Synthesis and biological evaluation of strigol analogues modified in the enol ether part

Several analogues of strigol, which is a germination stimulant for seeds of the parasitic weeds Striga and Orobanche, have been prepared. Structural modifications are introduced in the vinyl ether part and include: i, analogues containing an endocyclic vinyl ether double bond, using tetronic acids as precursors; ii, geometrical isomerization of the vinyl ether double bond; and iii, analogues containing a methyl substituent on the vinyl ether double bond. During coupling reactions to give compounds belonging to the last-mentioned class, undesired C-alkylation occurs, which can be minimized by choosing the appropriate reaction conditions. Bioassays reveal that the analogues prepared exhibit considerable activity in the stimulation of seed germination of Striga hermonthica and Orobanche crenata.