## 4 Discussion  
Once the planar molecular structures of the designated natural products were elucidated as depicted in **6**-**8** based on the analyses outlined above, our attention was diverted toward establishing their relative stereochemistry. Two out of the twenty-one carbons of **6** and **7** are stereogenic, and their location along the C8 acyclic tail made the configurational assignment somewhat difficult. Initially, the relative configurations of the stereocenters at C6 and C10 in compounds **6** and 7 were tentatively established through comprehensive analyses that included comparisons of proton and carbon chemical shifts along with proton NMR coupling data between each of these compounds (Table 1) and those reported for compound **1**[17]. Furthermore, a 2D NOESY experiment allowed us to reduce the number of possible stereoisomers surrounding the furanylidene moiety. Figure 5 highlights \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Concentration**} & \multicolumn{2}{c}{**Compound 6**} & \multicolumn{2}{c}{**Compound 7**} & \multicolumn{3}{c}{**Mixture of Compounds**} \\  & \multicolumn{2}{c}{} & \multicolumn{2}{c}{} & \multicolumn{2}{c}{**8 and 9**} \\ \cline{2-7}  & **\% Cell** & **\% CV** & \multicolumn{2}{c}{**\% Cell**} & \multicolumn{1}{c}{**\% CV**} & \multicolumn{1}{c}{**\% Cell**} & \multicolumn{1}{c}{**\% CV**} \\  & **Viability** & & & & & & **Viability** & **\% CV** \\ \hline
400 & 7 & 5 & 0 & 0 & 32 & 5 \\
200 & 43 & 5 & 10 & 3 & 71 & 5 \\
100 & 78 & 4 & 69 & 4 & 78 & 3 \\
50 & 83 & 5 & 82 & 5 & 85 & 5 \\
25 & 89 & 4 & 92 & 5 & 85 & 4 \\
13 & 88 & 3 & 103 & 3 & 84 & 2 \\
6 & 87 & 3 & 101 & 3 & 87 & 3 \\
3 & 90 & 3 & 101 & 2 & 89 & 4 \\
2 & 91 & 3 & 96 & 5 & 92 & 3 \\
0 & 100 & 4 & 100 & 4 & 100 & 5 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Percentages of cell viability of MCF-7 cells treated with compound **6** (gracilioether M), compound **7** (11,12-dihydroracilioether M), and the mixture of compounds **8** (9,10-dihydroracidone G) and **9** (plakortone G). Figure 4: Dose–response or IC\({}_{50}\) curves of the human breast cancer MCF-7 cell line for (red) compound **6** (gracilioether M), (green) compound **7** (11,12-dihydroracilioether M), (blue) the mixture of compound **8** and **9** (9,10-dihydroracidone G and plakortone G), and (black dashed lines) letrozole (10) as positive control. a few of the 2D NOESY cross-peaks that were observed for compound **6**. The actual 2D NOESY NMR spectrum of **6** (Figure S8) has been included in the Supplementary Materials. Most notably, the presence of cross-peaks between H\({}_{2}\)-15 and both H2 and H5 indicated that these protons lie within close proximity to each other in both gracilioether M (**6**) and 11,12-dihydroracilioether M (7). Regarding the C6 and C10 in compounds **6** and **7**, the absence of reliable cross-peaks that were ascribable to Hs near these centers made it impossible to assign their relative stereochemistry based on NOE correlations alone. Notwithstanding this, the fact that we managed to convert **6** into **7** by selective catalytic hydrogenation (vide infra) proved categorically that these compounds share a common stereochemistry at C6 and C10. At this time, we remark that this conclusion was later substantiated through detailed comparisons of relevant NMR chemical shifts of compound **7** with those of 9,10-dihydroracilio (**8**) (see Table 1). To finalize our configurational assignments for compounds **6** and **7**, we conducted a series of chemical correlation studies (Scheme 1) inspired by the prior work of [PERSON] and coworkers [35], who had firmly established the absolute configuration of plaktorone G (**9**) as 4\(R\),8\(R\). We commenced our investigation by conducting the catalytic hydrogenation of the mixture of 9,10-dihydroracilio G (**8**) and plaktorone G (**9**) (H\({}_{2}\), Pd-on-C, EtOAc, 8 h). This simple procedure led to compound **8** as the sole product and thus quickly established its absolute configuration as 4\(S\),8\(R\).