Phosphine-Catalyzed Enantioselective [4 + 2] Cycloaddition

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Phosphine-Catalyzed Enantioselective [4 + 2] Cycloaddition− Semipinacol-Type-Rearrangement Reaction of Morita−Baylis− Hillman Carbonates Yuan Zhong,†,‡,§ Xiaoyun Zhao,†,§ Lu Gan,† Sihua Hong,† and Xianxing Jiang*,† †

School of Pharmaceutical Sciences, Sun Yat-Sen University, Guangzhou, 510006, China Key Laboratory of Preclinical Study for New Drugs of Gansu Province, School of Basic Medical Sciences, Lanzhou University, Lanzhou, 730000, China

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‡

S Supporting Information *

ABSTRACT: The chiral phosphine-triggered electrophilic ylide intermediate for a Morita−Baylis−Hillman carbonates activation strategy provides a promising method for the design of organocatalytic intermolecular higher-order annulation processes.

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MBHCs upon exposure to phosphine catalysts could be generally initiated via nucleophilic allylic phosphonium ylides after deprotonation, which as nucleophilic C3 synthons could be reacted with electrophilic coupling partners to achieve annulations (Scheme 1a). Alternatively, it was proven to be

he continuous innovation of synthetic methodologies is a source of sustainable progress in modern organic chemistry. Among them, discovery of new tandem asymmetric transformations has successfully captured considerable interest from chemists in recent years.1 Apparently, it could contribute directly to the development of a new method to design a diverse array of complex molecules in a more convenient manner. Nucleophilic phosphine-triggered annulation reaction represents one of the most efficient approaches for the preparation of a wide range of carbo- and heterocyclic frameworks,2 some of which are commonly present in bioactive natural products and medicinally important compounds and can be easily acquired.3 In the wake of the emergence of the first chiral tertiary phosphine-catalyzed enantioselective acylation of secondary alcohols reported by the group of Vedejs,4 a variety of asymmetric variants of phosphine-catalyzed annulations have been presented involving [2 + 2],5 [3 + n] (n = 2, 3),6 [4 + n] (n = 1, 2),7 and other annulations.8 Although these efficient catalytic asymmetric reactions have been well-established, to date, catalytic asymmetric cycloaddition−semipinacol-type-rearrangement and higher-order annulation reactions such as [4 + 3]9 and [6 + 3]10 annulations, especially of catalytic enantioselective tandem variants, are much less developed and represent a challenging task. To the best of our knowledge, a phosphinecatalyzed enantioselective cycloaddition−rearrangement sequence has not yet been achieved. The Morita−Baylis−Hillman acetates (MBHAs) and carbonates (MBHCs) are generated from Morita−Baylis− Hillman alcohols by converting the hydroxyl group into an acetoxy or tert-butoxycarbonyloxy leaving group.11 Thus, they retain the electrophilicity at the terminal double bond and the allylic position. MBHAs and MBHCs as valuable and powerful reaction partners have emerged in Lewis base catalyzed annulations12 since Lu and co-workers reported the first MBHCs annulation by using a tertiary phosphine under mild conditions.13 In these phosphine-mediated systems, the © XXXX American Chemical Society

Scheme 1. Different Phosphonium Ylides for Asymmetric Annulation Reactions

feasible that the annulation reactions could also be promoted by electrophilic enolate ylides formed from MBHCs and tertiary amine when a chiral amine acts as a Lewis base instead of phosphine.14 To gain a better understanding of the scope of this conceptually catalytic system, we herein introduce a chiral phosphine-triggered electrophilic ylide intermediate for the MBHC activation strategy as a new platform for the design of Received: May 26, 2018

