N-methyl-2-pyrrolidonium chlorosulfonate: An efficient ionic-liquid catalyst and mild sulfonating agent for one-pot synthesis of δ-sultones

Article abstract of DOI:10.1016/j.molliq.2016.02.082

Addition of chlorosulfonic acid to N-methyl-2-pyrrolidone (NMP) resulted in the formation of a low-melting solid with the advantages of being reasonably stable for at least 4 months at ambient temperature and easy to handle. This solid forms homogeneous s

Full text of DOI:10.1016/j.molliq.2016.02.082

Journal of Molecular Liquids 218 (2016) 275–280
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Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
N-methyl-2-pyrrolidonium chlorosulfonate: An efficient ionic-liquid
catalyst and mild sulfonating agent for one-pot synthesis of δ-sultones
⁎
Kurosh Rad-Moghadam , Seyyed Ali Reza Mousazadeh Hassani, Saeedeh Toorchi Roudsari
Department of Chemistry, University of Guilan, Rasht, PO Box 41335-1914, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Addition of chlorosulfonic acid to N-methyl-2-pyrrolidone (NMP) resulted in the formation of a low-melting
solid with the advantages of being reasonably stable for at least 4 months at ambient temperature and easy to
handle. This solid forms homogeneous solutions with acetophenone derivatives, which on heating in the absence
of any additional solvent and catalyst lead to production of the desired 1,3-dienic δ-sultones in fairly high yields
and short reaction times. Beside its role as a solvent, the NMP-ClSO3H salt efficiently catalyzes the self-
condensation of acetophenones and plays as a mild sulfonating agent in conversion of the resulting condensates
into δ-sultones. The reaction protocol is simple and follows a green work-up procedure that enables to recover
the reaction residues and prevent the emission of wastes into the environment.
Received 14 September 2015
Received in revised form 22 January 2016
Accepted 15 February 2016
Available online xxxx
Keywords:
N-methyl-2-pyrrolidone
Ionic liquid
© 2016 Elsevier B.V. All rights reserved.
δ-Sultone
NMP-ClSO3H
1. Introduction
Certain members of these compounds are used as surfactants in the
manufacture of detergents and several others have shown antidiabetic,
The sulfonation of organic compounds has long been the subject of
most studies in particular, since these reactions are applied commercial-
ly for the preparation of surfactants [1], improving the properties of
polymers [2], and modification of dyes [3]. A variety of methods based
on using either organic or inorganic sulfonating agents have been de-
vised for synthesis of organic sulfonic acids, sulfones, sulfonates, and
sulfonate–sulfate anhydrides. Chlorosulfonic acid has been the most
widely used agent, after sulfuric acid and sulfur trioxide [4], for this pur-
pose. However, all these sulfonating agents react very vigorously with
arenes and olefins, usually giving rise to a mixture of products that the
composition of which depends on the reaction conditions [5]. Hence,
milder sulfonating agents were obtained by the addition of pyridine or
dioxane to sulfur trioxide or chlorosulfonic acid [6], and likewise by con-
version of chlorosulfonic acid to its silyl esters or silica sulfuric acid de-
rivatives [7]. Despite their own merit, application of these sulfonating
agents usually associate with one or more difficulties such as the risk
of releasing harmful pyridine and dioxane into environment during
workup step, using more excess or massive agents, and causing un-
wanted side-reactions [8]. Therefore, synthesis of greener, easy to han-
dle, preferably solid and mild sulfonating compounds are still in
demand. The mildness of such reagents is even crucial to supersede
the side reactions usually accompany the formation of sultones through
sulfonation of alkenes. The term “sultone” was initially introduced into
literature by Erdmann in 1888 [9]. Sultones are the cyclic esters of
hydroxysulfonic acids and regarded as the sulfur analogues of lactones.
antiviral, and inhibitory activities [10]. Moreover, sultones are valuable
sulfoalkylating agents and suitably flexible for manipulation into struc-
turally elaborate molecules [11,12]. Consequently, they have been used
widely as the key intermediates in the synthesis of many natural prod-
ucts as well as in unique synthetic organic transformations [13–20].
