Zwitterionic imidazolium salt: An efficient organocatalyst for tetrahydropyranylation of alcohols

Article abstract of DOI:10.1080/00397911.2017.1356334

An aprotic imidazole based zwitterionic-salt, 4-(3-methylimidazolium)-butane sulfonate has been found to be an efficient organocatalyst for tetrahydropyranylation by the reaction of 3,4-dihydro-2H-pyran (DHP) and different aliphatic alcohols as well as various phenolic compounds. The notable advantages of the present method are general applicability to various alcohols, clean reaction, production of no hazardous waste, open air reaction conditions and high yields. The catalyst can be reused without the loss of significant catalytic activity.

Full text of DOI:10.1080/00397911.2017.1356334

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
Chemistry
ISSN: 0039-7911 (Print) 1532-2432 (Online) Journal homepage: http://www.tandfonline.com/loi/lsyc20
Zwitterionic imidazolium salt: An efficient
organocatalyst for tetrahydropyranylation of
alcohols
Sachinta Mahato, Rana Chatterjee, Nirnita Chakraborty Ghosal & Adinath
Majee
To cite this article: Sachinta Mahato, Rana Chatterjee, Nirnita Chakraborty Ghosal & Adinath
Majee (2017): Zwitterionic imidazolium salt: An efficient organocatalyst for tetrahydropyranylation of
alcohols, Synthetic Communications, DOI: 10.1080/00397911.2017.1356334
To link to this article: http://dx.doi.org/10.1080/00397911.2017.1356334
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Date: 25 July 2017, At: 03:45

Zwitterionic imidazolium salt: An efficient organocatalyst
for tetrahydropyranylation of alcohols
Sachinta Mahato
Department of Chemistry, Visva-Bharati (A Central University), Santiniketan, India
Rana Chatterjee
Department of Chemistry, Visva-Bharati (A Central University), Santiniketan, India
Nirnita Chakraborty Ghosal
Department of Chemistry, Visva-Bharati (A Central University), Santiniketan, India
Adinath Majee*
Department of Chemistry, Visva-Bharati (A Central University), Santiniketan, India
Address correspondence to Adinath Majee, Department of Chemistry, Visva-Bharati (A Central
University), Santiniketan 731235, India. E-mail: adinath.majee@visva-bharati.ac.in
Full experimental detail, Copies of 1H and 13Cꢀ NMR spectra. This material can be found via the
“Supplementary Content” section of this article’s webpage.
ABSTRACT
An aprotic imidazole based zwitterionic-salt, 4-(3-methylimidazolium)-butane sulfonate
(MBS) has been found to be an efficient organocatalyst for tetrahydropyranylation by the reaction
1

of DHP and different aliphatic alcohols as well as various phenolic compounds The notable
advantages of the present method are general applicability to various alcohols, clean reaction,
production of no hazardous waste, open air reaction conditions and high yields. The catalyst can
be reused without the loss of significant catalytic activity.
GRAPHICAL ABSTRACT
KEYWORDS: chemoselective, organocatalyst, tetrahydropyranylation, unsaturated alcohols,
zwitterionic imidazolium salt
Introduction
In multistep organic synthesis common functional groups like carbonyls, hydroxyl, amine
and carboxylic acid etc. need protection.[1] Among various methods available in literature for
protection of hydroxyl group tetrahydropyranyl (THP) is the most useful due to its low cost and
stability under basic media, Grignard reagents, acylating agents, metal hydrides and alkyl
lithiums.[1] Glycosylation is arguably one of the most important and difficult reactions in
carbohydrate chemistry[2] which is also the formation of spiroacetal and similar to the THP ether.[3]
Several methods have been developed in literature for glycosylation reaction[3] but we focused our
target for tetrahydropyranylation of alcohols. Numerous reports are there in literature using
catalytic amount of acids,[4] Lewis acids,[5] polyaniline salt,[6] ferrous methanesulfonate,[7]
2