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DOI: 10.1021/acs.orglett.8b01661 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters organocatalytic intermolecular higher-order annulation processes. In our continuing efforts to achieve this type of reaction, the key point is to find a C4 synthon partner to couple with electrophilic phosphorus ylides formed from MBHCs and chiral phosphine. The functionalized 2-(acyl)but-2-enenitriles 1 bearing our desired multinucleophilic and electrophilic functions15 are an ideal candidate for this tandem transformation. We postulated that the reaction might be initiated by in situ generation of the electron-inverse allylic phosphonium ylide when phosphine serves as the Lewis base catalyst in combination with a Brønsted base. The electrophilic ylide might be subjected to the nucleophilic addition of enolate and subsequent cycloaddition to yield the zwitterionic intermediate, which is followed by a semipinacol-type sigmatropic 1,3-hydrogen shift via the ring-opening intermediate to afford the cyclization product. In this context, we document the first highly enantioselective [4 + 2] cycloaddition semipinacol-type rearrangement reaction 16 of MBHCs (Scheme 1b). To explore the possibility of the proposed annulation process, our investigation began with screening of a variety of chiral phosphine catalysts to evaluate their catalytic activities. The model reaction of (E)-2-benzoyl-3-phenylbut-2-enenitrile (1a) with MBHCs (2a) was performed at room temperature in the presence of a 10 mol % loading of ligand in CHCl3 (entries 1−8, Table 1). These results indicate that the (+)-DuanPhos is the best catalyst in terms of chemical yield and enantioselectivity, thus furnishing the products with excellent diastereoselectivity (dr >99:1) in 31% yield and 90% ee (entry 8). Subsequently, a survey of other solvents was carried out with (+)-DuanPhos (entries 10−14). We found that a change in the solvent has a significant effect on the reaction outcome. Among the solvents tested, DCE appeared to be the most suitable reaction media, thus giving the product in 50% yield with 96% ee (entry 14). Gratifyingly, the reaction can be further promoted when an inorganic base was employed in combination with (+)-DuanPhos, and the more favorable outcome of 62% yield was observed in the presence of 1.0 equiv of K2CO3 without a decrease in enantioselectivity (entry 15). The results of the experiments run under the optimized reaction conditions to probe the scope of the reaction are summarized in Scheme 2. In general, a variety of substituted MBHCs (2) including those bearing electron-withdrawing and -donating substituents at different positions on the aromatic ring, such as a 1-naphthyl substituent and heterocyclic as well as different ester groups, could be tolerated and gave the corresponding compounds 3a−h and 3z with high to excellent enantioselectivities (88−97% ee) and diastereoselectivities (>20:1 dr) in moderate to good yields (52−70%). Gratifyingly, the olefinic group substituted MBHCs also gave the corresponding product 3i, although the enantioselectivity was slightly diminished (81% ee). Subsequently, the substrate scope was extended to different substituents on 2-(acyl)but-2enenitriles (1). The results also show that variation of the aromatic electronic properties of the substituent at either R1 or R2 of the 2-(acyl)but-2-enenitriles (1) with different steric parameters was tolerated, affording the products (3j−3m, 3o, 3s−3u, 3w, 3x, and 3za) with excellent enantioselectivities (90%−97% ee) and diastereoselectivities in moderate to good yields (50−71%). The desirable products (3n, 3p, and 3v) were obtained in good yields (59−69%) with excellent

Table 1. Studies and Optimization of the Reaction Parametersa

entry

cat.

solvent

yield (%)b

ee (%)c

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

(S)-BINAP (S,Rp)-FePPh2 (S)-VPT (S, S)-DIPAMP (−)-Me-DuPhos (−)-Ph-BPE (+)-iPr-BPE (+)-DuanPhos (+)-DuanPhos (+)-DuanPhos (+)-DuanPhos (+)-DuanPhos (+)-DuanPhos (+)-DuanPhos (+)-DuanPhos (+)-DuanPhos

CHCl3 CHCl3 CHCl3 CHCl3 CHCl3 CHCl3 CHCl3 CHCl3 CH2Cl2 toluene THF Et2O MTBE DCE DCE DCE

14 16 nd 23 19 26 39 31 32 20 38 41 46 50 62 56

−23 17 − 5 0 14 −56 90 80 86 45 86 92 96 96 93

a Reaction conditions: unless specified, a mixture of 1a (0.1 mmol), 2a (0.2 mmol), and a catalyst (10 mol %) in a solvent (1.0 mL) was stirred at rt. bYields of isolated product after purification by flash column chromatography. Products were observed with >20:1 dr. cThe ee values were determined by HPLC. The dr values were determined by 1H NMR spectroscopy and HPLC. d1.0 equiv K2CO3 was used. e 1.0 equiv Cs2CO3 was used.

