Reagent-controlled regiodivergent intermolecular cyclization of 2-aminobenzothiazoles with β-ketoesters and β-ketoamides

Article abstract of DOI:10.3762/bjoc.13.270

Two regiodivergent approaches to intermolecular cyclization of 2-aminobenzothiazoles with β-ketoesters and amides have been developed, controlled by the reagents employed. With the Br?nsted base KOt-Bu and CBrCl₃ as radical initiator, benzo[d]imidazo[2,1-b]thiazoles are synthesized via attack at the α-carbon and keto carbon of the β-ketoester moiety. In contrast, switching to the Lewis acid catalyst, In(OTf)₃, results in the regioselective nucleophilic attack at both carbonyl groups forming benzo[4,5]thiazolo[3,2-a]pyrimidin-4-ones instead.

Full text of DOI:10.3762/bjoc.13.270

Reagent-controlled regiodivergent intermolecular cyclization
of 2-aminobenzothiazoles with β-ketoesters and β-ketoamides
Irwan Iskandar Roslan, Kian-Hong Ng, Gaik-Khuan Chuah* and Stephan Jaenicke*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2017, 13, 2739–2750.
Department of Chemistry, National University of Singapore, 3 Science
Drive 3, Singapore 117543
doi:10.3762/bjoc.13.270
Received: 13 October 2017
Accepted: 05 December 2017
Published: 18 December 2017
Email:
Gaik-Khuan Chuah* - chmcgk@nus.edu.sg; Stephan Jaenicke* -
chmsj@nus.edu.sg
Associate Editor: T. J. J. Müller
*
Corresponding author
©
2017 Roslan et al.; licensee Beilstein-Institut.
Keywords:
License and terms: see end of document.
cyclization; fused-ring systems; indium; radical; regiodivergent
Abstract
Two regiodivergent approaches to intermolecular cyclization of 2-aminobenzothiazoles with β-ketoesters and amides have been de-
veloped, controlled by the reagents employed. With the Brønsted base KOt-Bu and CBrCl3 as radical initiator,
benzo[d]imidazo[2,1-b]thiazoles are synthesized via attack at the α-carbon and keto carbon of the β-ketoester moiety. In contrast,
switching to the Lewis acid catalyst, In(OTf)3, results in the regioselective nucleophilic attack at both carbonyl groups forming
benzo[4,5]thiazolo[3,2-a]pyrimidin-4-ones instead.
Introduction
β-Ketoesters are versatile substrates frequently used in hetero- for the one-pot synthesis of imidazo[1,2-a]pyridines and
cyclic synthesis, having both electrophilic keto and ester thiazolamines by coupling β-ketoesters or their derivatives,
moieties as well as a nucleophilic α-carbon. They can act as di- phenylacetones and phenylacetophenones, with aminopyridines
nucleophiles [1-4], dielectrophiles [5-7] or ambiphiles [8,9] in [20,21] and thioureas [22]. The strategy involves in situ bromi-
the presence of their complementary coupling partners. Further- nation of the α-carbon using CBrCl3 as the Br source. This in
more, they can be prefunctionalized [10-13] with leaving situ halogenation strategy has been employed for the synthesis
groups, thus switching to an electrophile [14-16], or convert to of quinoxalines [23], oxazoles [24,25], pyrido[1,2-a]benzimida-
an α-radical carbon with an oxidant [17-19]. β-Ketoesters are zoles [26], imidazo[1,2-a]pyridines [27-30], thiazoles [31,32]
also inexpensive, abundant and commercially available, making and benzothiazoles [33,34]. With weak bicarbonate bases, direct
them attractive substrates.
bromination of the α-carbon does not occur. Instead, the Br is
shuttled to the α-carbon by its coupling partner. With this
In our continuing effort to develop green and atom-efficient tandem bromination and cyclization strategy, there is no need to
protocols, we have employed transition-metal-free approaches presynthesize substrates, thus reducing the number of synthetic
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Beilstein J. Org. Chem. 2017, 13, 2739–2750.
steps, time, chemicals and wastes. Here we describe the exten- KOt-Bu-mediated synthesis, especially after Itami’s group
sion of this α-bromination shuttle system to 2-aminobenzothia- showed that KOt-Bu provides a metal-free approach to the
zoles as substrates to synthesize benzo[d]imidazo[2,1-b]thia- traditional Pd-catalyzed aryl–heteroaryl coupling to biaryls
zoles.
