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Cu-Catalyzed Radical Cascade Annulations of AlkyneTethered N-alkoxyamides with Air: Facile Access to Isoxazolidine/1,2-Oxazinane-Fused Isoquinolin-1(2H)-ones Fei Chen, Sheng-Qiang Lai, Fei-Fei Zhu, Qiang Meng, Yu Jiang, Wei Yu, and Bing Han ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b02445 • Publication Date (Web): 21 Aug 2018 Downloaded from http://pubs.acs.org on August 21, 2018
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Cu-Catalyzed Radical Cascade Annulations of AlkyneTethered N-alkoxyamides with Air: Facile Access to Isoxazolidine/1,2-Oxazinane-Fused Isoquinolin-1(2H)-ones Fei Chen, Sheng-Qiang Lai, Fei-Fei Zhu, Qiang Meng, Yu Jiang, Wei Yu, and Bing Han* State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou, 730000, People’s Republic of China ABSTRACT: A series of structurally important isoxazolidine/1,2-oxazinane-fused isoquinolin-1(2H)-ones have been facilely synthesized via efficient Cu-catalyzed aerobic oxidative radical cascade annulations of alkyne-tethered N-alkoxyamides. This method features air as the environment-friendly oxidant, and has the merits of cheap catalyst, broad substrates scope, high atom economy and simple operation. KEYWORDS: copper/air catalytic system, cascade annulation, isoquinolin-1(2H)-one, isoxazolidine, 1,2-oxazinane Isoquinolin-1(2H)-one, as an essential framework, exists ubiquitously in natural products (Figure 1, I and II) and bioactive compounds.1 Therefore, its synthesis is greatly pursued by organic and pharmaceutical chemists.2 In recent years, transition-metal catalyzed intermolecular annulation of alkynes with benzamides and intramolecular annulation of alkyne-tethered benzamides via C−H activation have become powerful strategies to construct isoquinolin-1(2H)-one skeletons (Scheme 1a and 1b).2,3 Among multifarious benzamides, O-substituted N-hydroxybenzamides have proved to be excellent partners of alkynes because they can serve as internal oxidants in the transformation (Scheme 1c and 1d).4 Although elegant works have been provided, researches on this area are mainly centered on precious metal catalysis, whereas developing cheap metal catalysis still remains a challenging task.5
Scheme 1. Methods for Synthesis of Isoquinolin-1(2H)-ones
Figure 1. Bioactive molecules containing isoquinolin-1(2H)one/isoxazolidine/1,2-oxazinane moiety.
Isoxazolidine and 1,2-oxazinane structural motifs not only constitute the core of many drugs and natural products6 (Figure 1, III−VI), but also act as versatile synthetic intermediates.7 Some efficient methods, such as intermolecular cycloadditions and intramolecular cyclizations,8,9 have been developed to gain access to these two kinds of heterocycles. However, the protocol for the construction of structurally novel isoxazolidine/1,2-oxazinane-fused isoquinolin-1(2H)-
one skeletons by merging of isoxazolidine or 1,2-oxazinane with isoquinolin-1(2H)-one has not been explored so far to our knowlege. Polycyclization is a useful strategy for designing drug molecules. For example, cilansetron, a novel 5-HT3 receptor antagonist, is derived from polycyclic modification of ondansetron, while the activity of the former is ten times higher than its matrix.10 In this context, the novel structures of isoxazolidine or 1,2-oxazinane-fused polycyclic isoquinolin1(2H)-ones would draw more attention from pharmacologists for their potential bioactivity. Recently, metal-catalyzed aerobic oxidation for the synthesis of heterocycles has attracted more and more attention from chemists because of its sustainability and environmental friendliness.11 Among diverse metal catalysts, copper is considered as an ideal candidate owing to its low
