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DOI: 10.1039/D2RA03031D (Paper) RSC Adv., 2022, 12, 16684-16687

Diastereoselective synthesis of chroman bearing spirobenzofuranone scaffolds via oxa-Michael/1,6-conjugated addition of para-quinone methides with benzofuranone-type olefins†

Hongmei Qina, Qimei Xieb and Long He*ab
aCollege of Chemistry and Materials Engineering, Guiyang University, Guiyang, 550005, P. R. China. E-mail: longhe@cwnu.edu.cn
bCollege of Chemistry and Chemical Engineering, China West Normal University, Nanchong, 637002, P. R. China

Received 13th May 2022 , Accepted 29th May 2022

First published on 6th June 2022

Abstract

A simple and convenient cyclization of ortho-hydroxyphenyl-substituted para-quinone methides with benzofuran-2-one type active olefins via oxa-Michael/1,6-conjugated addition has been developed, which afforded an easy access to enriched functionalized chroman-spirobenzofuran-2-one scaffolds with good to excellent yields (up to 90%) and diastereoselectivities (up to >19[thin space (1/6-em)]:[thin space (1/6-em)]1 dr). This reaction provided an efficient method for constructing desired spirocyclic compounds combining both well-known heterocyclic pharmacophores chroman and benzofuran-2-one.

The chroman framework represents a privileged heterocyclic core commonly found within a wide variety of biologically active natural products1 and synthetic compounds of medicinal interest (Fig. 1).2 Owing to the wide application of these heterocyclic molecules, over the past few decades, numerous efforts have been devoted to the efficient synthesis of chroman nucleus motifs.3,4 In particular, incorporating chroman into spiro-bridged and spiro-fused heterocyclic systems is appealing due to its fascinating molecular architecture and proven biological activity.5 Among the existing methods, the [4 + m] cycloaddition of para-quinone methides (p-QMs) is found to be an efficient pathway to access these valuable spirocyclic skeletons.6 For instance, the Enders group synthesized functionalized chromans with an oxindole motif by the asymmetric organocatalytic domino oxa-Michael/1,6-addition reaction.7 After that, the Hao,8a Peng,8b Shi,8c,d Zhou,8e Liang8f and Wang8g groups developed convenient methods to construct chromans bearing spirocyclic skeletons from p-QMs, respectively. Despite all these shining achievements, however, it is still very challenging to simply and conveniently construct chromans bearing quaternary carbon spirals for organic chemical or drug discovery among these [4 + m] cycloaddition reactions.
image file: d2ra03031d-f1.tif
Fig. 1 Representative chroman compounds.

Benzofuran-2-(3H)-ones as one of the important oxygen-containing heterocycles that exist in a broad array of natural products9 and potential medicines.10 The streamlined synthesis of benzofuran-2-ones pose considerable challenge due to their quaternary carbon centers at the C-3 position,11 especially those featuring relatively congested spirocyclic motifs represent challenging synthetic targets.12,13 In our continuous interests in developing efficient method for the synthesis of spirocyclic compounds based on cyclization reaction,14 we wish to report a cycloaddition of para-quinone methides with benzofuranone derived olefins, affording the spiro-cycloadducts in good to excellent yields and diastereoselectivities. This cyclization features the simultaneous formation of chroman and spirobenzofuran-2-one skeletons in a single step (Scheme 1), which may be potentially applied as pharmaceutical agents.


image file: d2ra03031d-s1.tif
Scheme 1 Strategy for the synthesis of chroman-spirobenzofuran-2-one.

We initiated our investigations with the readily available ortho-hydroxyphenyl-substituted para-quinone methides 1a and 3-benzylidenebenzofuran-2-one 2a in toluene at room temperature in the presence of base. Unfortunately, no desired chroman derivatives bearing spirobenzofuranone scaffolds 3a was isolated in the presence of 2 eq. Na2CO3 after stirring at room temperature for 48 h (entry 1, Table 1). A base survey showed that K2CO3 and CsF led to desired cycloaddcuts 3a even if with a disappointing yield (entries 2–4, Table 1). Gratifyingly, Cs2CO3 furnished the desired product in 70% yield and with a generally acceptable 6[thin space (1/6-em)]:[thin space (1/6-em)]1 dr value (entry 5, Table 1). A solvent screening indicated that the yield and diastereoselectivity are both dependent on the solvent (Table 1). Thus, the oxygenated solvents such as THF, diethyl ether and 1,4-dioxane gave comparably high yields and diastereoselectivities than polar solvents as exemplified by CH3CN or DMF (entries 5–10). To our delight, performing the reaction in THF led to desired chroman-spirobenzofuranone 3a with an excellent diastereoselectivity, albeit with a very subtle erosion of the yield (entry 6). Further optimization of the reaction conditions by varying the temperature was also investigated. When higher temperature was used, no improvement in the final yield was observed (entry 11). Performing the reaction at lower 10 °C resulted in a increase of the yield accompanied with no erosion in diastereoselectivity (entry 12). However, the yield decreased to 64% when the reaction performed at lower 0 °C (entry 13).

