CuI-Mediated Synthesis of Sulfonyl Benzofuran-3-ones and Chroman

Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX

pubs.acs.org/OrgLett

CuI-Mediated Synthesis of Sulfonyl Benzofuran-3-ones and Chroman-4-ones Meng-Yang Chang,*,†,‡ Yan-Shin Wu,† and Han-Yu Chen† †

Department of Medicinal and Applied Chemistry, Kaohsiung Medical University, Kaohsiung 807, Taiwan Department of Medical Research, Kaohsiung Medical University Hospital, Kaohsiung 807, Taiwan

‡

S Supporting Information *

ABSTRACT: A concise, one-pot synthesis of sulfonyl benzofuran-3-ones and chroman-4-ones from α-sulfonyl ohydroxyacetophenones is described, using a combination of K2CO3, CuI, and dimethylsulfoxide (DMSO). This annulation protocol generates one carbon−oxygen (C−O) bond and one carbon−carbon (C−C) bond in a highly efficient manner.

C

armeniaspirols A, B, and C) and bioactive molecules.7 Consequently, methods that allow convenient access to functionalized benzofuran-3-ones in a chemoselective fashion are of great importance. Some methods for the synthesis of these compounds include one-pot bimetallic (Pd/Rh) catalytic reactions,7a amine-mediated Michael-aldol tandem cyclizations,7b and N-heterocyclic carbene (NHC)-promoted (3 + 2) annulations.7c,d Functionalized chroman-4-one 6 is a key fragment in naturally occurring oxygen-containing heterocyclic compounds that possess various biological activities.8 To date, a considerable number of synthetic routes to chromanone and its analogues have been developed.9,10 Although different routes have been reported for the formation of the ring systems of sulfonyl benzofuran-3-ones 4 and chroman-4-ones 6, the development of novel methods for constructing this core structure, especially α-sulfonyl-conjugated carbonyl frameworks, remains challenging. As a continuation of our research on the synthetic applications of β-ketosulfone frameworks,11 we herein present a straightforward synthesis of α-sulfonyl benzofuran-3-ones 4 and chroman-4-ones 6. The starting materials required (1) were easily prepared from o-hydroxyacetophenones, following a literature procedure11d that included bromination of ohydroxyacetophenones, followed by nucleophilic substitution of the resulting α-bromo-o-hydroxyacetophenones with RSO2Na in dioxane and H2O (v/v = 1/1). To examine the formation of benzofuran-3-ones, our study commenced with the treatment of model substrates 1a (R = Me, Ar = Ph, 0.5 mmol) and 2a (P = Ph, 1.0 mmol) with a combination of CuI (2.0 equiv) and DMSO (3 mL) in an open vessel, as shown in Table 1.6gh However, no reaction occurred in 5 h at 25 °C (entry 1 in Table 1). In particular, after the reaction mixture

opper-mediated cyclization reactions that use prefunctionalized substrates for the rapid and efficient construction of structurally diverse benzo-fused heterocyclic frameworks have been recognized as cost-effective alternatives to classic protocols that involve palladium-mediated C−H activation systems.1 In most copper-mediated chemical bond forming reactions, however, the focus has been on the formation of only one carbon−carbon (C−C)2 or carbon− heteroatom (C−N,3 C−O,4 or C−S5 for Ullmann-type reactions) bond. In contrast, cascade reactions are highly efficient for forming multiple bonds in a one-pot procedure. Among reported tandem annulations, the key combination of copper(I) iodide and dimethyl sulfoxide (CuI/DMSO) has attracted considerable interest, because of its use as an inexpensive copper complex, its high reactivity, and its operational simplicity.6 Based on previous reports of CuI/ DMSO-mediated syntheses of sulfonyl dihydrofurans6g and a benzo-fused dioxabicyclo[3.2.1]octane core,6h we herein report a K2CO3/CuI/DMSO-mediated double benzylation of αsulfonyl o-hydroxyacetophenones with benzyl bromides, followed by a debenzylative annulation for the synthesis of sulfonyl benzofuran-3-ones. Under similar one-pot conditions, synthesis of sulfonyl chroman-4-ones was also developed via the double Michael addition of α-sulfonyl o-hydroxyacetophenones and ynones (see Scheme 1). Benzofuran-3-one 4 is an important structural unit present in a wide variety of natural products (e.g., geodin, griseofulvin, and Scheme 1. Synthetic Routes to α-Sulfonyl Benzofuran-3-ones 4 and Chroman-4-ones 6