[57]. Since then, KOt-Bu has been used as a mediator for
various reactions including aryl–aryl coupling [58-63], inter-
The benzo[d]imidazo[2,1-b]thiazole backbone is found in many and intramolecular cyclizations [64-68], amidation [69], alkeny-
bioactive molecules and pharmaceutical compounds as evident lation [70], oxidation [71] and silylation [72]. In these reactions,
by its use as antimicrobial [35,36], antitumor [37-39], antibacte- the single electron transfer (SET) is initiated by KOt-Bu/DMF
rial [40], and anti-allergic agents [41]. In addition, compounds [63,67,69,71] or KOt-Bu in combination with additives such as
with this backbone are employed as kinase inhibitors and recep- bidentate diamine ligands [61-65], 18-crown-6 [70] or azobis-
tors [42-44] and as a tracer for PET imaging of β-amyloid isobutyronitrile (AIBN) [62,66].
plaques [45,46]. The conventional approach for the construc-
tion of benzo[d]imidazo[2,1-b]thiazole is the condensation of Herein, we report the synthesis of benzo[d]imidazo[2,1-b]thia-
2
-aminobenzothiazole with α-halo or tosyloxy ketone [47,48]. zoles via an in situ bromination strategy using a KOt-Bu/
The requirement for prior functionalization of the ketone moiety CBrCl3 system (Scheme 1a). By employing a base with radical
is a drawback, and several more direct methods have been de- susceptibility, the reaction mechanism is expected to differ sub-
veloped in recent years [49-52]. Zhang et al. coupled various stantially from that using bicarbonate bases which operates via
1
,2-dihaloarenes with 2-mercaptobenzimidazole in a nucleo- the α-Br shuttle mechanism [20-22]. Interestingly, by replacing
philic aromatic substitution reaction [53]. Zhu’s group used Cu the radical initiator and Brønsted base system with a Lewis acid
salts as a promoter for the cycloaddition of isocyanides with catalyst, benzo[4,5]thiazolo[3,2-a]pyrimidin-4-ones were
benzothiazoles [54]. Benzo[d]imidazo[2,1-b]thiazoles have also formed instead (Scheme 1b). This highlights the versatility of
been synthesized via coupling of 2-aminobenzothiazole with β-ketoesters where the regioselectivity of the reaction is
acetophenones [55] or aldehydes and nitroalkanes [56].
directed by the nature of the reagents.
Because the bicyclic structure of 2-aminobenzothiazole is Results and Discussion
more stable than aminopyridines and thioureas, the nucleophi- Optimization studies were carried out by reacting 1.2 mmol of
licity of the aromatic N atom is reduced. A stronger base, 2-aminobenzothiazole (1a) with 1.0 mmol of methyl aceto-
KOt-Bu, is therefore needed for in situ bromination to form the acetate (2a) in the presence of 2 equivalents of base using a sol-
benzo[d]imidazo[2,1-b]thiazole derivatives via coupling of vent mixture of CBrCl3/MeCN under reflux. After 16 h, the
2
-aminobenzothiazole with the brominated β-ketoesters and desired product 3a was not formed with KHCO3 (Table 1, entry
amides. Over the past decade, there has been a lot of interest in 1). Instead, the N-acetylated side product 4 was obtained in
Scheme 1: Two different intermolecular cyclization pathways controlled by reagents used.
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Beilstein J. Org. Chem. 2017, 13, 2739–2750.
moderate yields, together with trace amounts of the 4 (Table 1, entries 10–12). Unfortunately, further attempts to
benzo[4,5]thiazolo[3,2-a]pyrimidin-4-one side product 5a. increase the yields of 3a, including increasing the equivalents of
Even after switching to the stronger bases K2CO3 and K3PO4, CBrCl3, 1a and 2a proved futile (Table 1, entries 13–16).
only poor yields of 3a were obtained (Table 1, entries 2 and 3).
