SYNTHESIS OF ALL 8 STEREOISOMERS OF THE GERMINATION STIMULANT SORGOLACTONE

The naturally occurring sesquiterpene sorgolactone (2) belongs to the class of ''strigolactones'', which are highly potent germination stimu lants for seeds of the parasitic weeds Striga and Orobanche. The aim o f the present work was to synthesize all eight stereoisomers of sorgol actone and to evaluate their activities in the stimulation of germinat ion of S. hermontica and O. crenata. Two racemic diastereomers of the ABC part of sorgolactone, rac.10a and rac.10b respectively, were prepa red and coupled with homochiral latent D-ring synthons 12 and ent.12. In this manner, four mixtures of two separable (protected) sorgolacton e diastereomers were obtained. Deprotection gave all eight target comp ounds as single isomers. Bioassays revealed that only those isomers po ssessing the same stereochemistry as natural sorgolactone at two adjac ent chiral centers exhibit high biological activities.