Received: January 29, 2018

© XXXX American Chemical Society

A

DOI: 10.1021/acs.orglett.8b00316 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters Table 1. Reaction Conditionsa

Table 2. Synthesis of 4a

Yieldb (%) entry

base (equiv)

temperature (°C)

time (h)

3a

4a

1 2 3 4 5 6 7 8 9 10 11 12

c c c c c

25 80 150 80 80 80 80 80 80 80 80 80

5 5 5 15 30 15 15 15 15 15 15 15

d 30 22 18 20 15 8 d d 20 18 d

d 35 27 50 50 58 64 70 68 51 50 70

K2CO3 (1.0) K2CO3 (2.0) K2CO3 (2.2) K2CO3 (3.3) Ag2CO3 (2.2) Na2CO3 (2.2) Cs2CO3 (2.2)

a

The reactions were run on a 0.5 mmol scale with 1a, 2a (2.0 equiv), and CuI (2.0 equiv) in DMSO (3 mL). bIsolated yields. cNo base was added. dNo product was observed.

was heated to 80 °C for 5 h, two distinct products3a (30%) and 4a (35%)were isolated in a 1:1 ratio (entry 2 in Table 1). The formation of 3a under these conditions could have reasonably occurred via a CuI-mediated O-/C-benzylation, and the predicted double O- or C-benzylated products were not observed. Furthermore, methods for increasing the efficiency of the transformation of 3a to 4a needed to be investigated. Considering this, various reaction conditions were screened, as described below. Elevating the reaction temperature (80 °C → 150 °C) decreased the yields of 3a and 4a to 22% and 27%, respectively (entry 3 in Table 1). Extending the reaction time (5 h → 15 and 30 h) also impacted the transformation (entry 4 in Table 1). However, an extended reaction time (30 h) did not enhance the ratio of the products 3a and 4a yields (entry 5 in Table 1). Almost quantitative conversion of 3a to annulated product 4a was achieved by increasing the amount of K2CO3. From these experiments, we found that a base could drive the conversion of 3a to 4a to completion. Other inorganic bases such as Ag2CO3 and Na2CO3 are ineffective due to their weaker basicities. Cs2CO3 provided the same product ratio of 3a and 4a as that observed with K2CO3 (entry 8 in Table 1). Based on economic concerns, less-expensive K2CO3 was used as the preferred promoter to investigate the substrates scope of this reaction. While using K2CO3 as the base, other polar solvents (DMF, MeCN, and DMAc) were tested, and no improvements in the conversion of 3a to 4a were observed. On the other hand, replacing CuI with other metal iodides AgI, ZnI2, and BiI3 and two other copper(II) salts (Cu(OAc)2 and CuF2) did not promote the transformation, and no desired product 4a was isolated. From these observations, we concluded that the combination of CuI and DMSO in the presence of K2CO3 constitute the optimal conditions (80 °C, 15 h) for the one-pot double O-/C-benzylation and debenzylative cascade ringclosure procedure. To study the scope and limitations of this one-pot procedure, 1 and 2 were reacted with K2CO3/CuI/DMSO to afford a broad range of products 4 (see Table 2). With the optimal conditions established (Table 1, entry 8), we found that this route allowed a direct intramolecular annulation and de-O-

entry

1, Ar =, R =

2, P =

4, (%)b

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

1a, Ph, Me 1b, Ph, Ph 1c, Ph, Tol 1d, Ph, 4-FC6H4 1e, Ph, 4-MeOC6H4 1f, 4-FC6H3, Tol 1g, 5-MeOC6H3, Tol 1h, Ph, 4-EtC6H4 1i, Ph, 4-tBuC6H4 1j, naphthyl, Tol 1c, Ph, Tol 1c, Ph, Tol 1c, Ph, Tol 1c, Ph, Tol 1c, Ph, Tol 1b, Ph, Ph 1b, Ph, Ph 1b, Ph, Ph 1c, Ph, Tol 1c, Ph, Tol

2a, Ph 2a, Ph 2a, Ph 2a, Ph 2a, Ph 2a, Ph 2a, Ph 2a, Ph 2a, Ph 2a, Ph 2b, 3-ClC6H4 2c, 2-FC6H4 2d, 4-MeOC6H4 2e, 1-naphthyl 2f, 3,5-(MeO)2C6H3 2c, 2-FC6H4 2d, 4-MeOC6H4 2b, 3-ClC6H4 2g, allyl 2h, n-C3H7

