Design and synthesis of bile acid-peptide conjugates linked via triazole moiety

Article abstract of DOI:10.1039/c0ob01188f

A conjugation of bile acids with peptides via Cu(i)-catalyzed click chemistry has been described. Novel bile acid-peptide conjugates linked via a 1,2,3-triazole moiety based on cholic, deoxycholic and lithocholic acid derivatives were synthesized using Cu

Full text of DOI:10.1039/c0ob01188f

View Article Online / Journal Homepage / Table of Contents for this issue
Organic &
Biomolecular
Chemistry
Dynamic Article Links
Cite this: Org. Biomol. Chem., 2011, 9, 4921
www.rsc.org/obc
PAPER
Design and synthesis of bile acid–peptide conjugates linked via triazole
moiety†
Nadezhda V. Sokolova, Gennadij V. Latyshev, Nikolay V. Lukashev and Valentine G. Nenajdenko*
Received 16th December 2010, Accepted 11th April 2011
DOI: 10.1039/c0ob01188f
A conjugation of bile acids with peptides via Cu(I)-catalyzed click chemistry has been described. Novel
bile acid–peptide conjugates linked via a 1,2,3-triazole moiety based on cholic, deoxycholic and
lithocholic acid derivatives were synthesized using Cu(I)-catalyzed 1,3-dipolar cycloaddition (“click”
reaction). It was shown that up to three peptide fragments can be attached to a central steroid core, thus
forming complex three-dimensional polyconjugate structures, which can find important applications in
biochemistry, medicinal chemistry, and coordination chemistry.
The Ugi MCR is an efficient method for the construction of
Introduction
peptides and peptide-like molecules in one step.9 Recently, we
In recent years bile acids and their derivatives have become
leveraged on this multicomponent reaction for a straightforward
increasingly important in a number of fields such as medicine,
synthesis of “clickable” azidopeptides, starting from novel iso-
pharmacology, coordination and supramolecular chemistry, ma-
cyanoazides. We also demonstrated that the Ugi/click strategy
terials chemistry, and also in nanotechnology.1 Bile acids are a
is a powerful method for the decoration of biologically active
molecules with peptide residues.10
group of steroids formed during the catabolism of cholesterol
in the hepatocytes. The primary bile acids produced by living
organisms are usually conjugated with the amino acids glycine
and taurine.2 Bile acids play various physiological roles, such
as the digestion and absorption of lipids, cholesterol and lipid-
soluble vitamins, and influence intestinal and colonic epithelial
function and integrity. One of the most efficient processes in
the human body is the enterohepatic circulation of bile acids,
which is carried out by transport proteins.3 Synthesis and design
of new pharmacologically active hybrid molecules and prodrugs
based on bile acids are very important from the viewpoint of
improving intestinal absorption and increasing the metabolic
stability of pharmaceuticals specifically targeting enterohepatic
circulation.4
Herein, we apply our Ugi/click strategy to the conjugation
of azidopeptides with different bile acid derivatives. We believe
that due to cooperative interactions of flexible and functional-
ized peptide chains bile acid–peptide conjugates containing free
amine functions together with 1,2,3-triazole moieties may play
an important role in biochemistry, and medicinal chemistry, as
well as in coordination chemistry, because of their “tweezer-like”
structure.11 For example, they can find applications in “click-to-
chelate” approach to the preparation of ligand systems suitable
for complexation of metals12 and can be useful in finding of new
receptors for the selective recognition of anions.13
Examples of bile acid–peptide conjugates linked via amide
bonds have been reported in the literature.5 However, in all these
approaches classical methods of peptide chemistry have been used.
Herein, we disclose a conjugation of bile acids and peptides via a
1,2,3-triazole moiety using copper-catalyzed Huisgen 1,3-dipolar
cycloaddition (“click” reaction) of alkynes and azides.6
1,2,3-Triazole moieties are ideal linkers, because they are stable
under typical physiological conditions and form hydrogen bonds.7
Moreover, the ability of 1,2,3-triazoles to mimic topological and
electronic features of the amide bond can be used for design of
peptidomimetics with improved medicinal properties.8
Results and discussion
Starting azidopeptides 1a–f were synthesized using chiral iso-
cyanoazides, recently developed in our group.10 By reacting
different Boc-protected amino acids with chiral isocyanoazides
we prepared a series of novel azidopeptides 1 (Scheme 1).
