Facile and efficient synthesis of C2-symmetrical 1,3,5-triazine polycarboxylate ligands under microwave irradiation

Article abstract of DOI:10.1016/j.tet.2014.06.127

A rapid and efficient method for preparation of C₂-symmetrical 1,3,5-triazine polycarboxylate ligands was developed. The reactions included either selective mono- or di-substitution of 2,4,6-trichloro-1,3,5-triazine with various nucleophiles containing carboxyl group(s), followed by nucleophilic displacement of the remained chloride(s) in aqueous media under microwave irradiation. Novel C₂-symmetrical tripodal ligands were afforded in good yields and purities under short reaction time with simple work-up, which are potentially useful as structural directing units in metal-organic frameworks.

Full text of DOI:10.1016/j.tet.2014.06.127

Tetrahedron 70 (2014) 5415e5419
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Tetrahedron
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Facile and efficient synthesis of C2-symmetrical 1,3,5-triazine
polycarboxylate ligands under microwave irradiation
*
Watcharee Funfuenha, Wong Phakhodee, Mookda Pattarawarapan
Department of Chemistry and Center of Excellence for Innovation in Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand
a r t i c l e i n f o
a b s t r a c t
Article history:
A rapid and efficient method for preparation of C2-symmetrical 1,3,5-triazine polycarboxylate ligands
was developed. The reactions included either selective mono- or di-substitution of 2,4,6-trichloro-1,3,5-
triazine with various nucleophiles containing carboxyl group(s), followed by nucleophilic displacement
of the remained chloride(s) in aqueous media under microwave irradiation. Novel C2-symmetrical tri-
podal ligands were afforded in good yields and purities under short reaction time with simple work-up,
which are potentially useful as structural directing units in metal-organic frameworks.
Ó 2014 Elsevier Ltd. All rights reserved.
Received 28 April 2014
Received in revised form 20 June 2014
Accepted 30 June 2014
Available online 3 July 2014
Keywords:
1,3,5-Triazine derivatives
Polycarboxylated ligands
C2-symmetrical
Metal-organic frameworks
Microwave-assisted synthesis
1. Introduction
prepared from a cheap and readily available 2,4,6-trichloro-1,3,5-
triazine (TCT). However, due to synthetic difficulty, only one
Metal-organic frameworks (MOFs) or coordination polymers
have become increasingly important in many research fields due to
their interesting structural novelty and potential applications in
catalysis, fluorescent lighting, and chemical sensors.1e3 Based on
the shapes of the organic molecules and the specific interactions
within the frameworks, formation of complex structural architec-
tures with intriguing properties can be achieved.
study reported the use of C2-symmetrical triazine ligands including
1,3,5-triazine-2-iminodiacetic acid-4,6-biglycine, L1, and 1,3,5-
triazine-2-iminodiacetic acid-4,6-bis( -alanine), L2, in construct-
L
ing luminescent lanthanide coordination polymers (Fig. 1).6 It is
thus of great interest to develop a rapid and efficient method for
synthesis of C2-symmetrical polycarboxylic triazine-based ligands,
which will be potentially useful in MOFs design and synthesis.
Flexible ligands containing three or more carboxyl groups are
highly desirable since they can induce a number of coordination
modes allowing formation of interesting structures with higher
dimensionalities.4 In addition, each carboxyl groups can form dif-
ferent dihedral angles, which may coordinate to metal in various
orientations.
So far, a number of flexible C3-symmetrical polycarboxylic li-
gands based on 1,3,5-triazine core have been introduced as the
structural directing units in the construction of novel functional
MOFs.4e11 These ligands such as N,N0,N00-1,3,5-triazine-2,4,6-
triyltris-glycine (TTG),12e14 1,3,5-triazine-2,4,6-triamine hexa-
acetic acid (TTHA),14 1,3,5-triazine-2,4,6-piperidine-4-carboxylic
acid,15 1,3,5-triazine-2,4,6-trithiotri-3-benzoic acid,10 and 1,3,5-
triazine-2,4,6-triaminetri-4-benzoic acid16,17 could be easily
Fig. 1. Examples of C2-symmetrical 1,3,5-triazine ligands.