These results are not surprising as the substrate 1a is a weaker Hence, exploring the scope of the reaction was carried out using
dinucleophile than aminopyridine and thiourea used previously 1.0 mmol of methyl acetoacetate 2a with 1.2 mmol of CBrCl3/
[
(
20-22]. However, 56% yield of 3a was obtained with KOH MeCN solvent mixture at refluxing conditions for 16 h. A wide
Table 1, entry 4). The use of KOH as a base in the α-bromina- array of 2-aminobenzothiazoles was screened (Scheme 2). Good
tion of 1,3-dicarbonyl compounds has been reported previously to excellent yields were obtained with 2-aminobenzothiazoles
by Sasson’s group [73]. We propose that with a strong base like bearing electron donating methyl, dimethyl or methoxy groups
KOH, direct α-bromination of 2a occurs instead of relaying the (Scheme 2, 3b–d). Halogen substituents F, Cl and Br were well
Br through an α-bromination shuttle as observed in our previous tolerated under the optimized conditions. 2-Aminobenzothia-
studies [20-22]. Other strong bases like NaH and KOEt also zoles with an ester moiety also proceeded with good yields
proceeded with moderate yields of 3a (Table 1, entries 5 and 6). (Scheme 2, 3h). However, only trace amounts of 3i were ob-
Gratifyingly, an even higher yield of 3a, 86%, was obtained tained for 2-aminobenzothiazoles bearing the strongly electron-
with KOt-Bu (Table 1, entry 7).
withdrawing CF3 group. It was encouraging to see both that the
benzoxazole and thiazole derivatives reacted well with their
Next, various solvents besides MeCN were investigated. The respective β-ketoester coupling partners to form 3j and 3k with
polar aprotic solvents DMF and DMAc were suitable albeit 84% and 92% yields, respectively.
with slightly lower yields of 3a (Table 1, entries 8 and 9). On
the other hand, ethyl acetate and 1,2-dichloroethane fared A variety of β-ketoesters and β-ketoamides were tested using
poorly whereas toluene gave only the N-acetylated side product the optimized conditions. Alkyl acetoacetates, including pentyl,
Table 1: Optimization parameters for the synthesis of benzo[d]imidazo[2,1-b]thiazoles 3a.a
Entry
Base
Solvent
Yield of 3a (%)b,c
Yield of 4 (%)b,c
1
2
3
4
5
6
7
KHCO3
K2CO3
K3PO4
KOH
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
DMF
0
40 (31)
45 (37)
53 (46)
31
trace
5
56 (48)
69 (63)
61 (53)
86 (84)
77 (75)
74 (70)
33 (22)
9
NaH
20
KOEt
22
KOt-Bu
KOt-Bu
KOt-Bu
KOt-Bu
KOt-Bu
KOt-Bu
KOt-Bu
KOt-Bu
KOt-Bu
KOt-Bu
14
8
9
1
1
1
1
1
1
1
d
5
d
DMAc
EtOAc
DCE
7
0
38
1
24
2d
3e
4f
5g
6h
toluene
MeCN
MeCN
MeCN
MeCN
trace
44 (37)
11
89 (86)
65 (58)
78 (74)
46 (35)
23
22
6
aReaction conditions: 1a (1.2 mmol), 2a (1.0 mmol), base (2.0 equiv), in 3 mL of 1:9 (v/v) CBrCl3/solvent (3 mmol CBrCl3) for 16 h. bYield (from GC)
with respect to 2a, using biphenyl as an internal standard. cIsolated yields in parenthesis. dReaction conducted at 80 °C. e5.0 and f2.0 mmol of CBrCl3
was used. g2.0 mmol of 1a used. h1.0 mmol of 1a and 2.0 mmol of 2a used.
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Beilstein J. Org. Chem. 2017, 13, 2739–2750.
