New efficient synthetic pathways to tetrakis{p-[(diethylphosphono)methyl]} calix[4]arene

Article abstract of DOI:10.1002/hlca.201100413

Various operating conditions have been applied on tetrakis[p- (halogenomethyl)]- and tetrakis[p-(aminomethyl)]calix[4]arene derivatives to improve the synthesis of the 5,11,17,23-tetrakis[(diethylphosphono)methyl]-25, 26,27,28-tetrahydroxycalix[4]arene. Two new, high yield, synthetic pathways have been selected, involving, for the first one, the 25,26,27,28-tetrahydroxy-5,11, 17,23-tetrakis[(trimethylamino)methyl]calix[4]arene, tetraiodide, DMF, and 10 equiv. of triethyl phosphite ((EtO)₃P), and, for the other one, the 5,11,17,23-tetra(bromomethyl)-25,26,27,28-tetrahydroxycalix[4]arene, CH ₂Cl₂, and only 4 equiv. of (EtO)₂P. Copyright

Full text of DOI:10.1002/hlca.201100413

766
Helvetica Chimica Acta – Vol. 95 (2012)
New Efficient Synthetic Pathways to Tetrakis{p-[(diethylphosphono)-
methyl]}calix[4]arene
by Maxime Mourer*a), and Jean-Bernard Regnouf-de-Vainsa)
a
´
´
) SRSMC, UMR 7565 Nancy Universite – CNRS, Groupe dꢀEtude des Vecteurs Supramoleculaires du
´
´
Medicament, Faculte de Pharmacie, 5, rue Albert Lebrun, BP 80403, FR-54001 Nancy Cedex
(phone: þ 33 38 368 2367; fax: þ 33 38 368 2345; e-mail: maxime.mourer@pharma.uhp-nancy.fr)
Various operating conditions have been applied on tetrakis[p-(halogenomethyl)]- and tetrakis[p-
(aminomethyl)]calix[4]arene derivatives to improve the synthesis of the 5,11,17,23-tetrakis[(diethyl-
phosphono)methyl]-25,26,27,28-tetrahydroxycalix[4]arene. Two new, high yield, synthetic pathways have
been selected, involving, for the first one, the 25,26,27,28-tetrahydroxy-5,11,17,23-tetrakis[(trimethyl-
amino)methyl]calix[4]arene, tetraiodide, DMF, and 10 equiv. of triethyl phosphite ((EtO)3P), and, for
the other one, the 5,11,17,23-tetra(bromomethyl)-25,26,27,28-tetrahydroxycalix[4]arene, CH2Cl2, and
only 4 equiv. of (EtO)2P.
Introduction. – As recently reviewed by Cherenok and Kalchenko [1], calixarene
species bearing phosphor(II) or phosphor(V) at the upper or at the lower rim have
been involved in the complexation of metal ions, notably for extraction or catalysis
purposes, in the complexation of organic molecules such as amino acids, proteins,
nucleic bases derivatives, and some herbicides, and as biologically active substances, for
example, as phosphatase inhibitors or calcium pump effectors.
In these domains, our contributions were related to the synthesis of H2O-soluble
para-phosphonomethyl derivatives of calixarene-based bipyridyl chelators able to
complex and stabilize CuI in H2O, even in the presence of seric proteins, suggesting an
interesting role as cation carrier for biological applications [2], or derivatives of
calixarene-based bithiazolyl podands evaluated, among other anionic species, as anti-
infective agents [3], more precisely as anti-HIVagents on various HIV strains and cells
of the lymphocytic lineage.
The need of these derivatives in readily accessible quantities allowing the
development of our studies led us to seek a convenient synthetic pathway for the
key compound, i.e., the 5,11,17,23-tetrakis[(diethylphosphono)methyl]-25,26,27,28-
tetrahydroxycalix[4]arene.
The phosphonomethylation at the upper rim of unprotected calix[n]arene plat-
forms has been described by Ungaro and co-workers [4]; it involves a preliminary
chloromethylation by reaction of chloromethyl octyl ether and SnCl4, followed by
reaction of triethyl phosphite ((EtO)3P) under the conditions of Arbuzov reaction to
give the tetrakis{p-[(diethylphosphono)methyl]}calix[4]arene. At the same time,
Shinkai and co-workers described a similar protocol [5] applied on calix[6]arene
hexamethyl ether, using ClCH2OMe and ZnCl2 to obtain the hexakis(p-chloromethyl)
derivative, which was then reacted with (EtO)3P as described above to afford the
ꢁ 2012 Verlag Helvetica Chimica Acta AG, Zꢂrich

Helvetica Chimica Acta – Vol. 95 (2012)
767
hexakis{p-[(diethylphosphono)methyl]} analog. Similar methods were developed with
O-alkyl derivatives of p-(chloromethyl)calix[6]arenes or calix[4]arenes, involving here
a reaction with (iPrO)3P to give the corresponding p-[(diisopropylphosphono)methyl]
analog [6][7].