Recently, we have reported a microwave-assisted SNAr reaction
for the rapid synthesis of C3-symmetrical 1,3,5-triazine poly-
carboxylate ligands using water as the solvent.18 On continuing this
study, we describe the synthesis of C2-symmetrical carboxylate li-
gands based on a 1,3,5-triazine core by the selective formation of
mono- and di-substitutions triazine derivatives, followed by nu-
cleophilic displacement of the remained chloride(s) with various
amino acids under microwave irradiation.
* Corresponding author. Department of Chemistry, Faculty of Science, Chiang Mai
University, Chiang Mai 50200, Thailand. Tel.: þ66 5394 3341; fax: þ66 5389 2277;
e-mail address: mookdap55@gmail.com (M. Pattarawarapan).
http://dx.doi.org/10.1016/j.tet.2014.06.127
0040-4020/Ó 2014 Elsevier Ltd. All rights reserved.

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W. Funfuenha et al. / Tetrahedron 70 (2014) 5415e5419
2. Results and discussion
3g was obtained in 85% yield, while ligand 3g was obtained in 65%
yield emphasizing the effect of microwave in enhancing the rate of
Due to the difference in the reactivity of the chloride atoms of
TCT toward nucleophiles, ideally, C2-symmetrical carboxylate li-
gands should be accessible selectively by the stepwise addition of
different nucleophiles in one-pot.19 By the careful control of the
reaction temperature, it is commonly observed that mono-
substitution of chlorine with amine nucleophiles occurs at ca.
the reaction.6
It is worth noting that although it is possible to synthesize tri-
substituted ligands 3 from TCT 1 or 2 (see Table 1, entries 7 and 13, 9
and 11), the process is more feasible using 1 as the starting material.
The synthesis of monosubstituted TCT 1 is less complicated when
compared with that of the disubstituted TCT 2, while isolation of
the trisubstituted product 3 from the remained starting TCT 1
(whenever necessary) is much easier due to the difference in their
polarity and solubility. Nevertheless, whenever applicable, final
chloride substitution of disubstituted TCT derivatives with weak
nucleophiles should be avoided since this displacement step is of-
ten problematic requiring harsh conditions and long reaction time.
For example, in the stepwise synthesis of 1,3,5-triazine-2,4,6-
triaminetri-4-benzoic acid,17 the third substitution with 4-
0
ꢀC, di-substitution at room temperature, while tri-substitution
proceeds above 60 ꢀC.
Nevertheless, since most trisubstituted triazines containing
polycarboxylic acid are not soluble in common organic solvents
except in DMSO, the sequential addition of different nucleophiles to
TCT with <100% conversion in each step will results in a mixture of
trisubstituted products, which are unable to purify by conventional
means.
In fact, C2-symmetrical trisubstituted 1,3,5-triazine core have to
be synthesized from mono- or disubstituted TCT derivatives
(Scheme 1). Monosubstituted TCT 1a,17 1b,20 and 1c6 can be pre-
pared selectively by substitution of one chloride atom of TCT with
selected nucleophile using 1:1 mol ratio at 0e5 ꢀC according to the
reported procedures. Disubstituted TCTs 2a17 and 2b21 were also
similarly obtained by the reaction between TCT and amino acids
(1:2 mol ratio) at 0 ꢀC to room temperature.
aminobenzoic acid required 48
h
under refluxing in
watereacetone mixture. Using our developed procedure, ligand 3i
was rapidly obtained within 20 min either from the mono-
substituted TCT 1c or the disubstituted TCT 2a, though the yield
obtained from 1c (entry 9, 61%) was slightly lower than those
starting from 2a (entry 11, 75%). In an attempt to displace the two
chlorides of 1c with 4-hydroxybenzoic acid, pure product was un-
able to isolate due to incomplete conversion even after 30 min
Scheme 1. Synthesis of C2-symmetrical 1,3,5-triazine polycarboxylate ligand 3.
Finally, multidirectional ligands were synthesized under mi-
crowave irradiation at 180 W according to our previously reported
procedure for preparation of C3-symmetrical TCT carboxylate li-
gands. As summarized in Table 1, C2-symmetrical trisubstituted
triazine derivatives 3 can be obtained either by nucleophilic sub-
stitution of the two chloride atoms of TCT 1 using 2.4 equiv of
nucleophile (entries 1e9) or by displacement of the third chloride
atom of TCT 2 with 1.2 equiv of amino acid (entries 10e13)
according to Scheme 1.
