Protic ionic liquids as recyclable solvents for the acid catalysed synthesis of diphenylmethyl thioethers

Article abstract of DOI:10.1016/j.crci.2013.04.002

The acid catalysed formation of diphenylmethyl (DPM) thioethers was successfully achieved using the protic ionic liquid (pIL) triethylamine: methanesulfonic acid (TeaMs) as the reaction solvent under microwave irradiation. A slight excess of methanesulfon

Full text of DOI:10.1016/j.crci.2013.04.002

C. R. Chimie 16 (2013) 634–639
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Preliminary communication/Communication
Protic ionic liquids as recyclable solvents for the acid catalysed synthesis
of diphenylmethyl thioethers
b
a
a
Luke C. Henderson a,b, , Megan T. Thornton , Nolene Byrne , Bronwyn L. Fox ,
Kelsey D. Waugh b, Jennifer S. Squire b, Linden Servinis b, Joshua P. Delaney b,
Hannah L. Brozinski b, Luke M. Andrighetto b, Jarrad M. Altimari b
*
a Institute for Frontier Materials, Deakin University, Waurn Ponds Campus, Geelong, 3216, Victoria, Australia
b Strategic Research Center for Chemistry and Biotechnology, Deakin University, Pigdons Road, Waurn Ponds Campus, Geelong, 3216, Victoria, Australia
A R T I C L E I N F O
A B S T R A C T
Article history:
The acid catalysed formation of diphenylmethyl (DPM) thioethers was successfully
achieved using the protic ionic liquid (pIL) triethylamine:methanesulfonic acid (TeaMs) as
the reaction solvent under microwave irradiation. A slight excess of methanesulfonic acid
(10% v/v) was required to facilitate the reaction, which was applied to a variety of thiols.
Aliphatic, aromatic and heterocyclic aromatic thiols were converted to their corresponding
DPM thioethers in high yields (63–99%), in short reaction times (5–20 min) and using mild
temperatures (80–100 8C). Finally, the pIL (TeaMS) was recycled five times without loss of
yield.
Received 28 November 2012
Accepted after revision 3 April 2013
Available online 6 May 2013
Keywords:
Protic ionic liquid
Thioether
Thiol
´
ß 2013 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.
Microwave irradiation
1. Introduction
amidosulfones, and require the use of Lewis acids or
proton sources through an additional Friedel–Crafts
The application of thioethers in organic chemistry has
been extremely varied; they are crucial to the generation of
sulfur based heterocycles, are used in agrochemicals and
pharmaceuticals, and have been utilised as a protecting
group for thiols, all with great success [1,2]. Diphenyl-
methyl (DPM) thioether has been reported in several
medicinal applications [3]; it features in the recent
synthesis of novel benzothiazepines for the treatment of
type II diabetes [4], and forms the central core of Modafinil,
a compound used to treat narcolepsy [5].
Additionally, the oxidation of DPM thioethers to their
corresponding sulfones, such as compound 1, accesses
reagents which have been successfully employed in the
elaboration of both alkyl and olefinated triphenyl scaffolds,
via Friedel–Crafts reactions and the use of Grignard
synthetic step to synthesise the sulfone product. Given
their synthetic versatility, there is a constant demand for
rapid, high yielding and clean methods for synthesising
these intermediates.
The formation of DPM thioethers typically uses
diphenylmethanol 2 and employs strong Brønsted acids
and the desired thiol [5d,7], or conversely, the use of
diphenylmethylene halides and the corresponding thiolate
anion [8]. There have been recent examples of both ZrCl4
and HClO4-supported on silica [9] and Lewis acids [10] as
catalysts for this transformation, as well as AlPW12O40
catalysts [11], which are able to be recycled. The use of a
strong proton source takes the advantage of the rapid
formation of the diphenylmethylene cationic species 3,
which is then scavenged by the highly nucleophilic thiol
group (Scheme 1).
reagents [6]. These are commonly derived from
a-
In our continuing search for reactions novel to the
application of protic ionic liquid (pIL) solvents [12], our
attention turned to the formation of DPM thioethers. pILs
are a class of ionic liquids which are formed by mixing
equimolar amounts of Brønsted acids and bases [13]. Due
* Corresponding author. Institute for Frontier Materials, Deakin
University, Waurn Ponds Campus, Geelong, 3216, Victoria, Australia.