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toxicity, high earth abundance and low cost in comparison with precious metals.12 Furthermore, the use of air for aerobic oxidation is a better choice compared to pure oxygen, because it is safer, easier to operate, and almost costless.13 Therefore, the development of convenient and efficient copper/air catalytic systems to construct significant heterocycles is still highly in demand. As our lasting interest on the exploration of diversified radical cyclization strategies to prepare important heterocycles,14 herein we present a green, efficient, and practical tactic for the synthesis of structurally brand-new isoxazolidine/1,2-oxazinane-fused isoquinolin-1(2H)-ones via a radical cascade annulation of alkyne-tethered Nalkoxyamides using copper/air catalytic system (Scheme 1e). Although Park reported an exquisite work for the synthesis of alcohol-substituted isoquinolin-1(2H)-one using the same substrates, only the N-O cleavage product was obtained (Scheme 1d). Thus, this protocol not only provides a facile method for the construction of structurally novel polycyclic isoxazolidine/1,2-oxazinane-fused isoquinolin-1(2H)-ones, but also is a significant complement to the previous work. Table 1. Optimization of the Cascade Annulationsa
entry
catalyst
solvent
time (h)
yield (%)b
1
-
MeCN
10
3 (93)c
2
-
MeCN
48
46 (47)c
3
CuCl
MeCN
10
97
4
CuBr
MeCN
34
93
5
CuI
MeCN
27
91
6
CuCN
MeCN
18
96
7
CuSCN
MeCN
27
82
8
CuCl2
MeCN
48
95
9
CuBr2
MeCN
48
0
10
Cu(OAc)2
MeCN
48
53
11
FeCl2
MeCN
48
36
12
Fe(acac)3
MeCN
48
5
13
Co(acac)2
MeCN
10
76
14
Ni(acac)2
MeCN
10
86
15
CuCl
DMSO
10
95
16
CuCl
DMF
10
51
17
CuCl
toluene
10
93
18
CuCl
DCE
10
42
19d
CuCl
MeCN
6
94
e
20 CuCl MeCN 10 91 Reaction conditions: 1a (0.2 mmol, 1.0 equiv) and catalyst (5 mol%) in solvent (2 mL) was stirred at 80 oC under air for 10−48 h. bYield of the isolated product. acac = acetylacetonate, DMSO = dimethyl sulfoxide, DMF = N,N-dimethylformamide, DCE = 1,2dichloroethane. cThe yield of recovered substrate is listed in brackets. dCuCl (10 mol%) was used. eCuCl (1 mol%) was used. a
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To realize our hypothesis, N-((4-phenylbut-3-yn-1-yl)oxy) benzamide 1a was chosen as a model substrate and stirred in MeCN at 80 oC under air for the optimization investigation. In the absence of metal catalyst, the desired cyclizaition product 2a was obtained only in 3% yield and most of 1a was recovered after 10 h (Table 1, Entry 1). Even if prolonged the reaction time to 48 h, the yield of 2a was only increased to 46% accompanied by the recovery of 1a in 47% yield (Table 1, Entry 2). To improve the reaction efficiency, CuCl (5 mol%) was added in the reaction as the catalyst. Delightedly, 1a was almost quantitatively converted to the desired isoxazolidinefused isoquinolin-1(2H)-one 2a in 10 h (Table 1, Entry 3). Other copper salts promoted this annulation process as well, but no better result was gained (Table 1, Entries 4−10). In addition, other cheap metal catalysts such as FeCl2, Fe(acac)3, Co(acac)2 and Ni(acac)2 were also investigated, but they turned out to be less efficient in comparison with CuCl (Table 1, Entries 11−14). When other solvents such as DMSO, DMF, toluene and DCE served as the reaction medium, the yield of 2a was not further improved (Table 1, Entries 15−18). Moreover, the loading amount of catalyst was also explored. When the amount of CuCl was increased to 10%, the reaction was accomplished in 6 h and gave 2a in 94% yield (Table 1, Entry 19). Even if 1% of CuCl was used, the reaction could still proceed smoothly and gave 2a in 91% yield (Table 1, Entry 20).
