A new strategy for calixindole formation: Synthesis of a calix[3]indole with 2,2; 7,2; 7,7-methylene linkages and a new calix[4]indole with 2,2; 7,2; 7,7; 2,7-methylene linkages

Article abstract of DOI:10.1055/s-0032-1317858

A new strategy of acid-catalysed reactions of indole methanols with a 3-aryl-4,6-dimethoxyindole-7-aldehyde leads to an efficient synthesis of a calix[3]indole with 2,2; 7,2; 7,7-links. A new calix[4]indole containing four indole units with 2,2; 7,2; 7,7; 2,7-links has also been prepared by this strategy. Georg Thieme Verlag Stuttgart · New York.

Full text of DOI:10.1055/s-0032-1317858

24▌
LETTER
lAetteNr ew Strategy for Calixindole Formation: Synthesis of a Calix[3]indole with
2,2; 7,2; 7,7-Methylene Linkages and a New Calix[4]indole with 2,2; 7,2; 7,7;
2,7-Methylene Linkages
New Strategy for Calixindole Formation
Kittiya Somphol, Rui Chen, Mohan Bhadbhade, Naresh Kumar, David StC. Black*
School of Chemistry, The University of New South Wales, UNSW, Sydney NSW 2052, Australia
Fax +61(2)93856141; E-mail: d.black@unsw.edu.au
Received: 05.11.2012; Accepted: 19.11.2012
In principle, these calix[3]indoles can contain either three
Abstract: A new strategy of acid-catalysed reactions of indole
arylmethylene linkages between C2 and C7, or one such
methanols with a 3-aryl-4,6-dimethoxyindole-7-aldehyde leads to
linkage between C2 and C2, C2 and C7, and C7 and C7
(see Figure 1). Both types have already been synthesised.
an efficient synthesis of a calix[3]indole with 2,2; 7,2; 7,7-links. A
new calix[4]indole containing four indole units with 2,2; 7,2; 7,7;
2,7-links has also been prepared by this strategy.
Key words: indoles, diindolylmethanes, macrocyclic compounds,
acid-catalysed reactions, ipso-substitution reactions, formaldehyde
extrusion
HN
N
NH
H
HN
NH
HN
NH
Macrocycles containing pyrroles are well established in
the field of supramolecular chemistry for the reason that
the pyrrole NH groups readily form hydrogen bonds with
guest molecules or on deprotonation the nitrogen atoms
can bind to metal ions.1–3 Two well-known systems are
calix[4]pyrroles and porphyrins.4,5 However macrocycles
containing indoles have not been widely studied in this
context, despite the similar acidity of an indole NH group
to that of a pyrrole. The expansion of the five-membered
pyrrole ring system to the larger and more highly conju-
gated indole system creates new synthetic and structural
possibilities with great potential for supramolecular
chemistry.
H
N
2,2; 7,2; 7,7; 2,7
2,7; 2,7; 2,7; 2,7
NH
NH
HN
NH
NH
HN
HN
HN
2,2; 7,7; 2,2; 7,7
2,2; 7,2; 7,2; 7,7
Figure 2 Possible structural frameworks of calix[4]indoles
NH
HN
N
H
H
HN
Both 3-substituted 4,6-dimethoxyindole-7-carbaldehydes
and 4,6-dimethoxyindole-2-carbaldehydes can be pre-
pared via the Vilsmeier–Haack formylation reaction.13
Subsequent reduction of the formyl groups with sodium
borohydride affords the simple indole C7- and C2-metha-
nols, which can also undergo acid-catalysed cyclisation to
calix[3]indoles.14–17
N
NH
2,2; 7,2; 7,7
2,7; 2,7; 2,7
Figure 1 Possible structural frameworks of calix[3]indoles
However, in this process, calix[4]indoles can also be gen-
erated, and show four methylene linkages between C2 and
C7. There are four possible ways of linking four indoles
through the C2 and C7 positions with four methylene
links. These basic structures are shown in Figure 2.
The synthesis of 3-substituted 4,6-dimethoxyindoles has
enabled their enhanced activity at C2 and C7 to be inves-
tigated in a variety of ways.6–11 Acid-catalysed reactions
of these indoles with aryl aldehydes have led to the forma-
tion of calix[3]indoles, presumably via intermediate
indole methanols.12
We now report new, highly effective stepwise sequences
for the synthesis of a calix[3]indole and the first example
of a calix[4]indole with a 2,2; 7,2; 7,7; 2,7 set of methy-
lene linkages.
