Lewis Base-Catalyzed [4 + 3] Annulation of ortho-Quinone Methides

Letter pubs.acs.org/OrgLett

Cite This: Org. Lett. 2019, 21, 465−468

Lewis Base-Catalyzed [4 + 3] Annulation of ortho-Quinone Methides and MBH Carbonates: Synthesis of Functionalized Benzo[b]oxepines Bearing Oxindole Scaffolds Ji-Yuan Du,* Yan-Hua Ma, Fan-Xiao Meng, Rui-Rui Zhang, Ruo-Nan Wang, Hong-Liang Shi, Qi Wang, Ya-Xin Fan, Hong-Li Huang, Ji-Chun Cui, and Chun-Lin Ma College of Chemistry and Chemical Engineering, Liaocheng University, Liaocheng, Shandong 252059, China

Org. Lett. 2019.21:465-468. Downloaded from pubs.acs.org by UNIV DE BARCELONA on 01/18/19. For personal use only.

S Supporting Information *

ABSTRACT: A novel Lewis base-catalyzed [4 + 3] annulation process for the construction of benzo[b]oxepine scaffolds has been developed. 1,4-Diazabicyclo[2.2.2]octane (DABCO) promotes the union of o-QMs and Morita−Baylis−Hillman carbonates in reasonable to excellent yields and good stereoselectivities (dr > 20:1). This straightforward, catalytic approach offers access to a variety of synthetically useful benzo[b]oxepine derivatives bearing oxindole scaffolds containing all-carbon spiro-quaternary stereocenters.

T

Scheme 1. Previous Methods and Our Design for the Synthesis of Functionalized Benzo[b]oxepine Skeletons

he benzo[b]oxepine scaffolds represent attractive synthetic targets, which are often found in the structures of bioactive natural products and pharmaceuticals (Figure 1).1

Figure 1. Representative examples of natural products containing the benzo[b]oxepine scaffolds.

In particular, some functionalized benzo[b]oxepine derivatives have displayed a wide range of biological activities such as antifungal activity, antibiotic activity, and allelopathic activity, etc.2 Because of their biological properties, as well as potential synthetic value, this class of compounds has attracted growing interest from synthetic chemists. In this regard, as shown in Scheme 1, the current synthetic strategies primarily involve (a) multistep, traditional intramolecular Mitsunobu cyclization and Friedel−Crafts cyclization of phenol precursors,3 (b) ring closing metathesis (RCM) reactions of the corresponding dienes,4 (c) transition metal (Pd, Au, Ru, Rh, and Fe) catalyzed cyclization or annulation reactions using alkynes, alkenes, and allenes as reaction partners,5 (d) [3 + 2] photocycloaddition of oxidopyryliums,6 and (e) iodine(III) or Lewis acid promoted ring expansion and rearrangement reactions.7 Despite this progress, the construction of highly functionalized benzo[b]oxepine structures with all-carbon quaternary stereocenters still remains © 2019 American Chemical Society

challenging. Within this context, the development of novel, efficient methodologies for the assembly of such scaffolds is highly desirable. o-Quinone methides (o-QMs) are polarized and highly reactive species that have increasingly been investigated and applied in organic synthesis,8 chemical biology,9 and material chemistry.10 Recently, utilizing the highly electrophilic o-QMs, Received: November 20, 2018 Published: January 8, 2019 465

