Regioselective Synthesis of 5-Aminooxazoles via Cp?Co(III)-Catalyzed Formal [3 + 2] Cycloaddition of N-(Pivaloyloxy)amides with Ynamides

Article abstract of DOI:10.1021/acs.orglett.7b02959

A simple and efficient protocol for the regioselective synthesis of 5-aminooxazoles is disclosed. The reaction, catalyzed by a cheap Cp?Co(III) catalyst, starts from easily accessible N-(pivaloyloxy)amides and ynamides. Mild reaction conditions, a broad s

Full text of DOI:10.1021/acs.orglett.7b02959

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX-XXX
Regioselective Synthesis of 5‑Aminooxazoles via Cp*Co(III)-
Catalyzed Formal [3 + 2] Cycloaddition of N‑(Pivaloyloxy)amides with
Ynamides
Xiang-Lei Han,† Chu-Jun Zhou,† Xu-Ge Liu,† Shang-Shi Zhang,† Honggen Wang,†
and Qingjiang Li*,†,‡
†School of Pharmaceutical Sciences, Sun Yat-sen University, Guangzhou, 510006, China
‡State Key Laboratory of Natural and Biomimetic Drugs, Peking University, Beijing, 100191, China
S
* Supporting Information
ABSTRACT: A simple and efficient protocol for the regioselective
synthesis of 5-aminooxazoles is disclosed. The reaction, catalyzed by a
cheap Cp*Co(III) catalyst, starts from easily accessible N-(pivaloyloxy)-
amides and ynamides. Mild reaction conditions, a broad substrate scope,
good functional group tolerance, and good to excellent yields were
observed.
Scheme 1. Oxazoles Synthesis Using Ynamides and Cobalt-
Catalyzed Annulation of Benzamides with Alkynes
xazoles represent one of the most important heterocycles
Ofoundinnaturallyoccurringproductsandpharmaceutically
relevant molecules.1 They also serve as versatile building blocks
for efficient organic synthesis.2 Consequently, a variety of
methods,3 including the traditional Fischer,4 Robinson-Gabriel,5
and van Leusen6 oxazole syntheses, have been developed for their
preparation.7 Recently, with the development of organometallic
chemistry, transition-metal-catalyzed bimolecular cycloaddition
reactions provide an alternative method, which allows the
synthesis to take place under mild reaction conditions with a
broader substrate scope.3c
Alkynes, due to their versatile reactivities toward transition
metals, have been utilized as two-carbon synthons in the
construction of oxazoles under the catalysis of Ru,7o Pd,7i Hg,7s
Ag,7j Cu,7k,q or Au.7a,g,l,p Specifically, 1-amido-alkynes (yna-
mides), which could be activated by acids to form the
corresponding keteniminium ion species,8 have proven to be
versatile building blocks in organic synthesis, especially in
heterocyclic scaffolds synthesis.9 For instance, in oxazole
synthesis, Davies and co-workers disclosed a Au(III)-catalyzed
intermolecular formal [3 + 2] cycloaddition of pyridine-N-
aminides with ynamides, providing oxazoles with a useful amino
substituent (Scheme 1a).10 Very recently, similar [3 + 2]
cycloadditions catalyzed by Au(I)11 or Tf2NH12 using dioxazoles
as a nucleophilic N-acyl nitrene equivalent with ynamides had
been achieved (Scheme 1b). It should be mentioned that, in all
these cases, the same regioselectivity was observed, resulting in
the formation of 4-aminooxazoles exclusively.
In recent years, the cobalt-based complexes have been widely
utilized in the assembly of diverse C−C and C−X bonds.13 It has
been reported that the cobalt-catalyzed coupling reaction of
benzamide derivatives with alkynes could lead to the formation of
isoquinolones via C−H activation/annulation processes
(Scheme 1c and 1d).14 In continuation of our interest in
Cp*Co(III) catalysis as well as in the application of a
functionalized unsaturated carbon−carbon bond in C−H
activation reactions,15 we envisioned that the amino-substituted
isoquinolone might be accessible via a cobalt-catalyzed C−H
annulation of N-(pivaloyloxy)amide with ynamide (Scheme 1e,
path A). Interestingly, however, an unexpected 5-aminooxazole
product was obtained as the sole product (Scheme 1e, path B).
