Metal-free 5-exo-dig cyclization of propargyl urea using TBAF

Article abstract of DOI:10.1016/j.tetlet.2014.06.098

We present the first results of the intramolecular cyclization of propargyl ureas catalyzed by TBAF. Depending on the substituents close to the triple bond, imidazolone or methylene-imidazolidinone was obtained. The dual activation of the triple bond by interaction with TBAF via the cation-π is thought to be responsible.

Full text of DOI:10.1016/j.tetlet.2014.06.098

Accepted Manuscript
Metal-free 5-exo-dig cyclization of propargyl urea using TBAF
Florent Huguenot, Clémence Delalande, Michel Vidal
PII:
S0040-4039(14)01111-3
DOI:
http://dx.doi.org/10.1016/j.tetlet.2014.06.098
TETL 44823
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
19 May 2014
22 June 2014
24 June 2014
Please cite this article as: Huguenot, F., Delalande, C., Vidal, M., Metal-free 5-exo-dig cyclization of propargyl urea
using TBAF, Tetrahedron Letters (2014), doi: http://dx.doi.org/10.1016/j.tetlet.2014.06.098
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Metal-free 5-exo-dig cyclization of propargyl
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Florent Huguenot,∗ Clémence Delalande and Michel Vidal

1
Tetrahedron Letters
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Metal-free 5-exo-dig cyclization of propargyl urea using TBAF
Florent Huguenot,∗ Clémence Delalande and Michel Vidal and
CNRS UMR 8638, Université Paris Descartes, Faculté des sciences pharmaceutiques et biologiques, 4 avenue de l’observatoire, Paris 75006, France
ARTICLE INFO
ABSTRACT
Article history:
Received
We present the first results of the intramolecular cyclization of propargyl ureas catalyzed by
TBAF. Depending on the substituents close to the triple bond, imidazolone or methylene-
imidazolidinone was obtained. The dual activation of the triple bond by interaction with TBAF
via the cation-π is thought to be responsible.
Received in revised form
Accepted
Available online
2009 Elsevier Ltd. All rights reserved.
Keywords:
Propargyl urea
Intermolecular cyclization
TBAF
Cation-π interaction
Imidazolinone
Substituted imidazol-2-ones and other cyclic ureas are of
considerable interest in pharmaceutical and medicinal chemistry,
group. However, the cycloisomerization of 3a yielded the
imidazolone derivative 4c with the simultaneous hydrolysis of
the methyl ester moiety.
1
as many biologically active compounds contain these moieties.
Cyclic five-membered ring ureas have been demonstrated to act
as HIV protease inhibitors,2 receptor antagonists or antagonists,
and these structures have also been employed as monomeric units
for biopolymer scaffolds that exhibit greater stability than typical
peptides.3 In addition, substituted imidazolidin-2-ones have found
applications in organic synthesis as chiral auxiliaries,4 and as
precursors to α-amino acids5 and vicinal diamines.6
Due to the utility of imidazolidin-2-ones, a number of
methods have been developed for their construction. The most
commonly employed method for the synthesis of these
compounds involves the generation of 1,2-diamines, which are
then converted to cyclic ureas by treatment with phosgene or
phosgene equivalents such as carbonyl diimidazole,7 or through
other carbonylation methods.8 However, the preparation of 1,2-
diamines frequently requires several steps. Several alternative
strategies for the synthesis of imidazolidin-2-ones have been
developed by diamination of alkene or aziridine ring extension.6, 9
Scheme 1. Cyclization of the propargyl urea thiophene carboxylic
ester. Reagents and conditions: (a) KOHaq (1M), MeOH then HClaq
Intramolecular ring closure reactions are one useful method
for constructing targeted cyclic or polycyclic skeletons. The ring
size of the products can be predicted by Baldwin’s rule,12 based
on stereoelectronic effects. Starting from propargyl carbamates,
amides, amidines or amino alkynes, intramolecular ring closure
reactions catalyzed by different metallic species have been
widely described.13 Despite these advances, few existing methods
allow for ring closure of a simple, readily available acyclic urea
derivative, with the formation of both a C–C and a C–N bond in
the same step. Moreover, methods that accomplish this are
limited by the use of metallic species.9b
Our recent investigations into the discovery of a disrupting
reagent for the Vascular Endothelial Growth Factor
(VEGF)//VEGF-receptor 1 interaction10 required the synthesis of
a propargyl urea skeleton attached to a thiophene carboxylic
methyl ester. The alkyne moiety is used by click chemistry to
design new molecules that can be quickly diversified. A specific
reactivity of the carboxylic ester ureidothiophene
1 was
We decided to study the cyclization reaction of the propargyl
urea motif in a simpler model by replacing the thiophene-
carboxylic acid phenyl moiety. Only compound 5b, 1-methyl-4-
methylene-3-phenylimidazolidin-2-one was formed starting from
rediscovered during the hydrolysis step in basic medium to lead
to thienopyrimidinedione 2 (Scheme 1). 11 In order to avoid the
formation of pyrimidinedione, nitrogen was replaced by a methyl
———
∗ Corresponding author. Tel.: +33-1-537-315-65; fax: +33-1-432-914-03; e-mail: florent.huguenot@parisdescartes.fr

