Asymmetric α-Allylation of α-Substituted β-Ketoesters with Allyl

Oct 19, 2017 - Enantioselective α-allylation of α-substituted β-ketoesters with simple allyl alcohols was successfully performed by synergistic cat...
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Asymmetric α‑Allylation of α‑Substituted β‑Ketoesters with Allyl Alcohols Masanori Yoshida* Faculty of Mathematics and Science, National Institute of Technology, Asahikawa College, Shunkodai 2 jo 2-1-6, Asahikawa, Hokkaido 071-8142, Japan S Supporting Information *

ABSTRACT: Enantioselective α-allylation of α-substituted βketoesters with simple allyl alcohols was successfully performed by synergistic catalysis with the catalyst combination of a chiral primary amino acid and an achiral palladium complex without additional promotors like acids or bases. The allylation reaction and generation of a chiral quaternary carbon stereocenter proceeded smoothly to produce α,α-disubstituted β-ketoesters in high yields (91−99%) with high enantioselectivities (90−99% ee).

C

a long reaction time to complete the allylation reaction; for example, allylation of 2-oxocycloheptanecarboxylate required 5 days to consume the substrate completely.8−10 Tsuji−Trost allylation reaction is typically performed with a reactive allyl compound, such as an allyl ester and an allyl halide; therefore, the reaction produces a stoichiometric amount of acid as a byproduct. Although allyl alcohols are less reactive than allyl esters and allyl halides in Tsuji−Trost allylation reaction, the use of allyl alcohols directly for the allylation reaction has two advantages: (1) no requirement of the transformation of allyl alcohols to reactive allyl compounds before carrying out the allylation reaction and (2) the production of water instead of an acid as a waste byproduct after the reaction.11 In continued efforts to improve the reaction conditions for the enantioselective Tsuji−Trost allylation of α-substituted β-ketoesters,8 the allylation reaction and generation of a chiral quaternary carbon stereocenter were achieved by the direct use of simple allyl alcohols, and various α,α-disubstituted β-ketoesters were obtained in high yields with high enantioselectivities. Details of direct asymmetric αallylation of α-substituted β-ketoesters with allyl alcohols are described in this report. Allylation of ethyl 2-oxocyclopentancarboxylate (1a) was performed for investigation of the reactivity of allyl alcohol (2a) and its acetate (2a-Ac) in the presence of a catalytic amount of O-(tert-butyldiphenylsilyl)-L-threonine (3a) and tetrakis(triphenylphosphine)palladium(0) [Pd(PPh3)4, 4a] (Table 1). The allylation reaction of 1a with 2a-Ac proceeded readily at 25 °C to give α-allylated product 5a in 86% yield with 78% ee,8 while the reaction using 2a instead of 2a-Ac hardly proceeded under the same reaction conditions to give only a trace amount of 5a (Table 1, Entries 1 and 2). Fortunately, the low reactivity of 2a in the allylation reaction was overcome by carrying out

hiral quaternary carbon stereocenters can be found in many natural organic compounds with useful biological activity as pharmaceuticals and agrochemicals. Since the biological properties are often affected by the configuration of the chiral carbon atoms, the enantioselective synthesis of quaternary carbon stereocenters is critical in medicinal chemistry. Although various methodologies for the enantioselective synthesis of quaternary carbon stereocenters have been recently developed, an enantioselective catalytic method remains challenging in organic synthesis.1,2 Carbon−carbon bond formation at the tertiary carbon atom is a simple method to obtain the quaternary carbon atom; however, steric repulsion between the two reactants often makes this carbon−carbon bond formation difficult. Activation of both reactants is an approach to establish a carbon−carbon bond between the reactants and overcome this problem. In recent years, synergistic catalysis for the activation of both an electrophile and a nucleophile to produce a carbon−carbon bond has attracted the attention of synthetic organic chemists.3 The corporative catalysis consisted with organocatalysis and transition-metal catalysis, which were investigated individually in the past, led to a new era of enantioselective catalysis in organic synthesis.3 From the successful Tsuji−Trost allylation of aldehydes and ketones using an organic−transition-metal synergistic catalysis by Córdova’s group in 2006,4,5a several types of synergistic catalysis using the combination of an organocatalyst and a palladium-complex have been developed for enantioselective Tsuji−Trost allylation reaction.5,6 We recently reported that the catalyst combination of a chiral primary amino acid and an achiral palladium complex was effective for the enantioselective synthesis of a quaternary carbon stereocenter by Tsuji−Trost α-allylation of α-branched aldehydes.7 In our related research, α-branched ketones, such as 2-oxocycloalkanecarboxylates, were also found to be good substrates for the enantioselective synthesis of a quaternary carbon stereocenter. However, most of the substrates required © 2017 American Chemical Society

