Synthesis of diaziridines and diazirines via resin-bound sulfonyl oximes

Article abstract of DOI:10.1021/acs.orglett.6b03252

Diazirines are one of the most prominent functionalities in labeling experiments in vivo and in vitro because they allow photochemical generation of carbenes. The strategy presented herein describes the formation of diaziridines, being essential precursors in diazirine syntheses, using solid-supported procedures with immobilized sulfonyl oximes. The solid-supported building blocks have been shown to be valuable intermediates for CuAAC and amidation reactions, offering the possibility to build complex compounds with diverse functionalities.

Full text of DOI:10.1021/acs.orglett.6b03252

Letter
pubs.acs.org/OrgLett
Synthesis of Diaziridines and Diazirines via Resin-Bound Sulfonyl
Oximes
Irina Protasova,† Bekir Bulat,†,§ Nicole Jung,*,†,‡ and Stefan Brase*,†,‡
̈
†Institute of Toxicology and Genetics, Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, 76344
Eggenstein-Leopoldshafen, Germany
‡Institute of Organic Chemistry, Karlsruhe Institute of Technology, Fritz-Haber-Weg 6, 76131 Karlsruhe, Germany
S
* Supporting Information
ABSTRACT: Diazirines are one of the most prominent
functionalities in labeling experiments in vivo and in vitro
because they allow photochemical generation of carbenes. The
strategy presented herein describes the formation of
diaziridines, being essential precursors in diazirine syntheses,
using solid-supported procedures with immobilized sulfonyl
oximes. The solid-supported building blocks have been shown
to be valuable intermediates for CuAAC and amidation reactions, offering the possibility to build complex compounds with
diverse functionalities.
iazirines play a pivotal role in modern synthetic chemistry
Dand biochemistry because they are well-known precursors
for the syntheses of carbenes. They form, besides the compound
classes of benzophenones and azides, the most important group
of photoactive reagents being used for labeling experiments.1 The
diazirines gained particular importance due to their small size and
the advantageous excitation wavelengths of the three-membered
ring in biological experiments (ca. 360 nm).2 Therefore, in
comparison with other photoactivatable groups, diazirines offer
ideal conditions for use in vivo and in vitro due to a slight steric
influence in combination with minimum damage to the biological
system (Figure 1).3
precursors, which allow the synthesis of diaziridines 5 and their
oxidation to diazirines 3 (Scheme 1).
Scheme 1. Strategies for the Synthesis of Diaziridines 5 and
Diazirines 3
In a typical reaction sequence, ketones are converted into
sulfonyl ketoximes via an oxime formation and successive
tosylation or mesylation.9−11 Those tosyl and mesyl ketoximes
can be reacted in liquid ammonia to the corresponding
diaziridines 5 and are efficiently transformed in an additional
step to give diazirines 3. The key step of a diaziridine synthesis,
the ammonia-mediated formation of the three-membered ring,
requires equipment efforts because ammonia has to be
condensed at low temperature (−78 °C).12 Other alternatives
are known that describe the diaziridine formation via the use of
ammonia dissolved in solvents such as dioxane or methanol.13
Concerning the oxidation of diaziridine precursor 5 to the target
diazirine 3, one can choose among diverse well-established
procedures (Scheme 1).14 Formation of diazirines and their
reactions (e.g., attachment of biologically interesting compounds
for in vivo or in vitro investigations) are widely used;15 however,
to the best of our knowledge, methods allowing the generation of
this compound class on solid supports have not yet been
Figure 1. Three examples of functional groups that can be photochemi-
cally activated: benzophenones (1), azides (2), and diazirines (3).
Advantageous properties of labeling strategies with diazirines
have been proven in many biological applications. Synthesis of
the diazirine functionality and its chemical surrounding differs
slightly depending on the biological target. Aliphatic diazirines4
and aromatic derivatives5 were used, and new substituents on the
diazirine core continuously offer new applications in the field of
biology.6
The most important strategy for the generation of diazirines 3
via the oxidation of diaziridines 5 was developed in 19637 and has
since been an important reaction in the repertoire of organic
chemists. Besides some procedures demonstrating the synthesis
of diazirines from amidines,8 diazirines are usually synthesized via
a multistep procedure including the conversion of ketones into
Received: October 30, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b03252
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Organic Letters
Letter
developed. We investigated the potential of sulfonyl oxime
linkers for the generation of diaziridines, via a solid-supported
procedure, to provide an easily adaptable methodology for the
synthesis of structurally diverse molecules with diazirine labeling.
Development on solid supports aimed for the preparation of
flexible strategies for the attachment of diverse building blocks as
a basis for the combinatorial access to new diazirine−drug
conjugates.
Table 1. Solid-Supported Synthesis of Sulfonyl Ketoximes and
Their Cleavage for the Generation of Diaziridines 5
To develop solid-supported diazirine-forming procedures,
relying usually upon tosyl oxime or mesyl oxime preparation, we
envisaged the use of polystyrene−sulfonyl chloride as a polymer
equivalent to commonly used mesyl or tosyl chloride.
