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

Abstract : Since the C15 β-end-group aldehyde (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 can easily be synthesized from citral, the C15 + C5 route was applied to the syntheses of γ-retinal (I) and retinal (II), and therefore, by coupling (2 × C20 → C40), to the prepn. of lycopene and β-carotene. These new syntheses of retinal and γ-retinal use an extended aldol reaction with a C6 building block that incorporates a C5 unit after decarboxylation. [on SciFinder(R)]