DOI: 10.1021/acs.orglett.8b03709 Org. Lett. 2019, 21, 465−468

Letter

Organic Letters Table 1. Optimization of the Reaction Conditionsa

quite a few reactions, including 1,4-nucleophilic addition,11 [4 + 1] cycloaddition,12 and [4 + 2] cycloaddition reactions,13 have been developed to prepare a variety of diarylmethanes, chromanes, dihydrobenzofurans, and their derivatives. On the contrary, [4 + 3] cycloadditions have been particularly scarce,14 and to date there have been five groups who have reported the cycloaddition of o-QMs with three types of synthons. For instance, the groups of Ye and Scheidt have independently developed N-heterocyclic carbene catalyzed [4 + 3] annulation of o-QMs and enals.14a,b The group of Shi reported the Brønsted acid-catalyzed [4 + 3] cycloadditions of o-QMs with N,N′-cyclic azomethine imines to construct seven-membered heterocyclic scaffolds.14c Just recently, Lautens and co-workers disclosed a [4 + 3] cycloaddition between o-QMs and isomünchnones to yield oxa-bridged oxazocinecores.14e As part of our ongoing interest in the development of new tandem reactions based on o-QMs,15 as well as the fact that Morita−Baylis−Hillman (MBH) carbonates are highly reactive three-carbon synthons especially in tandem cyclization reactions under the catalysis of Lewis basic tertiary amines or phosphines,14f,16 we envisioned that benzo[b]oxepine frameworks featuring an oxindole scaffold could be constructed through [4 + 3] annulation between o-QMs and MBH carbonates catalyzed by tertiary amine catalysts (Scheme 1). This approach has the potential to assemble various functionalized benzo[b]oxepines from simple and readily available precursors in a single operation. To explore the feasibility of the [4 + 3] annulation, we employed stable o-QM 1a and MBH carbonate 2a as the model substrates to optimize this reaction under Lewis basic (50 mol %) conditions. Initially, the commonly used Lewis base triphenylphosphine and (±)-BINAP were employed as catalysts for this transformation in THF at 25 °C (Table 1, entries 1 and 2), and unfortunately, no desired product was obtained. In contrast, the desired [4 + 3] annulation proceeded smoothly in 2 h with catalyst DABCO in THF, affording the desired benzo[b]oxepine 3aa featuring a spirocyclic oxindole scaffold in 90% yield with good diastereoselectivity (10:1, dr; Table 1, entry 7), and the relative stereochemistry of 3aa has been confirmed by X-ray crystallographic analysis.17 Notably, we also tried other nucleophilic tertiary amine catalysts such as DMAP, quinine, NEt3, and DBU, and unfortunately, no positive results were observed in this transformation (Table 1, entries 3−6). We attempted to lower the temperature of the reaction to improve the yield and diastereoselectivity, but this resulted in a slight reduction in the diastereoselectivity (8:1, dr; Table 1, entry 8). We subsequently evaluated the nature of the solvent and its effect on the reaction stereoselectivity and yield. Acetonitrile was identified as the best solvent compared with other solvents, which facilitated an increased stereoselectivity (dr > 20:1; Table 1, entry 11). Additionally, when the catalyst loading was decreased to 20 mol %, the diastereoselectivity was maintained, and the yield was slightly increased to 92% (Table 1, entry 17). Consequently, we focused on the use of acetonitrile and DABCO (20 mol %) as shown in entry 17 to examine the potential of the [4 + 3] annulation. With the above optimized conditions in hand, the scope with respect to the Morita−Baylis−Hillman carbonates in this Lewis base-catalyzed annulation was first investigated (Scheme 2). Most reactions evaluated proceeded in moderate to good yields and stereoselectivities. Substrates with halogens (F, Cl, Br) on the aromatic ring were compatible with this method and afforded the corresponding benzo[b]oxepines (3ab−3ae, 3ag− 3ai) in 63−94% yields and excellent diastereoselectivities

entry

catalyst

time (h)

solvent

yield (%)b

drd

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

PPh3 (±)-BINAP DMAP quinine NEt3 DBU DABCO DABCO DABCO DABCO DABCO DABCO DABCO DABCO DABCO DABCO DABCO

2 24 2 2 10 2 2 20 2 4 2 6 2 2 2 6 2

THF THF THF THF THF THF THF THF CHCl3 EtOAc CH3CN toluene (CH2Cl)2 acetone dioxane PhCl CH3CN

c c c c NR c 90 92 85 85 89 75 93 86 90 67 92

10:1 8:1 9:1 12:1 >20:1 10:1 12:1 9:1 11:1 12:1 >20:1

a

Unless otherwise noted, all reactions were conducted with 0.20 mmol of 1a (1 equiv), 0.30 mmol of 2a (1.5 equiv), and 50 mol % of catalyst in the solvent (2.0 mL) at 25 °C for the indicated time. bYield of isolated 3aa. cOnly unidentified byproducts were obtained. d Determined by 1H NMR analysis. eThe reaction was conducted at 0 °C. f20 mol % DABCO was used. (±)-BINAP = (±)-2,2′Bis(diphenylphosphino)-1,1′-binaphthalene, DMAP = 4-dimethylaminopyridine, DBU = 2,2′-bis(diphenylphosphino)-1,1′-binaphthalene, DABCO = 1,4-diazabicyclo[2.2.2]octane, NR = no reaction.

