Stereoselective Synthesis of Homofarnesenes: Establishment of an Efficient Method for the Stereoselective Preparation of Acyclic Tetrasubstituted Olefins.

A new method for the stereoselective synthesis of tetrasubstituted olefins is described. β-Ketophosphonates are alkylated via conventional methods, and a Grignard reagent is used to diastereoselectively add to the carbonyl group of the resulting intermediates. The elimination of hydroxyl phosphonates yielded the desired tetrasubstituted olefins in a stereoselective manner. Thus, homofarnesenes of fire ant trail pheromones have been synthesized efficiently using this strategy.

Introduction

Tetrasubstituted olefins are present in certain natural products, for instance, the trail pheromones of fire ant Solenopsis invicta Buren isolated in 1959,[1] two structures of which were established in 1981[2] to be homofarnesenes (Figure a,b). However, Alvarez[3] et al. synthesized these two homofarnesenes in 1987 from geranyl bromide in five steps as a 1:2 (ZE/EE) mixture using Wittig olefination as a key reaction. Chou and Lee[4] reported in 1990 the synthesis in four steps using 3-sulfolene as a key building block as a 1:1.1 mixture. However, high performance liquid chromatography was used to separate components of the reaction mixture. Mateos[5] et al. stereoselectively synthesized homofarnesene 1a in two steps using Cu/Pd-catalyzed allylboration as a key reaction.

Figure 1

Structures of homofarnesenes 1a and 1b.

The stereoselective synthesis of tetrasubstituted olefins has been a challenge to the synthetic community.[6] Conventional methods of the preparation of tetrasubstituted olefins include the Wittig olefination[7] and the Horner–Wadsworth–Emmons reaction.[8] Less often used reactions are decomposition of diazo compounds[9] or extrusion of sulfur dioxide.[10] However, these methods are usually not stereoselective. Inspired by the findings reported by Reichwein and Pagenkopf[11] that β-hydroxy phosphonates produce olefins stereospecifically upon treatment with a dehydrating agent, we envisioned that the construction of tetrasubstituted olefins may also be feasible if the formation of the relevant stereochemical centers of the β-hydroxy phosphonate is under control. In this article, we report a stereoselective synthesis of homofarnesenes (1a and 1b) through the establishment of an efficient method for the preparation of tetrasubstituted acyclic olefins.

Results and Discussion

Our synthetic strategy of these two homofarnesenes is outlined in Scheme . Homofarnesene 1a may be synthesized from aldehyde 5 through a Wittig reaction. Aldehyde 5 may be obtained from the elimination of β-hydroxy phosphonate 4,[10] which may be produced from α-ketophosphonate 3 through a diastereoselective addition of a methyl Grignard reagent, whereas the formation of 3 may be accomplished via a two-step continuous alkylation of 2.

Scheme 1

Retrosynthetic Analysis of Homofarnesenes

Homofarnesene 1b may be synthesized from the elimination of β-hydroxy phosphonate 7,[11] which may be obtained from phosphonate 6 after a diastereoselective addition of the vinyl Grignard reagent. The preparation of phosphonate 6 is straightforward.

Synthesis of homofarnesene 1a is described in Scheme . Protection of the hydroxyl group of methyl glycolate with tert-butyldiphenylchlorosilane/Et3N/dimethylaminopyridine yielded ester 8.[12] Treatment of dimethyl methylphosphonate with n-BuLi followed by addition of ester 8 afforded 9 in 74% yield.[13] Alkylation of phosphonate 9 with geranyl chloride under the action of NaH/NaI/N,N-dimethylformamide (DMF) furnished 11 in 85% yield, which, after reaction with CH3I/NaH, produced 12 in 95% yield. When treated with methyl magnesium chloride, phosphonate 12 gave 13 in 94% yield.

Scheme 2

Synthesis of Homofarnesene 1a

Removal of the tert-butyldiphenylsilyl group of 13 with 1.1 equiv of tetrabutylammonium fluoride in tetrahydrofuran provided 14 in 70% yield. β-Hydroxy phosphonate 14 was hydrolyzed with NaOH in H2O/MeOH and then treated with dichloromethane (DIC) in CHCl3 to form alcohol 15 in 70% overall yield.[11] After a Dess–Martin oxidation, intermediate 5 was obtained in 86% yield. Finally, the desired homofarnesene 1a(3) was obtained when treated with the methylene Wittig reagent furnished in 81% yield (ZE/EE = 7.5:1).

