3′-(Phenyl alkynyl) analogs of abscisic acid: synthesis and biological activity of potent ABA antagonists

We report here the synthesis and biological testing of 3 '-(phenyl alkynyl) abscisic ABA analogs, a new class of potent ABA antagonists. These ABA analogs incorporate a rigid framework of eight carbon atoms attached at the 3 '-carbon atom of ABA that prevents folding of the ABA analog-bound receptor required for ABA signalling. The two-step synthesis is based upon the optimized conversion of natural (S)-ABA to 3 '-iodo ABA which can be coupled to phenyl acetylenes using Sonogashira conditions, or to styryl compounds through Suzuki chemistry. The parent 3 '-(phenyl alkynyl) ABA analog 7 was obtained in 29% yield, 74% yield based on recovered starting material. In a lentil seed germination assay, compound 7 was found to have more potent activity than other known 3 '-substituted ABA antagonists to date. In a structure activity study parasubstituted phenyl alkynyl analogs had comparable activity to the analog 7 while the 3 '-styryl ABA 18 was only slightly less active. Analog 7 overcame ABA inhibition of germination and seedling growth in a wide range of mono and dicot plant species, including canola, lentil, soybean, rice, wheat, barley, cannabis and canary seed. 3 '-(Phenyl alkynyl) ABA analogs have numerous potential practical agricultural applications including promoting ripening of crops, dormancy breaking of seeds and woody perennials, as well as promoting seed germination, and growth under stress conditions as demonstrated in this report.