Release of Terminal Alkynes via Tandem Photodeprotection and Decarboxylation of o -Nitrobenzyl Arylpropiolates in a Flow Microchannel Reactor

Article abstract of DOI:10.1021/acs.bioconjchem.7b00812

Photocleavable protecting groups (PPGs) offer a complementary protection paradigm compared to traditional protection groups. Herein, an o-nitrobenzyl (NB) PPG was employed to protect a variety of arylpropiolic acids. Upon a cascade of light-triggered photodeprotection in a microchannel reactor (residence times of 100-500 s), followed by Cu-catalyzed decarboxylation at 60 °C, the NB-protected arylpropiolic acid afforded a terminal alkyne. This terminal alkyne was further reacted in situ with an azide via click chemistry to yield a 1,2,3-triazole in a one-pot reaction. Furthermore, the effect of different substituents (methyl, vinyl, allyl, and phenyl) at the benzylic position on the rate of photodeprotection was studied. The quantum yields of photolysis for the benzylic-substituted esters were determined to be as high as 0.45 compared to the unsubstituted ester with a 0.08 quantum yield of photolysis.

Full text of DOI:10.1021/acs.bioconjchem.7b00812

Article
pubs.acs.org/bc
Cite This: Bioconjugate Chem. XXXX, XXX, XXX−XXX
Release of Terminal Alkynes via Tandem Photodeprotection and
Decarboxylation of o‑Nitrobenzyl Arylpropiolates in a Flow
Microchannel Reactor
Behabitu Ergette Tebikachew,† Karl Borjesson,‡ Nina Kann,† and Kasper Moth-Poulsen*,†
̈
†Department of Chemistry and Chemical Engineering, Chalmers University of Technology, 41296 Gothenburg, Sweden
‡Department of Chemistry and Molecular Biology, University of Gothenburg, 41296 Gothenburg, Sweden
S
* Supporting Information
ABSTRACT: Photocleavable protecting groups (PPGs) offer
a complementary protection paradigm compared to traditional
protection groups. Herein, an o-nitrobenzyl (NB) PPG was
employed to protect a variety of arylpropiolic acids. Upon a
cascade of light-triggered photodeprotection in a microchannel
reactor (residence times of 100−500 s), followed by Cu-
catalyzed decarboxylation at 60 °C, the NB-protected
arylpropiolic acid afforded a terminal alkyne. This terminal
alkyne was further reacted in situ with an azide via click
chemistry to yield a 1,2,3-triazole in a one-pot reaction.
Furthermore, the effect of different substituents (methyl, vinyl,
allyl, and phenyl) at the benzylic position on the rate of
photodeprotection was studied. The quantum yields of
photolysis for the benzylic-substituted esters were determined to be as high as 0.45 compared to the unsubstituted ester with
a 0.08 quantum yield of photolysis.
application range has been further extended to allow use
under physiological conditions.2
INTRODUCTION
■
Protecting groups are ubiquitous in modern synthetic organic
chemistry as some functional groups are susceptible to
undesired side reactions unless they are protected beforehand.
A requirement for a good protecting group is the ability to be
introduced and removed selectively and quantitatively.1
However, there is a lack of suitable groups that fulfill these
requirements in all situations. In many cases, their deprotection
requires harsh conditions and rarely proceeds quantitatively,
limiting their range of applications. As a result, milder
alternative methods for protection and deprotection have
been investigated by many researchers. Among these, photo-
cleavable protecting groups (PPGs)2−4 stand out because of
their ability to use light as a traceless reagent to trigger the
deprotection process. Besides providing orthogonal protec-
tion,5 PGGs also allow spatial and temporal control of the
process. Hence, they are very attractive for various applications
such as controlled drug delivery,6 total synthesis,7 photo-
responsive polymers,8 and surface modification processes such
as surface patterning and photolithograhy.9−11
Terminal alkynes have a prime importance in various
chemical transformations, such as Glaser coupling,17 the
Sonogashira−Hagihara protocol,18 and click chemistry.19−21
In addition, terminal alkynes are widely used as building blocks
in various molecular transformations that are pertinent for
biological22,23 as well as molecular electronics applications.24
However, some terminal alkynes are prone to oxidation and are
inconvenient to handle because of their high volatility.25 In
addition, during palladium-catalyzed cross coupling reactions
between aryl halides and terminal alkynes, dimerization of
alkynes often occurs as a side reaction,17 reducing the yield of
the cross coupling reaction. Consequently, an excess of alkyne
has to be used. To alleviate this use of an additional alkyne, an
elegant method has been reported by Kolarovic et al.,26 using
alkynoic acids in a stoichiometric amount as a source of
terminal alkyne via in situ decarboxylation. The generated
terminal alkyne was then further reacted with an azide to yield
1,2,3-triazoles in a one-pot process. However, the carboxylic
acid functionality may present compatibility issues and
limitations in terms of the reaction conditions used. Hence,
Despite the abundance of various PPGs in the litera-
ture,2,12−14 o-nitrobenzyl (NB) has been among the most
commonly used. This protecting group was first used by
Barltrop et al. to protect various carboxylic acids.15 Kaplan et al.
