Oxygen-involved oxidative deacetylation of α-substituted β-acetyl amides - Synthesis of α-Keto amides

Article abstract of DOI:10.1002/adsc.201300660

α-Substituted β-acetyl amides could undergo C-C bond cleavage to form α-keto amides when treated with copper(II) chloride (CuCl ₂) and boron trifluoride diethyl etherate (BF₃× OEt₂) under an oxygen atmosphere. The yield can be increased by the addition of tert-butyl hydroperoxide which alone can also effect the reaction. The reaction provides a new protocol for the synthesis of α-keto amides. Copyright

Full text of DOI:10.1002/adsc.201300660

UPDATES
DOI: 10.1002/adsc.201300660
Oxygen-Involved Oxidative Deacetylation of a-Substituted
b-Acetyl Amides – Synthesis of a-Keto Amides
Dianjun Lia and Wei Yua,*
a
State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou
University, Lanzhou 730000, Peopleꢀs Republic of China
Fax : (+86)-931-891-2582; e-mail: yuwei@lzu.edu.cn
Received: July 26, 2013; Revised: September 28, 2013; Published online: November 27, 2013
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201300660.
Abstract: a-Substituted b-acetyl amides could un-
dergo C C bond cleavage to form a-keto amides
Recently, Kꢁndig et al.[6] and Taylor et al.[7] inde-
pendently reported the synthesis of 3,3’-disubstituted
oxindoles from N-phenyl amides. This methodology
involves the oxidation of the N-phenyl amide-derived
enolate by copper salts to the corresponding carbon
radical, which then undergoes intramolecular attack
to the phenyl ring. Later on, Taylor et al. improved
the method by using copper as catalyst and oxygen as
oxidant.[8] These conditions work very well for a-sub-
stituted N-phenyl b-alkoxycarbonyl (or aminocarbon-
yl) amides. Besides its synthetic usefulness, it should
be pointed out that under an oxygen atmosphere, the
capture of a-alkyloxycarbonyl radicals or a-aminocar-
bonyl radicals by oxygen is not fast enough to com-
pete with the intramolecular cyclization. In continua-
tion of our interest in the free radical reactions of b-
carbonyl amides,[9] we recently found that when a-
À
when treated with copper(II) chloride (CuCl2) and
boron trifluoride diethyl etherate (BF3·OEt2) under
an oxygen atmosphere. The yield can be increased
by the addition of tert-butyl hydroperoxide which
alone can also effect the reaction. The reaction pro-
vides a new protocol for the synthesis of a-keto
amides.
Keywords: b-acetyl amides; tert-butyl hydroperox-
ide; copper catalysis; a-keto amides; oxidative de-
acetylation; oxygen
The single electron oxidation of enolates by transition substituted N-phenyl b-acetyl amides (1) were treated
metals such as Mn3+, Ce4+, Cu2+, Co3+, Fe3+ and Ag+ with CuCl2 and BF3·OEt2 under aerobic conditions,
etc. would generate a-carbonyl radicals, and this the oxidative deacetylation products a-keto amides
transformation forms the basis of a variety of synthet- (2) were generated. The formation of 2 involves the
ically useful intramolecular and intermolecular oxida- trapping of a-carbonyl radicals by molecular oxygen
[1]
À
tive C C bond forming methods. When the reac- as the key step. The yield of 2 can be improved by the
tions are carried out in the presence of oxygen, the a- addition of tert-butyl hydroperoxide (TBHP, 70% in
carbonyl radicals would be captured by oxygen to water). Herein we wish to report the detailed results.
form peroxy radicals and then a-peroxy ketones or al-
Our study was initiated by subjecting compound 1a
dehydes,[2] which would subsequently transform to to some common copper salts in an oxygen atmos-
À
1,2-dioxetane intermediates and then undergo C C phere under a variety of conditions. The results are
bond cleavage to yield two carbonyl fragments summarized in Table 1.
