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Synthesis 2010(21): 3663-3669  
DOI: 10.1055/s-0030-1258213
PAPER
© Georg Thieme Verlag Stuttgart ˙ New York

Yb(OTf)3-Promoted ZnCl2-Catalyzed Conia-Ene Reaction of Linear β-Alkynic β-Dicarbonyls

Yu Liu, Ren-Jie Song, Jin-Heng Li*
Key Laboratory of Chemical Biology & Traditional Chinese Medicine Research (Ministry of Education), Hunan Normal University, Changsha 410081, P. R. of China
Fax: +86(731)88872101; e-Mail: jhli@hunnu.edu.cn;

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Publication History

Received 25 June 2010
Publication Date:
13 August 2010 (online)

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Abstract

An atom-economical and solvent-free ytterbium(III) triflate promoted, zinc(II) chloride catalyzed Conia-ene method has been developed for the construction of five- to six-membered-ring carbocycles. In the presence of zinc(II) chloride and ytterbium(III) triflate, a variety of linear β-alkynic β-keto esters and β-alkynic β-diketones were cyclized under neat conditions in moderate to good yields. It is noteworthy that the selectivity toward five- or six-membered­-ring carbocycles depends on substituents at the terminal alkynes.

Key words

zinc(II) chloride - ytterbium(III) triflate - Conia-ene reaction - linear β-alkynic β-dicarbonyl - carbocycle

Scheme 1 Conia-ene reactions of alkynic dicarbonyl compounds

The development of new Lewis acid catalyzed Conia-ene route is of long-standing interest to organic chemists, because this process can construct new chemical bonds in an efficient and atom-economical manner. [¹-7] In recent years, several efficient catalytic systems, including Au/Ag, [4] Ni(acac)2/Yb(OTf)3, [5] In(NTf2)3, [6] and (CuOTf)2˙C6H6/Ag, [7] have been reported for the intramolecular Conia-ene reaction of numerous alkynic β-keto esters. [4-7] Despite the great advantages of these methods, certain drawbacks and limitations still exist: 1) the reported catalytic systems are highly expensive, 2) the used solvents are harmful in view of the environment, and 3) most are focused on α-alkynic β-keto esters, and only two papers have been reported on linear β-alkynic β-keto ester substrates. [4e] [7a] In 2006, Sawamura and co-workers reported a linear β-alkynic β-keto ester substrate for the Conia-ene reaction using [Au(N(SO2CF3)2(triethynyphosphine)] catalysis. [4e] Subsequently, we have developed an effective Cu/Ag-catalyzed protocol for the Conia-ene reactions of linear β-alkynic β-keto esters (Equations 1 and 2 in Scheme  [¹] ). [7a] In the presence of (CuOTf)2˙C6H6 and silver(I) (AgSbF6 or AgOAc), a variety of linear β-alkynic β-keto esters selectively underwent the Conia-ene intramolecular reaction to afford the corresponding exo- or endo-products in moderate to good yields. For the reaction of β-(substituted alkynic) β-keto esters, however, the yields are unsatisfactory. Recently, we reported a general and inexpensive Zn-catalyzed Conia-ene route, which allows a wide range of α-alkynic dicarbonyl compounds to proceed the Conia-ene reaction with inexpensive zinc(II) chloride under neat conditions (Equation 3, Scheme  [¹] ). [8] Thus, we decided to employ the reported zinc(II) chloride conditions for the Conia-ene reaction of linear β-alkynic β-keto esters and β-alkynic β-diketones. After a series of trials, we found that zinc(II) chloride combined with ytterbium(III) triflate displayed high efficiency for this purpose. Herein, we report an effective route to construct five- to six-membered-ring carbocycles by ytterbium(III) triflate promoted, zinc(II) chloride catalyzed Conia-ene reaction of linear β-alkynic β-dicarbonyls under neat conditions (Equation 4, Scheme  [¹] ).