Bu4NBr3,[8]
H14[NaP5W30O110],[9]
Pyridinium
chloride,[10]
Moβ,[11]
2,4,6-
trichloro[1,3,5]triazine,[12] Dowex 50WX4-100[13] and using molecular iodine[14] etc.. Recently,
development of environmentally benign and clean synthetic techniques has become the goal of
organic synthesis.[15] Particularly, the use of nonhazardous, inexpensive catalysts is now very
useful in organic synthesis and organocatalytic reactions are really meaningful to achieve this goal.
Organocatalysis has become very powerful synthetic technique[16] and is very much popular due
to the simple reaction conditions, tolerant of water and air. Several reports are available in literature
for tetrahydropyration using organocatlysis.[17] During preparation of our manuscript a method has
been reported under organocatalytic conditions using imidazolium based Brønsted–Lewis acidic
ionic liquid as catalyst.[18] Most of these proved to be efficient for this reaction. However, some of
them have several drawbacks, such as elevated temperature, long reaction time, harmful organic
solvent, expensive catalysts, and high catalyst to substrate ratio. Thus, there is still demand for the
introduction of mild and efficient method for this transformation.
In continuation of our research on green synthesis, we have explored imidazole-based
zwitterionic-type molten salts as a new class of organocatalyst in various chemical
transformations.[19] The requisite zwitterionic-type molten salts are obtained by the reactions of
the various imidazoles with 1,4-butane sultone/1,3-butane sultone.[20] Thus based on recent works
on green synthesis, imidazole-based zwitterionic-type molten salts, which are the important
precursors of room temperature ionic liquids (RTILs), have been explored as a new class of
organocatalyst in various chemical transformations.[21] Reactions of the various imidazoles with
1,4-butane sultone/1,3-propane sultone produce the requisite zwitterionic-type molten salts. The
synthetic strategy is outlined in Scheme 1.[20]
3

We have prepared different types of zwitterionic-type molten salts by varying imidazoles
and 1,4- butane sultone/1,3-propane sultone as shown in Figure 1. One of the most important
advantages of zwitterionic-type molten salt as catalysts is that they can easily be recovered from
the reaction media and it can be reused for subsequent reactions with high catalytic activity. In
addition, it is known that imidazolium motifs provide an unconventional C–H bond.[22d,f]
Herein, we are pleased to report a convenient approach for the tetrahydropyranylation of
alcohols by the reaction of 3,4-dihydro-2H-pyran (DHP) and various alcohols in presence of
10molꢀ% of zwitterionic-type salt as catalyst under mild and solvent-free conditions using (Scheme
2).
Results and Discussion
During our initial study, readily available benzyl alcohol (1a) and 1.2 equiv. of DHP were
taken as model substrates using 10molꢀ% of 4-(3-methylimidazolium)butane sulfonate (MBS, A)
as catalyst. The reaction proceeded smoothly at 60 °C and the corresponding THP ether (3a) was
isolated in 90% yield within 10hꢀ. We have also examined few other molten salts synthesized in
our laboratory as shown in the Table 1. It has been observed that other molten salts (B, C and D)
are not effective like MBS (A) for this tetrahydropyranylation. When 4-(1-imidazolium)butane
sulfonate (IBS, B) has been used (entry 2, Table 1) the yields are considerably lower (75%) than
A (90%) as the catalyst. Similarly 3-(1-imidazolium)propane sulfonate (C, entry 3, Table 1) also
less effective and observed the desired product with 70% yield. According to our previous
observation[15e] when we used 4-(2,3-dimethylimidazolium)butane sulfonate (D, entry 4, Table 1)
a trace amount of desired product was observed. As a result aprotic zwitterion A was found to be
an effective catalyst for this reaction. Effect of temperature has been studied as shown in the table
4