2
Tetrahedron Letters
5a, characterized unambiguously by 1H and 13C NMR. Various
a 67% yield (Table 1, Entry 11), without reflux and while
conserving the methyl ester moiety. The Bronsted basic activity
of fluoride was necessary to catalyze the reaction. The activation
of the triple bond was performed by the tetrabutylammonium
cation, and the nucleophilicity of the nitrogen was increased by
the fluoride anion. Although the lithium cation is described as an
excellent activator of the triple bond,16 LiF does not allow the
cyclization of 5b due to the very low dissociation of LiF in THF.
attempts to purify compound 5b led to the rapid formation of
isomer 5c, which is thermodynamically more stable (Scheme 2).
This behavior has already been observed for methylidene-2-
oxazolidinone.14
Next, we decided to focus our efforts on the preparation of an
imidazolidin-2-one skeleton with an exocyclic double bond using
ureas 8a-14a, prepared from 2-methylbut-3-yn-2-amine. In this
case, no isomerization of the double bond from the exocyclic to
the endocyclic position is possible. These imidazolidin-2-ones
can be evaluated by the formation of hydantoin (2,4-
imidazolidinedione), by the alkylation or acetylation of the
nitrogen NH, or by the reduction of the exocyclic double bond.
Also, this structure has an enamine function that may be used as
a nucleophile, or lead to hydrolysis and ring-opening. 8a and 9a
were synthesized by the condensation of phenyl or benzyl
Scheme 2. Cyclization and isomerization of phenyl propargyl urea,
5a
Initially, we undertook an examination of the base effect in
order to apply this methodology to substrates that are more
complex and therefore more sensitive to bases such as KOH or
NaH. Thus we synthesized 3-benzyl-1-methyl-1-(prop-2-yn-1-yl)
urea, 6a. We observed that no organic compound tested allowed
cyclization (Table 1, entries 1-5). The propargyl urea 6a was
fully recovered, even after a long time in refluxing MeOH or
THF solvent in the presence of a large excess of base. The
reactivity to KOH or other strong base appears to be limited to
ureas bearing an aromatic ring (or an electron-withdrawing
substituent) that can significantly reduce the pKa of the proton.15
The increase in the pKa of the urea due to the replacement of the
phenyl group by a benzyl group is responsible for the non-
reactivity of 6a. This aryl/alkyl selectivity should be of interest in
the case of polyurea type molecules.
isocyanate
respectively with
2-methylbut-3-yn-2-amine.
Compounds 10a-14a were prepared in two steps using a
phenyloxycarbonyl (Poc) strategy (Scheme 3). Phenylcarbamate
7 was prepared in large amounts by the condensation of phenyl
chloroformate with 2-methylbut-3-yn-2-amine without the
addition of a base, to avoid the release of the phenolate from the
carbamate. Then compound 7 was converted into ureas 8a–14a
by the substitution of the phenolate moiety by alkylamines, in
moderate to good yields.
The best strategy was the activation of the triple bond through
a cation-π interaction (Table 1).16 Only TBAF was able to
catalyze the intramolecular reaction at room temperature (Table
1, Entry 6). In 1995, Jacobi and Rajeswari reported the first
TBAF-promoted 5-exo-dig cyclization reaction starting from
alkynylamide.17 However, in spite of its efficiency, the
application of TBAF-promoted cyclization reactions to urea has
not been reported.18 We validated our strategy by transforming
ureas 5a and 3a, which generated, respectively, the aromatic
imidazol-2-ones 5c in a 66% yield (Table 1, Entry 10), and 3c in
Scheme 3. Strategy for the synthesis of ureas 8a-15a. Reagents and
conditions: (a) PhCO2Cl, THF, r.t., 24 h, 82%; (b) RNH2, Et3N,
CHCl3, reflux, 24 h, R : nPr 10a 98%, iPr 11a 96%, tBu 12a 88%,
CH2CO2Et 13a 94%, CH2CH2Cl 14a 57%.
Table 1. Reagent screening for the synthesis of imidazol-2-one derivatives produced via imidazolidin-2-one
Ratiob
Xa:Xb:Xc
Entry
Compound
R
Reagent (1.0 equiv.)
Conditions
Product
Yield %c
1
2
6a
6a
6a
6a
6a
6a
6a
6a
6a
5a
3a
Bn
KOH
NaH
THF or MeOH reflux, 48h
THF or MeOH reflux, 48h
THF or MeOH reflux, 48h
THF or MeOH reflux, 48h
THF or MeOH reflux, 48h
THF, RT, 8h
100:0:0
100:0:0
100:0:0
100:0:0
100:0:0
0:63:37
100:0:0
100:0:0
100:0:0
0:85:15
0:100:0
-
-
-
-
Bn
3
Bn
DBU
-
-
4
Bn
DBN
-
-
5
Bn
MeONa
TBAFa
TBAIa
LiF
-
-
6
Bn
6c
-
70
-
7
Bn
THF, reflux, 48h
8
Bn
THF, reflux, 48h
-
-
9
Bn
NH4F
TBAFa
TBAFa
THF, reflux, 48h
-
-
10
11
Ph
THF, RT, 8h
5c
3c
66
67
Methyl thiophene-3-carboxylate
THF, RT, 8h
aUsing freshly prepared 1M solution in THF.
bDetermined by 1H NMR analysis of the crude mixture.
c6c, 5c and 3c as isolated products.