Table 1 The Conia-Ene Reaction of Ethyl 3-Oxohept-6-ynoate (1a) in the Presence of Zinca

Entry [Zn] Additive (equiv) Temp (˚C) Yield (%)b
 1 ZnCl2 - 100 55
 2 ZnCl2 Yb(OTf)3 (0.2) 100 56
 3 ZnCl2 Yb(OTf)3 (0.5) 100 61
 4 ZnCl2 Yb(OTf)3 (1) 100 77
 5c ZnCl2 Yb(OTf)3 (1) 100 80
 6 ZnCl2 Mg(ClO4)2 (1) 100 52
 7 ZnCl2 CuCl2 (1) 100 38
 8 ZnI2 Yb(OTf)3 (1) 100 45
 9 Zn(OTf)2 Yb(OTf)3 (1) 100 36
10 ZnCl2 Yb(OTf)3 (1)  40 32
11 ZnCl2 Yb(OTf)3 (1) 120 72
a Reaction conditions: 1a (0.3 mmol), [Zn] (10 mol%), and additive in argon atmosphere under neat conditions for 24 h.
b Isoalted yield.
c DCE (4 mL) was added.
Table 2 Ytterbium(III) Triflate Promoted, Zinc(II) Chloride Catalyzed Conia-Ene Reactions of Terminal Alkynes 1 a
Entry Terminal alkyne 1 Product 2 Yield (%)b
1

1b

2b 		73
2

1c

2c 		66
3

1d

2d 		53
4

1e

2e 		85
5

1f

2f 		64
6

1g

2g 		75
7

1h

2h 		68
8

1i

2i 		81
a Reaction conditions: 1 (0.3 mmol), ZnCl2 (10 mol%), and Yb(OTf)3 (1.0 equiv) at 100 ˚C in argon atmosphere under neat conditions for 24 h.
b Isolated yield.

The reaction of ethyl 3-oxohept-6-ynoate (1a), a linear β-alkynic β-keto ester, was chosen as a model reaction to optimize the reaction conditions, and the results are summarized in Table  [¹] . Initially, the reaction of substrate 1a with zinc(II) chloride was carried out under the reported conditions, and the target exo-product 2a was isolated in 55% yield (Table  [¹] , entry 1). In the previously reported results, ytterbium(III) triflate was found to favor the generation of the enol form leading to the improvement of the corresponding reactions. [5] As expected, the yield was enhanced in the presence of ytterbium(III) triflate (entries 2-4). While 0.2 equivalent of ytterbium(III) triflate increased the yield slightly (entry 2), 1 equivalent enhanced the yield to 77% (entry 4). Two other additives, Mg(ClO4)2 and copper(II) chloride, were subsequently evaluated, and they were less effective (entries 6 and 7). The solvent, dichloroethane, has no effect on the reaction (entry 7). Both zinc(II) iodide and zinc(II) triflate displayed less efficiency than zinc(II) chloride in the presence of ytterbium(III) triflate (entries 8 and 9). Among the effect of the reaction temperatures examined, 100 ˚C turned out to be the most suitable for the reaction (entry 4 vs entries 10 and 11). It was noted that the yield was lowered to 32% when the reaction was carried out at 40 ˚C (entry 10), and to 72% at 120 ˚C (entry 11).

With the optimal conditions in hand, the scope for the Conia­-ene reaction was explored (Tables  [²] and  [³] ). As shown in Table  [²] , a variety of terminal alkynes were surveyed in the presence of zinc(II) chloride and ytterbium(III) triflate.

The results demonstrated that various β-alkynic β-keto esters 1b-g and β-diketones 1h-i were successfully reacted with zinc(II) chloride and ytterbium(III) triflate under neat conditions, affording the corresponding exo-products alone in moderate to good yields. We found that the methyl and isopropyl ester, 1b and 1c, could successfully undergo the reaction, although the yields were decreased to some extent (Table  [²] entries 1 and 2). Moderate to good yields were still achieved from γ-substituted substrates 1d-f (entries 3-5). Gratifyingly, a six-membered-ring product 2g was obtained from ethyl 3-oxooct-7-ynoate (1g) in 75% yield (entry 6). Diketones 1h and 1i could also be converted into the desired products in 68 and 71% yield, respectively (entries 7 and 8).

Table 3 Ytterbium(III) Triflate Promoted, Zinc(II) Chloride Catalyzed Conia-Ene Reactions of Internal Alkynes 1 a (continued)
Entry Internal alkyne 1 Product 2 Yield (%)b
1

1j

3j 		60
2

1k

3k 		59
3

1l

3l 		54
4

1m

3m 		36
5

1n

3n 		58
6

1o

3o 		70
7

1p

3p 		52
8

1q

3q 		80
9

1r

3r 		62
a Reaction conditions: 1 (0.3 mmol), ZnCl2 (10 mol%), and Yb(OTf)3 (1.0 equiv) at 100 ˚C in argon atmosphere under neat conditions for 24 h.
b Isolated yield.