Received: August 30, 2017 Published: October 19, 2017 12821

DOI: 10.1021/acs.joc.7b02188 J. Org. Chem. 2017, 82, 12821−12826

Note

The Journal of Organic Chemistry Table 1. Initial Study of the Allylation Reaction of 1a with 2a or 2a-Ac in the Presence of 3a and 4aa

Table 2. Screening of Palladium Complexes for the Allylation Reaction of 1a with 2a in the Presence of 3aa entry d

entry

allyl compound (equiv)

temp. (°C)

yield (%)b

ee (%)c

1 2 3 4 5 6 7d

2a-Ac (2.0) 2a (2.0) 2a (2.0) 2a-Ac (2.0) 2a (1.1) 2a-Ac (1.1) 2a (1.1)

25 25 40 40 40 40 40

86 trace 95 92 86 81 91

78 nd 81 75 90 72 92

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

a Unless otherwise mentioned, the reaction was carried out with 1a (0.5 mmol), 2 (0.55 mmol), 3a (0.1 mmol), 4a (0.025 mmol) in toluene (0.6 mL) for 16 h under N2 atmosphere. bYield of 5a based on 1a. cDetermined by chiral HPLC analysis. nd = not determined. dThe amount of toluene was increased to 1.0 mL.

ligand

yield (%)b

ee (%)c

none PPh3 (20) PPh3 (15) PPh3 (10) P(2-furyl)3 (15) P(4-FC6H4)3 (15) P(C6F5)3 (15) P(2-CH3C6H4)3 (15) P(cyclohexyl)3 (15) P(n-butyl)3 (15) dppee(10) dpppf(10) dppbg(10) dppfh(10) (±)-BINAPi(10)

91 86 88 43 81 92 trace nr nr nr trace trace trace 81 83

92 91 91 92 77 97 nd nd nd nd nd nd nd 88 71

a

Unless otherwise mentioned, the reaction was carried out with 1a (0.5 mmol), 2a (0.55 mmol), 3a (0.1 mmol), and Pd(OAc)2 (4b, 0.025 mmol) in toluene (1.0 mL) at 40 °C for 16 h. bYield of 5a based on 1a. nr = no reaction. cDetermined by chiral HPLC analysis. nd = not determined. dPd(PPh3)4 (4a) was used instead of Pd(OAc)2. e1,2Bis(diphenylphosphino)ethane. f1,3-Bis(diphenylphosphino)propane. g 1,4-Bis(diphenylphosphino)buthane. h1,1′-Bis(diphenylphosphino)ferrocene. i2,2′-Bis(diphenylphosphino)-1,1′-binaphthyl.

the reaction under gentle heating (40 °C), and 5a was obtained in 95% yield with moderate enantioselectivity (Table 1, Entry 3). When the reaction temperature of the allylation reaction with 2a-Ac was increased to 40 °C, the enantioselectivity was decreased (Table 1, Entry 4). By using a reduced amount of allyl compound, enantioselectivity of the allylation reaction with 2a was improved to 90% ee, while no improvement of the enantioselectivity was observed in the case of the reaction with 2a-Ac (Table 1, Entries 5 and 6). Thus, the allylation reaction with 2a provided better results than that with 2a-Ac. When an increased amount of solvent was used for the reaction of 1a with 2a in the presence of 3a and 4a, slightly better yield and enantioselectivity were obtained (Table 1, Entry 7). A catalyst screen of palladium complexes for the allylation reaction of 1a with 2a in the presence of 3a was then performed (Table 2). The use of palladium(II) acetate [Pd(OAc)2, 4b] with triphenylphosphine (PPh3) gave results similar to those obtained with 4a, and the use of three equivalents of PPh3 to 4b was optimal (Table 2, Entries 1−4). After screening of various monodentate and bidentate ligands with 4b, tris(4fluorophenyl)phosphine [P(4-FC6H4)3] was found to be the most effective ligand to give allylated compound 5a in high yield with high enantioselectivity (Table 2, Entries 5−15). Next, the screening of catalysts with amino acids 3 was examined in the presence of 4b and P(4-FC6H4)3, and Table 3 summarizes the results obtained. When lipophilic primary amino acids, O-silylated threonines 3a,b and β-homoserine 3c, were used as catalysts, the allylation reaction proceeded readily to give 5a in high yield with high enantioselectivity.12,13 Common primary and secondary amino acids 3d−g could not promote the allylation reaction due to low solubility in the solvent, and a large amount of 1a was recovered after the experiments. Using amino ester 3h, a methyl ester of 3a, as a catalyst, N-(2-ethoxycarbonylcyclopent-1-enyl) O-TBDPS-Lthreonine methyl ester (6),14 an enamine generated from 1a and 3h, was obtained in 12% yield and no allylated product was generated. Addition of a catalytic amount of acetic acid to the allylation reaction with 3h gave the same result, and enamine 6 was obtained instead of the allylated product 5a. These results

