Synthetic Photochemistry. LXIV. Mild Base-Induced <i>retro</i>-Benzilic-Acid Rearrangement of Isolated <i>proto</i>-Photocycloadducts of Methyl 2,4-Dioxopentanoate to Terpinolene. Facile Synthesis of <i>α</i>-Chamigrene and <i>α</i>-Chamigren-3-one

Abstract

Abstract In the photoaddition reaction of terpinolene with methyl 2,4-dioxopentanoate, the products derived from the tetrasubstituted C=C were four proto-[2+2] photocycloadducts, together with ene reaction products. When these proto-cycloadducts were treated with aqueous sodium carbonate, a facile retro-benzilic acid rearrangement to form substituted cyclopentenone derivatives occurred. The product from the other trisubstituted C=C was an ordinary retro-aldol product of the [2+2] cycloadduct and an ene reaction product. One of the proto-photocycloadducts was transformed into α-chamigren-3-one and further to α-chamigrene.