also used it to protect and release biologically relevant
molecules such as adenosine triphosphate (ATP).16 The
Received: December 20, 2017
Revised: February 19, 2018
Published: February 21, 2018
© XXXX American Chemical Society
A
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Article
protecting the carboxylic acid moiety is imperative, for instance,
with a PPG.
modification applications, for which mild conditions are
required. Moreover, to investigate the role of benzylic
substitution on the rate of deprotection, various substituents,
i.e., methyl, vinyl, allyl, and phenyl, were introduced at the
benzylic position (Scheme 1b), and the results from these
studies are disclosed herein.
Gschneidtner et al.27 have demonstrated the first example of
using PPGs to protect terminal alkynes via PPG-protected
propargylic alcohols. A series of tertiary propargylic ethers
containing an NB group were prepared. Upon photoirradiation
at 365 nm under alkaline reflux conditions, various terminal
alkynes were released in satisfactory yields. However, the long
reaction times and strong basic conditions employed in this
method may limit its application. Kakiuchi and co-workers have
also presented a novel thiochromone-based photocleavable
group, employing thiochromone S,S-dioxides to protect various
types of amines, alcohols, and carboxylic acids.28 They reported
an almost quantitative photodeprotection under ultraviolet
(UV) light. More recently, the same group also applied their
new PPG in the protection of propargylic alcohols in the form
of propargylic ethers.29 However, their PPG requires multistep
synthetic procedures, and more importantly, the purpose was
not for releasing terminal alkynes in this case.
RESULTS AND DISCUSSION
■
Synthesis of Starting Materials. The synthesis of
carboxylic acid-protected terminal alkynes 1a−c (Scheme 1a)
was achieved in satisfactory yields (65−70%) using a
Sonogashira reaction between the corresponding aryl iodides
and propiolic acid, using a modified literature method.30 While,
o-nitrobenzyl alcohol was obtained from a commercial source,
its benzyl-substituted analogues 2b−e (Scheme 1b) were
prepared via the reaction of o-nitrobenzaldehyde with the
corresponding Grignard reagents. With both precursors in
hand, the esterification reaction was then pursued to couple the
two moieties. This was achieved via a microwave-assisted
Steglich esterification31 at 40 °C, furnishing 3a−e in good
yields in a relatively short reaction time (30 min) as compared
to that under conventional conditions (>3 h) (Scheme 2). The
products were easily purified by flash chromatography, without
any complications caused by the side product dicyclohexylurea.
Characterization. All products were fully characterized by
1H NMR, 13C NMR, ATR-IR, and LC−MS. The UV−visible
absorption spectra of acetonitrile solutions of 3a−e as well as
1a are shown in Figure 1. Despite the benzylic substitution, the
esters have very similar absorption spectra as well as molar
extinction coefficients (ε), with absorption maxima around 260
nm. However, relative to the reference acid 1a, they all display a
hyperchromic as well as bathochromic shift.
To the best of our knowledge, photocleavable protection of
arylpropiolic acid [1a (Scheme 1a)] for the release of terminal
alkynes has not been reported hitherto.
Scheme 1. Substituted (a) Arylpropiolic Acids and (b) o-
Nitrobenzyl Alcohols Used as Precursors in This Study
To investigate the photodeprotection process, acetonitrile
solutions of 3a−e were prepared. Photoirradiation was
performed with a hand-held UV lamp operated at 254 nm.
The photoirradiation experiments demonstrate that NB-
protected esters (3a−e) have been photolyzed little as
compared to 1a, while the latter has been photolyzed
significantly during the photoirradation experiment time. This
is expected as decarboxylation of carboxylic moieties upon near-
UV irradiation has been demonstrated in other studies,
Here, we, for the first time, employ NB PPGs for the
protection of arylpropiolic acids and demonstrate the light-
triggered release of terminal alkynes “on demand” at 60 °C.