As seen above, the p-(chloromethyl)calixarene species are key compounds in the
synthesis of phosphono derivatives; they are readily obtained by reaction of the
calixarenes with chloromethyl octyl ether or ClCH2OMe in the presence of Lewis
acids; Huang et al. [8] proposed a cheaper, full or regioselective para-chloromethy-
lation of unprotected or partially OH-protected calix[4]arene derivatives, by reaction
of gaseous HCl and paraformaldehyde in a AcOH/H2SO4 mixture.
In our previous works, the key compound tetrakis{p-[(diethylphosphono)methyl]}-
calix[4]arene tetrol 6 was prepared in a two-step procedure involving first the
chloromethylation of ꢃde-terbutylatedꢀ calix[4]arene 1 according to [4], or to the less
expensive procedure in [8], to give the compound 2. The second step was performed via
a slight modification of the process of Ungaro and co-workers [4], consisting in
refluxing 2 and a moderate excess (40 equiv.) of (EtO)3P in CH2Cl2 (Entry 1);
elimination of most of residual phosphite was performed by lyophilization from
cyclohexane suspension and chromatography, to give 6 in a good yield, but always
polluted by traces of phosphite.
Nevertheless, the adaptation of the chloromethylation process of Huang et al. [8] to
large quantities gave some irregular results, and the stability of stocks of 2 was found
unsatisfactory, even when frozen. These findings and the aim of working without
chlorinated reagents led us to explore new approaches leading to 6, such as shown in the
Scheme together with the Table.
Scheme
i) 1 ! 2: According to [4], (HCHO)n, AcOH, conc. H2SO4, HCl gas, r.t.; 1 ! 3: according to [9],
(Me)2NH 40% in H2O, HCHO 37% in H2O, AcOH, r.t.; 1 ! 5: according to [11] (HCHO)n, HBr (g),
AcOH. ii) 3 ! 4: According to [10], MeI, DMSO, r.t. iii) From 2: CHCl3, 40 equiv. of (EtO)3P, reflux;
from 3: a) MeI, DMSO, then b) 20 equiv. (EtO)3P, 608, 4 h; from 4: DMF, 10 equiv. of (EtO)3P, 708; from
5: CH2Cl2, 4 equiv. of (EtO)3P, reflux.
In this sense, we attempted to use another intermediate with a leaving group
attached via a CH2 group to the upper rim of calixarene, and susceptible to react with
(EtO)3P. The different conditions employed in this study are compiled in the Table.

768
Helvetica Chimica Acta – Vol. 95 (2012)
Table. Resume of Operating Conditions
Entry
Substrate
Solvent
(EtO)3P (equiv.)
T [8]
Time [h]
Yield [%]
a
1
2
3
4
2
(EtO)3P/CH2Cl2
DMSOb)
DMFc)
DMFc)
)
40
5
5
reflux
80
60
6
5
7
4
85
31
n.d.d)
43
20
60
5
6
7
8
9
3 ! [4]
MeCNe)
MeCN
Sulfolanef)
Sulfolane
Sulfolane
Sulfolane
5
20
5
20
5
reflux
reflux
60
60
60
7
6
4
24
24
24
n.d.
32
37
trace
n.d.
n.d.
10
10
70
11
12
13
14
4
5
(EtO)3P/NMPg)
50
10
10
5
70
60
70
70
24
24
6
n.d.
57
81
Dimethylacetamideh)
DMF
DMF
6
75
15
16
CH2Cl2
CH2Cl2
5
4
reflux
reflux
6
6
78
89
c
a) CH2Cl2 distilled over CaCl2. b) DMSO dried over molecular sieves (4 ꢄ). ) DMF dried over CaSO4.
f
d) n.d.: Not determined. e) MeCN distilled over CaH2. ) Sulfolane (99%) was used without purification.
g) 1-Methylpyrrolidin-2-one. h) Dimethylacetamide (99%) was used without purification.
