SummaryRetro-Aldol reaction of retinylidene-dimedone is reported.We recently reported the preparation, characterization, and biological activity of a number of 2-retinylidene-1,3-diketones [ 11. Many of these compounds, especially those derived from 1,3-cyclohexanediones (1, Scheme I ) show considerable activity in the hamster tracheal organ culture (TOC.) assay. This activity, combined with their low toxicity, has made some of these retinoids excellent materials for in vitro anti-tumor experiments [2]. However, retinoids derived from 113-cyclopentanediones are markedly less active, 2-retinylidene-l , 3-cyclopentanedione (2, Scheme 1 ) itself having virtually no activity in the TOC. test. We now report a procedure for effecting a retro-aldol reaction of some of these retinoids which provides a chemical parallel to the observed variations in biological activities.Scheme I