Three in the spotlight: Photoinduced stereoselective synthesis of (Z)-acyloxyacrylamides through a multicomponent approach

Article abstract of DOI:10.1021/jo500535f

We report a straightforward approach to synthesize 2-acyloxyacrylamides, which are useful synthons in organic synthesis. This involves a photoactivated multicomponent reaction, performed both in batch and under continuous flow conditions. This process affords the desired compounds in a stereoselective fashion from readily available starting materials in one step, without the aid of metal catalysis. This paper illustrates the preliminary work, the extensive experiments carried out to understand the limitations of the approach, and the optimization of the conditions for the synthesis of these particular captodative olefins.

Full text of DOI:10.1021/jo500535f

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Article
Three in the spotlight: photoinduced stereoselective synthesis
of (Z)-acyloxyacrylamides through a multicomponent approach
Silvia Garbarino, Luca Banfi, Renata Riva, and Andrea Basso
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/jo500535f • Publication Date (Web): 25 Mar 2014
Downloaded from http://pubs.acs.org on March 29, 2014
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Three in the spotlight: photoinduced stereoselective synthesis of (Z)-
acyloxyacrylamides through a multicomponent approach
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Silvia Garbarino, Luca Banfi, Renata Riva and Andrea Basso*
Università degli Studi di Genova, Dipartimento di Chimica e Chimica Industriale, Via Dodecaneso
31, Genova (Italy)
andrea.basso@unige.it
O
R2
O
UV light
R4
OH
R3
R2
R3 NC
R1
R1
O
N
H
batch
or
O
N2
O
R4
Abstract
We report a straightforward approach to 2ꢀacyloxyacrylamides, useful synthons in organic synthesis. This
involves a photoactivated multicomponent reaction, performed both in batch and under continuous flow
conditions, and affords the desired compounds in a stereoselective fashion from readily available starting
materials in one step, without the aid of metal catalysis. This paper illustrates the preliminary work, the
extensive experiments carried out to understand the limitations of the approach and the optimization of the
conditions for the synthesis of these particular captodative olefins.
Introduction
Among acrylic derivatives, 2ꢀacyloxyacrylates have captured special interest owing to the opposite
electronic demand and to the synthetic potential displayed by the geminally substituted double
bond.[1] These captodative olefins have proved to be versatile synthons, efficiently employed in
DielsꢀAlder condensations, 1,3ꢀdipolacycloadditions and FriedelꢀCraft reactions, to name a few.[2]
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We have recently discovered that the corresponding amides can rearrange under basic conditions to
afford novel 5ꢀmembered ring heterocycles endowed with biological activity.[3] With the aim of
finding a general route to 2ꢀacyloxyacrylamides, we have discovered a multicomponent reaction
between ketenes, isocyanides and carboxylic acids. A preliminary report has been recently
published.[4] We wish now to report full details of this novel three component condensation.
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Results and Discussion
Use of acyl chlorides for the generation of ketenes. In 2008 Danishefsky reported the
sequential concerted rearrangement occurring in the coupling between isocyanides and carboxylic
acids, leading to Nꢀformylamides 1.[5] One year later we have observed that the reaction pathway is
different when arylacetic acids are employed; in this case, in fact, captodative olefins 2
incorporating two molecules of acid and one of isocyanide are formed (Scheme 1).[6] Reasoning on
the possible mechanism taking place in this instance, we postulated that the same outcome could be
achieved when an arylketene reacts with one molecule of arylacetic acid and one of isocyanide
through a formal Passerini reaction.[7]
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Scheme 1. The different fate of carboxylic acids and isocyanides. A possible alternative route to
compounds 2.
In the literature there are some precedents of the reactivity of ketenes and isocyanides. For example,
in 1961 Ugi reported the reaction between isocyanides and two equivalents of diphenylketene,
affording 1,3ꢀdioxolanꢀ4ꢀimine derivatives.[8] Twenty years later Capuano showed the versatility of
the reaction between isocyanides and ketenes obtaining, according to one of the first diversityꢀ
oriented approach, a plethora of different heterocycles.[9] More recently, Pirali reported that
isocyanoacetamides react with in situ generated ketenes, affording aminooxazoles.[10]
Nevertheless, to the best of our knowledge the use of ketenes in multicomponent reactions has
never been explored so far, if we exclude the condensation with thiazolium salts and dimethyl
acetylenedicarboxylate reported by Ma.[11]
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When we started this project we were aware of the difficulties we could have encountered, related
to the instability and high reactivity of ketenes. Indeed, our first attempts to generate acetoxyketene
from acetoxyacetyl chloride 3 in the presence of triethylamine at room temperature and to react it
with cyclohexyl isocyanide and phenylacetic acid failed to afford the desired product 4.
Interestingly, traces amounts of compound 5 were isolated (Scheme 2).
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Scheme 2.First attempts to the ketene three component reaction (Kꢀ3CR).
When the same reaction was attempted with phenylacetyl chloride and benzoic acid the
multicomponent adduct 6 was isolated in 30% yield, but compound 5 was also formed (15%)
(Scheme 3).
Scheme 3. Formation of heteroꢀadduct 6 and homoꢀadduct 5 from the Kꢀ3CR.
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These preliminary results already showed the main limitation of this approach. In order to explain
formation of homoꢀadduct 5 from both attempts, we postulated that, under the basic medium
generated by triethylamine, the ketene, once formed from the acyl chloride, can alternatively react
with the isocyanide or the carboxylic acid. In the latter case a mixed anhydride is produced and this
in turn can reꢀgenerate a molecule of ketene (the same one or the one deriving from the acid) or can
react with an additional molecule of carboxylic acid to form the symmetrical anhydride, thus
liberating the initial acyl chloride as its corresponding carboxylic acid. Reꢀgeneration of a ketene
explains formation of 5 from the first reaction: phenylketene, stabilized by the conjugation with the
aromatic ring, could have a longer lifeꢀtime compared to acyloxyketene, longer enough to react with
cyclohexylisocyanide and additional phenylacetic acid to give the Kꢀ3CR. On the other hand,
liberation of phenylacetic acid from the mixed benzylphenylanhydride can explain formation of 5
from the second reaction, where it can compete with benzoic acid to form the multicomponent
adduct (formation of phenylacetic acid as a consequence of the reaction of the chloride with traces
amounts of water was ruled out by employing molecular sieves) (Scheme 4).