This makes our approach attractive for future surface
Scheme 2. Microwave-Assisted Steglich Esterification Reaction and the Corresponding PPG-Protected Esters (3a−e)
B
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Bioconjugate Chemistry
Article
respectively, in the microchannel. The irradiated samples
1
were then studied by H NMR. The experiment led to 41,
74, and 100% photodeprotection for the 100, 167, and 500 s
1
UV irradiations, respectively, as shown in the H NMR spectra
in Figure 2. Moreover, the areas of the proton peaks from the
proton next to the nitro group (c) and the benzylic proton (b)
on the NB as well as the methyl doublet (a) have decreased
(Figure 2, spectrum 1), while a new aromatic peak (c′) at 7.0
ppm and a methyl peak (a′) at 2.7 ppm from one of the
photoproducts, nitrosoacetophenone, have also emerged
(Figure 2, spectrum 2). The second peak at 2.6 ppm labeled
d is most likely due to the methyl peaks of the dimer formed
from nitrosoacetophenone.37 However, because one of the
photoproducts, phenylpropiolic acid, is a carboxylic acid,38 its
protons were not observed, due to proton exchange with the
NMR solvent.
Figure 1. UV−visible absorption spectra of 1a and 3a−e in
acetonitrile.
To investigate the effect of substitution at the benzylic
position, the absorption values of 3a−e at 325 nm were plotted
versus time (Figure 3). The temporal evolution of the
highlighting the importance of protecting the carboxylic acid
moiety if one wishes to use photochemistry in the UV region.32
The implication of these results is that once the NB group is
photoremoved, the decarboxylation process is relatively
efficient. Hence, the total photodeprotection−decarboxylation
process to generate the terminal alkyne is governed by the first
photodeprotection process.
In addition, the thermal stability of 1a and 3a−e was also
tested by thermogravimetric studies. At 210 °C, a >90 wt % loss
was observed for 1a as compared to a <5 wt % loss for the
esters. This result indicates that NB-protected esters (3a−e)
are thermally more stable than 1a, showing the robustness of
our NB esters (see Figure S4).
It is known that the photoremoval of NBs at high
concentrations requires either high-intensity light33 or a large
surface area to be efficient.34,35 Henceforth, a quartz micro-
channel flow reactor (62.5 μL capacity) was utilized to
investigate the photodeprotection. The advantage of using
microchannel flow reactors is that over time, the whole solution
will be under uniform irradiation, leading to efficient photo-
reaction. Also, by controlling the flow rate and reactor volume,
one can precisely tune the photoirradiation time as desired.36
Consequently, a 5 mM CDCl3 solution of 3b was injected
through the reactor chip at rates of 2.0, 1.2, and 0.4 mL/h, with
residence times of approximately 100, 167, and 500 s,
Figure 3. Evolution of the absorption of compounds 1a−e at 325 nm
vs time during photolysis.
absorbance for the unsubstituted ester (3a) is the least
pronounced as compared to those of the compounds
Figure 2. 1H NMR spectra for the photodeprotection of 3b with a 254 nm UV lamp at a different exposure times. Spectrum 1 was recorded prior to
UV irradiation, while spectra 2−4 represent photodeprotection at residence times inside the microchannel of 100, 167, and 500 s, respectively,
corresponding to flow rates of 2, 1.2, and 0.4 mL/h, respectively. Furthermore, new peaks a′, c′, and d emerge, while peaks a−c disappear gradually.
C
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Figure 4. Picture of the chip-based microreactor setup. (a) Syringe pump for injecting the solutions into the microfluidic chamber. (b) Quartz
microreactor (62.5 μL) in which the PPG-protected esters are irradiated with a 254 nm UV lamp. (c) Collection vial that contains the reagents for
the CuAAC reaction.
Scheme 3. Tandem Reactions Involving Photodeprotection and Decarboxylation, Followed by CuAAC at 60 °C for 15 min in
CH2Cl2
substituted at the benzylic position (Figure 3). Furthermore,
the line for 3a is not straight. This may be explained by
undesired side reactions of the aldehyde byproduct. In the four
remaining molecules, as a result of the benzylic substitution, the
byproducts are less reactive ketones.
The faster rate for the allyl-substituted molecule 3d can also
be rationalized in terms of the favorable proximity of the
nitroso and allyl moiety for the nitroso-ene reaction.39 This
interaction traps the reactive nitroso moiety from undesired
side reactions, as demonstrated in the past by Pirrung et al.,
using a diene in this case.