(Scheme 1).[3,4,5] This process has found synthetic ap-
It can be seen from Table 1 that when treated with
plications by using cerium(IV) ammonium nitrate[4] or 0.1 equiv. of CuCl2 and 2.0 equiv. of BF3·OEt2 in
copper salts[5] as oxidant.
CH3CN, 1a was converted to a-keto amide 2a in mod-
erate yield (entries 3 and 5, Table 1). The reaction did
not take place without BF3·OEt2 (entry 1, Table 1).
Compound 2a was also generated with CuBr2,
CuACHTNUGTRENNUG(OAc)2 or CuACHTUNGTERN(NUGN OTf)2, but the yield was lower. Be-
sides 2a, a small amount of cyclization product 3a was
obtained in varying yields in most cases; 3a is appa-
rently derived from the acid-catalyzed intramolecular
Friedel–Crafts reaction of 2a.[10] Among the solvents
Scheme 1.
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Oxygen-Involved Oxidative Deacetylation of a-Substituted b-Acetyl Amides
Table 1. The reaction of 1a with oxygen under various conditions.
Entry CuX2 (equiv.)
Oxidant
Equiv. of BF3·OEt Solvent (Temp.) Reaction time [h] Products (yield [%])[a]
1
2
3
4
5
6
7
8
CuCl2 (0.1)
CuCl2 (0.1)
CuCl2 (0.1)
CuCl2 (1.0)
CuCl2 (0.1)
none
CuCl2 (0.1)
CuCl2 (0.1)
CuCl2 (0.1)
CuCl2 (0.1)
CuBr2 (0.1)
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
none
1.0
2.0
2.0
2.0
2.0
3.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
CH3CN (r.t.)
CH3CN (r.t.)
CH3CN (r.t.)
CH3CN (408C) 15
CH3CN (408C)
CH3CN (408C) 20
CH3CN (408C)
toluene (408C) 12
24
30
20
N.R.
2a (15), 3a (3)
2a (40), 3a (5)
2a (25), 3a (6)
2a (45), 3a (5)
2a (10), 3a (trace)
2a (40), 3a (6)
2a (5), 3a (0)
2a (30), 3a (3)
2a (0), 3a (0)
2a (30), 3a (10)
2a (10), 3a (25)
2a (22), 3a (10)
N.R.
8
8
9
THF (408C)
CH2Cl2 (r. t.)
12
24
10
11
12
13
14
15
16
17
18
19
20
21
CH3CN (408C) 10
CH3CN (408C) 10
CH3CN (408C) 12
CH3CN (408C) 12
CH3CN (408C) 12
CH3CN (r. t.)
CH3CN (408C)
CH3CN (408C)
CH3CN (408C)
CH3CN (408C)
CuOAc2·2H2O (0.1) O2
Cu
CuSO4·5H2O (0.1)
Cu(NO3)2 (0.1)
(OTf)2 (0.1)
O2
O2
O2
T
N.R.
CuCl2 (0.1)
CuCl2 (0.1)
O2 +TBHP[b] 2.0
O2 +TBHP[b] 2.0
O2 +TBHP[c] 2.0
O2 +TBHP[b] 2.0
O2 +TBHP[c] 2.0
O2 +TBHP[b] none
4
3
3
8
8
2a (61), 3a (trace)
2a (70), 3a (4)
2a (82), 3a (trace)
2a (42), 3a (4)
2a (48), 3a (6)
N.R.
CuCl
none
none
none
2 ACHTUNGTRENNUNG(0.1)
CH3CN (408C) 12
[a]
[b]
[c]
Isolated yields.
1.0 equiv. of TBHP was used.
2.0 equiv. of TBHP was used.
examined, CH3CN proved to be the most suitable re- all less effective. It is interesting to see that when trif-
action medium. The appropriate reaction temperature lic anhydride was used, the reaction afforded 4 rather
AHCTUNGTRENNUNG
was found to be from room temperature to 408C. At than 2a and 3a (Scheme 2), and the reaction proceed-
higher temperature decomposition became dominant. ed as well in the absence of CuCl2. This transforma-
In order to raise the yield of 2a, we tried using several tion is known to take place under strong acidic condi-
other acids such as TMSOTf, InCl3, BiCl3, ZnACTHNUTRGNEUNG
(OTf)2 tions.[11] The triflic anhydride-promoted process can
and CF3COOH in place of BF3·OEt2, but they were be rationalized by the mechanism shown in Scheme 2.
