Liquid crystalline bis(N-salicylideneaniline)s: Synthesis and thermal behavior of constitutional isomers

Article abstract of DOI:10.1016/j.tetlet.2013.04.071

The first examples of mesogenic bis(N-salicylideneaniline)s (BSANs), wherein two lipophilic (half-disk shaped) entities are interlinked through the dihydroxydiformylbenzene core, were synthesized and characterized. In particular, three constitutional (positional) isomeric BSANs were prepared by the facile twofold condensation of 3,4,5-tris(alkoxy)anilines with 2,3-dihydroxyterephthalaldehyde, 4,6-dihydroxyisophthalaldehyde, and 2,5-dihydroxyterephthalaldehyde and their structures were established by elemental analyses, FT-IR, ¹H NMR, and ¹³C NMR. Proton NMR experiments demonstrated their existence in enol-imine (OH) form solely. Polarizing optical microscopic, differential scanning calorimetric, and powder X-ray diffraction studies evidenced the occurrence of columnar mesomorphism in two sets of isomers.

Full text of DOI:10.1016/j.tetlet.2013.04.071

Tetrahedron Letters 54 (2013) 3419–3423
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Liquid crystalline bis(N-salicylideneaniline)s: synthesis and thermal behavior
of constitutional isomers
⇑
Uma S. Hiremath
Centre for Soft Matter Research, Prof. U.R. Rao Road, Jalahalli, Bangalore 560013, Karnataka, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The first examples of mesogenic bis(N-salicylideneaniline)s (BSANs), wherein two lipophilic (half-disk
shaped) entities are interlinked through the dihydroxydiformylbenzene core, were synthesized and char-
acterized. In particular, three constitutional (positional) isomeric BSANs were prepared by the facile two-
fold condensation of 3,4,5-tris(alkoxy)anilines with 2,3-dihydroxyterephthalaldehyde, 4,6-
dihydroxyisophthalaldehyde, and 2,5-dihydroxyterephthalaldehyde and their structures were estab-
lished by elemental analyses, FT-IR, 1H NMR, and 13C NMR. Proton NMR experiments demonstrated their
existence in enol–imine (OH) form solely. Polarizing optical microscopic, differential scanning calorimet-
ric, and powder X-ray diffraction studies evidenced the occurrence of columnar mesomorphism in two
sets of isomers.
Received 18 January 2013
Revised 16 April 2013
Accepted 18 April 2013
Available online 25 April 2013
Keywords:
Bis(N-salicylideneaniline)s
Shape-anisotropy
Hexacatenars
Intramolecular H-bonding
Columnar phase
Ó 2013 Elsevier Ltd. All rights reserved.
It is well-known that the facile reaction of salicylaldehydes (I-2)
with primary amines readily affords a unique class of organic com-
pounds called N-salicylideneanilines (SANs), the o-hydroxy Schiff
bases (II-2), where a highly stable, hydrogen (H)-bridged, quasi
six-membered ring exists due to intramolecular H-bonding be-
tween the H-atom of the hydroxy group and the N-atom of the
imine linkage.1,2 The significance of these systems, in the context
of both basic and applied research, primarily originates from the
fact that they show reversible proton-transfer processes both in
solution and solid state to form two tautomers viz., enol–imine
(OH) and keto-enamine (NH) forms. This unique behavior enables
SANs to exhibit technologically important phenomena such as
thermochromism and photochromism.2,3 These tautomers are ex-
pected to be in equilibrium with each other, but it is the (OH) form
that is commonly found and thus, well studied.