Scheme 2: Scope of reaction. Reaction conditions: 1 (1.2 mmol), 2 (1.0 mmol), KOt-Bu (2 mmol), in 3 mL CBrCl3/MeCN 1:9 (v/v) under reflux for
16 h.
isopropyl, tert-butyl and methoxyethyl, reacted smoothly with As trace amounts of benzo[4,5]thiazolo[3,2-a]pyrimidin-4-ones,
excellent yields (Scheme 2, 3l–3o). β-Ketoamides were well 5a, were observed in the synthesis of benzo[d]imidazo[2,1-b]-
tolerated (Scheme 2, 3h, 3p and 3q) while good yields were ob- thiazoles, we searched for a suitable reagent to regioselectively
tained with β-ketoesters containing n-propyl and n-butyl form this instead of 3a and 4. This tricyclic backbone can be
moieties (Scheme 2, 3r and 3s). However, significantly lower found in compounds that possess a wide range of medicinal
yields were obtained with the sterically more demanding iPr properties including those with antimalarial [74], anticancer
group (Scheme 2, 3t). Unfortunately, β-diketone (acetylacetone) [75,76], antiallergic [77,78], antibacterial [79], and antimicrobi-
was not suitable for the reaction as only trace amounts of 3u al properties [80,81]. In addition, they have been found to be bi-
were formed, with the bulk of the substrate being converted to ologically active antagonists of adenosine receptors [82], inhibi-
the N-acylated side product 4 instead.
tors of cyclic-AMP-diphosphoesterase [83], and benzodi-
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Beilstein J. Org. Chem. 2017, 13, 2739–2750.
azepine receptor ligands [84,85]. Reported methods to access entries 9 and 10). A wide range of solvents were also screened.
this structural motif include annulation between an aromatic Dioxane was fairly suitable for the reaction with moderate
amine and acid chloride [86] or via aza-Diels–Alder reaction yields of 5a whereas nitromethane fared poorly (Table 2, entries
[
87]. Coupling between alkynoic acid and 2-aminobenzothia- 11 and 12). Propionitrile, DMF and DMSO were also unsuit-
zole and the use of ionic liquids have also been developed [88]. able for the reaction (Table 2, entries 13–15). Hence, estab-
Heterogeneous catalysts such as kaolin and hydrotalcites have lishing the scope of reaction was carried out using In(OTf)3 as
been employed in the synthesis of benzo[4,5]thiazolo[3,2- catalyst and toluene as solvent.
a]pyrimidin-4-ones [89,90]. Polyphosphoric acid has also been
used to access 5a by coupling 2-aminobenzothiazole with Both ethyl acetoacetate and N,N-diethylacetamide reacted
β-ketoesters [79,91,92].
smoothly with 1a to give 5a in good yields (Scheme 3). A large
variety of substituted 2-aminobenzothiazoles were then
We have previously shown that bismuth salts can catalyze the screened in this reaction. Electron-donating groups including
intermolecular cyclization of 2-aminopyridines and β-ketoesters methyl and methoxy were well tolerated under the reaction
to pyrido[1,2-a]pyrimidin-4-ones [6]. Using Bi(OTf)3 in 1.5 conditions (Scheme 3, 5b–d). 2-Aminobenzothiazoles with
mmol 2-aminobenzolthiazole (1a) and 1.0 mmol methyl aceto- halogen substituents F, Cl and Br reacted smoothly with excel-
acetate (2a) gave a relatively poor yield of 5a (Table 2, entry 1). lent yields (Scheme 3, 5e–g). Good to excellent yields were also
This prompted us to switch the equivalents of 1a and 2a to 1.0 obtained with electron-withdrawing ester and CF3 moieties
and 1.5 mmol, respectively, which improved the yield tremen- (Scheme 3, 5h–j). Likewise, the reaction with 2-aminobenzoxa-
dously (Table 2, entry 2). Hence, the survey of Lewis acids was zole proceeded smoothly giving 5k in 79% yield.
