Tetrahedron Letters
A photocaged, cyclopropene-containing analog of the amino acid
neurotransmitter glutamate
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Pratik Kumar, David Shukhman, Scott T. Laughlin
Department of Chemistry, Stony Brook University, Stony Brook, New York 11794-3400, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Substituted cyclopropenes serve as compact biorthogonal appendages that enable analysis of biomole-
cules in complex systems. Neurotransmitters, a chemically diverse group of biomolecules that control
neuron excitation and inhibition, are not among the systems that have been studied using biorthogonal
chemistry. Here we describe the synthesis of cyclopropene-containing analogs of the excitatory amino
acid neurotransmitter glutamate starting from a Garner’s aldehyde-derived alkyne. The deprotected
cyclopropene glutamate was stable in solution but decomposed upon concentration. Appending a
light-cleavable group improved the stability of the cyclopropene while simultaneously caging the neuro-
transmitter. This strategy has the potential to permit deployment of cyclopropene-modified glutamate as
a bioorthogonal probe of the neurotransmitter glutamate in vivo with spatiotemporal precision.
Ó 2016 Elsevier Ltd. All rights reserved.
Received 5 October 2016
Revised 24 October 2016
Accepted 27 October 2016
Available online 11 November 2016
Keywords:
Cyclopropene
Bioorthogonal
Glutamate
Neurotransmitter
Tetrazine ligation
Cyclopropenes have become popular biorthogonal functional
groups due to their small size, inertness to biological functionality,
and reactivity with tetrazines1 and 1,3-dipoles.2 For example,
cyclopropene installation on monosaccharides3–6 and lipids7 has
promoted these biomolecules’ characterization in challenging
in vivo environs, and their use as chemical handles in proteomics
applications has facilitated analysis of proteomes with improved
spatiotemporal precision.8–10 Additionally, in efforts to employ
cyclopropene tags in studies of protein function, several groups
have synthesized amino acid analogs bearing a cyclopropene moi-
ety at the alpha carbon11 or terminal positions on amino acid side
chains, resulting in novel non-proteinogenic amino acid struc-
tures.2 However, there has been little exploration of cyclopropenes
and bio-orthogonal chemistry to analyze neurotransmitters, an
important and diverse class of biomolecules that modulate neuron
excitation and inhibition to control brain function.
analogs synthesized.13 Several of these structures were confirmed
as substrates for metabotropic glutamate receptors,14,15 which
detect glutamate release from synaptic vesicles at the neural
synapse to propagate neural signals, as well as glutamate trans-
porters that package glutamate into synaptic vesicles or remove
it from the synaptic cleft.13 These findings suggest that cyclo-
propene modified analogs of glutamate might serve as chemically
traceable neurotransmitter mimics. Here we describe the synthesis
of a light-activatable cyclopropene-containing glutamate analog
(Nvoc-cpGlu, Compound 1, Fig. 1), which holds promise to extend
the reach of biorthogonal chemistry into the realm of neurotrans-
mitter biology.
Initially, our strategy for the synthesis of cyclopropene gluta-
mate analogs employed Rh-catalyzed cyclopropenation of ethyl
diazoacetate to protected alkynyl glycine analogs (3a/b, Scheme 1).
Interestingly, our attempts at the preparation of an N-benzoyl alky-
The only documented synthesis of a cyclopropene-containing
neurotransmitter was performed by Reissig and coworkers, who
synthesized cyclopropene-containing analogs of the neurotrans-
mitter gamma-aminobutyric acid (GABA).12 However, there are
numerous neurotransmitters that have structures conducive to
modification with a reactive cyclopropene handle. For example,
glutamate, the subject of this study and an amino acid neurotrans-
mitter that leads to neuron excitation in most situations, has had a
complete panel of saturated, unreactive cyclopropane-containing
nyl glycine by addition of a tributyl tin acetylene to a-chloro-
glycine 2b according to Aldous and coworkers16,17 did not result
in the desired alkyne 3b. Instead, we observed a rapid cycloisomer-
ization-elimination affording oxazole 4b, similar to that reported
for other N-propargyl amides by Ciufolini and coworkers.18 Swap-
ping the amine protecting group from benzoyl to methyl carba-
mate, compound 2a, allowed the formation of the required
alkynyl glycine 3a. Subsequent Rh-catalyzed cyclopropenation of
ethyl diazoacetate produced a reaction mixture whose analysis
by mass spectrometry suggested the formation of cyclopropene
4a. However, our extensive efforts to purify the cyclopropene were
ultimately unsuccessful. The purification challenge resulted from
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Corresponding author.
0040-4039/Ó 2016 Elsevier Ltd. All rights reserved.