4a, 70 4b, 74 4c, 79 4d, 74 4e, 76 4f, 80 4g, 73 4h, 74 4i, 73 4j, c 4k, 73 4l, 78 4m, 76 4n, 73 4o, 70 4p, 73d 4q, 74 4r, 72 4s, e 4t, f

a The reactions were run on a 0.5 mmol scale with 1a−1j, 2a−2h (2.0 equiv), K2CO3 (2.2 equiv), and CuI (2.0 equiv) in DMSO (3 mL) for 15 h. bIsolated yields. cComplex mixture. d3p (6%) was isolated. e3s (66%) was isolated. f3t (40%) was isolated.

benzylative O → C (from phenoxide to α-carbon) bond formation under operationally simple open-vessel conditions in moderate to good yields (70%−80%). As shown in Table 2, entries 1−9 and 11−18, efficient formation of 4a−4i and 4k− 4r indicated that varied substituents (for 1a−1j, Ar and R; for 2a−2f, P) did not affect the yield of the desired product. A phenyl group, an electron-withdrawing 4-fluorophenyl group and an electron-donating 5-methoxyphenyl group were suitable aryl substituents (Ar) in the starting α-sulfonyl o-hydroxyacetophenones 1. However, when a naphthyl (for Ar) group was used under these reaction conditions, a complex mixture of products was isolated (entry 10 in Table 2). In addition, entry 16 in Table 2 shows the formation of trace amounts of 3p (6%). For sulfonyl substituents (R) on 1, both aliphatic (Me) and aromatic (Ph, Tol, 4-FC6H4, 4-MeOC6H4, 4-EtC6H4, and 4-tBuC6H4) groups were well-tolerated. Different aryl groups (P) in the benzyl bromides 2 were also suitable for the formation of 4; however, when P was an allyl or n-propyl group, it was found that 3s (66%) and 3t (40%) were generated instead of the desired 4s and 4t, respectively. The results show that nonaryl groups were less reactive than aryl groups. Furthermore, the structures of the products 3p, 4c, 4i, and 4m were determined by single-crystal X-ray crystallography. Based on the experimental results, a plausible reaction pathway for the formation of 4a was proposed, as illustrated in Scheme 2. Initially, CuI-mediated double cupration of 1a yielded A1 and HI in the absence of K2CO3 via O−Cu bond formation.12 Compound 3a could be generated by intermolecular double O-benzylation of A1 with 2 equiv of benzyl bromide (2a), along with the 2 equiv of CuBr formed in situ. We found that the reaction should readily equilibrate to the general reaction “HI + CuBr ⇆ HBr + CuI”.13 Under B

DOI: 10.1021/acs.orglett.8b00316 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters Table 3. Synthesis of 6a

Scheme 2. Plausible Mechanism

Sandmeyer reaction conditions, B, which involves a Cu(I)chelate, was then generated. Compound 3a was also produced in the presence of K2CO3 via a different pathway. K2CO3 deprotonates 1a to produce A2, followed by the double benzylation of A2 with 2a. B was then obtained from the CuI complexation of 3a. We envisioned that the addition of K2CO3 could increase the reaction efficiency and the yield of 3a. Subsequently, DMSO promoted the de-O-benzylation of B to provide C, which has a Cu-chelated phenoxide skeleton, via C− O bond cleavage and Cu−O bond formation.14a The Karlin group has reported a similar Cu complex-mediated debenzylation.14b Furthermore, the released halide group (bromide or iodide ion) could trap the benzylic proton of the in situgenerated sulfonium cation to produce HX, Me2S, and benzaldehyde as the byproducts. Because the byproducts of this reaction are volatile, only 3,5-dimethoxybenzaldehyde (from 2f) was isolated (66%) to confirm the reaction pathway. Finally, the efficient construction of benzofuran-3-one 4a was completed via the intramolecular annulation of D via the enolate, followed by the removal of Cu. To explore the K2CO3/CuI/DMSO-mediated reaction, a one-pot synthesis of chroman-4-ones 6 by a double Michael addition cascade15 of α-sulfonyl o-hydroxyacetophenones 1 with ynones 5 was explored, as shown in Table 3. With the above-mentioned conditions established, we found that the reaction directly afforded the six-membered ring and provided a broad range of chroman-4-ones 6 in moderate yields (65%− 86%). Diastereomeric ratios in the range of 3:2−8:1 were determined for 6a−6x from their NMR spectra. In addition, the two adjacent stereochemical centers of the major isomer were in trans-configuration due to the steric hindrance between the side chain (of C2) and the sulfonyl substituent (of C3). The efficient formation of 6a−6x showed that the substituents (R, P′, and Ar) did not affect the yields of the desired products (entries 1−24). The structure and relative stereochemistry of the products 6a, 6b, 6f, 6j, and 6k were determined by singlecrystal X-ray crystallography. As illustrated in Table 3, in the K2CO3/CuI-mediated deprotonation of 1 in DMSO, E1 (by Calkynylation) and E2 (by O-alkynylation) should be formed via the first intermolecular addition of 5 to the in-situ-generated A1 or A2. Then, 6 would be formed via the second intramolecular addition in E1 (by O-alkenylation) or E2 (by C-alkenylation). In summary, we have developed a K2CO3/CuI/DMSOmediated synthesis of sulfonyl benzo-fused oxygen-containing bicycles, benzofuran-3-ones 4, and chroman-4-ones 6, in moderate to good yields. The substrate scope and limitations of this facile and efficient transformation were investigated. The