Results and Discussion. – First approach. We first developed a strategy based on the
well explored ꢃMannichꢀ derivative tetrakis{p-[(dimethylamino)methyl]}calix[4]arene
3, involved in many functionalizations at the upper rim of calixarene backbones,
through conversion to the corresponding quaternary ammonium salt and replacement
of the ammonium moiety with an appropriate nucleophile [9].
The calixarene 3 was thus prepared according to the procedure of Gutsche and Nam
[10], by treatment of a THF/AcOH solution of 1, 37% aqueous HCHO, and 40%
aqueous Me2NH at room temperature. Compound 3 was then treated with MeI in
DMSO to give the quaternary ammonium salt 4.
In first assays, 4 was not isolated and was reacted directly with 5 equiv. of (EtO)3P
(Entry 2 of the Table) to afford the expected product 6 in low yield (31%) after a
laborious workup. The addition of H2O, commonly employed as counter-solvent for
DMSO solutions, did not give a precipitate. This solution, enriched in H2O to retain
DMSO, was finally extracted with CH2Cl2. The resulting yellow-brown organic solution
was evaporated, and the raw material was chromatographed to give 6 as a yellow oil.
This color that could result of the presence of iodine complexed with 6 disappeared
partially, when the crude material was washed by aqueous NaHSO3.
The same experiment was performed in dry DMF at 608 (Entries 3 and 4) during 7
or 4 h, with 5 or 20 equiv. of (EtO)3P, respectively. Only the excess conditions gave the
desired compound in 43% yield. Similar results were obtained in dry MeCN at reflux
(Entries 5 and 6) with 5 and 20 equiv. of (EtO)3P, respectively; the phosphonocalix[4]-
arene 6 was isolated with 32% under excess conditions. When the sulfolane (¼2,3,4,5-
tetrahydrothiophene 1,1-dioxide) was used as solvent (Entries 7 and 8), 6 was isolated

Helvetica Chimica Acta – Vol. 95 (2012)
769
in 37% yield, but only in the stoichiometric conditions. Most of experiments that
provided 6 as an isolated compound, gave good TLC control plates, but unsatisfactory
NMR analyses.
Gutsche and co-workers found that the isolation of the tetrakis(ammonium) salt 4
increase the flexibility of the next transformation processes [9]. For this purpose, 3 was
treated with MeI for ca. 4 h, and the addition of an excess of acetone resulted in the
precipitation of 4, which was recovered by filtration.
The first phosphonation assays were carried out with 4 in sulfolane at 60 and 708
(Entries 9 and 10) with moderate excess of (EtO)3P, or in 1-methylpyrrolidin-2-one
with an excess of the latter (Entry 11). Here also, precipitation of 6 by addition of H2O
failed. The oily materials thus obtained were recovered by extraction with CH2Cl2,
followed by a lyophilization from CH2Cl2/cyclohexane to remove (EtO)3P and
pyrrolidinone (Entry 11), and by a final column chromatography that was, however,
unable to remove residual traces of solvent from 6.
Treating 4 by a moderate excess (10 equiv.) of (EtO)3P in N,N’-dimethylacetamide
(Entry 12) gave interesting results. Evaporation of N,N’-dimethylacetamide, followed
by lyophilization of the residue from CH2Cl2/cyclohexane, gave a raw material that was
chromatographed (SiO2) to give 6 in a yield of 57%. However, a similar procedure
performed in dry DMF (Entry 13) gave much better results. Thus, the tetrakis(ammo-
nium) salt 4 was treated in DMF by a moderate excess of (EtO)3P (10 equiv. with
respect to 4) at 708 during 6 h. Compound 6 was isolated as a solid material, in a yield of
81%, after evaporation of DMF, followed by lyophilization cycles from a cyclohexane
suspension to remove residual (EtO)3P, and by a final chromatography.
The choice of the couple DMF/4 (Entry 13) gave the best results in this first
approach, with a high yield of ester 6. Nevertheless, the relatively difficult workup
prompted us to search for an easier procedure.
Second Approach. Here, the tetrakis[p-(bromomethyl)]calixarene 5 was chosen as
substrate of the phosphonation reaction. Contrary to its chloro analog 2, this
intermediate is easier to obtain and seems to be less unstable. According to the
procedure of Guo et al. [11], 5 was prepared in high yield from calix[4]arene 1 by
bubbling dry HBr gas in the presence of paraformaldehyde and glacial AcOH at room
temperature, and was recovered by a very simple workup.
