4-[(E)-(4-Fluoro-benzyl-idene)amino]-benzoic acid.

In the title compound, C(14)H(10)FNO(2), the benzene rings make a dihedral angle of 57.50 (13)°, and the molecule has an E configuration about the C=N bond. In the crystal, molecules are linked via pairs of O-H⋯O hydrogen bonds, forming inversion dimers.

Regioselective cleavage of rings E and F in sarsasapogenin.

Sapogenins from the 25R and 25S series show a marked difference on the E/F regioselectivity of the spiroketal cleavage with BF(3)/Ac(2)O. In contrast to the high yield of single E-ring cleavage products from diosgenin (3) and hecogenin (5), sapogenins of the 25R series (equatorial C-27 methyl), sarsasapogenin (1, 25S series, axial C-27 methyl) yields the corresponding acetyldihydropyran, (25S)-23-acetyl-22,26-epoxy-5beta-cholest-22-ene-3beta,16beta-diyl diacetate (8), two isomeric furostenes: (E)- and (Z)-(25S)-23-acetyl-5beta-furost-22-ene-3beta,26-diyl diacetate (9 and 10) and a third one bearing an additional acetyl group: (E)-(20S,25S)-20,23-diacetyl-5beta-furost-22-ene-3beta, 26-diyl diacetate (11). The structures of the compounds were unambiguously established using two dimensional NMR techniques.