Chemical Transformations of Phyllocladane (=13β‐Kaurane) Diterpenoids

Earlier phytochemical work on Plectranthus ambiguus (Lamiaceae) afforded a series of tetracyclic phyllocladane‐type (=13β‐kaurane) diterpenoids (see 1a–f). In the course of investigations concerning the reaction behavior of this rare natural‐products, a new constituent of P. ambiguus was isolated, (2S,3R,16R)‐phyllocladane‐2,3,16,17‐tetrol 2,3‐diacetate (1g), and another eighteen new phyllocladanes were prepared by chemical transformations and characterized. The main constituent 1b of P. ambiguus was chemically transformed to the known natural diterpenoid calliterpenone (=(16R)‐16,17‐dihydroxyphyllocladan‐3‐one; 2) thus unambiguously establishing its structure (Scheme 1). Epimerization at C(16) via the epoxy derivative 20 yielded 16‐epicalliterpenone (21), 17‐hydroxyphylloclad‐15‐ene‐3‐one (22), and (16R)‐3‐oxophyllocladan‐17‐al (23) (Scheme 6). Comparing this reaction sequence with the corresponding one starting from diastereoisomeric (16R)‐16,17‐dihydroxy‐ent‐kauran‐3‐one (=abbeokutone; 27) showed basically the same outcome (Scheme 7). Furthermore, three new C(16)‐substituted ent‐kauran‐3‐ones were characterized. Reliable spectroscopic arguments for the determination of the configuration at C(16) in phyllocladanes and kauranes as well as for the differentiation of the diastereoisomeric skeletons are presented.