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Scheme 4. The side formation of a mixed anhydride could explain formation of adduct 5 from the
previously described attempts.
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By searching alternative methods to generate ketenes, we came across the works by Lectka,
employing the “shuttle deprotonation” system.[12] By using a catalytic amount of a “kinetic base”
(quinine) and a stoichiometric amount of an insoluble “thermodynamic base” (proton sponge),
under anhydrous conditions, we could dramatically improve the outcome of the reaction.
Generation of the ketene from phenylacetyl chloride was performed in toluene at ꢀ78 °C, and a
DCM solution of cyclohexyl isocyanide and benzoic acid was added after 10 mins. at the same
temperature. The temperature was left rising overnight and, upon workꢀup and chromatographic
purification, compound 6 was isolated in 71% yield, although contaminated by a small amount (6%)
of ”homo” adduct 5. By doubling the amount of benzoic acid, the yield could be slightly improved
(82%) and the amount of 5 slightly reduced (4%).
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With these promising results in hand, we extended the improved methodology to other carboxylic
acids. However, results were disappointing and high amounts of “homo” adduct were isolated
together with the desired products. For example, when propionic acid was employed, crude material
contained the desired product 7 and compound 5 in a 2:1 ratio (Scheme 5). This reaction was taken
as a model to study in details the influence of the reaction conditions on the formation of 5. The
results are summarized in Table 1.
Scheme 5. Kꢀ3CR with phenylacetyl chloride, propionic acid and cyclohexyl isocyanide.
Table 1. Study of the influence of reaction parameters on the outcome of the Kꢀ3CR with
phenylacetyl chloride, propionic acid and cyclohexyl isocyanide.
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Entry Reagents ratioa Conc. Conditions
7/5 ratio (%)
1.0:0.50 (57%)
1.0:0.45 (62%)
1.0:0.40 (46%)
addition of isocyanide and acid in DCM at ꢀ78 °C, then
temperature gradually raised to r.t. in 24 hrs
1.0:0.9:2.0
1.0:0.9:2.0
1.0:0.9:2.0
0.2M
0.1M
0.2M
a
b
c
conditions a
conditions a, but reaction immediately at r.t. after
addition
conditions a, but acid is added after 2hrs at ꢀ70 °C, then
temperature gradually raised
conditions a, but propionic anhydride (1.0 eq) is used
instead of propionic acid
9
1.0:0.9:2.0
1.0:0.9:ꢀ
0.2M
0.2M
1.0:0.10 (10%)
ꢀ
d
e
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conditions a
1.0:0.9:0.9
1.0:0.9:5.0
1.0:0.9:2.0
0.1M
0.1M
0.2M
1.0:1.4 (64%)
1.0:0.20 (29%)
1.0:0.42 (73%)
f
g
h
conditions a
conditions a, but isocyanide and acid added neat
isocyanide and acid from the beginning, then acyl
chloride dropwise at ꢀ78 °C
1.0:0.9:2.0
1.0:0.9:2.0
1.0:0.9:2.0
0.1M
0.1M
0.1M
1.0:0.53 (75%)
i
conditions a, using Et3N as a base (no shuttle system)
ꢀ
j
conditions h, but acid added 20 mins. after the
isocyanide (at ꢀ70 °C)
aacyl chloride/isocyanide/carboxylic acid
1.0:0.49 (76%)
k
In light of these results it was clear that the 7/5 ratio could be improved only at the expense of the
overall yield, and that generation of the mixed anhydride, responsible for the formation of 5, was a
competing process difficult to be prevented and highly dependent on the nature of the carboxylic
acid. For example, acetyl chlorides lacking an αꢀaryl group failed to react under the above
mentioned conditions.
This novel strategy, leading to the desired captodative olefins, although straightforwardly, was not
pursued further due to this limitation, and alternative methods for the generation of ketenes were
taken into consideration.
Use of diazoketones for the generation of ketenes
Ketenes can be efficiently generated from diazoketones through the Wolff rearrangement. This
process can be performed in the presence of metal salts, induced by irradiation or by heating. Many
methods employing silver catalysts have been reported in the literature, although many of them use
an added base, that in our case would give the same side reactions of the previous methodology. We
therefore decided to employ the method developed by Sewald, with silver benzoate in dioxane.[13]
In a control experiment phenyldiazoketone was sonicated in the presence of a catalytic amount of
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silver benzoate and its disappearance was monitored by TLC. In a second experiment the reaction
was performed in the presence of methanol, and methyl phenylacetate was isolated as the sole
product. However, when the Wolff rearrangement was attempted in the presence of cyclohexyl
isocyanide and propionic acid, no reaction took place and the diazoketone was recovered unreacted.
The ability of isocyanides to complex metal ions is known, and probably also in this case silver ions
were sequestrated by the reagent.
