This work describes the application of vibrational (VCD) and electronic (ECD) circular dichroism spectroscopy to solve the longstanding debate around the absolute configuration of (+)-frondosin B (1). The absolute configuration of (+)-1 could confidently be assigned as (R) using these spectroscopic techniques. The discrepancy in the optical rotation (OR) values obtained in previous studies can be attributed to an undetected minor impurity (ca. 7%) that arose unexpectedly in a key step late in the synthesis. Additionally, the conditions used in the final step of the previous reports for demethylation to form the natural product proceeded with significant loss of enantiopurity. The large OR measured for the impurity at its observed level, when compared to the small rotation for the less enantiopure natural product 1, led to a measured OR value for the synthetic material that had the opposite sign of the natural product.

中文翻译：

---

超越光学旋转：总合成中剩下的并不总是正确的

这项工作描述了振动（VCD）和电子（ECD）圆二色光谱的应用，以解决有关（+）-frondosin B（1）的绝对构型的长期争论。使用这些光谱技术，可以肯定地将（+）- 1的绝对构型指定为（R）。先前研究中获得的旋光度（OR）值的差异可归因于未检测到的少量杂质（大约7％）在合成后期的关键步骤中出乎意料地出现。另外，在先前报道的最后步骤中用于脱甲基以形成天然产物的条件进行时，对映体纯度显着降低。与不那么纯净的天然产物1的较小旋转相比，在其观测水平下测得的杂质的大OR导致合成材料的测量OR值与天然产物的符号相反。