These outstanding features combined with the scarcity of general routes
to sultones have led to growing interests in investigation of multi-step
syntheses of this heterocyclic ring system. Recent advances in this re-
search frontier were based on improved and innovative reactions in-
volving carbanion-mediated sulfonate intramolecular cyclizations [21],
intramolecular Diels−Alder cycloaddition [22], ring-closing metathesis
[23], Pd-catalyzed intramolecular couplings [24], Rh-catalyzed C–H in-
sertion cyclization [25], Pd-catalyzed C–H activation [26], intramolecu-
lar Heck-type cyclization [27], and Rh-catalyzed carbene cyclization
cycloaddition cascade reactions [28]. Rogachev and coworkers were
the first who succeeded to set up a one-pot pseudo-three-component
synthesis of 1,3-dienic δ-sultones via sulfonation of arylacetylenes
with sulfur trioxide solution in dioxane [29]. Later, we developed a
similar pseudo-three-component approach based on using a sulfonic
ionic liquid both to catalyze the self condensation of the starting
acetophenone derivatives and to implement the following sulfonation
of the resulting condensates to give 1,3-dienic δ-sultones [30].
Prompted by this finding, we planned to simplify the synthesis and as
a result a much simple and available dual-functioning ionic liquid sys-
tem is introduced here for carrying out the method.
Since their early recognition as potent solvents, ionic liquids have
been continually evolving and today they found interesting catalytic ap-
plications either on their own [31], or as supports for catalysts [32]. They
⁎
Corresponding author.
E-mail address: radmm@guilan.ac.ir (K. Rad-Moghadam).
http://dx.doi.org/10.1016/j.molliq.2016.02.082
0167-7322/© 2016 Elsevier B.V. All rights reserved.

276
K. Rad-Moghadam et al. / Journal of Molecular Liquids 218 (2016) 275–280
Fig. 1. Preparation of the NMP-ClSO3H salt.
are capable to establish van der Waals and ionic interactions with sol-
utes where enabling them to serve as effective solvents for dissolving
both organic and inorganic compounds as well as being tunable for bi-
phasic separation from reaction mixtures. Catalytic application of ionic
liquids has even been widened in recent years since emerge of their
new generations which embed one or more catalytic functional groups,
especially the strong acidic groupings [33]. Despite these fascinating
developments, ionic liquids have received rare applications as reagents
in organic reactions [34]. Here, we introduce the salient advantages of
the low-melting ionic-liquid salt of N-methyl-2-pyrrolidone with
chlorosulfonic acid, [NMP-ClSO3H], in an efficient synthesis of the de-
sired 1,3-dienic δ-sultones, 4,6-diaryl-[1,2]-oxathiine-2,2-dioxides.
Selected spectroscopic data for [NMP-ClSO3H] are given below.
MS (70 eV): m/z (%) = 133 (1.7), 99 (100), 98 (83), 80 (69) [SO3], 64
(28) [SO2], 48 (62) [SO], 44 (88), 42 (94) [C3H6]+. FT-IR (as thin film on
KBr, cm−1):νmax = 3434, 1644, 1508, 1410, 1219, 1193, 1053, 896, 595.
1H NMR (400 MHz, CDCl3): δH = 2.32–2.42 (m, 2H), 3.03–3.22 (m, 5H),
3.88–3.94 (m, 2H), 12.55 (br s,) 12.95 (s, 1H, SO3H) ppm. 1H NMR
(400 MHz, D2O + CDCl3): δH = 3.42 (2H, t, J 7.0 Hz, 5-H), 2.87 (3H, s,
CH3), 2.43 (2H, t, J 8.0 Hz, 3-H), 2.05 (2H, q, J 7.6 Hz, 4-H). 13C NMR
(100.63 MHz, CDCl3), set 1 with low intensities: δC = 179.1, 77.9
(CDCl3), 53.6, 32.4, 30.8, 17.4 ppm; set 2 with high intensities: δC
=
178.2, 77.0 (CDCl3), 54.0, 32.5, 30.7, 17.4 ppm.