Scheme 2.
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Dianjun Li and Wei Yu
UPDATES
of cerium(IV) ammonium nitrate (CAN) as the single
electron oxidant.[4b] However, it was found that the
reaction did not take place when 1a was treated with
CAN under Nairꢀs conditions. On the other hand,
when our procedure was applied to compound 6, the
À
expected oxidative C C bond cleavage did not take
place. Instead, boron enolate 7 was obtained. The
structure of 7 proved to be very stable[12] and it
cannot be oxidized under the present conditions
Scheme 3.
Finally, we found that adding TBHP (70% in water) (Scheme 4).
to the reaction system could increase the yield of 2a
The formation of compound 2a can be rationalized
remarkably at 408C (entries 17 and 18, Table 1). Con- with the free radical mechanism illustrated in
trol experiment shows that TBHP alone in the ab- Scheme 5: compound 1a first tautomerizes to its eno-
sence of copper is capable of prompting the reaction, late form A[13] by the action of BF3·OEt2, and then is
albeit with lower yield (entries 19 and 20, Table 1). oxidized to a-carbonyl radical B via a single electron
But still BF3·OEt2 is a necessary condition (entry 21, transfer process. Control experiment indicates that
Table 1).
the tautomerization is a quite facile process. The
When the reaction was performed under an argon direct a-hydrogen abstraction can be ruled out as no
atmosphere under otherwise the same conditions, ox- reaction took place in the absence of BF3·OEt2. Both
indole product 5 was obtained (Scheme 3). This reac- CuCl2 and TBHP could act as oxidant. In addition,
tion was formerly realized by using Ag2O as the oxi- the reaction of CuCl2 and TBHP might produce
dant.[9a]
a more powerful oxidizing species, as the combination
Nair et al. previously reported a relevant transfor- of CuCl2 and TBHP led up to a much better yield. It
mation of a-unsubstituted acetoacetamides to oxa- might be Cu(II)OO-t-Bu, t-BuOC, or t-BuOOC, since
mates under an oxygen atmosphere by using 2 equiv. all these species might be generated in the reaction
system.[14] To evaluate the effect of t-BuOC on the rea-
tion, we did a control experiment by replacing CuCl2
with CuBr. The t-BuOC radical is believed to be
a major oxidizing species in the system of Cu(I) and t-
BuOOH. We found that the yield of 2a was only 44%
when CuBr and t-BuOOH were used. This result sug-
gests that t-BuOC may not be important in the present
case. A reasonable hypothesis is that Cu(II)OO-t-Bu
Scheme 4.
might be responsible for the enhanced performance
Scheme 5. Proposed mechanism for the formation of 2a, 3a and 5 from 1a.
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Oxygen-Involved Oxidative Deacetylation of a-Substituted b-Acetyl Amides
Table 2. CuCl2-catalyzed oxidative transformations from 1 to
of the CuCl2/t-BuOOH system. However, the partici-
pation of t-BuOOC cannot be ruled out despite the
fact that it does not usually act as a single electron ox-
idant.
2.[a]
Following the SET step, radical B would be trapped
by oxygen to generate peroxy radical C, and then per-
oxide D (D’), which is converted to 2a in two steps. It
is also possible that radical C might directly undergo
cyclization to afford the 1,2-dioxetane intermediate.[15]
In the absence of oxygen, B would undergo radical
cyclization to give 5. This result is different from that
reported by Taylor et al.[8] in that in their cases, a-car-
bonyl radicals can be converted to oxindoles via intra-
molecular oxidative coupling in high yields under an
oxygen atmosphere. Besides the structural features,
the acidic conditions adopted in the present work
might also have some influence on the reaction conse-
quences.
