3.1.6. Others

Eight new compounds, streptoglycerides A–H (**52**–**59**) (Figure 16) possessing a unique ring system, were obtained from *Streptomyces* sp. isolated from a mangrove sample collected on Kosrae Island [69,70]. This is the first report to describe a rare 6/5/5 tricyclic ring system consisting of a glycerol moiety from marine organisms. Streptoglyceride C (**54**) showed a weak inhibitory effect on nitric oxide production in BV-2 microglia cells. Compounds **56**–**59** showed significant anti-inflammatory activity by inhibiting lipopolysaccharide (LPS)- induced nitric oxide (NO) production in Raw 264.7 cells with IC50 values ranging from 3.5 to 10.9 *μ*M. It should be noted that **57** suppressed the transcription of iNOS and IL-6 without cytotoxicity.

**Figure 16.** Structures of compounds **52**–**63**.

Upon further investigation of the unusual strain, four new compounds, miharadienes A–D (**60**–**63**), possessing unique ring systems and a rare diene side chain, were isolated [71]. A plausible biosynthetic pathway of miharadienes and related compounds, streptoglycerides is proposed in the literature (Scheme 8a) [71]. However, the formation of **52**–**55** from **63** by attacking the nucleophilic hydroxy on the electron-rich furan ring seems inapplicable. Therefore, we proposed an optional pathway for **52**–**55** (Scheme 8b). In short, the starting lauryl alcohol derivative appears to react with dihydroxyacetone, an oxidation product of glycerol, to form the intermediate **int i** through Aldol type reaction of the C-4 active methylene of lauryl alcohol derivative with the carbonyl of the dihydroxyacetone. Then the hemiketal formation gives the tetrahydrofuran ring, and ether formation forms the other tetrahydrofuran ring. Afterward, the ketal formation by the interaction of the terminal hydroxyethylene with the hemiketal provides the pyran ring and affords the intermediate **int ii**, which could be further converted into **52**–**55**, possessing a rare 6/5/5 ring system.

 **Scheme 8.** (**a**) Proposed biosynthetic pathway for compounds **52**–**55** and **60**–**63** in the literature [71]; (**b**) proposed biosynthetic pathway for **52**–**55** in this review.