2.3. General procedure for synthesis of 4,6-diaryl-[1,2]-oxathiine-2,2-diox-
2. Experimental
ides (1,3-dienic δ-sultones)
2.1. General
A mixture of an acetophenone derivative (1.0 mmol), and [NMP-
ClSO3H] (0.5 mL, 511 mg, 2.4 mmol) was heated in an oil bath
(100 °C) for appropriate time. Progress of the reaction was monitored
by TLC on silica gel using petroleum ether:ethylacetate as eluent. After
completion, the reaction mass was cooled to room temperature
and washed with cold water to remove the excess ionic liquid and
the remaining NMP. All the solid products were purified by re-
crystallization in aqueous EtOH (90%) and identified by the interpreta-
tion of their FT-IR, 1H NMR, 13C NMR and mass spectral data as well as
elemental microanalysis. In the case of known compounds, the identity
of the products was also confirmed by comparison of the physical and
spectral data with those reported in literature. Selected data of new
compounds are presented below:
All the chemicals and solvents used in the present investigation
were of spectral grade (Merck & Aldrich) materials. Yields refer to the
isolated products. The formation of products was confirmed by compar-
ison of their physical and spectral (IR and NMR) data with those of
authentic samples as well as with the data reported in literature. Prog-
ress of the reactions was monitored by TLC on silica gel polygram
SILG/UV 254 plates. Melting points (mp) were recorded on a Barnstead
electrothermal instrument and are uncorrected. The absorption spectra
in the UV and visible regions were recorded by a Perkin Elmer LAMBDA
25 recording spectrophotometer. IR spectra were recorded in KBr wa-
fers on a FT-IR Perkin Elmer Spectrum One. The 1H NMR (400 MHz)
and 13C NMR (125 MHz) spectra were recorded on a Bruker DRX-400
spectrometer using DMSO as solvent and TMS as internal standard.
Electrical conductivity was measured with an Orion 101 conductometer
operating at 80 Hz and using a platinum conductivity cell (cell
constant = 1 cm−1). The cell was calibrated with KCl solutions of
known conductivity [35].
4,6-Di(4-Iodophenyl)-[1,2]-oxathiine-2,2-dioxide (7 g):
Mp 200–202 °C. IR (KBr) cm−1; 3070, 1624, 1358, 1163, 1001, 819,
775, 567. 1H NMR (CDCl3, 400.2 MHz); δH = 7.95 (d, J 10.8 Hz,
2H), 7.93 (d, J 10.8 Hz, 2H), 7.8 (d, J 0.8 Hz, 1H), 7.76 (d, J 8.6 Hz,
2H), 7.68 (d, J 8.6 Hz, 2H), 7.4 (d, J 0.8 Hz, 1H). 13C NMR (CDCl3,
100.63 MHz); δC = 153.3, 148.8, 144.9, 136.1, 134.5, 132.4, 131.2,
126.4, 125.9, 122.8, 120.8, 117.5. MS (70 eV): m/z (%) = 536 (M+
,
2.2. Preparation of [NMP-ClSO3H]
1.3), 409 (M+–I, 2.4), 368 (14), 333 (M+–C6H4I, 3.5), 313 (7), 239
(8), 183 (11). Anal. Calcd for C16H10I2O3S (536.12): C, 35.84; H,
1.88. Found: C, 35.69; H, 1.91.
Chlorosulfonic acid (2.33 g, 20 mmol) was added dropwise to a glass
tube (10 mL) containing a magnetic stirring bare and N-methyl-2-pyr-
rolidone (NMP) (1.98 g, 20 mmol) over a period of 5 min at ambient
temperature. Immediately after completion of the addition, the mixture
was transferred into a glass vial equipped with a pressure-tight septum
having a polytetrafluoroethylene-faced butyl rubber. The vial was
placed in a refrigerator at 0 °C, wherein the contained viscous adduct
[NMP-ClSO3H] get solidified after about 15 min (Fig. 1). Subject to be
kept away from moisture, the yellow solid adduct can be stored for
long periods of time at room temperature without appreciable loss of ef-
fectiveness. The ionic liquid was produced quantitatively and in high
purity as assessed by NMR spectroscopy. It is a low melting solid with
congealing range of 28 °C to 32 °C.
Scheme 1. (a) Lactim and (b) Lactam tautomers of protonated NMP with the counter
anion ClSO−3
.

K. Rad-Moghadam et al. / Journal of Molecular Liquids 218 (2016) 275–280
277
Fig. 2. The UV–visible absorption spectrum of p-nitroaniline (a) in CCl4 and (b) after adding an amount of NMP-ClSO4H in CCl4.
absorption bands located at 1053 cm−1 and 1193 cm−1 are clearly
assigned to symmetric and asymmetric S_O stretching vibrations and
the two weak bands observed at 595 cm−1 and 896 cm−1 are attributed
to SO−3 deformation and S–OH bending vibrations, respectively.
