New Syntheses of Retinal and Its Acyclic Analog γ-Retinal by an Extended Aldol Reaction with a C6 Building Block That Incorporates a C5 Unit after Decarboxylation. A Formal Route to Lycopene and β-Carotene

Since the C15 β‐end‐group aldehyde 10 ((β‐ionylidene)acetaldehyde), an excellent intermediate in the syntheses of retinoids, can be synthesized in many ways from β‐ionone, and since the corresponding acyclic C15 ψ‐end‐group aldehyde 5 can easily be synthesized from citral (1) (Scheme 3), we applied the C15+C5 route to the syntheses of γ‐retinal ((all‐E)‐8) (Scheme 3) and retinal ((all‐E)‐13) (Scheme 4), and therefore, by coupling (2×C20→C40), to the preparation of lycopene (14) and β‐carotene (15) (Scheme 5). Our new syntheses of retinal ((all‐E)‐13) and γ‐retinal ((all‐E)‐8 use an extended aldol reaction with a C6 building block that incorporates a C5 unit after decarboxylation.