Synthesis of hydroxyphthioceranic acid using a traceless lithiation–borylation–protodeboronation strategy
作者:Ramesh Rasappan、Varinder K. Aggarwal
DOI:10.1038/nchem.2010
日期:2014.9
In planning organic syntheses, disconnections are most often made adjacent to functional groups, which assist in CâC bond formation. For molecules devoid of obvious functional groups this approach presents a problem, and so functionalities must be installed temporarily and then removed. Here we present a traceless strategy for organic synthesis that uses a boronic ester as such a group in a one-pot lithiationâborylationâprotodeboronation sequence. To realize this strategy, we developed a methodology for the protodeboronation of alkyl pinacol boronic esters that involves the formation of a boronate complex with a nucleophile followed by oxidation with Mn(OAc)3 in the presence of the hydrogen-atom donor 4-tert-butylcatechol. Iterative lithiationâborylationâprotodeboronation allows the coupling of smaller fragments to build-up long alkyl chains. We employed this strategy in the synthesis of hydroxyphthioceranic acid, a key component of the cell-wall lipid of the virulent Mycobacterium tuberculosis, in just 14 steps (longest linear sequence) with full stereocontrol. Coupling of carbamates with boronic esters followed by protodeboronation creates a new carbonâcarbon bond, leaving behind no trace of the functional groups used to create it. Now, methodology for the protodeboronation of alkyl pinacol boronic esters has been developed and an iterative lithiationâborylationâprotodeboronation strategy used in a 14-step stereocontrolled synthesis of hydroxyphthioceranic acid.
在计划有机合成时,断链最常发生在官能团附近,有助于碳-碳键的形成。对于没有明显官能团的分子,这种方法提出了一个问题,因此必须暂时安装功能性基团,然后再移除。在这里,我们提出了一种无痕迹的有机合成策略,该策略使用硼酸酯作为在一锅锂化-硼化-去硼化序列中的这种功能基团。为了实现这一策略,我们开发了一种方法,用于对频哪醇硼酸酯的脱硼酸化,该方法涉及与亲核试剂形成硼酸盐络合物,然后在4-叔丁基邻苯二酚的氢原子供体存在下用Mn(OAc)3氧化。迭代锂化-硼化-脱硼化允许将较小片段耦合以构建长烷基链。我们将这一策略应用于制备强毒性结核杆菌细胞壁脂质的关键成分羟基紫胶虫酸的合成中,仅用14步(最长线性序列)就实现了立体控制。通过脱硼化将氨基甲酸酯与硼酸酯结合,可以创建新的碳-碳键,不会留下用于创建该键的官能团的任何痕迹。现在,已经开发出了频哪醇硼酸酯的脱硼化方法,并在14步立体控制合成羟基紫胶虫酸中使用了迭代锂化-硼化-脱硼化策略。