(entry 5–7, Table 1) and it has been observed that at higher temperature as well as lower
temperature the yields are not satisfactory. Finally, the catalyst loading was checked under this
reaction conditions using A as catalyst for the same reaction (entries 8 & 9, Table 1). We have
observed that 10molꢀ% of A as catalyst afforded better yield (90%) compare to that of 5molꢀ%
(74% yield). Similarly using 15molꢀ% of the catalyst no improvement was observed. In absence
of any catalyst no conversion has been detected (entry 10, Table 1). Finally, optimized reaction
conditions were achieved using benzyl alcohol (1a, 1 mmol) and DHP (2, 1.5 mmol) in presence
of 10molꢀ% zwitterionic imidazolium salt A at 60 °C for 10hꢀ (entry 1, Table 1) under neat
conditions in ambient air.
The scope and limitations of the optimized reaction conditions were examined by
employing various alcohols. First, the effect of various aliphatic, saturated as well as unsaturated
alcohols were tested (Table 2). It was observed aliphatic alcohols substituted with benzene moiety,
such as benzyl alcohol, styrene alcohol and 1-(p-tolyl)ethanol reacted efficiently with DHP to
afford the desired products with excellent yields under the present reaction conditions (3a-3c).
Diphenylmethanol, which is sterically hindered, also underwent facile tetrahydropyranylation to
afford the corresponding ether (3d). Acid sensitive piperonyl alcohol also afforded the desired
product with excellent yield (3e). Tert-amyl alcohol also reacted smoothly to give good yield (3f).
Alcohols present in unsaturated as well as alkyne systems were also investigated where both of
these functionalities were unaffected under the present reaction conditions and reacted smoothly
with DHP (3gꢀ-3k). We are pleased to inform that sterically hindered propargyl alcohol such as
1,1,3-triphenylpropargyl alcohol also afforded the desired product with good yield (3k). In
addition, Boc-protected amino ethanol underwent tetrahydropyranylation with moderate yield (3l).
5

To extend the scope of the present methodology, we turned our attention to use various
phenolic compounds (Table 3). To our delight, the desired products were obtained in good to
excellent yields (33mmꢀꢀ-3u). Simple phenol underwent smooth tetrahydropyranylation with good
yield (33mmꢀꢀ). Phenols bearing –Me and –OMe substituent efficiently reacted with DHP to afford
the products with high yields (3n-3p). Electron-withdrawing halogens like –F and –Br in phenol
also successfully gave the desired product without any difficulties (3qꢀ & 3r). This method was
also applicable to phloroglucinol, which reacted with three equivalents of DHP in one pot to afford
the corresponding product (3sꢀ). It is noteworthy to mention that 2-naphthol also reacted well under
the present reaction conditions (3t). In addition, Boc-protected 2-aminophenol underwent
tetrahydropyranylation with moderate yield (3u). All of the reported compounds have been
characterized by comparison of spectral data. Three newly compounds (3k, 3sꢀ, 3u) have been
characterized by spectral and analytical data. The reaction conditions are mild and give no
decomposition of the products or polymerization of the starting materials. We have not observed
any by-products for all reaction combinations which are supported with high yields.
Few control experiments have been carried out to prove the chemoselectivity of the present
method (Scheme 3). First, when the reaction was carried out under the standard conditions by
using a 1:1 mixture of phenol (11mmꢀꢀ) and benzyl alcohol (1a), we could not find any
tetrahydropyranylation of phenol (33mmꢀꢀ) but 3a was formed with high yield (Scheme 3a);
suggesting that benzyl alcohol is more reactive than phenol for the tetrahydropyranylation. Same
results were obtained using unsaturated crotyl alcohol (1hꢀ) as well as propargyl alcohol (1i)
(Scheme 3b,c). When styrene alcohol (1b) was used with crotyl alcohol (1hꢀ) and 1,1,3-
triphenylpropargyl alcohol (1k) separately both the corresponding products were obtained with
50:50 ratios (Scheme 3d,e). Whether using a 1:1 mixture of 1,1,3-triphenylpropargyl alcohol (1k)
6

and crotyl alcohol (1hꢀ); 1,1,3-triphenylpropargyl alcohol did not participate in the reaction
(Scheme 3f).
Reusability of the catalyst was also studied to make this protocol more effective. The
catalyst, left in the reaction vessel was dried under vacuum and was reused for subsequent
reactions. The catalyst was found to be effective up to fifth cycle giving a conversion of 82% in
the case of 3a (Table 4).
It is worthy to mention that the mechanistic path has not been studied but based on the
literature[22] and our previous works[19] we can propose the possible pathway as shown in Scheme
4. The zwitterionic-type molten salt may act a bifunctional organocatalyst.[22d–f] Simultaneously,
it could activate dihydropyran (DHP) through hydrogen bond formation between the C2–H of the
imidazolium cation and abstract the acidic hydrogen of alcohol by the sulphonate group.
Nucleophilic addition of the alkoxide ion with the activated double bond gives the desired THP
ether. Simple intramolecular proton transfer within the molten salt returns back to the zwitterion.
The pKa value of the molten salt (A) has been determined by pH titrimetric method and is 6.266.
Experimental
General
1H NMR spectra were determined on a Bruker 400 (400MHzꢀ) spectrometer as solutions
in CDCl3. Chemical shifts are expressed in parts per million (δ) and the signals were reported as s
13
(singlet), d (doublet), t (triplet), m (multiplet) and coupling constants J were given in Hz. Cꢀ
NMR spectra were recorded at 100MHzꢀ in CDCl3 solution. Chemical shifts are expressed in parts
per million (δ) and are referenced to CDCl3 (δ = 77.16) as internal standard. TLC was done on
silica gel coated glass slide (Merck, Silica gel G for TLC). Silica gel (60-120 mesh, SRL, India)
7