Comparable to the known reactions in solution phase
(compounds 4 and 5) (Scheme 1), we converted ketones into
the corresponding ketoximes (7), via reaction with hydroxyl
amine in a first step. We show that the resulting oximes can be
immobilized via reaction with commercially available sulfonyl
chloride linker 6 to give the desired solid-supported sulfonyl
oximes 8 in one step. Several attempts to develop a procedure for
the direct attachment of ketones, without the preparation of
precursors in solution, have been investigated, but the attachment
of presynthesized building blocks have been shown to be superior
to the direct immobilization of ketones onto sulfonyl hydroxyl-
amine linkers. While several different ketones have been used in
the past for similar conversions, α,α,α-trifluoroacetophenones
were selected as a model system for the establishment of this
methodology on solid phases because the resulting diazirines
with a 3-trifluoromethyl-3-phenyldiazirine core were investigated
with respect to their use as photoactivatable reagents and their
biological applications.16 The presence of the trifluoromethyl
group is particularly advantageous for the procedure presented
herein as it facilitates the cleavage of the immobilized precursors
from the solid supports and therefore allows good results for
diazirine formation. In addition, the CF3 group is known to
prevent the generation of byproducts during photolysis.13,17
The functional cleavage with ammonia/dioxane solution13 was
used in a second step to obtain a set of substituted diaziridines 5
(Table 1). Due to the use of a saturated solution of ammonia in an
organic solvent, instead of liquid ammonia via condensation, the
experimental effort has been minimized. This procedure allows,
in combination with the use of a solid support as starting material,
the application of this protocol to the synthesis of functionalized
molecules in a combinatorial manner by parallel reactions. We
have also been able to show that the described procedure is
suitable for the transformation of aromatic trifluoroacetophe-
nones with diverse substitution patterns and functional groups,
furnishing diaziridines in very good yields (compounds 5g and
5j) (Table 1).
yield, %
entry
8
R1
R2
R3
5
(6 to 8/8 to 5)
1
2
3
4
5
6
7
8
8a
8b
8c
8d
8e
8f
H
H
H
H
H
H
H
H
5a
5b
5c
5d
5e
5f
72 (88/82)
H
OMe
Cl
80 (quant/80)
63 (quant/63)
70 (quant/70)
69 (quant/69)
77 (quant/77)
96 (quant/96)
H
H
Br
H
−CCTMS
−CCH
H
−(CH)4−
H
8g
8h
5g
5h
a
H
1′-OMe-
63 (57/quant)
phenyl
9
8i
8j
H
H
H
H
4-bromophenyl 5i
4- 5j
ethinylphenyl
79 (98/81)
10
97 (quant/97)
a
Explanation for the discrepancy between the yield of single-step
calculations and overall yields is given in the Supporting Information.
Scheme 2. Strategy for the Syntheses of Immobilized Alkyne
and Amine Derivatives for the Syntheses of Complex
Molecules with Diazirine-Labeled 5l−z
azides and carboxylic acids or their chlorides are readily available
and commercial reagents can be gained easily, these reactions can
be used for efficient strategies on solid supports. The necessary
free amine functionality for an amidation strategy was gained by
immobilizing previously synthesized 4-aminophenyl trifluoroe-
thanone oxime (see Supporting Information) to solid support 8k
(Scheme 2). Both of the functionalized immobilized building
blocks, the alkyne-functionalized resin 8f and the amine
derivative 8k, were further modified on-bead in subsequent
experiments. The investigations presented here describe the first
CuAAC reactions and amidations involving sulfonyl oximes
(while retaining the sulfonyl oxime functionality).
Since CuAAC reactions can be performed under mild
conditions even on solid supports, they form a suitable method
for the efficient attachment of further functional groups, linkers,
or even small biologically active molecules. Out of many
protocols for the CuAAC reaction in solution,18 one of the
most common procedures using CuI without any additional
ligands was selected for the planned on-bead transformation. In a
typical on-bead CuAAC reaction, using CuI in DMF with
addition of water as solvent, diverse functionalized azides were
successfully immobilized on solid supports (resin-bound alkyne
8f) and then cleaved as triazole-derived diaziridines (Scheme 3).
Building block 1-(4-ethynylphenyl)-2,2,2-trifluoromethylketox-
Compatibility of the immobilization and cleavage protocols
with several aromatic building blocks (8a−d, 8g−i) and alkyne
groups (8e,f,j) has been shown. Target compound 5f (Table 1,
entry 6) is of particular interest for the improvement and
application of the developed method because the generated
linker 8f offers the possibility to immobilize additional building
blocks via Cu-catalyzed azide alkyne cycloaddition (CuAAC)
reactions.