fact, five constitutional (positional) isomers of BSANs, II-2,3, II-
4,6, II-2,5, II-2,4, and II-3,6, respectively, derived from
H
O
O
H
H
O
O
H
H
O
O
H
2 R-NH2
R
N
N
R
R
N
N
R
O
O
-2 H2
O
2,3-Dihydroxyterephthalaldehyde : I-2,3
BSAN - (OH) form: II-2,3
BSAN - (NH) form: II-2,3
O
O
O
O
O
O
H
N
H
2 R-NH2
-2 H2O
H
N
H
N
H
O
H
O
X
R
N
R
R
R
R
4,6-Dihydroxyisophthalaldehyde : I-4,6
BSAN - (OH) form: II-4,6
BSAN - (NH) form: II-4,6
O
O
R
O
R
2 R-NH2
-2 H2O
H
O
H
N
N
O
H
H
N
H
X
H
O
N
R
O
O
2,5-Dihydroxyterephthalaldehyde : I-2,5
BSAN - (OH) form: II-2,5
BSAN - (NH) form: II-2,5
R
R
O
H
O
H
O
H
N
O
O
H
O
H
N
O
O
H
O
O
2 R-NH2
-2 H2
N
R
H
N
R
N
R
O
O
H
R-NH2
-H2O
H
N
R
O
R
R
R
2,4-Dihydroxyisophthalaldehyde : I-2,4
BSAN - (OH) form: II-2,4
BSAN - (NH) form: II-2,4
SAN : II-2
SAN : II-2
R
R
N
R
R
Salicylaldehyde : I-2
Enol-imine (OH) form
Keto-enamine (NH) form
O
O
O
O
N
N
N
O
2 R-NH2
-2 H2O
H
H
H
H
O
H
H
Notably, analogous compounds namely, bis(N-salicylidene-ani-
line)s (BSANs)4–7 and tris(N-salicylideneaniline)s (TSANs)8–10 have
also been made to reveal their novel material and electronic prop-
erties. Recently, MacLachlan and co-workers have carried out pio-
neering work on BSANs which are made by the twofold
condensation of 1° amines with dihydroxydiformylbenzenes.5 In
O
O
3,6-Dihydroxyphthalaldehyde : I-3,6
BSAN - (OH) form: II-3,6
BSAN - (NH) form: II-3,6
R
H
R
H
O
N
O
H
O
O
O
H
O
N
O
3 R-NH2
-3 H2O
O
R
N
H
R
N
H
X
O
O
O
O
H
N
H
H
N
⇑
Tel.: +91 80 2838 1119x232; fax: +91 80 2838 2044.
E-mail addresses: hiremath@csmr.res.in, umahiremath66@gmail.com
R
R
1,3,5-triformylphloroglucinol : I-1,3,5
TSAN - (NH) form: II-1,3,5
TSAN - (OH) form: II-1,3,5
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.04.071

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U. S. Hiremath / Tetrahedron Letters 54 (2013) 3419–3423
dihydroxydiformylbenzenes, I-2,3, I-4,6, I-2,5, I-2,4, and I-3,6, are
known. The enol–imine (OH) form of BSANs II-2,3, II-2,4
and II-3,6 can tautomerize to give corresponding keto-enamine
(NH) form; however, such a process in BSANs II-4,6 and II-2,5 gives
rise to radical intermediates and thus, ruled out. Although the
exploration on BSAN materials is still in its infancy and thus, re-
mains elusive,5–7 they may be regarded as one of the significant
classes of supramolecular functional materials given the recent
observation that a specially designed BSAN, existing in (NH) form,
gives rise to H-bonded cleft in solution and an elongated, H-bond
assisted ladder-assembly in the solid state.5 Likewise, TSANs (II-
1,3,5), which are produced from the reaction of 1,3,5-triformylphl-
oroglucinol (I-1,3,5) with excess of aniline derivatives, have been
demonstrated by MacLachlan’s7 and Lee’s8 research groups as
functional motifs capable of yielding supramolecular networks.8,9
Interestingly enough, they generally exist in (NH) tautomeric
form,8–10 unlike BSANs. These systems with inter- and intra-molec-
ular H-bonding abilities, display tunable electronic and photo-
physical properties8–10 and thus, they are strongly gaining impor-
tance in material science.
Most importantly, since long ago, some of these motifs have
been incorporated in developing thermally and hydrolytically sta-
ble thermotropic mesogens exhibiting a rich variety of liquid crys-
tal (LC) phases.11–15 Specially, the molecular architecture of SAN
has been used extensively in synthesizing rod-like,11 banana-
shaped12 and dimeric13,14 bidentate ligands and their correspond-
ing metallo-mesogens.11,15 Due to the presence of the SAN core
some of these LCs have shown remarkable phase transitional
behavior.10–14 By utilizing the TSAN core we have developed an en-
tirely new class of disk-like (discotic) LCs existing exclusively in
ketoenamine tautomeric forms.10 These discotics show columnar
LC phases where protons and electrons can interact with each
other through the H-bonding environment. Markedly, the fluid/fro-
zen Col phases formed by these discotics show photolumines-
cence.9 On the other hand, to the best of our knowledge, liquid
crystallinity (mesomorphism) has not been induced in BSAN mate-
rials. That is, mesomorphic BSANs have not been reported hitherto
despite their attractive features in terms of both basic and applied
research points of view.
