A new efficient synthesis of GR24 and dimethyl A-ring analogues, germinating agents for seeds of the parasitic weeds Striga and Orobanche spp.

An efficient and high yielding preparation for the synthetic germination stimulant GR24 (5) and its A-ring dimethyl-substituted analogues 30–32 has been described. The first step involves a Stobbe condensation of benzaldehydes 9–11 with dimethyl succinate. Subsequent transposition of the ester and reduction of the double bond provides the building blocks 15–17 for an intramolecular Friedel–Crafts acylation. ABC-lactones 22–25 are prepared from γ-keto esters 18–21 by saponification, subsequent reduction with sodium borohydride followed by acid-catalyzed lactonization. Coupling of the lactones with the D-ring is accomplished by formylation and subsequent treatment with bromobutenolide 8 to give GR24 and its dimethyl analogues. Bioassays with Striga hermonthica seeds reveal that the dimethyl analogues are slightly less active than GR24 itself.