ABSCISIC ALDEHYDE - A NEW SYNTHESIS, ISOTOPIC LABELING, EXCHANGE-REACTIONS AND OXIDATION

Abscisic aldehyde(AB-ald) has been synthesized from(+/-)-R-S-1'-hydroxy,4'-oxo-alpha-ionone by a novel route (using a silyl ylid) that allows the ready incorporation of isotopes of C and H. SIM-NICI GC-MS of such labelled materials provides a sensitive and accurate method of measurement. AB-ald reacts with pentafluorobenzyl hydroxylamine to form a pentaflurobenzyl-oxime at C-1 and also at C-4' under basic conditions. These derivatives improve the sensitivity of detection by GC by ca five-fold in comparison with the underivatized compound. AB-ald exchanges its C-1 oxygen atom with the medium in basic (pH 8.5 t1/2 < 3 min) and acidic (pH 3 t1/2 < 3 min) conditions with a minimum at ca pH 5.5 (t1/2 = 50 min). Loss of O-18 from C-1 of AB-ald may distort apparent O-18 incorporation values into ABA obtained in biosynthetic experiments using O-18(2). Tomato shoots oxidized (+/-)-RS-AB-ald and (+/-)-RS-2-trans-AB-ald to ABA and 2-trans-ABA, respectively. When (+/-)- [6-H-2(3)] AB-ald was supplied and the ABA resolved as MeABA in a Chiralcel HPLC column, both (+)-S and (-)-R ABA were labelled but the (+) enantiomer predominated (ca 2: 1). AB-ald in aqueous solutions was slowly oxidized in air to ABA (16%) and other products (84%) and some of the ABA in the tomato shoots, therefore, may have been formed non-enzymically. It was also noted that when dilute solutions of AB-ald (0.1-10-mu-g ml-1) were exposed to air the 2-cis isomer was oxidized more rapidly than the 2-trans isomer with a concomitant change in the apparent ratios of the two isomers of AB-ald.