All stereoisomers of lariciresinol were synthesized to examine the effect of stereochemistry on plant growth. Configuration of benzylic 7-positions was constructed through S(N)1 or S(N)2 intramolecular etherification. 8- and 8'-position configurations were established from the starting material except for all cis stereoisomers, the 8-position configurations of which were achieved by employing stereoselective hydroboration. (-)-Lariciresinol and its 7S,8S,8'R stereoisomer inhibited the root growth of Italian ryegrass to 51-55% relative to the negative control, whereas other stereoisomers had less effect. These results demonstrate that the stereochemistry of lignans is one of the important factors influencing their inhibitory activity.
The syntheses of 55 lariciresinol derivatives containing derivatives on the 9-position and an aryl group at both 7- and 7'-positions were successful to examine the effect of structure of (-)-lariciresinol (1) on plant growth regulatory activity. (-)-(7R,8R,8'S)-9-Dehydroxylariciresinol 9 showed activity 2-fold more potent than that of natural (-)-lariciresinol (1) and -95% growth inhibitory activity to negative control against rye grass root at 1 mM. The derivatives bearing hydrophobic and smaller groups at the 9-position showed higher activity. The importance of 4- and 4'-hydroxy groups and 3- and 3'-small hydrophobic groups on 7- and 7'-phenyl groups for higher activity was also suggested.
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