Formation of Imidazolones by Ring Closure of α-Isocyanoamides: Exploring New Reactivities

Article abstract of DOI:10.1055/s-0040-1708015

Base-mediated ring closure of α,α-disubstituted α-isocyanoamides with further electrophilic trapping has previously been explored, but with limited applications. In this work, we wished to unravel the reactivities of these compounds and, in particular, to allow palladium-catalyzed coupling at the C2 position.

Full text of DOI:10.1055/s-0040-1708015

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© Georg Thieme Verlag Stuttgart · New York
2020, 31, A–E
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S. Frippiat et al.
Letter
Synlett
Formation of Imidazolones by Ring Closure of -Isocyanoamides:
Exploring New Reactivities
O
O
O
Bn
Steven Frippiat
Ph
Ph
1) n-BuLi
2) E+
N
N
N
H
Claire Leterrier
N+
N
N
E
R
C–
Christine Baudequin
Christophe Hoarau
Laurent Bischoff*
E, R = hydroxyalkyl, ketone, thioether, Br, I, Ar,
alkynes: 17 examples; yield = 66–88%
electrophilic trapping, arylation, Suzuki and Sonogashira couplings
Normandie University, COBRA, UMR 6014 et FR 3038,
University of Rouen; INSA Rouen; CNRS, IRCOF, 1 rue
Tesnière, 76821 Mont-Saint-Aignan Cedex, France
Laurent.bischoff@univ-rouen.fr
Received: 10.03.2020
Accepted after revision: 27.03.2020
Published online: 16.04.2020
alkylated to yield the required thioethers, but could itself
also undergo the same coupling reaction.4b
A few years ago, we succeeded in performing a direct
arylation at C2 starting from 2H substrates by using cooper-
ative Cu/Pd catalysis.5,6 4,4-Dialkylated imidazolones with a
benzyl group at N15 were sufficiently stable under the ary-
lation conditions, whereas 4-arylidene compounds6 re-
quired an ortho-directing group, such as a 2-picolinyl sub-
stituent, at this position. Although cooperative Cu/Pd catal-
ysis was found to be possible at C2, the direct lithiation of
2H-cycloalkylated imidazolones with BuLi gave no trace of
lithiation at this position, suggesting a very high pKa value.
In addition, the Nef reaction of -isocyano(secondary)am-
ides with electrophiles leads to highly reactive intermedi-
ates that undergo further attack at the oxygen atom, rather
than imidazolone formation (Scheme 1).7,8 This reactivity
DOI: 10.1055/s-0040-1708015; Art ID: st-2020-d0146-l
Abstract Base-mediated ring closure of ,-disubstituted -isocyano-
amides with further electrophilic trapping has previously been ex-
plored, but with limited applications. In this work, we wished to unravel
the reactivities of these compounds and, in particular, to allow palladi-
um-catalyzed coupling at the C2 position.
Key words isocyanides, imidazolones, lithium compounds, palladium
catalysis, coupling
Imidazolones are widely studied compounds, usually
prepared for their biological properties1 or as fluorescent
molecules.2 In the latter cases, a conjugated push–pull sys-
tem is required, necessitating the presence of a double bond
at C4. These imidazolones were first prepared over a centu-
ry ago by using Erlenmeyer’s method.3 4,4-Dialkylated im-
idazolones (generally cycloalkylated compounds, such as
cycloleucine derivatives, or spiro compounds with five- or
six-membered-ring carbocycles or heterocycles) are gener-
ally studied for their pharmaceutical properties, and there
are numerous reports and patents relating to their synthe-
sis.1 The main synthetic routes to these targets rely on con-
densation reactions that are often limited to reactive com-
pounds with low steric hindrance or that require harsh con-
ditions that are not amenable to the use of functionalized
molecules. Besides these methods, there are a few reports
of specific syntheses devoted to more convergent late-stage
functionalization at C2. Surprisingly, cross-coupling at C2
remains scarcely studied, the only example reported in the
literature being a Liebeskind–Srogl coupling using an S-
methyl group as the leaving group and coupling to a boron-
ic acid or stannane,4a or using an S-benzyl leaving group at
C2 and coupling to boronic acids. Thiohydantoin has been
O
O
Bn
Ph
n-BuLi, –78 °C
N
H
N
Li
O
N+
N+
C–
C–
E+
BnHN
O
Ph
N
N
N
n-BuLi
Li
E
O
N
E+
Ph
N
BnN
N
O
H
Ph
O
N
N
E
E
Nef-like reaction gives O-attack
No direct lithiation of imidazolones at C2
Few electrophiles described in the literature
Our work : - study other electrophiles
- develop palladium-catalyzed couplings at C2
Scheme 1 Electrophilic trapping at C2
© 2020. Thieme. All rights reserved. Synlett 2020, 31, A–E
Georg Thieme Verlag KG, Rüdigerstraße 14, 70469 Stuttgart, Germany