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We therefore moved to investigate the photochemical reaction of diazoketones, having access to a
Ryonet merryꢀgoꢀaround apparatus equipped with UV lamps at various wavelengths. Upon
irradiation of
a
heptane solution of equimolar quantitites of phenyldiazoketone,
cyclohexylisocyanide and propionic acid with Wood light (maximum wavelength at 365 nm, see
emission spectra in supporting information) for one hour, we were pleased to find that the desired
olefin 7 was formed and precipitated from the reaction medium. The reaction was not complete, as
ca. 50% of diazoketone remained unreacted, but at this stage no trace of compound 5 could be
detected on the crude, demonstrating that under neutral/slightly acidic conditions phenylacetic acid
was not generated as side product. In a second experiment the reaction mixture was irradiated for
four hours, time required by the diazoketone to be completely consumed. The product 7 was
1
isolated, after chromatographic purification, in 65% yield, however the HꢀNMR showed the
presence of two substances in a 1.0:0.2 ratio. The relatively similar NMR pattern of the two
compounds led us to hypothesize that we had obtained a mixture of the two stereomeric olefins 7a
and 7b (Scheme 6). It is worth noting that the previously developed reactions leading to captodative
olefins always afforded (Z)ꢀacrylamides as the sole geometric isomers, as demonstrated by NOE
experiments and theoretical calculations.[6]
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Scheme 6. The outcome of the photoinduced Kꢀ3CR
In light of this unexpected result, we performed different experiments using different diazoketones
8 in combination with various isocyanides and carboxylic acids (Scheme 7). As illustrated in Table
2, in all cases the desired captodative olefin could be isolated in moderate to good yield, although as
a, often unseparable, mixture of isomers. Only for compound 9 (entry a) the reaction was
completely stereoselective. Nevertheless, contamination by the “homo” adducts was never
observed, and no excess of one of the reagents was required to drive the reaction to completion, as
often occurs in multicomponent condensations. Moreover, also diazoketones lacking conjugation
with an aromatic ring (entries fꢀh) succeeded in giving the desired products, thus overcoming a
limitation of the previously developed methodologies and allowing for the preparation of
captodative olefins with increased structural diversity.
Scheme 7. General scheme of the photoactivated Kꢀ3CR
Table 2. Investigation of the scope of the photoactivated Kꢀ3CR
entry R1
R2
R3
compd. yield (Z)
Z/E
1:0
1:0.4
1:0.8
1:0.5
1:0.4
phenyl
phenyl
2ꢀthiophenyl cyclohexyl
cyclohexyl
cyclohexyl
phenyl
3’ꢀMeOꢀbenzyl
phenyl
72%
65%
37%
56%
56%
a
b
c
9
10
11
12
13
phenyl
phenyl
butyl
tertꢀbutyl
3’ꢀMeOꢀbenzyl
3’ꢀMeOꢀbenzyl
d
e
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4ꢀClꢀphenyl
4ꢀClꢀphenyl
benzyl
benzyl
3ꢀClꢀpropyl
2ꢀ(MeO)2ꢀethyl propyl
68%
75%
55%
62%
43%
1:0.2
1:0.1
1:0.1
1:0.1
1:0.1
3
4
5
6
7
8
f
14
15
16
17
18
cyclohexyl
cyclohexyl
cyclohexyl
cyclohexyl
propyl
phenyl
3’ꢀMeOꢀbenzyl
pentaꢀ1,3ꢀdienyl
g
h
i
j
9
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Our efforts were therefore directed towards the obtainment of a stereoisomerically pure product. In
the case of compound 12, we succeeded in separate the two isomers by chromatography, and we
could therefore confirm their structure as (Z)ꢀ12 (the major isomer) and (E)ꢀ12 (the minor one). In
particular, compound (Z)ꢀ12 showed a NOE interaction between the olefinic and amidic hydrogens,
while no interaction was observed in the case of (E)ꢀ12, confirming previous results and our initial
hypothesis. In light of this, it could be postulated that Z/E isomers could derive from a different Kꢀ
3CR mechanism occurring under UV light irradiation or, alternatively, the (E)ꢀisomer could be the
result of the isomerization of the (Z)ꢀmulticomponent adduct, generated according to the standard
mechanism. However, the following evidences prompted us to consider the second option more
plausible: a) irradiation of pure (Z)ꢀisomer under the same reaction conditions generated a Z/E
mixture (photoisomerization of alkenes is indeed a wellꢀknown process); b) when the
multicomponent reaction was performed with a delayed timing (1 hour of irradiation and 1 hour in
the dark repeated four times) the yield was comparable but the Z/E ratio was improved (prolonged
times facilitated precipitation of the product off the solvent, thus subtracting it from its
isomerization in solution); c) in the case of compound 9 only the (Z)ꢀisomer was isolated from the
multicomponent reaction, and in fact the (Z) to (E) photoisomerization was very slow under the
same conditions (no appreciable isomerization after irradiation with Wood’s light for 3 hours); d)
no Z/E isomerization was ever observed without irradiation (thus ruling out mechanisms different
from those previously illustrated), even upon microwave heating.
We took into consideration factors that could influence the alkene photoisomerization, such as
solvent, wavelength and presence of additives such as photosensitizers and photoquenchers. We
took as
a
model the reaction between phenyldiazoketone, butylisocyanide and 3’ꢀ
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methoxyphenylacetic acid (entry d) and carried out a detailed study, where the Z/E isomer ratios
4
5
6
1
were calculated by HꢀNMR on the crude and the yields were determined on the isomeric mixture
7
8
after chromatographic purification. The results are summarized in Table 3.
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Table 3. Investigation of the influence of reaction parameters on the outcome of the Kꢀ3CR leading
to 12
entry solvent
UVa Additive (eq)
Time (h) Z/E ratio yield (Z + E)
heptane
chloroform
diisopropylether
toluene
acetone
heptane
heptane
heptane
toluene
toluene
toluene
toluene
toluene
C
C
C
C
C
B
A
C
C
C
C
C
C
C
C
C
C
C
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
3h
3h
3h
3h
3h
1.5h
2h
3h
3h
3h
7h
4h
1:0.14
1:0.40
1:0.50
1:0.10
1:0.06
1:0.50
1:0.80
1:0.03
1:0.04
1:0.05
1:0
1:0.02
1:0.02
1:0.13
1:0.13
1:0.78
1:0.96
1:0.71
75%
73%
62%
73%
68%
46%
38%
62%
72%
65%
74%
77%
72%
64%
65%
60%
64%
68%
a
b
c
d
e
f
g
h
i
j
k
l
m
n
o
p
q
r
piperylene (10)
piperylene (10)
piperylene (5)
transꢀstilbene (5)
transꢀstilbene (1)
transꢀstilbene (0.2) 4h
toluene
toluene
heptane
heptane
toluene
pꢀterphenyl (5)
oꢀterphenyl (5)
benzophenone (5)
benzophenone (10) 5h
acetophenone (5) 3h
4h
3h
5h
aLamps with maximum wavelengths are C = 365 nm, B = 300 nm, A = 254 nm.