Table 1 Optimization of conditionsa

image file: d2ra03031d-u1.tif
Entry Base Solvent Yieldb (%) Drc
a Reaction conditions: p-QMs 1a (0.1 mmol), benzofuranones 2a (0.12 mmol) and base (0.2 mmol) in 2 mL of solvent for 6–48 h.b Isolated yields.c Determined by crude 1H NMR analysis.d Performed at 30 °C.e Performed at 10 °C.f Performed at 0 °C.
1 Na2CO3 Toluene — —
2 DMAP Toluene — —
3 K2CO3 Toluene 25 3[thin space (1/6-em)]:[thin space (1/6-em)]1
4 CsF Toluene 52 5[thin space (1/6-em)]:[thin space (1/6-em)]1
5 Cs2CO3 Toluene 70 6[thin space (1/6-em)]:[thin space (1/6-em)]1
6 Cs2CO3 THF 68 >19[thin space (1/6-em)]:[thin space (1/6-em)]1
7 Cs2CO3 Et2O 60 8[thin space (1/6-em)]:[thin space (1/6-em)]1
8 Cs2CO3 Dioxane 53 7[thin space (1/6-em)]:[thin space (1/6-em)]1
9 Cs2CO3 CH3CN 50 >19[thin space (1/6-em)]:[thin space (1/6-em)]1
10 Cs2CO3 DMF 31 15[thin space (1/6-em)]:[thin space (1/6-em)]1
11d Cs2CO3 THF 65 >19[thin space (1/6-em)]:[thin space (1/6-em)]1
12e Cs2CO3 THF 75 >19[thin space (1/6-em)]:[thin space (1/6-em)]1
13f Cs2CO3 THF 64 >19[thin space (1/6-em)]:[thin space (1/6-em)]1

With the optimized reaction conditions in hand, we explored the substrate scope of this cyclization reaction with a selection of benzofuranones. The results are shown in Table 2. To our pleasure, a wide range of benzofuran-2-ones derived olefins were compatible affording the corresponding chromans-spirobenzofuranone scaffolds 3 in good results. In detail, the steric hindrance of substituents had a significant impact on the cyclization reaction. The substrates with substituents on para- or meta-position on phenyl ring were tolerable affording the desired cycloadducts in good yields regardless of the electronic nature of substituents (entries 5–14). However, good diastereoselectivities were also observed in ortho-substituted substrates (entries 2–4). Furthermore, 2-naphthyl derived substrate gave also good yield and sole diastereoselectivity (entry 15). Multi-substituted substrate was also able to participate in this cyclization, for example, 3,4,5-trimethoxylphenyl substituted benzofuranone delivered cycloadduct 3p in 88% yield and with 12[thin space (1/6-em)]:[thin space (1/6-em)]1 dr value (entry 16). Interestingly, the extension of the reaction conditions to heteroaromatic substrates including 3-pyridyl and 3-thiophenyl benzofuranones were proceeded smoothly, giving rise to cyclization products 3q and 3r in 79% and 80% yield, respectively (entries 17–18).

Table 2 Substrate scope of diastereoselective synthesis of chroman-spirobenzofuran-2-onea