Importantly, the regioselectivity in this reaction was in contrast to
the previous observations wherein ynamides were used as
Received: September 21, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b02959
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
a
Table 1. Optimization of the Reaction Conditions
Scheme 2. Substrates Scope of N-(Pivaloyloxy)amides
b
entry
catalyst (x mol %)
additive (y mol %)
solvent
yield (%)
1
Cp*Co(CO)I2 (10)
Co2(CO)8 (10)
Co(OAc)2 (10)
Cp*Co(CO)I2 (2.5)
Cp*Co(CO)I2 (1)
[Cp*RhCl2]2 (2.5)
[Ir(COD)Cl]2 (2.5)
Cu(OTf)2 (10)
AgOTf (10)
NaOAc (20)
NaOAc (20)
NaOAc (20)
NaOAc (5)
NaOAc (5)
NaOAc (5)
NaOAc (5)
NaOAc (10)
NaOAc (10)
NaOAc (10)
−
TFE
76
0
2
TFE
3
TFE
0
4
TFE
TFE
88
71
0
5
6
TFE
7
TFE
0
8
TFE
0
9
TFE
0
10
11
12
13
14
15
16
17
18
19
20
IPrAuNTf2 (10)
IPrAuNTf2 (5)
TFE
0
DCE
DCE
DCE
HFIP
MeCN
DCE
HOAc
MeOH
TFE
0
Tf2NH (5)
−
−
0
Sc(OTf)3 (5)
0
Cp*Co(CO)I2 (2.5)
Cp*Co(CO)I2 (2.5)
Cp*Co(CO)I2 (2.5)
Cp*Co(CO)I2 (2.5)
Cp*Co(CO)I2 (2.5)
−
NaOAc (5)
NaOAc (5)
NaOAc (5)
NaOAc (5)
NaOAc (5)
NaOAc (5)
−
65
0
0
0
0
0
Cp*Co(CO)I2 (2.5)
TFE
62
a
General reaction conditions: 1a (0.2 mmol, 1.0 equiv), 2a (0.24
a
General reaction conditions: 1 (0.2 mmol, 1.0 equiv), 2a (0.24 mmol,
1.2 equiv), Cp*Co(CO)I2 (2.5 mol %), NaOAc (5.0 mol %), TFE (2.0
mL), 25 °C, 1−2 h under air, isolated yield.
mmol, 1.2 equiv), catalyst (x mol %), additive (y mol %), solvent (2.0
mL), 25 °C, 2 h. Isolated yield.
b
coupling partners (Scheme 1a and 1b).10−12 Herein, we disclose
our studies on the polysubstituted oxazole synthesis via a redox-
neutral Co(III)-catalyzed formal [3 + 2] cycloaddition of N-
(pivaloyloxy)amideswithynamides.Thereactionproceedsunder
mild reaction conditions, and a broad substrate scope, good to
excellent yields, and high regioselectivities are observed.
regioselective manner. The regioisomer 4-aminoxazole of 3aa
was not formed in our protocol.
With the optimized conditions in hand (Table 1, entry 4), we
next explored the substrate scope of the cycloaddition reaction.
The scope of N-(pivaloyloxy)amides 1 was first investigated by
reacting with ynamide 2a. As shown in Scheme 2, the reaction was
found to be very general and robust. A number of commonly
encountered functional groups, regardless of the electronic
nature, at different positions of the benzamide ring were well
tolerated, giving the corresponding products in generally good to
excellent yields (3ba−3ka). Substituents such as ether (3ba),
cyano (3ea), halides (3fa, 3ga, 3ka), andnitro (3ja)werevaluable
functional handles for further derivatization. Heteroaryl-sub-
stituted amides, such as furyl (3la, 3ma), thienyl (3na),
benzothienyl (3oa), and indolyl-substituted (3pa) substrates,
were also compatible with the reaction conditions, delivering a
bis-heteroaryl axis architecture. The regioselectivity of the
intermolecular cycloaddition was unambiguously confirmed by
X-ray analysis of compounds 3aa and 3la.17 The cycloaddition of
2-naphthyl (3qa) or alkenyl (3ra and 3sa) substituted amides
with 2a produced the desired oxazoles in good yields. In addition,
aliphatic substituents could be installed to the 2-position of
oxazoles (3ta and 3ua) by using alkyl-substituted amides.