conducted with 10 mol % of the Lewis acid at 100 °C using tol-
uene as solvent (Table 2). In(OTf)3 was a better catalyst for this Next, a variety of methyl or ethyl carboxylates of β-ketoesters
reaction with 95% yield (Table 2, entry 3). Reactions with ZnII were screened for this reaction. Good yields were obtained for
and YbIII triflates proceeded with moderate yields while AgI, β-ketoesters with n-propyl and n-butyl moieties at the C-4 car-
CuII and CoII were unreactive (Table 2, entries 4–8). Other InIII bon (Scheme 3, 5l and 5m). Bulkier isopropyl and tert-butyl
salts screened were not as effective as its triflate salt (Table 2, substituents were less well tolerated, with significantly lower
Table 2: Optimization parameters for the synthesis of benzo[4,5]thiazolo[3,2-a]pyrimidin-4-ones 5a.a
Entry
Catalyst
Solvent
Yield of 5ab,c (%)b,c
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
d
Bi(OTf)3
Bi(OTf)3
In(OTf)3
Yb(OTf)3
Zn(OTf)2
Cu(OTf)2
AgOTf
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
dioxane
NO2Me
EtCN
23
73 (70)
96 (95)
59 (52)
67 (57)
0
0
CoCl2
0
InCl3
63 (58)
50 (40)
39 (26)
10
0
1
2
3
4
5
InBr3
In(OTf)3
In(OTf)3
In(OTf)3
In(OTf)3
In(OTf)3
18
DMF
0
DMSO
0
aReaction conditions: 1a (1.0 mmol), 2a (1.5 mmol), catalyst (10 mol %), in 1.5 mL of solvent for 16 h at 100 °C. bYield (from GC) with respect to 1a,
using biphenyl as an internal standard. cIsolated yields in parenthesis. dReaction using 1.5 mmol of 1a and 1.0 mmol of 2a instead.
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Scheme 3: Scope of the reaction. Reaction conditions: 1 (1.0 mmol), 2 (1.5 mmol), In(OTf)3 (0.1 mmol), in 1.5 mL toluene at 100 °C for 16 h. a2 mL of
2-butanol/toluene 1:3 (v/v) used as solvent instead. bIsopentanol/toluene.
yields (Scheme 3, 5n and 5o). The best yield of 99% was ob- isopentanol, respectively. The Cl is utilized as a leaving group
tained when ethyl 4,4,4-trifluoroacetoacetate was employed during the nucleophilic attack by the corresponding alcohol.
while the reaction with diethyl 3-oxopentanedioate proceeded
with 92% yield (Scheme 3, 5p and 5q). β-Ketoesters with A series of control experiments were conducted to gain insights
methyl or ethyl substituent at the α-carbon were well suited for into the mechanism of the two contrasting reactions. Firstly,
this reaction (Scheme 3, 5r and 5s). Gratifyingly, the reaction two separate coupling reactions between 2-aminobenzothiazole
also proceeded smoothly with various cyclic β-oxo esters (1a) and methyl acetoacetate (2a) were conducted using the op-
ranging from cyclopentane to cyclooctane (Scheme 3, 5t–w). timized reaction conditions, one without KOt-Bu and the other
Interestingly, the ethers 5x and 5y were obtained when ethyl without CBrCl3 (Scheme 4a and b). The absence of 3a in both
4
-chloroacetoacetate was reacted with 2-amino-6-methylbenzo- cases shows that both KOt-Bu and CBrCl3 have to be present to
thiazole in a mixed solvent system of toluene with 2-butanol or form the desired product. Next, to confirm that the reaction
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Beilstein J. Org. Chem. 2017, 13, 2739–2750.
Scheme 4: Control experiments.
proceeds via a radical pathway, 2,2,6,6-tetramethylpiperidin-1- mination of 2a to form the intermediate 6 via single electron
yl oxyl (TEMPO, 2.0 equiv) was added to the reaction mixture transfer (SET). Recent mechanistic work by Murphy and
as a radical scavenger (Scheme 4c). After 24 h, none of the co-workers showed that in the presence of polyhalomethane
desired product 3a had formed, indicating that the reaction path- CBr4, KOt-Bu undergoes a SET reaction, forming •CBr3 and
way was a radical one in nature. Homolytic cleavage of the t-BuO• radicals [95]. Furthermore, with other strong bases of
C–Br bond in CBrCl3 can occur under UVC irradiation or ther- similar strength including NaH and KOEt the reaction
mally [21]. The cleavage is facile and was also observed under proceeded with significantly lower yields. This lends additional
blue LED light [93]. Finally, the α-brominated methyl aceto- support to the hypothesis that KOt-Bu, besides being a base, has
acetate 6 was detected in several reaction mixtures. To prove a secondary role of assisting in SET (Table 1, entries 5 and 6).