Advantageously, 2,4-dimethoxybenzyl or 4-methoxybenzyl pro-
tecting groups, were introduced via their corresponding amines.
Formaldehyde and acetone were used as carbonyl components to
form the central amino acid fragment of the azidopeptide skeleton.
As a result, azidopeptides 1 containing both natural and unnatural
amino acid residues were successfully synthesized. Therefore, we
have demonstrated that the Ugi reaction with isocyanoazides is
a convenient route for the introduction of orthogonal protecting
groups in the peptides 1.
Department of Chemistry, Moscow State University, 119991, Leninskie
Gory, Moscow, Russia. E-mail: nen@acylium.chem.msu.ru
† Electronic supplementary information (ESI) available: Full experimental
details and copies of NMR spectra. See DOI: 10.1039/c0ob01188f
Acetylenic derivatives of deoxycholic 2a–c, lithocholic 2d, and
cholic 2e acids containing one, two or three terminal alkyne
This journal is
The Royal Society of Chemistry 2011
Org. Biomol. Chem., 2011, 9, 4921–4926 | 4921
©

View Article Online
Scheme 1 Reagents and conditions: (a) MeOH, rt, 24 h.
groups were prepared via acylation of cholic acids with acyl
chlorides containing terminal alkynyl group using a modified
literature procedure.14 For this, the methyl and propargyl esters
of bile acids were treated with propargyloxyacetyl chloride or 4-
(propargyloxy)benzoyl chloride in the presence of different bases
to provide the required bile acid derivatives 2a–e in good yields.15
We envisioned that the geometry of peptide chains in the
target molecules can be controlled by the structure of starting
deoxycholic, lithocholic and cholic acids containing one, two or
three acetylenic fragments. In bile acids, the cis-coupled rings A
and B assume chair conformation, wherein the hydroxy group
in the 3-position takes an equatorial and the 7-hydroxy takes an
axial conformation, both pointing in the same direction. Thus, a
conjugation of peptide residues is likely to maintain an U-shaped
spatial arrangement.
Having the corresponding sets of alkynylated bile acids and
azidopeptides in hand we investigated a combinatorial approach
towards the conjugates. In a first series, deoxycholic acid derivative
2a bearing a monoacetylenic functionality on the C-3 position was
reacted with azidopeptides 1a–c (Scheme 2).
The click reaction proceeded regioselectively in good yields to
afford conjugates 3a–c. The optimal reaction conditions were
found to be EtOH–H2O (5 : 1)/10% of CuSO4·5H2O/40% of
sodium ascorbate for 12 h at 60 ◦C.
and excess of azidopeptides 1a,d–f (Scheme 3). After careful
optimization, heating the starting materials with Cu(II)/sodium
ascorbate in a biphasic mixture of CH2Cl2–H2O provided the
desired products 4a–d in good to excellent yields. An analogous
series of conjugates 5a–c replacing the 4-propargyloxy ben-
zoate linker with an propargyloxy glycolate linker was prepared
(Scheme 3).
Additionally, a series of bisconjugated derivatives was prepared.
Lithocholic acid derivative 2d containing two terminal alkyne
groups was reacted with azidopeptides 1a,d,e under the same
conditions afforded the corresponding conjugates 6a–c in high
yields (Scheme 4). In contrast to the compounds from series 4 and
5, which present an angular geometry, compounds 6a–c show a
linear arrangement.
In a final series, we even prepared cholic acid derivatives bearing
three peptide moieties. Thus, 1,3-dipolar cycloaddition between
the triynic cholic acid derivative 2e and excess of peptides 1a,c,e
afforded triplex conjugates 7a–c in high yields under standard
conditions (Scheme 5).
Both protecting groups of the amide and the amine func-
tions can be easily removed from the target conjugates, ei-
ther in an orthogonal mode under mildly acidic conditions
or in one step under more forcing conditions.16 As an ex-
ample, heating of the conjugate 6b in TFA led to the prod-
uct 8 containing free amide and amine groups in good yield
(Scheme 6).
A second series of conjugates was prepared starting from
deoxycholic acid derivative 2b bearing two alkyne appendages
Scheme 2 Reagents and conditions: (a) sodium ascorbate (40%), CuSO4·5H2O (10%), EtOH–H2O (5 : 1), 60 ◦C, 12 h.
4922 | Org. Biomol. Chem., 2011, 9, 4921–4926
This journal is
The Royal Society of Chemistry 2011
©

View Article Online
Scheme 3 Reagents and conditions: (a) sodium ascorbate (80%), CuSO4·5H2O (20%), CH2Cl2–H2O (10 : 1), 40 ◦C, 10 h.
Scheme 4 Reagents and conditions: (a) sodium ascorbate (80%), CuSO4·5H2O (20%), CH2Cl2–H2O (10 : 1), 40 ◦C, 10 h.
In general, “clickable” peptides based on chiral isocyanoazides
are promising candidates for bioconjugation reactions and syn-
thesis of hybrid peptide molecules with improved medicinal
properties.
well as in coordination chemistry, because of their “tweezer-like”
structure and hydrogen bond forming potential.
Experimental
General information
Conclusions
1H and 13C NMR spectra were recorded in deuterated solvents
on a Bruker Avance 400 MHz spectrometer. Chemical shifts
are reported in parts per million (ppm) or d values downfield
from TMS as internal standard. Deuterated solvent peaks were
used as internal references: CDCl3 at 7.25 and 77.00 ppm.
Analytical thin layer chromatography (TLC) was performed
using DC-Alufolien Kieselgel 60 F254 (Merck). The developed
chromatogram was analyzed by UV light and aqueous potassium
In conclusion, we described the conjugation of azidopeptides 1
with alkyne functionalized bile acids 2 via the Cu(I) catalyzed
Huisgen cycloaddition reaction. 1,2,3-Triazole formation pro-
ceeded smoothly “clicking” up to three alkyne moieties attached
to the steroid core. After straightforward deprotection, series of
mono-, duplex- and triplex-peptidyl bile acid conjugates may play
an important role in biochemistry, and medicinal chemistry, as
This journal is
The Royal Society of Chemistry 2011
Org. Biomol. Chem., 2011, 9, 4921–4926 | 4923
©

View Article Online
Scheme 5 Reagents and conditions: (a) sodium ascorbate (90%), CuSO4·5H2O (30%), CH2Cl2–H2O (10 : 1), 40 ◦C, 12 h.
General procedure for Ugi-4CC. Synthesis of peptides 1a–f
The corresponding amine (1 mmol) and acetone or CH2O (40%
in H2O) (1 mmol) were dissolved in 5 mL of MeOH and N-Boc
protected amino acid (1 mmol) and isocyanide (1 mmol) were
added at room temperature. The mixture was stirred for 24 h.
The solvent was removed in vacuo and the residue was purified by
column chromatography (hexanes–ethyl acetate 2 : 1).
N-(tert-butoxycarbonyl)-L-valyl-N1 -[(1S,2S)-1-(azidomethyl)-
2-methylbutyl]-N2-(4-methoxybenzyl)-2-methylalaninamide (1a).