The diastereoselectivity is presumably governed by a chelation control as is shown in Scheme .

Scheme 3

Possible Mechanism for the Origin of the Diastereoselectivity[14,15]

The synthesis of homofarnesene 1b is described in Scheme . Phosphonate 16 was alkylated by geranyl chloride under the action of NaH/DMF (58%). Phosphonate 17 was treated with vinylmagnesium bromide to give 7 in 88% yield. As shown in Scheme , chelation is also operative in this case. β-Hydroxy phosphonate 7 was hydrolyzed by NaOH in H2O/MeOH and then treated with DIC in CHCl3 to form the desired homofarnesene 1b (EE/ZE = 8:1) in 73% overall yield.

Scheme 4

Synthesis of Homofarnesene 1b

The geometric configuration of the two homofarnesenes was established on the basis of the nuclear Overhauser effect spectroscopy spectrum (see Supporting Information). In 1a, Ha is in close proximity to Hb and they produced a NOE, but it was not observed in 1b. In the structure of 1b, Ha is in close proximity to Hc and a NOE was observed (Scheme ). The ratio of the two homofarnesenes was determined via the integration of the protons.

Scheme 5

Nuclear Overhauser Effect (NOE) of Homofarnesenes 1a and 1b

Conclusions

In summary, two homofarnesenes have been synthesized in a highly stereoselective manner. In addition, during the synthesis, a very useful method for the highly stereoselective construction of acyclic tetrasubstituted olefins has been developed.

Experimental Section

General Experimental Methods

Reagents were obtained from commercial suppliers and used without further purification unless otherwise noted. NMR spectra were recorded on a Bruker Avance spectrometer at 400 or 500 MHz (1H NMR) and 125 MHz (13C NMR). CDCl3 and DMSO-d6 were used as solvents. 1H NMR spectra were referenced internally to the residual proton resonance in CDCl3 (δ 7.26 ppm) and DMSO-d6 (δ 2.54 ppm). Chemical shifts (δ) are reported as parts per million (ppm). 13C NMR spectra were referenced to CDCl3 (δ 77.16 ppm) and DMSO-d6 (δ 40 ppm). Spin multiplets are given as s (singlet), d (doublet), t (triplet), q (quartet), and m (multiplet). Coupling constants (J) are given in hertz (Hz). High-resolution mass spectra were recorded on an Agilent 6210 LC/MSD TOF spectrometer. Column chromatography was performed on silica gel (200–300 mesh).

Dimethyl (E)-(3,6,10-Trimethyl-2-oxoundeca-5,9-dien-3-yl)phosphonate (17)

A solution of dimethyl (3-oxobutan-2-yl)phosphonate (16) (10 g, 55.5 mmol) in 330 mL of anhydrous N,N-dimethylformamide was cooled to 0 °C, sodium hydride powder (60%, 2.44 g, 61 mmol) was quickly added to the flask, and the reaction mixture was stirred for 1 h before a solution of (E)-1-chloro-3,7-dimethylocta-2,6-diene (10) (19.2 g, 111 mmol) in 30 mL of anhydrous N,N-dimethylformamide was slowly added via a syringe. The reaction mixture was stirred for 1.5 h, and 500 mL of saturated ammonium chloride was added. The aqueous phase was extracted with ether (4 × 200 mL), and the extracts were combined with the organic phase. After the ether solution was dried over MgSO4, the drying agent was filtered off and the solution was concentrated in vacuo. Flash chromatography of the residue on silica gel (250 g, hexanes/EtOAc = 1:1 v/v) afforded phosphonate 17 (10.2 g, 32.3 mmol, 58% yield) as a yellowish oil. 1H NMR (500 MHz, CDCl3): δ (ppm) 5.01 (ddd, J = 8.6, 4.8, 1.5 Hz, 1H), 4.88 (td, J = 7.1, 6.7, 3.4 Hz, 1H), 3.78 (dd, J = 10.7, 6.2 Hz, 6H), 2.87 (ddd, J = 16.1, 10.4, 6.6 Hz, 1H), 2.46 (dt, J = 15.6, 8.3 Hz, 1H), 2.29 (s, 3H), 2.03 (t, J = 7.1 Hz, 2H), 1.99 (d, J = 7.0 Hz, 2H), 1.66 (d, J = 1.6 Hz, 3H), 1.61 (d, J = 1.5 Hz, 3H), 1.57 (s, 3H), 1.35 (d, J = 16.6 Hz, 3H). 13C NMR (125 MHz, CDCl3) δ (ppm) 206.01, 139.38, 131.44, 123.89, 117.45, 55.35, 54.33, 53.42, 39.75, 31.64, 27.94, 26.34, 25.55, 17.56, 16.34. ESI-HRMS (m/z) [M + H]+ calculated for C16H30O4P 317.1803; found 317.1807.