[NMP-ClSO3H] forms a fine dispersed liquid–liquid mixture after
shaking with CDCl3. The 1H NMR spectrum of this salt in CDCl3 exhibited
4,6-Di(3-Nitrophenyl)-[1,2]-oxathiine-2,2-dioxide (7d):
Mp 169–171 °C. IR (KBr) cm−1; 3085, 1629, 1527, 1352, 1170, 1106,
782, 736. 1H NMR (CDCl3, 400.2 MHz); δH = 8.76 (t, J 2.0 Hz, 1H),
8.72 (t, J 2.0 Hz, 1H), 8.46 (d, J 7.4 Hz, 1H), 8.43 (d, J 8.4 Hz, 2H),
8.37 (d, J 7.8 Hz, 2H), 8.1 (d, J 0.8 Hz, 1H), 7.9 (m, 2H), 7.8 (d, J
0.8 Hz, 1H). 13C NMR (CDCl3, 100.63 MHz); δC 154.8, 146.1, 138.4,
134.1, 130.5, 132.4, 129.7, 128.0, 114.8, 102.8, 99.7. MS (70 eV): m/
z (%) = 374 (M+, 12), 310 (M+–SO2, 45), 295 (MH+–SO3, 5), 189
(M+–SO3–C6H4NO2, 38), 150 ([O2N-C6H4CO]+, 18). Anal. Calcd for
C16H10N2O7S (374.32): C, 51.34; H, 2.69; N, 7.48. Found: C, 51.22;
H, 2.73; N, 7.45.
3. Results and discussion
Addition of N-methyl-2-pyrrolidone to an equimolar amount of
chlorosulfonic acid gave a yellow solid with congealing point of
28–32 °C. The IR spectrum of this low-melting solid displays a broad ab-
sorption band ranging from 2600 to 3400 cm−1 corresponding to
stretching vibration of acidic O-H groups. There is seen another strong
absorption band in this spectrum at 1644 cm−1 that accompanies by a
very weak shoulder centered at 1693 cm−1. These two overlapping
bands can be attributed to lactam and lactim tautomers of protonated
NMP (Scheme 1) [36]. The main band arise from stretching vibration
of internal iminium grouping in the lactim tautomer built on amidic
conjugation of the nitrogen with the protonated carbonyl group and
the weak shoulder probably results from stretching vibration of the car-
bonyl group in the N-protonated lactam or neutral molecules. The
dominant absorption of the former vibration can be ascribed to almost
complete O-protonation of NMP by chlorosulfonic acid. Two moderate
Table 1
The absorbance measurements at λmax = 328 nm for determining the Hammett acidity
value (Ho) of NMP-ClSO3H.
The acid
Amax
[B] %
100
[BH+] %
Ho
–
2.2161
0.4571
0
–
0.40
NMP-ClSO3H
20.63
79.37
The conditions adopted for UV–visible measurements in CCl4: 1.44 × 10−4 mol/L solution
(10 mL) of p-nitroaniline (pKa (BH+
)aq = 0.99) as the base indicator; NMP-ClSO3H (10
mg, 4.63 × 10−3 mol/L); 25 °C.
Fig. 3. Plots of conductivity against temperature (°C) for (up) the ionic liquid NMP-ClSO3H
and (down) for NMP (net).

278
K. Rad-Moghadam et al. / Journal of Molecular Liquids 218 (2016) 275–280
Scheme 2. A plausible fragmentation of [NMP-ClSO3H].
a sharp singlet at δ 12.94 which was flanked by a small broad peak at δ
12.56. These peaks can be assigned to the acidic protons of the two un-
mixed CDCl3 solution phases containing [NMP-ClSO3H] and [NMP-
H2SO4], respectively. The latter salt is thought to be formed on partial
hydrolysis of [NMP-ClSO3H] by moisture. Owing to formation of this bi-
phasic solution, other protons of NMP similarly appeared as two set of
neighboring signals. Certainly, NMP serves as the sole base in CDCl3
and its salts with ClSO3H and H2SO4 exist as solvated tight ion pair. Be-
cause of a different degree of solvation at the instant of formation, the
salt [NMP-H2SO4] builds a separate solution phase and exhibits different
chemical shifts from the bulk [NMP-ClSO3H] solution in CDCl3. It is note-
worthy that all the protons of [NMP-ClSO3H] appeared at larger chemi-
cal shifts relative to those reported for neutral NMP in CDCl3 [37]. The
magnitude of this down-field shift considerably varies among the pro-
tons of the lactim cation and seems to be depending on how close
they are to the protonated oxygen of this cation. As a result of proximity
to the protonated center (the carbonyl group), the two 3-H protons ex-
perience the largest shift (about 0.7 ppm) to lower field. Similarly, but
with less extent, the methyl protons showed 0.4 ppm shift toward
down-field and the 5-H protons, with a down-field shift of about
0.5 ppm, appeared at δ 3.92. Chemical shifts of all these protons return
back to those of neutral NMP if the 1H NMR spectrum is taken in
CDCl3 containing D2O. Moreover, each type of protons in D2O-CDCl3 so-
