Chemoselective Sequential Click Ligations Directed by Enhanced Reactivity of an Aromatic Ynamine

Article abstract of DOI:10.1021/acs.orglett.6b00635

Aromatic ynamines or N-alkynylheteroarenes are highly reactive alkyne components in Cu-catalyzed Huisgen [3 + 2] cycloaddition ("click") reactions. This enhanced reactivity enables the chemoselective formation of 1,4-triazoles using the representative aro

Full text of DOI:10.1021/acs.orglett.6b00635

Letter
pubs.acs.org/OrgLett
Chemoselective Sequential Click Ligations Directed by Enhanced
Reactivity of an Aromatic Ynamine
Marine Z. C. Hatit,† Joanna C. Sadler,†,‡ Liam A. McLean,† Benjamin C. Whitehurst,†,‡ Ciaran P. Seath,†
Luke D. Humphreys,‡ Robert J. Young,‡ Allan J. B. Watson,*,† and Glenn A. Burley*,†
†Department of Pure and Applied Chemistry, WestCHEM, University of Strathclyde, Glasgow, G1 1XL, U.K.
‡GlaxoSmithKline, Medicines Research Centre, Gunnels Wood Road, Stevenage, Hertfordshire, SG1 2NY, U.K.
S
* Supporting Information
ABSTRACT: Aromatic ynamines or N-alkynylheteroarenes are highly reactive alkyne components in Cu-catalyzed Huisgen [3 +
2] cycloaddition (“click”) reactions. This enhanced reactivity enables the chemoselective formation of 1,4-triazoles using the
representative aromatic ynamine N-ethynylbenzimidazole in the presence of a competing aliphatic alkyne substrate. The unique
chemoselectivity profile of N-ethynylbenzimidazole is further demonstrated by the sequential click ligation of a series of highly
functionalized azides using a heterobifunctional diyne, dispelling the need for alkyne protecting groups.
he Cu-catalyzed alkyne−azide cycloaddition (CuAAC) or
T“click” reaction is a powerful and robust method for the
rapid synthesis of 1,4-substituted triazoles.1−4 The bio-
orthogonality of this [3 + 2] cycloaddition reaction between
a terminal alkyne and an azide has been deployed for the
preparation of multifunctional biomaterials,5−7 as well as the
construction of discrete bioconjugates to probe cellular
processes.8−10 Despite its widespread use in these fields, the
exploration of chemoselectivity profiles of alkyne and azide
reagents has been limited.11 Zhu et al. reported a chelate-
directed strategy that highlighted the enhanced reactivity of
azide groups in close proximity to a metal-chelating atom
(Figure 1a).12,13 This chelate-assisted approach enhances the
reaction rate of picolyl azides and provides a reproducible
platform for sequential ligation of two different alkynes. At
present, the development of a cognate sequential ligation
strategy based on reactivity differences of terminal alkynes has
Figure 1. Protecting group-free chemoselective click reactions. (a)
Azide selectivity using chelate assistance. (b) Alkyne selectivity using
SPAAC. (c) Alkyne selectivity using the aromatic ynamine N-
ethynylbenzimidazole.
been confined to the use of protection/deprotection
methods,14−17 which disregards any potential reactivity
preferences that may exist between alkyne subtypes.
Strain-promoted alkyne−azide cycloaddition (SPAAC) offers
a copper-free route to discriminate between alkyne subtypes
(Figure 1b);18−21 however this produces a regioisomeric
mixture of triazole products, which is undesirable if discrete
products are required. Here, we describe an unprecedented
chemoselectivity profile of the aromatic ynamine N-ethynyl-
benzimidazole as a terminal alkyne surrogate in CuAAC
reactions (Figure 1c). Furthermore, we demonstrate the utility
of our strategy as a novel platform for sequential ligation.
Terminal ynamides have been previously shown to be
excellent substrates in a variety of Cu-catalyzed C−C bond
formations, such as Glaser couplings18−20 and nucleophilic
addition to mild electrophiles such as acyl chlorides and
chloroformates at room temperature and in high yield.21
Received: March 5, 2016
© XXXX American Chemical Society
A
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Letter
Furthermore, terminal ynamide substrates such as N-
tosylynamides have also been employed in CuAAC reactions
and tandem variations,22,23 although terminal N-ethynyl
imidazolidinones and N-ethynyl 1,3-oxazolidinones were
shown to decompose under conventional click chemistry
conditions.24 We surmised that the electronic bias present in
N-alkynylheteroarenes would strike a balance between
enhanced reactivity in CuAAC reactions relative to an aliphatic
alkyne. Indeed, in our previous work we observed an unusually
fast rate of reaction between 1 and 2a to form 3a,25 suggestive
of reactivity differences existing between 1 and 2a correspond-
ing to an aliphatic alkyne that could be used to undergo a
CuAAC reaction chemoselectively.
either MeCN or MeOH (Scheme 2). The reaction conditions
tolerated a variety of azides of varying steric bulk (e.g., b, c, d),
Scheme 2. Click Reactions of 1 with a Variety of Azides (Fc
= ferrocene)
To test this hypothesis, a reaction screen was undertaken
using 1 and benzyl azide 2a. The parameters of a copper source,
solvent, reductant, and ligand were surveyed (see Supporting
Information). The use of Cu(I) and Cu(II) salts were effective
in catalyzing this reaction (Table S1). We found that 5 mol %
Cu(OAc)2 was sufficient to provide high conversion to product