Table 3. Screening of 3 for the Allylation Reaction of 1a with 2a in the Presence of 4b and P(4-FC6H4)3a

a Unless otherwise mentioned, the reaction was carried out with 1a (0.5 mmol), 2a (0.55 mmol), 3 (0.1 mmol), 4b (0.025 mmol), and P(4-FC6H4)3 (0.075 mmol) in toluene (1 mL) at 40 °C for 16 h under N2 atmosphere. bYield of 5a based on 1a. cDetermined by chiral HPLC analysis. nd = not determined. dee of (S)-enantiomer. e Enamine 6 was obtained in 12% yield.

indicated that the carboxylic acid group of an amino acid catalyst was pivotal to the allylation reaction. It was confirmed that the allylation reaction does not proceed in the absence of an amino acid catalyst 3. Since the synthesis of 3a can be performed in shorter steps from a commercially available amino acid than that of 3c, 3a was chosen as the amino acid catalyst. The substrate scope of the allylation reaction was investigated with various β-ketoesters 1 and allyl alcohols 2 (Table 4). By using a reduced amount of solvent, the allylation reaction of 1a with 2a in the presence of a catalyst combination 12822

DOI: 10.1021/acs.joc.7b02188 J. Org. Chem. 2017, 82, 12821−12826

Note

The Journal of Organic Chemistry Table 4. Substrate Scopea

The allylation reactions with 1g or 2b−d required an increased amount of palladium catalyst 4b and P(4-FC6H4)3 to prevent prolonging the reaction time. By comparing the spectroscopic data with those previously reported, it was found that both the cyclic and acyclic allylated products, 5a and 5f, were R-enantiomers. Plausible mechanisms for the allylation reaction and its stereocontrol are proposed in Scheme 1. By formation of an intramolecular hydrogen bond Scheme 1. Plausible Mechanisms for the Allylation Reaction and Its Stereocontrol

entry

1

2

5

yield (%)b

ee (%)c

1 2 3 4 5 6d 7e 8e 9e 10e 11e 12e

1a 1b 1c 1d 1e 1f 1g 1h 1i 1a 1a 1a

2a 2a 2a 2a 2a 2a 2a 2a 2a 2b 2c 2d

5a 5b 5c 5d 5e 5f 5g

96 96 97 96 92 91 96 nr nr 96 99 96

97 97 97 97 99 90 96 nd nd 97 98 97

5h 5i 5j

a Unless otherwise mentioned, the reaction was carried out with 1 (0.5 mmol), 2 (0.55 mmol), 3a (0.1 mmol), 4b (0.025 mmol), and P(4FC6H4)3 (0.075 mmol) in toluene (0.6 mL) at 40 °C for 16 h under N2 atmosphere. bYield of 5 based on 1. nr = no reaction. cDetermined by chiral HPLC analysis. nd = not determined. dThe reaction was carried out for 24 h. eThe reaction was carried out with an increased amount of 4b (0.035 mmol) and P(4-FC6H4)3 (0.105 mmol) for 24 h.

