Biosynthesis of plant sterols. IX. The mode of oxygenation at carbon atom 26 in the formation of sapogenins from cholesterol.

It was suggested by previous workers that the C-26 oxygenation step in the biosynthetic conversion of cholesterol to tigogenin ((25R)-5a,220-spirostan-3-ol) requires the intermediacy of a C-24 double bond. To examine the validity of this hypothesis, (25-3H)-cholesterol was synthesized, and (4-'4 C)-(253 H)-cholesterol (H:14C ratio, 7.0) was administered to two Digitalis samen plants. A method of isolating the transformation products of tigogenin from the crude saponin mixture, under conditions which did not induce the loss of the C-25 proton, was developed. The (25)-(25-3H)(4-14C)-5a-cholestan-33, 16~,22 ,26-tetrol was isolated, and its H: 14C ratio (6.96) was found to be identical with that of the (253 H)-(4-14C)-cholesterol administered. Jones' oxidation of the tetrol gave (25f)-(25-3H)-(4-' 4C)-3,16,22-triketo5a-cholestan-26-oic acid, which on base-catalyzed equilibration lost all the tritium. It was concluded that the C-26 oxygenation of cholesterol during its biosynthetic conversion to tigogenin in D. samen, does not require the intermediacy of a C-24 (or C-25) double bond.