Visible Light-Promoted Synthesis of Spiroepoxy Chromanone Derivatives via a Tandem Oxidation/Radical Cyclization/Epoxidation Process

Article abstract of DOI:10.1002/adsc.201701072

A highly efficient and straightforward approach for the synthesis of spiroepoxy chroman-4-one derivatives was developed using a visible light-enabled tandem radical strategy. The reaction is initiated by the formation of an acyl radical that undergoes intramolecular radical cyclization and epoxidation. The optimal result was obtained with 1 mol% of Ru(bpy)₃Cl₂?6H₂O, TBHP, and K₂CO₃ in i-PrOAc at room temperature with irradiation from a blue LED. This unprecedented tandem approach utilizes a broad range of substrates, and provides a convenient and powerful synthetic tool for accessing the synthetically useful spiroepoxy chroman-4-ones and their nitrogen-containing derivatives. (Figure presented.).

Full text of DOI:10.1002/adsc.201701072

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DOI: 10.1002/adsc.201701072
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Visible Light-Promoted Synthesis of Spiroepoxy Chromanone
Derivatives via a Tandem Oxidation/Radical
Cyclization/Epoxidation Process
Sungwoo Jung,a, b Jiyun Kim,a, b and Sungwoo Honga, b,*
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a
Department of Chemistry, Korea Advanced Institute of Science and Technology, Daejeon, 305-701, Korea
Fax: (+82)-42-350-2810; phone: (+82)-42-350-2811; e-mail: hongorg@kaist.ac.kr
Center for Catalytic Hydrocarbon Functionalization, Institute for Basic Science (IBS), Daejeon, 305-701, Korea
b
Received: August 16, 2017; Revised: September 7, 2017; Published online: && &&, 0000
17 Supporting information for this article is available on the WWW under https://doi.org/10.1002/adsc.201701072
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mentally benign tools for synthesizing various com-
pounds under mild conditions.[6,7] We were intrigued
by the possibility of developing photochemical reac-
tions by employing a visible light-mediated protocol
and combining a multistep sequence to construct the
spiroepoxy chroman-4-one skeleton. Mechanistically,
both the oxidation of benzylic alcohol[8] and the
generation of the acyl radical are believed to proceed
by the action of peroxy radicals, which would set the
stage for intramolecular radical cyclization with inter-
nal alkenes.[9] Therefore, we envisioned that the
aldehyde group generated in situ as a result of visible
light-mediated oxidation of benzylic alcohol could be
further utilized for spontaneous intramolecular acyl
radical cyclization with internal olefins via a photo-
redox process. Subsequently, the generated b-peroxy
intermediate might further undergo a base-assisted
epoxidation to install a spiroepoxy group in the
chroman-4-one skeleton. If successful, this visible
light-enabled tandem radical strategy is highly desir-
Abstract: A highly efficient and straightforward
approach for the synthesis of spiroepoxy chroman-
4-one derivatives was developed using a visible
light-enabled tandem radical strategy. The reaction
is initiated by the formation of an acyl radical that
undergoes intramolecular radical cyclization and
epoxidation. The optimal result was obtained with
1 mol% of Ru(bpy)3Cl2·6H2O, TBHP, and K2CO3 in
i-PrOAc at room temperature with irradiation from
a blue LED. This unprecedented tandem approach
utilizes a broad range of substrates, and provides a
convenient and powerful synthetic tool for access-
ing the synthetically useful spiroepoxy chroman-4-
ones and their nitrogen-containing derivatives.
Keywords: acyl radical; radical cyclization; visible
light; tandem; photocatalysis
39 The chroman-4-one scaffolds constitute an important able and efficient from the perspectives of green
40 class of naturally occurring compounds and privileged process. Herein, we present the first example of a
41 scaffolds in medicinal chemistry.[1] Along these lines, a visible light photocatalyzed tandem synthesis of
42 series of synthetic methods to synthesize these scaf- diverse spiroepoxy chroman-4-one derivatives and
43 folds have been developed using various catalytic their nitrogen-containing counterparts (Scheme 1).
44 systems.[2] The spiroepoxy chroman-4-one moiety is Moreover, mild room temperature conditions are
45 also extremely fascinating because of its versatility in sufficient for the construction of the scaffolds in the
46 further synthetic transformation, allowing it to be overall reaction process.