Scheme 2. Synthesis of Isoxazolidine-Fused Isoquinolin1(2H)-ones a,b a Reaction conditions: 1 (0.4 mmol, 1.0 equiv), CuCl (5 mol%), in MeCN (4 mL) at 80 oC under air for 10 h. bIsolated yields. cAfter 20 h. dDMF (4 mL) was used instead of MeCN, at 120 oC for 10 h. e After 58 h. fAfter 87 h. gAfter 15 h. hTMS = trimethylsilyl. iThe diastereomer was derived from the axial chirality and the ratio was determined by 1H NMR spectroscopy.
With the optimal conditions established (Table 1, Entry 3), the scope of alkyne-tethered N-alkoxyamides was surveyed (Scheme 2). First, functional group tolerance at the amide moiety was explored. Para-substituted benzamides with a
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variety of electronic properties were well compatible in the reaction, delivering the desired products 2a−h in good to excellent yields. X-ray crystallography further validated the structure of 2g (see the Supporting Information (SI) for details). Meta-substitution lead to the generation of two regioisomers, as in the cases 2i and 2j. Ortho-substituted benzamides gave the desired products 2k−m in moderate to good yields. Polysubstituted phenyl, 2-indolyl and 2-thienyl involved amides were also suitable for this cascade procedure, affording the respective products 2n−p in excellent yields. Next, the alkyne moieties were also investigated. Nalkoxybenzamides with all kinds of substitution patterns on the phenyl of the alkyne moiety participated in this protocol very well, yielding the corresponding polycyclic products 2q−w with excellent reaction efficiency. 1-Naphthyl alkynyl and 2-thienyl alkynyl-tethered N-alkoxyamides were also good candidates for this transformation, providing 2x and 2y in 96% and 93% yields, respectively. Significantly, this cascade annulation strategy was applicable to alkyl alkyne-tethered Nalkoxybenzamides too, as exhibited in the cases 2z−ab. However, TMS-protected and terminal alkynes were found to be inert in the transformation, even at 120 oC. Notably, the estrone derivative could also undergo the cascade annulations to produce the complex product 2ae in 91% yield.
4e’−n’). The formation of the oxygen-trapped spirocyclic byproducts indicates that a radical process is involved in the reaction. Furthermore, the by-product can be efficiently inhibited by introducing a para-substituent on the benzene ring of amide moiety, as shown in the cases of 4b−d. However, for the para-Me substituted substrate 3b, there were still a tiny amount of by-product 4a’ formed by losing of the Me group. Similar aerobic demethylation has been observed by other groups.15
Scheme 4. Gram-Scale Reaction and Follow-up Transformations of 2a To further investigate the synthetic practicability and potentiality of this protocol, a gram-scale reaction of alkynetethered N-alkoxyamide 1a was carried out under the standard conditions. Delightedly, the reaction still took place very well and the desired polycyclic product 2a was successfully acquired in 90% yield (Scheme 4). In addition, compound 2a could be ulteriorly employed in synthetically useful transformations. For example, 2a could be easily converted to chlorinated product 5 in 82% yield by treatment with CuCl2. Remarkably, the reduction of the N−O bond of 2a could also be efficiently achieved, affording the corresponding 3-(2-hydroxyethyl)-4phenylisoquinolin-1(2H)-one 6 in 92% yield (Scheme 4). 1a
standard conditions TEMPO (3 equiv)
1a
1a
2a, 34% +
1a, 55% (recovered)
(1)
2a, 0% +
1a, 95% (recovered)
(2)
CuCl (5 mol%), Ar 80 oC, MeCN,10 h
O
CuCl2 (2.5 equiv), Ar
Cl O N H 7, 29% Cl
2a, 3% + Ph
80 oC, MeCN,10 h
Ph +
O 3a
standard conditions
Ph 4a, 60% + 4a', 12% + Ph
Ph
Ph N N
(3)
1a, 48% (recovered)
O 3 O
O
+
(4)
O 3 O Ph 9 8 detected by HRMS detected by HRMS Ph
3
O 3a
Scheme 3. Synthesis of 1,2-Oxazinane-Fused Isoquinolin1(2H)-onesa,b
Having successfully realized the synthesis of isoxazolidinefused isoquinolin-1(2H)-ones, we next investigated whether this tactic could be used to construct 1,2-oxazinane-fused isoquinolin-1(2H)-ones. To get the expected product, the length of tether between the oxygen atom and the alkyne in the substrate was adjusted to three carbon atoms. Gratifyingly, as illustrated in Scheme 3, a range of alkynyl-linked Nalkoxyamides 3a−n with various substituents participated smoothly in the reaction, providing the desired 1,2-oxazinanefused isoquinolin-1(2H)-ones 4a−n in moderate to good yields, some of them accompanied by a small amount of the unexpected oxygen-trapped spirocyclic by-products (4a’,
80 oC, MeCN,10 h
4a, 51% +
18
N
O
18
3a
a
Reaction conditions: 3 (0.4 mmol, 1.0 equiv), CuCl (5 mol%), in MeCN (4 mL) at 80 oC under air until 3 was completely consumed. bIsolated yields. cDMF (4 mL) was used instead of MeCN, at 120 oC.