Subsequently, the scope on the ynamides was also investigated
and the results were summarized in Scheme 3. The effect of the R1
group at the terminal alkyne was first examined. A wide range of
substituents, such as OMe, t-Bu, F, CN, Br, and Cl, at different
positions of the phenyl ring were all suitable for this process,
furnishing 3bb−3bi in 64−97% yields. It was noted that a lower
64% yield for 3bi was observed when o-Cl-phenyl alkyne was
employed, probably due to steric reasons. 2-Thienyl-substituted
Initially, the reaction of N-(pivaloyloxy)benzamide 1a and N-
methyl-N-(phenylethynyl)methanesulfonamide 2a was studied.
Under the conditions of Cp*Co(CO)I2 (10 mol %), NaOAc (20
mol %), TFE (2 mL), 25 °C, 2 h (Table 1, entry 1), oxazole 3aa
was obtained in 76% isolated yield, with no formation of amino-
substituted isoquinolone being detected. Other cobalt catalysts
tested such as Co2(CO)8 or Co(OAc)2 were ineffective for the
cycloaddition (entries 2 and 3). Interestingly, a higher yield was
obtained while lowering the loadings of both the catalyst (to 2.5
mol %) and additive (to 5 mol %) (entries 4 and 5). Further
studies demonstrated that some catalysts based on the siblings
(Rh and Ir) of cobalt and its neighbors (Cu, Ag, and Au) were
found to be inefficient (entries 6−10). Lewis acids such as
IPrAuNTf2, Tf2NH, and Sc(OTf)3, which were reported to be
effective in related cyclization reactions,11,12 showed no
reactivities for the transformation (entries 11−13). The solvent
effect was also examined. Hexafluoroisopropanol (HFIP)
exhibited less efficiency (entry 14), and there was no desired
product obtained in other solvents, such as acetonitrile,
dichloroethane, acetic acid, and methanol (entries 15−18).16
Control experiments showed that both the catalyst and additive
NaOAc were important for this transformation. The omission of
catalyst resulted in no formation of the oxazole product (entry
19), whereas the exclusion of NaOAc led to a lower yield (entry
20). Note that the transformation proceeded in a highly
B
DOI: 10.1021/acs.orglett.7b02959
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Scheme 3. Substrates Scope of Ynamides
Scheme 4. Possible Reaction Pathways for the Formation of
Oxazole 3aa
a
General reaction conditions: 1b (0.2 mmol, 1.0 equiv), 2 (0.24 mmol,
1.2 equiv), Cp*Co(CO)I2 (2.5 mol %), NaOAc (5.0 mol %), TFE (2.0
mL), 25 °C, 1−2 h under air, isolated yield.
alkyne was smoothly converted into the corresponding oxazole
3bj in high yield. Furthermore, alkyl-substituted ynamides could
also undergo reaction efficiently (3bk, 3bl). Most notably, the
desired oxazole was also formed by using terminal ynamide as a
reactionpartner, albeitinaloweryield(3bm, 29%). Finally, theR2
andR3 groupsatthenitrogenatomwereexplored. Thereactionof
N-tosyl ynamide 2n with 1b proceeded successfully, giving 3bn in
excellent yield. The 5-indolyl or pyrrolyl oxazole could be
preparedbyswitchingynamidetothecorrespondingalkyne(3bo,
3bp). The more electron-rich ynamide 2q with an oxazolidine
group was also applicable to the transformation, affording the
corresponding oxazole 3bq in a moderate yield. The chiral cyclic
ynamidewasalsoamenable for annulation(3br). Disappointedly,
the reaction of sulfur-substituted alkyne or diphenyl acetylene did
not give the corresponding oxazoles, only with near-quantitative
recovery of the starting materials.
The potential synthetic utility was demonstrated by a gram-
scale synthesis. Thus, 1.51 g of oxazole 3ba was obtained (94%
yield) via the cycloaddition of amide 1b with ynamide 2a under
the standard reaction conditions with a slightly prolonged
reaction time (to 3 h, eq 1). Remarkably, by using our protocol,
the oxazole moiety could be introduced to estrone in an excellent
95% yield (3va, eq 2). The reaction was also conducted with
different leaving groups on the nitrogen atom of amide. While the
use of OAc as the leaving group delivered a comparable yield, the
use of OH or OMe led to no reaction at all, indicating the leaving
ability of the substituent on nitrogen was important for reaction.