3
With these optimized reaction conditions in hand, we set out
to examine the generality and scope of this cyclization reaction
(Table 2). For all of ureas 8a-12a, the cyclization reaction
required refluxing THF. This suggests that the conformation of
the urea influences cyclization conditions. 8a-12a ureas are
predominantly in trans conformation, while the cyclization
requires the cis conformation. Despite the potential hydrolysis of
the N-acyl enamine moiety,19 imidazolidin-2-ones 8b-12b were
obtained in good yields after purification on silica gel. The
cyclization was carried out with both linear and hindered
substituents such as tert-butyl (Table 2, Entry 5).
Scheme 4. Synthesis and non-reactivity of propargyl urea 16a.
Reagents and conditions: (a) BnEtNH, Et3N, CHCl3, reflux, 24 h (b)
TBAF, THF, reflux, 48h.
Finally, we looked at the outcome of the cyclization reaction
from 13a and 14a (Scheme 5). 13a leads to the exclusive
formation of the imidazol-2-one 17 by a 5-exo-tet cyclization
process. Similarly, hydantoin 18 is the only product obtained
from 14a through a 5-exo-trig cyclization process. It is very
interesting to note that there is no competition between the
cyclization processes authorized by the rules of Baldwin. This
specificity may be of use later for the formation of a cyclic
pharmacophore (imidazolone, hydantoin) functionalized by a
triple bond.
Table 2. Substituent effect for cyclization to imidazolidin-2-one
8b-12b.
Entry
Substrate
R
Product
Yield %a
1
2
3
4
5
8a
Ph
8b
71
9a
CH2Ph
CH2CH2CH3
CH(CH3)2
C(CH3)3
9b
70
10a
10b
11b
12b
81
11a
98
12a
72b
a Yield of 8b-11b as isolated products.
b Yield of 12b in crude mixture by 1H NMR.
Interestingly, we did not observe the formation of a 6-
membered ring by the 6-endo-dig process,12 although the yield
was sometimes quite low for 12b. The reaction was very clean,
with the crude reaction mixture containing only the 5-exo-dig
cyclization product and TBAF. Almost quantitative presence of
TBAF in the crude reaction mixture showed that TBAF was not
consumed by the reaction and that it is thus possible to use it at
catalytic concentration. To provide a mechanism for this catalytic
cycloisomerization, 9a was set in the presence of 0.2 equivalent
of a freshly prepared TBAF solution in THF: after refluxing for 2
Scheme 5. Selective 5-exo-tet and 5-exo-trig cyclization reactions of
propargyl urea.
In summary, TBAF promotes the cyclization of propargyl
urea to an imidazolidin-2-one core with an exocyclic double bond
through a 5-exo-dig N-cyclization process. Depending on the
nature of the propargyl moiety, the imidazolidin-2-one is quickly
converted into the aromatic imidazol-2-one ring. The use of a
gem-dimethylpropyne pattern allows the preparation of an
imidazolidin-2-one core with a stable exocyclic double bond. We
have also shown in our case that the 5-exo-tet and 5-exo-trig
cyclization processes predominated over the 5-exo-dig
cyclization process.
1
hours, H NMR analysis showed the formation of 9b in 54%
yield. Upon purification by chromatography on silica gel, 12b
was relatively rapidly hydrolyzed to 3-oxo-urea 15 via ring
opening (Figure 1). Spectroscopic and physical data were in
agreement with the structure of 15.20
Acknowledgments
This work was supported by an ANR grant (SALSA project).
Figure 1. 1-(tert-butyl)-3-(2-methyl-3-oxobutan-2-yl)urea 15.
References and notes
Although NMR analyses are consistent with the structure of
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methyleneoxazolidin-2-imine (through an N-cyclization). The 5-
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geometrically impossible.22 For 16a, TBAF was not capable of
activating the O-cyclization reaction, whereas this was the only
5-exo-dig process possible (Scheme 4).
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