Synthesis of phthalides from bis-propargyl ethers: Use of Garratt-Braverman cyclization to construct the phthalans and IBX as a new reagent for subsequent oxidation

Article abstract of DOI:10.1016/j.tetlet.2015.09.036

An efficient route to phthalides is described starting from easily available bis-propargyl ethers. The method involves Garratt-Braverman cyclization of the ethers to phthalans which are then efficiently oxidized to the phthalides by iodoxy benzoic acid (IBX) which has been shown to be a superior reagent in terms of yield and reagent stoichiometry for such a transformation in comparison with more commonly used KMnO₄/CuSO₄ reagent.

Full text of DOI:10.1016/j.tetlet.2015.09.036

Accepted Manuscript
Synthesis of Phthalides from Bis-propargyl Ethers: Use of Garratt-Braverman
Cyclization to Construct the Phthalans and IBX as a New Reagent for Subse-
quent Oxidation
Arpita Panja, Eshani Das, Manasi Maji, Amit Basak
PII:
S0040-4039(15)30077-0
DOI:
http://dx.doi.org/10.1016/j.tetlet.2015.09.036
TETL 46708
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
8 August 2015
11 September 2015
13 September 2015
Please cite this article as: Panja, A., Das, E., Maji, M., Basak, A., Synthesis of Phthalides from Bis-propargyl Ethers:
Use of Garratt-Braverman Cyclization to Construct the Phthalans and IBX as a New Reagent for Subsequent
Oxidation, Tetrahedron Letters (2015), doi: http://dx.doi.org/10.1016/j.tetlet.2015.09.036
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Graphical Abstract
Synthesis of Phthalides from Bis-propargyl Ethers:
Use of Garratt-Braverman Cyclization to Construct
the Phthalans and IBX as a New Reagent for
Subsequent Oxidation Arpita Panja, Eshani Das,
Manasi Maji and Amit Basak*
Leave this area blank for abstract info.
O
R2
I = KOBut/toluene or DBU/toluene, reflux II = IBX/DMSO
III = KMnO4/CuSO4
O
R1
O
Exo
+
1
R2
R2
6
I
II/III
[O]
Yield for GB 60-80%Yield for IBX 75-90% for KMnO4/CuSO4
50-65%
O
GB
cyclization
R1
R1
R2
3
Exo/Endo Regioselectivity explained by computations
based on Cation or radical intermediate
O
GB = Garratt-Braverman
Exo-endo is with respect to the group (R1) at C-6
R1
Endo
O

Synthesis of Phthalides from Bis-propargyl Ethers: Use of Garratt-Braverman Cyclization to
Construct the Phthalans and IBX as a New Reagent for Subsequent Oxidation
Arpita Panjaa, Eshani Dasb, Manasi Maji and Amit Basak*
Department of Chemistry, Indian Institute of Technology Kharagpur, India
amajor contributor to the experimental part b major contributor to the computational part
*Corresponding author email: absk@chem.iitkgp.ernet.in Tel: +91 3222 283300
Abstract: An efficient route to phthalides is described starting from easily available bis-propargyl ethers. The method involves Garratt-Braverman
Cyclization of the ethers to phthalans which are then efficiently oxidized to the phthalides by Iodoxy Benzoic acid (IBX) which has been shown to be a
superior reagent in terms of yield and reagent stoichiometry for such a transformation in comparison to more commonly used KMnO4/CuSO4 reagent.
Phthalides have drawn special interest amongst the
scientific community due to their diverse array of biological
activities.1 Phthalide skeletons are found in various natural
products as well as unnatural molecules of pharmaceutical
importance.2 They also serve as key intermediate for the
synthesis of many natural products including important
antibiotics3 and related polyaromatic compounds.4 The
development of efficient methodology for the synthesis of this
important class of bio-active scaffold is a highly demanding
goal in organic synthesis. Synthetic strategy towards phthalide
is a well-exploited research area and several methods for their
synthesis are available in literature.5 Out of them, one common
and attractive strategy is construction of phthalans and
subsequent benzylic oxidation.6 The success of the strategy is
dependent upon the access to wide variety of phthalans and
also finding a suitable oxidizing system for their conversion to
reported till date. In addition, preliminary computations have
been carried out to explain the varying degree of observed
regioselectivity in the oxidation of phthalans (exo vs endo) on
the basis of cationic or radical intermediate.