Table 1. Optimization and Demonstration of the Scope of
the One-Pot Photodeprotection−Decarboxylation−CuAAC
Reaction
residence
time (s)
yield
a
entry
substrate
solvent
catalyst
CuI
(%)
1
3a
3b
3b
3b
3c
3d
3d
3d
3e
3e
3e
100
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
MeCN
5
2
100
167
500
100
100
100
500
100
100
100
CuI
CuI
CuI
CuI
CuI
21
27
41
19
17
3
4
5
6
b
Furthermore, assuming clean photoreaction at low photo-
7
Cu(acac)2 23
CuI 28
Cu(acac)2 20
conversion, the quantum yields of photolysis (Qp) for 3a−e
8
b
b
were determined by UV−visible (a) spectroscopy (λphotolysis
=
9
254 nm; λobs = 325 nm) to be 0.08, 0.41, 0.41, 0.44, and 0.45,
respectively (for details, see Figure S1). This result underscores
a significant improvement in Qp for the benzylic substituted
esters (3b−e) in comparison to that of the unsubstituted ester
(3a).
10
11
CuI
CuI
28
no
reaction
12
13
1a
not
CH2Cl2
CH2Cl2
CuI
CuI
45
available
ethynylbenzene not
available
54
To further demonstrate the release of the terminal alkyne
from the NB-protected arylpropiolates, a tandem photo-
deprotection−decarboxylation reaction followed by a copper-
catalyzed alkyne−azide cycloaddition (CuAAC) “click” reaction
was performed; 1 mM solutions of NB-protected phenyl-
propiolic acids 3a−e were injected into a flow microchannel
reactor chip (62.5 μL) while being irradiated with a 254 nm UV
lamp to trigger the photodeprotection of NB (Figure 4).
Then, the collected solution was transferred directly to a vial
a
Overall yields for the three step-reaction shown in Scheme 3
determined by 1H NMR, using methyl 3,5-dinitrobenzoate as the
internal standard. Yields are given as an average from two
b
experiments.
unsubstituted ester 3a (entry 1) is rather low. At lower flow
rates, the yields improve accordingly (entries 3, 4, and 8). In
particular, for 3b (entry 4) at a flow rate of 0.4 mL/h (500 s
residence time), the yield was significantly improved to 41%
after the three-step cascade photodeprotection−decarboxyla-
tion−CuAAC reaction, becoming comparable to that of the
control reaction of 1a (entry 12).
The versatility of our substrate has also been tested with a
Cu(acac)2-catalyzed decarboxylation−CuAAC reaction (entries
7 and 9). The desired triazole click product was obtained with
this method, as well, with yields comparable to those of the CuI
method at the corresponding flow rates. When acetonitrile was
used as a solvent instead of dichloromethane, the click reaction
containing a copper salt [CuI or Cu(acac) ], Hunig’s base, and
̈
2
acetic acid, followed by the addition of azidoanisole at 60 °C,
and reacted for 15 min (Scheme 3). LC−MS analysis as well as
1H NMR studies indicated the formation of the desired
cycloaddition product, [1-(4-methoxyphenyl)-4-phenyl]-1H-
1,2,3-triazole (4), in the crude reaction mixture (see the
Supporting Information for 1H NMR data). The cascade
reaction proceeded in modest yields for most of the substrates
at a flow rate of 2 mL/h (Table 1, entries 2, 5, 6, and 10) using
CuI as the catalyst (Table 1). At this flow rate, the yield for the
D
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did not yield the desired product (entry 11). This could be
explained by the inefficiency of the CuAAC click reaction in
dilute solutions.40 One reason that the click reaction works in
dichloromethane is that the solvent evaporates partially at the
temperature employed (60 °C), leading to a more concentrated
solution, which facilitates the reaction.
Hence, besides controlling the residence time through the
microchannel reactor, we found in our study that choosing the
right type of solvent is important for the entire photo-
deprotection−decarboxylation−click reaction cascade process
to proceed efficiently.
5:95) with 0.1% HCO2H. UV−visible spectra were recorded on
a PerkinElmer Lambda 900 UV/vis/NIR spectrometer.
Synthetic Procedures. General Procedure for the
Preparation of Various Arylpropiolic Acids via Sonogashira
Reaction. To a two-neck round-bottom flask provided with a
stirring bar containing a mixture of aryl iodide (10 mmol),
Pd(PPh3)2Cl2 (2 mol %), CuI (4 mol %), and DMF (15 mL)
was added propiolic acid (12 mmol) followed by the dropwise
addition of diisopropylamine under nitrogen. The mixture was
stirred for 5 h. At this point, the content was diluted with ethyl
acetate and passed through a short plug of silica gel. The filtrate
was washed with a cold aqueous KOH solution (1 M, 100 mL)
to extract the product, which was subsequently acidified with a
cold HCl solution (1 M) kept in an ice bath. Then, the organic
phase was extracted with dichloromethane and washed with
water and brine consecutively in a separatory funnel; the
organic phase was separated and dried over anhydrous MgSO4
and filtered, and the solvent was removed using a rotary
evaporator to yield the desired product.