The first phosphonation assay was carried out in dry DMF with 5 and only 5 equiv.
of (EtO)3P, at 708 during 6 h (Entry 14). Contrary to the ClCH2 analog 2 under the
same conditions, this procedure resulted in the rapid and easy conversion of 5 to 6 in a
yield of 75% after evaporation of DMF, rapid column chromatography, and
crystallization from pentane.
With the aim of relieving the workup, in particular, the time-consuming and delicate
high-vacuum evaporation of DMF, we performed the reaction in refluxing CH2Cl2, first
with 5, then with 4 equiv. of (EtO)3P, at reflux during 6 h (Entries 15 and 16, resp.).
It turned out that this new procedure, even with a stoichiometric amount of
(EtO)3P (4 equiv.), provided very good results in terms of reaction yields (78 and 89%,
resp.) and, more particularly, in the ease of product recovery. The tetrakis{p-
[(diethylphosphono)methyl]}calix[4]arene 6 is obtained as a pure, white powder after
evaporation of the solvent, and a simple crystallization of the raw material from
pentane.

770
Helvetica Chimica Acta – Vol. 95 (2012)
The compound 6 thus obtained was fully characterized, and gave satisfactory NMR
and electrospray (ESI) MS data, and elemental analyses. According to de Mendoza and
co-workers [12][13], who have shown that the resonance of the CH2 bridge C-atom is
near to 31 ppm, when the attached phenol groups are in the syn-orientation, and near
37 ppm, when they are in anti-orientation, we found that the cone conformation of the
calixarene subunit was conserved, as assessed by 13C-NMR resonance signals of
ArꢀCH2ꢀAr, observed between 31 and 33 ppm. An analytically pure sample of 6 was
obtained by chromatography (SiO2; CH2Cl2/MeOH 97:3).
Conclusions. – In conclusion, we have developed two new, high-yield synthetic
pathways leading to the tetrakis{p-[(diethylphosphono)methyl]}calix[4]arene 6. Var-
ious aprotic solvents and stoichiometric conditions have been evaluated for an easiest
introduction of the diethylphosphono group in active tetrakis{p-[(X)methyl]}ca-
lix[4]arene species, in which X is a halogen (Cl, Br), or a quaternary ammonium used
pure or generated in situ. For ammonium derivative, the results were satisfactory only
when used pure, in DMF; for halogeno derivatives, we found that the bromide was the
best candidate, first, due to its easy preparation, and second, for its excellent reactivity
with ca. stoechiometric amounts of (EtO)3P, in DMF or in CH2Cl2. The more
convenient procedure involved CH2Cl2 as solvent and 4 equiv. of (EtO)3P, to give the
tetrakis-phosphono ester in 89% yield with a very simple workup.
The authors are grateful to the MRES and to the CNRS for financial support, and thank Mr. E. Dubs
for the synthesis of some starting materials, Mrs. B. Fernette for NMR experiments, Mrs. S. Addach for
elemental analyses, and Mr. F. Dupire for MS recording.
Experimental Part
General. All commercially available products were used without further purification unless
otherwise specified. Chloromethyl, (dimethylamino)methyl, and (trimethylamino)methyl iodide deriv-
atives 2 [4], 3 [9], 4 [10], resp., and bromomethyl derivative 5 [11] were synthesized as described in the
literature. M.p.: Electrothermal 9200 in capillary apparatus; uncorrected. TLC: Merck plates (SiO2, ref
1.05554; Al2O3, ref 1.05581). UV Spectra: SAFAS UV mc2 apparatus, lmax in nm, e in molꢀ1 dm3 cmꢀ1. IR
Spectra: Bruker Vector 22 apparatus (KBr, n in cmꢀ1). 1H- and 13C-NMR spectra: Bruker DRX 400
(chemical shifts in ppm). MS (electronic ionisation (EI), and electrospray (ESI)): Micromass Platform II
´
apparatus, at the Service Commun de Spectrometrie de Masse Organique, Nancy. Elemental analyses:
performed at the Service de Microanalyse, Nancy.