The various phthalan derivatives, required for the
oxidation, could be easily accessed by a one-step base mediated
GB cyclization of bis-propargyl ethers (synthesis included in
Electronic Supplementary Information (ESI)) with one
propargyl arm conjugated to an alkene or aryl moiety (Scheme
2). This structural feature ensures the formation of only one
phthalans during GB cyclization.
R3
R3
O
R2
R2
R1
i
H
O
60-75%
R1
2a-d
1a-d
O
ii
H
O
NC
75%
NC
phthalides (Scheme 1).
1f
2f
O
O
H
O
ii
1
O
O
R
R
Garratt Braverman
Cyclization
80%
[O]
N
H
N
O
Exo
2e
1e
R
6
R
3
bis-propargyl ether
Phthalan
O
Endo
O
O
O
iii
H
Exo-Endo nomenclature is with respect to the substituent (R) at C-6
Phthalide
EtO2C
60%
CO2Et
1g
Scheme 1: Retrosynthesis of phthalides
2g
Although several reagents have been reported7 to carry out
such oxidations, the reactions do suffer from limitations like
low yields, poor regioselectivity in case of unsymmetrical
phthalans, use of excess reagents etc. In this paper, we report a
moderate to high yielding synthetic protocol for the synthesis
of phthalides employing a combination of base-mediated
Garratt-Braverman (GB) cyclization8 of bis-propargyl ethers
followed by oxidation of the resulting phthalans with iodoxy
benzoic acid (IBX) to the corresponding phthalides. Although
IBX has been successfully employed for carrying out benzylic
oxidations of various substrates,9 to the best of our knowledge,
oxidation of phthalans to phthalides by IBX has not been
1a R1= R2= R3= H; 1b R2= R3= H; R1= OCH3; 1c R2= H; R1= R3= OCH3;
1d R1= R2= R3= OCH3
Reagents and conditions i = KOBut, toluene, reflux, 4 h; iii = DBU,
tolune, reflux, 6 h
Scheme 2: Synthesis of phthalans by GB cyclization
With the phthalans in hand, we next proceeded for the
oxidation step. Initially the oxidation was carried out with most
commonly used reagent, namely KMnO4 on a CuSO4 support
as introduced by Noureldin et al.7a Apart from less than
satisfactory yields, the reaction suffered from over oxidation to
the dialdehyde (for electron rich systems) along with poor
regioselectivity and the need to use large excess reagent (~10
eq) for complete consumption of starting material. Other

2
reagents like Oxone/KBr7c-d were found to be not suitable for
oxidation of electron-rich phthalans because of nuclear
bromination. Because of these disadvantages, we looked for
newer reagents and realized the possibility of using IBX as an
oxidant for the conversion of phthalans to phthalides. IBX
could be easily prepared in laboratory and is broadly used for
intermediates for exo and endo products and their geometries of
these four intermediates were optimized and characterized as
minima using vibrational analysis (zero imaginary frequency).
Computational results as shown in Table 2 indicate the
following general features (barring minor differences): for
electron donating systems, in case of involvement of a cationic
intermediate, exo isomer is favoured as the exo cation is found
to be more stable (which can also be explained in terms of
electron delocalization as shown in Figure 1). The situation is
reversed in case of radical mediated pathway; in this case endo
isomer is calculated to be the preferred. For electron
withdrawing system, low extent of exo selectivity is expected
for both cationic and radical pathway because of low energy
difference.
selective oxidation of aromatic or aliphatic alcohols, lactols and
9d-e 10
benzylic oxidations etc.
,
An in situ generated bis-IBX
system have been developed by Moorthy et al.9f for the
conversion of diols to phthalides. However, it has not been
used for oxidation of phthalans which also involves oxidation
of benzylic ether and hence should be feasible. With this hope,
a DMSO solution of various phthalans and freshly prepared
o
IBX (4 eq added in portions) was stirred at 90 C for 72 h by
O
5
which the reaction was complete. The lactones were isolated
pure by column chromatography over silica gel. The yields are
significantly better as compared to those obtained by KMnO4-
CuSO4 mediated oxidation. Moreover, the reaction worked well
for both electron rich and electron poor phthalans without any
over-oxidation to the dialdehyde. The results for the oxidation
of phthalans to their phthalides for both the oxidizing systems;
KMnO4-CuSO4 and IBX, are shown in the Table 1.