In this Letter, the synthesis and thermal behavior of the first
examples of BSANs exhibiting LC behavior are reported. As shown
in Figure 1 (left portion), three constitutional (positional) isomeric
BSANs, II-2,3-a–b, II-4,6-a–b, and II-2,5-a–b were prepared from
the corresponding dihydroxydiformylbenzenes, I-2,3, I-4,6, and I-
2,5 and investigated for their mesomorphism.
The target BSANs were prepared via the synthetic route out-
lined in Scheme 1. The requisite 1,2-dimethoxybenzene (1), 1,3-
dimethoxybenzene (2), and 1,4-dimethoxybenzene (3) were pre-
pared in high yield by the reaction of respective benzenediols
viz., catechol, resorcinol, and hydroquinone with dimethyl-sulfate
using potassium carbonate (K2CO3) as a mild base and acetone as
solvent. The dilithiation of veratrole 1 with n-butyllithium (n-BuLi)
in the presence of tetramethylethylene-diamine (TMEDA)16a fol-
lowed by dimethylformamide (DMF) treatment yielded 2,3-dim-
ethoxyterephthalaldehyde (4).16b,c Subsequent boron tribromide
(BBr3) mediated O-demethylation of 4 afforded 2,3-dihydroxyte-
rephthalaldehyde (I-2,3) in 84% yield.16b,c Compound 2 was bromi-
nated using two equivalents of lithium bromide (LiBr) in the
presence of copper(II) bromide (CuBr2) and oxygen atmosphere
to obtain 1,5-dibromo-2,4-dimethoxybe-nzene (5) in 92% yield.17
The lithium–bromine exchange of 5 with n-BuLi followed by reac-
tion with N-methylformanilide gave 4,6-dimethoxyisophthalalde-
hyde (6); demethylation of
6 using aluminum chloride in
nitrobenzene furnished the requisite 4,6-dihydroxyisophthalalde-
hyde (I-4,6).18 Treating 1,4-dimethoxybenzene (3) first with n-BuLi
in the presence of TMEDA16a then with DMF gave 2,5-dimethoxy-
tere-phthalaldehyde (7). Compound 7 was demethylated with
48% HBr (aq) to get 2,5-dihydroxyterephthalaldehyde (I-2,5)19 in
42% yield. The reaction of pyrogallol with a large excess of 1-bro-
mooctane/1-bromodecane in the presence of K2CO3 using DMF as
solvent yielded 1,2,3-trialkoxybenzenes (8a–b)10c Two-phase
nitration10c,20a of 8a–b using aqueous solution of nitric acid (70%)
and sodium nitrite in dichloromethane gave 3,4,5-trialkoxynitro-
benzenes (9a–b). These nitro compounds were subjected to cata-
lytic (Pd–C/H2) hydrogenation to obtain 3,4,5-trialkoxyanlines
(10a–b).10c,20b
Finally, dihydroxydiformylbenzenes, I-2,3, I-4,6, and I-2,5 were
condensed with two equivalents of alkoxyanilines 10a–b in reflux-
RO
OR
OR
H
O
O
H
N
N
RO
OR
(a)
(b)
RO
II-2,3-a : R = C8H17 ; II-2,3-b : R= C10H21
O
O
H
N
H
N
RO
OR
OR
RO
OR
OR
II-4,6-a : R = C8H17 ; II-4,6-b : R= C10H21
OR
O
H
RO
N
OR
RO
N
OR
(c)
H
O
RO
II-2,5-a : R = C8H17 ; II-2,5-b : R = C10H21
Figure 1. Left portion: Molecular structures of three types of BSANs synthesized where the dotted lines represent H-bonds existing between N and O atoms. Right portion:
Space-filling energy minimized (all-trans) molecular models of II-2,3-a (a), II-4,6-a (b), and II-2,5-a (c) derived from MM2 method.