a-Keto amide is an important structural moiety oc-
curring in many bioactive natural products, and its
construction has long been of interest to synthetic
chemists.[16,17] Recently, Jiao et al.,[17a–c] Mai et al.,[17d]
and Ji et al.[17e] reported several efficient syntheses of
a-keto amides which employ the copper-catalyzed ox-
idative coupling reactions with molecular oxygen as
oxidant. In another study, a-keto amides were pre-
pared from the aerobic oxidation of arylacetamides.[18]
These methods are suitable for the synthesis of aro-
matic a-keto amides, but have not been used to pre-
pare their counterparts bearing alkyl groups at the a-
position. We expected that the reaction described
herein would constitute a complement to the above-
Entry
1
R1
H
R2 R3
Product 2 Yield [%][b]
1
2
3
4
5
6
7
8
1a
Bn Me
Bn Me
2a
2b
2c
2d
2e
2f
2g
2h
2i
82
78
76
81
66
58
77
83
89
61
81
42
44
1b p-Me
1c p-MeO Bn Me
1d p-Br
1e p-Cl
1f p-I
1g p-F
1h m-Me Bn Me
1i m-Br
1j o-Me
1k
Bn Me
Bn Me
Bn Me
Bn Me
9
Bn Me
Bn Me
Bn Et
Bn Bn
10
11
12
13
14
15
16
17
2j
2k
2l
H
H
H
H
H
H
H
1l
1m
1n
1o
1p
1q
Bn 4-MeBn 2m
[c]
Bn allyl
Me Me
Et Me
Bn n-Bu
2n
2o
2p
2q
–
71
65
58
[a]
The reaction was conducted on 0.5 mmol scale in 5 mL
CH3CN.
Isolated yield.
Complex mixture.
[b]
[c]
mentioned oxidative methods. With this in mind, next ents at the nitrogen atom. However, in the case of 1r,
we applied this protocol to a variety of N-aryl b- the steric hindrance gets relieved, and its trapping by
acetyl amides (1b–1q) to examine the scope of the re- Cu(II)OO-t-Bu is made possible. It should be noted
action, and the results are listed in Table 2.
Under the optimized reaction conditions (entry 18, of Cu(II)OO-t-Bu in the current reaction system.
Table 1), compounds 1 were converted to the corre- When compounds 10 were subjected to the condi-
that the formation of 8 is consistent with the presence
sponding a-keto amides in varying yields except for tions, mixed results were obtained (Scheme 7 and
1n (entry 14, Table 2), in which case the reaction only Scheme 8). In the case of 10a and 10b, the reaction
resulted in the formation of a complex mixture. The produced 12 besides the desired a-keto amides 11
yields were also low for compounds 1l and 1m (en- (Scheme 7), while a-keto amide products were ob-
tries 12 and 13, Table 2). Apart from these cases, a- tained predominantly when the a-methyl group was
keto amides 2 were obtained in good yields for other replaced by Bn (Scheme 8). The formation of 12a and
substrates.
12b might also be due to the lesser steric hindrance of
In contrast to these results, the reaction of the N- the methyl group in 10a and 10b. For these substrates,
monosubstituted compound 1r afforded a mixture of using 1.0 equiv. or 2.0 equiv. of TBHP did not make
a-keto amide (2r), a-tert-butylperoxy product (8) and apparent difference in the yields of the products.
a-hydroxylation product (9) under the same condi-
N-Aryl a-keto amides can react via the intramolec-
tions. Compound 8 was found to transform in situ into ular Friedel–Crafts reaction to form 3-hydroxyin-
9 (Scheme 6). The different behavior of 1r as com- doles.[10] As aforementioned, we did find a small
pared with 1a-1q might be explained by the lack of an amount of 3-hydroxyindole products 3 along with 2 in
alkyl group at the N atom in 1r. It is reasonable to be- the reaction mixture (Table 1). An attempt was then
lieve that compound 8 results from the oxidation of made to maximize the yields of 3. After some explo-
the 1r-derived radical by Cu(II)OO-t-Bu.[14] This reac- rations, we found that adding one more portion of
tion is supposed to be unfavorable for substrates 1a– CuCl2 and BF3·OEt2 to the reaction mixture after
1q due to the steric hindrance caused by the substitu- 1 was completely consumed and then allowing the re-
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Dianjun Li and Wei Yu
UPDATES
Scheme 6.