enantioselectivities (94%−98% ee) in the reactions of the oxyphenyl and heterocyclic group substituted substrate 1. Additionally, as expected, the catalytic system also proved to be efficient for aliphatic substituents, again leading to 3q, 3r, and 3y with excellent stereoselectivities (96%−98% ee and >20:1 dr) in moderate to good yields. The absolute and relative configurations of the products were unambiguously determined by X-ray crystallography (3za; see the Supporting Information (SI)). The transformation of the cyclization product to a synthetically useful compound was performed. As an illustration in Scheme 3, treatment of the product 3a with potassium tert-butoxide in THF at room temperature for 6 h led to the 1,3-shift of the benzoyl group, giving the product 5 with 85% ee and a 60% yield. The relative configurations of 5 were confirmed by X-ray crystallography (see the SI). On the basis of the experimental results described above and recent studies, a proposed possible mechanism is shown in B

DOI: 10.1021/acs.orglett.8b01661 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters Scheme 2. Scope of Substrates for Enantioselective Reaction of MBHCsa

Scheme 3. Synthetic Transformations of Product

Figure 1. Proposed mechanism for [4 + 2] cycloaddition semipinacoltype rearrangement reaction.

the elimination of the leaving group delivering the phosphonium salt intermediate, which could be deprotonated by an extra or in situ generated base to give allylic phosphonium ylide A.12,17 Then, the allylic phosphorus intermediate A might be subjected to the attack of the incoming nucleophiles on the (E)-2-benzoyl-3-phenylbut-2enenitrile 1a following a determined direction to generate intermediate B (the carbonyl group may be fixed and activated by the second phosphine moiety in the catalyst), and a subsequent stereoselective intramolecular cycloaddition to yield the zwitterionic intermediate C. Finally, a semipinacoltype sigmatropic 1,3-hydrogen shift18 might occur to give the ring-opening intermediate D, which is followed by intramolecular cycloaddition and elimination of phosphine to complete the catalytic cycle. To further identifying the mechanism of this transformation, a deuteration control experiment was conducted. The results indicate that the C-2, C-3, and C-5 positions of 3a could be successfully replaced by deuterium. The deuterium substitution at the C-3 position also confirmed that the intermediate C undergoes a semipinacoltype sigmatropic 1,3-hydrogen shift, rather than a simple 1,3isomerization, which is consistent with the proposed mechanism. In addition, a supplementary experiment of MS (ESI) analysis while monitoring the reaction was also performed, and the existence of main intermediates B, C,

a

Reaction conditions: unless specified, a mixture of 1 (0.1 mmol), 2 (0.2 mmol), (+)-DuanPhos (10 mmol %), and 1.0 equiv K2CO3 in a DCE (1.0 mL) was stirred at rt for 72 h. Yields of isolated product after purification by flash column chromatography. The ee values were determined by HPLC, and the configuration was assigned by comparison of HPLC data and X-ray crystal data of 3za. The dr values were determined by 1H NMR spectroscopy and HPLC.

Figure 1. The MBHCs 2a retain the electrophilicity at the terminal double bond and the allylic position. An initial addition or SN2′ attack of chiral phosphine on 2a will trigger C

DOI: 10.1021/acs.orglett.8b01661 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters

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and D in the reaction mixture was confirmed by MS (see the SI). In summary, we have disclosed a highly efficient chiral phosphine-triggered electrophilic ylide intermediate for an MBHC activation strategy that has enabled the development of the first highly enantioselective [4 + 2] cycloaddition semipinacol-type rearrangement reaction of MBHCs with high levels of enantio- and diastereoselectivity (up to 98% ee and >20:1 dr).

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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.8b01661. Experimental details and characterization data (PDF) Accession Codes

CCDC 1537111, 1546844, and 1549044 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, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Xianxing Jiang: 0000-0002-7508-2368 Author Contributions §

Y.Z. and X.Z. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We greatly appreciate the financial support from the Program for Guangdong Introducing Innovative and Enterpreneurial Teams (2016ZT06Y337). X.J. thanks the Thousand Young Talents Program for financial support. In Memory of Professor Carlos F. Barbas III in The Scripps Research Institute.

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DOI: 10.1021/acs.orglett.8b01661 Org. Lett. XXXX, XXX, XXX−XXX