gazvoda, M.; Kranjc, K.; Polanc, S.; Kočevar, M.
J Org Chem 2012, 77, 2857.
[16] Krivec, M.; Kranjc, K.; Polanc, S.; Kočevar, M. Curr Org
Chem 2014, 18, 1520.
[17] Štefane, B.; Požgan, F. Monatsh Chem 2014, 145, 1329.
[18] Michlik, S.; Kempe, R. Nat Chem 2013, 5, 140.
[19] Michlik, S.; Kempe, R. Angew Chem Int Ed 2013, 52,
6326.
[20] Zhang, M.; Fang, S.; Neumann, H.; Beller, M. J Am Chem Soc
2013, 135, 11384.
Acknowledgments. We gratefully acknowledge the Ministry of
Higher Education, Science and Technology of the Republic of
Slovenia and the Slovenian Research Agency for financial
support P1-0230-0103 (and young researcher grant for AJ) as
well as Basileus IV Erasmus Mundus Project (grant for JS). Dr.
B. Kralj and Dr. D. Žigon (Centre for Mass Spectroscopy,
"Jožef Stefan" Institute, Ljubljana, Slovenia) are gratefully
acknowledged for the mass measurements. This work was also
partially supported with infrastructure of EN → FIST Centre of
Excellence, Ljubljana.
[21] Gunanathan, C.; Milstein, D. Science 2013, 341, 249, article
number 1229712.
[22] Kranjc, K.; Kočevar, M. Collect Czech Chem Commun 2006,
71, 667.
REFERENCES AND NOTES
[23] Štefane, B.; Perdih, A.; Pevec, A.; Šolmajer, T.; Kočevar, M.
Eur J Org Chem 2010, 28, 5870.
[1] Gooßen, L. J.; Deng, G.; Levy, L. M. Science 2006, 313, 662.
[2] Glasnov, T. N.; Kappe, C. O. Adv Synth Catal 2010, 352, 3089.
[3] Felpin, F. X.; Fouquet, E.; Zakri, C. Adv Synth Catal 2009,
351, 649.
[4] Spivey, A. C.; Tseng, C. C.; Hannah, J. P.; Gripton, C. J. G.;
de Fraine, P.; Parr, N. J.; Scicinski, J. J. Chem Commun 2007, 2926.
[5] Caron, L.; Campeau, L. C.; Fagnou, K. Org Lett 2008, 10, 4533.
[6] Wetzel, A.; Ehrhardt, V.; Heinrich, M. R. Angew Chem Int Ed
2008, 47, 9130.
[7] Afarinkia, K.; Vinader, V.; Nelson, T. D.; Posner, G. H.
Tetrahedron 1992, 48, 9111.
[8] Woodard, B. T.; Posner, G. H. Recent advances in Diels–Alder
cycloadditions of 2-pyrones. In: Advances in cycloaddition; Harmata, M.
Ed.; JAI Press Inc.: Greenwich, 1999; Vol. 5, pp 47–83.
[24] Kranjc, K.; Kočevar, M. Curr Org Chem 2010, 14, 1050.
[25] Kranjc, K.; Kočevar, M. Bull Chem Soc Jpn 2007, 80, 2001.
[26] Kepe, V.; Kočevar, M.; Polanc, S.; Verček, B.; Tišler, M.
Tetrahedron 1990, 46, 2081.
[27] Kepe, V.; Kočevar, M.; Polanc, S. J Heterocycl Chem 1996,
33, 1707.
[28] Požgan, F.; Kranjc, K.; Kepe, V.; Polanc, S.; Kočevar, M.
Arkivoc 2007, (viii), 97.
[29] Kočevar, M. Acta Chim Slov 1996, 43, 143 Chem Abstr 1996,
125, 576047.
[30] Požgan, F.; Krejan, M.; Polanc, S.; Kočevar, M. Heterocycles
2006, 69, 123.
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet