A triazene-based new photolabile linker in solid phase chemistry

Article abstract of DOI:10.1016/j.tetlet.2004.02.002

An extension of the T2 linker methodology by showing its applicability as a photocleavable linker is reported. Photocleavage was carried out with 355nm UV laser irradiation (3ω Nd-YAG) in methanol/diethyl ether and is suitable for protected amino acid derivatives, as well as simple small organic molecules including resin-bound biotin. The linker is stable under a wide range of conditions with the exception of strongly acidic media.

Full text of DOI:10.1016/j.tetlet.2004.02.002

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 2839–2841
A triazene-based new photolabile linker in solid phase chemistry
a,
a
b
b
*
Dieter Enders, Christiaan Rijksen, Elke Bremus-K o€ bberling, Arnold Gillner
c
and Johannes K o€ bberling
a
Institut f €u r Organische Chemie, RWTH Aachen, Professor Pirlet Str. 1, 52074 Aachen, Germany
b
Fraunhofer Institut f €u r Lasertechnik, Steinbachstraße 15, 52074 Aachen, Germany
Bayer Healthcare, Pharma Research, 42096 Wuppertal, Germany
c
Received 23 December 2003; revised 2 February 2004; accepted 2 February 2004
Abstract—An extension of the T2linker methodology by showing its applicability as a photocleavable linker is reported.
Photocleavage was carried out with 355 nm UV laser irradiation (3x Nd-YAG) in methanol/diethyl ether and is suitable for pro-
tected amino acid derivatives, as well as simple small organic molecules including resin-bound biotin. The linker is stable under a
wide range of conditions with the exception of strongly acidic media.
Ó 2004 Elsevier Ltd. All rights reserved.
During the last years a wide range of new linkers,
including several photolabile linkers, have been
developed. The most important advantage is their re-
reaction conditions. The photochemistry of triazenes has
been thoroughly investigated during the nineteen-eight-
ies by Majer et al. and Julliard et al. independently.
1
4
5
agent-free cleavage, which leaves most functional and
protecting groups untouched. Orthogonal towards
basic, acidic, reductive and oxidative conditions, they
display wide applicability.
Majer proposed an ionic mechanism for the photolysis of
1-aryl-3,3-dialkyltriazenes based on spectroscopic stud-
ies. Julliard surmised that the reaction proceeded via a
radical mechanism, supported by the isolation of inter-
mediates that could have only been generated in radical
reactions. Both pathways involve an effervescence of
elementary nitrogen and give rise to amines. The chro-
mophore of triazenes, located in the near UV between
300 and 400 nm, depends on the substituents of the
aromatic ring. It was therefore anticipated that the
photocleavage of the triazene T2linker is possible.
Nevertheless, most photocleavable linkers suffer from
the disadvantage of a rather complicated synthesis,
involving multiple steps on the resin. Alternatively,
pathways featuring the synthesis of a precursor prior to
its attachment to the resin have also been reported.
Research has been conducted on triazene compounds as
protecting and anchoring groups for amines. Two dif-
ferent linkers for solid phase organic synthesis (SPOS)
3
The linker was synthesised following a known protocol.
The Merrifield resin 1 was transformed into the aniline
derivative 2 using WilliamsonÕs synthesis. It was then
converted into an immobilised diazonium salt 3 and
subsequently into the triazene moiety 4 by treatment
with a secondary amine (Scheme 1).
2
;3
have been described (T1 and T2linker).
We wish to give details on the photolabile properties of
the T2linker and its use for photoinduced cleavage. The
known linker is easily built in three steps and can be
cleaved with 10% TFA in dichloroethane. Such condi-
tions also affect functional groups such as certain car-
bamates, esters and silyl ethers. The triazene moiety was
found to be stable under a wide range of nonacidic
The optimum conditions in terms of yields of cleavage,
purity of the products and successive cleavage proce-
dures were determined by variation of wavelengths,
irradiation times, intensities and solvents. All reactions
were carried out in cooled open glass vessels containing
the resins in suspension.
Keywords: Solid phase synthesis; Photochemistry; Cleavage; Triazene;
Amino acid.
*
Corresponding author. Tel.: +49-241-8094676; fax: +49-241-8092-
27; e-mail: enders@rwth-aachen.de
As a source of light, a 1 kHz pulsed 3x Nd-YAG laser
(355 nm) as well as different excimer lamps (308 and
1
0
040-4039/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.02.002

2840
D. Enders et al. / Tetrahedron Letters 45 (2004) 2839–2841
moiety was further treated with different acid halides
HO
NH
2
including cyclohexanoyl chloride and pivaloyl chloride,
thus yielding resins 4a and 4b (Table 1). During the
photolytic experiments the power of the laser was var-
NaH, DMF
8h 60°C
Cl
O
NH2
1
2
1
2
ied, showing that a value below 1.0 W/cm did not yield
an adequate amount of product within the irradiation
(
90-100% loading)
Merrifield resin
.63-0.93 mmol/g
0
2
times considered. Values above 1.4 W/cm led to a sig-
nificant destruction of the resin matrix and to impurities.
Accordingly the power of the laser beam was found to
tBuONO 4 eq.,
BF .Et O 4 eq.
10°C, THF, 1 h
O
N
BF4
2
3
2
3
N
be optimum at 1.0 W/cm with a pulse of 1 kHz and a
pulse length of 20–25 ns, providing the desired products
-
(
stable for several
hours at -10°C)
6
a and 6b in moderate yields and high purities. The resin
loading has been determined by elementary analysis of
the nitrogen content. Photolysis rates and corresponding
yields of a selected reaction are shown in Table 2.
Reaction times were limited to 30 min for practical
reasons. In the current set-up only one sample at a time
could be irradiated, even small combinatorial libraries
would consume too much time. The use of a beam-
splitter for parallel processing is planned. The analytical
data of the cleavage products were identical to those of
the amides independently synthesised using standard
techniques.
excess
1
2
HNR R
N
R2
O
N
N
R1
4
Scheme 1. Synthesis of the T2linker.
R1
N
hν, λ = 355 nm
O
O
N
N
R2
4
R1
N
H
Table 1. Photocleavage of T2moieties with 3 x Nd-YAG laser at
+
N2 +
3
55 nm. (Solvent–purity relationship)
R2
4
/6
Resin 4
Product 6
Solvent
Purity
(%)
5
6
Scheme 2. General reaction scheme for the photolytic cleavage of
triazene T2bound compounds.
R =
R ¼ H
N
O
N
2
22 nm) were used. These investigations on the photo-
O
lysis wavelength showed that only the Nd-YAG laser at
a wavelength of 355 nm was capable of breaking the
nitrogen bonds without further decomposition of the
products (Scheme 2).
R
N
N
a
b
Et
Et
2
O/MeOH
O/MeOH
90
77
O
O
The vertical unfocused beam was directed from above
into the open vessel for irradiation times of 1–30 min.
Reactions were repeated using different laser power
R
R
N
N
N
H
2
CH
CH
3
3
C
3
2
ranging from 0.5 to 1.7 W/cm . Various solvents were
examined under these conditions, including THF,
methanol, diethyl ether and dichloromethane and mix-
tures thereof. Dichloromethane has the disadvantage of
leading to the formation of chlorinated side products
appearing upon UV irradiation. A mixture of methanol
and diethyl ether turned out to be the optimum solvent.
N
NH2
c
Et
2
O/MeOH
95
OtBu
When the cleavage is carried out with TFA rather than
photolysis, reactions can only be monitored by taking
out a sample from the reaction vessel and use destructive
analytical methods. In terms of combinatorial chemistry
this could be a laborious task. In the case of closed
O
R
N
N
d
Et
2
O/MeOH
97
6
microreactors or macrolithic resin portions like
H
H
N
Stratosphere Plugse monitoring was impossible without
severe altering of the reaction set-up. When photolysis is
used, a small fraction of the product is cleaved, reducing
slightly the resin loading. Most of the material is left
unchanged on the resin, available for further reactions.
O
S
NH
H
O
N
N
e
THF
100
O
CH3
CH
Acylated piperazines have been chosen as model com-
pounds for cleavage studies. The immobilised piperazine
H
3
C
3

D. Enders et al. / Tetrahedron Letters 45 (2004) 2839–2841
2841
Table 2. Photolysis rates of the resin 4b giving 6b
Irradiation time (min) Yield of 6b (%)
References and notes
1
. (a) Rich, D. H.; Gurwara, S. K. J. Chem. Soc., Chem.
Commun. 1973, 610–611; (b) Rich, D. H.; Gurwara, S. K.
J. Am. Chem. Soc. 1975, 97, 1575–1579; (c) Holmes, C. P.;
Jones, D. G. J. Org. Chem. 1995, 60, 2318–2319; (d)
Holmes, C. P. J. Org. Chem. 1997, 62, 2370–2380; (e)
Nicolaou, K. C.; Safina, B. S.; Winssinger, N. Synlett
1
2
3
0
24
0
38
0
45
Based on our experience of piperazine cleavage, amino
acids were immobilised at their C-terminus. Therefore
Fmoc-Tyr-(t-Bu)-OH was coupled onto the piperazine
2
001, 900–903; (f) Nicolaou, K. C.; Winssinger, N.;
Pastor, J.; DeRoose, F. J. Am. Chem. Soc. 1997, 119,
49–450; (g) Routledge, A.; Abell, C.; Balasubramanian,
4
resin using 2-fluoro-1-ethylpyridinium tetrafluoroborate
7
S. Tetrahedron Lett. 1997, 38, 1227–1230; (h) Glatthar, R.;
Giese, B. Org. Lett. 2000, 2, 2315–2317; (i) Peukert, S.;
Giese, B. J. Org. Chem. 1998, 63, 9045–9051; (j) Kumar,
K. S.; Roice, M.; Rajasekharan Pillai, V. N. Tetrahedron
2001, 57, 3151–3158; (k) Rodebaugh, R.; Fraser-Reid, B.;
Geysen, H. M. Tetrahedron Lett. 1997, 38, 7653–7656; (l)
For a review see: Bochet, C. G. J. Chem. Soc., Perkin
Trans. 1 2002, 125–142.
(
FEP) as a coupling reagent (Table 1). The photoclea-
vage of this tyrosine derivative 4c was compared with
the classical TFA cleavage. Upon treatment with TFA,
the acid-labile tert-butyl ether was destroyed, depro-
tecting the tyrosine hydroxyl moiety. Photocleavage,
however, left the ether unscathed. Two peaks were vis-
ible in the GC spectrum: one of them corresponding to
the Fmoc group, the other one to the Fmoc-deprotected
amino acid piperazide (as determined by GC–MS). The
basic moiety of the free piperazine amino group present
after photolysis, cleaved the Fmoc protecting group,
thus delivering the inversely protected product 6c as
obtained by acid cleavage.
2
3
. (a) Br €a se, S.; Enders, D.; K o€ bberling, J.; Avemaria, F.
Angew. Chem., Int. Ed. 1998, 37, 3413–3415; (b) Schunk,
S.; Enders, D. Org. Lett. 2000, 2, 907–910; (c) Br €a se, S.;
Dahmen, S. Chem. Eur. J. 2000, 6, 1; (d) Lazny, R.;
Nodzewska, A. Tetrahedron Lett. 2003, 44, 2441–2444.
. (a) Brase, S.; Dahmen, S.; Pfefferkorn, M. J. Comb. Chem.
2000, 2, 710–715; (b) Br €a se, S.; K o€ bberling, J.; Enders, D.;
Lazny, R.; Wang, M.; Brandtner, S. Tetrahedron Lett.
1999, 40, 2105–2108; (c) Rademann, J.; Smerdka, J.; Jung,
G.; Grosche, P.; Schmid, D. Angew. Chem., Int. Ed. 2001,
€
As the biotin–streptavidine system is a standard bio-
chemical couple for the investigation of enzyme–
substrate interactions, biotin is an interesting substrate
for solid phase chemistry. Biotin itself was nearly
insoluble in the solvents usually employed with this kind
of resins. The use of the biotin tetrafluorophenyl (TFP)
active ester, soluble in dichloromethane, allowed suc-
cessful attachment to the resin (4d). From this resin the
N-(+)-biotinylpiperazine 6d was cleaved using our
standard photolysis methodology yielding a very pure
product (97%).
40, 381–385; (d) Lazny, R.; Nodzewska, A.; Klosowski, P.
Tetrahedron 2004, 60, 121–130.
4
5
. Majer, J.; Reh ꢀa k, V.; Poskocil, J.; Cihlo, L. Wiss. Z. Tech.
Hochschule Leuna-Merseburg 1974, 16, 335–340; Chem.
Abstr. 1974, 24260D.
. (a) Julliard, M.; Scelles, M.; Guillemonat, A. Tetrahedron
Lett. 1977, 4, 375–378; (b) Julliard, M.; Vernin, G.;
Metzger, J. Helv. Chim. Acta 1980, 63, 456–466; (c)
Julliard, M.; Vernin, G.; Metzger, J. Helv. Chim. Acta
1980, 63, 467–473.
6
. (a) Bremus-K o€ bberling, E.; Gillner, A.; Wehner, M.;
Russek, U.; K o€ bberling, J.; Enders, D.; Brandtner, S. In
Interestingly, during the studies on solvent influence,
addition of THF to the cleaved amine was observed
when irradiation was performed in this solvent (Table
5
th International Conference on Microreaction Technology;
Matlosz, M., Ehrfeld, W., Baselt, J. P., Eds.; Springer:
Berlin, Heidelberg, 2001; pp 455–463; (b) K o€ bberling, J.;
Enders, D.; Bremus-K o€ bberling, E.; Gillner, A.; Brandt-
ner, S. GIT Lab. J. Int. 2000, 205.
1
). It is known that THF is prone to undergo a-hydro-
gen abstraction in radical reactions; the resulting
product 6e was detected by GC–MS. A similar photo-
chemical addition of THF onto nitrenes was observed
by Platz and co-workers. while Bumgardner et al.
9
reported a radical addition of THF onto diimides.
7. Li, P.; Xu, J. C. J. Pept. Res. 2001, 58, 129–139.
8. Poe, R.; Schnapp, K.; Young, M. J. T.; Grayzar, J.; Platz,
M. S. J. Am. Chem. Soc. 1992, 114, 5054–5067.
9. Bumgardner, C. L.; Purrington, S. T.; Huang, P. T. J. Org.
Chem. 1983, 48, 2287–2289.
8
1
0. General procedure for the photocleavage of resins with a
In summary, we have discovered a new advantageous
feature of the T2linker, widening its range of applica-
bility. The linker is selectively cleaved by UV irradiation
with a short reaction time to give the desired amines in
1
3
kHz pulsed 3x Nd-YAG laser at a wavelength of
55 nm: 100 mg of the resin was put into an open glass
vessel and suspended in 2mL of an appropriate solvent.
The inner diameter of the open vessel was as wide as
the unfocused vertical laser beam (10 mm). The suspen-
sion was stirred with a magnetic stirring bar and a
10
high purities.
â
H+P Variomag stirring device and cooled by means
Acknowledgements
of an aluminium cooling block. After irradiation, the
solvent was decanted and the resin was washed with
the solvent system used during the cleavage. Analyses
were performed using TLC, GC–MS and LC–MS
techniques.
This work has been supported by the Fonds der
Chemischen Industrie. We thank BASF AG and Bayer
AG for providing us with chemicals.