Dimethyl (E)-(3-Hydroxy-3,4,7,11-tetramethyldodeca-1,6,10-trien-4-yl)phosphonate (7)

A solution of dimethyl (E)-(3,6,10-trimethyl-2-oxoundeca-5,9-dien-3-yl)phosphonate (17) (1.0 g, 3.16 mmol) in 32 mL of anhydrous tetrahydrofuran was cooled to −20 °C before a solution of vinylmagnesium bromide (6.32 mmol) in 6.32 mL of anhydrous tetrahydrofuran was slowly added via a syringe. The reaction mixture was stirred for 1 h, and 50 mL of saturated ammonium chloride was added. The organic phase was separated, and the aqueous phase was extracted with ethyl acetate (3 × 20 mL). The extracts were combined with the organic phase. After the ethyl acetate solution was dried over MgSO4, the drying agent was filtered off and the solution was concentrated in vacuo. Flash chromatography of the residue on silica gel (20 g, hexanes/EtOAc = 1:1 v/v) afforded phosphonate 7 (0.96 g, 2.8 mmol, 88% yield, d.r. = 9:1) as a yellowish oil. 1H NMR (500 MHz, CDCl3) δ (ppm) 5.97 (dd, J = 17.1, 10.8 Hz, 1H), 5.33 (dd, J = 17.2, 1.8 Hz, 1H), 5.25 (t, J = 7.6 Hz, 1H), 5.15 (dt, J = 10.8, 1.6 Hz, 1H), 5.11–5.05 (m, 1H), 4.49 (s, 1H), 3.74 (d, J = 10.5 Hz, 6H), 2.43–2.22 (m, 2H), 2.07 (q, J = 7.4 Hz, 2H), 2.03–1.99 (m, 2H), 1.67 (s, 3H), 1.59 (s, 3H), 1.58 (s, 3H), 1.41 (s, 3H), 1.18 (s, 3H). 13C NMR (125 MHz, CDCl3) δ (ppm) 141.04, 137.06, 131.35, 124.24, 120.21, 114.01, 75.99, 53.37, 51.91, 48.31, 47.27, 40.05, 31.42, 26.62, 25.65, 24.92, 17.62, 16.16, 15.41. ESI-HRMS (m/z) [M – OH]+ calculated for C18H32O3P 327.2084; found 327.2057.

(3E,6E)-3,4,7,11-Tetramethyldodeca-1,3,6,10-tetraene (1b)

A solution of dimethyl (E)-(3-hydroxy-3,4,7,11-tetramethyldodeca-1,6,10-trien-4-yl)phosphonate (7) (200 mg, 0.58 mmol) in 2.7 mL of methanol was stirred at room temperature before 2.7 mL of aqueous sodium hydroxide (4 mol/L, 10.8 mmol) was slowly added. The solution was stirred for 16 h. Hydrochloric acid (2 mol/L) was slowly added to the reaction mixture to adjust the solution to pH 2. The aqueous phase was extracted with dichloromethane (4 × 10 mL), and the extracts were combined with the organic phase. After the dichloromethane solution was dried over MgSO4, the drying agent was filtered off and the solution was concentrated in vacuo. The residue was dissolved in 2.7 mL of chloroform, and the reaction mixture was stirred at room temperature before N,N-diisopropyl carbodiimide (0.18 mL, 1.16 mmol) was added. The reaction mixture was stirred for 4 h and then concentrated in vacuo. Flash chromatography on silica gel (3 g, n-pentane) afforded homofarnesene 1b (EE/ZE = 8:1) (93 mg, 0.42 mmol, 73%) as a colorless oil. 1H NMR (500 MHz, DMSO) δ (ppm) 6.83 (dd, J = 17.2, 10.9 Hz, 1H), 5.15 (dd, J = 17.2, 1.7 Hz, 1H), 5.12–4.97 (m, 3H), 2.91 (d, J = 7.1 Hz, 0.23H), 2.85 (d, J = 7.2 Hz, 1.84H), 2.08 (q, J = 7.3 Hz, 2H), 2.01 (t, J = 7.4 Hz, 2H), 1.78 (s, 3H), 1.76 (s, 3H), 1.68 (s, 3H), 1.66 (s, 3H), 1.59 (s, 3H). 13C NMR (125 MHz, DMSO) δ (ppm) 136.30, 135.53, 134.99, 131.06, 126.57, 124.45, 121.96, 112.22, 39.61, 34.21, 26.52, 25.87, 18.31, 17.92, 16.30, 13.58. ESI-HRMS (m/z) [M + H]+ calculated for C16H27 219.2035; found 219.2047.