SYNLETT 2013, 24, 0024–0028
Advanced online publication: 10.12.2012
DOI: 10.1055/s-0032-1317858; Art ID: ST-2012-D0947-L
© Georg Thieme Verlag Stuttgart · New York
These stepwise sequences involve the monoindole 2,7-di-
methanol 1 and the diindolyl 7,7′-dimethanol 2, which
have been previously reported.14 Reaction of dimethanol
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6

LETTER
New Strategy for Calixindole Formation
25
MeO
R
R
R
MeO
OMe
OMe
OH
N
MeO
H
NH
HN
OH
+
MeO
OHC
1
a
R
NH
MeO
R
4
MeO
CHO
N
MeO
MeO
MeO
H
CHO
b
R = 4-BrC6H4
3
R
R
R
R
OMe
MeO
OMe
NH
HN
HN
NH
MeO
R
OMe
H
MeO
OMe
OMe
c
N
OH
NH
MeO
R
MeO
OH
6
5
MeO
Scheme 1 Reagents and conditions: (a) p-TsOH, i-PrOH, r.t., 1 h, 90%; (b) NaBH4, THF, EtOH, r.t., 98%; (c) p-TsOH, DMSO, r.t., 5 min, 89%.
1 with two equivalents of the indole-7-aldehyde 3 in iso- observed at δ = 3.44, 3.53, 3.75, 3.80, 3.95, 3.99 and 4.01
propanol containing p-toluenesulfonic acid gave the tri- ppm for the eight methoxy groups. There are three differ-
indolyldimethane dialdehyde 4,18 which precipitated from ent methylene groups, the protons of which form AB dou-
the reaction mixture and was obtained in 90% yield. blets resonating at δ = 3.98, 3.99 and 4.16 ppm, and four
Reduction of the dialdehyde 4 with sodium borohydride in different H5 protons at δ = 5.91, 6.26, 6.33 and 6.48 ppm.
a mixture of ethanol and tetrahydrofuran gave the dimeth- Furthermore an X-ray crystal structure was obtained (see
anol 5 in 98% yield. Treatment of this dimethanol 5 with Figure 3), and showed a rigid cube-like structure, similar
p-toluenesulfonic acid in anhydrous dimethylsulfoxide to that of a calix[4]indole with four 2,7-methylene links.15
generated the calix[3]indole 619 in 89% yield (Scheme 1). It is noteworthy that both types of calix[4]indoles pre-
Compound 6 is a new example of this class of previously
reported calixindoles. Its 1H NMR spectrum showed pro-
ton resonances for six different methoxy groups at δ =
3.61, 3.62, 3.63, 3.64, 3.73, and 3.76 ppm, and three dif-
ferent methylene resonance singlets at δ = 3.93, 4.15, and
4.20 ppm.
Similar treatment of the 7,7′-dimethanol 2 with two equiv-
alents of the indole-7-carbaldehyde 3 in isopropanol con-
taining p-toluenesulfonic acid gave the tetraindole
dialdehyde 720 in 68% yield (Scheme 2). Reduction of di-
aldehyde 4 with sodium borohydride in a mixture of etha-
nol and tetrahydrofuran gave the dimethanol 8 in 98%
yield. The calix[4]indole 921 was formed in 69% yield
when the dimethanol 8 was treated with p-toluenesulfonic
acid in anhydrous dimethyl sulfoxide briefly at room
temperature.
The new calix[4]indole 9, and the first example of this
type, was characterised fully and its NMR spectra were
particularly revealing with regard to structural symmetry.
Figure 3 ORTEP diagram of calix[4]indole 9
Seven different methoxy proton resonances were
© Georg Thieme Verlag Stuttgart · New York
Synlett 2013, 24, 24–28

26
K. Somphol et al.
LETTER
R
R
MeO
OMe
OMe
R
R
NH
OH
HN
HO
MeO
OMe
MeO
NH
HN
MeO
OMe
R
2
+
R
a
NH
HN
MeO
R
MeO
OMe
CHO
OHC
N
MeO
MeO
OMe
H
CHO
7
3
b
R = 4-BrC6H4
R
R
R
OMe
R
MeO
MeO
OMe
NH
HN
HN
NH
OMe
MeO
MeO
R
OMe
R
c
NH
HN
HN
NH
R
R
MeO
OMe
OMe
MeO
OH HO
OMe
MeO
MeO
OMe
9
8
Scheme 2 Reagents and conditions: (a) p-TsOH, i-PrOH, r.t., 1 h, 68%; (b) NaBH4, THF, EtOH, r.t. 98%; (c) p-TsOH, DMSO, r.t., 5 min, 69%.