of 3a, 4b and P(4-FC6H4)3 gave 5a with an improved yield without reducing the enantioselectivity (Table 4, Entry 1). The effect of the alkoxycarbonyl group of ketoesters on the allylation reaction was investigated with 1a−c, and the results indicated that the bulkiness of the alkoxycarbonyl group did not significantly affect the yields and enantioselectivities of the allylated products 5a−c (Table 4, Entries 1−3). The allylation reaction of 2-oxocyclohexanonecarboxylate (1d) and 2oxocycloheptanonecarboxylate (1e) with 2a also readily proceeded to give the allylated products 5d and 5e, respectively, in high yields with high enantioselectivities (Table 4, Entries 4 and 5). Although acyclic α-substituted βketoesters, 2-methyl-3-oxobutanoate (1f) and 2-benzyl-3oxobutanoate (1g), required a longer reaction time, the allylation reactions with 2a were completed within 24 h to give the allylated products 5f and 5g (Table 4, Entries 6 and 7). The allylation reaction of 2-phenyl-3-oxobutanoate (1h) did not proceed due to the steric hindrance of the phenyl group (Table 4, Entry 8).8 2-Benzoyl propionate (1i) was also resulted in no reaction, since the reaction of aromatic ketones with the amino acid catalyst probably did not produce a sufficient amount of a corresponding enamine, which is a possible intermediate in allylation (Table 4, Entry 9).13c,d Substituted allyl alcohols like β-methallyl alcohol (2b), transcinnamyl alcohol (2c), and crotyl alcohol (2d) were also good allyl substrates for the allylation reaction of 1a, and the corresponding allylated products 5h−j were obtained in high yields with high enantioselectivities (Table 4, Entries 10−12).

between NH and CO, enamine Im-1, generated from βketoester 1 and amino acid 3a, would be a Z-isomer [Scheme 1 (a)].15 Then, the carboxylic acid group of the amino acid moiety assisted dehydration from the palladium olefin complex, which was generated from allyl alcohol 2a, Pd(OAc)2 4b and P(4-FC6H4)3, to afford Im-2, which can proceed to a rapid intramolecular allylation between the enamine and π-allylpalladium moiety.11b,16 A plausible mechanism of the stereocontrol is depicted in Scheme 1b. To avoid the steric repulsion between R1-substituent of the enamine moiety and the carboxylate group, the large amino acid side chain (Y) comes perpendicular to the Si-face of the α-carbon atom of the enamine moiety. Therefore, the Re-face of the α-carbon atom approached the πallylpalladium moiety rather than the Si-face to furnish an Renantiomer of allylated product 5 [Scheme 1b]. Since amino ester 3h contained no carboxylic acid groups to promote a rapid intramolecular allylation, the experiment with 3h would give an enamine instead of the allylated product as described in Table 3.11b In conclusion, the asymmetric α-allylation of α-substituted βketoesters with allyl alcohols was successfully carried out by a synergistic catalysis using the catalyst combination of a chiral primary amino acid, O-TBDPS-L-threonine, and an achiral palladium complex, Pd(OAc)2−P(4-FC6H4)3. The allylation reaction and generation of a chiral quaternary carbon 12823