According to the general procedure for Ugi-4CC, 1a was
obtained from p-methoxybenzylamine, acetone, N-Boc-L-valine
and (2S,3S)-1-azido-2-isocyano-3-methylpentane. White solid,
mp 78–78.8 ◦C, 50% yield; Rf (hexanes–ethyl acetate 1 : 1) 0.6;
[a]D -48.8 (c 1.0, MeOH, 25 ◦C); 1H NMR (400 MHz, CDCl3) d
0.86–0.92 (m, 12H), 1.10–1.17 (m, 1H), 1.28–1.35 (m, 1H), 1.39 (s,
3H), 1.43 (s, 9H), 1.47 (s, 3H), 1.54–1.61 (m, 1H), 1.91–1.99 (m,
1H), 3.30–3.40 (m, 2H), 3.79 (s, 3H), 3.82–3.87 (m, 1H), 4.42–4.48
(m, 1H), 4.59 (d, JAA¢ = 17.7 Hz, 1H), 4.73 (d, JAA¢ = 17.7 Hz,
1H), 5.16 (d, J = 8.8 Hz, 1H), 5.59 (d, J = 8.6 Hz, 1H), 6.89 (d,
J = 8.1 Hz, 2H), 7.29 (d, J = 8.1 Hz, 2H); 13C NMR (100 MHz,
CDCl3) d 11.0, 15.4, 17.2, 19.5, 24.0, 24.4, 25.2, 28.2, 32.2, 35.7,
46.9, 52.4, 55.3, 56.1, 63.3, 79.4, 114.4, 127.8, 130.1, 155.4, 159.1,
173.1, 174.0; IR (film, cm-1) 2108, 1710, 1682, 1640; Anal. Calcd
for C28H46N6O5 (546.70): C, 61.51; H, 8.48; N, 15.37; found: C,
61.35; H, 8.51; N, 15.20%.
Scheme 6 Reagents and conditions: (a) TFA, reflux, 7 h, 60%.
permanganate (KMnO4). Liquid chromatography was performed
using Fluka Silica gel 60 (0.063–0.200 mm). Infrared (IR) spectra
were recorded on a Nicolet IR2000 (Thermo Scientific). Optical
rotations were measured on a Perkin–Elmer 341 polarimeter at
589 nm. Melting points were determined with an Electrothermal
IA9100 Digital Melting Point Apparatus and are uncorrected.
High-resolution mass spectra (HRMS) were measured on a
Micr OTOF II (Bruker Daltonics) spectrometer. All commercial
solvents were distilled prior to use.
General procedure for the synthesis of 3a–c
The propargyl ester of bile acid derivative 2a (1 mmol),
CuSO4·5H2O (0.1 mmol) in 0.5 ml of H2O and sodium ascorbate
(0.4 mmol) in 0.5 ml of H2O were added successively to a solution
of the corresponding peptide (1 mmol) in 5 mL of EtOH. The
reaction mixture was stirred at 60 ◦C for 12 h. The solvent
4924 | Org. Biomol. Chem., 2011, 9, 4921–4926
This journal is
The Royal Society of Chemistry 2011
©

View Article Online
was removed in vacuo and the residue was purified by column
chromatography (hexanes–ethyl acetate 1 : 1).
White solid, mp 57–58 ◦C, 60% yield; Rf (CH3CN–NH3 9 : 1) 0.3;
◦
1
[a]D -5.3 (c 1.0, CH2Cl2, 25 C); H NMR (400 MHz, CDCl3):
d 0.61 (s, 3H), 0.87–0.95 (m, 18H), 1.04–1.13 (m, 15H), 1.34–
1.49 (m, 24H), 1.62–1.68 (m, 2H), 1.76–1.82 (m, 5H), 1.92–2.03
(m, 4H), 2.17–2.26 (m, 1H), 2.30–2.40 (m, 1H), 3.22–3.25 (m,
2H), 4.11–4.17 (m, 3H), 4.29–4.50 (m, 6H), 4.73–5.00 (m, 4H),
5.18 (br s, 2H), 7.07 (d, J = 5.7 Hz, 1H), 7.13 (d, J = 5.7 Hz,
1H), 7.68–7.73 (m, 4H); 13C NMR (CDCl3, 100 MHz) d 11.9,
12.0, 16.1, 17.5, 18.2, 20.8, 23.3, 23.9, 24.1, 24.8, 26.0, 26.3, 26.6,
27.0, 28.2, 30.8, 31.0, 32.2, 34.6, 34.9, 35.3, 35.8, 40.1, 40.4, 41.9,
42.7, 45.2, 45.3, 54.2, 55.9, 56.4, 56.9, 57.4, 58.4, 59.9, 64.6, 67.6,
75.2, 124.6, 125.0, 142.9, 144.1, 169.8, 174.1, 174.3, 174.9, 175.1;
HRMS (ESI) calcd for C58H98N12O9 [M + H]+ 1107.7658, found
1107.7639.
