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. The lower E/F selectivity in the cleavage of 1 is attributed to steric hindrance resulting from the axial methyl in F ring on a beta elimination forming the dihydropyran double bond in the major product 8.
Sandoval-Ramírez
Jesús
J
Facultad de Ciencias Qui;micas, Benemérita Universidad Autónoma de Puebla, Ciudad Universitaria, San Manuel, Pue., Mexico. jsandova@siu.buap.mx
Meza-Reyes
Socorro
S
del Río
Rosa E
RE
Hernández-Linares
Guadalupe
G
Suárez-Rojas
Angela
A
Rincón
Susana
S
Farfán
Norberto
N
Santillan
Rosa Luisa
RL
D009682:Magnetic Resonance Spectroscopy; D015394:Molecular Structure; D013150:Spirostans / Q000737:chemistry*
D013150:Spirostans; C046297:sarsasapogenin