Letter pubs.acs.org/OrgLett
Phosphine-Catalyzed [4 + 2] Annulation of Allenoate with SulfamateDerived Cyclic Imines: A Reaction Mode Involving γ′-Carbon of α‑Substituted Allenoate Biming Mao,† Wangyu Shi,† Jianning Liao,† Honglei Liu,† Cheng Zhang,† and Hongchao Guo*,†,‡ †
Department of Applied Chemistry, China Agricultural University, Beijing 100193, P. R. China State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, P. R. China
‡
S Supporting Information *
ABSTRACT: A phosphine-catalyzed [4 + 2] cycloaddition of cyclic α-substituted allenoates with sulfamate-derived cyclic imines has been reported. Using dibenzylphenylphosphine as the nucleophilic catalyst, the reaction worked efficiently to yield various fused multicyclic heterocyclic compounds in high yields with excellent diastereoselectivities. It undergoes a new reaction mode involving γ′-carbon of α-substituted allenoate. carbon,9 β and γ-carbon,10 α, β, and γ-carbon,11 α, β, γ, and β′carbon,8,12 and α, β, and β′-carbon13 were fused in the annulation products. However, a phosphine-catalyzed annulation reaction involving γ′-carbon of allenoates has not been reported. In this context, we conceived the possibility of introducing an electron-withdrawing group (EWG) onto γ′carbon, which might lead to proton transfer, thus forming a new zwitterionic intermediate (Scheme 1). Upon exposure of such a reactive intermediate to electrophilic-coupling partners, new annulation reactions involving γ′-carbon of α-substituted allenoates might be achieved (Scheme 1). On the basis of this idea, herein we report tetrahydrobenzofuranone-derived allenoates and the initial results with these new allenoates for phosphine-catalyzed [4 + 2] annulation with sulfamate-derived cyclic imines for the synthesis of biologically significant multicyclic heterocycles14 (Scheme 1). Initially, we carried out our study by examining the reaction of cyclic allenoate (1a)15 and sulfamate-derived cyclic imine (2a) with the use of dichloromethane as the solvent at room temperature. Typically, the nature of the tertiary phosphine catalyst has a tremendous impact on the efficiency of nucleophilic phosphine catalysis. Different phosphine catalysts were first screened (Table 1, entries 1−10). Among various tested phosphines, weak nucleophilic triphenylphosphine catalyzed the reaction to give the product as a single diastereomer in 39% yield (entry 1). Using NMR spectroscopy and X-ray crystallography, the new product was identified to be [4 + 2] cycloaddition product 3aa. Strangely, strong nucleophilic PBu3 and Me3P did not promote the reaction, affording no product (entries 2−3). In comparison with PBu3 and Me3P, less nucleophilic dialkylphenylphosphines (Me2PPh and Et2PPh) functioned in the reaction, albeit with poor 14%
I
n the past two decades, a variety of phosphine-catalyzed annulation reactions have been developed,1 and these phosphine-catalyzed reactions have become extremely versatile synthetic methodologies for the preparation of various carboand heterocyclic compounds1 and total synthesis of natural products.2 In the area of nucleophilic phosphine catalysis, the annulation reactions with the use of allenoates as phosphine acceptor are probably the most studied.1 Because unsubstituted allenoate (buta-2,3-dienoate) was first used in phosphinecatalyzed [3 + 2] annulation by Lu in 1995,3 several types of allenoates, including unsubstituted and γ- and α-substituted allenoates (Scheme 1), have been designed and exploited for various phosphine-catalyzed annulation reactions. Under phosphine catalysis conditions, diverse zwitterionic intermediates (Scheme 1), formed from the addition of phosphine to allenoates, play a key role in what annulations could be achieved. These zwitterionic intermediates have shown diverse reactivity and can work as 1-, 2-, 3-, or 4-carbon synthons to react with several types of electrophilic reaction partners, including activated alkenes, imines, aldehydes, aziridines, and 1,3-dipoles, to furnish [1 + n],4 [2 + n],5 [3 + n],6 and [4 + n]7 annulation reactions. Although significant progress has been made in nucleophilic phosphine catalysis, development of phosphine catalysis is meeting a bottleneck because nucleophilic phosphine catalysis works with single activation mode. Therefore, exploration of new activation modes and synthons constitutes a major challenge in the area of nucleophilic phosphine catalysis. Among several types of allenoates, α-substituted allenoates have attracted intense attention.1 Since Kwon first used αsubstituted allenoates for phosphine-catalyzed [4 + 2] annulation with activated imines for the synthesis of highly functionalized tetrahydropyridines in 2003,8 the potential of αsubstituted allenoates in phosphine catalysis has been fully developed. These allenoates work as 1,2-, 1,3-, and 1,4-dipoles to react with electrophilic coupling partners. The α and β© 2017 American Chemical Society
Received: October 11, 2017 Published: November 21, 2017 6340
DOI: 10.1021/acs.orglett.7b03175 Org. Lett. 2017, 19, 6340−6343
Letter
Organic Letters Scheme 1. Diverse Zwitterionic Intermediates from Various Allenoates under Phosphine Catalysis Conditions