Scheme 7.
droxyindoles from a-substituted b-acetyl amides has
been developed.
Experimental Section
General Methods
1
The H and 13C NMR spectra were recorded on a Bruker
AM-400 MHz spectrometer in CDCl3. The chemical shifts in
1H NMR spectra were determined with (CH3)4Si as the in-
ternal standard (d=0.00 ppm). The chemical shifts in
13C NMR spectra were determined based on that of CDCl3
(d=77.00 ppm). The EI-mass spectra were measured on an
HP 5988A spectrometer by direct inlet at 70 eV. The high
resolution mass spectra (HR-MS) were measured on
a Bruker Daltonics APEX II 47e spectrometer by ESI or
a Bruker micrOTOF QII by ESI. Melting points were mea-
sured on an XT-4 melting point apparatus and are uncor-
rected. Flash column chromatography was carried out with
silica gel (200–300 mesh). a-Substituted b-acetyl amides
were prepared according to the previous literatures.[7,19]
Scheme 8.
action to continue under an argon atmosphere at
608C gave 3 in moderate yields (Scheme 9).
In summary, we have demonstrated that the oxidiz-
ing system of CuCl2/t-BuOOH/O2 can affect the oxi-
dation of a-substituted b-acetyl amides in the pres-
ence of BF3·OEt2. The reaction results in the forma-
tion of oxidative deacetylation products a-keto
amides in moderate to good yields. The procedure de-
scribed herein provides an alternative protocol for the
synthesis of aliphatic a-keto amides. Furthemore, on
General Procedure for the Reactions of a-Substituted
b-Acetyl Amides (1)
To a 25-mL round-bottom flask equipped with a magnetic
stirring bar were added 1 (0.5 mmol), anhydrous CuCl2
(6.8 mg, 0.05 mmol), BF3·OEt2 (125 mL, 1.0 mmol), tert-butyl
hydroperoxide (TBHP, 70% in water) (140 mL, 1.0 mmol)
and CH3CN (5 mL). The mixture was stirred in an oil bath
the basis of this process, a one-pot synthesis of 3-hy- at 408C. After the reaction complete as indicated by TLC
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Oxygen-Involved Oxidative Deacetylation of a-Substituted b-Acetyl Amides
Scheme 9.
(generally 2–3 h), the reaction mixture was poured into satu- References
rated aqueous NH4Cl solution (15 mL), and was extracted
with EtOAc (10 mLꢃ3). The combined organic layers were
washed with brine (30 mL) and dried with anhydrous
Na2SO4. The solvent was removed under reduced pressure,
and the residue was treated with silica gel chromatography
to give products 2.
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General Procedure for the Preparation of 3-
Hydroxyindoles (3)
To a 25-mL round-bottom flask equipped with a magnetic
stirring bar was added 1 (0.5 mmol), anhydrous CuCl2
(6.8 mg, 0.05 mmol), BF3·OEt2 (125 mL, 1.0 mmol), tert-butyl
hydroperoxide (TBHP, 70% in water) (140 mL, 1.0 mmol)
and CH3CN (5 mL). The solution was stirred in an oil bath
at 408C. After the substrate was consumed as indicated by
TLC, another portion of anhydrous CuCl2 (5 mg) and
BF3·OEt2 (88 mL) was added to the reaction mixture, and
the stirring was continued at 608C under an argon atmos-
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and the mixture was extracted with EtOAc (10 mLꢃ3). The
combined organic layers were washed with brine (30 mL)
and dried with anhydrous Na2SO4. The solvent was removed
under reduced pressure, and the residue was treated with
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Acknowledgements
The authors thank the National Fund for Talent Training in
Basic Science (J1103307) from National Science Foundation
of China for financial support.
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3714
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