Copper-catalyzed synthesis of thiazol-2-yl ethers from oxime acetates and xanthates under redox-neutral conditions

Article abstract of DOI:10.1039/c8cc00445e

A novel copper-catalyzed annulation of oxime acetates and xanthates for the synthesis of thiazol-2-yl ethers with remarkable regioselectivity has been developed. Various oxime acetates, whether derived from aryl ketones or alkyl ketones, or natural product cores are suitable for this conversion. Unique dihydrothiazoles were also obtained when both reaction sites were methine. Mechanistic studies indicated that imino copper(iii) intermediates were involved. In addition, this protocol proceeded under redox-neutral conditions and did not require additives or ligands.

Full text of DOI:10.1039/c8cc00445e

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Copper-Catalyzed Synthesis of Thiazol-2-yl Ethers from Oxime
Acetates and Xanthates under Redox-Neutral Conditions
Received 00th January 20xx,
Accepted 00th January 20xx
Zhongzhi Zhu, Xiaodong Tang, Jinghe Cen, Jianxiao Li, Wanqing Wu, Huanfeng Jiang*
DOI: 10.1039/x0xx00000x
www.rsc.org/
thio group have been demonstrated as powerful tools for the
construction of C-C bonds9 and C-S bonds10 (Scheme 1a).
Noteworthy, Sekar and other chemists have reported several
examples triggered by Ullmann-type reaction with potassium
xanthates (Scheme 1b).11 However, the use of prefunctionalized
materials is relatively uneconomical and environmentally unfriendly.
Therefore, the developing of more reaction types of xanthates for
the preparation of sulfur-containing molecules is highly desirable.
A
novel copper-catalyzed annulation of oxime acetates and
xanthates for the synthesis of thiazol-2-yl ethers with remarkable
regioselectivity has been developed. Various oxime acetates,
whether derived from aryl ketones or alkyl ketones, or natural
product cores are suitable for this conversion. The unique
dihydrothiazoles were also obtained when both reaction sites are
methine. Mechanistic studies indicated that imino copper(III)
intermediates were involved. In addition, this protocol proceeded
under redox-neutral conditions and did not need for additives or
ligands.
Thiazole skeletons are versatile privileged motifs widely found in
natural products1 and exhibit a wide range of biological activities.2
Many natural products such as Apratoxins, Luciferin, Piscibactin,
Dolastatin E, Mirabazole, Tantazoles, Vitamin B1, Epothilones,
Thiostrepton and some of accepted drugs included dasatinib,
Ritonavir, nizatidine and fentiazac contain thiazole moieties.3,4
Further, they have been used as synthetic ligands, cosmetic
sunscreens and also applied to various biological evaluations.5
Many practical method including some of our group7a,7b have been
reported for the synthesis of thiazoles, especially 2-
aminothiazoles.6,7 However, the direct synthesis of thiazol-2-yl
ethers remains greatly underrepresented in the literature. In this
regard, only few methods are still limited to halogenated thiazoles
ring or α-haloketones as prefunctionalized substrates.5c,5d So far, it
is significant to develop more practical and alternative methods for
the construction of thiazol-2-yls from available raw materials.
Scheme 1. The utilization of thio part of thiocarbonyl for
xanthate derivatives
Oxime acetates as an internal oxidant have been found to be
versatile building blocks wildly applied in the field of transition
metal-catalyzed reactions due to their unique properties,12-14
especially under copper catalysis. Based on our previous work on
C-S bond as a basic chemical bond, its cleavage and formation
play crucial role in chemical reaction. During the past decades,
xanthate derivatives as an important sulfur-containing compound,
are widely used in organic synthesis, especially in radical addition
reactions.8 In this context, many kinds of radical reactions of
xanthate-based on the degenerative exchange of a thiocarbonyl
oxime derivatives7a,7b,14 herein, we describe
a novel [3+2]
annulation of oxime acetates with xanthates via organo-copper(III)
strategy (Scheme 1c). This transformation provides a simple way for
the preparation of various thiazol-2-yl ethers with good
stereoselectivity.