3a in MeCN (92%) or MeOH (quantitative; Scheme 1). The
Scheme 1. Click Reactions of 1 and Aliphatic Alkyne 4 with
Benzylazide 2a
a
b
Isolated yield. NMR yield.
a
Isolated yields.
oxidizable groups (e, l), potentially Cu-chelating substrates (e,
f, h, i, l, m, n), and fluorophores (m, n).
reaction in MeOH was notably faster, requiring only 2 h for the
consumption of 1 and the concomitant formation of product 3a
by HPLC analysis compared to 4 h for MeCN. In contrast, the
corresponding reaction using aliphatic alkyne 4 under
equivalent conditions in either MeOH or MeCN delivered
no product after 4 h with reactions requiring 16 h in MeOH to
proceed to useful levels of conversion (Scheme 1b and Table
S2).
A solvent screen for the reaction of alkyne 4, using a standard
16 h reaction time and Cu(OAc)2, afforded triazole product 5a
only when MeOH (76%), DMSO (79%), and aqueous
mixtures (1:1 MeOH/H2O, 88%; DMSO/H2O, 84%) of
these two solvents were used (Table S2). Solvents such as
MeCN, EtOH, i-PrOH, and DMF using Cu(OAc)2 did not
produce product 5a after 16 h. As expected, standard CuAAC
conditions using CuSO4, a reductant (NaAsc), and TBTA
produced 5 in quantitative yield. Taken collectively, the
reaction rate of 1 in CuAAC reactions was considerably faster
than the aliphatic alkyne 4.25 Furthermore, the pairing of
Cu(OAc)2 and MeCN suggested conditions for the develop-
ment of a chemoselective platform of N-alkynylheteroarenes in
the presence of an aliphatic alkyne.
With the substrate scope of 1 established, the chemo-
selectivity profile was explored in competition experiments
using equal stoichiometries of 1 and 4 and a corresponding
azide (Scheme 3a). Using benzyl azide (2a), full conversion to
triazoles 3a was observed with high selectivity for click adduct
3a vs aliphatic alkyne adduct 5a in both MeCN (100:0) and
MeOH (90:10) (Scheme 3a (i)). Complete conversion was
observed using cyclohexyl azide 2c to form 3c, in both MeCN
and MeOH, which is likely due to the increased steric bulk of
2c slowing the rate of the CuAAC reaction.
Based on these encouraging results, we investigated whether
the enhanced reactivity of the N-ethynylbenzimidazole would
be retained in an intramolecular context. To test this
hypothesis, the bifunctional scaffold 6 was prepared (see
synthesis in Supporting Information). Two azides of functional
significance were chosen to test the chemoselectivity of N-
ethynylbenzimidazole relative to the aliphatic alkyne both
present in 6. For example, ferrocene azide 2g has been used
extensively as an electrochemical reporter,26−28 whereas the
green dye 2m is widely used as a fluorescent probe for lipid
membranes.29,30 Consistent with the intermolecular competi-
tion reactions (Scheme 3a), the reaction of 6 with 2g, followed
by 2m, resulted in the exclusive formation of 7a in 96% yield
over the two steps. A workup procedure was conducted after
The generality of 1 using a series of azide substrates was then
explored using optimized conditions of 5 mol % Cu(OAc)2 in
B
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Scheme 3. (a) Intermolecular Competition Click Reactions Using 1 vs 4 and (i) Azide 2a and (ii) Azide 2c; (b) Chemoselective
Sequential and One-Pot Click Reactions of Diyne 6
the first click reaction to confirm that the first reaction occurred
exclusively at the N-ethynylbenzimidazole. Reversing the
sequence of addition (i.e., 2m followed by 2g) produced the
reverse click product 7b in 92% yield, further exemplifying the
flexibility of the sequential ligation approach. Finally, we
showed that the formation of 7a and 7b was possible in a one-
pot process, controlled simply by the sequence of addition of
the corresponding azide. Exclusive formation of the first
CuAAC reaction at the N-ethynylbenzimidazole was observed
in both cases using 1 equiv of azide and Cu(OAc)2. Addition of
the second azide, ligand AMTC, and NaAsc resulted in the
formation of 7a and 7b (respectively 97% and 92%).
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.6b00635.
■
S
Experimental procedures, characterization data for all
compounds (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: allan.watson.100@strath.ac.uk.
*E-mail: glenn.burley@strath.ac.uk.
Notes
In summary, we have demonstrated a modular, step-efficient,
and robust sequential CuAAC-based ligation platform that
exploits the inherent differences in the reactivity of N-
ethynylbenzimidazole relative to aliphatic alkynes. Using the
bifunctional system 6 we show that these reactivity differences
enable discrimination of the two alkynes in a simple one-pot
two-step procedure. We envisage that exploiting the reactivity
differences of alkyne substrates could have the potential for
utility in bioconjugation applications,31 particularly where dual
differential modification of biomolecules32−34 or sequential
modification35−38 is required.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
G.A.B. thanks the Alexander von Humboldt Foundation
(Visiting Research Fellowship, LMU Munich). G.A.B. and
A.J.B.W. thank GlaxoSmithKline (EPSRC Industrial CASE
studentship for B.C.W. and J.C.S.) for funding. C.P.S. thanks
the Carnegie Trust for a PhD scholarship.
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