A novel concept in combinatorial chemistry in solution with the advantages of solid-phase synthesis: Formation of N-betaines by multicomponent domino reactions

Article abstract of DOI:10.1002/1521-3773(20010302)40:5<903::AID-ANIE903>3.0.CO;2-7

Advantages of solid-phase and liquid-phase synthesis are combined in a new concept of combinatorial chemistry: a domino sequence comprising Knoevenagel and hetero-Diels-Alder reactions, with subsequent hydrogenation starting from protected aminoaldehydes, 1.3-dicarbonyl compounds, and enol ethers leads to N-heterocycles of high diversity with a betaine structure, which are isolated in highly pure form by precipitation with diethyl ether (see scheme), Cbz = benzylocycarbonyl, Bn = benzyl.

Full text of DOI:10.1002/1521-3773(20010302)40:5<903::AID-ANIE903>3.0.CO;2-7

COMMUNICATIONS
Table 1. Amino aldehydes used.
A Novel Concept in Combinatorial Chemistry
in Solution with the Advantages of Solid-Phase
Synthesis: Formation of N-Betaines by
Multicomponent Domino Reactions**
Aminoaldehyde
R1
R2
n
1a
1b
1c
1d
1e
1 f
1g
2
H
H
Me
iPr
iBu
Bn
H
Me
H
H
H
0
0
0
0
0
0
0
1
2
Lutz F. Tietze,* Holger Evers, and Enno Töpken
Dedicated to Professor Ivar Ugi
on the occasion of his 70th birthday
H
CH2CH2CH2
H
H
H
H
3
Combinatorial chemistry is an important method, both in
the search for lead structures of pharmacologically active
compounds and in their optimization,[1] and it also finds
application in supramolecular chemistry,[2] in catalyst devel-
opment,[3] and in material science.[4] Within this context
combinatorial solid-phase synthesis[5] has the advantage of
simple automation, since reagents can be used in excess, and
impurities and by-products can be removed by simple
filtration and washing procedures. Frequently, however, the
transfer of reactions established in solution to the solid phase
is fraught with difficulties; an additional disadvantage is that
the synthesis is extended by two steps. Moreover, monitoring
the course of the reaction on the solid phase is problematic
using normal methods. Therefore, parallel to solid-phase
synthesis new combinatorial approaches to synthesis in
solution, to reactions on soluble resins,[6] and in the perfluori-
nated phase,[7] as well as separation of by-products by binding
to polymers[8] have been developed.
dimethylbarbituric acid (5a), the cyclohexan-1,3-diones 5b
and 5c, and 4-hydroxycoumarin (5d) were used as 1,3-
dicarbonyl components. The differently substituted benzyl
enol ethers 4a ± d[11] served as dienophiles.
O
N
N
O
O
O
O
5a
5b
O
O
O
O
OH
Herein we describe a new, simple concept in combinatorial
chemistry which combines the advantages of reactions in
solution with those of solid-phase synthesis. The reagents can
be used in excess, and the products can be obtained by simple
precipitation in purities of up to 99%. The procedure involves
a multicomponent domino Knoevenagel/hetero-Diels ± Alder
reaction[9] of a 1,3-dicarbonyl compound with an amino
aldehyde and an enol ether, followed by a reductive amination
with the formation of a betaine which can be precipitated
from the solution in high purity. Organic by-products and
excess reagents are separated by simple filtration. By the use
of a-, b-, and g-amino aldehydes libraries of pyrrolidine,
piperidine, and azepane derivatives, respectively, may be
obtained.
The required amino aldehydes 1 ± 3 are accessible in high
yields simply by reaction of a wide spectrum of available
natural and nonnatural a-, b-, and g-amino acids with benzyl
chloroformate, followed by esterification, and reduction with
diisobutylaluminum hydride (DIBAL-H) in hexane solution
(Table 1).[10] The compounds are stable and storable. N,N'-
5c
5d
The amino aldehydes 1 ± 3 were treated with the 1,3-
dicarbonyl components 5a ± d (1 equiv) and the benzyl enol
ethers 4a ± d (4 equiv) in toluene in the presence of catalytic
amounts of EDDA and trimethyl orthoformate as dehydrat-
ing agent in an ultrasonic bath at 508C (Scheme 1). Knoeve-
nagel condensation of the amino aldehyde with the 1,3-
dicarbonyl component occurs first with the formation of an
electron-deficient, sterically fixed 1-oxo-1,3-butadiene
6
which then reacts with the benzyl enol ether in a Diels ±
Alder reaction with inverse-electron demand to afford a
benzyl-protected acetal 7. Subsequent hydrogenation with
palladium on carbon as catalyst leads to hydrogenolysis of the
benzyl acetal and the Cbz protecting group with release of an
amine and an aldehyde, which under the reaction conditions
lead to the desired highly substituted nitrogen heterocycles
8 ± 10 of different ring size in an intramolecular reductive
amination (Scheme 2, Table 2); mixtures of diastereoisomers
are normally formed in the reaction.
The products thus obtained contain a 1,3-dicarbonyl unit
with an C ± H-acidic methylene group and an amino group;
this leads to the formation of a betaine which is readily soluble
in water and methanol, but can be precipitated by the addition
of diethyl ether. This represents a new concept in combina-
torial chemistry which should find general applicability. The
scope of the procedure is considerable; for example, sterically
demanding amino aldehydes and benzyl enol ethers could be
used. The use of 1,3-dicarbonyl compounds with reduced
reactivity did not result in a deterioration of the purity of the
[*] Prof. Dr. Dr. h.c. L. F. Tietze, Dipl.-Chem. H. Evers,
Dipl.-Chem. E. Töpken
Institut für Organische Chemie der Universität
Tammannstrasse 2, 37077 Göttingen (Germany)
Fax : (‡49)551-39-9476
E-mail: ltietze@gwdg.de
[**] This work was supported as part of the BMBF Project ªKombinatori-
sche Chemieº (grant number 03D00562). We thank BASF AG,
Ludwigshafen, and the Fonds der Chemischen Industrie for generous
support.
Supporting information for this article is available on the WWW under
http://www.angewandte.com or from the author.
Angew. Chem. Int. Ed. 2001, 40, No. 5
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001
1433-7851/01/4005-0903 $ 17.50+.50/0
903

COMMUNICATIONS
Table 2. Products of the domino sequence: pyrrolidine, piperidine, and
azepane derivatives 8, 9, and 10, respectively.
Product
R1
R2
R3
n
Purity [%][a]
Yield [%]
8a
8b
8c
8d
8e
8f
8g
8h
8i
H
H
Me
iPr
iBu
iBu
H
Me
H
H
H
Et
iPr
Me
H
H
Et
H
H
H
H
H
0
0
0
0
0
0
0
0
0
0
0
1
1
2
2
97
> 90[b]
97
95
99
> 95
> 90
97
> 95
> 95
95
> 95
> 95
98
53
47
45
55
59
41
51
65
71
59
61
60
69
58
55
H
CH2CH2CH2
Bn
Me
Me
Me
H
H
H
H
H
H
H
H
H
H
H
H
8j
8k
9a
9b
10a
10b
H
H
H
Me
> 95
[a] Determined by HPLC unless otherwise indicated. [b] Purity deter-
1
mined by H NMR spectroscopy.
cycles of different ring sizes and substitution patterns with a
betaine structure which can be obtained in high purity by
precipitation in a process which is also suitable for
automation.
Experimental Section
Scheme 1. Domino sequence comprising Knoevenagel, hetero-Diels ± Alder
reaction, and hydrogenation starting from amino aldehydes, 1,3-dicarbonyl
compounds, and enol ethers. a) toluene, EDDA, ultrasound, 508C, 15 h; b) Pd/
C, H2, 1 bar, RT, 24 h. EDDA ˆ ethylenediammonium diacetate, Cbz ˆ benzyl-
oxycarbonyl, Bn ˆ Benzyl.
Synthesis of 8h: 1 f (35.4 mg, 125 mmol), N,N'-dimethylbarbituric acid
(5a) (19.5 mg, 125 mmol) and benzylvinyl ether (4a) (134 mg,
1.00 mmol) were allowed to react in toluene (0.5 mL) and trimethyl
orthoformate (0.1 mL) with a few crystals of EDDA in an argon
atmosphere in a 10-mL pressure flask in an ultrasonic bath for 15 h at
50 ± 608C. After removal of the solvent in vacuo, the residue was
dissolved in methanol (4 mL) and stirred in a hydrogen atmosphere in
the presence of palladium on carbon (10%, 12.5 mg) for 24 h.
Subsequently, the catalyst was removed by filtration through a little
celite, washed with methanol, and the solvent removed in vacuo. The
residue was dissolved in a little methanol and the product was
precipitated by the addition off diethyl ether. The white, amorphous
precipitate was filtered off and washed with diethyl ether. Compound
8h was 97% pure as determined by HPLC. Decomposition >2508C;
1H NMR (300 MHz, CD3OD): d ˆ 2.05 ± 2.45 (2m, 2H; 4-H), 2.90 ± 2.95
(m, 2H; PhCH2), 3.20 (s, 6H; 2 Â NCH3), 3.30 ± 3.55 (m, 3H; 2-H, 5-H),
4.22 ± 4.32 (m, 1H; 3-H), 7.15 ± 7.30 (m, 5H; Ph-H); 13C NMR (75 MHz,
[D6]DMSO): d ˆ 26.17 (2  NCH3), 27.21 (C-4), 39.87 (C-3), 35.28,
43.49 (C-1'', C-5), 61.83 (C-2), 81.15 (C-5'), 125.5, 128.3, 128.7 (C-3'',
C-4'', C-5'', C-6'') 131.0 (C-2''), 152.6 (C-2'), 162.0 (C-4', C-6'); MS
O
N
N
O
O
O
O
O
O
O
O
O
R3
R3
R3
R3
n
n
n
n
N
R1
N
R1
N
R1
N
R1
R2
R2
R2
R2
8a-h, 9a,
10a-b
8i
8j
8k, 9b
Scheme 2. Products from the domino Knoevenagel/hetero-Diels ± Alder/hydro-
genation sequence.
‡
‡
(70 eV): m/z (%): 315.2 (1) [M ], 224.1 (100) [M
Bn], 91.0 (10)
‡
[C7H7 ]; HR-MS: calcd for C17H21N3O3: 315.1582, found: 315.1582.
products, in spite of the required extended reaction times.
Only reaction with the disubstituted N-Cbz-2-amino-2-meth-
ylpropionaldehyde and benzyl-2-methylprop-1-enyl ether did
not lead to the desired products, presumably because of steric
overload.
Unlike solid-phase combinatorial chemistry the reactions
may be monitored at all times by thin-layer chromatography,
and where required the intermediates investigated without
difficulty by normal spectroscopic methods. Finally, one
further advantage is the lack of requirement for the use of
expensive resins as well as the development and optimization
of suitable coupling and decoupling reactions. The domino
sequence of Knoevenagel, hetero-Diels ± Alder reaction, and
hydrogenation allows rapid access to a number of N-hetero-
Received: October 2, 2000 [Z15881]
[1] a) M. A. Gallop, R. W. Barret, W. J. Dower, S. P. A. Fodor, E. M.
Gordon, J. Med. Chem. 1994, 37, 1233 ± 1280; b) L. A. Thompson, J. A.
Ellman, Chem. Rev. 1996, 96, 555 ± 602; c) L. F. Tietze, M. Lieb, Curr.
Opin. Chem. Biol. 1998, 2, 363 ± 371; d) F. Guillier, D. Orain, M.
Bradley, Chem. Rev. 2000, 100, 2091 ± 2157.
[2] a) Y. Cheng, T. Suenaga, W. C. J. Still, J. Am. Chem. Soc. 1996, 118,
1813 ± 1814; b) M. T. Burger, W. C. J. Still, J. Org. Chem. 1995, 60,
7382 ± 7383; c) M. S. Goodmann, V. Linton, A. D. Hamilton, J. Am.
Chem. Soc. 1995, 117, 11610 ± 11611.
[3] R. Kranich, K. Eis, O. Geis, S. Mühle, J. W. Bats, H.-G. Schmalz, Chem.
Eur. J. 2000, 6, 2874 ± 2894.
[4] B. Jandeleit, D. S. Schaefer, T. S. Powers, H. W. Turner, W. H.
Weinberg, Angew. Chem. 1999, 111, 2648 ± 2689; Angew. Chem. Int.
Ed. 1999, 38, 2476 ± 2514.
904
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001
1433-7851/01/4005-0904 $ 17.50+.50/0
Angew. Chem. Int. Ed. 2001, 40, No. 5

COMMUNICATIONS
tion.[6] In these experiments, the electrochemical sensorÐ
usually a microdisk electrode with a tip diameter of a few
micrometersÐhas to be positioned in close proximity to the
object. The approach of the microelectrode to the biological
object is, in general, performed under control of an optical
microscope using manual- or piezo-actuated micromanipula-
tors for positioning of the sensor tip. The electrode is slowly
moved towards the cell until it slightly touches the cell
membrane, which is visualized by its bending. The tip is then
retracted for a defined distance allowing, in principal, adjust-
ment of the membrane-to-electrode distance within a range of
several hundred nanometers to two micrometers. Although
this approach has been successfully used in recent years,[3]
there are significant drawbacks, such as a) possible contam-
ination of the electrode surface due to its contact with the cell
membrane, b) insufficient reproducibility of the tip-to-cell
distance, which becomes even more difficult using smaller
microelectrodes, that are hardly visible in the optical micro-
scope, c) eventual deterioration of the cell at the contact point
or mechanical depolarization of the cell, and d) the impossi-
bility to investigate sequentially different spots on the same
cell.
After initial investigations of biological samples by means
of scanning electrochemical microscopy (SECM),[7] attempts
have been recently undertaken to visualize the metabolism
and the redox activity of individual cells.[8] However, one of
the major problems in conventional SECM experiments is the
constant z-height of the microelectrode, which does not allow
differentiation between variations in the tip-to-sample dis-
tance and changes of the local electrochemical activity. In
order to overcome these limitations, we have introduced a
shear-force based constant-distance control into the SECM.[9]
The benefits and limitations had been described in detail
recently.[10] In short, the microelectrode vibrates at its
resonance frequency with typical amplitudes of only a few
nanometers with use of a piezo-pusher. Simultaneously, a
laser beam is focused onto the very end of the vibrating
electrode and the resulting Fresnel diffraction pattern is
projected onto a split photodiode. Amplitude and phase
information about the vibrating tip is obtained by the
amplification of the difference signal from the split photo-
diode with respect to the agitation signal using a lock-in
amplifier. With decreasing tip-to-sample distance, increasing
shear forces between tip and sample surface lead to a damping
of the vibration amplitude and to a phase shift, which can be
used to continuously keep a predefined damping value related
to a constant distance of about 50 ± 100 nm by means of a
software-controlled feedback loop.
[5] a) R. C. D. Brown, J. Chem. Soc. Perkin Trans. 1 1998, 3293 ± 3320;
b) A. R. Brown, P. H. H. Hermkens, H. C. J. Ottenheijm, D. C. Rees,
Synlett 1998, 817 ± 827; c) P. H. H. Hermkens, H. C. J. Ottenheijm,
D. C. Rees, Tetrahedron 1997, 53, 5643 ± 5678; d) J. S. Früchtel, G.
Jung, Angew. Chem. 1996, 108, 19 ± 43; Angew. Chem. Int. Ed. Engl.
1996, 35, 17 ± 40; e) F. Balkenhohl, C. von dem Bussche-Hünnefeld,
A. Lansky, C. Zechel, Angew. Chem. 1996, 108, 2436 ± 2502; Angew.
Chem. Int. Ed. Engl. 1996, 35, 2288 ± 2354.
[6] a) D. J. Gravert, K. D. Janda, Chem. Rev. 1997, 97, 489 ± 509; b) U.
Grether, H. Waldmann, Angew. Chem. 2000, 112, 1688 ± 1691; Angew.
Chem. Int. Ed. 2000, 39, 1629 ± 1632.
[7] a) A. Studer, S. Hadida, R. Ferritto, S. Y. Kim, P. Jeger, P. Wipf, D. P.
Curran, Science 1997, 275, 823 ± 826; b) A. Studer, P. Jeger, P. Wipf,
D. P. Curran, J. Org. Chem. 1997, 62, 2917 ± 2924; d) A. Studer, D. P.
Curran, Tetrahedron 1997, 53, 6681 ± 6696; e) J. J. J. Juliette, D.
Â
Rutherford, I. T. Horvath, J. A. Gladysz, J. Am. Chem. Soc. 1999,
121, 2696 ± 2704.
[8] A. G. M. Barret, M. L. Smith, F. J. Zecri, Chem. Commun. 1998,
2317 ± 2318.
[9] Reviews: a) L. F. Tietze, F. Haunert in Stimulating Concepts in
Chemistry (Eds.: F. Vögtle, J. F. Stoddart, M. Shibasaki), Wiley-
VCH, Weinheim, 2000, pp. 39 ± 64; b) L. F. Tietze, U. Beifuû, Angew.
Chem. 1993, 105, 137 ± 170; Angew. Chem. Int. Ed. Engl. 1993, 32,
131 ± 164; c) L. F. Tietze, Chem. Rev. 1996, 96, 115 ± 136; d) L. F.
Tietze, A. Modi, Med. Res. Rev. 2000, 20, 304 ± 322. Further
publications: e) L. F. Tietze, Y. Zhou, Angew. Chem. 1999, 111,
2076 ± 2078, Angew. Chem. Int. Ed. 1999, 38, 2045 ± 2047; f) L. F.
Tietze, Y. Zhou, E. Töpken, Eur. J. Org. Chem. 2000, 2247 ± 2252;
g) L. F. Tietze, J. Bachmann, J. Wichmann, Y. Zhou, T. Raschke,
Liebigs Ann. 1997, 881 ± 886.
[10] a) NaOH (1n), CbzCl, RT, 15 h; b) ROH, camphor sulfonic acid,
reflux, 15 h, quantitative over both stages; c) DIBAL-H (1n in
hexane), CH2Cl2, 788C, 2 h, 60 ± 84%: Y. Tajeuchi, A. Yamada, T.
Suzuki, T. Koizumi, Tetrahedron, 1996, 52, 225 ± 232.
[11] G. Hesse, Methoden Org. Chem. (Houben-Weyl) 4th ed. 1952-, Vol. 6/
1d, 1978, pp. 136 ± 139.
Spatially Resolved Detection of
Neurotransmitter Secretion from Individual
Cells by Means of Scanning Electrochemical
Microscopy
Andreas Hengstenberg, Andrea Blöchl,
Irmgard D. Dietzel, and Wolfgang Schuhmann*
Microelectrochemical methods, especially constant-poten-
tial amperometry and fast-scan cyclic voltammetry,[1] provide
possibilities to investigate biological systems with cellular or
subcellular spatial resolution determined mainly by the size of
the available microelectrodes.[2] Among the biological phe-
nomena studied so far at single cells or substructures of single
cells are individual exocytosis events,[3] oxygen consump-
tion,[4] photosynthetic activity,[5] and ion channel distribu-
Thus, adaptation of the shear-force based constant-distance
control of the tip-to-sample gap to biological preparations at
the single-cell level should, on the one hand, allow the
problems occurring with manual microelectrode positioning
to be overcome and, on the other hand, enable detectable
variations of chemical species at different sites of a biological
preparation.
Using platinum microelectrodes sealed in glass capillaries,
which can be easily positioned over hard sample surfaces
using the shear-force positioning mode, no satisfying results
could be obtained. In preliminary experiments using adher-
[*] Prof. Dr. W. Schuhmann, Dr. A. Hengstenberg[‡]
Analytische ChemieÐElektroanalytik & Sensorik
Ruhr-Universität Bochum, 44780 Bochum (Germany)
Fax : (‡49)234-321-4683
E-mail: woschu@anachem.ruhr-uni-bochum.de
Priv.-Doz. Dr. A. Blöchl, Priv.-Doz. Dr. I. D. Dietzel
Lehrstuhl für Molekulare Neurobiochemie
Ruhr-Universität Bochum, 44780 Bochum (Germany)
‡
[ ] Present address:
BioCurrents Research Center (NIH:NCRR)
Marine Biological Laboratory
7 MBL Street, Woods Hole, MA 02543 (USA)
Angew. Chem. Int. Ed. 2001, 40, No. 5
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001
1433-7851/01/4005-0905 $ 17.50+.50/0
905