## 3 Results  
### Chemical Structural Analysis  
During this study, three new polykeptides possessing a furanylidene motif were isolated from the marine sponge _P. halichondioides_ along with previously known plakortone G (**9**) [22], thus totaling four compounds. Gracilioether M (**6**) was obtained as a colorless optically active oil: [\(\alpha\)]\({}^{20}\)D-80.5 (c 0.96, CHCl\({}_{3}\)); UV (MeOH) \(\lambda_{\text{max}}\) (log \(\varepsilon\)) 285 (3.45), 240 (2.55), 201 (3.17) nm; and \(\
u_{\text{max}}\) (thin film) 2964, 2936, 2874, 1715, 1689, 1626, 1459, 1164 cm\({}^{-1}\). High-resolution electrospray ionization mass spectrometry (HRESIMS) indicated a single [M+Na]\({}^{+}\) ion peak at \(m/z\) 357.2401, suggesting a molecular formula of C\({}_{21}\)H\({}_{34}\)O\({}_{3}\) (calculated as 357.2400). Likewise, 11,12-dihydroxforteo M (**7**) was obtained as a colorless optically active oil: [\(\alpha\)]\({}^{20}\)D-47.1 (c 0.70, CHCl\({}_{3}\)); UV (MeOH) \(\lambda_{\text{max}}\) (log \(\varepsilon\)) 286 (3.15), 240 (2.31) nm; and \(\
u_{\text{max}}\) (thin film) 2961, 2929, 2873, 2859, 1716, 1688, 1638, 1626, 1458, 1273, 1164 cm\({}^{-1}\). HRESIMS indicated a single [M+Na]\({}^{+}\) ion peak at \(m/z\) 359.2553, suggesting a molecular formula of C\({}_{21}\)H\({}_{36}\)O\({}_{3}\) (caled 359.2557). All the proton and carbon resonances of **6** and 7 were assigned by detailed 1D \({}^{1}\)H- and \({}^{13}\)C-NMR analysis and a combination of 2D NMR [\({}^{1}\)H-\({}^{1}\)H-correlated spectroscopy (\({}^{1}\)H-\({}^{1}\)H COSY), heteronuclear single-quantum coherence spectroscopy (HSQC), and heteronuclear multiple-bond correlation (HMBC)]. The \({}^{1}\)H and \({}^{13}\)C NMR data for compounds **6** and **7** are shown in Table 1.  
The two-dimensional molecular structures for gracilioether M (**6**) and 11,12-dihy drogracilioether M (**7**) were successfully determined based on gradient-selected correlation spectroscopy (gCOSY), total correlation spectroscopy (TOCSY), HSQC, and HMBC data. Copies of all the spectra can be found in the supplementary materials as Figures S1-S14. Careful spectroscopic analyses clearly show a distinction between the two compounds: gracilioether M (**6**) possessed an additional olefin at the C11-C12 position (\(\delta_{\rm H}\) 5.02, H11; \(\delta_{\rm H}\) 5.34, H12; \(\delta_{\rm C}\) 133.3, C11; \(\delta_{\rm C}\) 132.2, C12) when compared against 11,12-dihydrogracilioether M (**7**) (\(\delta_{\rm H}\) 1.10-1.20, H11\(\alpha\); \(\delta_{\rm H}\) 1.10-1.20, H11\(\beta\); \(\delta_{\rm H}\) 1.10-1.20, H12; \(\delta_{\rm C}\) 32.7, C11; \(\delta_{\rm C}\) 28.8, C12). However, it is important to mention that the furanylidene functionality present in **6** and **7** is the same as that reported for compounds **1-5**[17; 18; 19; 20] on account of the fact that all these compounds display the same key resonances, which are ascribable to that functionality. Conversely, despite our best efforts, 11,12-unsaturated butylrolactones 9,10-dihydr oplakortone G (**8**) and plakortone G (**9**) were isolated as an inseparable mixture. This mixture was obtained as a yellowish optically active oil: [\(\alpha\)]\({}^{20}_{\rm D}\)-38.3 (c 0.87, CHCl\({}_{3}\)).