pared so far show rigid structures leading to non-identical and one 7,7-link, and calix[4]indoles containing one 2,2-,
methylene protons, whereas the calix[3]indoles, such as one 7,7-, and two 7,2-links. Although all four substituents
compound 6 are quite flexible, allowing their methylene at C3 are the same in the case of compounds 6 and 9, there
link protons to equilibrate on the NMR time scale and is the possibility to introduce two different substituents by
show singlet resonances.
this stepwise procedure. Furthermore, although the indole
methanols 1 and 2 undergo ready reaction with 3-substi-
tuted 4,6-dimethoxyindoles,14 linkages occur at both C2
and C7 and product mixtures result. The use of the 7-alde-
hyde 3 provides much cleaner and regiospecific linkage
formation.
The cyclisation reactions of compound 5 to 6 and of com-
pound 8 to 9 are typical of 4,6-dimethoxyindoles that are
substituted at both C2 and C3 and bear a hydroxymethyl
group at C7. This process involves a mildly acid-catalysed
ipso-substitution in which the restoration of the indole
aromaticity requires the loss of formaldehyde. A postulat-
ed mechanism, exemplified for the conversion of com-
pound 8 to macrocycle 9, is shown in Scheme 3. Although
the indole C5 position is unsubstituted, it is protected
against electrophilic substitution by the adjacent methoxy
groups, which are in turn buttressed by the C3 and C7 sub-
stituents. We have observed many examples of this re-
markably effective and irreversible formaldehyde
extrusion process for the formation of 7,7′-diindolylmeth-
anes. The formation of compounds 6 and 9 demonstrate
the effectiveness of this synthetic strategy for macrocycli-
sation reactions.
Acknowledgment
Research support from the Australian Research Council (Project
number: DP1095159) and the University of New South Wales is
gratefully acknowledged.
References and Notes
(1) Gale, P. A.; Sessler, J. L.; Král, V.; Lynch, V. J. Am. Chem.
Soc. 1996, 118, 5140.
(2) Allen, W. E.; Gale, P. A.; Brown, C. T.; Lynch, V. M.;
Sessler, J. L. J. Am. Chem. Soc. 1996, 118, 12471.
(3) Davis, N. K. S.; Thompson, A. L.; Anderson, H. L. J. Am.
Chem. Soc. 2010, 133, 30.
In summary, this synthetic methodology has the capacity
to generate calix[3]indoles containing one 2,2-, one 7,2-
(4) Gale, P. A.; Sessler, J. L.; Kral, V. Chem. Commun. 1998, 1.
Synlett 2013, 24, 24–28
© Georg Thieme Verlag Stuttgart · New York

LETTER
New Strategy for Calixindole Formation
27
R
R
R
R
MeO
MeO
OMe
OMe
NH
NH
HN
HN
MeO
MeO
OMe
R
OMe
R
H+
R
R
NH
NH
HN
HN
– H2O
MeO
MeO
OMe
OMe
OH
HO
OH
MeO
MeO
OMe
OMe
R = 4-BrC6H4
8
R
R
OMe
R
R
MeO
MeO
OMe
NH
HN
HN
NH
OMe
MeO
MeO
OMe
R
R
HN
NH
R
– H+
NH
HN
R
MeO
OMe
– HCHO
OMe
MeO
OMe
MeO
MeO
O
OMe
H
9
Scheme 3 Postulated mechanism for the formation of compound 9
(5) Kurotobi, K.; Kim, K. S.; Noh, S. B.; Kim, D.; Osuka, A.
Angew. Chem. Int. Ed. 2006, 45, 3944.
(6) Black, D. StC.; Gatehouse, B. M. K. C.; Theobald, F.; Wong,
L. C. H. Aust. J. Chem. 1980, 33, 343.
(7) Black, D. StC.; Rothnie, N. E.; Wong, L. C. H. Tetrahedron
Lett. 1980, 21, 1883.
(8) Black, D. StC.; Rothnie, N. E.; Wong, L. C. H. Aust. J.
Chem. 1983, 36, 2407.