47 converted into a diverse range of useful molecular
To corroborate this hypothesis, the feasibility of
48 entities.[3] Despite these important contributions, exist- using a photoreaction to convert aldehyde substrate
49 ing methods for preparing this motif suffer from 1a into spiroepoxy chroman-4-one 2a via a radical-
50 multiple steps, and harsh reaction conditions.[4] In this based cyclization was initially investigated, and se-
51 context, a more straightforward approach for synthe- lected data are listed in Table 1. After screening some
52 sizing structurally diverse spiroepoxy chroman-4-ones potential catalytic systems, the corresponding product
53 under mild reaction conditions is a topic of consid- 2a was obtained upon irradiation of 1a in the
54 erable importance.[5]
presence of 1 mol% of Ir(ppy)3 at room temperature
Strategies using visible light-driven photoredox under irradiation by a household fluorescent light
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56 catalysis emerge as the promising alternative to the (Table 1, entries 3 and 4), thus highlighting that the
57 existing methods, as they are powerful and environ- overall process was operating effectively. It was
Adv. Synth. Catal. 2017, 359, 1–6
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Table 1. Optimization of the reaction conditions.[a]
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entry Photocat (1 mol%) base
(3 quiv)
light source yield
(%)
1
Ir(ppy)3
KOtBu
DBU
23 W CFL
23 W CFL
trace
–
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2
Ir(ppy)3
3
Ir(ppy)3
Ir(ppy)3
Ir(ppy)3
[Ir(dF(CF9)ppy)2
(bpy)]PF6
Ru(bpz)3PF6
Ru(bpy)3Cl2·6H2O K2CO3
Ru(bpy)3Cl2·6H2O K3PO4
Cs2CO3 23 W CFL
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–
4
K2CO3
–
K2CO3
23 W CFL
23 W CFL
23 W CFL
5[b]
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K2CO3
23 W CFL
23 W CFL
23 W CFL
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72
65
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–
Ru(bpy)3Cl2·6H2O Cs2CO3 23 W CFL
Scheme 1. Synthetic approach to the spiroepoxy chroman-4-
ones and their nitrogen-containing derivatives via a visible
light-induced tandem oxidation/radical cyclization/epoxida-
tion sequence.
Ru(bpy)3Cl2·6H2O K2CO3
K2CO3
Ru(bpy)3Cl2·6H2O K2CO3
blue LED
blue LED
–
–
trace
[a] Procedure A: 1 (1.0 equiv), photocatalyst (1.0 mol%),
TBHP (5.0 equiv), and base (3.0 equiv) in i-PrOAc
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(0.1 M) at rt under N2 for 24 h with light irradiation. H
26 determined that the use of base promoted the
27 formation of 2a, and the intermediate b-peroxy
28 ketone was detected in the absence of base, indicating
29 that b-peroxy ketone is a key intermediate in the
30 reactions. (entry 5). Among the bases screened,
31 K2CO3 was most effective in terms of overall con-
32 version. With this encouraging preliminary result,
NMR yield using an internal standard.
[b] b-Peroxy ketone intermediate was isolated in 42% yield
with 40% recovery of starting material 1a. TBHP=tert-
butyl hydrogen peroxide, DBU=1,8-diazabicyclo[5.4.0]
undec-7-ene.
33 thorough screening of reaction media with photo- ization. In a similar fashion, substrates bearing sub-
34 catalysts was conducted with the goal of optimizing stituted alkenes are viable precursors for products 2p
35 the reaction outcome. More promising results were and 2q. Importantly, expanding the scope from
36 obtained using Ru(bpy)3Cl2·6H2O, which afforded 2a allyloxy- to allylamino-compounds was also possible
37 in 72% yield under an N2 balloon (entry 8). Further and gave rise to the corresponding spiroepoxy enami-
38 investigations revealed that i-PrOAc was most effi- none products (2r and 2s), thereby significantly
39 cient in this reaction. Various light sources were expanding the scope and synthetic utility of this
40 further screened, and the use of a blue LED resulted reaction. The structure of the product 2d was
41 in an improvement in yield (entry 11). Subsequent unambiguously confirmed by X-ray diffraction.[10]
42 control experiments confirmed the essential role of
43 visible light and the photocatalyst in this reaction our attention to the proposed three-step tandem
44 (entries 12, and 13). process mentioned previously by investigating the
The optimal conditions were then applied to a optimal reaction conditions under blue LED irradi-
Encouraged by the above results, we next turned
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46 variety of substrates to demonstrate the utility and ation at room temperature. To this end, we screened a
47 generality of this method, the results of which are number of reaction conditions. When the amount of
48 summarized in Table 2. We were pleased to observe TBHP was increased to 8.0 equiv, the benzylic alcohol
49 that by this catalytic method, spiroepoxy chroman-4- was completely converted to the corresponding alde-
50 one scaffolds could easily be prepared with either hyde, and overall reaction yields improved. When
51 electron-rich or electron-deficient substituents alcohol 3 was subjected to the optimized conditions,
52 (methyl, methoxy, fluoro, chloro, bromo, and ester) the desired sequential oxidation/intramolecular radical
53 with the desired products being generated in moderate cyclization /epoxidation process proceeded, thus ex-
54 to good yields at room temperature. Notably, halides hibiting that this method allows the synthesis of
55 such as Cl, Br and I were well tolerated under the spiroepoxy chroman-4-one derivatives. Subsequently,
56 reaction conditions and provided the desired products we examined the scope of this protocol with respect to
57 (2j, 2k, and 2l), thereby facilitating further functional- the substituents and were delighted to observe that a
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Table 2. Substrate scope of intramolecular radical cycliza-
Table 3. Substrate scope of the tandem process involving
tion/epoxidation reactions.[a]
oxidation/intramolecular radical cyclization/epoxidation.[a]
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[a] Procedure B: 3 (1.0 equiv), Ru(bpy)3Cl2·6H2O (1.0 mol%),
TBHP (8.0 equiv), and K2CO3 (3.0 equiv) in i-PrOAc
(0.1 M) at rt under N2 for 30–36 h with light irradiation
using blue LED. Yields of isolated products.