Syntheses. 5,11,17,23-Tetrakis[(diethylphosphono)methyl]-25,26,27,28-tetrahydroxycalix[4]arene
(¼Octaethyl {[25,26,27,28-Tetrahydroxypentacyclo[19.3.1.13,7.19,13.115,19]octacosa-1(25),3(28),4,6,9(27),
10,12,15(26),16,18,21,23-dodecaene-5,11,17,23-tetrayl]tetramethanediyl}tetrakis(phosphonate); 6): from
5,11,17,23-tetra(chloromethyl)-25,26,27,28-tetrahydroxycalix[4]arene (¼ 5,11,17,23-tetrakis(chlorome-
thyl)pentacyclo[19.3.1.13,7.19,13.115,19]octacosa-1(25),3(28),4,6,9(27),10,12,15(26),16,18,21,23-dodecaene-
25,26,27,28-tetrol; 2). A mixture of 2 (1 g, 1.62 · 10ꢀ3 mol) and (EtO)3P (11 ml, 64.0 · 10ꢀ3 mol, 40 equiv.)
in CH2Cl2 (15 ml) was refluxed during 6 h under Ar. The solvent was evaporated, and the residue was
concentrated under high vacuum at 508 to remove most of residual phosphite. The resulting oily material
was lyophilized from a cyclohexane suspension to remove residual (EtO)3P (1H-NMR control; 2 or 3
times). The residue was chromatographed (SiO2; CH2Cl2/MeOH 97:3) to give 6 (1.34 g, 85%; pure on
the basis of TLC and 1H-NMR analysis).
From 5,11,17,23-Tetrakis[(dimethylamino)methyl]-25,26,27,28-tetrahydroxycalix[4]arene (¼ 5,11,
17,23-Tetrakis[(dimethylamino)methyl]pentacyclo[19.3.1.13,7.19,13.115,19]octacosa-1(25),3(28),4,6,9(27),
10,12,15(26),16,18,21,23-dodecaene-25,26,27,28-tetrol; 3). To a soln. of 3 (0.2 g, 0.31 · 10ꢀ3 mol) in anh.

Helvetica Chimica Acta – Vol. 95 (2012)
771
DMF (5 ml) was added MeI (0.10 ml, 1.53 · 10ꢀ3 mol). The mixture was stirred at r.t. during 30 min.
(EtO)3P (1.05 ml, 6.13 · 10ꢀ3 mol, 20 equiv.) was added, and the soln. was heated at 608 during 4 h. After
cooling to r.t., DMF was evaporated under high vacuum. The crude material was dissolved in CH2Cl2
(50 ml), then washed with H2O (50 ml). The org. phase was dried (Na2SO4), filtered, and concentrated.
The residue was chromatographed (SiO2; CH2Cl2/MeOH 97:3) to give 6 (0.14 g, 43%; pure on the basis
of TLC and 1H-NMR analysis).
From 25,26,27,28-Tetrahydroxy-5,11,17,23-tetrakis[(trimethylamino)methyl]calix[4]arene tetraiodide
(¼ [25,26,27,28-Tetrahydroxypentacyclo[19.3.1.13,7.19,13.115,19]octacosa-1(25),3(28),4,6,9(27),10,12,15(26),
16,18,21,23-dodecaene-5,11,17,23-tetrayl]tetrakis(N,N,N-trimethylmethanaminium) Tetraiodide; 4). To a
soln. of 4 (4.77 g, 3.91 · 10ꢀ3 mol) in anh. DMF (45 ml) was added (EtO)3P (6.7 ml, 39.11 · 10ꢀ3 mol,
10 equiv.); the mixture was heated at 708 under Ar during 6 h. After cooling, DMF was evaporated under
high vacuum, and the residue was lyophilized from a cyclohexane suspension, then chromatographed
1
(SiO2; CH2Cl2/MeOH 97:3) to give 6 (3.21g, 81%; pure on the basis of TLC and H-NMR analysis).
From 5,11,17,23-Tetrakis(bromomethyl)-25,26,27,28-tetrahydroxycalix[4]arene (¼ 5,11,17,23-Tetra-
kis(bromomethyl)pentacyclo[19.3.1.13,7.19,13.115,19]octacosa-1(25),3(28),4,6,9(27),10,12,15(26),16,18,
21,23-dodecaene-25,26,27,28-tetrol; 5). To a soln. of 5 (0.15 g, 0.19 · 10ꢀ3 mol) in CH2Cl2 (5 ml) was added
(EtO)3P (0.13 ml, 0.75 · 10ꢀ3 mol, 4 equiv.); the mixture was refluxed during 6 h. After cooling to r.t., the
solvent was evaporated, and the residue was dried under vacuum to give an oil. An excess of pentane
(50 ml) was then added, and the oily material was triturated and sonicated. After 24 h, the resulting solid
that was formed was filtered off, washed with pentane, and dried under vacuum to give 6 (0.17 g, 89%).
