Absolute Configuration of Highly Flexible Natural Products by the Solid-State ECD/TDDFT Method: Ximaolides and Sinulaparvalides

The quantum-mechanical calculation of chiroptical properties such as electronic circular dichroism (ECD) is an increasingly popular tool for establishing the absolute configuration of organic compounds, including natural products. However, this approach is often limited by conformational flexibility, which can be overcome by considering the solid crystalline state instead of solution. By knowing the solid-state geometry, one may employ it as the input structure for ECD calculations and then comparing this with the ECD spectrum measured on a microcrystalline sample. We report here the application of this solid-state ECD approach to establish the absolute configuration of three natural products, namely ximaolide A, sinulaparvalide A and B; all display pronounced flexibility and belong to two families of compounds with remarkable biological activity. The present configurational assignment may also be extended to several analogues whose absolute configurations have not yet been reported.