[a] 1 (0.20 mmol, 1.0 equiv), Ru(bpy)3Cl2·6H2O (1.0 mol%),
TBHP (5.0 equiv), and K2CO3 (3.0 equiv) in i-PrOAc
(0.1 M) at rt under N2 for 20–24 h with light irradiation
using a blue LED. Yields of isolated products.
43 range of substrates were tolerated in this tandem
44 process and successfully afforded the corresponding
45 products, as shown in Table 3.
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When aldehyde 1a was subjected to the standard
47 reaction conditions without adding base, b-peroxy
48 ketone 4 was obtained in 71% yield. The a-carbonyl
49 epoxide 2a was then synthesized through a base-
50 induced epoxidation of the separated b-peroxy ketone
Scheme 2. Control experiments.
51 4 (Scheme 2, eq 1).[9a,11] Next, the complete quenching ation may act as a single electron transfer (SET)
52 of reactivity in the presence of 2,2,6,6-tetramethyl- reagent. The catalytic process can be initiated by
53 piperidine-1-oxyl (TEMPO) supports the involvement oxidation of the hydroxyl group by the tert-butoxy
54 of a radical intermediate (Scheme 2, eq 2).
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56 ated tandem process is depicted in Scheme 3. The 1a is then engaged in the second catalytic cycle, and
57 photoexcited Ru(II)* species under blue LED irradi- its acyl radical species II is presumably generated by
radical that is formed by SET-induced scission of the
A plausible mechanism for the visible light medi- weak OÀO bond of TBHP.[12] The resulting aldehyde
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Scheme 3. Proposed mechanism of the present tandem catalysis
reaction mixture was diluted with ethyl acetate (25 mL x 3)
and washed with brine (40 mL). The organic layer was dried
over Na2SO4. After removal of solvent, the residue was
purified by flash chromatography on silica gel (n-hexane/
ethyl acetate=10:1) to give the desired product compound
2a (25.7 mg, 73%) as a white solid.
26 hydrogen abstraction of the aldehydic hydrogen.
27 Afterward, intramolecular cyclization of the acyl
28 radical II with the internal olefin gives rise to the
29 radical intermediate III, which undergoes radical
30 coupling with a peroxy radical as the Li’s group
31 reported.[9a] Finally, the base-assisted epoxidation of
32 the resulting b-peroxy intermediate IV yields product
33 2a.
Procedure B: Reactions were conducted in test tube sealed
with rubber septa. The mixture of alcohol 3a (32.8 mg,
0.20 mmol), Ru(bpy)3Cl2·6H2O (1.5 mg, 0.002 mmol), tert-
butyl hydroperoxide (5.5 M in decane, 0.29 mL, 1.6 mmol)
and K2CO3 (83.1 mg, 0.60 mmol) were combined in i-PrOAc
(2.0 mL) under N2 atmosphere. The mixture was placed in
the irradiation apparatus equipped with a blue LED. The
resulting mixture was stirred at room temperature. The
reaction mixture was monitored by TLC using 91% hexane
and 9% ethyl acetate as the mobile phase. After 36 h, the
reaction mixture was diluted with ethyl acetate (25 mL x 3)
and washed with brine (40 mL). The organic layer was dried
over Na2SO4. After removal of solvent, the residue was
purified by flash chromatography on silica gel (hexane/ethyl
acetate=10:1) to give the desired product compound 2a
(16.8 mg, 48%) as a white solid.
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In summary, we developed a straightforward and
35 efficient tandem reaction for the direct construction of
36 spiroepoxy chroman-4-ones with light irradiation using
37 a blue LED under mild reaction conditions. During
38 the reaction, the aldehyde generated in situ from the
39 benzyl alcohol could further undergo visible light-
40 mediated internal radical cyclization and epoxidation.
41 The present strategy allows the rapid generation of
42 valuable and synthetically useful spiroepoxy chroman-
43 4-ones and their nitrogen-containing derivatives.
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Experimental Section
47 General Procedure
Acknowledgements
This research was supported financially by Institute for Basic
Science (IBSÀR010-G1).
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Procedure A: Reactions were conducted in test tube sealed
with rubber septa. The mixture of aldehyde 2a (32.4 mg,
0.20 mmol), Ru(bpy)3Cl2·6H2O (1.5 mg, 0.002 mmol), tert-
butyl hydroperoxide (5.5 M in decane, 0.18 mL, 1.0 mmol)
and K2CO3 (82.9 mg, 0.60 mmol) were combined in i-PrOAc
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Visible Light-Promoted Synthesis of Spiroepoxy Chro-
manone Derivatives via a Tandem Oxidation/Radical
Cyclization/Epoxidation Process
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S. Jung, J. Kim, S. Hong*
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