Compound 3a. White solid, mp 77–79 ◦C, 63% yield; Rf (ethyl
acetate) 0.7; [a]D +10.5 (c 1.5, MeOH, 25 ◦C); 1H NMR (CDCl3,
400 MHz) d 0.63 (s, 3H), 0.82–0.85 (m, 6H), 0.89–0.92 (m, 6H),
0.95 (d, J = 5.8 Hz, 3H), 1.01 (d, J = 6.3 Hz, 3H), 1.15–1.40
(m, 26H), 1.46–1.58 (m, 8H), 1.64–1.93 (m, 9H), 2.17–2.27 (m,
1H), 2.30–2.39 (m, 1H), 3.64 (s, 3H), 3.78 (s, 3H), 3.94 (br s, 1H),
4.01–4.12 (m, 2H), 4.13–4.23 (m, 1H), 4.40–4.61 (m, 4H), 4.66–
4.81 (m, 4H), 5.20 (d, J = 9.3 Hz, 1H), 5.72 (d, J = 8.3 Hz, 1H),
6.86 (d, J = 8.1 Hz, 2H), 7.23–7.25 (m, 2H), 7.75 (br s, 1H); 13C
NMR (CDCl3, 100 MHz) d 11.0, 12.6, 15.5, 17.2, 19.4, 23.0, 23.6,
23.9, 24.1, 24.9, 25.9, 26.5, 26.9, 27.4, 28.2, 28.6, 29.6, 30.9, 31.0,
32.1, 32.2, 33.6, 34.1, 34.8, 35.1, 35.9, 41.8, 46.4, 47.0, 47.1, 47.9,
51.2, 51.5, 55.3, 55.2, 55.8, 62.9, 64.5, 67.4, 72.9, 75.0, 79.5, 114.2,
124.1, 127.9, 130.0, 144.2, 155.5, 159.0, 169.7, 173.0, 174.4, 174.7;
HRMS (ESI) calcd for C58H92N6O11 [M + Na]+ 1071.6722, found
1071.6690.
Acknowledgements
The authors would like to thank Dr T. Schotten (CAN GmbH,
Germany) and Dr A. V. Gulevich (University of Illinois at
Chicago) for fruitful discussions.
General procedure for the synthesis of 4–7
The corresponding peptide (3 mmol for 2b–d and 4 mmol for 2e),
CuSO4·5H2O (0.2 mmol for 2b–d and 0.3 mmol for 2e) in 0.5 ml
of H2O and sodium ascorbate (0.8 mmol for 2b–d and 0.9 mmol
for 2e) in 0.5 ml of H2O were added successively to a solution of
the corresponding propargyl ester of bile acid derivative (1 mmol)
in 10 mL of CH2Cl2. The reaction mixture was stirred at 40 ◦C for
10 h for 4, 5, 6 and 12 h for 7. The solvent was removed in vacuo
and the residue was purified by column chromatography (ethyl
acetate or CH2Cl2–MeOH 10 : 1).
Notes and references
1 (a) Nonappa and U. Maitra, Org. Biomol. Chem., 2008, 6, 657–669;
(b) E. Virtanen and E. Kolehmainen, Eur. J. Org. Chem., 2004, 3385–
3399; (c) Y. Li and J. R. Dias, Chem. Rev., 1997, 97, 283–304; (d) P.
Wallimann, T. Marti, A. Fu¨rer and F. Diederich, Chem. Rev., 1997, 97,
1567–1608.
2 D. Stamp and G. Jenkins, Bile Acids: Toxicology and Bioactivity, Royal
Society of Chemistry, Cambridge, 2008, pp. 1–13.
3 (a) W. A. Alrefai and R. K. Gill, Pharm. Res., 2007, 24, 1803–1823;
(b) A. Enhsen, W. Kramer and G. Wess, Drug Discovery Today, 1998,
3, 409–418.