U. S. Hiremath / Tetrahedron Letters 54 (2013) 3419–3423
3421
HO
OH
HO
HO
HO
OH
(a)
HO
OH
HO
(i)
(i)
(i)
(viii)
H3CO
H3CO
H3CO
OCH3
OCH3
RO
RO
H3CO
1 (91 %)
2 (90 %)
3
(92 %)
(ii)
OR
8a: R= C8H17 (71 %)
8b: R= C10H21 (65 %)
(ii)
(iv)
CHO
OCH3 OHC
OCH3
(ix)
H3CO
Br
H3CO
H3CO
RO
RO
NO2
CHO
H3CO
Br
34.0
33.5
33.0
32.5
32.0
31.5
31.0
7 (92 %)
5 (92 %)
(b)
CHO
(v)
(vii)
4 (38 %)
OR
9a: R= C8H17 (73 %)
9b: R= C10H21 (71 %)
(iii)
OCH3
CHO
H3CO
OHC
OHC
HO
OH
(x)
CHO
CHO
HO
HO
6
(61 %)
I-2,5 (42 %)
RO
RO
NH2
(xi)
(vi)
CHO
II-2,5-a II-2,5-b
;
OR
OH
HO
I-2,3 (84 %)
10a
10b
: R= C8H17 (93 %)
: R= C10H21 (91 %)
(xi)
(xi)
OHC
CHO
I-4,6 (72 %)
35
40
45
50
55
60
65
70
75
80
II-2,3-a II-2,3-b
II-4,6-a II-4,6-b
;
;
Temperature °C
Scheme 1. Synthesis of BSANs. Reagents and conditions; (i) (CH3)2SO4, anhyd
K2CO3, acetone, reflux, 12 h; (ii) (a) n-BuLi, TMEDA, Et2O, 0 °C, then reflux 18 h, (b)
DMF, rt, 12 h then H2O; (iii) BBr3, CH2Cl2, rt, 4 h; (iv) LiBr (2 equiv), CuBr2, AcOH, O2
(1 atm; balloon), 24 h; (v) n-BuLi, N-methylformanilide, Et2O, rt, 5 min then 10%
HCL; (vi) AlCl3, nitrobenzene, 120 °C, 1 h; (vii) 48%, HBr (aq), reflux, 24 h; (viii) 1-
bromooctane/1-bromodecane, anhyd K2CO3, KI, DMF, 80 °C, 12 h; (ix) 70%, HNO3,
NaNO2, CH2Cl2, rt, 1 h; (x) 10% Pd–C, THF, rt, 12 h; (xi) 10a/10b, EtOH, reflux, 12 h.
Figure 2. (a) Microphotograph of the optical texture observed at 65 °C for the Col
phase of II-2,3-a. (b) DSC traces of the first heating–cooling cycles recorded for II-
2,3-a at a rate of 5 °C/min.
texture at about 57 and 53 °C, respectively; on further heating the
samples, a transition to the isotropic liquid state occurred at 74.5
and 70.9 °C. On cooling from the isotropic phase, a transition to
the LC phase occurred at about 73 °C (
DH = 2.8 J/g) and 69 °C
Table 1
(D
H = 2.8 J/g), which remains unchanged until crystallization at
Phase sequences, transition temperatures (°C)a and enthalpies (J/g) associated with
ꢀ48 and 44 °C for samples II-2,3-a and II-2,3-b, respectively. As
a representative case, the photomicrograph of a textural pattern
seen at 65 °C for II-2,3-a is shown in Figure 2a. This pattern com-
prising the regions of uniform extinction and tiny fan-shaped as
well as rectilinear defects is typical of an uniaxial, hexagonal Col
(Colh) phase. Calorimetric study complemented the transition tem-
peratures noted during the POM study; as a representative case,
the DSC thermograms obtained for II-2,3-a are shown in Figure 2b.