DOI: 10.1021/acs.joc.7b02188 J. Org. Chem. 2017, 82, 12821−12826

Note

The Journal of Organic Chemistry

ketone), 1639 (C = C); HRMS (ESI-TOF) m/z: [M+Na]+ Calcd for C16H18O3Na 281.1148; Found 281.1147. Ethyl (R)-1-(Prop-2-enyl)-2-oxocyclohexanecarboxylate (5d). Spectroscopic data are in agreement with the published data.9g 96% yield (100.4 mg). Colorless oil; [α]58925.4 = +126.5 (c 1.0, CHCl3); Rf = 0.31 (n-hexane−EtOAc, 9:1). 1H NMR (400 MHz, CDCl3) 1.23 (3H, t, J 7.2 Hz), 1.40−1.48 (1H, m), 1.55−1.78 (3H, m), 1.96−2.03 (1H, m), 2.29−2.34 (1H, m), 2.42−2.49 (3H, m), 2.57−2.62 (1H, m), 4.17 (2H, q, J 7.2 Hz), 5.00−5.04 (2H, m), 5.67− 5.78 (1H, m); 13C{1H} NMR (100 MHz, CDCl3) 14.3, 22.6, 27.6, 35.9, 39.4, 41.2, 60.9, 61.3, 118.4, 133.4, 171.6, 207.7.; ν(neat)/cm−1 1745 (C = O of CO2Et), 1711 (C = O of ketone), 1639 (C = C); HRMS (ESI-TOF) m/z: [M+Na]+ Calcd for C12H18O3Na 233.1148; Found 233.1148. Ethyl 1-(Prop-2-enyl)-2-oxocycloheptanecarboxylate (5e). Spectroscopic data are in agreement with the published data.18d 92% yield (103.2 mg). Colorless oil; [α]58925.4 = +79.7 (c 1.0, CHCl3); Rf = 0.69 (n-hexane−EtOAc, 4:1). 1H NMR (400 MHz, CDCl3) 1.23 (3H, t, J 7.2 Hz), 1.34−1.43 (1H, m), 1.58−1.81 (6H, m), 2.04−2.12 (1H, m), 2.29−2.47 (2H, m), 2.61−2.75 (2H, m), 4.15 (2H, q, J 7.2 Hz), 5.02−5.06 (2H, m), 5.65−5.76 (1H, m); 13C{1H} NMR (100 MHz, CDCl3) 14.2, 24.6, 25.6, 30.0, 32.1, 39.7, 42.2, 61.3, 62.9, 118.7, 133.7, 172.1, 209.3.; ν(neat)/cm−1 1732 (C = O of CO2Et), 1708 (C = O of ketone), 1639 (C = C); HRMS (ESI-TOF) m/z: [M+Na]+ Calcd for C13H20O3Na 247.1305; Found 247.1313. (R)-3-Ethoxycarbonyl-3-methylhex-5-en-2-one (5f). Spectroscopic data are in agreement with the published data.18e,f 91% yield (83.7 mg). Colorless oil; [α]58925.6 = +24.2 (c 1.0, CHCl3); Rf = 0.37 (n-hexane−EtOAc, 9:1). 1H NMR (400 MHz, CDCl3) 1.24 (3H, t, J 7.2 Hz), 1.30 (3H, s), 2.13 (3H, s), 2.45−2.64 (2H, m), 4.17 (2H, q, J 7.2 Hz), 5.05−5.10 (2H, m), 5.57−5.68 (1H, m); 13C{1H} NMR (100 MHz, CDCl3) 14.2, 19.0, 26.3, 39.4, 59.5, 61.5, 119.1, 132.7, 172.6, 205.2.; ν(neat)/cm−1 1740 (C = O of CO2Et), 1711 (C = O of ketone), 1641 (C = C); HRMS (ESI-TOF) m/z: [M+H]+ Calcd for C10H17O3 185.1172; Found 185.1174. 