Abscisic aldehyde: a new synthesis, isotopic labelling, exchange reactions and oxidation

Abscisic aldehyde (AB-ald) has been synthesized from (±)-R-S-1′-hydroxy,4′-oxo-α-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 t_1/2 < 3 min) and acidic (pH 3 t_1/2 < 3 min) conditions with a minimum at ca pH 5.5 (t_1/2 = 50 min). Loss of ¹⁸O from C-1 of AB-ald may distort apparent ¹⁸O incorporation values into ABA obtained in biosynthetic experiments using ¹⁸O₂. Tomato shoots oxidized (±)-RS-AB-ald and (±)-RS-2-trans-AB-ald to ABA and 2-trans-ABA, respectively. When (+)- [6-²H₃] 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,μg 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.