10-Undecenoic Acid, an Inexpensive Source for the Synthesis of the Pheromones of Cotton Pests, Peach Tree Borer and Cherry Tree Borer

The aldehyde 7, derived from 10-undecenoic acid (5) on cis-selective Wittig reaction with pentylidenetriphenylphosphorane, subsequent deprotection and oxidation gave the pheromone (11Z)-hexadecenal (1). Wittig-Horner olefination of 1 with the phosphonate 9 furnished the conjugated ester 10 which on base catalyzed isomerization to the (3Z)-ester 11 followed by LAH reduction and acetylation gave (3Z,13Z)-octadeadien-1-yl acetate (2). Compound 10 on the other hand was chemoselectively reduced and acetylated to furnish the pheromone (2E,13Z)-octadeadien-1-yl acetate (4). For the synthesis of (3E,13Z)-octadeadien-1-yl acetate (3), 1 was condensed with malonic acid under modified condition to afford the acid 13 which was converted to 3 by standard reaction protocol.


Introduction
In the quest for practical syntheses of agriculturally important insect pheromones, 10-undecenoic acid (5 ) seems to be an ideal starting material.Besides its easy availability from castor oil, the terminal bifunctionality of 5 is amenable to synthetic maneuvers to different target long chain olefinic compounds.Earlier we have used [1][2][3][4] it extensively for the syntheses of some plant growth regulators, insect pheromones, marine demospongic acids, prostaglandin synthon, etc. Herein, we report a divergent synthetic strategy for some other pheromones of destructive pests of cash crops and fruits viz.(11Z)-hexadecenal (1 ), (3Z,13Z)-octadecadienyl acetate (2 ) and its 3E-isomer (3) and (2E,13Z)-octadecadienyl acetate (4 ).While compound 1 is a pheromone component [5] of the cotton bollworm, Heliothis armigera, 2-4 have been isolated as the pheromones of various orchard pests in the genera, Synanthedon [6], Sanninoidea [6], Melittia and Vitacea [7].In view of their importance, several individual synthesis of 1 [8a], 2-4 [8b-e] have been reported.However, to the best of our knowledge, this is the first divergent synthesis (Scheme 1) of all of these from a single commercially available precursor involving extremely simple stereoselective methodologies and hence seems suitable as a practical route.

Results and Discussion
The acid 5 on LAH reduction followed by tetrahydropyranylation gave 6 .Its hydroboration-oxidation furnished the known aldehyde 7 [9] which on Z-selective Wittig olefination with n-pentylidenetriphenylphosphorane and subsequent deprotection gave the alcohol 8.This was oxidized with PCC to furnish the title pheromone 1.
Compound 1 thus obtained, on Wittig-Horner reaction with the hindered phosphonoacetate 9 [10] furnished the (2E)-C 18 ester 10.Its base catalyzed deconjugation [10] with potassium hexamethyldisilazide afforded the (3Z)-ester 11 exclusively.This on LAH reduction gave the alcohol 12 which was acetylated to furnish the pheromone 2. On the other hand, compound 1 when subjected to modified Doebner condensation [11] gave the (3E)-acid 13 in excellent yield.Its LAH reduction followed by acetylation provided the pheromone 3. Likewise, chemoselective reduction of the ester functionality in 10 with AlH 3 [12] gave the alcohol 15 which on acetylation afforded the title pheromone 4. The spectral and physical data of the pheromones 1-4 were in good agreement with those reported [8a-e] in literature.The olefinic stereochemistry of the pheromones were assigned [4]

(11Z)-Hexadecenol (8 )
To a solution of dimsyl anion [prepared from NaH (0.86 g, 50% suspension in oil, 0.018 mol) in DMSO (30 mL)] was added n-pentyltriphenylphosphonium bromide (7.44 g, 0.018 mol) at room temperature.After 1 h, THF (20 mL) was introduced in the flask, the reaction mixture cooled to -20 °C and the aldehyde 7 [9] (4.32 g, 0.016 mol) in THF (40 mL) added in dropwise fashion.Stirring was continued for 2 h at -20 °C and 1 h at room temperature.The mixture was poured in large excess of water and extracted with hexane.The hexane layer was washed with water, brine and dried.The crude product obtained on concentration, was reextracted with cold hexane.Removal of solvent gave a residue which was dissolved in MeOH (50 mL) containing dil.HCl (50%, 2-3 drops) and heated to reflux for 6 h.Most of the solvent was removed under vacuum, the residue dissolved in ether and washed with aqueous 10% NaHCO 3 , water and brine.After drying, the solvent was removed in vacuo and the crude compound purified by column chromatography over silica gel (0-15% EtOAc/hexane) to furnish pure 8 (2.7 g, 70%): bp 100-102 °C/ 0.

(11Z)-Hexadecenal (1 )
To a stirred suspension of pyridinium chlorochromate (PCC) (5.39 g, 0.025 mol) in CH 2 Cl 2 (50 mL) was added the compound 8 (4.0 g, 0.017 mol) in one lot.After completion of the reaction (~ 3 h), the mixture was diluted with 50 mL of ether and the supernatant passed through a 2" pad of silica gel which was thoroughly eluted with ether.The eluent on concentration furnished pure 1 (

(2E,13Z)-Octadecadienyl acetate (4 )
To a stirred suspension of AlH 3 [12] (9.0 mmol) in THF (20 mL) was added 10 (2.46 g, 6.5 mmol) in THF (20 mL) and stirring continued till the completion of the reaction (cf.tlc).The reaction mixture was worked up by first adding saturated aqueous Na 2 SO 4 solution followed by dil.HCl (2 N) and extraction with ether.The ether layer was washed with water and dried.After concentration, the residue was purified by careful chromatography over silica gel (0-15% EtOAc/hexane) to give the alcohol 15 (1.35