(9) Black, D. StC.; Kumar, N.; Wong, L. C. H. Aust. J. Chem.
1986, 39, 15.
(10) Black, D. StC.; Bowyer, M. C.; Bowyer, P. K.; Ivory, A. J.;
Kim, M.; Kumar, N.; McConnell, D. B.; Popiolek, M. Aust.
J. Chem. 1994, 47, 1741.
(11) Pchalek, K.; Jones, A. W.; Wekking, M. T.; Black, D. StC.
Tetrahedron 2005, 61, 77.
(12) Black, D. StC.; Craig, D. C.; Kumar, N. Aust. J. Chem. 1996,
49, 311.
(13) Black, D. StC.; Bowyer, M. C.; Catalano, M. M.; Ivory, A.
J.; Keller, P. A.; Kumar, N.; Nugent, S. J. Tetrahedron 1994,
50, 10497.
(14) Black, D. StC.; Bowyer, M. C.; Kumar, N.; Mitchell, P. S. R.
J. Chem. Soc., Chem. Commun. 1993, 819.
(15) Black, D. StC.; Craig, D. C.; Kumar, N. Tetrahedron Lett.
1995, 36, 8075.
(16) Black, D. StC.; Craig, D. C.; Kumar, N.; McConnell, D. B.
Tetrahedron Lett. 1996, 37, 241.
MHz, CDCl3): δ = 3.66, 3.70, 3.82, 3.95, 3.99, 4.07 (6 × s,
18 H, OMe), 3.82 (s, 2 H, CH2, 2,2′-link), 4.18 (s, 2 H, CH2,
2,7′-link), 6.05, 6.15, 6.30 (3 × s, 3 H, indolyl H5), 6.91 (s, 1
H, NH), 6.96–7.32 (m, 12 H, ArH), 9.98, 10.59 (2 × s, 2 H,
NH), 10.28, 10.32 (2 × s, 2 H, CHO). 13C NMR (75 MHz,
CDCl3): δ = 20.9, 23.7 (CH2), 55.1, 55.2, 55.3, 56.3, 56.4,
56.7 (OMe), 86.6, 86.8, 89.1 (indolyl C5), 130.0, 130.3,
130.7, 132.1, 132.2, 132.4 (ArCH), 101.3, 104.2, 104.3,
110.9, 111.2, 111.9, 113.5, 114.6, 119.7, 119.9, 120.1,
129.1, 130.5, 130.7, 132.1 (2×), 133.6, 134.0, 134.3, 134.4,
136.3, 153.0, 153.3, 160.2, 160.5, 162.3, 162.4 (ArC), 187.8,
188.0 (CHO). IR (KBr): 3409, 3342, 2934, 2843, 1643,
1593, 1562, 1511, 1487, 1464, 1434, 1393, 1367, 1353,
1247, 1214, 1120, 994, 817, 794 cm–1. UV–Vis (CH2Cl2):
λmax = 234 (ε = 83,900 cm–1M–1), 257 (72,400), 317 (35,200)
nm. MS (ESI): m/z (%) = 1074 [M, 79/79/79Br] (10), 862 (15),
796 (20), 663 (80), 647 (30), 429 (60), 135 (100), 108 (70).
Anal. Calcd for C52H42Br3N3O8: C, 58.0; H, 3.9; N, 3.9.
Found: C, 57.9; H, 4.0; N, 3.7.
(19) Compound 6 was prepared by treatment of the triindolyl
dimethanol 8 (0.300 g, 0.280 mmol) in anhyd
dimethylsulfoxide (5 mL) with a catalytic amount of p-
toluenesulfonic acid monohydrate and stirred at r.t. After 5
min, H2O was added and the resulting green precipitate was
filtered off, washed with H2O, dried and recrystallised from
EtOAc–hexane to yield the calixindole 9 (0.26 g, 89%) as a
yellow-green solid; mp >300 °C. 1H NMR [300 MHz,
(CD3)2SO]: δ = 3.61, 3.62, 3.63, 3.64, 3.73, 3.76 (6 × s, 18
H, OMe), 3.93, 4.15, 4.20 (3 × s, 6 H, CH2), 6.33, 6.34 (2 ×
s, 3 H, indolyl H5), 7.10–7.15 (m, 4 H, ArH), 7.31–7.36 (m,
4 H, ArH), 7.45 (d, J = 8.3 Hz, 2 H, ArH), 7.54 (d, J = 8.3
Hz, 2 H, ArH), 9.49, 9.62, 9.95 (3 × s, 3 H, NH). 13C NMR
[75 MHz, (CD3)2SO]: δ = 19.2, 21.6, 23.3 (CH2), 55.3, 55.4,
55.5, 56.8, 57.4, 57.7 (OMe), 89.7, 90.5, 90.8 (C5), 125.9,
(17) Black, D. StC.; Kumar, N.; McConnell, D. B. Tetrahedron
2000, 56, 8513.