O
O
O
O
7
exo cation from 1b
exo cation from 1c
Blue and pink arrows indicate the electron flow
from the 7-methoxy and 5-methoxy respectively
Figure 1: resonance stabilization of exo-cation
The above predictions were more or less in agreement with the
experimental result in case of electron donating systems for
which exo selectivity has been observed. The situation is more
complex in case of electron withdrawing systems.
Computational calculation did not show any preference
regardless of the nature of intermediate (radical or cation),
except exo-radical of 1g. For cyano-substituted phthalan 1f,
almost no selectivity was observed for both KMnO4 and IBX
mediated oxidation. The higher level of exo selectivity
observed for the phthalan ester 1g for both the reagents was
perhaps due to the direct attachment of ester group to the
phthalan benzene ring due to which the electron withdrawing
effect of the ester group is more strongly felt. In this case, the
exo-endo ratio can be best explained if a radical intermediate is
operative. The exo radical, being nucleophilic is stabilized by
the ester (captodative effect11-see Figure 2). This is also
understandable as two-electron transfer to generate cation is
less likely in case of electron poor system. However, for
pyridine-fused phthalan 1e, the endo selectivity for KMnO4
was not clear from computation.
. Regarding the extent of regioselectivity, both IBX and
KMnO4 reagent gave less than satisfactory level of selectivity.
However, analysis of the results as mentioned in Table 1
revealed some interesting features. These are: for electron
donating systems, exo selectivity was observed for both IBX
and KMnO4, the former giving slightly better selectivity as
revealed in the oxidation of 1c and 1d; for the mono methoxy
phthalan 1b, complete exo selectivity was observed for both the
reagents. For phthalans 1e and 1f with electron withdrawing
groups, IBX failed to show any selectivity whereas KMnO4
showed significant endo selectivity for 1e (re 34%). For the
other electron withdrawing system 1g, both the reagents gave
similar extent of exo selectivity. The structures of the exo and
endo isomers were established by extensively analysing the
respective 2D NMR spectra (NOESY) (included in ESI).
We also have carried out DFT-based computations (details
given under Experimental) to explain the results. As a starting
point, it was perhaps logical to assume that the nature of
intermediates (radical or cation) and their relative stability
(between exo and endo) would control the regioselectivity.
Thus we have considered both radical and cationic
O
O
EtO
EtO
O
O
exo radical from 1g
Figure 2: stabilization of exo-radical

OMe
OMe
MeO
MeO
O
O
O
O
3
MeO
O
MeO
O
1a
1b
1c
1d
O
EtO C
NC
N
2
1g
1e
1f
Combined % yield of Combined % yield of
Individual % yields phthalides (Exo-Endo phthalides (Exo-Endo
Individual % yields
Substrate
Products
ratio)
ratio)
KMnO4/CuSO4
31
IBX
80
KMnO4/CuSO4
IBX
O
O
31
80
3a
1a
(Not applicable)
(Not applicable)
CHO
5
Nil
90
CHO 5a
O
46
MeO
MeO
O3b
O
46
90
Nil
14
Nil
Nil
1b
1c
1d
(only exo)
(only exo)
O
4b
CHO
CHO
MeO
5b
OMe
OMe
O
35
21
5
51
20
O3c
MeO
MeO
56
(1.6:1)
71
(2.5:1)
O
O
4c
OMe
CHO
Nil
CHO5c
MeO
OMe
OMe
O
MeO
MeO
31
27
6
50
35
Nil
40
40
38
38
53
O
3d
MeO
MeO
58
(1.3:1)
85
(1.5:1)
O
O
4d
OMe
MeO
MeO
CHO
CHO5d
O
22
43
24
21
40
O
3e
N
N
65
(1:2)
80
(1:1)
1e
1f
O
O
4e
O
NC
NC
O3f
45
(1.1:1)
76
(1:1)
O
O
4f
O
O
EtO2C
3g
60
(2:1)
80
(2:1)
1g
O
20
27
EtO2C
O
4g
Table 1: Results of oxidation [for experimental details see references 12 (a) and (b)].