3-Ethoxycarbonyl-3-(phenylmethyl)hex-5-en-2-one (5g). 96% yield (124.7 mg). Colorless oil; [α]58917.9 = −24.1 (c 1.0, CHCl3); Rf = 0.57 (n-hexane−EtOAc, 4:1). 1H NMR (400 MHz, CDCl3) 1.23 (3H, t, J 7.2 Hz), 2.11 (3H, s), 2.55−2.57 (2H, m), 3.11−3.24 (2H, m), 4.11−4.21 (2H, m), 5.11−5.15 (2H, m), 5.63−5.74 (1H, m), 7.05−7.08 (2H, m), 7.21−7.26 (3H, m); 13C{1H} NMR (100 MHz, CDCl3) 14.1, 27.6, 36.1, 37.6, 61.5, 64.8, 119.4, 127.0, 128.4, 130.1, 132.4, 136.3, 171.6, 204.6.; ν(neat)/cm−1 1740 (C = O of CO2Et), 1710 (C = O of ketone), 1640 (C = C); HRMS (ESI-TOF) m/z: [M +H]+ Calcd for C16H21O3 261.1485; Found 261.1471. Ethyl 1-(2-Methylprop-2-enyl)-2-oxocyclopentanecarboxylate (5h). 96% yield (100.6 mg). Colorless oil; [α]58925.5 = −39.8 (c 1.0, CHCl3); Rf = 0.79 (n-hexane−EtOAc, 4:1). 1H NMR (400 MHz, CDCl3) 1.26 (3H, t, J 7.2 Hz), 1.65 (3H, s), 1.88−2.06 (3H, m), 2.23−2.32 (2H, m), 2.37−2.45 (1H, m), 2.54−2.61 (1H, m), 2.81− 2.84 (1H, m), 4.16 (2H, q, J 7.2 Hz), 4.71 (1H, s), 4.84 (1H, s); 13 C{1H} NMR (100 MHz, CDCl3) 14.0, 19.4, 23.4, 31.8, 37.6, 41.4, 60.1, 61.6, 114.8, 141.3, 170.5, 214.4.; ν(neat)/cm−1 1750 (C = O of CO2Et), 1717 (C = O of ketone), 1645 (C = C); HRMS (ESI-TOF) m/z: [M+H]+ Calcd for C12H19O3 211.1329; Found 211.1339. Ethyl 1-(3-Phenylprop-2-enyl)-2-oxocyclopentanecarboxylate (5i). Spectroscopic data are in agreement with the published data.18b 99% yield (135.3 mg). Colorless oil; [α]58925.5 = −76.8 (c 1.0, CHCl3); Rf = 0.54 (n-hexane−EtOAc, 4:1). 1H NMR (400 MHz, CDCl3) 1.25 (3H, t, J 7.2 Hz), 1.86−2.08 (3H, m), 2.19−2.29 (1H, m), 2.39−2.55 (3H, m), 2.78−2.83 (1H, m), 4.14−4.20 (2H, m), 6.08 (1H, dt, J 7.2, 16.0 Hz), 6.44 (1H, d, J 16.0 Hz), 7.18−7.33 (5H, m); 13 C{1H} NMR (100 MHz, CDCl3) 14.2, 19.7, 32.3, 37.1, 38.2, 60.4, 61.6, 124.6, 126.3, 127.5, 128.6, 134.2, 137.1, 171.1, 214.9.; ν(neat)/ cm−1 1748 (C = O of CO2Et), 1720 (C = O of ketone); HRMS (ESITOF) m/z: [M+Na]+ Calcd for C17H20O3Na 295.1305; Found 295.1310. Ethyl 1-(But-2-enyl)-2-oxocyclopentanecarboxylate (5j). Obtained as a mixture of (E)- and (Z)-isomers. 96% yield (101.3 mg). Colorless oil; [α]58918.0 = −43.6 (c 1.0, CHCl3); Rf = 0.57 (n-hexane−