Compound 4a. White solid, mp 97–99 ◦C, 93% yield; Rf (ethyl
acetate) 0.7; [a]D +28.8 (c 1.5, MeOH, 25 ◦C); 1H NMR (CDCl3,
400 MHz) d 0.79–0.83 (m, 18H), 0.87–0.90 (m, 6H), 0.93 (s, 1H),
0.99–1.04 (m, 6H), 1.12–1.28 (m, 26H), 1.39 (s, 18H), 1.47–1.58
(m, 8H), 1.65–1.89 (m, 10H), 2.06–2.16 (m, 1H), 2.22–2.33 (m,
1H), 3.59 (s, 3H), 3.76 (s, 3H), 3.77 (s, 3H), 4.12–4.24 (m, 2H),
4.33–4.68 (m, 9H), 4.77–4.88 (m, 1H), 5.07–5.17 (m, 5H), 5.33 (br
s, 1H), 5.65–5.73 (m, 1H), 5.89–5.99 (m, 1H), 6.87–6.90 (m, 6H),
7.02 (d, J = 8.6 Hz, 2H), 7.22–7.26 (m, 4H), 7.79 (d, J = 8.6 Hz,
2H), 7.85 (s, 1H), 7.87 (s, 1H), 8.03 (d, J = 8.6 Hz, 2H); 13C NMR
(CDCl3, 100 MHz) d 11.0, 12.5, 14.1, 15.5, 17.2, 17.5, 19.5, 22.9,
23.5, 23.9, 24.1, 24.9, 25.7, 26.0, 26.4, 26.7, 27.4, 28.2, 29.6, 30.8,
31.0, 32.1, 32.2, 33.9, 34.5, 34.7, 35.7, 35.9, 36.1, 41.7, 45.5, 46.8,
47.9, 50.2, 51.0, 51.4, 53.2, 53.3, 55.2, 56.0, 60.3, 61.8, 62.0, 62.9,
74.2, 76.1, 79.4, 114.2, 114.3, 114.5, 123.7, 123.8, 124.1, 127.7,
129.9, 131.4, 143.0, 143.2, 155.3, 159.0, 161.7, 162.0, 165.4, 165.5,
173.1, 174.3, 174.5; HRMS (ESI) calcd for C101H146N12O18 [M +
Na]+ 1838.0776, found 1838.0696.
4 E. Sieva¨nen, Molecules, 2007, 12, 1859–1889.
5 (a) P. W. Swaan, K. M. Hillgren, F. C. Szoka and J. S. Øie, Bioconjugate
Chem., 1997, 8, 520–525; (b) E. Petzinger, A. Wickboldt, P. Pagels,
D. Starke and W. Kramer, Hepatology, 1999, 30, 1257–1268; (c) W.
Kramer, G. Wess, G. Schubert, M. Bickel, F. Girbig, U. Gutjahr, S.
Kowalewski, K.-H. Baringhaus, A. Enhsen, H. Glombik, S. Mu¨llner,
G. Neckermann, S. Schulz and E. Petzinger, J. Biol. Chem, 1992, 267,
18598–18604; (d) G. Wess, W. Kramer, G. Schubert, A. Enhsen, K.-H.
Baringhaus, H. Glombik, S. Mu¨llner, K. Bock, H. Kleine, M. John,
G. Neckermann and A. Hoffmann, Tetrahedron Lett., 1993, 34, 819–
822; (e) S. N. Bavikar, D. B. Salunke, B. G. Hazra, V. S. Pore, R.
H. Dodd, J. Thierry, F. Shirazi, M. V. Deshpande, S. Kadreppa and
S. Chattopadhyay, Bioorg. Med. Chem. Lett., 2008, 18, 5512–5517;
(f) D. G. Rivera and L. A. Wessjohann, Molecules, 2007, 12, 1890–
1899.
6 (a) C. W. Tornøe, C. Cristensen and M. Meldal, J. Org. Chem., 2002,
67, 3057–3064; (b) V. V. Rostovtsev, L. G. Green, V. V. Fokin and K. B.
Sharpless, Angew. Chem., 2002, 114, 2708–2711; (c) J. E. Moses and A.
D. Moorhouse, Chem. Soc. Rev., 2007, 36, 1249–1262; (d) M. Meldal
and C. W. Tornøe, Chem. Rev., 2008, 108, 2952–3015.
7 (a) H. C. Kolb and K. B. Sharpless, Drug Discovery Today, 2003, 8,
1128–1137; (b) D. K. Dalvie, A. S. Kalgutkar, S. C. Khojasteh-Bakht,
R. S. Obach and J. P. O’Donnell, Chem. Res. Toxicol., 2002, 15, 269–
299; (c) W. S. Horne, M. K. Yadav, C. D. Stout and M. R. Ghadiri, J.
