Chemical transformations of phyllocladane (=13 beta-kaurane) diterpenoids

Earlier phytochemical work on Plectranthus ambiguus (Lamiaceae) afforded a series of tetracyclic phyllocladane-type (= 13beta-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.