Dimethyl (E)-(1-((tert-Butyldiphenylsilyl)oxy)-6,10-dimethyl-2-oxoundeca-5,9-dien-3-yl)phosphonate (11)

A solution of dimethyl (3-((tert-butyldiphenylsilyl)oxy)-2-oxopropyl)phosphonate (9) (6.72 g, 16 mmol) in 65 mL of anhydrous N,N-dimethylformamide was cooled to 0 °C, and sodium hydride powder (60%, 0.77 g, 19.2 mmol) was quickly added to the reaction mixture. After the mixture was stirred for 1 h, a solution of (E)-1-chloro-3,7-dimethylocta-2,6-diene (10) (3.6 g, 20.8 mmol) in 15 mL of anhydrous N,N-dimethylformamide was slowly added via a syringe and sodiumiodide powder (1.2 g, 8 mmol) was quickly added. The reaction mixture was stirred for 10 min before it was warmed up to room temperature and stirred for 1.5 h. Saturated ammonium chloride (200 mL) was added. The aqueous phase was extracted with ether (4 × 50 mL), and the extracts were combined with the organic phase. After the ether solution was dried over MgSO4, the drying agent was filtered off and the solution was concentrated in vacuo. Flash chromatography of the residue on silica gel (150 g, hexanes/EtOAc = 2:1 v/v) afforded phosphonate 11 (7.6 g, 13.6 mmol, 85% yield) as a colorless oil. 1H NMR (400 MHz, CDCl3) δ (ppm) 7.76–7.57 (m, 4H), 7.52–7.33 (m, 6H), 5.13–4.99 (m, 1H), 4.94 (t, J = 7.4 Hz, 1H), 4.42–4.18 (m, 2H), 3.72 (dd, J = 14.8, 11.0 Hz, 6H), 3.61 (ddd, J = 23.0, 10.8, 3.8 Hz, 1H), 2.70 (dq, J = 18.8, 9.1 Hz, 1H), 2.49 (tdd, J = 14.7, 8.9, 4.5 Hz, 1H), 1.99 (q, J = 7.5 Hz, 2H), 1.91 (dd, J = 9.2, 6.2 Hz, 2H), 1.70–1.63 (m, 3H), 1.59 (s, 3H), 1.56 (s, 3H), 1.09 (s, 9H). 13C NMR (125 MHz, CDCl3) δ (ppm) 204.12, 139.57, 135.59, 133.20, 131.47, 129.69, 127.64, 123.97, 117.45, 117.34, 77.20, 67.94, 53.53, 53.47, 53.37, 39.78, 31.41, 31.38, 26.71, 26.50, 25.65, 19.35, 17.65, 16.18, 15.74, 15.70. ESI-HRMS (m/z) [M + H]+ calculated for C31H46O5PSi 557.2774; found 557.2793.

Dimethyl (E)-(1-((tert-Butyldiphenylsilyl)oxy)-3,6,10-trimethyl-2-oxoundeca-5,9-dien-3-yl)phosphonate (12)

