A Facile Asymmetric Synthesis of (S)-14-Methyl-1-Octadecene, the Sex Pheromone of the Peach Leafminer Moth

An asymmetric synthesis of 14-methyl-1-octadecene, the sex pheromone of the peach leafminer moth has been achieved. The target molecule was synthesized in six linear steps and in 30.3% overall yield from commercially available hexanoyl chloride, (S)-4-benzyloxazolidin-2-one and 1,9-nonanediol. The hexanoyl chloride was connected with (S)-4-benzyloxazolidin-2-one, and with the induction of the chiral oxazolidinone auxiliary, after chiral methylation, LAH reduction and then tosylation gave the chiral key intermediate 5 in high stereoselectivity. 1,9-Nonanediol, was selectively brominated, THP protected and subjected to Li2CuCl4-mediated C-C coupling to afford a C12 intermediate. The target molecule, (S)-14-methyl-1-octadecene, was obtained after the two parts were subjected to a second Li2CuCl4-mediated C-C coupling. Our synthetic approach represents the first time a substrate-control asymmetric synthesis of (S)-14-methyl-1-octadecene has been reported.


Introduction
Nowadays, there is a sharp conflict between intense agricultural production and environmental issues. We all are facing a big challenge caused by the detrimental effects resulting from traditional agricultural production [1], therefore, there is an urgent need to develop more green production modes, such as the use of environmentally benign, low-dosage sex pheromones to control pests [2,3]. This mode plays an important role in so-called Integrated Pest Management (IPM). The peach leafminer moth, Lyonetia clerkella, is one of the most destructive pests in peach orchards in East Asia. It causes defoliation when the leaves are infested by only a few larvae of this insect and Chinese orchardists use traditional highly toxic pesticides to control it. We envisaged that an ideal way to control it could be by using its sex pheromone to disrupt its mating process and to trap it at a suitable time.
The sex pheromone of peach leafminer was first identified as (S)-14-methyl-1-octadecene (1, Figure 1) by Sugie et al. [4] and syntheses of 1 in racemic form have been achieved by two teams [5,6]. Recently, Ando disclosed an informal synthesis of (±)-1 through a hard-to-get intermediate [7]. The asymmetric syntheses of 1 were also reported by Mori [8] and Kharisov [9], who both used expensive chiral starting materials. However, the existing synthetic pathways have some drawbacks such as requiring too many chemical operations or lack of cost-effectiveness, so the development of a more economical synthesis of 1 in consideration of applying this kind of chemical in the pesticide industry is highly desirable. Our research group is interested in finding new agrochemicals and lowering the cost of existing lead compounds or pesticides [3,10]. We are interested in an efficient and convenient synthesis of sex pheromones through more economical approaches. Herein, we wish to report a synthesis of (S)-14-methyl-1-octadecene (1) from inexpensive starting materials by a substrate-induction strategy.

Results and Discussion
The retrosynthesis of 1 is shown in Scheme 1. First, the molecule was disconnected into a chiral C 7 synthon A1 and a C 12 moiety. Then the C 12 moiety in turn could be disconnected into two parts to be joined through a Li 2 CuCl 4 mediated C-C coupling protocol, a C 9 subunit A2 and an allyl bromide which is highly active in S N 2 reactions and helpful to apply the strategy.