Results in table 3 showed that hydrocarbon solvents such as heptane and toluene were giving the
best compromise between conversion and (Z)ꢀselectivity (entries a and d) compared to halogenated
solvents (entry b) or ethers (entry c). An interesting result was obtained when acetone was used as
the solvent (entry e). Although not ideal, as it could react with the isocyanide and the carboxylic
acid to give a Passerini adduct, it afforded a very good Z/E ratio. Irradiation using UV lamps with a
maximum at 365 nm was giving better results, compared to shorter wavelengths (entries f and g),
both for Z/E ratios and yields. The reactions at 254 or 300 nm were faster, as a consequence of the
higher kinetics of the Wolff rearrangement, but also decomposition products could be observed in
the crude. In a second set of experiments molecules known to act as photosensitizers or
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photoquenchers were added to the reaction mixture. From a general point of view it was observed
that when the reaction was performed in the presence of molecules such as piperylene or transꢀ
stilbene a higher (Z)ꢀselectivity was observed (entries hꢀm). On the contrary, photosensitizers such
as benzophenone or acetophenone partially reversed the selectivity in favor of the (E)ꢀisomer
(entries pꢀr). Unfortunately, the (E)ꢀisomer could not be obtained as the major product, as
isomerization reached a stationary state and prolonged irradiation caused partial decomposition of
the isomeric mixture. Terphenyl derivatives, although known to be photoquenchers, did not produce
appreciable differences (entries n and o) compared to the reactions without additives. The amount
of additive was also important, as its effect increased when increasing the number of equivalents.
Using five equivalents of transꢀstilbene it was possible to obtain selectively the (Z)ꢀolefin; however,
from a practical point of view, purification of the product in the presence of a large excess of
additive could be a problem. Moreover, some additives were only partially soluble in heptane, and
therefore toluene was often preferred. The use of piperylene, a low boiling point solvent, was
indeed an advantage (using 10 equivalent, the undesired (E)ꢀisomer was only 3%), since it could be
easily removed by evaporation. Unfortunately, although enquiring various chemical suppliers, we
never succeeded to get an affordable price for this compound, and for this reason an extensive use
of piperylene was economically unpractical. We therefore moved back to transꢀstilbene and found
that its amount could be reduced to 0.2 equivalents without a detectable loss of selectivity (entry
m).
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As the molecules used as additives in these experiments act as quenchers or sensitizers of triplet
excited states, it could be rationalized that the Z/E photoisomerization of captodative olefins is
proceeding, at least partially, through triplet excited states that could therefore be affected by the
presence of substances able to enhance or suppress their formation. On the other hand, the Wolff
rearrangement of diazoketones was partially slowed down by the presence of additives, but this
effect was observed both with quenchers and sensitizers. This can be explained with the capture of
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part of the incident photons by the molecules of additive, independently by their nature. Indeed, the
absence of triplet excited states involved in the Wolff rearrangement has been demonstrated.[14]
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Development of a flow system for the photochemical reaction
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In parallel with the investigation of the effect of quenchers and sensitizers as additives, we also
reasoned that a higher level of (Z)ꢀselectivity could be obtained if we could reduce the irradiation
times. Recently, continuousꢀflow chemistry has attracted the attention of many research groups,
both in academia and industry. In particular, it has been proven effective in photochemical
transformations,[15] as the reduced size of reaction channels allows a more efficient penetration of
light through the reaction mixture. Moreover, continuous removal of products from the irradiated
area can suppress secondary photoreactions, increasing yield and purity of the desired products.
Many examples of homogeneous and heterogeneous photochemical reactions under flow conditions
have been recently described, although to the best of our knowledge there are no examples of
multicomponent condensations. We therefore set up an in house flow system made of a syringe
pump, a flow reactor assembled by wrapping a FEP tube (Φ = 1.6 mm, length= 2.5m) around a
hollow cylinder hanging in the center of the Rayonet cavity, and a collection flask (the assemblage
is illustrated in the supporting information). A toluene solution of the three reagents
(phenyldiazoketone, butylisocyanide and 3’ꢀmethoxyphenylacetic acid) was then syringed in the
tube and the elute was analyzed by TLC. After various attempts at different concentrations and
elution times, best results were obtained using a 0.034 M solution of reagents and a flow rate of 15
mL/h. Under these conditions, in the absence of additives, formation of the E isomer was almost
completely suppressed, paralleling the results obtained in batch conditions in the presence of an
additive.
We then moved to compare these two alternative methodologies, in terms of selectivity and yield,
with a representative set of compounds, using different diazoketones, isocyanides and carboxylic
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acids (at this stage it was decided not to investigate the full scope of the reaction, as this was already
done in a precedent work[4]). Results are summarized in Table 4.
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9
Table 4. Comparison of batch and flow conditions in the synthesis of captodative olefins. Method
A: batch reaction with 0.2 eq of transꢀstilbene (conditions m of Table 3). Method B: flow reaction
(see text for conditions).