yield (entries 4 and 5). With the use of moderate nucleophilic alkyldiphenylphosphines such as MePPh2, EtPPh2, and nPrPPh2 as catalysts, cycloaddition product 3aa was obtained with 37, 64, and 32% yields, respectively (entries 6−8). Although benzyldiphenylphosphine promoted the reaction to give cycloadduct 3aa in only 30% yield (entry 9), to our delight, dibenzylphenylphosphine proved to be the most efficient catalyst, showing excellent catalytic activity to give product 3aa in 90% yield (entry 10). Having determined a promising phosphine, we next evaluated additional parameters including solvent and temperature (entries 11−13). Polar solvents such as methanol and acetonitrile gave inferior results. Toluene and THF turned out to be inefficient (entries 14−17). Other chlorinated solvents such as chloroform and 1,2-dichloroethane (DCE) were also compatible solvent (entries 11 and 12). Particularly, chloroform was the optimal choice in terms of the yield (entry 11). Higher reactivity was observed at elevated an temperature of 40 °C, and the product was obtained in 99% yield (entry 13). On the basis of the above experimental results, the optimal reaction conditions were determined to be with the use of Bn2PPh (20 mol %) as the catalyst at 40 °C in CHCl3. We tried to develop the asymmetric variant of this phosphinecatalyzed [4 + 2] cycloaddition of cyclic allenoate 1a with sulfamate-derived cyclic imine 2a. However, the attempt was not satisfactory, and up to 36% ee was obtained (see Supporting Information for details). After the optimized conditions were established, various sulfamate-derived cyclic imines 2 were investigated in the reaction of cyclic allenoate 1a (Table 2). With the use of Bn2PPh as the catalyst, various sulfamate-derived cyclic imines 2 carried out [4 + 2] cycloaddition reaction with allenoate 1a in CHCl3 at 40 °C for 12−24 h, providing multicyclic heterocyclic
Table 1. Optimization of the Reaction Conditionsa
Table 2. Scope of Cyclic Imines and Allenoatesa
entry
PR3
solvent
temp/°C
t/h
yield (%)b
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
PPh3 PBu3 Me3P Me2PPh Et2PPh MePPh2 EtPPh2 n-PrPPh2 BnPPh2 Bn2PPh Bn2PPh Bn2PPh Bn2PPh Bn2PPh Bn2PPh Bn2PPh Bn2PPh
CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CHCl3 DCE CHCl3 CH3OH CH3CN toluene THF
25 25 25 25 25 25 25 25 25 25 25 25 40 25 25 25 25
4 24 24 4 3 4 4 4 4 12 24 12 24 48 48 24 24
39 0 0 14 14 37 64 32 30 90 91 77 99 0 trace 17 38
a
All reactions were performed with 1a (0.15 mmol), 2a (0.1 mmol), and phosphine (0.02 mmol) in solvent (3 mL). bIsolated yield.
a
All reactions were performed with 1 (0.15 mmol), 2 (0.1 mmol), and Bn2PPh (0.02 mmol) in CHCl3 (3 mL) at 40 °C. bIsolated yield.
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DOI: 10.1021/acs.orglett.7b03175 Org. Lett. 2017, 19, 6340−6343
Letter
Organic Letters compounds (3aa−aq) with four chiral centers in high to excellent yields (71−99%) (entries 1−17). Sulfamate-derived cyclic imines 2 with either electron-withdrawing or -donating groups on the benzene ring, generally worked efficiently in the reaction, affording the product in high yields (entries 1−16). The position of the substituents at the benzene ring in sulfamate-derived cyclic imines 2 seems to have no remarkable effect on the yield (entries 1−16). A special substrate 2q also performed the reaction well to produce the corresponding product 3aq in 77% yield (entry 17). Unfortunately, under the optimal conditions, γ-methyl allenoate (1b) and γ-phenyl allenoate (1c), did not react with the benzol[e][1,2,3]oxathiazine 2,2-dioxide (2a) (entries 18 and 19). As shown in Scheme 2, the reaction on the gram scale still worked very efficiently and was completed in 24 h to provide
Scheme 3. A Plausible Mechanism
Scheme 2. Reaction on the Gram Scale and Further Transformation of the Cycloadduct
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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.7b03175. Experimental procedure, characterization data, NMR spectra, and crystallographic data for compounds 1a and 3aa (PDF) Accession Codes
CCDC 1577273−1577274 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.
product 3aa in 81% yield. In the presence of Pd/C catalyst, the carbon−carbon double bond of the cyclohexanone part in product 3aa could easily be reduced to give derivative 4 in 73% yield. On the basis of previous mechanistic studies,8,16 a plausible mechanism for the reaction was proposed in Scheme 3. Initially, phosphine catalyst undergoes nucleophilic addition to the allenoate 1a, leading to the formation of the zwitterionic intermediates (A↔A′). Intermediate B is formed from the intermediate through proton transfer. Subsequent isomerization and proton transfer leads to intermediate D. It attacks the sulfamate-derived cyclic imine 2a to afford intermediate E, which performs an intramolecular cyclization followed by expulsion of phosphine catalyst to give annulation product 3aa. When intermediate D reacts with the cyclic imine, the imine preferably approaches the up face of intermediate D due to steric hindrance from the methyl and phosphonium group, which results in the excellent stereoselecitivity. In conclusion, a novel phosphine-catalyzed [4 + 2] cycloaddition of cyclic allenoates and sulfamate-derived cyclic imines has been achieved under mild reaction conditions, leading to the multicyclic heterocyclic compounds, 3a,6,13b,13c-tetrahydro-1H,5H-benzo[5,6][1,2,3]oxathiazino[4,3-a]furo[4,3, 2-de]isoquinoline derivatives with four chiral centers in good to excellent yields. In particular, it is the first example of the γ′-carbon of α-substituted allenoates merging in the [4 + 2] cycloaddition product under phosphine catalysis conditions.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Cheng Zhang: 0000-0002-8760-8152 Hongchao Guo: 0000-0002-7356-4283 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is supported by the NSFC (Nos. 21372256, 21572264) and the State Key Laboratory of Chemical Resource Engineering.
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