The conformation of abscisic acid by n.m.r. and a revision of the proposed mechanism for cyclization during its biosynthesis

Abstract

The n.m.r. spectrum of abscisic acid (ABA) formed from [1,2-13C2]acetate by the fungus Cercospora rosicola shows 13C-13C coupling between C-6′ (41.7 p.p.m.; 36 Hz) and the downfield 6′-methyl group (6′-Me) (24.3 p.p.m, 36 Hz). This 6′-Me, therefore, is derived from C-3′ of mevalonate [Bennett, Norman & Maier (1981) Phytochemistry 20, 2343-2344]. An i.n.e.p.t. (insensitive nuclei enhanced by polarization transfer) pulse sequence demonstrated that the downfield 13C signal is produced by the 6′-Me that gives rise to the upfield 1H 6′-Me signal (23.1 d). The absolute configuration of this, the equatorial 6′-Me group, was determined as 6′-pro-R by decoupling and n.O.e. (nuclear-Overhauser-enhancement) experiments at 300 MHz using ABA, ABA in which the axial 6′-pro-S 5′-hydrogen atom had been exchanged with 2H in NaO2H and the 1′,4′-cis- and 1′,4′-trans-diols formed from these samples. The configuration at C-1′ and at C-6′ are now compatible with a chair-folded intermediate during cyclization, as proposed for beta- and epsilon-rings of carotenoids. ABA in solution exists, as in the crystalline form, with the ring in a pseudo-chair conformation. The side chain is axial and the C-3 Me and the C-5 hydrogen atoms are predominantly cis(Z).