Attempted stereocontrol at C-2′ of strigol-type compounds by a Michael reaction/elimination approach

Simplified strigolactams as potent analogues of strigolactones for the seed germination induction of Orobanche cumana Wallr.

“…GR‐28 isomers 12a and 12b were prepared in four steps following a procedure reported by Jonhson et al Commercially available 2‐cyclopentene‐1‐acetic acid 7 (Scheme ) was transformed into lactone 8 via iodolactonisation in excellent yield (98%), and simple treatment with DBU afforded key unsaturated lactone 9 (Scheme ). Formylation ( t BuOK/ethyl formate) followed by alkylation with bromobutenolide 10 led to the two diastereomers of GR‐28 12a and 12b in 16 and 9% yield respectively.…”

“…Natural strigolactones display very strong activity for the seed germination induction of parasitic weed seeds of the Orobanche , Striga and Phelipanche genera . Most of this high activity has been retained in various simplified analogues of the natural strigolactones as GR‐24, GR‐7 and GR‐28 . We believe that the simplification of the natural strigolactone structures will be required to secure attractive cost of goods for potential commercial applications.…”

“…We report here the efficient synthesis of the GR‐28 strigolactam 23 (Schemes and ) and of their derivatives 37 to 43 (Schemes 8 to 10). To evaluate the importance of the stereochemistriy of the strigolactones, we developed a novel, highly efficient stereoselective synthesis of all four stereoisomers of the GR‐28 strigolactams 24 to 27 (Schemes and ) and assessed their activity to induce O. cumana seed germination. We have shown that almost all biological activity resides in a single stereoisomer of the GR‐28 lactam 24 (Table ), showing a much higher stereochemical discrimination compared with the GR‐28 strigolactone derivatives (Table ).…”

“…The naturally occurring strigolactones have limited use for field applications owing to their restricted synthetic accessibility and short soil persistence. Therefore, simplified strigolactone derivatives have received considerable attention . For example, the natural strigolactone strigol ( 1 , Scheme ) can be mimicked for its seed‐germination‐inducing ability by the simplified analogue GR‐24 2 , which retains high biological activity and is synthetically more accessible, because several stereoselective syntheses have been reported .…”

“…For example, the natural strigolactone strigol ( 1 , Scheme ) can be mimicked for its seed‐germination‐inducing ability by the simplified analogue GR‐24 2 , which retains high biological activity and is synthetically more accessible, because several stereoselective syntheses have been reported . A further structural simplification was achieved by removing the A‐ring of GR‐24, and the corresponding tricyclic derivatives GR‐7 4 and GR‐28 5 displayed promising parasitic weed seed‐germination‐inducing abilities …”