(18) Compound 4 was prepared from the indole-7-aldehyde 3
(0.900 g, 2.50 mmol), diindolyl dimethanol 1 (0.490 g, 1.25
mmol), p-toluenesulfonic acid monohydrate and isopropanol
(100 mL). After stirring for 1 h, filtration and drying, the
compound 7 (1.21 g, 90%) was obtained as a yellow solid;
mp 256–257 °C (CH2Cl2–light petroleum). 1H NMR (300
© Georg Thieme Verlag Stuttgart · New York
Synlett 2013, 24, 24–28

28
K. Somphol et al.
LETTER
precipitate was filtered off, washed with H2O, dried and
128.4, 130.1, 132.9, 133.2, 133.5 (ArCH), 102.0, 104.3,
105.0, 111.4, 111.8 (2 ×), 114.0, 119.1, 131.4, 132.1, 132.3,
132.9, 134.8, 135.2, 135.4, 135.6, 136.1, 136.7, 137.0,
138.0, 152.0 (2 ×), 152.3, 152.5, 153.3, 154.1, 154.3 (ArC).
IR (KBr): 3419, 2933, 2835, 1619, 1595, 1520, 1488, 1463,
1434, 1393, 1341, 1295, 1207, 1141, 1112, 1073, 997, 819
cm–1. UV–Vis (CH2Cl2): λmax = 233 (ε = 111,200 cm–1M–),
289 (41,800) nm. MS (ESI): m/z (%) = 1036 [M + 1,
81/81/81Br] (24), 1034 [M + 1, 79/81/81Br] (43), 1032 [M + 1,
79/79/81Br] (20), 1030 [M + 1, 79/79/79Br] (20). Anal. Calcd for
C51H42Br3N3O6·1.5 H2O: C, 57.8; H, 4.3; N, 4.0. Found: C,
57.8; H, 4.2; N, 3.9.
recrystallised from EtOAc–hexane to yield the calixindole 9
(67.0 mg, 69%) as a pale yellow solid; mp >300 °C. 1H NMR
(300 MHz, CDCl3): δ = 3.44, 3.53, 3.75, 3.80 (2 ×), 3.95,
3.99, 4.01 (8 × s, 24 H, OMe), 3.98 (d, J = 12.7 Hz, 2 H,
CH2), 3.99 (d, J = 16.0 Hz, 2 H, CH2), 4.16 (d, J = 13.7 Hz,
4 H, CH2), 5.91, 6.26, 6.33, 6.48 (4 × s, 4 H, indolyl H5),
6.95 (d, J = 8.3 Hz, 4 H, ArH), 7.03 (d, J = 8.3 Hz, 4 H, ArH),
7.24 (d, J = 8.3 Hz, 4 H, ArH), 7.40 (d, J = 8.3 Hz, 4 H, ArH),
8.31, 8.81 (2 × s, 4 H, NH). 13C NMR (75 MHz, CDCl3): δ =
21.3, 21.5, 29.6 (CH2), 55.2, 55.5, 57.4, 57.5 (OMe), 88.8,
89.8 (C5), 130.3, 130.7, 132.1, 132.7 (ArCH), 102.3, 111.8,
112.0, 112.3, 119.3, 119.7, 119.9, 130.5, 132.5, 136.2,
136.6, 136.9, 137.0, 152.2 (2 ×), 152.3, 152.5, 152.6 (ArC).
IR (KBr): 3415, 2933, 2837, 1620, 1596, 1489, 1463, 1434,
1342, 1296, 1257, 1213, 1147, 1114, 996 cm–1. UV–Vis
(CH2Cl2): λmax = 279 nm (ε = 111,100 cm–1M–). MS
(MALDI): m/z (%) = 1379 [M – 1, 81/81/81/81Br] (27), 1377 [M
– 1, 79/81/81/81Br] (70), 1375 [M – 1, 79/79/81/81Br] (100), 1373
[M – 1, 79/79/79/81Br] (72), 1371 [M – 1, 79/79/79/79Br] (35).
