Structural studies on the biosides of Digitalis lanata: bisdigitoxosides of digitoxigenin, gitoxigenin and digoxigenin.

Abstract

The crystal structures and conformations of bisdigitoxosides of digitoxigenin (I), gitoxigenin (II) and digoxigenin (III and IV) have been determined using single-crystal X-ray crystallographic techniques. Crystals of (I), (II) and (IV) were grown from ethyl acetate solutions of the glycosides while (III) was grown from a solution of the digitoxoside in ethanol. As in other cardiac glycosides the ring junctions A-B and C-D are cis. The D ring in these structures shows different conformations while the A, B and C rings remain conformationally similar. Although digitoxigenin bisdigitoxoside and gitoxigenin bisdigitoxoside differ from each other in the absence and presence of a hydroxyl group at C(16) of the D ring, these two biosides crystallize in the space group P2(1)2(1)2 [corrected] and are isomorphous. The presence of the hydroxyl group at C(16) does not affect the orientation of the lactone ring and the conformation of the molecule. Digoxigenin bisdigitoxoside crystallizes in two different crystal systems with four molecules of water in the orthorhombic form and one molecule of ethyl acetate in the triclinic form. In both forms the hydroxyl at C(3') of the first sugar forms a hydrogen bond with the ring oxygen of the second sugar. This has also been observed in the trioside digoxin. The torsion angle C(13)-C(17)-C(20)-C(22) in the two forms differs by 7 degrees.(ABSTRACT TRUNCATED AT 250 WORDS)