A solution of dimethyl (E)-(1-((tert-butyldiphenylsilyl)oxy)-6,10-dimethyl-2-oxoundeca-5,9-dien-3-yl)phosphonate (11) (4.42 g, 7.95 mmol) in 35 mL of anhydrous N,N-dimethylformamide was cooled to 0 °C, and sodium hydride (60%, 334 mg, 8.35 mmol) was quickly added to the reaction mixture. The mixture was stirred for 1 h before a solution of iodomethane (0.6 mL, 9.54 mmol) in anhydrous N,N-dimethylformamide (5 mL) was slowly added via a syringe. The reaction mixture was stirred for 1.5 h before 100 mL of saturated ammonium chloride was added. The aqueous phase was extracted with ether (4 × 30 mL), and the extracts were combined with the organic phase. After the ether solution was dried over MgSO4, the drying agent was filtered off and the solution was concentrated in vacuo. Flash chromatography of the residue on silica gel (100 g, hexanes/EtOAc = 2:1 v/v) afforded phosphonate 12 (4.33 g, 7.6 mmol, 95% yield) as a colorless oil. 1H NMR (400 MHz, CDCl3) δ (ppm) 7.71–7.61 (m, 4H), 7.46–7.35 (m, 6H), 5.07–4.99 (m, 1H), 4.94 (t, J = 7.4 Hz, 1H), 4.39–4.18 (m, 2H), 3.72 (dd, J = 14.7, 11.0 Hz, 6H), 3.66–3.55 (m, 1H), 2.70 (dq, J = 18.8, 9.1 Hz, 1H), 2.49 (tdd, J = 14.7, 8.9, 4.5 Hz, 1H), 1.99 (d, J = 7.8 Hz, 2H), 1.91 (dd, J = 9.2, 6.2 Hz, 2H), 1.67–1.64 (m, 3H), 1.59 (s, 3H), 1.56 (s, 3H), 1.09 (s, 9H). 13C NMR (125 MHz, CDCl3) δ (ppm) 204.12, 139.57, 135.59, 133.19, 131.46, 129.69, 127.63, 123.97, 117.40, 67.94, 53.43, 39.78, 31.49, 26.71, 26.50, 25.65, 19.35, 17.65, 16.18, 15.72. ESI-HRMS (m/z) [M + H]+ calculated for C32H48O5PSi 571.2930; found 571.2933.

Dimethyl (E)-(1-((tert-Butyldiphenylsilyl)oxy)-2-hydroxy-2,3,6,10-tetramethylundeca-5,9-dien-3-yl)phosphonate (13)

A solution of dimethyl (E)-(1-((tert-butyldiphenylsilyl)oxy)-3,6,10-trimethyl-2-oxoundeca-5,9-dien-3-yl)phosphonate (12) (4.1 g, 7.2 mmol) in anhydrous tetrahydrofuran (70 mL) was cooled to −20 °C before a solution of methylmagnesium chloride (14.4 mmol) in anhydrous tetrahydrofuran (4.8 mL) was slowly added via a syringe. The reaction mixture was stirred for 30 min, and saturated ammonium chloride (50 mL) was added. The organic phase was separated, the aqueous phase was extracted with ethyl acetate (3 × 30 mL), and the extracts were combined with the organic phase. After the ethyl acetate solution was dried over MgSO4, the drying agent was filtered off and the solution was concentrated in vacuo. Flash chromatography of the residue on silica gel (100 g, hexanes/EtOAc = 1:1 v/v) afforded phosphonate 13 (4.0 g, 6.8 mmol, 94% yield) as a yellowish oil. 1H NMR (400 MHz, CDCl3) δ (ppm) 7.68 (dt, J = 7.9, 1.8 Hz, 4H), 7.47–7.33 (m, 6H), 5.26 (t, J = 7.8 Hz, 1H), 5.09 (t, J = 6.6 Hz, 1H), 3.98–3.74 (m, 3H), 3.72–3.51 (m, 6H), 2.51 (ddt, J = 23.3, 11.9, 6.3 Hz, 1H), 2.33 (ddd, J = 33.3, 16.7, 9.1 Hz, 1H), 2.06 (q, J = 7.2 Hz, 2H), 2.02–1.91 (m, 2H), 1.67 (s, 3H), 1.60 (s, 3H), 1.55 (d, J = 7.2 Hz, 3H), 1.40 (m, 3H), 1.29–1.20 (m, 3H), 1.09 (d, J = 2.4 Hz, 9H). 13C NMR (125 MHz, CDCl3) δ (ppm) 136.67, 135.71, 135.63, 133.45, 133.12, 131.22, 129.63, 129.60, 127.62, 124.27, 120.40, 120.35, 77.20, 76.13, 69.34, 53.20, 53.14, 51.72, 46.58, 40.01, 31.20, 26.90, 26.58, 25.66, 20.82, 20.76, 19.31, 17.61, 16.09, 15.69. ESI-HRMS (m/z) [M + H]+ calculated for C33H52O5PSi 587.3243; found 587.3257.