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entry R1
R2
R3
compd. yield (Z)
Z/E
method
phenyl
phenyl
4ꢀClꢀphenyl
4ꢀClꢀphenyl
benzyl
benzyl
3ꢀClꢀpropyl
3ꢀClꢀpropyl
tertꢀbutyl
tertꢀbutyl
cyclohexyl
cyclohexyl
cyclohexyl
cyclohexyl
cyclohexyl
cyclohexyl
3’ꢀMeOꢀbenzyl
3’ꢀMeOꢀbenzyl
propyl
propyl
phenyl
phenyl
pentaꢀ1,3ꢀdienyl
pentaꢀ1,3ꢀdienyl
60%
46%
78%
48%
75%
71%
80%
78%
1:0.02
1:0.03
1:0
1:0.05
1:0
1:0
1:0
1:0
A
B
A
B
A
B
A
B
a
b
c
d
e
f
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16
18
18
g
h
Both approaches afforded captodative olefins almost exclusively as Z isomers and in the case of
compounds 16 and 18 also yields were comparable (and higher than those obtained with
unoptimized conditions reported in Table 2). On the other hand, when aromatic diazoketones were
employed, probably due to their higher reactivity, crude NMRs after reactions under continuous
flow conditions showed impurities belonging to products of decomposition of ketene, as well as
variable amounts of unreacted acid and isocyanide. This resulted in a diminished yield of the
multicomponent adducts, as highlighted by the comparison of entry a with b and entry c with d. We
are currently optimizing the continuous flow apparatus in order to reduce or suppress these side
reactions.
Synthesis of tetrasubstituted olefins
Finally, we have investigated the possibility to use our methodology also to synthesize
tetrasubstituted olefins by employing disubstituted diazoketones. To this purpose we have prepared
compound 19, according to known procedures,[16] and subjected it to the multicomponent reaction
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with an isocyanide and a carboxylic acid under batch conditions. With our delight, compounds 20-
22 could be isolated in 54ꢀ80% yield as single geometric isomers (Scheme 8). NOE experiments
helped to determine the Z configuration of the product.
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O
R1
N2
Wood light
toluene, r.t.
N
H
O
R2
OH
R1 NC
O
O
O
19
20-22
R2
20: R1 = cHex, R2 = Et, yield = 55%
21: R1 =cHex, R2 = Bn, yield 80%
22: R1 = nBu R2 = Bn, yield 54%
Scheme 8. Synthesis of tetrasubstituted alkenes through the Kꢀ3CR approach
Conclusion
2ꢀAcyloxyacrylamides have been employed in the past as substrates for baseꢀmediated
rearrangements to pyrrolones and pyrrolidinediones,[6] as well as for the PADAM approach to γꢀ
acylaminoꢀαꢀoxoamides.[4] These compounds have shown biological activity in various fields as
antitumor agents[3] or protease inhibitors.[17] The availability of a straightforward methodology for
the obtainment, under batch or flow conditions, of such compounds could open up the route to
novel applications. Indeed, we are currently investigating reactions at the double bond such as
epoxidations, cyclopropanations and the transition metal catalyzed additions, as well as radical
reactions. We will report the results in due course.
Experimental Section
General Experimental Methods. NMR spectra were recorded at 300 MHz (1H) and 75 MHz (13C)
and the chemical shifts (δ) are expressed in parts per million relative to tetramethylsylane (TMS) as
internal standard (0.00 ppm). Coupling constants are reported in hertz. NMR acquisitions were
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performed at 295 K and CDCl3 was used as solvent. HRMS were performed employing an ESI+
ionization method and TOF as analyzer.
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Photoinduced reactions were performed with a Rayonet instrument, equipped with 16 lamps
(technical features: power 8W, maximum wavelength 365 nm).
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Reactions were monitored by TLC. TLC analyses were carried out on silica gel plates (thickness =
0.25 mm), viewed at UV (λ = 254 nm) and developed with Hanessian stain (dipping into a solution
of (NH4)4MoO4•4H2O (21 g) and Ce(SO4)2•4H2O (1 g) in H2SO4 (31 mL) and H2O (469 mL) and
warming). Column chromatography was performed with the “flash” methodology using 220−400
mesh silica. Solvents employed as eluents and for all other routinary operations, as well as
anhydrous solvents and all reagents used were purchased from commercial suppliers and employed
without any further purification. Diazoketones 8 were prepared from the corresponding acyl
chlorides according to Ref. [18], while diazoketone 19 was prepared according to Ref. [16].
General procedure for the preparation of K-3CR adducts. Diazoketone 8 (1.0 eq) and
carboxylic acid (1.0 eq) were dissolved in the solvent (5mL/0.35mmol) within a glass test tube and
degassed under argon atmosphere. The isocyanide (1.0 eq) and the additive (no additive in the case
of compound 7) were added and the solution irradiated at 365 nm under magnetic stirring for three
to five hours. The reaction was monitored by TLC: when completed, volatiles were removed under
reduced pressure and the crude product purified by means of flash chromatography to afford
adducts 7a/7b, 9-18 and 20-22.
Mixture of (Z)- and (E)- 3-(cyclohexylamino)-3-oxo-1-phenylprop-1-en-2-yl propionate
(7a/7b). Yellow oil (67 mg, 65% yield) upon flash chromatography (silica gel, DCM/EtOAc/PE
1
1:1:5.5). Rf = 0.33 (DCM/EtOAc/PE 1:1:5.5). H NMR δ 7.50 ꢀ 7.47 [m, 2H], 7.37 ꢀ 7.27 [m, 3H],
7.16 [s, 0.83H], 6.65 [s, 0.17H], 5.84 [d br, J = 8.1, 0.83H], 5.47 [d br, J = 8.1, 0.17H], 3.92 ꢀ 3.65
[m, 1H], 2.60 [q, J = 7.5, 1.66H], 2.55 [q, J = 7.5, 0.34H], 2.02 ꢀ 1.13 [m, 10H], 1.24 [t, J = 7.5,
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2.49H], 1.23 [t, J = 7.5, 0.51H]. 13C NMR δ 173.2 (7b), 171.5 (7a), 161.9 (7a), 161.5 (7b), 142.9
(7b), 140.2 (7a), 132.6 (7a), 132.3 (7b), 129.5 (7a/7b), 129.1 (7a/7b), 128.7 (7a/7b), 122.9 (7a),
122.4 (7b), 48.6 (7a), 48.3 (7b), 33.0 (7a), 32.3 (7b), 27.7 (7a), 27.3 (7b), 25.5 (7a), 25.2 (7b), 24.8
(7a), 24.6 (7b), 9.1 (7a) 9.0 (7b). HRꢀMS (7a) found: [M+H]+ 302.1756; C18H24NO3 requires
302.1756.