Due to the difference in nucleophilicity between an amino
group and a carboxylate ion, chemoselectivity of the substitutions
can be achieved without the requirement of protecting groups. A
variety of amino acids containing primary, secondary, or aromatic
amine nucleophile were reacted smoothly with the TCT derivatives
under short reaction times (within 10e20 min). With a simple
aqueous work-up, the corresponding trisubstituted products were
afforded in good yields and high purities (>95% based on 1H NMR
analysis).
microwave irradiation at 180 W. Attempts to drive the reaction to
completion by raising the microwave power under prolonged ir-
radiation still failed to give good conversion.
3. Conclusions
In summary, a fast and effective method to prepare C2-sym-
metrical 1,3,5-triazine polycarboxylate ligands has been developed.
A variety of multipodal ligands with potential applications in the
MOFs design were readily obtained in good yields from low-cost
commercially available materials in short reaction times using
water as the eco-friendly solvent. All products were obtained in
high purities from a simple aqueous work-up without requirement
of column chromatography. Study on the complexation of these
multidirectional ligands with various metals is underway and will
be reported in due course.
4. Experimental
Compared with conventional stirring and heating, using mi-
crowave conditions allows various polycarboxylate ligands to be
prepared in high yields with very short reaction times. For example,
within 10 min microwave irradiation, ligand 3g and 3h were ob-
tained in 84% and 75%, respectively (Table 1, entries 7 and 8),
whereas under the conventional heating at 105 ꢀC for 12 h, ligand
4.1. Material and methods
All reagents were purchased from Fluka and Aldrich and were
used without further purification. Microwave reactions were con-
ducted using a modified microwave oven (Samsung GB872, 850 W,

W. Funfuenha et al. / Tetrahedron 70 (2014) 5415e5419
5417
Table 1
Synthesis of C2-symmetrical trisubstituted 1,3,5-triazine polycarboxylate ligandsa
Entry
1 or 2
Nu1H or Nu2H
3Ref
3a
Time
Yield (%)
1
5ꢁ2 min
82
78
75
2
3
1a
1a
3b
3c
5ꢁ2 min
10ꢁ2 min
4
5
1a
3d
3e
10ꢁ2 min
10ꢁ2 min
81
76
6
7
1b
3f
10ꢁ2 min
5ꢁ2 min
72
84
3g6
8
1c
1c
3h6
5ꢁ2 min
75
9
3i
3j
10ꢁ2 min
5ꢁ2 min
61
85
10b
11b
12b
2a
3i
5ꢁ2 min
75
81
3k
10ꢁ2 min
13b
2b
3g
5ꢁ2 min
73
a
Unless otherwise indicated, all reactions were carried out using triazine 1 (1.0 mmol), amine (2.4 mmol), and NaOH (7.2 mmol) in 2 mL H2O at 180 W.
The reaction was performed using amine (1.2 mmol) and NaOH (3.6 mmol).
b

5418
W. Funfuenha et al. / Tetrahedron 70 (2014) 5415e5419
2.54 GHz). The reaction was monitored by thin-layer chromatog-
raphy carried out on silica gel plates (60F254, MERCK, Germany) and
visualized under UV light (245 nm). Melting points were de-
termined using Mettler Toledo DSC equipment at a heating rate of
10 ꢀC/min and are uncorrected. NMR spectra were determined
using a Bruker AVANCEÔ (400 MHz for 1H). Chemical shifts were
2% NaOH in D2O) 178.5, 165.5, 164.1, 142.3, 130.0, 119.2, 118.7, 51.9;
HRMS (ESI): MNaþ, found 501.0985. [C18H18N6O10Na]þ requires
501.0982.
Compound 3c: White solid, Rf (60% MeOH/EtOAc) 0.44; mp
(dec)ꢃ270 ꢀC; nmax (KBr) 3209, 3092, 1771, 1774, 1615, 1514, 1455,
1416, 1330, 1246 cmꢂ1
; dH (400 MHz 2% NaOH in D2O) 7.87 (d,
reported in parts per million (ppm,
d
) downfield from TMS. Split-
J¼8.8 Hz, 2H), 7.62 (d, J¼8.8 Hz, 2H), 7.02 (d, J¼8.4 Hz, 4H), 6.62 (d,
J¼8.4 Hz, 4H), 3.44 (dd, J¼7.2, 5.2 Hz, 2H), 2.88 (dd, J¼13.7, 5.2 Hz,
2H), 2.71 (dd, J¼13.7, 7.2 Hz, 2H); dC (100.6 MHz 2% NaOH in D2O)
182.8, 180.3, 174.9, 166.2, 164.2, 141.2, 130.5, 129.6, 123.6, 121.8,
120.8, 118.6, 57.3, 39.7; HRMS (ESI): MHþ, found 575.1882.