Dimethyl (E)-(1,2-Dihydroxy-2,3,6,10-tetramethylun-deca-5,9-dien-3-yl)phosphonate (14)

A solution of tetrabutylammonium fluoride (2.6 mmol) in anhydrous tetrahydrofuran (2.6 mL) was slowly added to the solution of dimethyl (E)-(1-((tert-butyldiphenylsilyl)oxy)-2-hydroxy-2,3,6,10-tetramethylundeca-5,9-dien-3-yl)phosphonate (13) (1.5 g, 2.56 mmol) in anhydrous tetrahydrofuran (15 mL). The solution was stirred at room temperature for 15 min before saturated ammonium chloride (10 mL) was added. The organic phase was separated, and the aqueous phase was extracted with ethyl acetate (3 × 10 mL). The extracts were combined with the organic phase. After the ethyl acetate solution was dried over MgSO4, the drying agent was filtered off and the solution was concentrated in vacuo. Flash chromatography of the residue on silica gel (30 g, hexanes/EtOAc = 2:1 v/v) afforded phosphonate 14 (620 mg, 1.78 mmol, 70% yield, d.r. = 7.4:1) as a colorless oil. 1H NMR (500 MHz, CDCl3) δ (ppm) 5.25 (td, J = 7.1, 3.9 Hz, 1H), 5.08 (tq, J = 5.5, 1.5 Hz, 1H), 4.06 (s, 1H), 3.93–3.83 (m, 1H), 3.76 (dd, J = 18.4, 10.6 Hz, 6H), 3.49 (d, J = 11.5 Hz, 1H), 3.20 (s, 1H), 2.43–2.19 (m, 2H), 2.08 (q, J = 7.2, 6.7 Hz, 2H), 2.05–1.97 (m, 2H), 1.70–1.66 (m, 3H), 1.64–1.58 (m, 6H), 1.28 (d, J = 17.4 Hz, 3H), 1.23 (s, 3H). 13C NMR (125 MHz, CDCl3) δ (ppm) 137.47, 131.54, 124.10, 119.67, 76.41, 67.46, 53.56, 52.30, 40.06, 31.39, 26.52, 25.69, 20.15, 17.65, 16.17, 15.41. ESI-HRMS (m/z) [M + H]+ calculated for C17H34O5P 249.2066; found 249.2057.

(2Z,5E)-2,3,6,10-Tetramethylundeca-2,5,9-trien-1-ol (15)

A solution of dimethyl (E)-(1,2-dihydroxy-2,3,6,10-tetramethylundeca-5,9-dien-3-yl)phosphonate (14) (600 mg, 1.72 mmol) in methanol (8 mL) was stirred at room temperature before aqueous sodium hydroxide (4 mol/L, 8 mL, 32 mmol) was slowly added. The mixture was stirred for 16 h before hydrochloric acid (2 mol/L) was slowly added to the reaction mixture to adjust the solution to pH 2. The aqueous phase was extracted with dichloromethane (6 × 20 mL), and the extracts were combined with the organic phase. After the dichloromethane solution was dried over MgSO4, the drying agent was filtered off and the solution was concentrated in vacuo. The residue was dissolved in chloroform (8 mL), and the solution was stirred at room temperature before N,N-diisopropylcarbodiimide (0.53 mL, 3.44 mmol) was added. The reaction mixture was stirred for 4 h before it was concentrated in vacuo. Flash chromatography of the residue on silica gel (20 g, hexanes/EtOAc = 1:1 to 1:5 v/v) afforded alcohol 15 (202 mg, 0.91 mmol, 53% yield) as a colorless oil. 1H NMR (400 MHz, CDCl3) δ (ppm) 5.12–4.99 (m, 2H), 4.13 (s, 2H), 2.81 (d, J = 7.1 Hz, 2H), 2.07 (q, J = 7.4 Hz, 2H), 2.03–1.95 (m, 2H), 1.76 (s, 3H), 1.70–1.64 (m, 9H), 1.59 (s, 3H). 13C NMR (125 MHz, CDCl3) δ (ppm) 135.59, 133.14, 131.43, 127.87, 124.20, 122.79, 63.80, 39.72, 32.73, 26.61, 25.68, 18.80, 17.67, 16.84, 16.08. ESI-HRMS (m/z) [M – OH]+ calculated for C15H25 205.1951; found 205.1934.

