Syntheses of (±)- and (+)-Sorgolactone, the Germination Stimulant fromSorghum bicolor

Abstract

Syntheses of (±)‐2a, the racemate of the structure proposed for sorgolactone, and its three racemic stereoisomers have been accomplished with confirmation of the stereostructures of the intermediate (±)‐10 and the final product (±)‐2a by X‐ray analysis. Its optically active form, (3aR,8S,8bS,2′R)‐(+)‐2a, has also been prepared from (S)‐(–)‐citronellal by employing radical cyclization of 18 to 19 as the key step. Spectroscopic properties of the synthetic products are compared with those reported for natural sorgolactone. Bioassays using clover broomrape (Orobanche minor) seeds have revealed that all the stereoisomers strongly stimulate their germination.