(>20:1, dr). A substrate with a strong electron-withdrawing group (NO2) also reacted, affording the benzo[b]oxepine derivative 3aj (76% yield; >20:1, dr). Substrates bearing electron-donating groups (Me, OMe) on the aromatic ring also reacted under the standard reaction conditions, and the desired products 3af and 3ak were isolated in 93% and 89% yields, respectively, with good diastereoselectivities. Increasing the size of the ester group of the MBH carbonates (R3 = Et, Bn) resulted in a decrease in the yield, without any discernible impact on the stereoselectivity (3al, 3am). Furthermore, the effect of the N-substituent (R4 = allyl, benzyl, and propargyl) of the MBH carbonates was examined, and the benzo[b]oxepines 3an−3ap were obtained with good yields and stereoselectivities. Next, the scope of this transformation with respect to the stable o-QMs was preliminarily investigated using Morita− Baylis−Hillman carbonates 2. As shown in Scheme 2, o-QM substrate 1b bearing a 3,4-methylenedioxyphenyl substituent cyclized with 2a, 2b, and 2g to give the benzo[b]oxepines 3ba, 3bb, and 3bg, respectively, albeit in moderate yields. Additionally, the reaction of o-QM 1c (R1 = OMe) and MBH carbonates (2a and 2b) to give the desired products 3ca and 3cb proved more efficient, resulting in slightly higher yields and good stereoselectivities. To demonstrate the potential of this methodology as a tool to construct benzo[b]oxepine scaffolds bearing diverse substitution patterns, two chemical transformations of the products 3aa and 3ai were pursued. As shown in Scheme 3, exposure of spirooxindole benzo[b]oxepine 3aa to DIBAL-H at −40 °C gave the highly functionalized alcohol 4 in 65% yield. Additionally, in 466

DOI: 10.1021/acs.orglett.8b03709 Org. Lett. 2019, 21, 465−468

Letter

Organic Letters Scheme 2. Scope of Morita−Baylis−Hillman Carbonates and o-QMsa,b

Scheme 4. Proposed Mechanism

1a. Due to the steric hindrance between the bulky ammonium ylide and p-methoxyphenyl of o-QM in intermediate B′, intermediate B is more favored. A subsequent oxa-Michael addition reaction of the resulting C leads to the intermediate D, which releases the DABCO and furnishes the benzo[b]oxepine 3aa. In conclusion, we have developed an efficient Lewis basecatalyzed [4 + 3] annulation of o-QMs and Morita−Baylis− Hillman carbonates to the synthesis of benzo[b]oxepine scaffolds. Utilizing this approach, a series of benzo[b]oxepine derivatives bearing oxindole scaffolds were achieved with moderate to excellent yields and good diastereoselectivities (dr > 20:1). This methodology is not only complementary to methods reported for the benzo[b]oxepine synthesis but also manifests the novel reactivity of ortho-quinone methides (oQMs) in methodology design.

a Performed with o-QMs 1 (0.20 mmol) and MBH carbonates 2 (0.30 mmol) in the presence of DABCO (0.04 mmol) in MeCN (2 mL) at 25 °C. bYield of isolated product.

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ASSOCIATED CONTENT

S Supporting Information *

Scheme 3. Synthetic Transformations of Benzo[b]oxepines 3aa and 3ai

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b03709. Experimental procedure and spectra data (PDF) Accession Codes

CCDC 1879405 contains 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 data_ [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].

the presence of the catalyst Pd(PPh3)4, Cs2CO3, and phenylboronic acid in a mixed solvent of THF and H2O, 3ai was transformed into the Suzuki coupling product 5 in 89% yield. Next, we turned our attention to the mechanism of this [4 + 3] annulation process (Scheme 4). Based on previous reports of Lewis base catalyzed reactions,16 the reaction is initiated by the nucleophilic addition of the tertiary amine DABCO to the MBH carbonate 2a, followed by the elimination of t-butylcarbonate moiety and deprotonation, to deliver N-allylic ylide A. Subsequent γ-selective nucleophilic addition would be preferred because of less steric hindrance between the amine and o-QM

ORCID

Ji-Yuan Du: 0000-0002-3059-9830 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for financial support from NSFC (21602094) and Natural Science Foundation of Shandong Province (ZR2016BB11). 467

DOI: 10.1021/acs.orglett.8b03709 Org. Lett. 2019, 21, 465−468

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Organic Letters

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DOI: 10.1021/acs.orglett.8b03709 Org. Lett. 2019, 21, 465−468