CuCl (5 mol%), 18O2
standard conditions H218O (20 equiv), 17 h O
Ph O-4a', 11%
4a, 61% + 4a', 12% + 18O-4a' (undetected)
2
(6) O
CuCl (5 mol%), air
O
1a, [D]5-1a
80 oC, MeCN, 1 h 64% conv. KH/KD = 1.07 (1H NMR)
H4/D4
N O
(7)
Ph 2a, [D]4-2a O
O Ph
(5)
Ph N H
H5/D5
O
N H
CuCl (5 mol%), air 10
o Ph 80 C, MeCN,10 h or 120 oC, DMF,10 h
N
+ 10, 100% (recovered)
(8)
Ph 11, 0%
Scheme 5. Control Experiments To gain insights into the reaction mechanism, a series of control experiments were conducted as shown in Scheme 5 (see the SI for details). When TEMPO (2,2,6,6-tetramethyl-1piperidinyloxy), a well-known radical scavenger, was added into the reaction system, the yield of 2a was decreased
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dramatically to 34%, with 55% of 1a being recovered (Scheme 5, eq 1). This result reveals that a radical course might be involved in this transformation. When the reaction was carried out under argon atmosphere, no product was formed, and 1a was almost completely recovered (Scheme 5, eq 2). When CuCl2 (2.5 equiv) was used as the oxidant, 2a was acquired only in 3% yield and the dichlorination product 7 was isolated in 29% yield as a main product (Scheme 5, eq 3). These consequences suggest that the essential oxidant might be the species produced by the reaction of CuI with O2. In addition, when 3a was subjected to the standard conditions, besides the desired products 4a and 4a’, by-product 8 and its denitrogenated derivative 9 were detected by HRMS (Scheme 5, eq 4, see SI for details). Apparently, 8 was formed by the homo-coupling of N-centered radical which was derived from the oxidation of 3a. Similar phenomena were also observed for the oxidation of other N-alkoxy-benzamides.16,19b-e The 18Olabeling experiment further indicated that the O-atom of the formed carbonyl group of 4a’ was derived from air (Scheme 5, eqs 5 and 6). Moreover, the kinetic isotope effect was measured by performing the reaction with a 1:1 mixture of 1a and [D5]-1a (Scheme 5, eq 7). The relative rate constant was observed to be 1.07. This result suggests that the cleavage of C(sp2)−H bond on the benzamide moiety is not the ratedetermining step. When alkyne-tethered benzamide 10 was subjected to the standard conditions, no reaction occurred, and the substrate was completely recovered. This consequence not only indicates that N-O linkage is necessary for the reaction, but also implies that the captodative effect might be involved for making the substrate easier to be converted to the Ncentered radical.17
Figure 2. The electron paramagnetic resonance (EPR) spectra (X band, 9.4 GHz, rt) of A): reaction mixture of CuCl and DMPO in MeCN under air at 80 oC; B): the enlargement of the triple signal in box of A); C): reaction mixture of 1a, CuCl, and DMPO in MeCN under air at 80 °C; D) simulated spectrum of trapping the in-situ formed N-centered radical. To further prove that the reaction involves the generation of N-centered radical, the electron paramagnetic resonance (EPR) experiments were conducted using DMPO (2,2-dimethyl-3,4dihydro-2H-pyrrole 1-oxide) as the radical trapping reagent