E-mail address: luke.henderson@deakin.edu.au (L.C. Henderson).
1631-0748/$ – see front matter ß 2013 Acade´mie des sciences. Published by Elsevier Masson SAS. All rights reserved.
http://dx.doi.org/10.1016/j.crci.2013.04.002

L.C. Henderson et al. / C. R. Chimie 16 (2013) 634–639
635
Scheme 1. Acid catalysed formation of DPM thioethers.
Table 1
to their highly polar nature, they are excellent solvents for
Optimisation of thioether formationa.
organic transformations facilitated by microwave irradia-
tion. This manuscript presents an investigation into the
formation of thioethers facilitated by pILs doped with a
slight excess of methansulfonic acid (10% v/v). These
reactions occur rapidly (10–20 min) at mild temperatures
(80–100 8C), and demonstrate an excellent tolerance for
alkyl, aryl and heteroaromatic thiols.
Entry
Solvent
Acid (v/v%)
Time (min)b
Conv. (%)c
1
TeaMs
TeaFa
0
0
20
20
20
20
10
10
10
10
5
0
2
0
3
TeaTFA
TeaH2SO4
TeaMs
TeaMs
TeaMs
Emim
TeaMs
TeaMs
TeaMs
TeaFa
0
0
4
0
0
5
3
34
65
99
58
82
43d
39d
10
Trace
0e
6
5
7
10
10
10
10
10
10
10
10
10
0
8
2. Results and discussion
9
10
11
12
13
14
15
16
2
We chose diphenylmethanol 2 and b-mercaptoethanol
1
as our model reactants foroptimisation, due to the synthesis
of the bifunctional DPM thioether product 5, which holds
significant synthetic utility (as the alcohol moiety offers a
range of versatile functional group inter-conversions).
Additionally, this compound has been reliably synthesised
in high yield by other groups and as such provides a
literature reference for identification [5c,9]. Using triethy-
lamine:methanesulfonic acid (TeaMs) at 80 8C for 20 min
(Table 1, entry 1), we were surprised to see that there was no
trace of the product observed in the crude reaction mixture.
It is not uncommon that one pIL is optimal for a given
transformation and as such, we tried a selection of other
commonly employed pILs (triethylamine:formic acid
(TeaFa), triethylamine:trifluoroacetic acid (TeaTFA) and
triethylamine:sulfuric acid (TeaH2SO4) (Table 1, entries 2–
4)), which showed no improvement in any case (Scheme 2).
As a result, we considered that in this instance, a
catalytic amount of acid is required to facilitate this
10
10
10
10
10
TeaTFA
CH2Cl2
Et2O
10d
0f
Neat
a
Experimental procedure: DPM-OH 2 (100 mg) was placed into a
microwave reactor vessel charged with thiol (0.1 mL) and treated with
acid doped TeaMs (0.25 mL) with a stirrer bar. The vessel was then heated
to the desired temperature for the desired time. The solution was then
diluted with water and diethyl ether, 5 mL of NaOH (2 M solution) was
added and the aqueous phase extracted three times with diethyl ether.
