Synthesis of Bifunctional Potassium Acyltrifluoroborates

Article abstract of DOI:10.1021/acs.orglett.6b02652

New bifunctional potassium acyltrifluoroborate (KAT) substrates have been synthesized in gram scale using optimized reaction conditions. Chemoselective transformation of functional groups in the presence of an acyltrifluoroborate has been demonstrated, an

Full text of DOI:10.1021/acs.orglett.6b02652

Letter
pubs.acs.org/OrgLett
Synthesis of Bifunctional Potassium Acyltrifluoroborates
Sizhou M. Liu, Dmitry Mazunin, Vijaya R. Pattabiraman, and Jeffrey W. Bode*
Laboratorium fur Organische Chemie, Department of Chemistry and Applied Biosciences, ETH Zurich, 8093 Zurich, Switzerland
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* Supporting Information
ABSTRACT: New bifunctional potassium acyltrifluoroborate
(KAT) substrates have been synthesized in gram scale using
optimized reaction conditions. Chemoselective transformation
of functional groups in the presence of an acyltrifluoroborate
has been demonstrated, and orthogonal reactions of bifunctional KAT reagents are reported. This allows for the incorporation of
KAT moieties into peptides and dyes.
Scheme 1. Synthetic Access to Bifunctional KATs
hemoselective ligation reactions have transformed the
Cpractice of organic synthesis by enabling covalent bond
construction without the need for reagents or protection of
common functional groups.1 Easy to use chemistry such as
azide−alkyne click reactions and thio-Michael additions are
commonly employed not only for organic chemistry but also in
biology and materials.2 The popularity of these methods,
particularly azide/alkyne reactions, lies in the facile introduction
of the requisite functional groups.
Recently, we reported ligations from potassium acyltrifluor-
oborates (KATs), a unique class of functional groups that
undergo rapid, chemoselective amide-forming reactions with
hydroxylamines under aqueous conditions without the need for
activating agents or protecting groups.3 With second-order rate
constants of up to 20 M−1 s−1 under acidic conditions and
complete compatibility with azides, strained alkynes, thiols, and
acrylates, this reaction complements contemporary conjugation
methods.4−6 The KAT functional groups are extremely stable
and can be introduced into simple organic molecules by several
methods. However, their use for bioconjugation is currently
hampered by the lack of multifunctional reagents for
incorporating KATs into more complex molecules.7
In an effort to expand the synthetic repertoire of bifunctional
KAT reagents to meet the demand for ease of incorporation
into organic molecules, we envisioned bifunctional reagents
that bear the unique KAT functionality as well as an easily
modifiable functional group that can undergo chemoselective
reaction in the presence of the KAT. We now report the
preparation and use of six key reagents for this purpose
(Scheme 1), suitable for reactions including copper-catalyzed
azide−alkyne cycloaddition (CuAAC), acylation, Wittig reac-
tion, alkylation, and nucleophilic substitution reactions.
To date, we have employed only one bifunctional KAT for
elaborating other molecules. Potassium 4-fluorobenzoyl-
trifluoroborate is an excellent substrate for nucleophilic
aromatic substitution (SNAr) reactions due to the electron-
withdrawing nature of the KAT moiety. Using this reagent, our
group successfully prepared KATs bearing PEG-chains as well
as markers including dyes and biotin.4 These studies
demonstrated that KATs can tolerate different reaction
conditions and reagents, including strongly basic and strongly
acidic conditions, and encouraged us to prepare a number of
other bifunctional reagents.
As depicted in Scheme 1, two general methods for KAT
synthesis were exploited for the synthesis of such bifunctional
reagents. In the first approach, KATs were accessed through
lithium−halogen exchange and the subsequent quenching of
the metalated species with reagent 27 (Scheme 2). This reagent
allows for the synthesis of a broad range of aromatic KATs
including phenyl, pyridine, quinoline, and thiophene KAT
substrates in one step and has been adapted by Sigma-Aldrich
for commercial purposes (CAS No.: 1622923-36-7). The
second approach, via benzotriazole chemistry, allows access to
both aromatic and aliphatic KATs by utilizing the N,O-acetal as
an acyl anion equivalent.8
Received: September 4, 2016
© XXXX American Chemical Society
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Letter
ature to −110 °C had a favorable effect on the overall yield of
the reaction. Using a modified procedure with a prolonged
lithiation time, 11 was isolated in 62% yield and with high
purity. Subsequent substitution of the chloride with an azide
provided a handle for further functionalization via a CuAAC
reaction. KAT 13 was synthesized in a one-pot fashion
including an in situ TMS deprotection (see Supporting
Information). The overall isolated yield for the alkyne KAT
formation was 55%.
Scheme 2. Preparation of Bifunctional KATs
In order to demonstrate the orthogonality of the newly
installed functional group in the presence of the acyltrifluor-
oborate, chemoselective reactions were performed for each of
the substrates (Scheme 3). In some cases, the product mixtures
Scheme 3. Chemoselective Transformation of Reagents
Bearing Acyltrifluoroborate Moieties
The reagent-based approach was used to prepare a KAT
bearing an activated ester via the route shown in Scheme 2a.
Hydrolysis of 3 gave the carboxylate salt, which was easily
converted to the corresponding perfluorophenyl ester in
excellent yield. For preparation of the structurally related