lution of the NMP salts showed only one 1H NMR signal with appropri-
ate coupling to the neighboring atoms. This seems to be also the case for
1H NMR of other NMP salts [38–40], indicating that the lactim cation of
these salts is deprotonated by water or a slight water impurity. In other
words, the lactim cation is a stronger acid than H3O+. A quantity suit-
able for estimating the proton donating power of the lactim cation in
nonaqueous media is obtained by Hammett's acidity function. In this
regard, the function is expressed as follow,
À
Á
À
Â
ÃÁ
Ho ¼ pKa BHþ aq þ log ½Bꢀ= BHþ
where pKa(BH+)aq refers to pKa = 0.99 of p-nitroaniline in aqueous so-
lution and [B] /[BH+] stands for the molar concentration ratio of p-
nitroaniline to its conjugate acid in nonaqueous media. This concentra-
tion ratio was readily determined by measuring the UV–visible absor-
bance of the indicator base (p-nitroaniline) at the maximal absorption
wave length (λmax) of 328 nm before and after adding [NMP-ClSO3H]
to its solution in CCl4 (Fig. 2). As summarized in Table 1, the data obtain-
ed from these evaluations afford the value of Ho = 0.40 for acidic
strength of the ionic liquid.
This non-aqueous acid exhibits significant electric conductivity as a
signature of its strong ionic character. Fig. 3 displays the electrical con-
ductivity (EC) of [NMP-ClSO3H] as a function of temperature and pro-
vides a comparison with that of NMP. It is evident that [NMP-ClSO3H]
is about three orders of magnitude more electrically conductive than
NMP. The EC of [NMP-ClSO3H] is increased by increasing the mobility
of its ionic components at elevated temperatures and reaches to maxi-
mum of 1.25 ms·cm−1 at 68 °C. After this temperature the conductance
curve goes flattened and follows a slight decrease at above 80 °C due to
slight decomposition of [NMP-ClSO3H] at higher temperatures. On the
other hand, the EC of NMP can mainly be referred to nearly 0.3% water
impurity of this hygroscopic liquid, as measured by Karl-Fischer meth-
od. Nevertheless, the NMP sample shows greater EC (6.2 μs·cm−1) at
68 °C relative to that of deionized water (0.33 μs·cm−1) at the same
temperature [41]. The additional conductivity can be attributed to
Table 2
Determination of the optimal condition for the trial synthesis of 4,6-diphenyl-[1,2]-
oxathiine-2,2-dioxide.
Entry
Catalyst (mmol)
Temperature (°C)
Time (min)
Yield (%)a
1
2
3
4
5
2.4
4.8
4.8
4.8
2.4
r.t.
r.t.
60
100
100
120
120
120
5
Trace
Trace
40
95
94
5
Scheme 3. Synthesis of 4,6-diaryl-[1,2]-oxathiine-2,2-dioxides via reaction of
acetophenones with [NMP-ClSO3H].
a
Isolated yields.

K. Rad-Moghadam et al. / Journal of Molecular Liquids 218 (2016) 275–280
279
Table 3
this spectrum is consistent with the salt of NMP and the superacid
ClSO3H. Accordingly, NMP seems to be too weak a nucleophile to
solvolize chlorosulfonic acid in the applied condition and so appeared
as the base peak at m/z = 99. Presumably, the pattern of fragmentation,
as suggested in Scheme 2, can be interpreted as involving the initial loss
of hydrogen from the lactim cation followed by dissociation of
chlorosulfonic moiety and the concomitant addition of chlorine atom
to the preformed cyclic iminium cation.
In the next phase of this investigation, we aimed to evaluate the
sulfonating capacity as well as the acid catalytic efficiency of [NMP-
ClSO3H] in synthesis of 1,3-dienic δ-sultones via reaction with
acetophenone derivatives. To commence with and to find an optimized
condition, the model reaction of acetophenone and [NMP-ClSO3H] was
carried out with varied ratios of these reactants at some different tem-
peratures. As shown in Table 2, the best result was obtained by treating
1 mmol of acetophenone with 0.5 mL (2.4 mmol) of [NMP-ClSO3H] at
100 °C. Under solventless condition (Scheme 3), the reaction undergoes
smoothly to give a fairly high yield of the desired 1,3-dienic δ-sultone in
a short reaction time. It is worth to note that multiple by-products are
formed when acetophenone is treated with pure chlorosulfonic acid
or its solution in CH2Cl2 even at 0 °C.