Am. Chem. Soc., 2004, 126, 15366–15367.
Synthesis of compound 8
8 (a) Y. L. Angell and K. Burgess, Chem. Soc. Rev., 2007, 36, 1674–1689;
(b) A. L. Jochim, S. E. Miller, N. G. Angelo and P. S. Arora, Bioorg.
Med. Chem. Lett., 2009, 19, 6023–6026; (c) Ahsanullah, P. Schmieder,
R. Ku¨hne and J. Rademann, Angew. Chem., Int. Ed., 2009, 48, 5042–
5045; (d) N. G. Angelo and P. S. Arrora, J. Am. Chem. Soc., 2005, 127,
17134–17135; (e) H.-F. Chow, K.-N. Lau, Z. Ke, Y. Liang and C.-M.
Lo, Chem. Commun., 2010, 46, 3437–3453; (f) C. O. Kappe and E. V.
Eycken, Chem. Soc. Rev., 2010, 39, 1280–1290; (g) J. M. Holub and K.
Kirshenbaum, Chem. Soc. Rev., 2010, 39, 1325–1337.
A solution of 6b (70 mg, 0.044 mmol) in trifluoroacetic acid
(0.5 ml, 6.5 mmol) was refluxed for 7 h. The reaction mixture
was diluted with DCM (5 ml) and neutralized with an aqueous
saturated solution of sodium bicarbonate until the violet color
disappeared. The layers were separated and the aqueous layer was
extracted with DCM (2 ¥ 10 ml). The combined organic extracts
were dried (Na2SO4) and evaporated. The residue was purified by
column chromatography (CH3CN–NH3 90 : 1).
9 (a) A. Domling and I. Ugi, Angew. Chem., Int. Ed., 2000, 39, 3168–3120;
(b) A. Domling, Chem. Rev., 2006, 106, 17–89; (c) A. V. Gulevich, A. G.
This journal is
The Royal Society of Chemistry 2011
Org. Biomol. Chem., 2011, 9, 4921–4926 | 4925
©

View Article Online
Zhdanko, R. V. A. Orru and V. G. Nenajdenko, Chem. Rev., 2010, 110,
5235–5331; (d) A. V. Gulevich, N. E. Shevchenko, E. S. Balenkova, G.-
V. R o¨schenthaler and V. G. Nenajdenko, Synlett, 2009, 403–406; (e) A.
V. Gulevich, N. E. Shevchenko, E. S. Balenkova, G.-V. Ro¨schenthaler
and V. G. Nenajdenko, Tetrahedron, 2008, 64, 11706–11712; (f) L. A.
Wessjohann, D. G. Rivera and O. E. Vercillo, Chem. Rev., 2009, 109,
796–814.
12 T. L. Mindt, H. Struthers, L. Brans, T. Anguelov, C. Schweinsberg,
V. Maes, D. Tourwer and R. Schibli, J. Am. Chem. Soc., 2006, 128,
15096–15097.
13 V. K. Khartri, S. Upretti and P. S. Pandey, Org. Lett., 2006, 8, 1755–
1758.
14 (a) N. Vijayalakshmi and U. Maitra, J. Org. Chem., 2006, 71, 768–774;
(b) S.-Y. Liu, Y.-B. He, W.-H. Chan and A. W. M. Lee, Tetrahedron,
2006, 62, 11687–11696.
10 V. G. Nenajdenko, A. V. Gulevich, N. V. Sokolova, A. V. Mironov and
E. S. Balenkova, Eur. J. Org. Chem., 2010, 1445–1449.
11 C.-W. Chen and H. W. Whitlock, J. Am. Chem. Soc., 1978, 100, 4921–
4922.
15 See ESI for more details.
16 M. Mroczkiewicz and R. Ostaszewski, Tetrahedron, 2009, 65, 4025–
4034.
4926 | Org. Biomol. Chem., 2011, 9, 4921–4926
This journal is
The Royal Society of Chemistry 2011
©