B
S. Frippiat et al.
Letter
Synlett
has been exploited with tertiary and secondary amides to
obtain oxazoles and 4-imino-4H-3,1-benzoxazines, respec-
tively.9,10
imidazolone with a leaving group at such a position being
used in a Suzuki coupling;4 in that work, the authors pre-
pared the substrate through S-alkylation of the correspond-
ing thiohydantoin.
Thus, high conjugation of the amide bond does not favor
the attack by the nitrogen atom that would lead to the im-
idazolone core. However, the nitrogen’s reactivity can be re-
stored through base-mediated cyclization of the amide an-
ion onto an adjacent isocyanide, as described by Marcaccini
and co-workers.11 However surprisingly, this trapping of
the intermediate lithiated species with electrophiles has
rarely been exploited, the reported reactions being limited
to protonation and trapping with various aldehydes.
O
O
Bn
Ph
1) n-BuLi, –78 °C
N
N
H
2) E+, –78 °C to 0 °C
N+
N
E
C-
1
2a–h
Ph
Ph
Ph
O
O
O
N
N
N
OH
OH
OH
N
N
N
In the present work, we wished to exploit the potential
of other electrophiles. As shown in Scheme 2, we first reex-
amined the reactivity of classical electrophiles. Initially, we
checked the reaction of -isocyanoamide 1 with acids. Ace-
tic acid gave none of the expected 3-benzyl-1,3-di-
azaspiro[4.4]non-1-en-4-one (2) at room temperature, and
partial degradation occurred. Treatment with 4M HCl in
1,4-dioxane at 50 °C mainly led to degradation, with only
21% of 2 being isolated, with no trace of the isocyanide re-
maining. Similar results were obtained with Lewis acids
such as LiCl and BF3·OEt2. We then confirmed that, after pri-
or deprotonation of 1 with butyllithium, trapping with ace-
tic acid at –78 °C gave a 91% yield of the 2H-imidazolone 2,
in agreement with previous findings; suggesting low-tem-
perature ring closure before reaction with the electrophile.
As far as the scope of electrophiles is concerned, to the best
of our knowledge, only the reactions of a series of aldehydes
have been described, with no reports of reactions of cyclo-
leucine derivatives. We tested a few aldehydes such as
benzaldehyde and anisaldehyde, which gave 2a and 2b in
yields of 80 and 66%, respectively. As expected, the pres-
ence of an ester group at the para-position led to a good
chemoselectivity in favor of the aldehyde (2c; 71% yield).
Nonenolizable ketones such as fluorenone and benzophe-
none also provided the corresponding tertiary alcohols 2d
and 2e in high yields. To our surprise, only one example of a
4,4-dialkylated imidazolone containing a ketone group at
C2 is described in the literature, and this was prepared by a
completely different pathway.12 This encouraged us to ex-
amine trapping with a Weinreb amide, which afforded a
useful 69% yield of 2f, compared with the 66% yield ob-
tained through reaction with the acyl chloride. However, at-
tempts at formylation with DMF, N-formylpiperidine, N-
formylmorpholine, or the more-reactive N-formylcarbazole
were unsuccessful. The use of TMSCl or diethylphosphoryl
chloride as an electrophile also failed, leading only to the
2H-imidazolone 2. Finally, quenching with diphenyl disul-
fide or dibenzyl disulfide afforded the corresponding thio-
ethers 2g and 2h in yields of 81 and 73%, respectively. Note
that the imidazolone substituted with a C2 S-benzyl group
is almost the only example described in the literature of an
Ph
2a: 80%
2b: 66%
2c: 71%
Ph
MeO
MeO2C
O
N
Ph
O
N
N
OH
N
OH
Ph
2d: 73%
2e: 83% Ph
Ph
O
O
N
N
Ph
Ph
N
2g: 81%
O
N
S
O
2f: 69%a
66%b
Ph
N
2h: 73%
N
S
MeO
Ph
Scheme 2 Reactivity of the lithiated imidazolone ring towards various
electrophiles. a Reaction with the Weinreb amide. b Reaction with the
acyl chloride.
