Determination of structure and stereochemistry of tomentosic acid by X-ray crystallography. A novel mechanism for transformation of arjungenin to tomentosic acid

Abstract

The molecular geometry of tomentosic acid (1), a tetrahydroxy triterpene acid obtained by acid hydrolysis of arjunglucoside-I, isolated from Terminalia bellerica, has been determined by single-crystal X-ray analysis. The triterpene, C₃₀H₄₈O₆, is orthorhombic with space group P2₁2₁2₁ and lattice constants a= 15.994(2), b= 14.682(3), c= 11.494(2)Å, cell volume V= 2 699.0 ų, and Z= 4. Diffractometer intensity measurements were made with Ni-filtered Cu-K_α radiation, and least-squares adjustment of the atomic parameters converged to a final R-value of 0.036. ¹³C NMR data are satisfactorily interpreted within terms of the structure. The mechanism of the transformation of arjungenin to tomentosic acid via formation of the 28→19 lactone is elucidated. The epimerisation of the 16β-OH group of cochalic acid to its 16α-isomer is also briefly described.