(Figure 2, see SI for details). Stirring a mixture of CuCl and DMPO under air at 80 oC gave a triplet signal with aN = 13.98 G and g = 2.0056 as well as a weak signal of Cu2+ (Figure 2A). The expanded triple signal is depicted in Figure 2B. This triplet signal was attributed to a nitroxyl radical which was derived from the decomposing of the adduct of CuOO· and DMPO.18 No EPR signal was obtained by treatment of 1a with CuCl in MeCN at 80 oC for 2 h. However, when DMPO was introduced to the reaction system, besides the aforementioned triplet signal, a new set of signals of the trapping radical were observed (Figure 2C). Data analysis suggests that an in-situ generated N-centered radical is promptly trapped by DMPO to produce the metastable radical RN (aH = 16.59 G, aN1 = 13.73 G, aN2 = 2.86 G, and g = 2.0058). Similar signal was also obtained by using of DMPO to trap N-centered radical derived from the oxidation of other N-alkoxybenzamide.19d In addition, the formation of trapping radical RN was also confirmed by HRMS (see SI). The simulated EPR spectra of RN is shown in Figure 2D, which is well-matched with the experimental one.
Scheme 6. Proposed Mechanism Based on the experimental results and literatures,8h,19 a proposed mechanism is delineated in Scheme 6. Initially, CuI reacts with oxygen molecule to produce a CuII peroxo species A or B.20 Substrate 1 or 3 is oxidized by A or B via a hydrogen atom transfer (HAT) process to generate the CuII species C or D and the N-centered radical E; the latter experiences intramolecular 5/6-exo-dig cyclization to afford the intermediate F. The intermediate F may also be produced via a competitive CuII-promoted aminocupration of internal alkyne process, which cannot be completely excluded.8d,8g,19j-l Subsequently, F undergoes aromatic homolytic substitution (AHS) to form the intermediate G, which is finally aromatized to yield product 2 or 4, along with the regeneration of CuI. In addition, F can also experience a competitive AHS process to generate the intermediate H when substrate 3 is involved in the reaction. Trapping of H by oxygen molecule produces peroxy radical I, which is finally converted to the by-product 4’. In summary, an efficient, environmentally friendly, and practical aerobic cascade reaction has been developed for the synthesis of structurally brand-new isoxazolidine/1,2oxazinane-fused isoquinolin-1(2H)-ones. This method employs inexpensive CuCl as the catalyst and air as the oxidant, and has the merits of operational simplicity and extensive functional group tolerability. Preliminary mechanism studies reveal that a radical-mediated cascade annulation is involved in the reaction. To the best of our
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knowledge, the present protocol not only represents the first example of isoxazolidine/1,2-oxazinane-fused isoquinolin1(2H)-one preparation, but also a cheap metal-catalyzed synthesis of isoquinolin-1(2H)-one skeletons. Further studies on the applications of copper/air catalytic systems in the synthesis of heterocycles are currently underway in our laboratory.
ASSOCIATED CONTENT Supporting Information. Detailed experimental procedures and spectral data for all products are provided. This material is available free of charge via the Internet at http://pubs.acs.org. Experimental details and characterization data (PDF) X-ray crystallographic data for 2g (CIF)
AUTHOR INFORMATION Corresponding Author *E-mail:
[email protected] Notes The authors declare no competing financial interest.
ACKNOWLEDGMENT We thank the National Natural Science Foundation of China (Nos. 21422205 and 21632001), the Fundamental Research Funds for the Central Universities (Nos. lzujbky-2016-ct02 and lzujbky2016-ct08), the Changjiang Scholars and Innovative Research Team in University (IRT-15R28), and the “111” Project for financial support.
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