Anal. Calcd for C68H56Br4N4O8: C, 59.3; H, 4.1; N, 4.1.
Found: C, 59.6; H, 4.3; N, 4.0.
Single crystals were obtained from a solvent mixture of
CH2Cl2 and EtOAc: C68H56Br4N4O8·CCl3, M = 1495.17,
monoclinic, a = 25.00(3) Å, b = 19.47(2) Å, c = 12.594(10)
Å, α = 90.00°, β = 102.39(3)°, γ = 90.00°, V = 5986(10) Å3,
T = 150(2) K, space group C2/c, Z = 4, μ(MoKα) = 2.887
mm–1, 19259 reflections measured, 5245 independent
reflections (Rint = 0.2689). The final R1 values were 0.0824
[I >2σ(I)]. The final wR(F2) values were 0.0864 [I > 2σ(I)].
The final R1 values were 0.3272 (all data). The final wR(F2)
values were 0.1306 (all data). The goodness of fit on F2 was
0.988 (all data). Crystallographic data for compound 9 has
been deposited with the Cambridge Crystallographic Data
Centre as supplementary publication No. CCDC 894925.
Copies of the data can be obtained free of charge on
application to CCDC, 12 Union Road, Cambridge CB21EZ,
UK [fax: +44(1223)336033; email:
(20) Compound 7 was prepared from the indole-7-aldehyde 3
(49.0 mg, 0.136 mmol), diindolyl dimethanol 2 (50.0 mg,
0.068 mmol), p-toluenesulfonic acid monohydrate and
isopropanol (30 mL). After stirring for 1 h, the yellowish
precipitate was filtered off to yield compound 7 (66.0 mg,
68%); mp 292 °C (dec.; CH2Cl2–light petroleum). 1H NMR
(300 MHz, CDCl3): δ = 3.57 (s, 2 H, CH2, 2,2′-link), 3.63,
3.70, 3.94, 4.08 (4 × s, 24 H, OMe), 4.07 (s, 4 H, CH2, 2,7′-
link), 5.99, 6.36 (2 × s, 4 H, indolyl H5), 6.82 (s, 2 H, NH),
6.85–6.89 (m, 6 H, ArH), 6.98–7.01 (m, 6 H, ArH), 7.17 (d,
J = 8.3 Hz, 4 H, ArH), 10.26 (s, 2 H, CHO), 10.53 (s, 2 H,
NH). 13C NMR (75 MHz, CDCl3): δ = 21.0, 23.9 (CH2),
55.0, 55.2, 56.2, 56.9 (OMe), 86.7, 89.3 (indolyl C5), 129.9,
130.4, 132.1, 132.3 (ArCH), 101.3, 104.1, 110.8, 112.1,
112.5, 114.5, 119.4, 120.0, 129.2, 132.2, 133.8 (2×), 134.0,
136.2, 152.8, 153.3, 160.3, 162.3 (ArC), 188.0 (CHO). IR
(KBr): 3414, 2934, 2839, 1722, 1644, 1591, 1561, 1517,
1487, 1464, 1434, 1393, 1367, 1352, 1327, 1291, 1250,
1213, 1119, 1071, 993, 817, 794 cm–1. UV–Vis (CH2Cl2):
λmax = 239 (ε = 147,800 cm–1M–1), 312 (55,800) nm. MS
(MALDI): m/z (%) = 1137 (100), 1059 (25). Anal. Calcd for
C69H56Br4N4O10·H2O: C, 57.6; H, 4.1; N, 3.9. Found: C,
57.5; H, 4.3; N, 3.6.
(21) Compound 9 was prepared by treatment of the tetraindolyl
dimethanol 8 (100 mg, 0.070 mmol) in anhyd
dimethylsulfoxide (5 mL) with a catalytic amount of p-
toluenesulfonic acid monohydrate and stirred at r.t. After
5 min, H2O was added and the resulting yellow-green
deposit@ccdc.cam.ac.uk].
Synlett 2013, 24, 24–28
© Georg Thieme Verlag Stuttgart · New York

Copyright of Synlett is the property of Georg Thieme Verlag Stuttgart and its content may not be copied or
emailed to multiple sites or posted to a listserv without the copyright holder's express written permission.
However, users may print, download, or email articles for individual use.