The cyclization of internal alkynes 1j-q were subsequently investigated under the optimal conditions, and the selectivity was shifted toward the endo-products 3 (Table  [³] ). Compared with the reported Cu/Ag catalytic system, the present Zn/Yb system is more efficient. In the presence of zinc(II) chloride and ytterbium(III) triflate, diketones 1j-p, bearing either electron-rich or electron-deficient aryl groups at the terminal alkyne, underwent the reaction smoothly to afford the corresponding endo-products 3j-p in moderate to good yields (Table  [³] , entries 1-7). However, both the steric hindrance and the electron-deficient aryl groups disfavored the reaction. We found that the active order of methyl-substituted substrates 1k-m is para-1k > meta-1l > ortho-1m in terms of yields (entries 2-4). While substrate 1o bearing an electron-donating aryl group afforded the desired product 3o in 70% yield (entry 6), electron-deficient diketone 1p lowered the yield to 52% (entry 7). To our delight, another diketone 1q with a methyl group at the terminal alkyne was also cyclized in good yield (entry 8). Keto ester 1r, the reportedly less active substrate, [7a] was also compatible with the optimal conditions to afford the desired endo-product in 62% yield (entry 9).

A possible mechanism as outlined in Scheme  [²] was proposed on the basis of the present results and the reported mechanisms. [¹-8] Generally, two mechanisms are described for the Conia-ene reaction: 1) nucleophilic attack on an M-alkyne complex by the enol form of the keto ester, and 2) an M-enolate formation by directly complex with the β-keto ester, followed by a cis-carbon metalation of the alkyne. [¹-8] The present results suggest that the intermediate A may be generated by the complexation of zinc with alkyne, and another metal (Zn or/and Yb) with dicarbonyl, which is similar to the mechanism proposed by Toste. [²a] [7] Subsequently, the intramolecular attack of the M-enolate on the Zn-alkyne complex takes place to form the intermediates B (terminal alkynes) and C (internal alkynes), followed by protonation/isomerization sequence to give the corresponding exo- or endo-products. The present results show that the selectivity toward exo- or endo-products depends on the properties of alkynes: Zn often adds to the less sterically hindered position of the alkyne during the intramolecular attack (intermediate B); however, the formation of less strained ring is preferable when there are similar substituents at the ends of the CºC bond (intermediate C). We deduce that ytterbium(III) triflate can improve the reaction by complexing with dicarbony groups leading to the formation of enol form.

Scheme 2 A possible mechanism

A radical cyclization mechanisn for the endo-cyclization has been proposed. [9] Thus, two controlled experiments using the substrate 1j were carried out in the presence of two radical inhibitors, 1,1-diphenylethylene or TEMPO. We found that these radical inhibitors could not affect this reaction, suggesting that the free radical mechanism can be ruled out.

In summary, we have developed a zinc(II) chloride and ytterbium(III) triflate catalytic system for the Conia-ene intramolecular reaction of linear β-alkynic β-dicarbonyls under neat conditions. In the presence of zinc(II) chloride and ytterbium(III) triflate, the present Conia-ene route allows the selective construction of five- to six-membered-ring carbocycles from various linear β-alkynic β-keto esters and β-alkynic β-diketones. It is noteworthy that this protocol is atom-economical and environmentally benign (solvent-free).

All melting points are uncorrected. IR spectra were recorded using KBr optics. NMR spectroscopy was performed on a Bruker-500 spectrometer operating at 500 MHz (¹H NMR) and 125 MHz (¹³C NMR). TMS was used an internal standard and CDCl3 was used as the solvent. Mass spectrometric analysis was performed on GC-MS analysis (Shimadzu GCMS-QP2010 plus).

Conia-Ene Reaction; General Procedure

To a Schlenk tube was added dicarbonyl 1 (0.3 mmol), ZnCl2 (10 mol%), and Yb(OTf)3 (1.0 equiv). Then the mixture was stirred in argon atmosphere under neat conditions at 100 ˚C for the indicated time until complete consumption of the starting material as monitored by TLC. After completion of the reaction, the mixture was poured into EtOAc (10 mL) and the EtOAc layer was washed with brine (3 × 5 mL). The aqueous washings were extracted with EtOAc (3 × 5 mL), the combined organic layers were dried (Na2SO4), and evaporated under vacuum. The residue was purified by flash column chromatography on silica gel (hexane-EtOAc) to afford the pure product.

