Sandoval-Ramírez J, Meza-Reyes S, del Río RE, Hernández-Linares G, Suárez-Rojas A, Rincón S, Farfán N, Santillan RL. Regioselective cleavage of rings E and F in sarsasapogenin.
Steroids 2003;
68:199-204. [PMID:
12606011 DOI:
10.1016/s0039-128x(02)00170-8]
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Abstract
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.
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