The synthesized 4,6-diaryl-[1,2]-oxathiine-2,2-dioxides.
Entry Aldehyde
Product Time (min) Yield (%) a Mp°C
Found
Rep. Ref.
1
2
3
4
5
6
7
8
C6H5COMe
3-ClC6H4COMe
4-ClC6H4COMe
7a
7b
7c
10
8
88
94
86
90
89
92
91
87
154–155 156 30
152–154 153 30
219–221 222 30
169–171 New
221–223 220 30
216–217 216 30
200–202 New
194–196 195 30
10
15
15
10
12
15
3-NO2C6H4COMe 7d
4-FC6H4COMe
4-BrC6H4COMe
4-IC6H4COMe
7e
7f
7g
4-MeC6H4COMe 7h
a
Isolated yields.
further ionization of the water impurity, induced by NMP and possible
traces of its other impurities.
The 13C NMR study displayed two distinct peaks with different in-
tensities for CDCl3 and for each carbon of the lactim cation, affirming
the formation of a fine biphasic liquid–liquid mixture. As expected, by
long-term shaking of the salt with CDCl3 the signals partially merged
into one set due to anion exchange and union of the two liquid phases.
In either case, the carbon atoms were observed at appropriate chemical
shifts with a slight displacement to low-fields in comparison with the
corresponding peaks of neutral NMP [37].
Encouraged by this result, we set out to examine the substrate scope
of this reaction by examining several acetophenone derivatives in reac-
tion with [NMP-ClSO3H]. As tabulated in Table 3, fairly high yields of
4,6-diaryl-[1,2]-oxathiine-2,2-dioxides were obtained rapidly by this
method from the readily available acetophenone substrates. Notably,
hydroxyacetophenones gave no desirable results when applied to this
The mass spectrum of the salt has not shown a molecular ion peak
attributable to solvolysis of chlorosulfonic acid by NMP and also is de-
void of any fragment splitting off from such a molecular ion. Instead,
Scheme 4. A proposed mechanism for the synthesis of 4,6-diaryl-[1,2]-oxathiine-2,2-dioxides via reaction of acetophenones with [NMP-ClSO3H].

280
K. Rad-Moghadam et al. / Journal of Molecular Liquids 218 (2016) 275–280
synthetic method and attempts to resolve the δ-sultone product of 4-
methoxyacetophenone by TLC remained unsuccessful due to its proba-
ble decomposition [22].
References
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Separate experiments disclosed that [NMP-ClSO3H] decomposes at
above 80 °C and liberates a gaseous mixture of HCl and sulfurtrioxide.
This finding verifies [NMP-ClSO3H] as the potent source of SO3 under
the conditions set forth in this study. Formation of the resonance stabi-
lized pyrrolidonium chloride is probably the driving force of [NMP-
ClSO3H] decomposition and liberation of sulfurtrioxide. As suggested
in Scheme 4, it is likely that acetophenone derivatives undergo self con-
densation in acidic and dehydrating medium of [NMP-ClSO3H] to give
the intermediate enones 3. Addition of SO3 to the protonated oxygen
of enones 3 results in formation of intermediate dienylsulfates 4. Sulfo-
nation once again occurs, this time at CH2-termini of the diene interme-
diates 5, where the electron-donating conjugation of the ester oxygen
with the diene segment makes the carbon atom of this end more suit-
able for electrophilic attack by sulfurtrioxide. The resulting double
sulfonated intermediate 6 then undergoes cyclization followed by elim-
ination of a molecule of sulfuric acid to deliver the product 7.
Progress of the reaction was monitored by TLC using silica gel coated
sheets and EtOAc:petroleum ether (1:4) as eluents. After completion of
the reaction, cold distilled water was added to the reaction mixture and
the solid product was separated by filtration, dried in air, and recrystal-
lized from EtOH (95%). For recovery of the residual NMP, left by the re-
action, the filtrate was neutralized by NaOH and then subjected to
extraction with Et2O. The NMP separated by evaporation of the ether so-
lution at 50 °C and then under reduced pressure is pure enough to be
recycled in the next use.
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Acknowledgments
The authors are grateful to the Research Council of University of
Guilan for the partial support of this work.
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Appendix A. Supplementary data
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.molliq.2016.02.082.