[C28H27N6O8]þ requires 575.1890.
ting patterns are described as singlet (s), doublet (d), triplet (t),
quartet (q), multiplet (m), broad (br), and doublet of doublet (dd).
Fourier-transform infrared (FTIR) spectra were recorded in the
range of 4000e400 cmꢂ1 on a Bruker, TENSOR 27 spectrometer in
KBr pellet. High resolution mass spectra (HRMS) were recorded
using the LC-DAD-ESI-MS/MS system consisted of a Waters Alliance
2695 LC-DAD and a Q-TOF 2 (quadrupole mass filter-time-of-flight)
mass spectrometer with a Z-spray ES source.
Compound 3d: White solid, Rf (60% MeOH/EtOAc) 0.57; mp
(dec)ꢃ270 ꢀC; nmax (KBr) 3483, 3077, 1735, 1675, 1643, 1595, 1449,
1414, 1331, 1262 cmꢂ1
; dH (400 MHz 2% NaOH in D2O) 7.75 (d,
J¼8.8 Hz, 2H), 7.52 (d, J¼8.8 Hz, 2H), 2.33 (m, 2H), 4.16 (m, 2H), 1.87
(m, 2H), 2.04 (m, 2H); dC (100.6 MHz 2% NaOH in D2O) 182.2, 180.6,
175.3, 167.4, 165.5, 141.6, 129.9, 129.6, 120.2, 118.9, 56.0, 31.5, 28.6;
HRMS (ESI): MNaþ, found 529.1307. [C20H22N6O10Na]þ requires
529.1295.
4.2. Synthesis of 4-[(4,6-dichloro-1,3,5-triazin-2-yl)oxy]-ben-
zoic acid (1b)
To a solution of TCT (1.84 g, 0.01 mol) in acetone (10 mL),
a
mixture of sodium hydroxide (0.8 g, 0.02 mol), and 4-
Compound 3e: White solid, Rf (60% MeOH/EtOAc) 0.45; mp
hydroxybenzoic acid (1.38 g, 0.01 mol) in deionized water
(10 mL) was added slowly at 0e5 ꢀC with continuous stirring. The
reaction was continued further for 3 h. At the end of the reaction,
the mixture was poured into acidified water and the whitish solid
was collected by filtration. It was washed several times with hot
water and dried at 80 ꢀC, giving the product in 82% yield. 1H NMR
(dec)ꢃ270 ꢀC; nmax (KBr) 3374, 1758, 1673, 1538, 1405, 1316,
1279 cmꢂ1
;
dH (400 MHz 2% NaOH in D2O) 7.74 (d, J¼8.4 Hz, 2H),
6.64 (d, J¼8.4 Hz, 2H), 3.89 (s, 4H); dC (100.6 MHz 2% NaOH in D2O)
178.4, 176.2, 170.2, 167.1, 165.5, 131.6, 120.9, 118.4, 44.1; HRMS (ESI):
MHþ, found 364.0893. [C14H14N5O7]þ requires 364.0893.
Compound 3f; White solid (0.121 g, 0.25 mmol, 72%). Rf (60%
(D2O, 400 MHz):
d
6.56 (d, J¼8.4 Hz, 2H), 7.67 (d, J¼8.4 Hz, 2H).
MeOH/EtOAc) 0.35; mp (dec)ꢃ270 ꢀC; nmax (KBr) 3112, 1746, 1699,
1662, 1583, 1497, 1382, 1310, 1214 cmꢂ1
; dH (400 MHz 2% NaOH in
4.3. Synthesis of N,N0-(6-chloro-1,3,5-triazine-2,4-diyl)bis-
glycine (2a)
D2O) 7.68 (d, J¼7.2 Hz, 2H), 6.58 (d, J¼7.2 Hz, 2H), 3.95 (s, 4H), 4.08
(s, 4H); dC (100.6 MHz 2% NaOH in D2O) 179.4, 176.5, 170.6, 166.8,
164.1, 131.6, 121.2, 118.4, 51.7; HRMS (ESI): MNaþ, found 502.0827.