(2Z,5E)-2,3,6,10-Tetramethylundeca-2,5,9-trienal (5)

A solution of (2Z,5E)-2,3,6,10-tetramethylundeca-2,5,9-trien-1-ol (15) (100 mg, 0.45 mmol) in anhydrous dichloromethane (5 mL) was stirred at room temperature before Dess–Martin periodinane (380 mg, 0.9 mmol) was added. After 20 min, saturated sodium bicarbonate (5 mL) and saturated sodium thiosulfate (5 mL) were added. The organic phase was separated, and the aqueous phase was extracted with dichloromethane (3 × 10 mL). The extracts were combined with the organic phase. After the dichloromethane solution was dried over MgSO4, the drying agent was filtered off and the solution was concentrated in vacuo. Flash chromatography of the residue on silica gel (3 g, hexanes/EtOAc = 10:1) afforded aldehyde 5 (85 mg, 0.386 mmol, 86% yield) as a colorless oil. 1H NMR (500 MHz, CDCl3) δ (ppm) 10.14 (d, J = 4.0 Hz, 1H), 5.06 (q, J = 6.6 Hz, 2H), 3.29 (d, J = 7.1 Hz, 1.77H), 2.97 (d, J = 7.2 Hz, 0.27H), 2.08 (q, J = 7.3 Hz, 2H), 2.02 (q, J = 7.5 Hz, 2H), 1.93 (s, 3H), 1.74 (s, 3H), 1.69 (s, 3H), 1.67 (s, 3H), 1.59 (s, 3H). 13C NMR (125 MHz, CDCl3) δ (ppm) 190.86, 158.07, 137.57, 132.17, 131.64, 123.90, 120.87, 39.64, 31.57, 26.48, 25.68, 21.53, 17.67, 16.30, 10.94. ESI-HRMS (m/z) [M + H]+ calculated for C15H25O 221.1827; found 221.1814.

(3Z,6E)-3,4,7,11-Tetramethyldodeca-1,3,6,10-tetraene (1a)

A solution of n-BuLi in anhydrous hexane (0.5 mL, 0.8 mmol) was added to a solution of methyltriphenylphosphonium bromide in anhydrous tetrahydrofuran (2 mL), and the resulting solution was cooled to −15 °C and stirred for 5 min before it was warmed up to room temperature and stirred for 1 h. A solution of (2Z,5E)-2,3,6,10-tetramethylundeca-2,5,9-trienal (5) (67 mg, 0.3 mmol) in anhydrous tetrahydrofuran (1 mL) was slowly added via a syringe, and the reaction mixture was heated under reflux. After 1.5 h, saturated ammonium chloride (5 mL) was added. The organic phase was separated, and the aqueous phase was extracted with n-pentane (3 × 5 mL). The extracts were combined with the organic phase. After the n-pentane solution was dried over MgSO4, the drying agent was filtered off and the solution was concentrated in vacuo. Flash chromatography of the residue on silica gel (3 g, n-pentane) afforded homofarnesene 1a (ZE/EE = 7.5:1) (53 mg, 0.243 mmol, 81% yield) as a colorless oil. 1H NMR (500 MHz, DMSO) δ (ppm) 6.94–6.79 (m, 1H), 5.14 (dd, J = 17.3, 1.6 Hz, 1H), 5.11–5.05 (m, 1H), 5.05–4.96 (m, 2H), 2.90 (d, J = 7.2 Hz, 1.79H), 2.84 (d, J = 7.2 Hz, 0.24H), 2.06 (q, J = 7.4 Hz, 2H), 1.99 (q, J = 7.4 Hz, 2H), 1.75 (s, 3H), 1.74 (s, 3H), 1.67 (s, 3H), 1.65 (s, 3H), 1.58 (s, 3H). 13C NMR (125 MHz, DMSO) δ (ppm) 135.82, 135.52, 135.22, 131.20, 126.72, 124.54, 122.75, 112.27, 39.70, 32.66, 26.60, 26.00, 20.00, 18.05, 16.40, 14.11. ESI-HRMS (m/z) [M + H]+ calculated for C16H27 219.2035; found 219.2047.