Ethyl 2-Methyl-5-oxocyclopent-2-enecarboxylate (2a) [7a]

Colorless oil.

IR (film): 1749, 1717 cm-¹.

¹H NMR (500 MHz, CDCl3): δ = 4.34-4.30 (m, 2 H), 2.66 (t, J = 5.0 Hz, 2 H), 2.49 (t, J = 5.0 Hz, 2 H), 2.39 (s, 3 H), 1.35 (t, J = 7.5 Hz, 3 H).

¹³C NMR (125 MHz, CDCl3): δ = 203.5, 184.2, 163.3, 132.8, 60.8, 35.0, 32.6, 19.3, 14.2.

LRMS (EI, 70 eV): m/z (%) = 168 (M+, 13), 123 (83), 96 (100).

Methyl 2-Methyl-5-oxocyclopent-1-enecarboxylate (2b) [7a]

Colorless oil.

IR (film): 1741, 1720 cm-¹.

¹H NMR (500 MHz, CDCl3): δ = 3.85 (s, 3 H), 2.68 (t, J = 5.0 Hz, 2 H), 2.50 (t, J = 5.0 Hz, 2 H), 2.41 (s, 3 H).

¹³C NMR (125 MHz, CDCl3): δ = 203.5, 185.6, 163.7, 132.3, 51.8, 35.0, 32.8, 19.4.

LRMS (EI, 70 eV): m/z (%) = 154 (M+, 40), 123 (100).

Isopropyl 2-Methyl-5-oxocyclopent-1-enecarboxylate (2c)

Colorless oil.

IR (film): 1740, 1718 cm-¹.

¹H NMR (500 MHz, CDCl3): δ = 5.22-5.17 (m, 1 H), 2.65 (t, J = 5.0 Hz, 2H), 2.49-2.47 (m, 2 H), 2.36 (s, 3 H), 1.34 (s, 3 H), 1.33 (s, 3 H).

¹³C NMR (125 MHz, CDCl3): δ = 203.5, 182.8, 162.8, 133.2, 68.4, 34.9, 32.5, 21.8, 19.1.

LRMS (EI, 70 eV): m/z (%) = 182 (M+, 4), 123 (100).

HRMS (EI): m/z calcd for C10H14O3 (M+): 182.0942; found: 182.0940.

Methyl 2,4-Dimethyl-5-oxocyclopent-1-enecarboxylate (2d) [7a]

Colorless oil.

IR (film): 1740, 1718 cm-¹.

¹H NMR (500 MHz, CDCl3): δ = 3.85 (s, 3 H), 2.93, 2.89 (dd, J = 7.0, 7.0 Hz, 1 H), 2.50-2.47 (m, 1 H), 2.40 (s, 3 H), 2.30, 2.26 (dd, J = 2.5, 2.5 Hz, 1 H), 1.21 (d, J = 7.5 Hz, 3 H).

¹³C NMR (125 MHz, CDCl3): δ = 205.9, 183.8, 163.7, 130.9, 51.7, 41.6, 40.3, 19.2, 16.2.

LRMS (EI, 70 eV): m/z (%) = 168 (M+, 66), 137 (100).

Ethyl 4-Ethyl-2-methyl-5-oxocyclopent-1-enecarboxylate (2e)

Colorless oil.

IR (film): 1741, 1721 cm-¹.

¹H NMR (500 MHz, CDCl3): δ = 4.33-4.29 (m, 2 H), 2.84, 2.81 (dd, J = 6.5, 6.5 Hz, 1 H), 2.38-2.33 (m, 5 H), 1.89-1.82 (m, 1 H), 1.45-1.39 (m, 1 H), 1.35 (t, J = 7.5 Hz, 3 H), 0.96 (t, J = 7.5 Hz, 3 H).

¹³C NMR (125 MHz, CDCl3): δ = 205.4, 183.0, 163.4, 132.1, 60.7, 47.2, 39.1, 24.3, 19.2, 14.2, 11.3.

LRMS (EI, 70 eV): m/z (%) = 196 (M+, 4), 168 (44), 122 (100).

HRMS (EI): m/z calcd for C11H16O3 (M+): 196.1099; found: 196.1096.

