Neue Diterpenoide aus Blattdrüsen einer <i>Plectranthus sp.</i> aus Rwanda

Abstract

Novel Diterpenoids from Leaf‐Glands of a Plectranthus sp. from Rwanda More than 20 highly modified abietanoid compounds belonging to 6,7‐dioxocoleons, serveral kinds of royleanones, spirocoleons, extended quinones, quinone methides, and 1,4‐phenanthraquinones have been isolated from a Plectranthus sp. from the borders of Lake Kiwu (Rwanda). The structure of plectranthon A ( 1 ), a 5,7,8‐trimethyl‐1,4‐phenanthraquinone, has been confirmed by an X‐ray analysis of its p ‐bromobenzoate. It is twisted around the C(4)–C(5) moiety; this fact renders the molecule chiral. Notable features of the hydroxylation pattern in all of the isolated compounds are: (a) a predominant hydroxylation at C(16) (abietan numbering) leading to the (15 S )‐configuration of the 2‐hydroxy‐l‐methylethyl side‐chain and to the (13 S , 15 R )‐configuration of the derived spirocyclopropanes and to the derived ( S )‐2‐hydroxypropyl side‐chain; (b) the 6β,7α‐dihydroxylation which, together with further hydroxylations at C(3) and/or C(2), are considered as a prerequisite for the formation of the conjugated system extending over rings A, B and C ( e.g. in the coleons E and F), hence allowing a complete aromatization of the basic ring skeleton. The reasons for these extensive dehydrogenation and oxygenation reactions that cost the plant a great many of oxidation equivalents are not clear. Presumably, these modifying reactions preceed the degradation of the ring system.