The combined organic phases were then dried (MgSO4), filtered and the
filtrate removed in vacuo to give clear oil.
b
Does not take into account heating and cooling times.
c
Determined by integration of key peaks in the 1H NMR spectrum.
d
The remaining components of this mixture were starting material
and dimerised 2.
e
A complex mixture of products was obtained from this reaction.
f
The crude material from this reaction was only starting alcohol 2.
samples were then doped with 5% and 10% (v/v) acid,
respectively, and upon repeating the reaction, increased
conversions within the 10 min reaction duration were
transformation, thus, we added
a slight excess of
methanesulfonic acid to TeaMS and repeated this reaction
(Table 1, entry 5). We were pleased to observe, in the
presence of acid catalyst (3% v/v), the formation of the
desired compound 5 after 10 min at 80 8C, though in a
moderate conversion (34%). Rather than increase reaction
time to encourage further conversion, we increased the
amount of methanesulfonic acid in the TeaMs solvent. The
noted (Table 1, entries
6 and 7), of 65% and 99%,
respectively. Thus, we determined 10% v/v methanesulfo-
nic acid to be optimal for this system.
Given these conditions, we set out to determine if there
was any benefit of using the pIL in this system. Therefore,
Scheme 2. Thioether formation using
b-mercaptoethanol.

636
L.C. Henderson et al. / C. R. Chimie 16 (2013) 634–639
Scheme 3. Recycling protic ionic liquids and yields after five cycles.
the reaction was repeated in 1-ethyl-3-methylimidazo-
lium methane sulfonate (EMimSO4, an aprotic ionic liquid),
with 10% v/v of methanesulfonic acid doped into the system
(Table 1, entry 8). These conditions gave the desired product
albeit with an inferior conversion (58%), suggesting that
aprotic ionic liquids may be suitable for this transformation,
though further optimisation would be required.
We considered the use of pIL solvents superior for this
transformation as they have several advantages over
imidazole-based ionic liquids:
(Table 1, entry 16) and found that the reaction did not
proceed at all.
Therefore, the optimal reaction conditions were deter-
mined to be: 10 min, 80 8C, TeaMs with 10% methane-
sulfonic acid 10% v/v (Table 1, entry 7). This example offers
not only extremely rapid and clean formation of the
desired product, but also demonstrates selectivity of the
formation of DPM thioether over the ether product.
Removal of the pIL from the reaction mixture can be
carried out via a number of ways:
ꢀ
they are composed of reagents which are ubiquitous
throughout organic chemistry laboratories;
they are relatively cheap;
pILs are the result of equimolar mixing rather than
several synthetic steps which are required for the
isolation of imidazole-based ILs.
ꢀ
ꢀ
aqueous work-up removes the polar pIL by placing the
pIL into the water layer or;
by filtration of the crude reaction mixture through a
silica plug, giving the desired product in the presence of
excess thiol.
ꢀ
ꢀ
Though these are very easy means to remove the pIL, it
Given the nucleophilic nature of thiols in comparison to
is quite wasteful and thus, our attention turned at
investigating the recycling potential of the TeaMs reaction
solvent.
alcohols, it was thought that the formation of the DPM
thioether might be taking place much faster than the
corresponding symmetric ether formation. Halving the
reaction time to 5 min (Table 1, entry 9) gave a similar
To carry out this study (Scheme 3), we chose the same
reactants that were used to optimise the reaction
conditions (as comparison TLC and 1H NMR spectra were
at hand). To recycle TeaMs, the first reaction was carried
out as per usual (10 min, 80 8C, TeaMs with 10% additional
methanesulfonic acid v/v). The reaction mixture was then
cooled to room temperature and the pIL was washed with
diethyl ether twice. The ethereal layer was removed by
pipette and reduced to dryness under vacuum, giving the
desired product as clear oil. The starting reagents were
added to the remaining pIL and the reaction cycle was
repeated.
result to the previous entry, though in
a reduced
conversion (82%). Continuing along this line of investiga-
tion, further reductions in reaction duration to 2 and 1
min(s) (Table 1, entries 10 and 11) gave partial conversion
to the desired thioether 5 (43% and 39%, respectively).