Phenylpropiolic Acid (1a). Phenyl iodide (2.04 g, 10 mmol)
was reacted with propiolic acid (0.79 mL, 12 mmol),
Pd(PPh3)2Cl2 (2 mol %), CuI (4 mol %), and diisopropylamine
(15 mL) as described in the general procedure to afford the
product as an off-white solid (0.95 g, 65%). NMR data were
consistent with literature data:30 1H NMR (400 MHz, CDCl3)
δ 7.57 (m, 2H), 7.42 (m, 1H), 7.35 (m, 2H); 13C NMR (101
MHz, CDCl3) δ 156.2, 133.1, 130.6, 128.6, 120.0, 86.3, 81.3.
p-Bromophenylpropiolic Acid (1b). p-Bromophenyl iodide
(2.83 g, 10 mmol) was reacted with propiolic acid (0.79 mL, 12
mmol), Pd(PPh3)2Cl2 (2 mol %), CuI (4 mol %), and
diisopropylamine (15 mL) as described in the general
procedure to afford the product as a gray solid (1.53 g, 68%).
NMR data were consistent with literature data:41 1H NMR
(400 MHz, CDCl3) δ 7.52 (m, 2H), 7.44 (m, 2H); 13C NMR
(101 MHz, CDCl3) δ 155.9, 134.5, 132.1, 125.5, 118.8, 85.4,
82.0.
CONCLUSION
■
In summary, a series of o-nitrobenzyl phenylpropiolates was
synthesized for investigation of light-triggered release of
terminal alkynes. The photodeprotection at 254 nm was
followed by 1H NMR, indicating the release of phenylpropiolic
acid, which further underwent copper-catalyzed decarboxylative
cleavage in situ to yield a terminal alkyne, phenylacetylene. In
addition, the benzylic position of the NB group was modified
with methyl, vinyl, allyl, and phenyl moieties. The quantum
yields of photolysis for the benzylic substituted esters (3b−e)
were determined to be as high as 0.45. As a proof of concept,
solutions of the alkyne esters were pumped via a flow
microchannel reactor while being irradiated with a 254 nm
UV lamp to increase the efficiency of photodeprotection. At a
rate of 0.4 mL/h (500 s residence time), quantitative
photodeprotection of the NB group was found. A tandem
photodeprotection−decarboxylation reaction, followed by Cu-
catalyzed click reaction on the liberated alkyne in situ with
azidoanisole, afforded the anticipated 1,4-disubstituted 1,2,3-
triazole product at 60 °C. This way of releasing terminal
alkynes could find potential applications in organic synthesis as
well as in surface modification by incorporation of thiol-
functionalized substrates.
p-Aminophenylpropiolic Acid (1c). Boc-p-aminophenyl
iodide (1.5 g, 4.9 mmol) was reacted with propiolic acid (0.4
mL, 6 mmol), Pd(PPh3)2Cl2 (2 mol %), CuI (4 mol %), and
diisopropylamine (10 mL) as described in the general
procedure to afford the product as an off-white solid (0.9 g,
70%): 1H NMR (400 MHz, DMSO) δ 7.66 (m, 2H), 7.55 (m,
2H), 1.48 (s, 9H); 13C NMR (101 MHz, DMSO) δ 164.4,
163.1, 152.9, 144.4, 128.5, 123.1, 96.5, 90.9, 90.04.
General Procedure for the Synthesis of o-Nitrobenzyl
Alcohols (2e−h). To a stirring solution of o-nitrobenzaldehyde
(33 mmol) in THF (70 mL) at −78 °C in an acetone/liquid
nitrogen bath was added dropwise a Grignard reagent (35
mmol). The color of the solution changed from yellow to
brown, and the mixture was allowed to react for 1 h. Then, the
solution was warmed to room temperature and left to stir
overnight. To this solution was added diethyl ether (50 mL),
and the reaction was quenched with a saturated ammonium
chloride solution. Any precipitated solids were filtered off, and
the solvents were removed using rotary evaporation. The crude
product was re-dissolved in dichloromethane (30 mL) and
washed with distilled water and brine in a separatory funnel.
The organic phase was then collected, dried over anhydrous
MgSO4, and filtered. The solvent was removed using a rotary
evaporator to afford a crude product, which was subsequently
purified by automated flash chromatography with a gradient of
0 to 10% ethyl acetate in hexane.
EXPERIMENTAL SECTION
■
Materials and Methods. Starting materials were purchased
from commercial suppliers and used without further
purification unless stated otherwise. All dry solvents were
obtained using an mBraun MB SPS-800 solvent purification
system. Dichloromethane was dried over 3 Å molecular sieves.
