LAUSR

All four stereoisomers of naturally occurring 6-(2-hydroxy-6-phenylhex-1-yl)-5,6-dihydro-2H-pyran-2-one (1) were synthesized by employing yeast-reduction products with high optical purity (95%ee-more than 99% ee), and then their phytotoxicities against lettuce and Italian ryegrass were evaluated. In the Italian ryegrass seedlings test, (6S,2´R)-1 showed the most potent and stereospecific activity against the shoots (IC50 = 260 µM) and roots (IC50 = 43.2 µM) with a significant difference from other stereoisomers. The highest seed germination inhibitory activity against Italian ryegrass seed was also observed in (6S,2´R)-1, showing 53% germination ratio from control at 1000 µM. This advantageous (6S,2'R)-stereochemistry was employed in the syntheses of α,β-dihydro, 2´-dehydroxy, and 2´-methoxy derivatives 13-15. By the test using these derivatives, the importance of α,β-unsaturated double bond and a hydroxy group bonding to a chiral center on the 6-alkyl chain of 5,6-dihydro-α-pyrone for phytotoxicity was determined. In the test against lettuce, 6S-configuration and (6S,2'S)-configuration were necessary for growth inhibition (IC50 = ca. 60 µM) and germination inhibition (63% germination ratio at 1000 µM), respectively.