stereocenter proceeded readily without additional promotors or harsh conditions to give α,α-disubstituted β-ketoesters in high yields with high enantioselectivity. By using allyl alcohols directly instead of allyl esters or allyl halides, the allylation reaction could be performed without generation of a stoichiometric amount of an acid waste byproduct.

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EXPERIMENTAL SECTION

Materials. β-Ketoesters 1a,b,d,f−i were purchased and used after distillation; other ketoesters 1c17a and 1e17b were synthesized according to the literature procedures. Allyl alcohols 2a−d were purchased and used without purification. O-Silylated L-threonines (3a and 3b)12 and β-homoserine 3c13 were synthesized according to the literature procedures; other amino acids 3d−g were purchased and used without purification. Palladium catalysts 4 and phosphine ligands were purchased and were used without purification. Purification of the products was accomplished by column chromatography on Kanto Chemical Co., Inc. Silicagel 60N (spherical, neutral; 63-210 μm). Proton NMR (1H NMR) and proton-decoupled carbon NMR [13C{1H} NMR] spectra were recorded on a JNM-ECS400 FT NMR. Chemical shifts, δ are referred to TMS. Specific rotation was measured by a HORIBA SEPA-500 polarimeter. HPLC was carried out using a JASCO PU-2089 Plus intelligent pump and a UV-2075 Plus UV detector. HRMS was mesured by a Hitach HighTechnologies NanoFrontier eLD. Typical Procedure for the Allylation Reaction. In a 7 mL vial, Pd(OAc)2 (4b, 5.6 mg, 0.025 mmol), P(4−F-C6H4)3 (23.7 mg, 0.075 mmol), O-TBDPS-L-threonine (3a, 35.7 mg, 0.1 mmol) and toluene (0.6 mL) were placed, and the atmosphere in the vial was replaced with nitrogen. To the reaction mixture, allyl alcohol (1a, 32 mg, 0.55 mmol) and ethyl 2-oxocyclopentanecarboxylate (2a, 78 mg, 0.5 mmol) were added with stirring. The reaction mixture was then stirred for 16 h at 40 °C. The resulting mixture was filtered through a small plug of silica gel, eluted with Et2O (1 mL × 4) and concentrated under reduced pressure. Ethyl (R)-1-(prop-2-enyl)-2-oxocyclopentanecarboxylate (5a) was isolated by column chromatography (silica gel, hexane−Et2O 9:1) in 96% yield (93.7 mg) as colorless oil. The enantioselectivity was determined by chiral HPLC analysis (97% ee). The absolute configuration was determined by comparison of the specific rotation with that of the literature.18a Spectroscopic data are in agreement with the published data.18b Colorless oil; [α]58924.9= −38.2 (c 1.0, CHCl3); Rf = 0.43 (n-hexane−EtOAc, 4:1). 1H NMR (400 MHz, CDCl3) 1.25 (3H, t, J 7.2 Hz), 1.88−2.06 (3H, m), 2.20−2.29 (1H, m), 2.34−2.50 (3H, m), 2.64−2.70 (1H, m), 4.13−4.20 (2H, m), 5.09−5.13 (2H, m), 5.64−5.75 (1H, m); 13C{1H} NMR (100 MHz, CDCl3) 14.1, 19.5, 32.1, 37.8, 38.1, 59.9, 61.4, 119.0, 133.0, 170.9, 214.7.; ν(neat)/cm−1 1749 (C = O of CO2Et), 1720 (C = O of ketone), 1640 (C = C); HRMS (ESI-TOF) m/z: [M+Na]+ Calcd for C11H16O3Na 219.0992; Found 219.0997. Methyl (R)-1-(Prop-2-enyl)-2-oxocyclopentanecarboxylate (5b). Spectroscopic data are in agreement with the published data.18c 96% yield (87.0 mg). Colorless oil; [α]58925.0 = −51.9 (c 1.0, CHCl3); Rf = 0.46 (n-hexane−EtOAc, 4:1). 1H NMR (400 MHz, CDCl3) 1.85−2.04 (3H, m), 2.18−2.27 (1H, m), 2.32−2.49 (3H, m), 2.62−2.68 (1H, m), 3.69 (3H, s), 5.06−5.11 (2H, m), 5.61−5.71 (1H, m); 13C{1H} NMR (100 MHz, CDCl3) 19.6, 32.2, 38.0, 38.2, 52.7, 60.1, 119.3, 133.0, 171.4, 214.7.; ν(neat)/cm−1 1750 (C = O of CO2Me), 1724 (C = O of ketone), 1639 (C = C); HRMS (ESI-TOF) m/z: [M+Na]+ Calcd for C10H14O3Na 205.0835; Found 205.0835. Benzyl 1-(Prop-2-enyl)-2-oxocyclopentanecarboxylate (5c). Spectroscopic data are in agreement with the published data.11c 97% yield (125.0 mg). Colorless oil; [α]58925.3 = −27.3 (c 1.0, CHCl3); Rf = 0.51 (n-hexane−EtOAc, 4:1). 1H NMR (400 MHz, CDCl3) 1.84−2.03 (3H, m), 2.18−2.27 (1H, m), 2.32−2.49 (3H, m), 2.65−2.70 (1H, m), 5.04−5.09 (2H, m), 5.13 (2H, s), 5.60−5.71 (1H, m), 7.28−7.37 (5H, m); 13C{1H} NMR (100 MHz, CDCl3) 19.6, 32.2, 37.9, 38.2, 60.0, 67.2, 119.3, 128.0, 128.4, 128.7, 133.0, 135.7, 170.9, 214.5.; ν(neat)/cm−1 1750 (C = O of CO2Bn), 1724 (C = O of 12824