Beyond the introduction of additional building blocks in
CuAAC reactions, the preparation of a sulfonyl-oxime-bound
functional group for the modification of building blocks via amide
formation was intended (Scheme 2). Advantages of CuAAC
reactions and amidations revealing these transformations to be
highly suitable for the attachment of new building blocks are mild
reaction conditions and easily accessible reagents. Since many
B
DOI: 10.1021/acs.orglett.6b03252
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Scheme 3. Formation of 1,2,3-Triazole-Substituted
Diaziridines via Modification of Resin-Bound Sulfonyl
Oximes in CuAAC Reactions
Scheme 4. Syntheses of Diaziridines 5v−5z via Functional
Cleavage of Resin-Bound Amides
*
Yield calculated over all steps on solid phases, starting with polymer
supported sulfonyl chloride (6).
of diaziridines with an unprotected alkyne functionality (5f, 5j)
(Table 1), as the oxidation with iodine resulted in the occurrence
of diverse byproducts (Scheme 5). Concerning the latter
*
Yields refer to the conversion of 8f to 5l−5u over two steps (see
Supporting Information for details of the calculation of yields on solid
supports).
Scheme 5. Oxidation of Diaziridines 5 Gained via Solid Phase
Synthesis To Give Corresponding Diazirines 3
ime, which was necessary for the synthesis of alkyne-modified
sulfonyl oxime resin 8f, was gained in a three-step synthetic
sequence (for the synthesis of resin 8f, see the Supporting
Information). We demonstrated that azides without any
functionalization (target compounds 5n, 5s, 5t; Scheme 3), as
well as functionalized derivatives with the option for further
modifications (5l, 5o−q, Scheme 3) can be attached successfully
to the solid support. Additionally, synthesis of diaziridines 5r and
5u (Scheme 3) demonstrates the integration of naturally
occurring building blocks (e.g., sugar units or nucleosides). As
the immobilized sulfonyl oximes are prone to react with strongly
nucleophilic reagents, building blocks bearing such function-
alities have to be protected adequately prior to their use in this
protocol.
Besides the on-bead modification via CuAAC reaction, we
aimed for the development of amidations on solid support by
conversion of immobilized amines with carboxylic acid
derivatives. The building block necessary for the synthesis of
amine-derived sulfonyl oxime resin 8k was obtained in a synthetic
sequence consisting of four steps (for synthesis of resin 8k, see
the Supporting Information). To avoid any influence of
additional coupling reagents on the sulfonyl-based linker during
the amidation, a conversion of the resin-bound amine with acid
chlorides was chosen in initial experiments. Reactions of the
linker 8k with four different acid chlorides furnished the
immobilized amides 8v, 8w, 8y, and 8z. In addition, conversion
of 8k as an example with a carboxylic acid and the peptide
coupling reagents DIC/HOBT has been demonstrated success-
fully (5x) (Scheme 4). Subsequently, all of the resin-bound
amides were cleaved from the solid phases by addition of
ammonia dioxane solution, and the corresponding diaziridines
5v−5z were obtained in moderate to good yields (Scheme 4).
A selection of the diaziridines that were synthesized during this
work was oxidized to give the diazirines 3a−3j. In the chosen
protocol, which is used as a standard procedure for most of the
diaziridine to diazirine conversions, oxidation is conducted with
elemental iodine (3.3 equiv) in diethyl ether at room temper-
ature. The reaction was shown to yield good to very good results
for derivatives 3a−3f, 3i, and 3j with reaction times of 12−18 h.
The standard procedure had to be modified only for the oxidation
[a] refers to method 1), [b] refers to method 2).
diaziridines, the reaction with freshly prepared manganese
dioxide was found to be suitable for a conversion to give the
diazirines 3g and 3h in very good yields (Scheme 5).
Since the synthesis of the diazirines primarily aims for the
investigation of receptor−ligand interactions via covalent binding
of a carbene precursor to a biological target, the successful
generation of the active carbene species is a prerequisite for the
obtained diazirines. Successful activation of the diazirine
functionality and its cleavage for the generation of carbenes was
shown with two diaziridines, which were isolated after their
formation on solid phases. For this purpose, the corresponding
diazirines 3a and 3f were photochemically activated by exposure
to irradiation in methanol-d4. It was proven that the target
compounds 10a and 10b can be obtained in very good yield and
purity by irradiation with UV light at 360 nm, in accordance with
known reactions of comparable systems,19 which enables them
for a use in labeling experiments (Scheme 6).
To the best of our knowledge, the present study describes for
the first time the synthesis of diazirines via a solid-supported
procedure. The given examples show that a synthetic sequence
consisting of (1) attachment of ketoximes to the solid phases, (2)
functionalization of the immobilized building blocks, (3)
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ASSOCIATED CONTENT
* Supporting Information
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compounds and intermediates and copies of H and 13C
spectra of the obtained diaziridines and diazirines (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
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*E-mail: nicole.jung@kit.edu.
*E-mail: braese@kit.edu.
ORCID
Stefan Brase: 0000-0003-4845-3191
̈
Present Address
§Molekulare Physikalische Chemie, Heinrich-Heine-Universitat
̈
Dusseldorf, 40225 Dusseldorf, Germany.
̈
̈
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Helmholtz program
Biointerfaces in Technology and Medicine (BIFTM). Research
data have been stored and registered with support of the DFG-
funded project Chemotion.
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