Regioselective one-pot three component synthesis of chiral 2-iminoselenazolines under sonication

Article abstract of DOI:10.1039/c5ra18763j

A one-pot multi component reaction of selenoureas, which are in situ generated from l-amino esters and isoselenocyanates, with α-bromoketone under ultrasonication. Selenourea and α-bromoketones formed 2-iminoselenazoles through a Hantzsch selenazole-type reaction. The steric effect of the α-substituted bromoketones on the rate of the tandem reaction was studied to understand the reaction mechanism by isolating the key reaction intermediate, 2-iminoselenol.

Full text of DOI:10.1039/c5ra18763j

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Regioselective One-pot Three Component
Synthesis of Chiral 2-Iminoselenazolines by
Sonication
Wong-Jin Chang1, Manohar V. Kulkarni1 and Chung-Ming Sun*1,2
1Department of Applied Chemistry, National Chiao-Tung University, Hsinchu 300,
Taiwan, R.O.C. 2Department of Medicinal and Applied Chemistry, Kaohsiung
Medical University, 100, Shih-Chuan 1st Road, Kaohsiung, 80708, Taiwan, ROC
E-mail : cmsun@mail.nctu.edu.tw

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Regioselective OneꢀPot Three Component Synthesis of
Chiral 2ꢀIminoselenazolines by Sonication
WongꢀJin Chang1, Manohar V. Kulkarni1 and ChungꢀMing Sun*1,2
1Department of Applied Chemistry, National ChiaoꢀTung University, Hsinchu 300ꢀ10, Taiwan
2Department of Medicinal and Applied Chemistry, Kaohsiung Medical University, 100, Shihꢀ
Chuan 1st Road, Kaohsiung, 807ꢀ08, Taiwan
Eꢀmail : cmsun@mail.nctu.edu.tw
Graphical Abstract
Abstract
A oneꢀpot multiꢀcomponent reaction of selenourea which is in situ generated from
Lꢀamino esters and isoselenocyanate reacted with αꢀbromoketone under
ultrasonication. Selenourea and αꢀbromoketone formed 2ꢀimino selenazoles by
Hantzsch selenazoleꢀtype reaction. The steric effect of αꢀsubstituted bromoketone
1

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on the rate of tandem reaction was studied to understand the reaction mechanism
by isolation of key reaction intermediate, 2ꢀiminoselenol.
Introduction
Functionalized organoselenium compounds serve as useful synthetic reagents and
exhibit a wide range of biological properties such as antitumor, antimicrobial and
antioxidant activities.1 Synthesis of isosteric selenium analogs of biologically
active sulphur compounds has often resulted in molecules with higher potency than
their parent counterpart.1d, 2 For example, imidazoleneꢀ2ꢀselone as the inhibitor of
thyroid peroxidase (TPO) displayed higher antioxidant activity than that of thione
analogs.3 There are few representatives of bioactive organoselenium compounds,
such as imidazoline 2ꢀselone and selenouracil are potent inhibitiors of LPOꢀ
catalyzed oxidation for antiꢀthyroid.4ꢀ5 Selenazofurin is the blocker of IMP
dehydrogenase and 2ꢀacylamido selenazoles display anti SOC channel activity
(Figure 1).6ꢀ7 Other structure such as selenocysteine is employed in the synthesis of
artificial proteins to probe the function of target selenoprotein in human body.8
This motivated us to study the synthesis of structurally related 2ꢀ
iminoselenazolines which may provide new pharmaceutical profiles. Some
publications report to use thiourea and αꢀbromoketone to synthesize 2ꢀ
2