Methyl 2,4,4-Trimethyl-5-oxocyclopent-1-enecarboxylate (2f) [¹0]

Colorless oil.

IR (film): 1747, 1721 cm-¹.

¹H NMR (500 MHz, CDCl3): δ = 3.85 (s, 3 H), 2.55 (s, 2 H), 2.40 (s, 3 H), 1.13 (s, 6 H).

¹³C NMR (125 MHz, CDCl3): δ = 207.8, 182.6, 163.8, 129.4, 51.6, 49.5, 43.8, 24.8, 19.1.

LRMS (EI, 70 eV): m/z (%) = 182 (M+, 69), 167 (100).

Ethyl 2-Methyl-6-oxocyclohex-1-enecarboxylate (2g) [6a]

White solid; mp 35.2-36.7 ˚C.

IR (KBr): 1713, 1692 cm-¹.

¹H NMR (500 MHz, CDCl3): δ = 4.33-4.29 (m, 2 H), 2.44-2.38 (m, 4 H), 2.04-2.00 (m, 5 H), 1.33 (t, J = 7.5 Hz, 3 H).

¹³C NMR (125 MHz, CDCl3): δ = 195.0, 166.8, 160.0, 133.2, 61.2, 36.8, 31.5, 22.0, 21.6, 14.1.

LRMS (EI, 70 eV): m/z (%) = 182 (M+, 24), 136 (76), 99 (100).

2-Acetyl-3-methylcyclopent-2-enone (2h) [7a]

Colorless oil.

IR (film): 1716, 1699 cm-¹.

¹H NMR (500 MHz, CDCl3): δ = 2.68 (t, J = 5.0 Hz, 2 H), 2.49-2.47 (m, 5 H), 2.41 (s, 3 H).

¹³C NMR (125 MHz, CDCl3): δ = 205.6, 197.4, 186.1, 138.7, 34.9, 33.0, 30.8, 19.7.

LRMS (EI, 70 eV): m/z (%) = 138 (M+, 84), 123 (100).

2-Benzoyl-3-methylcyclopent-2-enone (2i) [¹¹]

Colorless oil.

IR (film): 1699, 1653 cm-¹.

¹H NMR (500 MHz, CDCl3): δ = 7.81 (d, J = 8.5 Hz, 2 H), 7.59 (t, J = 7.5 Hz, 1 H), 7.46 (t, J = 7.5 Hz, 2 H), 2.76 (t, J = 5.0 Hz, 2 H), 2.60 (t, J = 5.0 Hz, 2 H), 2.16 (s, 3 H).

¹³C NMR (125 MHz, CDCl3): δ = 205.0, 193.3, 179.4, 141.3, 136.6, 133.8, 129.4, 128.5, 35.2, 32.6, 18.6.

LRMS (EI, 70 eV): m/z (%) = 200 (M+, 68), 199 (100).

2-Benzoyl-3-phenylcyclohex-2-enone (3j)

White solid; mp 110.3-111.4 ˚C.

IR (KBr): 1681, 1644 cm-¹.

¹H NMR (500 MHz, CDCl3): δ = 7.73 (d, J = 7.0 Hz, 2 H), 7.45-7.41 (m, 1 H), 7.30 (t, J = 7.5 Hz, 2 H), 7.26-7.24 (m, 2 H), 7.21-7.19 (m, 3 H), 2.87 (t, J = 6.0 Hz, 2 H), 2.64 (t, J = 6.0 Hz, 2 H), 2.31-2.26 (m, 2 H).

¹³C NMR (125 MHz, CDCl3): δ = 197.2, 196.6, 159.3, 138.5, 137.4, 136.7, 133.2, 129.3, 128.9, 128.4, 128.3, 127.0, 37.4, 31.7, 22.4.

LRMS (EI, 70 eV): m/z (%) = 276 (M+, 88), 105 (96), 77 (100).

HRMS (EI): m/z calcd for C19H16O2 (M+): 276.1150; found: 276.1153.

2-Benzoyl-3- p -tolylcyclohex-2-enone (3k)

White solid; mp 85.3-86.3 ˚C.

IR (KBr): 1681, 1653 cm-¹.