[C18H17N5O11Na]þ requires 502.0822.
A solution of glycine (6.00 g, 80 mmol) and sodium hydroxide
(12.1 g, 288 mmol) in 50 mL of water was added dropwise into TCT
(7.51 g 40 mmol) in 50 mL at 0 ꢀC under stirring. After 3 h stirring at
room temperature, the pH value was adjusted to about 2 using
concentrated HCl, standing at 0e5 ꢀC for 20 min. The white crystal
powder was collected by filtration, washed with water and then
dried at 80 ꢀC, giving the product in 84% yield. 1H NMR (D2O,
Compound 3g:6 White solid, Rf (60% MeOH/EtOAc) 0.32; nmax
(KBr) 1724, 1674, 1623, 1522, 1435, 1401, 1365, 1319, 1293,
1233 cmꢂ1
;
dH (400 MHz 2% NaOH in D2O) 4.08 (s, 4H), 3.20 (s, 4H).
Compound 3h:6 White solid, Rf (60% MeOH/EtOAc) 0.29; nmax
(KBr) 1714, 1621, 1557, 1488, 1401, 1327, 1232 cmꢂ1
dH (400 MHz 2%
;
NaOH in D2O) 4.05 (s, 4H), 3.30 (m, 2H), 1.21 (d, J¼7.0 Hz, 6H).
400 MHz):
d 3.74 (s, 4H).
4.5. General procedure for the microwave-assisted synthesis
of ligand 3 from disubstituted TCT 2
4.4. General procedure for the microwave-assisted synthesis
of ligand 3 from monosubstituted TCT 1
Amino acid (1.2 mmol) and disubstituted triazine 3 (1.0 mmol)
were added with NaOH (3.6 mmol) and 2 mL of H2O before mi-
crowave irradiation at 180 W for the specified time. The product
was isolated and characterized according to the above described
procedure.
A solution of amino acid (2.4 mmol) and NaOH (7.2 mmol) in
1 mL H2O was slowly added into a vessel containing a 1 mL
aqueous solution of the monosubstituted triazine 1 (1.0 mmol).
After completed addition, boiling chips were added and the
reaction vessel was microwave irradiated at 180 W for the
prescribed time. After cooling down, the reaction mixture was
acidified using concentrated HCl. The precipitate was recovered
by filtration, washed successively with H2O and acetone before
drying at 80 ꢀC to afford pure product. New compounds were
fully characterized by 1H NMR, 13C NMR, FTIR, and HRMS anal-
yses. Spectroscopic data for known compounds were consistent
with those reported in the literature.
Compound 3i: White solid, Rf (60% MeOH/EtOAc) 0.14; mp
(dec)ꢃ300 ꢀC; nmax (KBr) 3385, 3103, 1698, 1639, 1593, 1496, 1412,
1240 cmꢂ1
;
dH (400 MHz 2% NaOH in D2O) 7.74 (d, J¼8.4 Hz, 4H),
7.45 (d, J¼8.4 Hz, 4H), 3.77 (s, 2H); dC (100.6 MHz 2% NaOH in D2O)
178.2,175.2, 165.5,163.6, 141.3, 129.5, 119.2, 118.9, 44.6; HRMS (ESI):
MHþ, found 425.1212. [C19H17N6O6]þ requires 425.1210.
Compound 3j: White solid, Rf (60% MeOH/EtOAc) 0.12; mp
(dec)ꢃ270 ꢀC; nmax (KBr) 3389, 3078, 1717, 1684, 1594, 1557, 1497,
Compound 3a: White solid, Rf (60% MeOH/EtOAc) 0.28;
1416, 1232 cmꢂ1
;
dH (400 MHz 2% NaOH in D2O) 7.80 (d, J¼8.4 Hz,
mpꢃ300 ꢀC; nmax (KBr) 3334, 3099, 1730, 1675, 1595, 1447, 1412,
4H), 7.58 (d, J¼8.4 Hz, 4H), 4.20 (s, 4H); dC (100.6 MHz 2% NaOH in
D2O) 177.8, 175.0, 165.5, 163.7, 141.7, 130.0, 119.9, 119.2, 52.6; HRMS
(ESI): MHþ, found 483.1268. [C21H19N6O8]þ requires 483.1264.
