Regioselective Cationic 1,2- and 1,4-Additions Forming Carbon−Carbon Bonds to Methyl Santalbate, a Conjugated Enyne

Lewis acid-induced carbocationic addition reactions to methyl santalbate [methyl (E)-octadec-11-en-9-ynoate] [(E)-1] give products by regioselective formation of a new carbon−carbon bond at C-9 of the molecule chain. The allenic fatty acid derivatives methyl 12-chloro-9-(1-oxoheptyl)-9,10-octadecadienoate (2) and methyl 9-isopropyl-9,10-octadecadienoate (3) were obtained by Friedel−Crafts acylation of 1 with heptanoyl chloride and alkylation of 1 with isopropyl chloroformate, respectively. While the reaction between 1 and formaldehyde induced by Me2AlCl or Et3Al2Cl3 gives a mixture of the conjugated chlorodienes 5a and 5b, the corresponding reaction carried out in the presence of AlCl3 affords the α,β-unsaturated ketone 6.