DETERMINATION OF STRUCTURE AND STEREOCHEMISTRY OF TOMENTOSIC ACID BY X-RAY CRYSTALLOGRAPHY - A NOVEL MECHANISM FOR TRANSFORMATION OF ARJUNGENIN TO TOMENTOSIC ACID

The molecular geometry of tomentosic acid (1), a tetrahydroxy triterpene acid obtained by acid hydrolysis of arjunglucoside-1, 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 Å3, and Z = 4. Diffractometer intensity measurements were made with Ni-filtered Cu-Kα 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.