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(Z)-3-(cyclohexylamino)-3-oxo-1-phenylprop-1-en-2-yl benzoate (9). White solid (86 mg, 72%
yield) upon flash chromatography (silica gel, DCM/EtOAc/PE 1:1:5.5), m.p.: 163.7ꢀ165.2 °C. Rf =
1
0.34 (DCM/EtOAc/PE 1:1:5.5). H NMR δ 8.19 [dd, J= 8.4, 1.5, 2H], 7.68 [tt, J = 7.5, 1.2, 1H],
7.56 ꢀ 7.50 [m, 4H], 7.33 [s, 1H], 7.28 ꢀ 7.24 [m, 3H], 6.03 [d br, J = 8.0, 1H], 3.94 ꢀ 3.81 [m, 1H],
1.97ꢀ1.09 [m, 10H]. 13C NMR δ 163.7, 161.7, 140.0, 134.3, 132.5, 130.3, 129.6, 129.1, 129.0,
128.7, 128.4, 123.6, 48.6, 32.9, 25.5, 24.8. HRꢀMS found: [M+H]+ 350.1757; C22H24NO3 requires
350.1756.
(Z)-3-(cyclohexylamino)-3-oxo-1-phenylprop-1-en-2-yl 2-(3-methoxyphenyl)acetate (10). White
solid (100 mg, 75% yield) upon flash chromatography (silica gel, DCM/EtOAc/PE 1:1:5.5), m.p.:
1
89.0ꢀ91.2 °C. Rf = 0.27 (DCM/EtOAc/PE 1:1:5.5). H NMR δ 7.43 ꢀ 7.25 [m, 7H], 7.00 ꢀ 6.89 [m,
3H], 5.46 [d br, J = 8.4, 1H], 3.80 [s, 3H], 3.79 [s, 2H], 3.76 ꢀ 3.65 [m,1H], 1.77 ꢀ 0.73 [m, 10H].
13C NMR δ 167.7, 161.2, 160.2, 139.4, 134.2, 132.5, 130.4, 129.6, 129.2, 128.7, 124.0, 121.7,
115.0, 113.4, 55.3, 48.2, 41.9, 32.7, 25.5, 24.9. HRꢀMS found: [M+H]+ 394.2012; C24H28NO4
requires 394.2018.
(Z)-3-(cyclohexylamino)-3-oxo-1-(thiophen-2-yl)prop-1-en-2-yl benzoate (11). Brown solid (43
mg, 37% yield) upon flash chromatography (silica gel, DCM/EtOAc/PE 1:1:5.5), m.p.: 129.5ꢀ131.5
1
°C. Rf = 0.32 (DCM/EtOAc/PE 2:1:7). H NMR δ 8.29 [dd, J = 8.4, 1.5, 2H], 7.72 [tt, J = 7.5, 1.5,
1H], 7.64 [s, 1H], 7.58 [tt, J = 7.8, 1.5, 2H], 7.33 [dt, J = 5.1, 1.2, 1H], 7.28 ꢀ 7.27 [m, 1H], 7.02
[dd, J = 5.1, 3.7, 1H], 5.80 [d br, J = 7.8, 1H], 3.95 ꢀ 3.82 [m, 1H], 1.97 ꢀ 1.07 [m, 10H]. 13C NMR
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δ 163.9, 161.3, 137.9, 134.9, 134.4, 131.7, 130.7, 129.5, 129.0, 128.6, 127.2, 118.2, 48.7, 33.0,
25.6, 24.8. HRꢀMS found: [M+H]+ 356.1322; C20H22NO3S requires 356.1320.
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(Z)-3-(butylamino)-3-oxo-1-phenylprop-1-en-2-yl 2-(3-methoxyphenyl)acetate (Z-12). Yellow
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oil (70 mg, 56% yield) upon flash chromatography (silica gel, EtOAc/PE 3:7). Rf = 0.27
1
(EtOAc/PE 3:7). H NMR δ 7.43 ꢀ 7.29 [m, 7H], 7.00 ꢀ 6.89 [m, 3H], 5.52 [t br, J = 5.1, 1H], 3.81
[m, 5H], 3.13 [q, J = 6.9, 2H], 1.33 ꢀ 1.14 [m, 4H], 0.88 [t, J = 6.9, 3H]. 13C NMR δ 167.8, 162.3,
160.3, 139.4, 134.2, 132.5, 130.4, 129.6, 129.3, 128.8, 124.2, 121.7, 115.0, 113.6, 55.4, 42.0, 39.6,
31.5, 20.1, 13.9. HRꢀMS found: [M+H]+ 368.1852; C22H26NO4 requires 368.1862.
(E)-3-(butylamino)-3-oxo-1-phenylprop-1-en-2-yl
2-(3-methoxyphenyl)acetate
(E-12).
Obtained according to entry q of Table 3.Yellow oil (30 mg, 24% yield) upon flash chromatography
1
(silica gel, EtOAc/PE 3:7). Rf = 0.36 (EtOAc/PE 3:7). H NMR δ 7.50 ꢀ 7.20 [m, 6H], 7.00 ꢀ 6.75
[m, 3H], 6.64 [s, 1H], 5.53 [s br, 1H], 3.82 [s, 3H], 3.81 [s, 2H], 3.13 [dt, J = 6.9, 6.0, 2H], 1.33 ꢀ
1.14 [m, 4H], 0.80 [t, J = 7.2, 3H]. 13C NMR δ 170.1, 162.2, 159.9, 142.0, 134.6, 132.2, 129.9,
129.2, 128.7, 128.6, 124.0, 121.8, 115.0, 113.2, 55.4, 41.0, 39.3, 30.9, 20.0, 13.8. HRꢀMS found:
[M+H]+ 368.1852; C22H26NO4 requires 368.1862.