This unexpected lack of availability of substrates for the
synthesis of imidazolones through palladium-catalyzed
coupling at C2 motivated us to clarify the reactivity of such
compounds. As stated above, Liebeskind–Srogl coupling
with substrates possessing an S-benzyl group has already
been described, so we therefore focused on halogenated
substrates.
In our initial attempts using the same conditions as
above, N-chlorosuccinimide gave only the 2H-imidazolone
2, with no reaction at the chlorine atom. However, high 80
and 88% isolated yields were obtained by using N-bromo-
or N-iodosuccinimide, respectively (Scheme 3). Note that
these products are the first examples of imidazolones bear-
ing a halogen atom at C2.
Ph
Ph
O
O
O
Bn
N
N
N
H
1) n-BuLi, –78 °C
+
N+
2) X+, –78 °C to 0 °C
N
N
X
H
C–
1
2
2i–j
with NBS: 2i: 80%
NIS: 2j: 88%
Scheme 3 Halogenation reactions
© 2020. Thieme. All rights reserved. Synlett 2020, 31, A–E

C
S. Frippiat et al.
Letter
Synlett
We also checked whether strong bases other than butyl-
lithium would be suitable for this reaction. Unfortunately,
only butyllithium proved efficient (Table 1). This confirmed
that the acidity of hydrogen atom at C2 might be very low,
leading to in situ quenching of the C2-lithiated species aris-
ing from ring closure with the amine generated by depro-
tonation with LDA, LiHMDS, or even LiTMP, and this be-
comes the only pathway in the case of LiHMDS. In addition,
magnesium or sodium species also gave low yields and, sur-
prisingly, compound 2 was obtained even when no proton
source was available (entries 5 and 6), suggesting that the
carbanion at C2 might itself deprotonate the starting amide
as soon as it is generated. In agreement with these results,
attempts at deprotonating imidazolone 2 with butyllithium
and quenching with D2O gave no trace of the 2-deuterated
product.
a) Our previous work: C–H arylation/vinylation of imidazolones
O
O
R2
R2
R1
Pd/Cu cat
R1
+
Ar-X
N
R3
N
R3
DBU, DMF, Δ
N
N
H
Ar
b) Pirali et al.: ring-closure of α-isocyanoamides with arynes
O
R2
O
R3
R2
R1
OTf
R1
N
H
KF, 18-crown-6
THF, 0 °C
N
R3
+
N+
N
TMS
C–
Ar
c) Zhu et al.: one-pot arylative ring closure of α-isocyanoesters with amines
O
O
R2
N
R2
R1
R1
OMe
Pd cat, Cu
or AgNO3 (R4 = H)
N
R3
R4-X
R3-NH2
+
+
N+
C–
R4
d) Our present work: arylation of α-isocyanoamides
O
O
1) n-BuLi then NBS
R3
R4
Bn
2) Suzuki or Sonogashira coupling
N
H
N
Table 1 Other Strong Bases Used in the Deprotonation of the Amide
N+
cat. Pd(OAc)2, Cu2O, 130 °C
N
C–
or 1 equiv Ar-Pd-I⋅TMEDA
CH2Cl2, rt
Entry
Base
Yield (%) of 2i
Yield (%) of 2
Scheme 4 Various routes available for the synthesis of imidazolones
substituted at C2; Ar = Ph, 4-Me-C6H4, 4-MeO-C6H4, 4-CN-C6H4
1
2
3
4
5
6
BuLi
80
55
53
0
0
33
33
98
67
67
LiTMP
LDA
LiHMDS
i-PrMgCl
NaH
um complex, and the byproduct 4 resulting from hydrolysis
was observed in only small amounts. Pleasingly, good yields
(68 to 91%) of compounds 3a–d were obtained with neu-
tral, electron-withdrawing, or electron-donating groups at
the para-position of the aryl group. Although Suzuki cou-
pling is a widely used tool in organic synthesis, such reac-
tions of 2-bromoimidazolones have not previously been de-
scribed in the literature. In addition, a one-pot procedure16
30
27
Encouraged by the availability of 2-bromo- and 2-iodo-
imidazolones 2i and 2j, we next turned our attention to-
wards the classical, although previously unreported, Suzuki
and Sonogashira couplings of these compounds.
