General approach for the synthesis of ajmaline-related alkaloids. Enantiospecific total synthesis of (-)-suaveoline, (-)-raumacline, and (-)-Nb-methylraumacline
作者:Xiaoyong Fu、James M. Cook
DOI:10.1021/jo00055a019
日期:1993.1
(-)-Raumacline (3) and (-)-Nb-methylraumacline (4), isolated from plant cell cultures of Rauwolfia serpentina Benth., have been synthesized from D-(+)-tryptophan in enantiospecific fashion. In addition, (-)-suaveoline (1) has also been prepared via this strategy. The synthesis of these indole alkaloids employed a stereospecific Pictet-Spengler/Dieckmann protocol to prepare the key intermediate, (-)-N(b)-benzyltetracyclic ketone 12. This ketone 12 was converted into the C-15 functionalized system 16 via an anionic oxy-Cope rearrangement; 16 had also been obtained by 1,4-addition of a Grignard reagent to the alpha,beta-unsaturated aldehyde (-)-8 generated from (-)-12. Conversion of 16 into either (-)-N(b)-benzylsuaveoline (23) or (-)-N(b)-benzylraumacline (28), respectively, is described. Catalytic debenzylation of the hydrochloride salts of 23 and 28 in ethanol provided the corresponding alkaloids (-)-1 and (-)-3 in excellent yield. In contrast to the published reports, the optical rotation of (-)-suaveoline (>98% ee) was found to be [alpha]25D = -9.3-degrees (c = 0.30, CHCl3) rather than 0-degrees as previously reported. Treatment of 23 or 28 (individually) with excess Pd/C and hydrogen in methanol yielded (-)-N(b)-methylsuaveoline and (-)-N(b)-methylraumacline in excellent yield, respectively.
本研究成功地通过不对称方式利用D-(+)-色氨酸合成了从Rauwolfia serpentina Benth.植物细胞培养物中分离出的(-)-Raumacline(3)和(-)-Nb-甲基raumacline(4)。此外,(-)-Suaveoline(1)也通过该方法成功制备。在吲哚类生物碱的合成过程中,采用了一种立体特异的Pictet-Spengler/Dieckmann协议来制备关键中间体(-)-N(b)-苄基四环酮(12)。随后,通过阴离子氧-Cope重排,将酮12转化为C-15功能化系统16;另外,通过将Grignard试剂与从(-)-12生成的α,β-不饱和醛(-)-8进行1,4-加成,也获得了16。将16分别转化为(-)-N(b)-苄基suaveoline(23)或(-)-N(b)-苄基raumacline(28)的过程得到了详细描述。通过在乙醇中使用乙醇的催化脱苄基反应,分别以优异的产率得到了相应的生物碱(-)-1和(-)-3。与已发布的报告不同,(-)-Suaveoline的光学旋转被发现为[α]D25 = -9.3度(c = 0.30,CHCl3),而不是先前报告的0度。将23或28分别与过量的Pd/C和氢气在甲醇中处理,分别以优异的产率得到了(-)-N(b)-甲基suaveoline和(-)-N(b)-甲基raumacline。