4
Experimentally
observed preference
Substrates
with
For involvement of
cationic intermediate
For involvement of radical
intermediate
Nature of
intermediate
supported by both
experimental and
computational result
optimized
geometry
KMnO4-
IBX
ΔE (Exo-
Endo in
kcal/mol)
Preference
according to
computation
ΔE (Exo-
Endo in
kcal/mol)
Preference
according to
computation
CuSO4
Cationic
Cationic
No
preference
-4.69
-7.53
Exo
+0.27
+0.13
Exo
Exo
Exo
Exo
1b
1c
No
preference
Exo
Cationic
No
preference
-4.09
Exo
+0.08
Exo
Exo
1d
1e
1f
Inconclusive
Inconclusive
No
preference
No
preference
No
preference
-0.71
-0.18
-0.06
-0.89
Endo
No
preference
No
preference
No
No
preference preference
No
preference
+0.58
-2.56
Exo
Exo
Exo
Radical
1g
Table 2: Computational results [for computational details see reference 12 (c)].
In conclusion, for the first time, we could successfully access the
regioisomeric phthalides via GB cycization as the key intermediate
step followed by oxidation with varying selectivity. IBX has been
found to be a useful reagent for the conversion of phthalan to
phthalide. The high yield and ease of operation should make it an
attractive reagent for such transformation. We have been able to
rationalize the obtained selectivity on the basis of electronic and
steric grounds. The extent of regioselectivity is shown to be
dependent upon the electronic nature of the aryl ring (donor or
acceptor) and the results have been further elucidated by considering
the nature of the intermediate (cation or radical) involved in the
oxidation. Future study will aim towards improving the extent of
regioselectivity for oxidation of phthalans by perturbing the reaction
condition as well as by screening other potent oxidants.
Acknowledgement
We express our thanks to Dr. A. Anoop for his help at different stages
of computation. Author A.B. is grateful to CSIR, Govt. of India, for
funding (No. 02(0014)/11/EMR-I) and to DST, Govt. of India for the
JC Bose fellowship. AP thanks CSIR, Govt. of India (NET) and ED
thanks IIT Kharagpur for research fellowships. Authors also thank
IIT Kharagpur for the 600 MHz facility and DST for 400 MHz
facility under the IRPHA program.
References
1. (a) Barton, D. H. R.; Vries, J. D. J. Chem. Soc. 1963, 1961-1919;
(b) Devon, T. K.; Scott, A. I. In Handbook of Naturally Occurring
Compounds; Academic Press: New York, 1975; 1, 249; (c) Puder, C.;
Zeeck, A.; Beil, W. J. J. Antibiot. 2000, 53, 329; (d) Strobel, G.;
Ford, E.; Worapong, J.; Harper, J. K.; Arif, A. M.; Grant, D. M.;
Fung, P. C. W.; Chau, R. M. W. Phytochemistry 2002, 60, 179; (e)
Arnone, A.; Assante, G.; Nasini, G.; Strada, S.; Vercesi, A. J. Nat.

5
9. (a) Nicolaou, K. C.; Zhong, Y. L.; Baran, P. S. J. Am. Chem. Soc.
2000, 122, 7596; (b) Nicolaou, K. C.; Baran, P. S.; Zhong, Y. L. J.
Am. Chem. Soc. 2001, 123, 3183; (c) Nicolaou, K. C.; Montagnon,
T.; Baran, P. S.; Zhong, Y. L. J. Am. Chem. Soc. 2002, 124, 2245; (d)
Moorthy, J. N.; Singhal, N.; Venkatakrishnan, P. Tetrahedron Lett.
2004, 45, 5419; (e) Moorthy, J. N.; Singhal, N.; Mal, P. Tetrahedron
Lett. 2004, 45, 309; (f) Seth, S.; Jhulki, S.; Moorthy, J. N. Eur. J.
Org. Chem. 2013, 2445.
Prod. 2002, 65, 48; (f) Yoganathan, K.; Rossant, C.; Ng, S.; Huang,
Y.; Butler, M. S.; Buss, A. D.. J. Nat. Prod. 2003, 66, 1116; (g)
Kuriyama, M.; Ishiyama, N.; Shimazawa, R.; Shirai, R.; Onomura, O.
J. Org. Chem. 2009, 74, 9210; (h) Tang, Y.; Zhu, M.; Yu, S.; Hua,
Y.; Duan, J. A.; Su, S.; Zhang, Y. L.; Ding, A. Molecules 2010, 15,
341.