Arylations of imidazolones at C2 by following different
strategies have been recently described in the literature
(Scheme 4). Our group reported the arylation and vinyla-
tion of 4,4′-disubstituted imidazolones5 under cooperative
Cu/Pd catalysis with various halides. By starting from an
isocyanoamide precursor instead of imidazolone, Pirali and
co-workers13 proposed the trapping of highly-reactive
arynes to give ring closure with arylation at C2. Recently,
Zhu and co-workers developed an interesting ring-closure
protocol starting from readily available isocyanoesters, in-
cluding the reaction of an activated isocyanide with an
amine before ring closure through amidification of the es-
ter. This reaction can be carried out with no arylation by sil-
ver nitrate activation,14 or with concomitant arylation if
Pd/Cu cooperative catalysis is involved.15 This strategy also
provided high structural diversity at N1.
Suzuki couplings:
Ph
Ph
O
O
N
N
1.5 equiv Ar-B(OH)2, 4 equiv K2CO3
10 mol% PdCl2⋅dppf
1,4-dioxane/H2O (2:1)
N
N
Br
Ar
2i
3a–d
Ar =
OMe
Ph
CN
3a: 68%
3b: 76%
3d: 76%
3c: 91%
or 68%a
O
possible byproduct:
N
N
Sonogashira couplings:
O
H
4
Ph
O
Ph
N
O
(2 equiv)
H
R
N
4 mol% CuI, 5 mol% PdCl2⋅dppf
Br
N
N
dioxane/Et3N = 3:2
As shown in Scheme 5, we used several commercially
available boronic acids in Suzuki coupling reactions with 2-
bromoimidazolone 2i. Despite its moderate stability, the
latter undergoes rapid oxidative addition with the palladi-
2i
R
R = Ph: 5a: 82 %
R = TMS: 5b: 79 %
Scheme 5 Palladium-catalyzed couplings at the C2 position: Suzuki
and Sonogashira reactions. dppf = 1,1′-bis(diphenylphosphino)ferro-
cene. a One-pot procedure.
© 2020. Thieme. All rights reserved. Synlett 2020, 31, A–E

D
S. Frippiat et al.
Letter
Synlett
was also developed, albeit giving a lower, but still useful,
68% yield of 3c. In addition, Sonogashira couplings of 2i
with ethynylbenzene or ethynyl(trimethyl)silane (the most
frequently used substrates for this reaction) gave 5a and 5b,
respectively, in yields of 82 and 79% yields. We believe that
these compounds are the first reported imidazolones con-
taining an alkyne substituent at C2.
Finally, we were interested in comparing our protocol
with the method developed by Zhu and co-workers,15 in-
volving -isocyanoesters, with an arylation procedure
starting from the analogous -isocyanoamide under the
same experimental conditions. The 51% yield of compound
3b (Scheme 6) shows that these conditions afforded a simi-
lar yield when using the amide substrate to that previously
obtained starting from the methyl ester (57%). Arylation
starting from the ester is more convenient because the
need for prior preparation of the amide is avoided although,
in our protocol, no further amidification of the transient
amidine is required.
Funding Information
This work has been partially supported by INSA Rouen, Rouen Univer-
sity, CNRS, EFRD, European INTERREG IV A France (Channel) and
Labex SynOrg (ANR-11-LABX-0029). S.F. is grateful to the Region Nor-
mandie for a grant.
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Supporting Information
Supporting information for this article is available online at
https://doi.org/10.1055/s-0040-1708015.
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References and Notes
(1) (a) Chen, Y.; O’Connor, S. J.; Gunn, D.; Newcom, J.; Chen, J.; Yi, L.;
Zhang, H.-J.; Hunyadi, L. M.; Natero, R. WO 2004058727, 2004.