¹H NMR (500 MHz, CDCl3): δ = 7.74 (d, J = 7.5 Hz, 2 H), 7.45 (t, J = 7.5 Hz, 1 H),7.33 (t, J = 8.0 Hz, 2 H), 7.15 (d, J = 8.5 Hz, 2 H), 7.01 (d, J = 8.0 Hz, 2 H), 2.87 (t, J = 6.5 Hz, 2 H), 2.63 (t, J = 7.0 Hz, 2 H), 2.30-2.25 (m, 2 H), 2.23 (s, 3 H).

¹³C NMR (125 MHz, CDCl3): δ = 197.3, 196.9, 159.4, 139.6, 137.1, 136.8, 135.6, 133.2, 129.1, 129.0, 128.4, 127.1, 37.5, 31.9, 22.5, 21.2.

LRMS (EI, 70 eV): m/z (%) = 290 (M+, 73), 275 (93), 105 (100).

HRMS (EI): m/z calcd for C20H18O2 (M+): 290.1307; found: 290.1306.

2-Benzoyl-3- m -tolylcyclohex-2-enone (3l)

Colorless oil.

IR (film): 1682, 1649 cm-¹.

¹H NMR (500 MHz, CDCl3): δ = 7.73 (d, J = 8.0 Hz, 2 H), 7.46-7.42 (m, 1 H), 7.32 (t, J = 7.5 Hz, 2 H), 7.09-7.00 (m, 4 H), 2.87 (t, J = 6.0 Hz, 2 H), 2.64 (t, J = 6.0 Hz, 2 H), 2.30-2.25 (m, 2 H), 2.20 (s, 3 H).

¹³C NMR (125 MHz, CDCl3): δ = 197.2, 196.7, 159.6, 138.5, 138.1, 137.4, 136.9, 133.1, 130.0, 128.9, 128.3, 128.3, 127.7, 124.2, 37.5, 31.8, 22.5, 21.2.

LRMS (EI, 70 eV): m/z (%) = 290 (M+, 46), 119 (100).

HRMS (EI): m/z calcd for C20H18O2 (M+): 290.1307; found: 290.1303.

2-Benzoyl-3- o -tolylcyclohex-2-enone (3m)

Colorless oil.

IR (film): 1681, 1652 cm-¹.

¹H NMR (500 MHz, CDCl3): δ = 7.66 (t, J = 8.5 Hz, 2 H), 7.46-7.43 (m, 1 H), 7.32 (t, J = 8.0 Hz, 2 H), 7.09-7.03 (m, 2 H), 6.96 (t, J = 7.5 Hz, 1 H), 6.91 (d, J = 7.5 Hz, 1 H), 2.70-2.67 (m, 4 H), 2.32-2.27 (m, 2 H), 2.25 (s, 3 H).

¹³C NMR (125 MHz, CDCl3): δ = 197.1, 195.6, 161.5, 138.7, 137.9, 137.0, 133.7, 133.1, 130.3, 128.7, 128.4, 126.6, 125.5, 37.7, 31.9, 22.7, 19.6.

LRMS (EI, 70 eV): m/z (%) = 290 (M+, 34), 175 (25), 119 (100).

HRMS (EI): m/z calcd for C20H18O2 (M+): 290.1306; found: 290.1303.

2-Benzoyl-3-(2-methoxyphenyl)cyclohex-2-enone (3n)

Colorless oil.

IR (film): 1676, 1654 cm-¹.

¹H NMR (500 MHz, CDCl3): δ = 7.73 (d, J = 8.0 Hz, 2 H), 7.43-7.40 (m, 1 H), 7.28 (t, J = 7.5 Hz, 2 H), 7.16-7.12 (m, 1 H), 7.04-7.02 (m, 1 H), 6.78 (t, J = 7.5 Hz, 1 H), 6.68 (d, J = 8.5 Hz, 1 H), 3.66 (s, 3 H), 2.80 (t, J = 6.0 Hz, 2 H), 2.65 (t, J = 6.0 Hz, 2 H), 2.29-2.23 (m, 2 H).

¹³C NMR (125 MHz, CDCl3): δ = 197.2, 195.8, 159.8, 155.1, 138.1, 136.8, 132.9, 130.2, 128.8, 128.7, 128.0, 127.5, 120.4, 110.5, 55.0, 37.9, 31.3, 22.6.

LRMS (EI, 70 eV): m/z (%) = 306 (M+, 6), 275 (100).

HRMS (EI): m/z calcd for C20H18O3 (M+): 306.1256; found: 306.1251.

