New synthesis of catalpalactone

Abstract

Various alkylidenephthalides (E- and Z-5,6 and 7) were prepared by Wittig reaction of phosphoranes 4a, 4b and 4c respectively with phthalic anhydride. The products were then lactonised, using chloro(trimethyl)silane and sodium iodide, to the corresponding dilactones 8 and 3. Attempts to isomerise the dilactone 3 to catalpalactone 1 failed.

Phthalides 13a, 13b and 13c were prepared by Wittig reaction of phosphoranes 4a, 4b and 4c respectively with phthalaldehydic acid. Lactonisation of 13a gave the dilactone 15, while lactonisation of 13b or 13c gave dihydrocatalpalactone 9. Compound 9, on treatment with NaH and careful acidification with acetic acid, gave acid (E)12, which on selenolactonisation furnished selenide 2. Selenide 2 gave catalpalactone 1 on treatment with H₂O₂–AcOH.