that 6 is indeed a reaction intermediate, the starting material 2a
was replaced with 6 (Scheme 4d). After 16 h, 93% of 3a was The following control experiments were carried out for the syn-
formed, showing unambiguously that the reaction proceeds via thesis of benzo[4,5]thiazolo[3,2-a]pyrimidin-4-ones. A blank
6
as an intermediate. We propose that KOt-Bu assists in α-bro- reaction was done without the InIII catalyst in toluene under the
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Beilstein J. Org. Chem. 2017, 13, 2739–2750.
optimized conditions (Scheme 4e). No desired product 5a was thiazole N atom, forming an adduct A [98]. This activates the
formed, implying the need for In(OTf)3 as catalyst in the reac- N–H bonds and its subsequent cleavage by the triflate forms B
tion. A radical trapping experiment with 2 equivalents of [99]. The InIII metal center also coordinates to the keto carbon
TEMPO was also conducted using the optimized conditions of methyl acetoacetate (2a) and thus brings both substrates in
(
Scheme 4f). 89% of 5a was formed, implying that the reaction close proximity to one another [100,101]. An addition reaction
does not proceed via a radical pathway but an ionic one.
ensues the formation of C which is followed by proton abstrac-
tion, releasing the catalyst as In(OTf)2OH [102] and forming D.
Based on these observations, the syntheses of benzo[d]imi- An intramolecular condensation reaction occurs which forms
dazo[2,1-b]thiazoles and benzo[4,5]thiazolo[3,2-a]pyrimidin-4- the desired product 5a with extrusion of MeOH. The In(OTf)3
ones are proposed to occur via the following mechanisms is regenerated by TfOH with release of a water molecule, and
(
Figure 1 and Figure 2). For the former, the reaction is initiated the catalytic cycle is repeated.
by SET from the tert-butoxide anion to CBrCl3, forming the
tert-butoxy radical [94]. This radical attacks the α-hydrogen of Conclusion
2
a via hydrogen atom transfer (HAT), to form intermediate A We have developed two regiodivergent protocols for the inter-
with a radical at the α-carbon. A then undergoes α-bromination molecular cyclization of 2-aminobenzothiazoles with
to form the intermediate 6 [95]. Attack at the α-carbon of 6 by β-ketoesters and β-ketoamides that are determined by the
2
-aminobenzothiazole (1a) via an Ortoleva–King type of reac- reagents used. This is possible due to the versatility of
tion forms B [96,97]. This is followed by a nucleophilic addi- β-ketoesters in switching polarities and reactivities in the pres-
tion and dehydration to form C. Upon deprotonation of the ence of different reagents. With the Brønsted base and radical
acidic proton at C by KOt-Bu, the desired product 3a is formed initiator system of KOt-BU/CBrCl3, in situ α-bromination
with release of KBr and tert-butanol. As proposed by Zeitler’s occurs and nucleophilic attacks at the α-carbon and keto carbon
group, the •CCl3 radicals are quenched to CHCl3 via HAT [93]. lead to the formation of benzo[d]imidazo[2,1-b]thiazoles. On
the other hand, the Lewis acidic catalyst In(OTf)3 allows for
The proposed catalytic cycle for the synthesis of benzo[4,5]thia- nucleophilic attacks at both carbonyl groups to form
zolo[3,2-a]pyrimidin-4-ones is as follows. The Lewis acidic benzo[4,5]thiazolo[3,2-a]pyrimidin-4-ones. The scope of these
InIII metal center coordinates to the more nucleophilic benzo- regiodivergent protocols was demonstrated with 19 examples of
Figure 1: Proposed mechanism (benzo[d]imidazo[2,1-b]thiazoles).
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Beilstein J. Org. Chem. 2017, 13, 2739–2750.
Figure 2: Proposed mechanism (benzo[4,5]thiazolo[3,2-a]pyrimidin-4-ones).
tricyclic benzo[d]imidazo[2,1-b]thiazoles and 27 examples of Methyl 2-methylbenzo[d]imidazo[2,1-b]thiazole-3-carboxyl-
tricyclic and tetracyclic benzo[4,5]thiazolo[3,2-a]pyrimidin-4- ate (3a): Obtained as a yellow semi-solid (206 mg, 84%);
ones.
