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Cite This: Org. Lett. 2019, 21, 330−334

Organocatalyzed Intermolecular Asymmetric Allylic Dearomatization of Both α- and β‑Naphthols Binmiao Yang, Xuejie Zhai, Shubo Feng, Dongyan Hu, Yuhua Deng, and Zhihui Shao* Key Laboratory of Medicinal Chemistry for Natural Resource, Ministry of Education, School of Chemical Science and Technology, Yunnan University, Kunming 650091, China

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S Supporting Information *

ABSTRACT: The first highly stereoselective intermolecular catalytic asymmetric dearomatization (CADA) of α-naphthols through C−C formation and the first asymmetric allylic dearomatization of naphthols by chiral organocatalysis have been achieved. These new and complete atom-economic reactions provide enantioriched α- and β-naphthalenones bearing an all-carbon quaternary center. Scheme 1. Intermolecular CADA Reactions of α-Naphthols

P

henols are cheap, abundant, and readily available chemical feedstocks as well as widely used as important starting materials in chemical synthesis. Catalytic asymmetric dearomatization (CADA) reactions of phenols are of wide interest because of their potential for the synthesis of functionalized chiral cyclic enones bearing a quaternary stereogenic center, which appears frequently as basic skeletons in biologically active natural products and pharmaceuticals.1 Recently, great efforts have been devoted to the development of reactions involving naphthols. As a result, a series of intermolecular CADA reactions have been elegantly developed.2,3 However, almost all of those reactions have been focused on β-naphthols, thus leading to the formation of chiral β-naphthalenones. In sharp contrast, the intermolecular CADA reactions employing α-naphthols remain elusive. Such reactions could potentially furnish chiral α-naphthalenones bearing an all-carbon quaternary center, which are important structural motifs prevalent in various biologically active natural products and therapeutic reagents.4 However, unlike β-naphthols, the use of α-naphthols in intermolecular CADA reactions has proved to be more challenging. Indeed, so far, only few scattered examples have been reported, and they have been limited to the formation of C−X bonds with use of heteroatom-based electrophiles (Scheme 1a).5a,b However, to our knowledge, a general and highly enantioselective intermolecular CADA of α-naphthols through C−C formation has not been realized but would enable the construction of a chiral all-carbon quaternary center, which is one of the most challenging research areas in modern organic synthesis. In our continued interest in developing new catalytic asymmetric reactions,6 herein, we report a catalytic method that enables the first asymmetric intermolecular C−C bondforming dearomatization of α-naphthols (Scheme 1b). This method is also applicable to β-naphthols. This is the first successful example of catalytic asymmetric intermolecular C− C bond-forming dearomatization applicable to both α- and βnaphthols. The process proceeds through allylic substitution © 2018 American Chemical Society

Received: December 10, 2018 Published: December 26, 2018 330

DOI: 10.1021/acs.orglett.8b03934 Org. Lett. 2019, 21, 330−334

Letter

Organic Letters Table 1. Substrate Scope of α-Naphtholsa

reaction and provides enantioriched α- and β-naphthalenones bearing an all-carbon quaternary center (Scheme 1c) with high chemo- and enantioselectivity. The allylic substitution is one of the most powerful tools for enantioselective C−C bond formation.7 A variety of catalytic enantioselective allylic substitution reactions have been creatively developed in the past decades. One of the significant challenges in this area is the utilization of naphthols as nucleophilic partners for the intermolecular allylic substitution dearomatization due to the competitive O-allylation and Friedel−Crafts-type allylic alkylation reaction pathways. In 2013, You and co-workers pioneered a Pd-catalyzed intermolecular asymmetric allylic dearomatization of β-naphthols with allyl carbonates.2c Very recently, the groups of You2l and Zhong2m independently reported impressive Ir-catalyzed asymmetric allylic dearomatization of β-naphthols with allylic alcohols. However, α-naphthols have remained elusive substrates for this transformation so far. Moreover, a complementary chiral organocatalyzed variant has not been realized. Inspired by these challenges and in combination with our interest in developing catalytic asymmetric allylic substitution to construct chiral cyclic enones bearing an allcarbon quaternary stereocenter8 and in exploring the chemistry of N,O-acetals,9 and our efforts toward the development of intermolecular CADA of α-naphthols,10 we began our studies. After extensive efforts, we found for the first time that chiral phosphoric acids11 can catalyze the unreported asymmetric allylic substitution dearomatization reactions of both α- and βnaphthols with allylic N,O-acetals as allylating electrophiles generated in situ from allenamides.12,13 We initiated our study with the α-naphthol 1a and the allenamide 2 as the model substrates to optimize the reaction conditions (Scheme 2). When the reaction was performed in

entry

1, R1, R2, R3, R4

product 3

yield (%)b

e.r.c

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

1a, allyl, H, H, H 1b, 2-methylallyl, H, H, H 1c, (CH2)2Ph, H, H, H 1d, nPr, H, H, H 1e, Me, H, H, H 1f, allyl, H, Me, H 1g, allyl, H, OMe, H 1h, Me, H, OMe, H 1i, allyl, H, C6H5, H 1j, allyl, H, 2-OMeC6H4, H 1k, allyl, H, 3-MeC6H4, H 1l, allyl, H, 3-OMeC6H4, H 1m, allyl, H, 3-FC6H4, H 1n, allyl, H, 4-OMeC6H4, H 1o, allyl, H, 4-ClC6H4, H 1p, allyl, Me, OMe, H 1q, allyl, H, H, 6-OMe 1r, allyl, H, H, 7-OMe