As shown in Scheme 2, our synthesis commenced with (S)-4-benzyloxazolidin-2-one, which is cheap and can be recycled in industry. After deprotonation, it reacted with hexanoyl chloride to give 2 in 70% yield. Compound 2 was deprotonated at −78 °C in the presence of a strong base such as LDA, then it was stereoselectively methylated to afford 3 [11][12][13]. Because of the steric effect of chiral benzyl group in the oxazolidine ring, the methyl groups was directed to the -face in d.r. = 1:50 and 78% yield. Then the chiral oxazolidine auxiliary was smoothly removed by LAH reduction at −10 °C. However, in attempting to purify compound 4 in the laboratory, this was found to be very laborious and less effective, so the crude 4 was directly treated with TsCl in pyridine, thus the key intermediate 5 was obtained in 55% yield in two steps. 9-Bromononan-1-ol was prepared in large quantities from nonane-1,9-diol through selective bromination of the α,ω-diol by a known method reported by Kang et al. [14]. The hydroxyl group of 7 was protected as its tetrahydropyranyl ether in 92% yield by 3,4-dihydro-2H-pyran under acidic conditions. Compound 8 can be converted into a Grignard reagent, and then this was coupled with allyl bromide under Li 2 CuCl 4 catalysis to form 9 in 66% yield [15,16]. Afterwards, the THP group was removed in methanol in the presence of 4-methyl-benzenesulfonic acid [17,18]. Compound 11 was produced in 90% yield after 10 was subjected to a Corey-Fuji protocol. With compound 11 in hand, it was converted into the corresponding Grignard reagent and coupled with 5 in the presence of Li 2 CuCl 4 to afford (S)-1 in 65% yield. Thus, the asymmetric total synthesis of (S)-1 in an overall yield of 30.3% was achieved in six linear steps. Compared with other asymmetric synthesis of the target, our synthesis is apparently more cost-effective, even if the chiral oxazolidinone auxiliary were not recycled.

General
The 1 H-NMR and 13 C-NMR data were recorded in CDCl 3 solution with Bruker NMR spectrometers (DRX 500, AM 300) if not noted otherwise. The chemical shifts are measured relative to TMS  = 0) or chloroform ( = 7.26) and the coupling J is expressed in Hertz. Mass spectra were recorded on a Thermo Scientific TSQ Quantum Access MAX mass spectrometer (ESI, positive or negative). Standard flash chromatography was employed to purify the crude reaction mixture using 200-300 mesh silica gel (Tsingdao Ocean Company, Tsingdao, China) under a positive nitrogen pressure. Tetrahydrofuran (THF) and diethyl ether were freshly distilled from lithium aluminium hydride under an argon atmosphere. Dichloromethane, hexane and toluene were freshly distilled from calcium hydride under argon. (2). At −78 °C, to a solution of (S)-4-benzyloxazolidin-2-one (5.32 g, 0.03 mol) in anhydrous THF (50 mL) was added BuLi (0.036 mol), then freshly distilled hexanoyl chloride (4.63 g, 0.033 mol) in THF (25 mL) was added after half an hour. The mixture was gradually warmed to 25 °C and maintained for 12 h. After completion, the reaction was quenched by addition of saturated aqueous NH 4 Cl, and the volatiles were evaporated. The residue was extracted with ethyl acetate (100 mL × 3), and the combined organic layers were washed with dilute NaOH and brine, and dried over anhydrous MgSO 4 . Then the extract was evaporated, and purified by column chromatography (15-25% gradient, EtOAc-hexane) to give 2 as a yellowish oil (5.78 g, 70%).  (5). To a cooled (0 °C) suspension of LiAlH 4 (606 mg, 16 mmol) in anhydrous THF (20 mL) was added a solution of the imide 3 (1.4 g, 5 mmol) in THF (30 mL) over a 15 min period. After an additional 30 min of stirring, the cold (0 °C) reaction was slowly quenched with water (0.6 mL), then 10% aqueous NaOH (1.2 mL) and water (1.8 mL) to precipitate the aluminum salts, which were then filtered. The filtrate was dried (MgSO 4 ), and the solution was concentrated in vacuo. Because of the volatility of the product, the crude product was used directly for the next step. To the cooled (−10 °C) solution of the above alcohol 4 in pyridine (20 mL) was added 4-methylbenzene-1-sulfonyl chloride (1.0 g, 5.1 mmol). After the solution was stirred an additional 30 min at −10 °C, the reaction mixture was slowly warmed