A new synthetic scheme for 5Z,9E-tridecadien-1-ylacetate, an attractant of the cabbage looper Trichoplusia ni (Huebner) was developed using a highly stereoselective Claisen rearrangement.Keywords: insect pheromones, cabbage looper (Trichoplusia ni), 5Z,9E-tridecadien-1-ylacetate, Claisen rearrangement, Wittig reaction.5Z,9E-Tridecadien-1-ylacetate (1) is an attractant of the male cabbage looper (Trichoplusia ni Huebner) that affects their mating behavior [1]. Only one synthetic method for this compound that uses the so-called acetylene method has been reported [1].We developed a new synthetic scheme for 1 that uses as the key step the construction of a double bond with the E-configuration via the orthoester version of the Claisen rearrangement.The starting material in the proposed scheme was 1-hexen-3-ol (2), which was readily obtained from reaction of propylmagnesium bromide and acrolein. The Claisen rearrangement occurred easily upon heating allylic alcohol 2 with triethylorthoacetate in the presence of a catalytic amount of propionic acid. This produced the ethyl ester of 4E-octenoic acid (3). The stereochemical purity of 3 was confirmed by GC and IR and PMR spectral data. In particular, the IR spectrum of 3 exhibited a single absorption band with Q = 965 cm -1 in the region of double bond bending vibrations. This was characteristic of trans compounds. A band with Q = 780 cm -1 , characteristic of cis compounds, was not observed. Next, 3 was converted to the target molecule using a Wittig reaction through the two intermediates 4E-octen-1-ol (4) and 4E-octenal (5). The reaction of 5 with the ylide generated from 1-bromo-5-(tetrahydropyran-2-yl)oxypentane produced 5Z,9E-tridecadien-1-ol tetrahydropyranyl ether (6), which was converted in one step to the target compound 1 upon reaction with AcCl-AcOH. The overall yield of 1 was 15.5% calculated per starting acrolein. EXPERIMENTALIR spectra were recorded in thin layers on a Beckman Microlab 620 MX instrument. PMR spectra were taken in (CD 3 ) 2 CO solution with HMDS internal standard on a Tesla BS-587A spectrometer (operating frequency 80 MHz). Chemical shifts are given on the G scale. GC was performed on a Chrom-5 instrument with a flame-ionization detector, column (3500 u 3 mm), silicone liquid SE-30 (5%) stationary phase on chromaton N-AW-DMCS (0.16-0.20 mm), operating temperature 50-250°C, and He carrier gas (30 mL/min). TLC was performed on Silufol UV-254 plates with a fixed layer of SiO 2 .1-Hexen-3-ol (2). The Grignard reagent obtained from propylbromide (24.6 g, 0.2 mol), Mg (4.86 g, 0.2 mol), and anhydrous Et 2 O (300 mL) was stirred vigorously, cooled (-10° to -15°C), treated dropwise with acrolein (11.2 g, 0.2 mol) in