An improved method for the prepn. of Me2C:CFCO2Et is described. The two stereoisomers (E)- and (Z)-BrCH2CMe:CFCO2Et obtained by g-bromination are sepd. and can be individually converted into the corresponding phosphonates. The PO-ylides generated by a-deprotonation of the latter lose their stereochem. integrity by rapid torsional equilibration of the ester tail. Thus, Horner-Wittig reactions accomplished with PhCHO and (Z)- or (E)-(b-ionylidene)acetaldehyde lead inevitably to stereo-randomization at the terminal, CO2Et-bearing double bond affording (E)- and (Z)-isomers in 50:50 to 15:85 ratios, depending on the reaction conditions. The new double bond, however, is formed with perfect trans-selectivity. [on SciFinder (R)]