was used for column chromatography. Petroleum ether refers to the fraction boiling in the range
of 60-80 °C unless otherwise mentioned. All solvents were dried and distilled before use.
Commercially available substrates were freshly distilled before the reaction. Solvents, reagents
and chemicals were purchased from Aldrich, Merck, and Spectrochem Chemicals. All reactions
involving moisture sensitive reactants were executed using oven dried glassware.
General procedure for the synthesis of 3
A mixture of corresponding alcohol (1, 1 mmol) and 3,4-dihydro-2H-pyran (2, 100mgꢀ, 1.2
mmol) was stirred in presence of zwitterionic-salt A (10mgꢀ, 10molꢀ%) at 70-80 °C (oil bath) for
10-12hꢀ in a seal tube. After completion of the reaction (TLC), the reaction mixture was cooled to
room temperature and diluted with water (10mLꢀ) and extracted with ethyl acetate (20mLꢀ).
Organic layer was dried over anhydrous Na2SO4. After evaporation of solvent the crude product
was purified by column chromatography on silica gel using petroleum ether/ethyl acetate (4% to
5%) as eluent.
Spectral data of 2-(1,1,3-Triphenylprop-2-ynyloxy)-tetrahydro-2H-pyran (3k)
1
Yield: 76%, white solid, M.P. (53-55 °C); H NMR (CDCl3, 400MHzꢀ): δ 7.57 (d, J =
7.6Hzꢀ, 4H), 7.43-7.41 (m, 2H), 7.26-7.12 (m, 9H), 5.06 (t, J = 7.2Hzꢀ, 1H), 3.92-3.87 (m, 1H),
3.34 (d, J = 11.6Hzꢀ, 1H), 1.90 (t, J = 14.4Hzꢀ, 1H), 1.65 (d, J = 2.8Hzꢀ, 2H), 1.46 (t, J = 10.4Hzꢀ,
3H); 13Cꢀ NMR (CDCl3, 100MHzꢀ): δ 144.7, 144.0, 131.7, 128.5, 128.3, 128.1, 127.9, 127.6, 127.4,
127.1, 126.9, 122.8, 95.6, 89.7, 89.1, 79.7, 62.5, 31.6, 25.5, 19.6. Anal. Calcd for C26H24O2: C,
84.75; H, 6.57%; Found: C, 84.68; H, 6.51%.
Conclusions
8