(Z)-3-(tert-butylamino)-3-oxo-1-phenylprop-1-en-2-yl 2-(3-methoxyphenyl)acetate (13). White
solid (70 mg, 56% yield) upon flash chromatography (silica gel, PE/Et2O 7:3), m.p.: 103.5ꢀ105.1
°C. Rf = 0.36 (PE/Et2O 1:1).1H NMR δ 7.41ꢀ 7.26 [m, 6H], 7.22 [s, 1H], 6.99 ꢀ 6.86 [m, 3H], 5.49
[s, 1H], 3.80 [s, 3H], 3.78 [s, 2H], 1.22 [s, 9H]. 13C NMR δ 167.8, 161.5, 160.2, 140.1, 134.1,
132.6, 130.3, 129.5, 129.1, 128.8, 123.2, 121.7, 115.0, 113.5, 55.4, 51.4, 41.9, 28.5. HRꢀMS found:
[M+H]+ 368.1864; C22H26NO4 requires 368.1862.
(Z)-1-(4-chlorophenyl)-3-((2,2-dimethoxyethyl)amino)-3-oxoprop-1-en-2-yl butyrate (14).
White solid (58 mg, 59% yield) upon flash chromatography (silica gel, from PE/Et2O 1:1 to
PE/EtOAc 3:7), m.p.: 85.1ꢀ86.8°C. Rf = 0.43 (PE/EtOAc 1:1). 1H NMR δ 7.46 ꢀ 7.41 [m, 2H], 7.35
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ꢀ 7.31 [m, 2H], 7.18 [s, 1H], 6.21 [t br, J = 5.6, 1H], 4.41 [t, J = 5.3, 1H], 3.50 [t, J = 5.6, 2H], 3.42
[s, 6H], 2.55 [t, J = 7.4, 2H], 1.75 [sest, J = 7.4, 2H], 1.01 [t, J = 7.4, 3H]. 13C NMR δ 170.5, 162.7,
140.1, 135.2, 131.0, 130.8, 129.1, 122.7, 102.6, 54.8, 41.4, 36.1, 18.3, 13.8. HRꢀMS found: [M+H]+
356.1265; C17H23ClNO5 requires 356.1265.
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(Z)-1-(4-chlorophenyl)-3-(cyclohexylamino)-3-oxoprop-1-en-2-yl butyrate (15). White solid (76
mg, 78% yield) upon flash chromatography (silica gel, DCM/EtOAc/PE 1:2:8.5), m.p.: 126.0ꢀ127.5
1
°C. Rf = 0.55 (PE/EtOAc 7:3). H NMR δ 7.44 – 7.31 [m, 4H], 7.13 [s, 1H], 5.83 [d br, J = 7.9,
1H], 3.92ꢀ3.79 [m, 1H], 2.54 [t, J = 7.3, 2H], 2.00ꢀ1.94 [m, 2H], 1.81ꢀ1.60 [m, 6H], 1.47ꢀ1.34 [m,
13
2H], 1.26ꢀ1.11 [m, 2H], 1.01 [t, J = 7.4, 3H]. C NMR δ 170.4, 161.7, 140.6, 134.9, 131.2, 130.7,
129.0, 121.9, 48.7, 36.1, 33.0, 25.5, 24.8, 18.3, 13.7. HRꢀMS found: [M+H]+ 350.1520;
C19H25ClNO3 requires 350.1523.
(Z)-1-(cyclohexylamino)-1-oxo-4-phenylbut-2-en-2-yl benzoate (16). White solid (74 mg, 55%
yield) upon flash chromatography (silica gel, DCM/EtOAc/PE 1:1:5.5), m.p.: 125.5ꢀ126.3 °C. Rf =
1
0.44 (DCM/EtOAc/PE 1:1:5.5). H NMR δ 8.17 [dd, J = 8.2, 1.1, 2H], 7.67 [tt, J = 7.4, 1.2, 1H],
7.53 [t, J = 7.6, 2H], 7.31 ꢀ 7.18 [m, 5H], 6.68 [t, J = 7.6, 1H], 5.84 [d br, J = 8.0, 1H], 3.89 ꢀ 3.77
[m, 1H], 3.44 [d, J = 7.6, 2H], 1.94 ꢀ 1.05 [m, 10H]. 13C NMR δ 163.9, 161.0, 141.3, 138.1, 134.3,
130.4, 128.9, 128.7, 128.7, 128.4, 126.7, 125.5, 48.5, 33.0, 32.5, 25.5, 24.8. HRꢀMS found: [M+H]+
364.1909; C23H26NO3 requires 364.1913.
(Z)-1-(cyclohexylamino)-1-oxo-4-phenylbut-2-en-2-yl 2-(3-methoxyphenyl)acetate (17). Yellow
oil (78 mg, 62% yield) upon flash chromatography (silica gel, DCM/EtOAc/PE 1:1.5:5). Rf = 0.61
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(DCM/EtOAc/PE 1:2:4.5). H NMR δ 7.34 ꢀ 7.14 [m, 6H], 6.99 ꢀ 6.86 [m, 3H], 6.64 [t, J = 7.6,
1H], 5.31 [d br, J = 8.2, 1H], 3.80 [s, 3H], 3.75 [s, 2H], 3.71 ꢀ 3.59 [m, 1H], 3.34 [d, J = 7.6, 2H],
1.72 ꢀ 0.69 [m, 10H]. 13C NMR δ 168.0, 160.5, 160.2, 140.5, 138.0, 134.6, 130.4, 128.7, 126.6,
125.9, 121.6, 115.0, 113.3, 55.3, 48.0, 41.6, 32.7, 32.4, 25.5, 24.9. HRꢀMS found: [M+H]+
408.2180; C25H30NO4 requires 408.2175.