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iminthiazoles.9 In addition, the synthesis of 2ꢀimino selenazoles is rarely studied,
and only few methods for the synthesis of seleniumꢀnitrogen heterocycles were
reported.10 The reactions of Nꢀcontaining nucleophiles such as βꢀhaloamines and
propargyl amines with aryl or alkyl isoselenocyanates were utilized for the
synthesis of 1,3ꢀselenazolidinꢀ2ꢀimines.11 Other reported synthesis of pure chiral
selenazolidines was employed reaction of chiral isoselenocyanates in aqueous
ammonia.12 Reaction of readily available enantiopure αꢀaminoacids/esters is
relatively unexplored for such transformations. Therefore, development an
efficient method to synthesis 2ꢀiminoselenazoline is of great interest to academia
and pharmaceutical industry. Herein, we studied to incorporate selenium atom into
the heterocyclic rings from aryl or alkyl isoselenocyanates to the synthesis the 2ꢀ
iminoselenazoline by multicomponent coupling reactions.
Figure 1. Structure related biologically active organoselenium compounds
Multicomponent reaction (MCR) is a powerful synthetic tool for the rapid and
efficient construction of complicated molecular frameworks. Their flexibility to
3

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generate structural diversity and incorporation of molecular complexity in oneꢀpot
operation is wellꢀrecognized.13 An essential rule to avoid isolation and purification
of the intermediates in one pot reaction is a major factor to speed up synthesis of
drugꢀlike compounds.14 MCRs are strategically amenable with modern synthetic
tools such as microwave irradiation, ultrasonication, polymer and ionic liquidꢀ
supported synthesis. Combination to apply of these techniques is a driving force to
discover new MCRs. Ultrasonication employs a nonꢀelectromagnetic radiation
source of sound energy to induce chemical reactions by acoustic cavitations.
Ultrasonication also accelerates chemical reactions by improvement of mixing
reactants of mass and heat transfer in the reaction medium.15 Ultrasound
accelerated three component coupling synthesis of tetrahydropyrimidines,16
phthalazinones,17 spirooxindoles18 and azoles19 has been demonstrated their
efficiency in multicomponent reactions.
In view of the power associated with ultrasonication and the biological importance
of selenium analogs, we report a three component coupling reaction toward
seleniumꢀcontaining heterocycles. These telescoped reactions of Lꢀamino esters,
isoselenocyanates and αꢀsubstituted bromoketones for regioselective synthesis of
enantiopure 2ꢀiminoselenazolenes by ultrasonication were explored in one pot.
4

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Result and Discussion
Initially, we tested the reactivity of Lꢀaminoesters 2 with various aryl and
alkylꢀisoselenocyanates 1, for the synthesis of selenium containing heterocycles
from selenourea.20 The required isoselenocyanates were synthesized using
modified literature procedures from Nꢀformylamines with various amines (R1NH2)
in sonication. Continuously, Nꢀformamide, triphosgene and triethylamine in
dichloromethane were refluxed to generate intermediate isocyanide, which was
further reacted with selenium powder to produce isoselenocyanates in one pot.
(Scheme 1) The isolated yields were much higher than that of previous report by
the application of sonication.21 The structures of compound 1f were confirmed by
Xꢀray analysis (Figure 2).
5

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O
O
triphosgene, Et3N
Ultrsonication
2hr
Se powder
R1
H2N R1
C N R1
+
Se C N R1
H
N
H
O
CH3
H
CH2Cl2, reflux, 5hr
1
CH2Cl2, reflux, 8hr
O
O
SeCN
SeCN
SeCN
1b, 70%
1c, 65%
1a, 75%
SeCN
SeCN
SeCN
CH3
CH3
1d,
46%
1e, 41%
1f,
69%
Scheme 1. Synthesis of isoselenocyanate 1 in oneꢀpot
N C Se
1f
Figure 2. Xꢀray crystal structures of 1f
With successful preparation of isoselenocyanates, we began to react Lꢀ
phenylalanine methyl ester 2d with phenyl selenoisocyanate 1d to deliver
6