Compound 3k: White solid, Rf (80% MeOH/EtOAc) 0.28; mp
(dec)ꢃ300 ꢀC; nmax (KBr) 3374, 3042, 1739, 1681, 1618, 1592, 1514,
1260 cmꢂ1
;
dH (400 MHz 2% NaOH in D2O) 7.80 (d, J¼8.4 Hz, 2H),
7.57 (d, J¼8.4 Hz, 2H), 3.84 (s, 4H); dC (100.6 MHz 2% NaOH in D2O)
178.3, 175.3, 165.1, 163.6, 141.5, 129.8, 120.6, 119.3, 44.5; HRMS (ESI):
MHþ, found 363.1051. [C14H15N6O9]þ requires 363.1053.
Compound 3b: White solid, Rf (80% MeOH/EtOAc) 0.43; mp
1454, 1433, 1331, 1244 cmꢂ1
; dH (400 MHz 2% NaOH in D2O) 6.87 (d,
(dec)ꢃ270 ꢀC; nmax (KBr) 3342, 3096, 1730, 1681, 1549, 1504, 1404,
J¼8.0 Hz, 2H), 6.47 (d, J¼8.0 Hz, 2H), 3.75 (s, 4H), 3.26 (dd, J¼7.4,
5.2 Hz, 1H), 2.74 (dd, J¼13.7, 5.2 Hz, 1H), 2.56 (dd, J¼13.7, 7.4 Hz,
1H); dC (100.6 MHz 2% NaOH in D2O) 182.2, 178.4, 168.0, 165.2,
1330, 1237 cmꢂ1
;
dH (400 MHz 2% NaOH in D2O) 7.82 (d, J¼8.8 Hz,
2H), 7.62 (d, J¼8.8 Hz, 2H), 4.08 (s, 4H), 4.13 (s, 4H); dC (100.6 MHz

W. Funfuenha et al. / Tetrahedron 70 (2014) 5415e5419
5419
163.0, 130.6, 124.0, 118.0, 57.3, 44.1, 39.7; HRMS (ESI): MNaþ, Found
429.1126. [C16H18N6O7Na]þ requires 429.1135.
5. Huang, Y.-h.; Sheng, T.-l.; Zhu, Q.-l.; Tan, C.-h.; Fu, R.-b.; Hu, S.-m.; Wu, X.-t.
Chin. J. Struct. Chem. 2013, 32, 1572.
6. Huang, Y.; Zhu, Q.; Sheng, T.; Hu, S.; Fu, R.; Shen, C.; Tan, C.; Wen, Y.; Bai, S.; Wu,
X. CrystEngComm 2013, 15, 3560.
7. Zhu, Q.-L.; Sheng, T.-L.; Fu, R.-B.; Hu, S.-M.; Chen, L.; Shen, C.-J.; Ma, X.; Wu, X.-T.
Chem.dEur. J. 2011, 17, 3358.
Acknowledgements
8. Zhu, Q.; Sheng, T.; Tan, C.; Hu, S.; Fu, R.; Wu, X. Inorg. Chem. 2011, 50, 7618.
9. Zhu, Q.; Sheng, T.; Fu, R.; Hu, S.; Shen, C.; Ma, X.; Wu, X. CrystEngComm 2011, 13,
2096.
We thank the Center of Excellence for Innovation in Chemistry
(PERCH-CIC), the Graduate School, Chiang Mai University, and the
National Research University Project under Thailand’s Office of the
Higher Education Commission for financial support to this research.
Financial support from Rajamangala University of Technology
Lanna Chiang Mai (to W. Funfuenha) is also acknowledged.
10. Shen, C.; Sheng, T.; Fu, R.; Hu, S.; Zhu, Q.; Ma, X.; Huang, Y.; Wu, X. Inorg. Chem.
Commun. 2011, 14, 1119.
11. Zhu, Q.; Sheng, T.; Fu, R.; Hu, S.; Chen, J.; Xiang, S.; Shen, C.; Wu, X. Cryst. Growth
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14. Ghanashyam Acharya, S. N.; Srinivasa Gopalan, R.; Kulkarni, G. U.; Venkatesan,
Supplementary data
K.; Bhattacharya, S. Chem. Commun. 2000, 1351.
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.tet.2014.06.127.
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