In light of these results for TeaMs, whereby a slight
excess of acid component was required to facilitate this
reaction, we returned to the previously investigated pILs,
which proved unable to facilitate this reaction on their
own. In each case, the acid component of the pIL was added
to each neutral ionic liquid with the exception of
Tea:H2SO4, as the aqueous portion of this acid can compete
with the thiol in the etherification reaction. Applying the
same reaction conditions elucidated earlier (Table 1, entry
7) to each of these now acid doped pILs, very little
conversion to the desired product was observed (Table 1,
entries 12 and 13). Finally, to verify that this reaction could
not be performed in a standard organic solvent under the
same reaction conditions, we repeated this methodology
using dichloromethane and diethyl ether to replace the pIL
solvent and found that in these instances, a complex
mixture of products and starting material 2 were isolated
in the crude reaction mixture in each case (Table 1, entries
14 and 15). Similarly, we repeated the reaction in neat thiol
To our surprise, the reaction progression increased with
each reuse of the pIL, culminating in progressive conver-
sion increase from 84% (first use) to 99% (after fifth use)1.
Though counter-intuitive, this result could be explained by
1
pIL recycling procedure: DPM-OH 2 (100 mg) was placed into a
microwave reactor vessel charged with b-mercaptoethanol (0.1 mL) with
a stirrer bar and treated with TeaMs (0.25 mL). The vessel was then heated
to 80 8C for 10 min under microwave irradiation. The resulting clear
solution was then washed with diethyl ether 2 Â 5 mL) while being
vigorously stirred. The combined diethylether layers were dried (MgSO4)
and solvent removed in vacuo to give clear oil. The remaining ionic liquid
was then retreated with DPM-OH
2 (100 mg), b-mercaptoethanol
(0.1 mL) and the reaction procedure repeated.

L.C. Henderson et al. / C. R. Chimie 16 (2013) 634–639
637
Scheme 4. Scope of thioether formation.
Fig. 1. Structures of Modafinil, an anti-narcoleptic agent, and
diphenylsulfone used in Friedel–Crafts reactions as derived from
DPM thioether.
a
a
the reaction ‘‘work-up’’ procedure; the b-mercaptoethanol
is more polar than both starting material 2 and the desired
product 5. As such, the diethylether washes to remove both
suitable for less reactive thiols. This important intermedi-
ate 4c has been used in several studies on the development
of Modafinil analogues, and as a substrate upon which
chiral oxidations have been carried out to investigate the
active stereochemistry of the pharmaceutical [14].
Isopropyl thiol and cyclohexanethiol (Table 2, entries 5
and 6) gave poor conversion to 4d (11%) and 4e (23%),
respectively at 80 8C. We attributed this lower conversion
to the increased steric bulk of these thiols as they were the
first secondary thiols investigated in this series.
2 and 5 may not remove all of the
the TeaMs layer. Therefore, after several recycles of the pIL,
the residual amount of -mercaptoethanol left behind
b-mercaptoethanol from
b
from each previous use will accumulate in the pIL,
resulting in a faster and more efficient reaction. Analysis
by 1H NMR of the TeaMs layer after the fifth use confirmed
the presence of residual b-mercaptoethanol in the pIL. Our
focus then turned to the scope of the reaction, by
incorporating other thiols (Scheme 4).
Applying the modified conditions used for methylthio-
Primary thiols 1-mercaptobutanol and benzyl-mercap-
tan proceeded under the optimised reaction conditions to
give their corresponding DPM thioethers 4a and 4b in good
to excellent yields (Table 2, entries 1 and 2), respectively.
Our initial attempt employing methylthioglycolate under
the same reaction conditions (Table 2, entry 3) gave very
poor conversion (ꢁ 5%); this was attributed to the
deactivating effect of the methyl ester functionality. We
considered this example extremely important, as it forms
the structural backbone of Modafinil (Fig. 1). In an attempt
to boost the reaction conversion, the reaction temperature
was increased to 100 8C and the reaction time to 20 min
(Table 1, entry 4) and we were pleased to see that under
these slightly modified conditions, the conversion of
methylglycolate to 4c proceeded quantitatively. This also
elucidated a set of reaction conditions which would be
glycolate (100 8C and 20 min) showed a substantial
increase in reaction conversion (Table 2, entries 7 and 8)
for each product, 4d (77%) and 4e (63%). Thioacetic acid
also required an extended reaction time of 20 min (Table 2,
entry 9) to get a synthetically viable yield (75%) of 4f.