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(2E,4E)-(Z)-6-chloro-1-(cyclohexylamino)-1-oxohex-2-en-2-yl hexa-2,4-dienoate (18). Yellow
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oil (15 mg, 43% yield) upon flash chromatography (silica gel, DCM/EtOAc/PE 1:1:6). Rf = 0.22
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(DCM/EtOAc/PE 1:1:6). H NMR δ 7.49 ꢀ 7.40 [m, 1H], 6.47 [t, J = 7.7, 1H], 6.36 ꢀ 6.22 [m, 2H],
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5.92 [d, J = 14.8, 1H], 5.75 [d br, J = 7.8, 1H], 3.87 ꢀ 3.74 [m, 1H], 3.53 [t, J = 6.4, 2H], 2.21 [m,
2H], 1.95 ꢀ 1.86 [m, 7H], 1.70 ꢀ 1.07 [m, 8H]. 13C NMR δ 164.2, 160.9, 148.3, 142.1, 141.5, 129.6,
125.3, 116.4, 48.3, 44.3, 33.0, 31.0, 25.6, 24.8, 23.4, 18.9. HRꢀMS found: [M+H]+ 340.1678;
C18H27ClNO3 requires 340.1679.
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(Z)-1-(cyclohexylamino)-1-oxo-3-phenylbut-2-en-2-yl propionate (20). White foam (80 mg, 55%
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yield) upon flash chromatography (silica gel, PE/EtOAc 8:2). Rf = 0.36 (PE/EtOAc 8:2). H NMR
(300 MHz, CDCl3) δ 7.36 ꢀ 7.17 [m, 5H], 5.93 (d br, J = 7.7, 1H), 3.92 ꢀ 3.80 [m, 1H], 2.40 [s, 3H],
2.16 [q, J = 7.6, 2H], 1.98 ꢀ 1.93 [m, 2H], 1.74 ꢀ 1.59 [m, 2H], 1.47 ꢀ 1.12 [m, 6H], 0.93 [t, J = 7.6,
3H]. 13C NMR δ 172. 9, 162.7, 139.7, 136.3, 135.3, 128.3, 127.7, 127.2, 48.1, 33.0, 27.4, 25.6, 24.8,
19.8, 8.9. HRꢀMS found: [M+H]+ 316.1919; C19H26NO3 requires 316.1913.
(Z)-1-(cyclohexylamino)-1-oxo-3-phenylbut-2-en-2-yl 2-phenylacetate (21). White solid (75 mg,
75% yield) upon flash chromatography (silica gel, DCM/EtOAc/PE 1:1:5.5), m.p.: 124.0 ꢀ 125.4
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°C. Rf = 0.42 (PE/EtOAc 8:2). H NMR δ 7.33 ꢀ 7.28 [m, 6H], 7.21 ꢀ 7.13 [m, 4H], 5.45 [d br, J =
8.0, 1H], 3.77 ꢀ 3.64 [m, 1H], 3.41 [s, 2H], 2.45 [s, 3H], 1.77 ꢀ 1.71 [m, 2H], 1.64 ꢀ 1.52 [m, 2H],
1.36 ꢀ 1.20 [m, 2H], 1.13 ꢀ 0.99 [m, 2H], 0.86 ꢀ 0.73 [m, 2H]. 13C NMR δ 169.2, 162.1, 139.9,
137.0, 135.5, 133.0, 129.3, 129.1, 128.3, 127.8, 127.6, 127.2, 47.9, 41.2, 32.7, 25. 6, 24.9, 19.9.
HRꢀMS found: [M+H]+ 378.2076; C24H28NO3 requires 378.2069.
(Z)-1-(butylamino)-1-oxo-3-phenylbut-2-en-2-yl 2-phenylacetate (22). White solid (65 mg, 54%
yield) upon flash chromatography (silica gel, DCM/EtOAc/PE 1:1:5.5), m.p.: 72.3 ꢀ 73.4 °C. Rf =
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0.34 (DCM/EtOAc/PE 1:1:5.5). H NMR δ 7.35 ꢀ 7.11 [m, 10H], 5.56 [t br, J = 6.0, 1H], 3.42 [s,
2H], 3.12 [dt, J = 6.9, 6.0, 2H], 2.46 [s, 3H], 1.32 ꢀ 1.14 [m, 4H], 0.87 [t, J = 7.0, 3H]. 13C NMR δ
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169.3, 163.0, 139.8, 137.3, 135.4, 132.9, 129.2, 129.0, 128.3, 127.8, 127.6, 127.1, 41.2, 39.0, 31.4,
20.1, 20.0, 13.8. HRꢀMS found: [M+H]+ 352.1919; C22H26NO3 requires 352.1913.
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Acknowledgments
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The authors wish to thank students Andrea Bozzano, Thomas Virdis and Marta Nola for their
valuable contribution. The flow apparatus was set up thanks to the invaluable advice of Prof.
Alessandro Massi. PRIN 2009 ("Synthetic Methodologies for Generation of Biologically Relevant
Molecular Diversity") and University of Genova (“Progetti di Ateneo 2011 and 2012”) are
acknowledged for financial support. A.B. wish to thank Lamberti SpA for a generous donation.
Supporting Information availability statement
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Copies of H NMR and 13C NMR spectra for compounds 7, 9-18 and 20-22. Set up of the flow
apparatus and emission spectra of UV lamps. This material is available free of charge via the
Internet at http://pubs.acs.org.
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4071.
[7] La Spisa, F.; Tron, G. C.; El Kaïm, L. Synthesis, 2014, 46, 829–841.
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[8] Ugi, I.; Rosendahl, K. Chem. Ber. 1961, 94, 2233–2238. The initial structure of the ketene
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