INTRAMOLECULAR ENE REACTION OF VINYLPHOSPHONATES - SYNTHETIC APPLICATION TO BICYCLIC COMPOUNDS AND CADALANE AND VALERENIC ACID TERPENOIDS

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'-yl)phosphonoacetates 8-10 in high yields. The Wittig-Horner reaction of the phosphonates 8-10 with paraformaldehyde led to 1,5-diene compounds 12a,b and 13b in 87-90% yield. Subsequent Lewis-acid-catalyzed cyclization of the compounds 12, 13 or 25, 27 afforded 1,6-dimethyloctahydronaphthalene-4-carboxylates 17, 22, or carbaldehydes 28, 29 and/or 8-methyl-2,5-dimethylenebicyclo[4.4.0]decan-4-ol (30). Palladium-catalyzed metallo-ene reaction of the acetate 32 afforded 2,5,8-trimethyl-1,2,3,4-tetrahydronaphthalene (34). The bicyclic compounds 17a and 29 were applied to the synthesis of cadalane and valerenic acid sesquiterpenoids.