Next we examined a series of aromatic thiols, and
considering their steric influence, we chose to only employ
the longer reaction conditions (100 8C, 20 min) to these
examples. Both aromatic thiols, 1-naphthalenethiol and 4-
fluorothiophenol gave their corresponding DPM thioethers
4g and 4h, respectively, in very high yield (99%, Table 2,
entries 10 and 11).
Our attention then turned to the use of heterocyclic
scaffolds, which bear more than one heteroatom in their
core, as complex small molecules of this nature feature
heavily in naturally occurring compounds and in medicinal
Scheme 5. Proposed mechanism of 6 formation including diagnostic shifts of the C2 carbon.

638
L.C. Henderson et al. / C. R. Chimie 16 (2013) 634–639
chemistry [15]. Employing 4-methyl-2-mercaptothiazole
gave the desired product 4i in excellent yield of 94% (Table
2, entry 12) and the application of these same conditions to
2-mercapto-1,3,4-thiadiazole consumed all of the starting
DPM-OH 2 (Table 2, entry 13), though it gave two products.
and the analysis of 6 by 13C NMR spectroscopy confirmed
the identity of the thiocarbonyl ( 185 ppm) of 6 when
compared to the peak C2 of 4j ( 165 ppm). The formation
of this compound is thought to arise via the interception of
the benzhydryl cation by the nitrogen at the 3-position of
the heterocycle, followed by collapsing of the thiol S–H
bond to give the diagnostic thiocarbonyl (Scheme 5). The
N-alkylated thiazole core of 6 is a central component to key
compounds, which have been used to synthesise crown
d
d
This mixture consisted of
a 2:1 ratio of diphenyl-
methylthioether 4j to 6, respectively, as determined by
the integration of key peaks in the 1H NMR spectrum.
These compounds were separated chromatographically
Table 2
Reaction scope of thioether formationa.
Entry
1
Thiol
Temp (8C)
Time (min)
5
Product
Yield (%)b
68
80
4a
2
80
5
4b
99
3
4
80
10
20
4c
4c
5
100
99
5
6
80
80
2
5
4d
4e
11
23
7
8
100
100
20
20
4d
4e
77
63
9
100
100
20
20
4f
75
99
10
4g
11
12
100
100
20
20
4h
4i
99
94
13
100
20
4j
96c
Characterization data for novel compounds: 4a 1H NMR (270 MHz, CDCl3):
d
= 7.30 (m, 10H, ArH), 5.11(s, 1H, CR3H), 3.58 (t, 2H,3JHH = 6.21, CH2OH),
= 141.56, 128.64, 128.38, 127.25, 62.46, 54.22, 32.13,
31.89, 25.35; HRMS, m/z found: MNa+295.11216, (C17H21OS)Na+ requires 295.11271; 4g 1H NMR (270 MHz, CDCl3):
= 7.98 (1H, m, ArH), 7.83–7.55 (6H, m,
ArH), 7.50–7.15 (10H, m, ArH), 5.68 (1H, s, CHPh); 13C NMR (100 MHz, CDCl3):
= 141.1, 133.7, 133.6, 132.1, 129.5–125.5 (18 Â C), 57.5; HRMS, m/z found:
MH+, (C23H19S) requires 327.1202, found 327.1235; 4h m.p. C; 1H NMR (270 MHz, CDCl3):
= 7.50–7.25 (12H, m, ArH), 6.93 (2H, dd, J = 5.4, 5.4 Hz, ArH), 5.51
= 162.4 (d, JC–F = 246 Hz), 141.0, 134.2, 130.8, 128.7, 128.5, 127.5, 116.0 (d, JC–F = 88 Hz), 58.9; HRMS, m/z
= 7.50–5.35 (4H, m, ArH), 7.30–7.15 (6H, m, ArH), 6.69 (1H, s,
= 161.9, 153.2, 142.9, 140.1, 129–127 (10 Â C), 115.0, 57.9;. HRMS, m/z found:
a
2.41(t, 2H,3JHH = 7.02, CH2S), 1.78 (br.s, 1H, CH2OH), 1.59 (m, 4H, CH2). 13C NMR (100 MHz, CDCl3):
d
d
d
d
˚
(1H, s, CH(Ph)2); 13C NMR (100 MHz, CDCl3):
d
found: MH+, (C19H16FS) requires 295.0951, found 295.0962; 4i 1H NMR (270 MHz, CDCl3):
d
thiazole H), 6.96 (1H, s, CHPh2), 2.35 (3H, s, CH3); 13C NMR (100 MHz, CDCl3):
d
MH+, (C17H16NS2) requires 298.0718, found 298.0725; 4j m.p. 67–68 C; 1H NMR (270 MHz, CDCl3):
d
= 8.90 (1H, s, N = CH), 7.46–7.20 (10H, m, ArH), 6.21
˚
(1H, s S–CH(Ph)2); 13C NMR (100MHz, CDCl3):
d
:
= 164.7, 152.1, 139.3, 129–127 (10 Â C), 57.4;. HRMS, m/z found: MNa+, (C15H13N2S2) requires 307.0334,
˚
found 307.0352; 6 m.p. 129–130 C;
13C NMR (100 MHz, CDCl3):
n
(max)cm –1
1H NMR (270 MHz, CDCl3):
d
= 8.24 (1H, s, thiadiazole CH), 7.71 (1H, s, CHPh2), 7.42–7.22 (10H, m, 2 Â Ph);
d
= 185.8, 143.7, 143.5, 137.8, 129.0–128.0 (10 Â C), 65.8, 65.6. HRMS, m/z found: MH+, (C15H13N2S2) requires 285.0514, found
285.0532.
b
Determined by integration of key peaks in the 1H NMR spectrum.
c
Reaction conversion based on residual 2, this conversion consisted of a 2:1 mixture of 4j and 6.

L.C. Henderson et al. / C. R. Chimie 16 (2013) 634–639
639
U. Maitra, S. Chandrasekaran, W.H. Moos, S.S. Ghosh, J. Org. Chem. 68
(2002) 92.
[8] (a) S. Chowdhury, S. Roy, Tetrahedron Lett. 38 (1997) 2149 ;
(b) R.E. Weisburger, J. Chem. Soc. (1964) 515 ;
ethers and other heterocycles, which have demonstrated
important properties, such as anti-microbial, anti-inflam-
matory and anti-cancer amongst others [16].
(c) R.W. Hanson, H.D. Law, J. Chem. Soc. (1965) 7285 ;
(d) S. Banerjee, J. Das, R.P. Alvarez, S. Santra, New J. Chem. 34 (2010)
302 ;
3. Conclusions
(e) K.K. Bhasin, N. Singh, R. Kumar, D. Gupta Deepali, S.K. Mehta, T.M.
Klapoetke, M.J. Crawford, J. Organomet. Chem. 689 (2004) 3327 ;
(f) A. Altieri, V. Aucagne, R. Carrillo, G.J. Clarkson, D.M. D’Souza, J.A.
Dunnett, D.A. Leigh, K.M. Mullen, Chem. Sci. 2 (2011) 1922.
[9] (a) B.P. Bandgar, S.S. Gawande, D.B. Muley, Green Chem. Lett. Rev. 3
(2010) 49 ;
(b) H. Firouzabadi, N. Iranpoor, M. Jafarpour, Tetrahedron Lett. 47
(2006) 93.