Onthebasisofthe results obtainedand preceding reports,14d,18
the possible reaction pathways are outlined in Scheme 4. Initially,
the coordination of N-(pivaloyloxy)benzamide 1a to the CoIII
catalyst delivers the amido complex A. Thereafter, the
nucleophilic attack of ynamide 2a to A leads to the formation of
keteniminium ion species B. An intramolecular 6-exo-trig
cyclization generates a cyclocobalt species C. At this stage, the
reductive elimination would generate the N-pivaloyloxy oxazole
cation DandaCoI species. TheCoI isthenoxidizedtoCoIII bythe
N−O bond (path a). Alternatively, the nucleophilic displacement
of the N−O bond with a Co−C bond could produce 3aa and
release thecatalystdirectly(pathb). Anotherpossibilityisthatthe
CoIII might be oxidized to CoV prior to a C−N bond reductive
elimination (path c).
Inconclusion, wehavedevelopedasimpleandefficientmethod
for the synthesis of 5-aminooxazoles by using a Cp*Co(III)-
catalyzed regioselective formal [3 + 2] cycloaddition reaction of
N-(pivaloyloxy)amides with ynamides. The reaction proceeds
under mildreactionconditions, withgoodtolerance toavarietyof
functional groups and good to excellent yields being observed. In
consideration of the ready availability of the starting materials and
the importance of the products, we anticipate this method will
find applications in organic synthesis and medicinal chemistry.
ASSOCIATED CONTENT
* Supporting Information
■
S
TheSupportingInformationisavailablefreeofchargeontheACS
Publications website at DOI: 10.1021/acs.orglett.7b02959.
C
DOI: 10.1021/acs.orglett.7b02959
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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Detailed experimental procedures, characterization of all
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Crystallographic data for 3aa (CIF)
Crystallographic data for 3la (CIF)
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AUTHOR INFORMATION
Am. Chem. Soc. 2011, 133, 191. (r) Egorova, O. A.; Seo, H.; Kim, Y.;
Moon, D.;Rhee, Y. M.;Ahn, K. H. Angew. Chem., Int. Ed. 2011, 50, 11446.
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■
Corresponding Author
*E-mail: liqingj3@mail.sysu.edu.cn.
ORCID
Xiang-Lei Han: 0000-0003-1951-3765
Chu-Jun Zhou: 0000-0003-0759-3219
Xu-Ge Liu: 0000-0003-1878-2525
Shang-Shi Zhang: 0000-0002-9247-1373
Honggen Wang: 0000-0002-9648-6759
Qingjiang Li: 0000-0001-5535-6993
Notes
(9)Forsomereviews: (a)Evano, G.;Coste, A.;Jouvin, K. Angew. Chem.,
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R.; Lu, Z.; Zhang, Y.; Hsung, R. P. Chem. Rev. 2010, 110, 5064. (c) Wang,
X.-N.; Yeom, H.-S.; Fang, L.-C.; He, S.; Ma, Z.-X.; Kedrowski, B. L.;
Hsung, R. P. Acc. Chem. Res. 2014, 47, 560. (d) Pan, F.; Shu, C.; Ye, L. W.
Org. Biomol. Chem. 2016, 14, 9456. (e) Hu, L.; Zhao, J. Synlett 2017, 28,
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(10)(a)Davies, P.W.;Cremonesi, A.;Dumitrescu, L. Angew. Chem., Int.
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The authors declare no competing financial interest.
(11) Chen, M.; Sun, N.; Chen, H.; Liu, Y. Chem. Commun. 2016, 52,
6324.
(12) Zhao, Y.; Hu, Y.; Wang, C.; Li, X.; Wan, B. J. Org. Chem. 2017, 82,
3935.
ACKNOWLEDGMENTS
■
We are grateful for the support of this work by the National
Natural Science Foundation of China (21502242, 81402794, and
21472250) and the State Key Laboratory of Natural and
Biomimetic Drugs (K20150215).
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DOI: 10.1021/acs.orglett.7b02959
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