(b) Murugesan, N.; Tellew, J. E.; Macor, J. E.; Gu, Z. WO
200001389, 2000. (c) Murugesan, N.; Tellew, J. E.; Macor, J. E.;
Gu, Z. US 20020143024, 2002. (d) Ye, P.; Sargent, K.; Stewart, E.;
Liu, J.-F.; Yohannes, D.; Yu, L. J. Org. Chem. 2006, 71, 3137.
(e) Malamas, M. S.; Erdei, J. J.; Fobare, W. F.; Quagliato, D. A.;
Antane, S. A.; Robichaud, A. J. US 20070072925, 2007. (f) Berg,
S.; Holenz, J.; Hoegdin, K.; Kihlstroem, J.; Kolmodin, K.;
Lindstroem, J.; Plobeck, N.; Rotticci, D.; Sehgelmeble, F.;
Wirstam, M. WO 2007145569, 2007. (g) Berg, S.; Hallberg, J.;
Hoegdin, K.; Karlstroem, S.; Kers, A.; Plobeck, N.; Rakos, L. WO
2008076044, 2008. (h) Bignan, G. C.; Connolly, P. J.; Lu, T. L.;
Parker, M. H.; Ludovici, D.; Meyer, C.; Meerpoel, L.; Smans, K.;
Rocaboy, C. WO 2014039769, 2014.
(2) (a) Yampolsky, I. V.; Kislukhin, A. A.; Amatov, T. T.; Shcherbo, D.;
Potapov, V. K.; Lukyanov, S.; Lukyanov, K. A. Bioorg. Chem. 2008,
36, 96. (b) Wu, L.; Burgess, K. J. Am. Chem. Soc. 2008, 130, 4089.
(3) (a) Erlenmeyer, E. Jr. Liebigs Ann. Chem. 1893, 275, 1. For exam-
ples of transformations of oxazolones into imidazolones by con-
densation reactions, see: (b) Muselli, M.; Colombeau, L.;
Hédouin, J.; Hoarau, C.; Bischoff, L. Synlett 2016, 27, 2819; and
references cited therein.
(4) (a) Oumouch, S.; Bourotte, M.; Schmitt, M.; Bourguignon, J.-J.
Synthesis 2005, 25. (b) Gosling, S.; Rollin, P.; Tatibouet, A. Syn-
thesis 2011, 3649.
(5) Muselli, M.; Baudequin, C.; Hoarau, C.; Bischoff, L. Chem.
Commun. 2015, 51, 745.
(6) Muselli, M.; Baudequin, C.; Perrio, C.; Hoarau, C.; Bischoff, L.
Chem. Eur. J. 2016, 22, 5520.
(7) Bossio, R.; Marcaccini, S.; Pepino, R.; Polo, C.; Valle, G. Synthesis
1989, 641.
(8) García-Valverde, M.; Marcaccini, S.; González-Ortega, A.; Javier
Rodríguez, F.; Rojo, J.; Torroba, T. Org. Biomol. Chem. 2013, 11,
721.
(9) Mossetti, R.; Pirali, T.; Cesare Tron, G.; Zhu, J. Org. Lett. 2010, 12,
820.
(10) Bonne, D.; Dekhane, M.; Zhu, J. Org. Lett. 2005, 7, 5285.
(11) (a) Bossio, R.; Marcaccini, S.; Papaleo, S.; Pepino, R. J. Heterocycl.
Chem. 1994, 31, 397. (b) Bossio, R.; Marcaccini, P.; Paoli, P.;
Papaleo, S.; Pepino, R.; Polo, C. Liebigs Ann. Chem. 1991, 843.
(12) Yan, Q.; Carroll, P. J.; Gau, M. R.; Winkler, J. D.; Joullié , M. M. Org.
Lett. 2019, 21, 6619.