[10] (a) Y. Bikard, R. Mezaache, J.M. Weibel, A. Benkouider, C. Sirlin, P. Pale,
Tetrahedron Lett. 64 (2008) 10224–10232 ;
In this manuscript, we have described the use of pILs as
recyclable solvent to facilitate the acid catalysed
a
formation of diphenylmethylthioethers. These reactions
proceeded quickly, in high yields and were successfully
applied to the etherification of a wide range of thiols. In
addition, we have demonstrated the reusability of TeaMs
in this protocol, which demonstrated an increase in
reaction progression with each reuse.
(b) B.C. Ranu, R. Jana, Adv. Synth Catal. 347 (2005) 1811 ;
(c) G.J. Sterk, J. Koper, H. Van Der Goot, H. Timmerman, Eur. J. Med.
Chem. 22 (1987) 491 ;
(d) C.-R. Liu, M.-B. Li, C.-F. Yang, S.-K. Tian, Chem. Eur. J. 15 (2009) 793.
[11] H. Firouzabadi, N. Iranpoor, A.A. Jafari, Tetrahedron Lett. 46 (2005)
2683.
Acknowledgements
[12] (a) J.M. Altimari, J.P. Delaney, L. Servinis, J.S. Squire, M.T. Thornton, S.K.
Khosa, B.M. Long, M.D. Johnstone, C.L. Fleming, F.M. Pfeffer, S.M. Hickey,
M.P. Wride, T.D. Ashton, B.L. Fox, N. Byrne, L.C. Henderson, Tetrahedron
Lett. 53 (2012) 2035 ;
The authors would like to thank the Institute for
Frontier Materials and the Strategic Research Centre for
Chemistry and Biotechnology for financial assistance. NB
acknowledges the Australian Research Council for an APD.
(b) L.C. Henderson, N. Byrne, Green Chem. 13 (2011) 813 ;
(c) M.T. Thornton, L.C. Henderson, N. Byrne, F.M. Pfeffer, Curr. Org.
Chem. 16 (2012) 121.
References
[13] (a) A.R. Hajipour, F. Rafiee, Org. Prep. Proced. Int. 42 (2010) 285 ;
(b) J. Akbari, A. Heydari, L. Ma’mani, H. Hassan, C. R. Chimie 13 (2010)
544 ;
[1] T.W. Greene, P.G.M. Wuts, Protective Groups in Organic Synthesis, 3rd
ed., J. Wiley & Sons, New York, 1999.
[2] W. Ge, I.J. Clifton, A.R. Howard-Jones, J.E. Stok, R.M. Adlington, J.E.
Baldwin, P.J. Rutledge, ChemBioChem 10 (2009) 2025.
[3] E. Carceller, M. Merlos, M. Giral, C. Almansa, J. Bartroll, J. Garcia-
Rafanell, J. Forno, J. Med. Chem. 6 (1993) 2984.
[4] Y. Pei, M.J. Lilly, D.J. Owen, L.J. D’Souza, X.-Q. Tang, J. Yu, R.N.A. Hunter,
C.M. Anderson, S. Glasco, N.J. Ede, I.W. James, U. Maitra, S. Chandrase-
karan, W.H. Moos, S.S. Ghosh, J. Org. Chem. 68 (2003) 92.
[5] (a) A. Bicherov, A. Akopova, V. Spiglazov, A. Morkovnik, Russ. Chem.
Bull. 59 (2010) 91 ;
(c) C.P. Gordon, N. Byrne, A. McCluskey, Green Chem. 12 (2010) 1000 ;
(d) R. Atkin, G. Warr, J. Phys. Chem. B 112 (2008) 4164 ;
(e) C.A. Angell, N. Byrne, J.-P. Belieres, Acc. Chem. Res. 40 (2007) 1228 ;
(f) J.-P. Belieres, C.A. Angell, J. Phys. Chem. B 111 (2007) 4926 ;
(g) T.L. Greaves, A. Weerawardena, C. Fong, I. Krodkiewska, C.J. Drum-
mond, Chem. Commun. 110 (2006) 22479 ;
(h) H. Luo, G.A. Baker, J.S. Lee, R.M. Pagni, S. Dai, J. Phys. Chem. B 113
(2009) 4181.
