Abstract Depending on the steric hindrance of the reaction centers, hemiacetal vinylogs 1 in the presence of BF 3. OEt 2 can, with enol ethers, lead to a Michael-type addition (4) or an aldolisation-type reaction (3). Hemiacetal vinylog 1b always yields the second reaction leading to β-methoxy-γ, δ-ethylenic carbonyl compounds 4 precursors of polyenic carbonyl compounds 5. Crotonaldehyde and methanol can be used instead of compound 1b.