All reactions were performed under nitrogen media in a fume
hood. The progress of the reaction was followed by thin layer
chromatography (TLC) on precoated aluminum plates, Merck
Silica gel 60 F254, and the spots were visualized with the aid of a
hand-held UV lamp (254 and 365 nm). Phenylpropiolic acid
was prepared starting from iodophenol and propiolic acid via a
Sonogashira reaction. Purification of the compounds was
performed using automated flash chromatography (Biotage
Isolera Spektra One). 1H and 13C NMR spectra were recorded
by an automated Agilent (Varian) MR 400 (400 MHz)
spectrometer equipped with a “One-probe”. The chemical shifts
are given in parts per million downfield from tetramethylsilane
using the residual solvent signal as a reference. IR spectra were
acquired on a PerkinElmer Frontier FTIR spectrometer
equipped with an ATR device. The data were processed with
Spectrum software. LC−MS (ESI) data were acquired on a
PerkinElmer PE SCIEX API 150 EX instrument with a
Turbolon spray ion source equipped with a Gemini 5 mm
RP-C18 110 Å column, using a H2O/MeCN gradient (95:5 to
E
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1-(2-Nitrophenyl)ethan-1-ol (2b). Methylmagnesium bro-
mide (3 M in diethyl ether, 11.7 mL, 35 mmol) was used,
affording 2e as a yellowish oil (1.52 g, 28%). NMR data were
consistent with literature data:42 1H NMR (400 MHz, CDCl3)
δ 7.84 (dd, 1H), 7.65 (tdd, J = 7.9, 1.4, 0.5 Hz, 1H), 7.42 (ddd,
J = 8.1, 7.3, 1.5 Hz, 1H), 5.41 (q, J = 6.4 Hz, 1H), 1.57 (d, J =
6.4 Hz, 3H); 13C NMR (101 MHz, CDCl3) δ 148.0, 141.0,
133.7, 128.2, 127.7, 124.4, 65.7, 24.3.
1-(2-Nitrophenyl)prop-2-en-1-ol (2c). Vinylmagnesium bro-
mide (1 M in THF, 35 mL, 35 mmol) was used, affording 2f as
a yellowish oil (3.86 g, 65%). NMR data were consistent with
literature data:43 1H NMR (400 MHz, CDCl3) δ 7.91 (m, 1H),
7.76 (m, 1H), 7.64 (tdd, J = 7.4, 7.4, 1.3, 0.5 Hz, 1H), 7.44 (m,
1H), 6.07 (dddd, J = 17.2, 10.5, 5.2, 0.8 Hz, 1H), 5.79 (dt, J =
5.4, 1.4, 1.4 Hz, 1H), 5.41 (dtd, J = 17.2, 1.4, 1.3, 0.8 Hz, 1H),
5.26 (dtd, J = 10.5, 1.3, 1.3, 0.8 Hz, 1H), 2.63 (bs, 1H); 13C
NMR (101 MHz, CDCl3) δ 148.1, 137.8, 137.3, 133.4, 128.7,
128.3, 124.4, 116.0, 69.8.
1-(2-Nitrophenyl)but-3-en-1-ol (2d). Allylmagnesium bro-
mide (1 M in THF, 35 mL, 35 mmol) was used, affording 2g as
a colorless oil (0.97 g, 16%). NMR data were consistent with
literature data:44 1H NMR (400 MHz, CDCl3) δ 7.92 (m, 1H),
7.82 (m, 1H), 7.65 (m, 1H), 7.42 (m, 1H), 5.89 (m, 1H), 5.31
(dd, J = 8.4, 3.7 Hz, 1H), 5.21 (m, 1H), 5.18 (t, J = 1.2, 1.2 Hz,
1H), 2.70 (m, 1H), 2.42 (m, 2H); 13C NMR (101 MHz,
CDCl3) δ 147.8, 139.2, 133.9, 133.4, 128.1, 128.1, 124.4, 119.1,
68.4, 42.8.
(2-Nitrophenyl)(phenyl)methanol (2e). Phenylmagnesium
bromide [freshly prepared from phenyl bromide (3.5 mL, 35
mmol) and magnesium turnings (0.9 g, 35 mmol) in THF] was
used, affording 2e as an orange oil (5.47 g, 72%). NMR data
were consistent with literature data:45 1H NMR (400 MHz,
CDCl3) δ 7.91 (dd, J = 8.2, 1.3 Hz, 1H), 7.72 (dd, J = 7.9, 1.5
Hz, 1H), 7.61 (td, J = 7.8, 7.7, 1.3 Hz, 1H), 7.43 (m, 1H), 7.32
(m, 6H), 6.40 (d, J = 2.5 Hz, 1H), 2.77 (s, 1H); 13C NMR (101
MHz, CDCl3) δ 148.5, 141.6, 138.6, 133.5, 129.5, 128.7, 128.6,
128.2, 127.0, 124.8, 71.6.
