Synthesis of labile all-trans-7,8,7′,8′-bis-acetylenic carotenoids by bi-directional Horner–Wadsworth–Emmons condensation

A new stereoselective synthesis of the C-40-bis-acetylenic carotenoids all-trans-(3R,3'R)-alloxanthin and all-trans-3,4,7,8,3', 4', 7', 8'-octadehydro-beta, beta-carotene, both compounds featuring a stereochemically labile C7-C10 enyne, based on a bi-directional Horner-Wadsworth-Emmons (HWE) reaction of a C-15-phosphonate and a central C-10-dialdehyde, is reported. The triene unit of the latter fragment was synthesized using the acyclic metathesis/dimerization reaction.