[14] (a) J. Ternois, F. Guillen, J.-C. Plaquevent, G. Coquerel, Tetrahedron:
Asymmetry 18 (2007) 2959 ;
(b) J. Cao, T.E. Prisinzano, O.M. Okunola, T. Kopajtic, M. Shook, J.L. Katz,
A.H. Newman, ACS Med. Chem. Lett. 2 (2010) 48 ;
(c) W. Hauck,P. Adam, C. Bobier, N. Landmesser, Chirality20 (2008) 896 ;
(d) H.F. Olivo, A. Osorio-Lozada, T.L. Peeples, Tetrahedron: Asymmetry
16 (2005) 3507 ;
(b) J.-C. Jung, Y. Lee, J.-Y. Son, E. Lim, M. Jung, S. Oh, Molecules 17
(2012) 10446 ;
(c) J. Ternois, F. Guillen, G. Piacenza, S. Rose, J.-C. Plaquevent, G.
Coquerel, Org. Proc. Res. Dev. 12 (2008) 614.
[15] (a) Z. Jin, Z. Li, R. Huang, Nat. Prod. Rep. 19 (2002) 454 ;
(b) X. Gao, Y.-M. Pan, M. Lin, L. Chen, Z.-P. Zhan, Org. Biomol. Chem. 8
(2010) 3259 ;
(c) M.-X. Wei, L. Feng, X.-Q. Li, X.-Z. Zhou, Z.-H. Shao, Eur. J. Med. Chem.
44 (2009) 3340.
[16] (a) S. Pappalardo, F. Bottino, C. Tringali, F.R. Fronczek, J. Org. Chem. 52
(1987) 3409 ;
(d) T. Prisinzano, J. Podobinski, K. Tidgewell, M. Luo, D. Swenson, Tetra-
hedron: Asymmetry 15 (2004) 1053–1058 ;
(e) S. Shirakawa, S. Kobayashi, Org. Lett. 9 (2006) 311–314.
[6] (a) P. Thirupathi, S.S. Kim, J. Org. Chem. 74 (2009) 7755 ;
(b) P. Thirupathi, S.S. Kim, Eur. J. Org. Chem. (2010) 1798.
[7] (a) S.M. Okarvi, P. Torfs, P. Adriaens, A.M. Verbruggen, J. Labelled
Compd. Radiopharm. 45 (2002) 407 ;
(b) N. Aggarwal, R. Kumar1, P. Dureja, J. Khurana, Chem. Biol. Drug Des.
79 (2012) 384–397 ;
(b) K. Manabe, S. Iimura, X.-M. Sun, S. Kobayashi, J. Am. Chem. Soc. 124
(2002) 11971 ;
(c) J. Zhang, P. Zhang, X. Liu, K. Fang, G. Lin, Bioorg. Med. Chem. Lett. 17
(2007) 3769 ;
(d) V. Kumar, B.A. Averill, J. Heterocycl. Chem. 25 (1988) 241 ;
(e) Y. Pei, M.J. Lilly, D.J. Owen, L.J. D’Souza, X.-Q. Tang, J. Yu, R.
Nazarbaghi, A. Hunter, C.M. Anderson, S. Glasco, N.J. Ede, I.W. James,
(c) A.A. Kadi, E.S. Al-Abdullah, I.A. Shehata, E.E. Habib, T.M. Ibrahim,
A.A. El-Emam, Eur. J. Med. Chem. 45 (2010) 5006 ;
(d) R.P. Bhole, K.P. Bhusari, Med. Chem. Res. 20 (2011) 695 ;
(e) K.M. Khan, N. Fatima, M. Rasheed, S. Jalil a, N. Ambreen, S. Perveen,
M.I. Choudhary, Biorg. Med. Chem. 17 (2009) 7816.