Isomerization-based strategies to enable the stereodivergent construction of complex polyenes from geometrically defined alkene linchpins remain conspicuously underdeveloped. Mitigating the thermodynamic constraints inherent to isomerization is further frustrated by the considerations of atom efficiency in idealized low-molecular weight precursors. In this work, we report a general ambiphilic C-3 scaffold that can be isomerized and bidirectionally extended. Predicated on highly efficient triplet energy transfer, the selective isomerization of beta-borylacrylates is contingent on the participation of the boron p orbital in the substrate chromophore. Rotation of the C(sp(2))-B bond by 90 degrees in the product renders re-excitation inefficient and endows directionality. This subtle stereoelectronic gating mechanism enables the stereocontrolled syntheses of well-defined retinoic acid derivatives.