Elucidation of the Structure of Pseudorubriflordilactone B by Chemical Synthesis

Rubriflordilactone B (1) is a schinortriterpenoid isolated by Sun and colleagues, which possesses a tetrasubstituted benzene moiety and eight stereocenters. The previous synthesis of 1 by Li and co-workers uncovered the existence of its naturally occurring stereoisomer “pseudorubriflordilactone B”. Here we report a collaborative study by the two groups that elucidates the structure of pseudorubriflordilactone B to be 16,17-bis-epi-rubriflordilactone B (26). Chemical synthesis served as an important tool in the structure determination. Taking advantage of a modular synthetic route, we systematically “mutated” the configurations of C-23, C-22, C-20, and C-16/C-17 located at the right-hand domain of 1, and thus prepared its 15 stereoisomers for spectrum comparison. The 1H NMR spectra of synthetic 26 in deuterated chloroform and pyridine were identical to those of authentic pseudorubriflordilactone B, respectively. This synthetic sample displayed anti-HIV activity (EC50 = 0.288 μM) in vitro.红佛地内酯B（Rubriflordilactone B）(1)是由孙及其团队分离得到的裂环三萜（schinortriterpenoid），其分子结构含有四取代苯环结构（tetrasubstituted benzene moiety）与八个手性中心（stereocenters）。此前李团队在化合物1的全合成研究中，发现了其天然存在的立体异构体——伪红佛地内酯B（pseudorubriflordilactone B）。本研究由两个团队联合完成，明确了伪红佛地内酯B的结构为16,17-双差向异构红佛地内酯B（16,17-bis-epi-rubriflordilactone B）(26)。化学合成在该结构解析工作中扮演了关键工具角色。我们借助模块化合成路线（modular synthetic route），对化合物1右侧结构域内的C-23、C-22、C-20以及C-16/C-17的手性构型进行系统性改造，由此制备得到该分子的15种立体异构体用于光谱比对。合成所得的26在氘代氯仿（deuterated chloroform）与氘代吡啶中测得的1H核磁共振（1H NMR）光谱，分别与天然伪红佛地内酯B的光谱完全一致。该合成样品在体外实验中展现出抗HIV活性，其半数有效浓度EC50为0.288 μM。