2-Benzoyl-3-(4-methoxyphenyl)cyclohex-2-enone (3o)

Colorless oil.

IR (film): 1679, 1654 cm-¹.

¹H NMR (500 MHz, CDCl3): δ = 7.75 (d, J = 8.5 Hz, 2 H), 7.45-7.42 (m, 1 H), 7.32 (t, J = 8.0 Hz, 2 H), 7.24-7.21 (m, 2 H), 6.73-6.71 (m, 2 H), 3.70 (s, 3 H), 2.87 (t, J = 6.5 Hz, 2 H), 2.62 (t, J = 6.5 Hz, 2 H), 2.29-2.23 (m, 2 H).

¹³C NMR (125 MHz, CDCl3): δ = 197.1, 197.0, 160.3, 158.8, 136.6, 136.4, 133.1, 128.8, 128.8, 128.3, 113.7, 55.0, 37.3, 31.5, 22.3.

LRMS (EI, 70 eV): m/z (%) = 306 (M+, 100), 250 (46), 105 (68).

HRMS (EI): m/z calcd for C20H18O3 (M+): 306.1256; found: 306.1258.

3-(4-Acetylphenyl)-2-benzoylcyclohex-2-enone (3p)

White solid; mp 130.6-131.8 ˚C.

IR (KBr): 1683, 1653, 1647 cm-¹.

¹H NMR (500 MHz, CDCl3): δ = 7.80 (d, J = 8.5 Hz, 2 H), 7.73 (d, J = 7.0 Hz, 2 H), 7.47 (t, J = 7.0 Hz, 1 H), 7.36-7.27 (m, 4 H), 2.87 (t, J = 5.0 Hz, 2 H), 2.67 (t, J = 6.0 Hz, 2 H), 2.51 (s, 3 H), 2.33-2.31 (m, 2 H).

¹³C NMR (125 MHz, CDCl3): δ = 197.3, 196.9, 196.2, 157.8, 143.1, 138.3, 137.2, 136.6, 133.6, 129.0, 128.6, 128.4, 127.3, 37.4, 31.6, 26.6, 22.5.

LRMS (EI, 70 eV): m/z (%) = 319 (M+, 73), 318 (100), 105 (95).

HRMS (EI): m/z calcd for C21H18O3 (M+): 318.1256; found: 318.1253.

2-Benzoyl-3-methylcyclohex-2-enone (3q) [¹²]

White solid; mp 71.0-72.3 ˚C.

IR (KBr): 1650, 1659, 1641 cm-¹.

¹H NMR (500 MHz, CDCl3): δ = 7.84 (t, J = 8.0 Hz, 2 H), 7.56 (t, J = 7.5 Hz, 1 H), 7.45 (t, J = 8.0 Hz, 2 H), 2.52-2.48 (m, 4 H), 2.15-2.10 (m, 2 H), 1.87 (s, 3 H).

¹³C NMR (125 MHz, CDCl3): δ = 197.1, 196.8, 159.8, 137.6, 136.7, 133.6, 129.0, 128.7, 37.2, 31.9, 22.0, 21.8.

LRMS (EI, 70 eV): m/z (%) = 214 (M+, 40), 213 (100).

Methyl 2-(4-Acetylphenyl)-6-oxocyclohex-1-enecarboxylate (3r) [7a]

Colorless oil.

IR (film): 1745, 1721, 1716 cm-¹.

¹H NMR (500 MHz, CDCl3): δ = 7.97 (d, J = 8.5 Hz, 2 H), 7.44 (d, J = 8.0 Hz, 2 H), 3.60 (s, 3 H), 2.76 (t, J = 6.0 Hz, 2 H), 2.63 (s, 3 H), 2.58 (t, J = 7.2 Hz, 2 H), 2.22-2.17 (m, 2 H).

¹³C NMR (125 MHz, CDCl3): δ = 197.2, 194.9, 166.7, 158.2, 143.3, 137.6, 129.8, 128.6, 128.8, 52.2, 36.9, 31.1, 26.6, 22.1.

LRMS (EI, 70 eV): m/z (%) = 272 (M+, 100).

Acknowledgment

The authors thank the Scientific Research Fund of Hunan Provincial Education Department (No. 08A037) and National Natural Science Foundation of China (No. 20872112) for financial support.

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Scheme 1 Conia-ene reactions of alkynic dicarbonyl compounds

Scheme 2 A possible mechanism