to 20 °C for an additional 30 min period. The reaction was then quenched with brine (30 mL), and the aqueous layer was extracted with three 30 mL portions of CH 2 Cl 2 . The combined organic extracts were washed with brine, diluted HCl and saturated aqueous CuSO 4  9-Bromononan-1-ol (7). A mixture of nonane-1,9-diol 6 (24 g, 0.15 mol), a catalytic amount of iodine (0.5 g, 2 mmol) and 40% HBr (33 mL) in toluene (240 mL) was heated to reflux and the water formed was separated continuously for 30 h. Then the mixture was washed successively with water, aqueous NaOH, HCl, water and brine. The organic phase was concentrated in vacuo, and chromatographed (10% EtOAc-hexane) to give 30 g of pure 7 as a colorless oil in 89% yield. 1 (8). To a solution of 9-bromononan-1-ol (7, 25 g, 0.112 mol) in anhydrous CH 2 Cl 2 (150 mL) was added 4-methylbenzenesulfonic acid (0.3 g) and freshly distilled 3, 4-dihydro-2H-pyran (11.24 mL, 0.123 mol) in CH 2 Cl 2 (10 mL), at 0 °C. After addition, the mixture was allowed to stir for 5 h at room temperature and monitored by TLC. Then the solution was diluted by addition of another portion of CH 2 Cl 2 (150 mL) and then the reaction mixture was washed with saturated aqueous NaHCO 3 and brine, dried over anhydrous MgSO 4 and then concentrated in vacuo and chromatographed to afford 31.6 g (92% yield) of pure 8 as a colorless oil.  (9). Under a nitrogen atmosphere at 20 °C, Mg turnings (56 mmol, 1.37 g), a catalytic amount of iodine and anhydrous THF (25 mL) were placed in a dry 500 mL three-necked round-bottomed flask equipped with a condenser and a dropping funnel. A solution of compound 8 (13 g, 42 mmol) in THF (150 mL) was added dropwise to the above mixture (approx. 20 min). Then the mixture was allowed to reflux for another 5 h. Another dry 500 mL three-necked round-bottomed flask was charged with allyl bromide (84 mmol), NMP (4 mL) and Li 2 CuCl 4 (0.3 M in THF, 4 mL). The above Grignard reagent was transferred to this flask through a double-ended needle with stirring. After transfer, the reaction mixture was warmed to 85 °C for 12 h. After cooling, the reaction mixture was quenched by dropwise addition of saturated aqueous NH 4 Cl solution. The THF was recovered on a rotavapor and the residue was extracted with ethyl acetate (100 mL × 3). The combined organic extracts were dried (MgSO 4 ), concentrated in vacuo, and chromatographed (15-25% gradient, EtOAc-hexane) to provide 7.5 g (66%) of pure 9 as a colorless oil. 1  Dodec-11-en-1-ol (10). To a solution of compound 9 (7.0 g, 26 mmol) in MeOH (100 mL) was added a catalytic amount of PPTS with stirring. Then the reaction mixture was heated to 50 °C overnight and monitored by TLC. After completion, the methanol was removed on a rotavapor, And the residue was purified by column chromagraphy (15-25% gradient, EtOAc-hexane) to give dodec-11-en-1-ol (4.3 g, 96% yield) as a colorless oil. 1  12-Bromododec-1-ene (11). A dry 250 mL three-necked round-bottomed flask was charged with PPh 3 (14.7 g, 56 mmol), dichloromethane (80 mL) and CBr 4 (22.2 g, 67 mmol) and the mixture was allowed to stir for 20 min. To the above solution was added compound 10 portionwise at −10 °C. After completion, the volatiles were removed on a rotavapor, and the residue was purified by column chromagraphy directly to give 12-bromododec-1-ene (11, 90% yield) as a colorless oil. 1  (S)-14-Methyloctadec-1-ene (1). Under nitrogen atmosphere at 20 °C, Mg turnings (0.2 g, 8.4 mmol), a catalytic amount of iodine, and anhydrous THF (5 mL) were placed in a dry 250 mL three-necked round-bottomed flask equipped with a condenser and a dropping funnel. A solution of compound 11 (1.6 g, 6.5 mmol) in THF (50 mL) was added dropwise to the above mixture (approx. 20 min). Then the mixture was allowed to reflux for another 5 h. Another dry 500 mL three-necked round-bottomed flask was charged with the chiral intermediate 5 (1.8 g, 6.5 mmol), NMP (2 mL) and Li 2 CuCl 4 (0.3 M in THF, 2 mL). The above Grignard reagent was transferred to this flask through a double-ended needle with stirring. After transfer, the reaction mixture was warmed to 85 °C for 3 days. After cooling, the reaction mixture was quenched by dropwise addition of saturated aqueous NH 4 Cl solution.