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intermediate selenourea 3d within 30 min. Addition of phenacyl bromide to the
aboveꢀmentioned reaction mixture led to the formation of 2ꢀseleniimidazoles 5d in
52 % yield. Base on this observation, we attempted to develop a oneꢀpot procedure
without prior isolation of selenourea 3. The 2ꢀseleniimidazoles 5d was obtained in
72 % yield by one pot protocol which is higher than that of stepwise reaction
(52 %). To explore the reaction conditions, a series of experiments to use Lꢀamino
acid methyl esters, isoslenocyanates, and αꢀbromoketones with variation of
reaction parameters were performed (see SI). In order to accelerate the coupling
reactions, we also applied microwave in the various solvents such as THF,
acetonitrile, MeOH, DMF, and H2O for the same reaction stoichiometry. When the
external base was added to the reaction mixture, intermolecular cyclization was
performed first to deliver selenohydantoin which was not our desired selenourea 3.
Although microwave irradiation increased the reaction yield and
diminished the reaction time in some cases,22 this harsh condition could cause
decomposition and racemization of the temperature sensitive compounds such as
chiral amino acids and carbonyl compounds…etc.23 Moreover, selenourea is wellꢀ
known air and light sensitive molecule, it is necessary to develop a mild reaction
tool such as sonication to synthesize 2ꢀiminoselenazolines.24 We used
ultrsonication to accelerate the reaction progress in various solvents such as THF,
7

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acetonitrile, MeOH, DMF, and H2O. The compound 5a was finally obtained in 80 %
yield by sonication in acetonitrile.25
To ensure the stereochemistry of 5d remains unaffected during these
transformations, the other enantiomer 5d
ˊ
was synthesized from Dꢀphenylalanine
confirmed complete
methyl ester. Chiral HPLC analysis of the products 5d and 5d
ˊ
conservation of enantiomeric purity (see SI). As shown in Table1, we summarized
this reaction with miscellaneous reagents and starting material. Treatment of αꢀ
aminoesters 2 with isoselenocyanates 1 under ultrasonication at room temperature
to deliver corresponding selenourea 3 within 5 min in acetonitrile. The same
reaction required 30 min reflux to reach completion. Further treatment of crude
reaction mixture with various bromoketones 4aꢀl led to the formation of 2ꢀ
iminoselenazoles 5aꢀl in 30 min by sonication (Table 1). This observation also
contributes to study the ultrasonic effect and thermal refluxing condition on rates
of reactions. For examples by the same stoichiometry, it took 3 hours to reflux in
CH3CN to synthesize 5a. The structure of compound 5f was further confirmed by
Xꢀray analysis (Figure 3).
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N
Se
Se
O
H3C
N
O
N
O
H
=
O
CH3
5f
Figure 3. Xꢀray crystal structures of 5f
Table 1. One pot, two step to synthesis of 2ꢀiminoselenazole by αꢀbromoketone,
isoselenocyanate, and Lꢀaminoester
O
4: R4 = H
Br
4
R3
6:
R = CH3
R2
O
N C Se
O
H
H
N
H
R4
8: R4 = Ph, R3 = Ph
H3C
N
NH2
H3C
1
R2
O
O
Se
R1
R1
CH3CN, sonication
rt,
CH3CN, sonication, rt
5min
or reflux, 30 min
2
3
R2
R2
R2
N
N
N
Se
Se
Se
R3
O
O
O
R4
H3C
N
H3C
N
N
H3C
O
O
O
or
or
R3
R1
R1
R1
5
7
9
9