In conclusion we have reported, imidazole-based zwitterionic type molten salt to be an
efficient catalyst for the tetrahydropyranylation of alcohols. High yields, chemoselectivity,
reusability of the catalyst, general applicability, clean reaction, production of no hazardous waste
and open air reaction conditions are the advantages of our methodology.
Acknowledgments
A. Majee acknowledges the financial support from the DST-RSF Major Research Project
(Ref. No. INT/RUS/RSF/P-08).
References
[1] Greene, T. W.; Wuts, P. G. M. Protective groups in organic synthesis, 2nd ed.; Wiley: New
York, 1991; p 346.
[2] (a) He, X.; Liu, H. W. Curr. Opin. Chem. Biol. 2002, 6, 590–597; (b) Kirschning, A; Bechthold,
A. F. W.; Rohr, J. Bioorg. Chem. Deoxysug. Polyket. Relat. Classes Synth. Biosynth.
Enzym. 1997, 188, 1–84.
[3] (a) Das, S.; Pekel, D.; Neudörfl, J. M.; Berkessel, A. Angew. Chem. Int. Ed. 2015, 54, 12479–
12483; (b) Balmond, E. I.; Coe, D. M.; Galan, M. C.; McGarrigle, E. M. Angew. Chem.,
Int. Ed. 2012, 51, 9152–9155.
[4] (a) Bernady, K. F.; Floyd, M. B.; Poletto, J. F.; Weiss, M. J. J. Org. Chem. 1979, 44, 1438–
1447; (b) Miyashita, M.; Yoshikoshi, A.; Grieco, P. A. J. Org. Chem. 1977, 42, 3772–
3774; (c) Pore, D. M.; Desai, U. V.; Mane, R. B.; Wadgaonkar, P. P. Synth. Commun. 2004,
34, 2135–2142; (d) Yang, J. H.; Zhang, X.; Liu, W. Y. Chinese Chem. Lett. 2008, 19, 893–
896.
[5] (a) Karimi, B.; Maleki, J. Tetrahedron Lett. 2002, 43, 5353–5355; (b) Wang, Y. G.; Wu, X.
X.; Jiang, Z. Y. Tetrahedron Lett. 2004, 45, 2973–2976; (c) Heravi, M. M.; Behbahani, F.
K.; Oskooie, H. A.; Shoar, R. H. Tetrahedron Lett. 2005, 46, 2543–2545; (d) Choudary, B.
M.; Neeraja, V.; Lakshmi Kantam, M. J. Mol. Catal. A Chem. 2001, 175, 169–172; (e)
Pachamuthu, K.; Vankar, Y. D. J. Org. Chem. 2001, 66, 7511–7513; (f) Shirini, F.;
Zolfigol, M. A.; Abri, A. R. Chin. Chem. Lett. 2007, 18, 803–806; (g) Khan, A. T.; Ghosh,
S.; Choudhury, L. H. Euro. J. Org. Chem. 2005, 22, 4891–4896; (h) Kamal, A.; Khan, M.
N.; Srikanth, Y. V.; Reddy, K. S. Can. J. Chem. 2008, 86, 1099–1104; (i) Williams, D. B.
G.; Simelane, S. B.; Lawton, M.; Kinfe, H. H. Tetrahedron 2010, 66, 4573–4576; (j) Ranu,
B. C.; Saha, M. J. Org. Chem. 1994, 59, 8269–8270.
[6] Palaniappan, S.; Ram, M. S.; Amarnath, C. A. Green Chem. 2002, 4, 369–371.
[7] Wang, M.; Song, Z.; Wan, X.; Zhao, S. RSC Adv. 2011, 1, 1698–1700.
[8] Naik, S.; Gopinath, R.; Patel, B. K. Tetrahedron Lett. 2001, 42, 7679–7681.
9

