4 -Dihydrophaseic acid

Phaseic acid (305) and dihydrophaseic acid (306a) are formed when (+)-abscisic acid (304) is fed to certain plants (e.g. tomato, bean, etc.). The stereochemistry of these compounds has recently been established131 by spectroscopic analysis (n.m.r. and i.r.) of the epimeric 4 -dihydrophaseic acids (306a and b) derived by reduction (NaBH4) of phaseic acid (305). 

ABA = abscisic acid IAA = indole-3 acetic acid DPA = dihydrophaseic acid PA = phaseic acid. 2GPC = gel permeation chromatography. 

ABA absorbs ultraviolet with maxima at 240 nm (e 2.1 x 104, a shoulder peak), 260 nm (e 2.6 x 104), and 320 nm (e 50) in an acidic methanol solution.590 Irradiation with UV with a wavelength shorter than 305 nm isomerizes the 2-(Z)-double bond to (E) to give an equilibrium mixture of ABA and its 2-( )-isomer with a ratio of 1 1, and also causes decomposition of ABA to unidentified compounds by the excitation of the 7t-7t transition of the side chain and the enone groups. ABA has a strong optical activity, and its specific optical rotation is +430° in an acidic methanol solution.591 In the optical rotatory dispersion (ORD)591 and the circular dichroism (CD) spectra,558 ABA shows a positive Cotton effect from 300 to 200 nm. Phaseic acid and dihydrophaseic acid and its epimer, which did not have the enone group, show a small specific optical rotation with a minus value and also a negative plain curve in the ORD. 

The catabolic inactivation of ABA would be achieved in a stepwise manner. The biological activity of 8 -hydroxy-ABA cannot be tested due to its instability, but several studies have suggested that the activity of 8 -hydroxy-ABA is about 20% of that of ABA.626 683 684 The activity of phaseic acid is low, about 5% of that of ABA, in many biological assays,548 and may be contributed by 8 -hydroxy-ABA present in a sample of phaseic acid. Dihydrophaseic acid and its epimer are almost inactive in assays tested, meaning that the catabolic inactivation is completed in the step reducing phaseic acid to dihydrophaseic acid and its epimer. 

HOABA is enzymatically or spontaneously isomerized to phaseic acid (PA, 25), followed by reduction to dihydrophaseic acid (DPA, 26), as shown in Figure (10) [1,7]. 

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