Pestic. zyxwvutsrqp Sci. zyxwvutsrq 1987, zyxwvuts 21, 253-267 zyxwvuts Comparative Activity of the Enantiomers of Triadimenol and Paclobutrazol as Inhibitors of Fungal Growth and Plant Sterol and Gibberellin Biosynthesis" Raymond S. Burden, Geoffrey A. Carter, Terence Clark, David T. Cooke, Steven J. Croker, Adrian H. B. Deas, Peter Hedden, Carolyn S. James and John R. Lenton Department of Agricultural Sciences, University of Bristol, AFRC Institute of Arable Crops Research, Long Ashton Research Station, Long Ashton, Bristol BS18 9AF, UK (Received 3 June 1987; accepted 19 June 1987) ABSTRACT The nature and degree of the biological activity shown by triadimenol and paclobutrazol are influenced considerably by the absolute configuration of the two asymmetric carbon atoms present in the molecules. The order of the activity of triadimenol enantiomers was found to be: fungitoxicity- IS, 2RBIR, 2R>IR, 2S>IS, 2s; inhibition of gibberellin biosynthesis- IR, 2S>IS, 2S>IR, 2R>IS, 2R; and inhibition of plant sterol bio- synthesis-IR, 2S=1S, 2R>lR, 2R>1S, 2s. The relative activity of paclobutrazol enantiomers was: fungitoxicity- 2R, 3R>2S, 3R>2R, 3S=2S, 3s; inhibition of gibberellin biosynthesis- 2S, 3S>2R, 3SB2R, 3R>2S, 3R; and inhibition of plant sterol bio- synthesis-2Ri, 3S>2R, 3R>2S, 3S>2S, 3R. These data indicate that the R configuration at the chiral carbon bearing the hydroxyl group is the prime determinant for fungitoxicity whereas enantiomers having the S configuration at this carbon are effective inhibitors of gibberellin bio- synthesis. This agrees well with published data for the structurally related vinylazoles. Plant sterol C-14a demethylation was riot always inhibited by enan- tiomers having high fungicidal activity thus indicating that the structural features necessary for binding to the plant C-14a demethylase may differ from those needed for the analogous fungal enzyme. *Based on a lecture presented in London at the symposium Gibberellin zyxw and Sterol Synrhesis Inhibitors on 5 March 1987, organised by the Physicochemical and Biophysical Panel of the Pesticides Group, Society of Chemical Industry. 253 Pesfic. Sci. 0031-613X/87/$03,50 zyxwvu 0 Society of Chemical Industry. 1987. Printed in Great Britain