Abstract. Racemic 4,4,4-trifluoro-3-(indole-3-)butyric acid (TFIBA) has been synthesized and shown to inhibit Avena coleoptile elongation. (S)-( +)-TFIBA ( fig. 1), which was prepared by an enzymatic method and markedly promotes root growth of Chinese cabbage, lettuce and rice plants, is a novel fluorinated plant growth regulator. Activity of the (S)-( +)-enantiomer of TFIBA was 10-fold greater than that of the (R)-( -)-enantiomer in the first two plant species and 5-fold greater in rice. Key words. Plant growth regulator; trifluoro,3-(indole-3-)butyric acid; fluorinated PGR; root growth promotion; Arena coleoptile elongation. Biologically active substances that contain fluorine, e.g., 5-fluorouracil, ofloxacin, diflubenzuron, fluazifop-butyl, have been synthesized by medical and agricultural researchers ~-4. The introduction of the fluorine atom(s) to biologically important molecules has dramatic effects on the properties of those molecules. In our studies on the syntheses of plant growth regulators which contain fluorine atom(s) and which promote biomass growth, we synthesized 4,4,4-trifluoro-3-(indole-3-)butyric acid (TFIBA) by introducing one trifluoromethyl and one methylene group to the side chain of indole-3-acetic acid, the first natural plant hormone identified. We here report the synthesis and biological activities of racernic and opitcally active TFIBA, a novel fluorinated plant growth regulator.
MethodsRacemic TFIBA was synthesized as follows: 2,2,2-trifluoro-l-(indole-3-)ethanoP, obtained from the condensation of indole with an excess of trifluoroacetaldehyde ethyl hemiacetal, was coupled with the sodium salt of *To whom correspondence should be addressed. diethyl malonate in toluene at 100~ giving a diester. The ester was hydrolyzed and decarboxylated with potassium carbonate in methanol-water under refluxing conditions, after which methanol was removed in vacuo. The resulting aqueous solution was acidified with hydrochloric acid, giving crude TFIBA which was chromatographed on silica gel with ethyl acetate-nhexane to give pure racemic TFIBA in high yield. TFIBA: m.p. 117-119~ 1H-NMR spectrum (360MHz, acetone-d6, TMS, ppm) 3.04 (1H, dd, J= 16.1, 9.3 Hz), 3.13 (1H, dd, J= 16.1, 5.2 Hz), 4.36 (1H, ddq, J = 5.2, 9.3, 9.4 Hz), 7.09 (1H, ddd, J = 7.8, 6.8, 1.1 Hz), 7.14 (1H, ddd, J = 8.0, 6.8, 1.1 Hz), 7.41 (1H, d, J=8.0Hz), 7.47 (1H, s), 7.70 (1H, d, J=7.8Hz), 10.39 (1H, br.s); ~9F-NMR spectrum (84.7MHz, acetone-d6, TFA, pprn) 6.65 (d, J= 9.4Hz); mass spectrum (70eV, m/z, relative intensity(%)) 257 (M +, 86), 237(31), 198(100), 188(22); IR spectrum Vmax(KBr)(cm -1) 3430, 1722, 1460, 1438, 1422, 1380, 1326, 1313, 1296, 1280, 1155, 1113, 962, 823, 745, 664, 618.
ResultsBiological activities of racemic TFIBA were compared with those of racemic 3-(indole-3-)butyric acid (3-IBA), synthesized according to refs 6 and 7, 4-(indole-3)-butyric acid (4-IBA), 3-(indole-3-)propionic acid (IPA) and indole-3-acetic acid (IAA), using two kinds of auxin bioassays. The result of the bioassay using elongation of Arena s...