[9] Heravi, M. M.; Haghighi, M.; Derikvand, F.; Bamoharram, F. F. Synth. Commun. 2006, 36,
3103–3107.
[10] Hajipour, A. R.; Kargosha, M.; Ruoho, A. E. Synth. Commun. 2009, 39, 1084–1091.
[11] Narender, N.; Suresh Kumar Reddy, K.; Arun Kumar, M.; Rohitha, C. N.; Kulkarni, S. J.
Catal. Lett. 2010, 134, 175–178.
[12] Akhlaghinia, B.; Roohi, E. Turkish J. Chem. 2007, 31, 83–88.
[13] Poon, P. S.; Banerjee, A. K.; Bedoya, L.; Laya, M. S.; Cabrera, E. V.; Albornoz, K. M. Synth.
Commun. 2009, 39, 3369–3377.
[14] Tamami, B.; Parvanak Borujeny, K. Tetrahedron Lett. 2004, 45, 715–718.
[15] (a) Gawande, M. B.; Bonifácio, V. D. B.; Luque, R.; Branco, P. S.; Varma, R. S. Chem. Soc.
Rev. 2013, 42, 5522–5551; (b) Ramón, D. J.; Yus, M. Angew. Chem. Int. Ed. 2005, 44,
1602–1634; (c) Simon, M.-O.; Li, C.-J. Chem. Soc. Rev. 2012, 41, 1415; (d) Gu, Y. Green
Chem. 2012, 14, 2091–2128.
[16] Giacalone, F.; Gruttadauria, M.; Agrigento, P.; Noto, R. Chem. Soc. Rev. 2012, 41, 2406–
2447.
[17] (a) Kotke, M.; Schreiner, P. R. Synthesis 2007, 779–790; (b) Khomutnyk, Y. Y.; Argüelles,
A. J.; Winschel, G. A.; Sun, Z.; Zimmerman, P. M.; Nagorny, P. J. Am. Chem. Soc. 2016,
138, 444–456; (c) Madarász, A.; Dosa, Z.; Varga, S.; Soos, T.; Csampai, A.; Papai, I. ACS
Catal. 2016, 6, 4379–4387.
[18] Azizi, N.; Abdoli-Senejani, M.; Abbasi, F. Tetrahedron Lett. 2016, 57, 5009–5011.
[19] (a) Kundu, D.; Debnath, R. K.; Majee, A.; Hajra, A. Tetrahedron Lett. 2009, 50, 6998–7000;
(b) Kundu, D.; Majee, A.; Hajra, A. Catal. Commun. 2010, 11, 1157–1159; (c) Kundu, D.;
Bagdi, A. K.; Majee, A.; Hajra, A. Synletter 2011, 8, 1165–1167; (d) Rahman, M.; Bagdi,
A. K.; Kundu, D.; Majee, A.; Hajra, A. J. Heterocycl. Chem. 2012, 49, 1224–1228; (e)
Rahman, M.; Roy, A.; Ghosh, M.; Mitra, S.; Majee, A.; Hajra, A. RSC Adv. 2014, 4, 6116–
6119; (f) Ghosal, N. C.; Santra, S.; Das, S.; Hajra, A.; Zyryanov, G. V.; Majee, A. Green
Chem. 2016, 18, 565–574.
[20] (a) Cole, A. C.; Jensen, J. L.; Ntai, I.; Tran, K. L. T.; Weaver, K. J.; Forbes, D. C.; Davis, J.
H. J. Am. Chem. Soc. 2002, 124, 5962–5963; (b) Yoshizawa, M.; Hirao, M.; Ito-Akita, K.;
Ohno, H. J. Mater. Chem. 2001, 11, 1057.
[21] Das, S.; Santra, S.; Mondal, P.; Majee, A.; Hajra, A. Synth. 2016, 48, 1269–1285.
[22] (a) Kumar, A.; Srivastava, S.; Gupta, G.; Kumara, P.; Sarkar, J. RSC Adv. 2013, 3, 3548–
3552; (b) Aggarwal, A.; Lancaster, N. L.; Sethi, S. R.; Welton, T. Green Chem. 2002, 4,
517–520; (c) Cammarata, L.; Kazarian, S. G.; Salter, P.; Welton, T. Phys. Chem. Chem.
Phys. 2001, 3, 5192–5200; (d) Sarkar, A.; Roy, S. R.; Chakraborti, A. K. Chem. Commun.
2011, 47, 4538–4540; (e) Chakraborti, A. K.; Roy, S. R.; Kumar, D.; Chopra, P. Green
Chem. 2008, 10, 1111–1118; (f) Chakraborti, A. K.; Roy, S. R. J. Am. Chem. Soc. 2009,
131, 6902–6903; (g) Zhang, L. X.; Gao, G. Chem. Cat. Chem. 2011, 3, 1359–1364; (h)
Gholap, A. R.; Venkatesan, K.; Daniel, T.; Lahoti, R. J.; Srinivasan, K. V. Green Chem.
2003, 5, 693–696; (i) Tsuzuki, S.; Tokuda, H.; Mikami, M. Phys. Chem. Chem. Phys. 2007,
9, 4780–4784.
[23] Bartoli, G.; Giovannini, R.; Giuliani, A.; Marcantoni, E.; Massaccesi, M.; Melchiorre, P.;
Paoletti, M.; Sambri, L. Euro. J. Org. Chem. 2006, 1476–1482.
[24] Hamada, N.; Sato, T. Synlett. 2004, 1802–1804.
[25] Stephens, J. R.; Butler, P. L.; Clow, C. H.; Oswald, M. C.; Smith, R. C.; Mohan, R. S. Euro.
J. Org. Chem. 2003, 3827–3831.
10

