Biosynthesis of cardenolides, bufadienolides and steroid sapogenins

Abstract

It has been known for several years that pregnenolone is the biogenetic precursor of hormones of the adrenal cortex and of progesterone in the animal kingdom. The route to the androgenous and oestrogenous hormones also goes via this intermediate. However, in the plant kingdom it was only later that the role of pregnenolone (1) was investigated. It was first isolated by Tschesche & Snatzke (1960) from the roots of Xysmalobium undulatum where it occurs in small amounts together with the glucoside of allopregnanolone (a ring A/B trans derivative). The main fraction of the steroid glycosides from this root consists of the cardenolide derivatives uzarigenin and xysmalogenin (2). Since this time many nitrogen-free C 21 steroids have been isolated from plants; these are collectively called digitenol derivatives by us as they were first obtained from Digitalis species. Some examples are diginigenin, digifologenin, digipurpurogenin (Tschesche, Brugmann & Snatzke 1964) and digacetigenin (Tschesche 1966). In Apocynaceae there are many C 21 steroid alkaloids, of which conessin is the best known representative. Up to the present time more than 90 different alkaloids of this type have been isolated from the Apocynaceae family. It is of considerable importance that in some cases nitrogen-free precursors of these alkaloids have also been found in the same plant, for example holadysone (3) (Tschesche, Mörner & Snatzke 1963; Tschesche, Knittel & Snatzke 1965), which is a probable precursor of holonamine (4) (Tschesche & Ockenfels 1964) isolated by us and certain lactones of pregnenone-18-carboxylic acid by Potier, Kan & Le Man (1964) which are related to the paravallarin type of alkaloids (5). The isolation of progesterone (6) from Holarrhena floribunda by Goutarel’s group (Leboeuf, Cave & Goutarel 1964) has special importance in this connexion. This was the first report of the occurrence of this animal hormone in the plant kingdom.