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10

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O
Se C N
Br
5l
78%
O
Se
N
C
7a
7b
Br
S
66%
85%
CH3
CH3
Br
O
Se C N
Br
CH3
O
7c
7d
7e
Br
Br
83%
80%
Se C N
CH3
CH3
O
S
CH3
CH3
Se C N
CH3
O
Se C N
CH3
CH3
Br
78%
CH3
O
Se C N
7f
Br
Br
S
O
78%
82%
CH3
CH3
O
O
CH3
CH3
9a
Se C N
CH3
O
O
Se C N
Br
Br
9b
9c
42%
67%
Se C N
S
CH3
a
b
2
1
Reaction Conditions : Lꢀaminoesters ( , 1.0mmol), isoselenocyanate ( , 1.5mmol), αꢀbromoketone
4
( , 1.5mmol), MeCN (10ml), rt, step two 30min, total 35min.
2
1
Reaction Conditions : Lꢀaminoesters ( , 1.0mmol), isoselenocyanate ( , 1.5mmol), αꢀbromoketone
6
( , 1.5mmol), MeCN (10ml), rt, step two 90min, total 95min
c
2
1
eaction Conditions : Lꢀaminoesters ( , 1.0mmol), isoselenocyanate ( , 1.5mmol), αꢀbromoketone
(8, 1.5mmole), MeCN (10ml), rt, step two 60min, total 65min
dIsolation yields
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Formation of selenazoles is favorable through Seꢀalkylation over Nꢀ
alkylation followed by the regioselective cyclization to generate product 5,
according to a recent report regarding reaction of N, Nˊꢀbiaryl selenourea with
phenacyl bromide.9a Such a condensation of selenourea, with enolizable ketones
and bromine is wellꢀknown as Hantzsch synthesis.26 However, the interplay of
ambident N vs Se nucleophilicity leading to ambiguous structural interpretation of
imidazolꢀ2ꢀselenone or selenoneꢀ2ꢀimine prompted us to study the reaction
mechanism in more details.27 We utilized condensation of selenourea with αꢀ
bromoketone to synthesize 2ꢀiminoselenazolines in 90 min (Table 1, 7a~7f). The
structure of compound 7a was confirmed by Xꢀray analysis (see SI). Secondary αꢀ
bromoketones are rarely used for Hantzsch selenazole type condensations due to
their low reactivity. The αꢀsubstituted bromoketones required higher sonication
time compared to that of unsubstituted bromoketones due to steric hindrance. We
utilized this fact to study the reaction mechanism with possible isolation of
reaction intermediates. The treatment of selenourea 3 with αꢀphenyl phenacyl
bromide 8 under ultrasonication in acetonitrile for 40 min confirmed complete
conversion into a new product, which is characterized to reveal the formation of
4, 5ꢀdiphenyl 2ꢀimino 4ꢀhydroxy 1, 3ꢀselenazolidin 10 as possible reaction
intermediate (Scheme 2). The structural characterization of 10 was also confirmed
by Xꢀray crystallographic analysis (Figure 4). These similar results for the
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observations also made by Egan and Tadanier in Hantzsch thiazole synthesis.25a
We synthesize a series of compounds 9a, 9b, 9c to test the reactivity difference of
R1 and R2 group with 2ꢀbromoꢀ1,2ꢀdiphenylethanone. The dehydrated 2ꢀ
iminoselenazole 9 (Table 1, 9a~9c) was eventually obtained by prolonged
sonication for 60 min. Compound 9a was less steric bulky to obtain higher yields,
but compound 9b with steric bulk groups R3 and R1 leads to less yield. Current
studies have been restricted to benzoin condensation, ester hydrolysis,
dichlorocyclopropanation and there are paucity of data to rationalize the steric
effects in terms of the bulk of the two carbon electrophiles in such cyclization
reactions.28ꢀ30 Summarization the reaction time of selenourea with three kinds of αꢀ
bromoketone shows that the primary αꢀbromoketone was rapid (35 min), the 2ꢀ
bromoꢀ1,2ꢀdiphenylethanone was milder (65 min), and 2ꢀbromopropiophenone
was slower (95 min).
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Scheme 2. One pot synthesis of 4, 5ꢀdiphenyl 2ꢀimino ꢀ4ꢀhydroxy 1, 3ꢀ