3a 3b 3c 3d 3e 3f 3g 3h 3i 3j 3k 3l 3m 3n 3o 3p 3q 3r

95 90 94 94 87 93 94 90 97 94 91 94 95 95 95 97 96 95

96:4 94.5:5.5 94.5:5.5 92:8 90:10 96:4 95:5 97:3 95:5 93:7 96:4 96:4 96.5:3.5 95.5:4.5 96:4 92.5:7.5 96:4 92:8

a

Reaction conditions: 1 (0.1 mmol), 2 (0.15 mmol), A2 (10 mol %), 5 Å MS (50 mg), PhMe (0.25 mL), −20 °C, 4 h. bIsolated yield. c Determined by chiral HPLC.

underwent the desired reaction smoothly, delivering the corresponding α-naphthalenone products in perfect chemoselectivity and high yields with good enantioselectivity (87− 95% yield, 90:10−96:4 e.r., entries 1−5). 2-Allyl-1-naphthol derivatives bearing 4-Me, 4-MeO, and 4-aryl groups led to the corresponding dearomatized products in up to 97% yield and 97:3 e.r. (entries 6−15). The asymmetric allylic dearomatization also occurred with trisubstituted α-naphthol 1p with excellent yield and good enantioselectivity (entry 16). Lastly, the substituent could be tolerated at other positions to furnish the dearomatization products 3q−r. The absolute configuration of 3h was determined by X-ray crystal analysis. Next, we examined the compatibility of our reaction protocol with β-naphthols. So far, there has been no successful example of catalytic asymmetric intermolecular C−C bondforming dearomatization applicable to both α- and βnaphthols. We established that a variety of β-naphthols can react with allenamide 2 smoothly to provide the desired chiral β-naphthalenones (5a−i) containing an all-carbon quaternary center in high yields (up to 98% yield) with good chemo- and enantioselectivity (up to 97:3 e.r.) (Scheme 3). The absolute configuration of 5a was determined by X-ray crystal analysis. To further evaluate the utility of our protocol, we performed a gram-scale reaction and derivatizations (Scheme 4). Pleasingly, 3a was obtained with the almost same yield and enantioselectivity at 2 mmol scale. The catalytic hydrogenation of 3a afforded saturated protected amine 6 in 98% yield. Interestingly, olefin-regioselective hydrogenation of 3g could be accomplished, providing compound 7 in 90% yield. Diastereoselective reduction of the ketone group of 3a with NaBH4 gave alcohol 8 in 98% yield with 10:1 d.r. Finally, treatment of 5a with 1 N HCl provided aldehyde 9 in 80% yield.

Scheme 2. Catalysts and Model Reaction Employed for the Reaction Condition Optimization

the presence of chiral phosphoric acid A1 as the catalyst, 4 Å molecular sieve (MS) as the additive in toluene at −5 °C for 4 h, the α-naphthalenone 3a was obtained in 70% yield with 80:20 e.r. After screening a series of reaction conditions to improve the chemical yield and enantioselectivity (for details, see the Supporting Information), we ultimately determined that the designed reaction could afford 3a in 95% yield with 96:4 e.r. in the presence of A2 catalyst and 5 Å MS at −20 °C. With the optimized conditions in hand, we investigated the substrate scope of α-naphthols (Table 1). α-Naphthols bearing different allyl, alkyl, and methyl groups at the 2-position 331

DOI: 10.1021/acs.orglett.8b03934 Org. Lett. 2019, 21, 330−334

Letter

Organic Letters Scheme 3. Substrate Scope of β-Naphthol Reaction Conditions: 4 (0.1 mmol), 2 (0.15 mmol), B1 (10 mol %), 4 Å MS (50 mg), PhMe (0.25 mL), −5 °C, 4 ha

Scheme 5. Proposed Reaction Mechanism

In summary, we have successfully developed the first catalytic asymmetric intermolecular C−C bond-forming dearomatization of α-naphthols. This intermolecular asymmetric allylic dearomatization reaction provided a new and complete atom-economic method for the synthesis of two important structural classes, chiral α- and β-naphthalenones bearing an all-carbon quaternary center. In contrast to previous intermolecular catalytic asymmetric allylic dearomatization of naphthols, our study not only obviates the need for transitionmetal catalysts but also expands the scope of both reaction partners. Our study demonstrates the synthetic potential of αnaphthols in catalytic asymmetric intermolecular C−C bondforming dearomatization.

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a

Yield corresponds to that of isolated products. The e.r. value was determined by chiral HPLC analysis.

ASSOCIATED CONTENT

* Supporting Information S

Scheme 4. Gram-Scale Reaction and Derivatizations

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b03934. Experimental details, characterization data for all of the new compounds, and copies of NMR and HPLC spectra (PDF) Accession Codes

CCDC 1867075 and 1867086 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], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Zhihui Shao: 0000-0003-4531-3417 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by NSFC (21672184, 21861042), the Program for Changjiang Scholars and Innovative Research Team in University (IRT13095), the Program for Innovative Research Team (in Science and Technology) in University of Yunnan Province, Yunling Scholar of Yunnan Province, Yunnan Province Government (2018FY001(016)), the National Postdoctoral Program for Innovative Talents (BX201600129), and China Postdoctoral Science Foundation funded project (2016M602726).

To gain insight into the reaction mechanism, we performed HRMS analysis and NMR analysis (for details, see the Supporting Information). Based on these experimental results and previous related work,13 a possible reaction mechanism involving a concerted asynchronous SN2′-like displacement is proposed in Scheme 5. However, we could not completely rule out possible participation of an α,β-unsaturated iminium ion formed upon protonation by the allenamide. 332

DOI: 10.1021/acs.orglett.8b03934 Org. Lett. 2019, 21, 330−334

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Organic Letters

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