2. (a) Bellasio, E. German Patent 2422193, 1974; (b) Dang, Q.;
Brown, B.S.; Poelje, P. D. V.; Colby, T. J.; Erion, M. D. Bioorg.
Med. Chem. Lett. 1999, 9, 1505; (c) Matsuda, H.; Shimoda, H.;
Yoshikawa, M. Bioorg. Med. Chem. 1999, 7, 1445; (d) Bentley, R.
Chem. Rev. 2000, 100, 3801; (e) Rayabarapu, D., Chang, H.; Cheng,
C. Chem. Eur. J. 2004, 10, 2991; (f) Zhang, H.; Zhang, S.; Liu, L.;
Luo, G.; Duan, W.; Wang, W. J. Org. Chem. 2010, 75, 368-374. (g)
Nalini, V. P.; Poonam, Y. Res. J. Chem. Sci. 2012, 2, 57; (h)
Teixeira, R. R.; Bressan, G. C.; Pereira, W. L.; Ferreira, J. G.;
Oliveira, F. M. D.; Thomaz, D. C. Molecules 2013, 18, 1881-1896.
3. (a) Watanabe, H.; Furuuchi, T.; Yamaguchi, T.; Kitahara, T. Org.
Lett. 1999, 1, 1079; (b) Matsumoto, T.; Yamaguchi, H.; Tanabe, M.;
Yasui, Y.; Suzuki, K. Tetrahedron Lett. 2000, 41, 8393; (c) Hayat, S.;
Rahman, A. U.; Choudhary, M. I.; Khan, K. M.; Bayer, E.
Tetrahedron Lett. 2001, 42, 1647; (e) Mukhopadhyay, R.; Kundu, N.
G. Tetrahedron 2001, 57, 9475; (f) Kawasaki, T.; Saito, S.;
Yamamoto, Y. J. Org. Chem. 2002, 67, 2653; (g) Kelly, T. R.; Xu, D;
Martínez, G.; Wang, H. Org. Lett. 2002, 4, 1527; (h) Fei, Z.;
McDonald, F. E. Org. Lett. 2007, 9, 3547.
4. (a) Hauser, F. M.; Rhee, R. P. J. Org. Chem., 1978, 43, 178; (b)
Hauser, F. M.; Prasanna, S. J. Org. Chem. 1979, 44, 2597; (c) Yang,
X.; Rotter, T.; Piazza, C.; Knochel, P. Org. Lett. 2003, 5, 1229.
5 (a) Sakamoto, M.; Takahashi, M.; Moriizumi, S.; Yamaguchi, K.;
Fujita, T.; Watanabe, S. J. Am. Chem. Soc. 1996, 118, 8138; (b)
Sakamoto, M.; Sekine, N.; Miyoshi, H. Mino, T.; Fujita, T. J. Am.
Chem. Soc. 2000, 122, 10210; (c) Everaere, K.; Scheffler, J.-L.;
Mortreux, A.; Carpentier, J.-F. Tetrahedron Lett. 2001, 42, 1899 (d)
Lei, J.-G.; Hong, R.; Yuan, S.-G.; Lin, G.-Q. Synlett 2002, 6, 927; (e)
Seki, T.; Tachikawa, H.; Yamada, T.; Hattori, H. J. Cat. 2003, 217
117; (f) Everaere, K.; Mortreux, A.; Carpentier, J.-F. Adv. Synth. Cat.
2003, 345, 67; (g) Chang, H.-T.; Jeganmohan, M.; Cheng, C.-H.
Chem. Eur. J. 2007, 13, 4356; (h) Phan, D. H. T.; Kim, B.; Dong, V.
M. J. Am. Chem. Soc. 2009, 131, 15608; (i) Omura, S.; Fukuyama,
T.; Murakami, Y.; Okamoto, H.; Ryu, I. Chem. Commun. 2009, 6741;
(j) Zhang, H.; Zhang, S.; Liu, L.; Luo, G.; Duan, W.; Wang, W. J.
Org. Chem. 2010, 75, 368; (k) Mangas-Sanchez, J.; Busto, E. Gotor-
Fernandez, V.; Gotor, V. Org. Lett. 2012, 14, 1444; (l) Gerbino, D.
C.; Augner, D.; Slavov, N.; Schmalz, H.-G. Org. Lett. 2012, 14,
2338; (m) Fujita, K.; Ito, W.; Yamaguchi, R. Chem. Cat. Chem.