image file: d2ra03031d-u2.tif
Entry R 3 Yieldb (%) Drc
a Reaction conditions: p-QMs 1a (0.1 mmol), benzofuranones 2 (0.12 mmol) and Cs2CO3 (0.2 mmol) in 2 mL of THF.b Isolated yields for 4–48 h.c Determined by crude 1H NMR analysis.
1 C6H5 3a 75 >19[thin space (1/6-em)]:[thin space (1/6-em)]1
2 2-ClC6H4 3b 69 >19[thin space (1/6-em)]:[thin space (1/6-em)]1
3 2-BrC6H4 3c 67 15[thin space (1/6-em)]:[thin space (1/6-em)]1
4 2-CH3C6H4 3d 64 >19[thin space (1/6-em)]:[thin space (1/6-em)]1
5 3-CH3C6H4 3e 75 >19[thin space (1/6-em)]:[thin space (1/6-em)]1
6 3-MeOC6H4 3f 72 >19[thin space (1/6-em)]:[thin space (1/6-em)]1
7 3-NO2C6H4 3g 71 >19[thin space (1/6-em)]:[thin space (1/6-em)]1
8 3-BrC6H4 3h 75 >19[thin space (1/6-em)]:[thin space (1/6-em)]1
9 4-CH3C6H4 3i 73 10[thin space (1/6-em)]:[thin space (1/6-em)]1
10 4-MeOC6H4 3j 86 >19[thin space (1/6-em)]:[thin space (1/6-em)]1
11 4-CF3C6H4 3k 82 12[thin space (1/6-em)]:[thin space (1/6-em)]1
12 4-NO2C6H4 3l 85 12[thin space (1/6-em)]:[thin space (1/6-em)]1
13 4-ClC6H4 3m 83 >19[thin space (1/6-em)]:[thin space (1/6-em)]1
14 4-BrC6H4 3n 81 12[thin space (1/6-em)]:[thin space (1/6-em)]1
15 2-Naphthyl 3o 74 >19[thin space (1/6-em)]:[thin space (1/6-em)]1
16 3,4,5-(OMe)3C6H2 3p 88 12[thin space (1/6-em)]:[thin space (1/6-em)]1
17 3-Pyridinyl 3q 79 >19[thin space (1/6-em)]:[thin space (1/6-em)]1
18 3-Thiophenyl 3r 80 >19[thin space (1/6-em)]:[thin space (1/6-em)]1

Subsequently, the generality of this cyclization reaction was further evaluated through varying p-QMs 1. As shown in Table 3. It turned out that various p-QMs 1 can be employed to the reaction, which delivered functionalized chroman-spiro-benzofuran-2-ones scaffolds 4 in high yields (up to 90%) and with good diastereoselectivities. It seems that the position of the substituents had some delicate influence on the reaction. The C5-methoxyl- or methyl-substituted substrates p-QMs 1 generated the products with a higher yield than those of the C4- substituted counterparts (entries 3–6). Moreover, the C5-chloro- and bromo-substituted p-QMs 1 afforded the products 4a and 4b in moderate yield (entries 1–2).

Table 3 Substrate scope of p-QMsa

image file: d2ra03031d-u3.tif
Entry R 4 Yieldb (%) Drc
a Reaction conditions: p-QMs 1 (0.1 mmol), benzofuranones 2a (0.12 mmol) and Cs2CO3 (0.2 mmol) in 2 mL of THF for 6–48 h.b Isolated yields.c Determined by crude 1H NMR analysis.
1 5-Cl 4a 73 >19[thin space (1/6-em)]:[thin space (1/6-em)]1
2 5-Br 4b 65 >19[thin space (1/6-em)]:[thin space (1/6-em)]1
3 5-CH3 4c 90 >19[thin space (1/6-em)]:[thin space (1/6-em)]1
4 5-OCH3 4d 80 10[thin space (1/6-em)]:[thin space (1/6-em)]1
5 4-CH3 4e 72 >19[thin space (1/6-em)]:[thin space (1/6-em)]1
6 4-OCH3 4f 66 10[thin space (1/6-em)]:[thin space (1/6-em)]1

The structure and relative configuration of 3a were determined by HRMS, NMR spectroscopy and single-crystal X-ray analysis.15 The relative configuration of other cycloadducts were tentatively assigned by analogy (Fig. 2 and see ESI†).


image file: d2ra03031d-f2.tif
Fig. 2 X-ray crystal structure of 3a.

Conclusions

In conclusion, we described a cyclization reaction of ortho-hydroxyphenyl-substituted para-quinone methides with benzofuran-2-one derived olefins via oxa-Michael/1,6-conjugated addition, which efficiently constructed enriched functionalized spirocyclic compounds combining both well-known heterocyclic pharmacophores chroman and benzofuran-2-one in good to excellent yields (up to 90%) and diastereoselectivities (up to 19[thin space (1/6-em)]:[thin space (1/6-em)]1 dr).

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We are grateful to the National Natural Science Foundation of China (21772158), Scientific Research Funds of Guiyang University [GYU-KY-2022] and Guizhou Education Department Youth Science and Technology Talents Growth Project KY[2019]094.

Notes and references

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  15. The relative configuration of 3a was confirmed by X-ray crystallography experiments: CCDC 2128983 contains the supplementary crystallographic data for this paper.†.

Footnote

† Electronic supplementary information (ESI) available. CCDC 2128983. For ESI and crystallographic data in CIF or other electronic format see https://doi.org/10.1039/d2ra03031d
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