White solid. M.p.: 139 – 1408. 1H NMR (400 MHz, CDCl3): 1.20 (t, J ¼ 7.1, 8 MeCH2O); 2.89 (d, J ¼ 21.2, 4
CH2P); 3.48 – 4.18 (br. ꢃqꢀ, AB, JAB ¼ 13.4, 4 ArCH2Ar); 3.79 (m, 8 MeCH2O); 6.96 (d, J ¼ 1.5, 8 arom. H);
10.04 (s, 4 OH). 13C NMR (100 MHz, CDCl3): 16.8 (d, J ¼ 5.9, MeCH2O); 32.0 (ArCH2Ar); 33.1 (d, J ¼
138.4, CH2P); 65.5 (d, J ¼ 16.6, MeCH2O); 125.4 (d, J ¼ 8.8, Cp); 128.6 (d, J ¼ 2.9, Co); 130.7 (d, J ¼ 6.6,
Cm); 148.2 (d, J ¼ 3.7, Cipso). ES-MS (pos.): 1047.33 ([M þ Na]þ), 1025.35 ([M þ H]þ), 997.32 ([M ꢀ Et þ
2 H]þ), 969.28 ([M ꢀ 2 Et þ 3 H]þ), 941.26 ([M ꢀ 3 Et þ 4 H]þ), 913.23 ([M ꢀ 4 Et þ 5 H]þ). ES-MS
(neg.): 1023.34 ([M ꢀ H]ꢀ), 885.29 ([M ꢀ (PO(OEt)2) ꢀ 2 H]ꢀ), 857.24 ([M ꢀ (PO(OEt)2) ꢀ Et ꢀ H]–),
811.18 ([M ꢀ (PO(OEt)2) ꢀ Et ꢀ EtO ꢀ 2 H]ꢀ), 783.13 ([M ꢀ (PO(OEt)2) ꢀ 2 Et ꢀ EtO ꢀ H]ꢀ), 747.25
([M ꢀ 2 (PO(OEt)2) ꢀ 3 H]ꢀ), 719.19 ([M ꢀ 2 (PO(OEt)2) ꢀ Et ꢀ 2 H]ꢀ), 673.17 ([M ꢀ 2
(PO(OEt)2) ꢀ Et ꢀ EtO ꢀ 3 H]ꢀ). Anal calc. for C48H68O16P4 (1024.94): C 56.25, H 6.69; found: C
56.30, H 6.54.
REFERENCES
[1] S. Cherenok, V. Kalchenko, Top. Heterocycl. Chem. 2009, 20, 229.
[2] N. Psychogios, J.-B. Regnouf-de-Vains, Tetrahedron Lett. 2002, 43, 7691.
[3] M. Mourer, N. Psychogios, G. Laumond, A.-M. Aubertin, J.-B. Regnouf-de-Vains, Bioorg. Med.
Chem. 2010, 18, 36.
[4] M. Almi, A. Arduini, A. Casnati, A. Pochini, R. Ungaro, Tetrahedron 1989, 45, 2177.
[5] T. Arimura, T. Nagasaki, S. Shinkai, T. Matsuda, J. Org. Chem. 1989, 54, 3766.
[6] Y. I. Rudzevich, A. B. Drapailo, V. L. Rudzevich, V. I. Miroshnichenko, V. I. Kalꢀchenko, I. V.
Smirnov, V. A. Babain, A. A. Varnek, G. Wipff, Russ. J. Gen. Chem. 2002, 72, 1736.
[7] O. Klimchuk, L. Atamas, S. Miroshnichenko, V. Kalchenko, I. Smirnov, V. Babain, A. Varnek, G.
Wipff, J. Inclusion Phenom. Macrocycl. Chem. 2004, 49, 47.
[8] Z.-T. Huang, G.-Q. Wang, L.-M. Yang, Y.-X. Lou, Synth. Commun. 1995, 25, 1109.
[9] I. Alam, S. K. Sharma, C. D. Gutsche, J. Org. Chem. 1994, 59, 3716.
[10] C. D. Gutsche, K. C. Nam, J. Am. Chem. Soc. 1988, 110, 6153.
[11] T.-D. Guo, Q.-Y. Zheng, L.-M. Yang, Z.-T. Huang, J. Inclusion Phenom. Macrocycl. Chem. 2000, 36,
327.
[12] C. Jaime, J. de Mendoza, P. Prados, P. Nieto, C. Sanchez, J. Org. Chem. 1991, 56, 3372.
[13] J. O. Magrans, J. de Mendoza, M. Pons, P. Prados, J. Org. Chem. 1997, 62, 4518.
Received October 21, 2011