2014, 6, 109; (n) Karmakar, R.; Pallab Pahari, P.; Mal, D. Chem.
Rev. 2014, 114, 6213.
10. More, J. D.; Finney, N. S. Org. Lett. 2002, 4, 3001.
11. (a) Vieche, H.G.; Janousek, Z.; Merenyi. R. Acc. Chem. Res.
1985, 18, 148; (b) Ito, O.; Arilo, P.; Matsuda, M. J. Chem. Soc.
Perkin Trans 2: 1988, 869; (c) Bordwell, F. G.; Lynch, T. Y. J. Am.
Chem. Soc. 1989, 111, 7558.
12. (a) General procedure for oxidation with KMnO4/CuSO4
reagent: Equal amounts of potassium permanganate and copper
sulphate pentahydrate are grounded together in a mortar pestle. The
resulting mixture was used as a heterogeneous oxidant (2.3 g for 1
mmol of substrate). The oxidant was added to the solution of
phthalans (1 mmol) in dry DCM (6 mL) and refluxed for 72 h. Fresh
batch of oxidant (1.15 g) was added in between (after 24 h of
refluxing). The reaction mixture was then filtered through a celite pad
and after evaporating the solvent, the crude products were purified by
column chromatography using Si-gel, PE - EtOAc as eluent.
(b) General procedure for oxidation with IBX
To the solution of phthalans (1 mmol) in DMSO (4 mL), IBX (1.1 g,
4 eq.) was added and heated to ~85C. After completion of the
reaction (18-24 h), the reaction mixture was quenched with saturated
solution of NaHCO3 and extracted with DCM. The crude product,
obtained after evaporation of the solvent, was purified by column
chromatography using Si- gel and PE - EtOAc as eluent.
(c) Methods used for computations
All computations were carried out with TURBOMOLE V6.513
software package. Geometry optimizations were done using Density
Functional Theory (DFT). B3-LYP15 functional with def2-TZVP14
basis set in combination with the third version of the empirical
dispersion correction (DFT-D316) was used for all calculations.
Restricted formalism was used for the closed shell structures (cation
intermediate), whereas unrestricted formalism used for open shell
radical intermediate. The nature of stationary point was characterised
by vibrational frequency analysis.
13. (a) Turbomole, V 6.5; Turbomole GmbH : Karlsruhe, Germany,
2013; (b) Ahlrichs, R.; Bar, M.; Haser, M.; Horn, H.; Kolmel, C.
Chem. Phys. Lett. 1989, 162, 165.
14. (a) Becke, A.D. J. Chem. Phys. 1993, 98, 5648; (b) Lee, C.;
Yang, W.; Parr, R. G. Phys. Rev. B. 1988, 37, 785
15. Zheng, J.; Xu, X.; Truhlar, D. G. Theor. Chem. Acc. 2011, 128,
295.
6. (a) Shaabani, A.; Farhangi, E.; Rahmati, A. Monatshefte fuer
Chemie 2008, 139, 905; (b) Yoshinori, E.; Backvall, J. E. Chem. Eur.
J. 2011, 17, 12596; (c) Cui, L. Q.; Liu, K.; Zhang, C. Org. Biomol.
Chem. 2011, 9, 2258.
16. Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. J. Chem. Phys.
2010, 132, 154104.
7. (a) Noureldin, N. A.; Zhao, D.; Lee, D. G. J. Org. Chem. 1997, 62,
8767; (b) Dohi, T.; Takenaga, N.; Goto, A.; Fujioka, H.; Kita, Y. J.
Org. Chem. 2008, 73, 7365; (c) Moriyama, K.; Takemura, M.; Togo,
H. Org. Lett. 2012, 14, 2414; (d) Yin, L.; Wu, J.; Xiao, J.; Cao, S.
Tetrahedron Lett. 2012, 53, 4418; (e) Nammalwar, B.; Fortenberry,
C.; Bunce, R. A.; Lageshetty, S. K.;.Ausman, K. D. Tetrahedron Lett.
2013, 54, 2010; (f) Chen, K.; Zhang, P.; Wang, Y.; Li, H. Green
Chem. 2014, 16, 2344;
8. (a) Mondal, S.; Mitra, T.; Mukherjee, R.; Addy, P. S.; Basak, A.
Synlett 2012, 23, 2582.

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