entry

1, Ar =, R =

5, P′ =

6, (%),b ratio

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

1a, Ph, Me 1b, Ph, Ph 1c, Ph, Tol 1d, Ph, 4-FC6H4 1e, Ph, 4-MeOC6H4 1f, 4-FC6H3, Tol 1g, 5-MeOC6H3, Tol 1h, Ph, 4-EtC6H4 1i, Ph, 4-tBuC6H4 1j, naphthyl, Tol 1k, Ph, 3-MeC6H4 1l, Ph, 4-nBuC6H4 1m, Ph, 4-iPrC6H4 1n, 4-BrC6H3, Tol 1o, Ph, nBu 1p, 4-ClC6H3, Tol 1c, Ph, Tol 1c, Ph, Tol 1c, Ph, Tol 1c, Ph, Tol 1c, Ph, Tol 1c, Ph, Tol 1c, Ph, Tol 1c, Ph, Tol

5a, Me 5a, Me 5a, Me 5a, Me 5a, Me 5a, Me 5a, Me 5a, Me 5a, Me 5a, Me 5a, Me 5a, Me 5a, Me 5a, Me 5a, Me 5a, Me 5b, 3,4-CH2O2C6H3 5c, 2-naphthyl 5d, 1-naphthyl 5e, 4-MeOC6H4 5f, 3,4-(MeO)2C6H3 5g, 3,4,5-(MeO)3C6H2 5h, Ph 5i, 4-PhC6H4

6a, 76 (2:1) 6b, 86 (3:1) 6c, 83 (4:1) 6d, 74 (4:1) 6e, 78 (3:1) 6f, 82 (3:1) 6g, 81 (8:1) 6h, 72 (3:1) 6i, 78 (3:1) 6j, 65 (7:3) 6k, 72 (3:2) 6l, 76 (2:1) 6m, 82 (3:1) 6n, 72 (3:1) 6o, 70 (3:2) 6p, 76 (3:1) 6q, 76 (3:1) 6r, 73 (3:1) 6s, 70 (3:1) 6t, 80 (5:3) 6u, 77 (3:1) 6v, 70 (2:1) 6w, 72 (2:1) 6x, 74 (2:1)

a

The reactions were run on a 0.5 mmol scale with 1a−1p, 5a−5i (1.0 equiv), K2CO3 (2.2 equiv), and CuI (2.0 equiv) in DMSO (3 mL) over 15 h. bIsolated yields.

structures of the key products were confirmed by X-ray crystallography. Further investigations regarding the synthetic applications of β-ketosulfones is currently underway.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b00316. Detailed experimental procedures and spectroscopic data for all new compounds and X-ray analysis data of 3p, 4c, 4i, 4m, 6a, 6b, 6f, 6j, and 6k (PDF) Accession Codes

CCDC 1574346−1574349 and 1574355−1574359 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/ data_request/cif, or by emailing [email protected]. uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, U.K.; fax: +44 1223 336033. C

DOI: 10.1021/acs.orglett.8b00316 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Meng-Yang Chang: 0000-0002-1983-8570 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank the Ministry of Science and Technology of the Republic of China for financial support (No. MOST 106-2628-M-037-001-MY3).

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REFERENCES

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DOI: 10.1021/acs.orglett.8b00316 Org. Lett. XXXX, XXX, XXX−XXX