General Procedure for the Microwave-Assisted Esterifica-
tion Reaction (1d−h). To a stirring solution of phenylpropiolic
acid (2 mmol, 0.32 g) in dry CH2Cl2 cooled in an ice bath were
added the alcohol (2 mmol), DCC (2 mmol), and DMAP (10
mol %), in that order. The microwave vial was sealed, and the
mixture was heated in a microwave reactor at 40 °C for 30 min.
The reaction mixture was then purified by automated flash
chromatography with 0−10% ethyl acetate in hexane. The
product was collected as an oil after the solvent had been
removed using a rotary evaporator.
2-Nitrobenzyl-3-phenylpropiolate (3a). o-Nitrobenzyl alco-
hol (0.3 g, 2 mmol) was reacted with the acid as described in
the general procedure to afford 1d as a yellowish oil (0.2 g,
72%). NMR data were consistent with literature data:46 1H
NMR (400 MHz, CDCl3) δ 8.17 (dq, J = 8.2, 0.9, 0.9, 0.9 Hz,
1H), 7.70 (m, 2H), 7.62 (m, 2H), 7.49 (m, 2H), 7.40 (m, 2H),
5.70 (s, 2H); 13C NMR (101 MHz, CDCl3) δ 153.16, 147.16,
133.80, 132.93, 131.23, 130.74, 128.83, 128.45, 125.04, 119.12,
87.42, 79.89, 63.95; νmax (neat) 2215, 1707, 1524.
MHz, CDCl3) δ 152.83, 147.48, 137.30, 133.84, 133.02, 130.78,
128.59, 128.57, 127.25, 124.58, 119.38, 86.95, 80.37, 69.92,
21.99; νmax (neat) 2210, 1704, 1523.
1-(2-Nitrophenyl)allyl-3-phenylpropiolate (3c). 2c (0.36 g,
2 mmol) was reacted with the acid as described in the general
1
procedure to afford 3c as a yellow oil (0.41 g, 66%): H NMR
(400 MHz, CDCl3) δ 8.01 (ddt, J = 8.1, 1.0, 0.5, 0.5 Hz, 1H),
7.71 (m, 2H), 7.60 (m, 2H), 7.48 (m, 2H), 7.39 (m, 2H), 7.01
(dq, J = 5.2, 0.9, 0.9, 0.7 Hz, 1H), 6.12 (dddd, J = 17.3, 10.5,
5.7, 0.6 Hz, 1H), 5.41 (m, 2H); 13C NMR (101 MHz, CDCl3)
δ 152.36, 133.78, 133.66, 133.50, 132.91, 130.69, 128.81,
128.44, 128.30, 124.57, 119.19, 118.66, 87.21, 80.03, 72.51; νmax
(neat) 2210, 1705, 1523.
1-(2-Nitrophenyl)but-3-en-1-yl-3-phenylpropiolate (3d).
2d (0.13 g, 0.6 mmol) was reacted with the acid as described
in the general procedure to afford 1g as a yellow oil (0.11 g,
51%): 1H NMR (400 MHz, CDCl3) δ 8.02 (dd, J = 8.2, 1.3 Hz,
1H), 7.72 (dd, J = 7.9, 1.6 Hz, 1H), 7.67 (ddd, J = 7.6, 7.1, 1.3
Hz, 1H), 7.60 (m, 2H), 7.46 (m, 2H), 7.39 (m, 2H), 6.53 (dd, J
= 8.1, 4.2 Hz, 1H), 5.91 (ddt, J = 17.2, 10.2, 7.0, 7.0 Hz, 1H),
5.16 (m, 2H), 2.77 (m, 2H); 13C NMR (101 MHz, CDCl3) δ
152.92, 147.56, 135.72, 133.68, 133.04, 132.42, 130.81, 128.70,
128.57, 127.72, 124.73, 119.36, 118.92, 87.19, 80.27, 72.52,
40.15; νmax (neat) 2211, 1706, 1524.
(2-Nitrophenyl)(phenyl)methyl-3-phenylpropiolate (3e).
