Catalytic base-induced decarboxylation of polyunsaturated α-cyano-β-methyl acids derived from malonic acid led to the corresponding nitriles 3 (Schemes 2 and 3), 6 (Scheme 5), and 9 (Scheme 6). This decarboxylation occurred with previous deconjugation of the α,β-alkene moiety of the α-cyano-β-methyl acid, leading to an α-cyano-β-methylene propanoic acid which was easily decarboxylated (see Scheme 2). β-Methylene intermediates, in some cases, could be isolated; mechanistic pathways are proposed. The nitriles 3, 6, and 9 were reduced to the sesquiterpene aldehydes 4 (β-end group), 7 (φ-end group), and 10 (ψ-end group), respectively.