As can be seen in Table 1, BSANs II-4,6-a and II-4,6-b, derived
from 4,6-dihydroxyisophthalaldehyde, behave identically exhibit-
ing a metastable LC phase. When a thin film of any of these two
BSANs, held between two plates, is gradually cooled from the iso-
tropic phase, a texture appears sharply manifesting optically in the
form of regions of uniform extinction and tiny fan-shaped/rectilin-
ear defects. Figure 3 depicts the optical texture observed for II-4,6-
a at 50 °C. It must be remarked here that this textural pattern is
virtually identical to the ones seen for the Col phase of II-2,3-a
and II-2,3-b. This implies that BSANs II-2,3-a, II-2,3-b, II-4,6-a,
and II-4,6-b show an identical Col phase. However, in the latter
two cases the LC phase freezes into a glassy state (see Table 1),
which is noteworthy given the fact that frozen Col phases are
promising for practical use.21
phase transitions of BSANs
Phase sequence
BSANs
Heating; cooling
II-2,3-a
II-2,3-b
II-4,6-a
II-4,6-b
II-2,5-a
II-2,5-b
Cr 57.3 [13.6] Col 74.5 [2.9] I; I 72.5 [2.8] Col 48.2 [13.4] Cr
Cr 52.7 [17.9] Col 70.9 [2.9] I; I 68.7 [2.8] Col 43.9 [16.3] Cr
Cr 64.5 [39.3] I; I 52.6 [2.2] Colb
Cr 51.8 [7.9] Cr1 56.3 [18] Cr2 61.1 [0.8] I; I 60.9 [2.1] Colb
Cr 139.6 [38.2] I; I 132.5 [15.4] Cr
Cr 127.9 [34.9] I; I 123.1 [35.9] Cr
a
Peak temperatures in the DSC thermograms obtained during the first heating–
cooling cycles at 5 °C/min. Cr = crystal; Col = columnar phase; I = isotropic liquid
state.
b
The Col phase did not crystallize upon cooling down to room temperature.
ing ethanol to obtain three isomeric BSANs, II-2,3-a–b, II-4,6-a–b,
and II-2,5-a–b. The structures of the BSANs synthesized were
established by elemental analysis and spectroscopic (IR, 1H, and
13C NMR) data (see Supplementary data). As expected these com-
pounds were found to exist in enol-imine (OH) form that was as
ascertained by 1H NMR spectral studies.
The phase behavior of all the newly synthesized BSANs was
investigated by means of polarizing optical microscopy (POM), dif-
ferential scanning calorimetry (DSC) and X-ray diffraction (XRD).
The results derived from these complementary studies are summa-
rized in Table 1.
BSANs II-2,5-a and II-2,5-b derived from 2,5-dihydroxy-tereph-
thalaldehyde did not show liquid crystalline properties. This
behavior is rather surprising given the fact that the anisotropic
shape of these compounds, which is similar to that of II-2,3-a
and II-2,3-b, appears to be conducive in exhibiting mesomorphism.
In particular, BSANs II-2,3-a, II-2,3-b, II-2,5-a, and II-2,5-b belong
Compounds II-2,3-a and II-2,3-b, sandwiched between two
glass plates, upon heating, melted into a LC phase with an atypical

3422
U. S. Hiremath / Tetrahedron Letters 54 (2013) 3419–3423
(a)
(b)
7
4
1
5
10
15
2θ (deg)
20
25
Figure 4. (a) XRD pattern (inset) and 1D-intensity versus 2h profile obtained for the
Colh phase of II-2,3-b at 60 °C. (b) Schematic representation of the Colh phase where
the individual columns formed by the piling up of clusters of hexacatenars is
shown.
Figure 3. Photomicrograph of the texture observed at 50 °C for the Col phase of
compound II-4,6-a.
to a special class of hybrid mesogenic motifs called hexacatenars
where an elongated rod-like (calamitic) rigid core comprise half-
disk (in the form of three paraffinic chains) entities at both ends.22
This can be visualized in energy minimized (all-trans) molecular
models of II-2,3-a and II-2,5-a shown in Figure 1a and c, respec-
tively. As can be seen, three aliphatic chains tethered at the
extremities are disposed symmetrically that enable hexacatenar
mesogens to exhibit Col mesomorphism predominantly.22 In fact,
the organization of these non-classical mesogens in the individual
columns is quite different when compared to conventional discotic
LCs. To begin with, a number of hexacatenars (on average three or
four) self-assemble in a side by side manner (laterally) to form
columnar clusters (slices). These slices with the terminal tails
forming a liquid matrix around, then pile one upon another to give
indefinitely long columns where long axis of the constituent elon-
gated rigid cores remain perpendicular to the columnar axis. Such
an organization of BSANs II-2,5-a and II-2,5-b would lead to Col
phase formation. However, they do not exhibit the anticipated
Col behavior that can possibly be attributed to the overall geome-
try (in all-trans conformation) where the phenyl rings present on
the either side stay away from the long molecular axis. Conse-
quently, BSANs II-2,5-a and II-2,5-b, unlike II-2,3-a and II-2,3-b,
are not linear (elongated rod-like) mesogens and thus, fail to stack
together to form columnar slices.
