jo052081t_si_001.pdf (177.71 kB)
General Approach for the Synthesis of 12-Methoxy-Substituted Sarpagine Indole Alkaloids Including (−)-12-Methoxy-Nb-methylvoachalotine, (+)-12-Methoxy-Na-methylvellosimine, (+)-12-Methoxyaffinisine, and (−)-Fuchsiaefoline
journal contribution
posted on 2006-01-06, 00:00 authored by Hao Zhou, Xuebin Liao, Wenyuan Yin, Jun Ma, James M. CookThe enantiospecific synthesis of 7-methoxy-d-tryptophan ethyl ester was completed by combination of
the Larock heteroannulation process with a Schöllkopf-based chiral auxiliary in good yield. This ester
was then employed in the first regiospecific, stereospecific total synthesis of (+)-12-methoxy-Na-methylvellosimine, (+)-12-methoxyaffinisine, (−)-fuchsiaefoline, and 12-methoxy-Nb-methylvoachalotine
in excellent overall yield. The asymmetric Pictet−Spengler reaction and enolate-driven palladium-catalyzed
cross-coupling processes served as key steps. The quaternary center at C(16) of 12-methoxy-Nb-methylvoachalotine was established via the Tollens reaction between (+)-12-methoxy-Na-methylvellosimine
and formaldehyde to form diol 17. The two prochiral primary alcohols in diol 17 were differentiated by
the oxidative cyclization(DDQ) of the hydroxyl group at the axial position of 17 with the benzylic postion
at [C(6)] to form a cyclic ether [C(6)−O(17)]. After oxidative formation of the α-ester at C(16), the
ether bond was reductively cleaved with TFA/Et3SiH in high yield. The DDQ-mediated oxidative
cyclization and TFA/Et3SiH reductive cleavage served as protection/deprotection steps in order to provide
a versatile entry into the voachalotine alkaloids.