[26] (a) Chaturvedi, D.; Kumar, A.; Ray, S. Indian J. Chem. 2004, 43B, 437–438; (b) Kometani,
T.; Kondo, H.; Fujimori, Y. Synthesis 1998, 1005–1007; (c) Giles, R. G. F.; Sargent, M.
V.; Sianipar, H. J. Chem. Soc. Perkin Trans.1 1991, 1571–1579.
[27] (a) Narender, M.; Reddy, M. S.; Kumar, V. P.; Nageswar, Y. V. D.; Rao, K. R. Tetrahedron
Lett. 2005, 46, 1971–1973; (b) Narender, M.; Reddy, M. S.; Kumar, V. P.; Nageswar, Y.
V. D.; Rao, K. R. Tetrahedron Lett. 2005, 46, 1971–1973.
[28] (a) Khan, A. T.; Mondal, E.; Borah, B. M.; Ghosh, S. Euro. J. Org. Chem. 2003, 4113–4117;
(b) Kumar, H. M. S.; Reddy, B. V. S.; Reddy, E. J..; Yadav, J. S. Chem. Lett. 1999, 28,
857–858. (c) Hayashi, T.; Konishi, M.; Yokota, K. I.; Kumada, M. J. Organomet. Chem.
1985, 285, 359–373; (d) Boruwa, J.; Bhuyan, R.; Gogoi, N.; Barua, N. C. Indian J. Chem.-
Sect. B Org. Medicinal Chem. 2005, 44, 331–334; (e) Varala, R.; Adapa, S. R. Can. J.
Chem. 2006, 84, 1174–1179.
11

Table 1. Optimization of the reaction conditionsa
Entry
Catalyst (mol%)
A (10)
B (10)
C (10)
D (10)
A (10)
A (10)
A (10)
A (5)
Temp (°C)
Time (h)
10
Yieldsb (%)
1
2
3
4
5
6
7
8
9
10
60
60
60
60
80
40
rt
90
10
75
10
70
10
Trace
84
10
12
60
10
25
60
60
60
10
74
A (15)
–
10
90
12
NDc
aCarried out with 0.5 mmol of 1a and 0.5 mmol of 2inꢀ the presence of 10molꢀ% catalyst in neat conditions.
bIsolated yields.
cND = Not detected on TLC.
12

Table 2. Studies on the scope of aliphatic alcoholsa
Entry Alcohols (1)
Products (3)
Yieldsb (%)
90
References
[5e]
1
[5f]
2
3
4
93
92
70
[27a]
[5a]
[5f]
5
94
[6]
6
7
82
78
[5f]
[28a]
[5d]
8
9
75
78
13

[24]
10
78
76
11c
–
[23]
12
67
aReaction Conditions: Alcohols 1 (1 mmol), DHP 2 (1.2 mmol), zwitterionic salt A (10molꢀ%) at 60 °C for 10hꢀ.
bAll are isolated yields.
c0.5mLꢀ THP was used as solvent.
14

Table 3. Studies on the scope of aryl alcoholsa
Entry
1
Phenols
Products
Yieldsb (%)
82
References
[5d]
[5a]
[5f]
2
82
80
75
3c
4
[26a]
[5a]
[5a]
5c
6
84
71
74
7c,d
-
[5a]
8c
78
15

9
60
–
aReaction Conditions: Alcohols 1 (1 mmol), DHP 2 (1.2 mmol), zwitterionic salt A (10molꢀ%) at 60 °C for 10hꢀ.
bAll are isolated yields.
c0.5mLꢀ THP was used as solvent.
d3 equiv. DHP was used.
16

Table 4. Recycling of the catalyst A for synthesizing 3aa
No of cycles
Yieldsb (%)
Time (h)
1
2
3
4
5
90
90
88
85
82
10
10
10
12
12
aCarried out with 1 mmol of 1a and 1.2 mmol of 2inꢀ the presence of 10molꢀ% zwitterionic salt A at 60 °C.
bIsolated yields.
17

Figure 1. Different types of zwitterionic-type molten salts.
18

Scheme 1. Synthetic strategy for zwitterionic-type molten salts.
19

Scheme 2. Zwitterionic salt-catalyzed tetrahydropyranylation of alcohols.
20

Scheme 3. Control experiments for chemoselectivity test.
21

Scheme 4. Probable mechanistic pathway.
22