Key Laboratory of Functional Molecular Engineering of Guangdong Province,
School of Chemistry and Chemical Engineering, South China University of
Technology, Guangzhou 510640, China. E-mail: jianghf@scut.edu.cn
Fax: +86 20-87112906; Tel: +86 20-87112906
Electronic supplementary information (ESI) available: Experimental section,
characterization of all compounds, and copies of 1H and 13C NMR spectra for
selected compounds. For ESI and other electronic format See
DOI:10.1039/x0xx00000x
After extensive screening of different parameters, the optimal
reaction conditions were determined (see the Supporting
Information for details). Then the substrate scope of aryl oxime
acetates was investigated and the results were summarized in Table
1. Firstly, a series of para-substituted acetophenone oxime acetates,
including electron-donating groups (Me, Et, OMe, SMe) and
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electron-withdrawing groups (F, Cl, Br, I, CF3, NO2) were converted acetyl oxime and nonan-5-one O-acetyl oxime could react with 2a
into the corresponding thiazol-2-yl ethers (3ab-3ak). Furthermore, smoothly and afforded the corresponding products in good yields
ortho, meta-substituted and poly-substituted acetophenone oxime (3bb-3bd). In addition, the cyclic alkyl oxime esters were also
acetates were able to give the desired products in moderate to compatible with this reaction, providing the polycyclic products in
good yields (3al-3aq). Moreover, oxime acetates derived from other moderate to good yields (3be-3bg). Additionally, oxime acetates
aromatic ketones such as propiophenone, butyrophenone, 1,2- substituted with alkenyl groups such as cyclohex-2-en-1-one O-
diphenylethan-1-one and 3,4-dihydronaphthalen-1(2H)-one could acetyl oxime and 4-phenylbut-3-en-2-one O-acetyl oxime, were
be accessed through this route to generate the annulation products smoothly transformed into the corresponding alkenyl products in
in good yields (3ar-3au). In addition, the heteroarene oxime 53% and 42% yields, respectively (3bh, 3bi).
When oxime acetates bore different two reaction sites (C(sp3)-H
acetates were compatible with this transformation (3av-3ay). When
1-(benzo[b]thiophen-2-yl)ethan-1-one and 1-acetylnaphthalene
oxime acetate were used as the substrate, 3az and 3ba could be
isolated in 65% and 64% yields, respectively.
bonds) at the α position of the C=N group, the major product is the
one that has the fewest hydrogen substituents. In particular, when
one of them is methyl, only the single product is present (3bj-3bn).
And when both of them are methylenes, there will be different
products appear at a certain rate (3bo and 3bo’).
Table 1. Substrate scope of aryl oxime acetates a
OEt
NOAc
S
N
CuBr2 (20 mol %)
DCM, 115 o
S
+
KS
OEt
R
C
Table 3. The substrate scope and stereoselectivity of oxime
acetates a
1
2a
3
OEt
OEt
S
OEt
S
S
CuBr2 (20 mol %)
115 oC, DCM
N
NOAc
OEt
S
OEt
S
+
S
N
N
R
KS
OEt
2a
R
N
N
1
3
R
R
R
3aa, R = H, 83%
3ab, R = Me, 85%
3ac, R = Et, 76%
3al, R = 2-Me, 81%
3am, R = 2-Cl, 56%
3an, R = 3-Me, 81%
3ar, R = Me, 72%
3as, R = Et, 68%
3at, R = Ph, 61%
OEt
OEt
N
3aq, 82%
OEt
N
OEt
N
S
N
S
S
S
3ad, R = OMe, 65% 3ao
, R = 3-F, 66%
3ap, R = 3-Cl, 63%
3ae, R = SMe, 62%
3af, R = F, 67%
OEt
OEt
OEt
3ag, R = Cl, 71%
3ah, R = Br, 59%
3ai, R = I, 51%
3aj, R = CF3, 75%
3ak, R = NO2, 75%
N
S
N
S
N
S
3bm, 68%
3bl, 61%
3bj, 51%
3bk, 63%
S
OEt
N
OEt
O
OEt
S
S
N
N
3aw, 73%
OEt
3au, 84%
3av
, 76%
S
OEt
S
OEt
OEt
N
N
N
N
S
3bo, 41%
3bo', 22%
3bn, 73%
S
S
S
N
S
a Reaction conditions: 1 (0.2 mmol), 2a (0.3 mmol), CuBr2 (0.04 mmol), in 2.0
3ba, 64%
3ax, 71%
3ay, 54%
3az, 65%
mL DCM at 115 °C for 6 h. Isolated yield.
a Reaction conditions: 1 (0.3 mmol), 2a (0.36 mmol), CuBr2 (0.06 mmol), in
2.0 mL DCM at 115 °C for 6 h. Isolated yield.