DOI: 10.1021/acs.joc.7b02188 J. Org. Chem. 2017, 82, 12821−12826

Note

The Journal of Organic Chemistry EtOAc, 4:1). 1H NMR (400 MHz, CDCl3) 1.25 (3H, t, J 7.2 Hz), 1.65 (3H, d, J 6.4 Hz), 1.85−2.05 (3H, m), 2.18−2.33 (2H, m), 2.37−2.45 (2H, m), 2.56−2.62 (1H, m), 4.12−4.20 (2H, m), 5.26−5.34 (1H, m), 5.48−5.57 (1H, m); 13C{1H} NMR (100 MHz, CDCl3) 14.2, 18.1, 19.6, 32.1, 36.7, 38.3, 60.3, 61.5, 125.4, 129.9, 171.1, 215.0.; ν(neat)/ cm−1 1749 (C = O of CO2Et), 1720 (C = O of ketone), 1667 (C = C); HRMS (ESI-TOF) m/z: [M+Na]+ Calcd for C12H18O3Na 233.1148; Found 233.1145. Synthesis of O-TBDPS-L-threonine Methyl Ester (3h). Nbenzyloxycarbonyl O-TBDPS-L-threonine was synthesized according to the literature. 12 To a DMF solution (17 mL) of Nbenzyloxycarbonyl O-TBDPS-L-threonine (4.11 g, 8.4 mmol), K2CO3 (1.16 g, 8.4 mmol) and MeI (1.19 g, 8.4 mmol) were added. After stirring for 6 h at room temperature, the reaction mixture was poured into H2O (100 mL) and extracted with EtOAc (20 mL × 3), washed with brine, dried over MgSO4 and concentrated under reduced pressure to give N-benzyloxycarbonyl O-TBDPS-L-threonine methyl ester. The obtained N-benzyloxycarbonyl O-TBDPS-Lthreonine methyl ester was dissolved in MeOH (15 mL), and the solution was added into a mixture of Pd/C (10%, 1 g) and MeOH (5 mL) under a nitrogen atmosphere in a round-bottomed flask. After the atmosphere in the flask was replaced with hydrogen, the reaction mixture was stirred for 15 h under a hydrogen atmosphere at 40 °C. Pd/C was filtered, and the filtrate was concentrated under reduced pressure. O-TBDPS-L-threonine methyl ester (3h) was obtained after purification by column chromatography (silica gel, hexane−EtOAc) in 72% yield (2 steps, 2.26 g) as colorless oil. [α]58916.4 = −1.1 (c 1.0, CHCl3); Rf = 0.24 (n-hexane−EtOAc, 4:1). 1H NMR (400 MHz, CDCl3) 1.02 (9H, s), 1.12 (3H, d, J 6.4 Hz), 3.28 (1H, d, J 2.8 Hz), 3.61 (3H, s), 4.34 (1H, dq, J 2.8, 6.4 Hz), 7.36−7.46 (6H, m), 7.62− 7.68 (4H, m); 13C{1H} NMR (100 MHz, CDCl3) 19.4, 20.7, 26.9, 52.0, 60.6, 70.9, 127.6, 127.7, 129.7, 129.9, 133.3, 134.2, 135.9, 136.0, 175.1.; ν(neat)/cm−1 1740 (C = O of CO2Me); HRMS (ESI-TOF) m/z: [M+Na]+ Calcd for C21H29NSiO3Na 394.1809; Found 394.1798. Synthesis of Enamine (6), N-(2-Ethoxycarbonylcyclopent-1enyl) O-TBDPS-L-threonine Methyl Ester. To a solution of OTBDPS-L-threonine methyl ester (3h, 74.2 mg, 0.2 mmol) in toluene (1 mL), ethyl 2-oxocyclopentanecarboxylate (1a, 31.2 mg, 0.2 mmol), MgSO4 (24 mg, 0.2 mmol) and AcOH (6.0 mg, 0.1 mmol) were added. After stirring for 24 h at 40 °C, the resulting mixture was filtered through a small plug of silica gel, eluted with Et2O (1 mL × 4) and concentrated under reduced pressure. N-(2-ethoxycarbonylcyclopent-1-enyl) O-TBDPS-L-threonine methyl ester (6) was isolated by column chromatography (silica gel, hexane−Et2O 9:1) in 95% yield (96.8 mg) as colorless oil. [α]58915.0 = −43.8 (c 1.0, CHCl3); Rf = 0.59 (n-hexane−EtOAc, 4:1). 1H NMR (400 MHz, CDCl3) 1.03 (9H, s), 1.07 (3H, d, J 6.4 Hz), 1.27−1.30 (3H, m), 1.72−1.83 (2H, m), 2.23− 2.38 (2H, m), 2.52−2.56 (2H, m), 3.67 (3H, s), 3.82 (1H, dd, J 3.2, 10.8 Hz), 4.16−4.24 (2H, m), 4.32 (1H, dq, J 3.2, 6.4 Hz), 7.36−7.46 (6H, m), 7.66−7.71 (4H, m), 7.85 (1H, br); 13C{1H} NMR (100 MHz, CDCl3) 14.9, 19.3, 20.2, 20.7, 26.8, 29.6, 32.3, 52.4, 58.7, 63.4, 70.7, 95.3, 127.6, 127.8, 129.8, 129.9, 133.1, 133.9, 136.0, 136.1, 161.8, 167.9, 171.7.; ν(neat)/cm−1 3310 (N−H), 1750 (C = O of CO2Me), 1663 (C = O of CO2Et), 1604 (C = C of enamine); HRMS (ESITOF) m/z: [M+Na]+ Calcd for C29H39NSiO5Na 532.2490; Found 532.2486.

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ORCID

Masanori Yoshida: 0000-0001-9481-6462 Notes

The author declares no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by JSPS KAKENHI Grant Number JP17K05872. The author thanks Mr. Hiroaki Akutsu (Center for Advanced Research and Education, Asahikawa Medical University) for the measurements of HRMS.

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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.joc.7b02188. HPLC data of 5, 1H and 13C{1H} NMR spectra of 5, 3h, and 6 (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*[email protected] 12825

DOI: 10.1021/acs.joc.7b02188 J. Org. Chem. 2017, 82, 12821−12826

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