1H NMR (300 MHz, CDCl3) 8.95 (d, J = 8.1 Hz, 1H), 7.66 (d,
J = 7.8 Hz, 1H), 7.45 (t, J = 8.0 Hz, 1H), 7.34 (t, J = 7.5 Hz,
1H), 3.97 (s, 3H), 2.63 (s, 3H); 13C NMR (75 MHz, CDCl3)
Experimental
Representative procedure for the synthesis of benzo[d]imi- 161.1, 154.4, 151.7, 134.0, 129.7,126.3, 125.0, 123.6, 118.3,
dazo[2,1-b]thiazole: A 25 mL two-neck round-bottomed flask 117.6, 51.6, 16.9; HRMS–ESI (m/z): [M + H]+: calcd for
was charged with 2-aminobenzothiazole (1a, 180 mg, C12H11N2O2S, 247.0536; found, 247.0533.
1
3
.2 mmol), methyl acetoacetate (2a, 108 μL, 1.0 mmol), in
mL of CBrCl3/MeCN 1:9 (v/v) solvent mixture. KOt-Bu Representative procedure for the synthesis of benzo[d]imi-
(
224 mg, 2.0 mmol) was added slowly at room temperature and dazo[2,1-b]thiazole: A 10 mL round-bottomed flask was
the reaction mixture was stirred under reflux for 16 h. Upon charged with 2-aminobenzothiazole (1a, 150 mg, 1.0 mmol),
completion, the reaction mixture was diluted with 30 mL of methyl acetoacetate (2a, 162 μL, 1.5 mmol) and indium(III) tri-
ethyl acetate, filtered through a short pad of silica gel and fluoromethanesulfonate (56 mg, 0.1 mmol) in 1.5 mL of tolu-
washed down with an additional 60 mL ethyl acetate. The ene. After stirring at 100 °C for 16 h, the reaction was diluted
filtrate was washed with distilled water (3 × 30 mL) and the with water and extracted with EtOAc (15 mL × 5). The
organic phase was dried with anhydrous Na2SO4. After filtra- combined organic layers were washed with brine and dried with
tion, the solvent was removed by rotary evaporation and the anhydrous Na2SO4. After filtration, the solvent was removed by
residue was purified by column chromatography using hexane rotary evaporation, and the residue was cleaned up by column
and ethyl acetate (v/v = 8:1) as eluent to afford 3a with 84% chromatography using hexane and ethyl acetate (v/v = 4:1) as
yield.
eluent to afford 5a with 95% yield.
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Beilstein J. Org. Chem. 2017, 13, 2739–2750.
1
1
1
4. Wang, C.; Zhang, J.; Wang, S.; Fan, J.; Wang, Z. Org. Lett. 2010, 12,
2
-Methyl-4H-benzo[4,5]thiazolo[3,2-a]pyrimidin-4-one (5a):
2338–2341. doi:10.1021/ol100688c
Obtained as a light yellow solid (206 mg, 95%); mp
5. Sadeghi, M.; Safari, J.; Zarnegar, Z. RSC Adv. 2016, 6, 64749–64755.
doi:10.1039/C6RA11175K
2
02–204 °C; 1H NMR (300 MHz, CDCl3) δ 8.94 (d, J = 7.5 Hz,
H), 7.57 (d, J = 7.2 Hz, 1H), 7.44–7.33 (m, 2H), 6.16 (s, 1H),
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HRMS–ESI (m/z): [M + H]+ calcd for C11H9N2OS, 217.0430;
1
1
2
2
2
2
found, 217.0432.
1
9
Supporting Information
2
Supporting Information File 1
1. Roslan, I. I.; Ng, K.-H.; Wu, J.-E.; Chuah, G.-K.; Jaenicke, S.
J. Org. Chem. 2016, 81, 9167–9174. doi:10.1021/acs.joc.6b01714
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2017, 704–709. doi:10.1002/ejoc.201601410
Experimental procedure, analytical data and NMR spectra.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-13-270-S1.pdf]
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Acknowledgements
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Financial support from the Ministry of Education ARC Tier 1
Grant numbers R-143-000-603-112 and R-143-000-667-114 is
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ORCID® iDs
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