The Lewis-acid-catalyzed intramolecular ene reaction of vinylphosphonates 4, 6, and 7, prepared by the Knoevenagel condensation of triethyl phosphonoacetate (3) with citronellal (1) or 2, 6-dimethyl-5-heptenal (2), stereoselectively gave 2-(8'-p-menthen-3'-yl) or 2-(2'- isopropenyl-5'-methylcyclopent-1'-y1) phosphonoacetates 8-10 in high yields. The Wittig- Horner reaction of the phosphonates 8-10 with paraformaldehyde led to 1, 5-diene ...
