New Syntheses of Retinal and Its Acyclic Analog γ ‐Retinal by an Extended Aldol Reaction with a C 6 Building Block That Incorporates a C 5 Unit after Decarboxylation. A Formal Route to Lycopene and β ‐Carotene

Since the C 15 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 C 15 ψ ‐end‐group aldehyde 5 can easily be synthesized from citral ( 1 ) ( Scheme 3 ), we applied the C 15 +C 5 route to the syntheses of γ ‐retinal (( all‐E )‐ 8 ) ( Scheme 3 ) and retinal (( all‐E )‐ 13 ) ( Scheme 4 ), and therefore, by coupling (2×C 20 →C 40 ), 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 C 6 building block that incorporates a C 5 unit after decarboxylation.