On the other hand, the occurrence of Col phase in materials II-
4,6-a and II-4,6-b can be explained based on their shape. These
compounds can be regarded as bent-rod hexacatenars23 and the
organization of such motifs within the columnar phase is well
known to be originating from the antiparallel organization of near-
est neighboring molecules.23
As discussed earlier, BSANs II-2,3-a, II-2,3-b, II-4,6-a, and II-
4,6-b show an indistinguishable columnar behavior that was
established based on the observation of akin optical textures. In
fact, miscibility study by means of contact preparation carried
out suggested that the Col phases formed by II-2,3-a, II-2,3-b, II-
4,6-a, and II-4,6-b are identical. Therefore, as a representative case,
the structure of Col phase formed by II-2,3-b was probed with the
help of XRD. The measurement was performed on an unaligned
fact, the presence of this broad peak indicates the association of
three molecules in
a columnar slice. The low angle region
(0 < 2h < 5°) shows a strong reflection corresponding to a Bragg
spacing (d) of 26.8 Å. The nonexistence of multiple reflections in
this Col phase can be attributed to a minimum in the form factor.
While the presence of only one (d100) peak excludes the unambig-
uous assignment, such a pattern has been assigned to a two-
dimensional hexagonal lattice (Colh phase).10,24 Besides, in the
present case, the optical textural pattern also supports the hexag-
onal columnar structure. Thus, the XRD study coupled with optical
texture confirms the occurrence of Colh phase.
Summarizing, in this Letter, the syntheses and characterization
of three constitutional isomeric bis(N-salicylideneaniline)s are de-
scribed. Mesogens derived from 2,3-dihydroxyterephthalalde-hyde
and 4,6-dihydroxyisophthalaldehyde show columnar mesomor-
phism convincingly whereas the compounds obtained from 2,5-
dihydroxyterephthalaldehyde do not show LC behavior. To the best
of our knowledge the occurrence of Col phase in such a class of
materials reported herein is first of its kind. Given the fact that
such motifs have ability to undergo reversible proton-transfer pro-
cesses, they may be regarded as important functional materials.
Further studies through the rational molecular design and synthe-
sis of new LC BSANs, including motifs derived from
dihydroxydiformylbenzenes I-2,4 and I-3,6 are necessary to reveal
their true significance both in fundamental and applied sciences.
Acknowledgments
The author sincerely thanks the Department of Science and
Technology (DST), New Delhi, Govt. of India, for providing financial
support through WOS-A scheme (SR/WOS-A/CS-19/2010); Dr. P.
Asthana and Professor K. A. Suresh for their support and encour-
agement; Dr. G. G. Nair for her support as mentor of the project;
Dr. D. S. S. Rao for providing XRD data; Professor M. J. MacLachlan
for providing authentic sample of aldehydes.
Supplementary data
(powder) sample with CuK (k = 0.15418 nm) radiation using an
/
image plate arrangement. The sample was placed in a Lindemann
capillary tube. The LC phase achieved at ꢀ60 °C by slow cooling
from the isotropic phase of the sample was used for the study.
The XRD pattern obtained, along with the extracted intensity ver-
sus 2h profile is shown in Figure 4. The XRD pattern essentially
confirmed the presence of a LC state, specially the Colh phase. As
can be seen, the wide angle region (2h ꢁ 20°) of the diffractogram
consisted of a diffuse peak at 4.6 Å that can be ascribed to the li-
quid-like correlation of the discs within the column. Another dif-
fuse peak seen at 13.1 Å suggests that the cores of a few
molecules within the column form loosely defined aggregates. In
Supplementary data associated with this article can be found, in
the
online
version,
at
http://dx.doi.org/10.1016/
j.tetlet.2013.04.071.
References and notes
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