selenazolidin 10.
a
O
Br
N
O
Se
H
O
O
N C Se
H
H
H3C
NH2
H3C
N
H3C
N
N
8
O
O
O
OH
CH3CN, sonication
rt, 40 min
Se
CH3CN, sonication
rt, 5 min
10, 67%b
a
2
1
8
Reaction Conditions : Lꢀaminoesters ( , 1.0 mmol), isoselenocyanate( , 1.5 mmol), 2ꢀbromoꢀ1,2ꢀdiphenylethanone( , 1.5 mmol),
CH3CN (10ml), rt, 45 min
bIsolated yields
H3C
O
O
N
H
N
HO
Se
H
10
Figure 4. Structure of the 4ꢀhydroxyꢀ4,5ꢀdiphenylꢀ2ꢀ(phenylimino)ꢀ1,3ꢀ
selenazolidinꢀ3ꢀyl)ꢀ3ꢀphenylpropanoate 10
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Hard nucleophile
Hard electrophile
O
H
N
1
H
N
2
O
Br
O
R2
R4
R1 Se
R3
Soft electrophile
Soft nucleophile
Figure 5. Various reactive sites in selenourea and αꢀbromoketone
O
H
H
O
H3C
N1
N2
O
R3
Br
R3
R1
Se
R4
3
4
-HBr
-HBr
R2
N
R2
H
N
O
H
O
Br
H3C
NH
N
O
H3C
O
R3
O
O
R1
e
S
R1
R3
Se
R4
R4
H Br
R2
N
R2
N
O
N
H3C
Se
O
H
N
R3
O
R4
OH
R3
H3C
R1
O
Se
H
OH
R1
10'
R4
10
-H2O
R2
-H2O
R2
N
O
H3C
N
N
O
Se
R3
O
R3
R4
R1
Se
N
H3C
O
R4
R1
5', 7', 9'
5, 7, 9
Scheme 3. A plausible mechanism for the formation of 2ꢀiminoselenazoles 5, 7, 9
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Thus in presence of anhydrous acetonitrile, the selenourea reacts via soft
nucleophile Se with soft electrophilic CꢀBr bond (Figure 5). The subsequent
transformation involves loss of proton from N1 and intramolecular nucleophilic
attack of N2 (hard nucleophile) on carbonyl group of ketone (hard electrophile) to
deliver 2ꢀiminoselenol 10 that on subsequent dehydration releases the observed
product 2ꢀiminothiozole 5, 7, 9 (Scheme 3). From our supposition, the N2 was more
reactive than N1 because N1 was near electronꢀwithdrawing carbonyl group of
amino ester to cause regioselectivity. Therefore, formation of the 2ꢀ
iminoselenazoles 5, 7, 9 is due to preferential attack of selenium of its enhanced
nucleophilicity and the driving force for the selective N1 attack on the alkyl carbon
to eliminate a stable hydrobromide salt. Furthermore, influence of steric factors on
the reaction time has been effectively to characterize the intermediate 2ꢀiminoꢀ5ꢀ
selenol 10 which is undoubtedly confirmed that selenourea reacts via soft
nucleophile (i. e. selenium atom).
In conclusion, we explored a regioselective synthesis of polysubstituted 2ꢀ
iminoselenazoles by reaction of in situ generated selenoureas with αꢀbromoketones
under environmentally benign ultrasonic activation at room temperature. We
understand reaction mechanism through N1 selenourea by reaction of soft
nucleophile Se with soft electrophilic CꢀBr bond to form 2ꢀiminoselenazoles.
Intermediate of dihydro selenol in the construction of 2ꢀimino selenazolines has
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been conclusively established in the seleno Hantzsch reaction. An efficient
synthesis of enantiopure 2ꢀiminoselenazoles from ready available building blocks
may help to discover novel biological profiles of these skeletal diverse scaffolds.
Acknowledgement: The authors thank the Ministry of Science and Technology
(MOST, Taiwan) for the financial assistance. Many thanks for the help of Xꢀray
crystallographic operators PeiꢀLin Chen (NTHU) and TingꢀShen Kuo (NTNU) to
obtain 1f, 5f and 10. This work is particularly supported by "Center for
Bioinformatics Research of Aiming for the Top University Program" of the
National Chiao Tung University and Ministry of Education, Taiwan.
Supporting Information: Spectroscopic data of essential intermediates and final
compounds as well as Xꢀray data of compounds 1f, 5f, 7a, 9b are included in the
Supporting Information.
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