2e (0.15 g, 0.6 mmol) was reacted with the acid as described
in the general procedure to afford 1h as a yellow thick oil (0.53
1
g, 70%): H NMR (400 MHz, CDCl3) δ 8.03 (dd, J = 8.2, 1.3
Hz, 1H), 7.83 (dd, J = 8.0, 1.4 Hz, 1H), 7.71 (m, 2H), 7.60 (m,
2H), 7.47 (m, 4H), 7.38 (m, 5H); 13C NMR (101 MHz,
CDCl3) δ 152.43, 147.68, 137.12, 134.38, 133.40, 132.83,
130.67, 128.80, 128.52, 128.44, 128.42, 128.38, 127.79, 124.81,
119.06, 87.35, 80.03, 73.78; νmax (neat) 2214, 1707, 1525.
General Procedure for the One-Pot CuAAC Reaction.
Three milliliters of a 1 mM solution of the esters in
dichloromethane was injected through a microchannel reactor
chip (Syrris, 62.5 μL) while being irradiated at 254 nm using a
hand-held UV lamp (UVP; UVGL-25 Compact UV Lamp, 4
W, 254/365). The photoproduct was collected in a vial
containing the click reagents [CuI (3.8 mg, 0.02 equiv), DIPEA
(5.2 mg, 0.04 equiv), and HOAC (2.4 mg, 0.04 equiv)]. The
vial was heated to 60 °C until the amount of solvent was
reduced by half. Then, azido anisole (0.1 mL) was added and
the mixture stirred until it was dry (15 min). The crude product
was re-dissolved in CDCl3 and passed through a short pipet
plug containing anhydrous MgSO4 into an NMR tube. Then
methyl-3,5-dinitrobenzoate (30 μL) was added as an internal
1
standard to determine the yield by quantitative H NMR (32
scans, 10 s ds, 45° pulse angle).47
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.bioconj-
chem.7b00812.
Photolysis experiment and quantum yield measurement,
1-(2-Nitrophenylethyl-3-phenylpropiolate) (3b). 2b (0.32
UV−visible absorption spectra, FTIR spectra, thermog-
g, 2 mmol) was reacted with the acid as described in the general
1
ravimetric spectra, and H and 13C spectra (PDF)
1
procedure to afford 1e as a yellow oil (0.4 g, 70%): H NMR
(400 MHz, CDCl3) δ 8.00 (ddt, J = 8.2, 1.2, 0.5, 0.5 Hz, 1H),
7.76 (ddd, J = 8.0, 1.5, 0.6 Hz, 1H), 7.68 (dddt, J = 7.9, 7.3, 1.2,
0.5, 0.5 Hz, 1H), 7.59 (m, 2H), 7.46 (m, 2H), 7.38 (m, 2H),
6.51 (m, 1H), 1.74 (dd, J = 6.5, 0.5 Hz, 3H); 13C NMR (101
AUTHOR INFORMATION
Corresponding Author
*E-mail: mkasper@chalmers.se.
■
F
DOI: 10.1021/acs.bioconjchem.7b00812
Bioconjugate Chem. XXXX, XXX, XXX−XXX

Bioconjugate Chemistry
Article
(17) Siemsen, P., Livingston, R. C., and Diederich, F. (2000)
Acetylenic Coupling: A Powerful Tool in Molecular Construction.
Angew. Chem., Int. Ed. 39, 2632−2657.
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a booming methodology in synthetic organic chemistry. Chem. Rev.
107, 874−922.
ORCID
Kasper Moth-Poulsen: 0000-0003-4018-4927
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
(19) Kolb, H., Finn, M., and Sharpless, K. (2001) Click Chemistry:
Diverse Chemical Function from a Few Good Reactions. Angew.
Chem., Int. Ed. 40, 2004−2021.
■
The authors are grateful to the European Research Council
(ERC-StG #337221, SIMONE) for funding. Dr. P. Jarvoll, Dr.
T. Gschneidtner, and Dr. M. Jevric are kindly acknowledged for
fruitful discussions.
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̊
Kann, N. (2016) Ruthenium-Catalyzed Azide Alkyne Cycloaddition
Reaction: Scope, Mechanism, and Applications. Chem. Rev. 116,
14726−14768.
ABBREVIATIONS
■
PPGs, photocleavable protected groups; NB, o-nitrobenzyl; ε,
molar extinction coefficient; λ, wavelength; LC−MS, liquid
chromatography−mass spectrometry; NMR, nuclear magnetic
resonance; CuAAC, Cu(I)-catalyzed azide−alkyne cycloaddi-
tion
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Azide-Alkyne Cycloaddition-Click Chemistry over the Recent Decade.
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Silylation as a Protective Method for Terminal Alkuynes in Oxidative
Couplings: A General Synthesis of Polyenes. Tetrahedron 28, 4601−
4616.
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