Table 4. Synthesis of thiazol-2-yl ether derivatives derived from
natural product cores a
Table 2. Substrate scope of alkyl oxime acetates a
a Reaction conditions: 1 (0.2 mmol), 2a (0.3 mmol), CuBr2 (0.04 mmol), in 2.0
a Reaction conditions: 1 (0.2 mmol), 2a (0.3 mmol), CuBr2 (0.04 mmol), in 2.0
mL DCM at 115 °C for 6 h. Isolated yield.
mL DCM at 115 °C for 6 h. Isolated yield.
To further demonstrate the synthetic utility of this strategy, the
structure modification was also applied to some natural product
cores. As shown in Table 4, oxime acetates derived from β-ionone,
Isophorone, Flavanone and Nootkatone smoothly participated in
Next, we examined the scope of aliphatic oxime acetates (Table
2). To our delight, a large number of aliphatic oxime acetates are
compatible with this transformation. Different chain ketoxime
esters, for example, pentan-3-one O-acetyl oxime, heptan-4-one O-
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the transformation, affording the thiazide-skeleton molecules in 1]. When reducing the temperature, 5a was obtained in 42% yield,
moderate to good yields (3bp-3bs). In addition, when oxime acetate which may come from the reductive elimination of organo-
derived from Progesterone was used as the substrate, the copper(III) intermediates [eq. 2]. Moreover, the treatment of 2a
cyclization reaction only reacts at one end of the six-membered ring with large steric substrate camphorone oxime acetate under the
and a single product 3bt was obtained in 62% yield, which might be standard conditions led to the product 5b in 78% yield [eq. 3]. This
due to the preference to enolization or the steric hindrance.
result indicated that large steric hindrance inhibited the
Furthermore, the substrate scope with regard to xanthates was isomerization of imino copper(III) intermediates. In addition, the
evaluated (Table 5). Different straight chain alkyl potassium treatment of 5a under the standard conditions, 3aa could not be
xanthates with 1aa were reacted smoothly, and the corresponding obtained [eq. 4].
products could be obtained in moderate yields (3bu–3bw). In
addition, branched alkyl potassium xanthates could also react
smoothly under the standard conditions (3bx, 3by). Moreover,
potassium xanthate with alkenyl was also compatible in this
reaction, affording the desired product 3bz in 64% yield.
Regrettably, the aryl potassium xanthate did not react in this
strategy (3ca).
Table 5. Substrate scope of potassium xanthates a
OR
NOAc
S
N
CuBr2, DCM
115 o
+
S
KS
OR
C
1aa
2
3
O
S
O
O
S
O
S
N
N
N
N
S
3bx, 52%
3bv, 75%
3bw, 62%
3bu, 83%
O
S
O
S
Ph
Scheme 3. Control experiments
O
S
N
N
N
On the basis of these experimental results as well as previous
works,12-14
a tentative mechanism involved imino copper(III)
3ca, 0%
3bz, 64%
3by, 56%
intermediates is proposed in Scheme 4. First, intermediate A is
formed by oxidative addition of the N-O bond of 1aa to copper(I). 7b,
a Reaction conditions: 1aa (0.2 mmol), 2 (0.3 mmol), CuBr2 (0.04 mmol), in
2.0 mL DCM at 115 °C for 6 h. Isolated yield.
13e, 14a
Coordination of potassium xanthates to A gives intermediate
B
and simultaneously releases KOAc. Tautomerization and
In addition, when oxime acetates bearing two methines at α
position of the C=N group reacted under the standard conditions,
the corresponding dihydrothiazoles were obtained (Scheme 2). For
example, using oxime acetate 1cb as a substrate, the reaction
proceeded smoothly and the corresponding product 4a was
obtained in 76% yield. Similar results were observed when oxime
acetate 1cc was used under the standard condition, the desired
activation of the vinyl C-H bonds provides copper(III) intermediate
D, which would transform to E and release copper(I) catalyst via the
reductive elimination process. And intramolecular cyclization of E
generates intermediate F. Finally, hydrogen sulfide was removed at
high temperature and gave the product 3. In addition, when the
reaction temperature was reduced or the substrate steric hindrance
was increased greatly, by-product 5a tended to be produced via
direct reductive elimination of intermediate C.
product 4b was achieved in 51% yield
.
Scheme 2. The synthesis of dihydrothiazoles 4
To gain more insight into the reaction mechanism, several
experiments were performed in Scheme 3. First, when TEMPO and
BHT were added, this conversion was not inhibited completely,
showing that the generation of radicals should not be involved [eq.
Scheme 4. Proposed mechanism
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