KAT aldehyde 8, we found that the best, although still low
yielding, approach was via the benzotriazole chemistry, as
shown in Scheme 2b. In the original procedure,9 the
deprotonation step was carried out at −78 °C, but we found
that these conditions led to decomposition of the lithiated
intermediate. Decreasing the temperature to −110 °C proved
more effective, but required a prolonged lithiation step to
ensure complete consumption of n-butyllithium to avoid the
formation of the side product n-BuBF3K, upon quenching with
aqueous KHF2.
The synthesis of bifunctional, aliphatic KATs begins with
readily available starting material 1-(phenoxymethyl)-1H-
benzotriazole 9, which is deprotonated with n-buthyllithium,
resulting in an anion which is allowed to react with a variety of
alkyl halides.10 The resulting KAT precursors (e.g., 10) were
deprotonated a second time, trapped with B(OMe)3, and
converted to the potassium acyltrifluoroborates. While the
synthesis of the precursors were straightforward, the subse-
quent reactions to form the KATs often led to poor or no yield
of the desired product when following the original protocol.
For instance, KAT 11 was isolated in 30% yield while KAT 13
was only observed in trace amounts. Decreasing the temper-
could not be purified by column chromatography and relied
instead on a trituration-based purification. As such, full
consumption of the KAT-containing starting material is vital,
and these were kept as limiting reagents throughout. Refluxing
KAT 8 and reagent 15 for 16 h gave exclusively the Wittig
product 16, which was isolated by trituration in 77% yield with
an E/Z- isomer ratio of 36:1.
O-Alkylation of the phenol KAT 12 with coumarin 1711 was
performed in acetone in the presence of a slight excess of
Cs2CO3 to give 18 in good yield. Substitution of 19 with 12
gave the protected hydroxylamine-KAT 20 in good yield,
demonstrating that that substitution works equally well on
aliphatic mesylates.
Encouraged by these findings, we attempted to modify larger
molecules using the bifunctional KATs and test their utility in
sequential coupling reactions (Scheme 4a). Alkyne 13 could be
joined with fluorescein azide 21,12 followed by in situ KAT
ligation to give amide 23.
The same approach was repeated with azide 14 and alkyne-
rhodamine 2413 to form compound 25. These ligations were
performed in situ, as purification and isolation of the KAT-
CuAAC products proved challenging. Interestingly, the 1,3-
dipolar cycloaddition using copper(II) sulfate and sodium
ascorbate as the Cu (I) source failed to give the desired
product, yet the reaction proceeded to completion in the
presence of an equimolar amount of copper(I) iodide and an
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Scheme 4. Orthogonal Reaction of Potassium Acyltrifluoroborates
amine base. Adapting the protocol described above allowed
cycloaddition and KAT ligation with O-carbamoyl hydroxyl-
amine 224 to form the corresponding amide adduct in a one-
pot fashion. Purification by preparative HPLC afforded
compound 23 in 59% and 25 in 56% isolated yield.
To demonstrate that this strategy is also applicable to the
modification of peptides we coupled KAT 5 to a resin-bound
17-residue segment of SUMO2-26 in the presence of DMAP in
DMF (Scheme 4b). Cleavage from the resin using TFA with 10
mM potassium acetate and subsequent ligation with a
hydroxylamine led to the isolation of the desired modified
peptide after purification by preparative HPLC. To avoid
reduction of the KAT group by hydride-based scavengers, we
found that cleavage with TFA alone or with electron-rich
aromatic scavengers is preferred. The addition of a potassium
salt, in this case KOAc, was also beneficial for the stability of the
product. This was particularly true during preparative HPLC,
where we found it essential to use a buffer system containing
potassium for isolation of KATs. This straightforward protocol
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(3) Dumas, A. M.; Molander, G. A.; Bode, J. W. Angew. Chem., Int.
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allows for incorporation of KATs into peptides and subsequent
labeling or conjugation.
(4) Noda, H.; Ero
̋
s, G.; Bode, J. W. J. Am. Chem. Soc. 2014, 136,
In summary, we have developed convenient syntheses of
bifunctional KAT reagents and demonstrated that the KAT
groups are stable to a wide range of transformations. We also
improved the benzotriazole route to aliphatic KATs, which
complements our established reagent-based route for the
synthesis of neutral and electron-deficient aromatic KATs. Six
new bifunctional KAT reagents were developed by these routes,
allowing for the incorporation of the KAT moiety into more
complex molecules, including peptides. These reagents can be
prepared on a multigram scale in a few synthetic steps from
readily available starting materials. As showcased by their use in
the incorporation of KATs into peptides and dyes, these new
bifunctional molecules will allow for expansion of the current
library of potassium acyltrifluoroborates, a class of molecules
with great potential for bioconjugation and convergent
synthesis.
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.6b02652.
Experimental procedures and characterization of bifunc-
tional KAT reagents (PDF)
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: bode@org.chem.ethz.ch.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by an ETH Research Grant (ETH-43
13-2). We thank the LOC Mass Spectrometry Service and the
LOC NMR Service for analysis and Iain Stepek (ETH) for
helpful discussions.
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