I
O
Bn
+
N
H
5 mol% Pd(OAc)2, 10 mol% PPh3
O
Ph
N+
2 equiv Cu2O, DMF, 130 °C
C–
N
1
R = Me: 3b, yield = 51%
N
(1.5 equiv)
1 equiv Ar-Pd-I⋅TMEDA complex
CH2Cl2, rt, 30 min
R
R = H: 3a: 79%; R = Me: 3b: 86%; R = OMe: 3c: 87%; R = CN: 3d: 76%
Scheme 6 Ring closure with palladium complexes, under catalytic or
stoichiometric conditions; Ar = Ph, 4-Me-C6H4, 4-MeO-C6H4, 4-CN-C6H4
Nevertheless, we were interested in knowing whether
the reaction with the isocyanide itself required high tem-
peratures. For this purpose, we focused on various previ-
ously prepared stable palladium complexes described in the
literature,17 which were used in stochiometric amounts in a
direct reaction with the isocyanoamide 1. This allowed us
to gain a better understanding of the reactivity of such sub-
strates, because high yields of the isolated compounds were
obtained under mild conditions (room temperature, di-
chloromethane, 30 minutes), indicating the isocyanide has
a high reactivity and that ring closure with the nitrogen
atom of the amide occurs immediately.
In conclusion, we have revealed a previously unexplored
aspect of the formation of imidazolones through a ring-
closing reaction of isocyanoamide 1. Surprisingly, given the
potential of such molecules as biologically active materials,
some simple conversions involving this method in this se-
ries have not been previously developed. This protocol
should pave the way towards a higher molecular diversity
in this field.
(13) Gesù , A.; Pozzoli, C.; Torre, E.; Aprile, S.; Pirali, T. Org. Lett. 2016,
18, 1992.
(14) Clemenceau, A.; Wang, Q.; Zhu, J. Org. Lett. 2017, 19, 4872.
(15) Clemenceau, A.; Wang, Q.; Zhu, J. Org. Lett. 2018, 20, 126.
© 2020. Thieme. All rights reserved. Synlett 2020, 31, A–E

E
S. Frippiat et al.
Letter
Synlett
(16) 3-Benzyl-2-(4-methoxyphenyl)-1,3-diazaspiro[4.4]non-1-
en-4-one (3c); One-Pot Procedure
CH2Cl2. The solvents were removed under reduced pressure, and
the crude product was purified by gradient column chromatog-
raphy [silica gel, PE to PE–EtOAc (80:20)] to give a pale yellow
oil (199 mg, 68%).
A 2.5 M solution of BuLi in hexanes (0.42 mL, 1.056 mmol, 1.2
equiv) was added dropwise to a solution of isocyanoamide 1
(200 mg, 0.88 mmol) in dry THF (35 mL) at –78 °C, and the
mixture was stirred for 1 h at –78 °C. NBS (188 mg, 1.056 mmol,
1.2 equiv) was introduced into the solution, and the mixture
was allowed to warm to r.t. The mixture was then concentrated
to ~1 mL and introduced into a sealed tube containing a mag-
netic stirrer bar. (4-Methoxyphenyl)boronic acid (1.32 mmol,
1.5 equiv), K2CO3 (4.24 mmol, 4 equiv), and PdCl2(dppf) (0.1
equiv, 0.088 mmol) were added, followed by 1,4-dioxane (4 mL)
and H2O (2 mL). The mixture was degassed and placed under N2,
and the tube was sealed and heated at 80 °C for 18 h. The
mixture was then filtered through a Celite pad, washing with
1H NMR (300 MHz, DMSO-d6):  = 7.47 (d, J = 8.79 Hz, 2 H,
H
Arom), 7.31–7.21 (m, 3 H, HArom), 7.00–6.97 (m, 2 H, HArom), 6.95
(d, J = 8.79 Hz, 2 H, HArom), 4.76 (s, 2 H, HBn), 3.77 (s, 3 H, HMeO),
1.95–1.80 (m, 8 H, HCy). 13C NMR (75 MHz, DMSO-d6):  = 186.3,
161.0, 159.5, 137.0, 129.6, 128.7, 127.2, 126.0, 121.9, 113.9,
76.3, 55.3, 44.1, 37.2, 25.5.
(17) (a) Markies, B. A.; Canty, A. J.; de Graaf, W.; Boersma, J.; Janssen,
M. D.; Hogerheide, M. P.; Smeets, W. J. J.; Spek, A. L.; van Koten,
G. J. Organomet. Chem. 1994, 482, 191. (b) Ball, N. D.; Gary, J. B.;
Ye, Y.; Sanford, M. S. J. Am. Chem. Soc. 2011, 133, 7577.
© 2020. Thieme. All rights reserved. Synlett 2020, 31, A–E