3.1.5. Macrolides

Macrolides, especially those possessing 10- to 19-membered ring systems, have diversified structural features and constitute a prominent group of active secondary metabolites. Since the discovery of well-known progenitor macrolide antibiotic pikromycin in 1950 and the second generation of macrolides such as azithromycin and clarithromycin, naturally occurring macrolides have been found today due to their diverse structures and promising biological properties [54]. A total of 63 macrolides have been isolated from the mangrove-associated fungi.

Sumalarins A−C (**44**−**46**) (Figure 14) were identified from the cytotoxic extract of *Penicillium. sumatrense* MA-92 from the rhizosphere of the mangrove *Lumnitzera racemose*. Notably, they were the unusual and rare examples of sulfur-containing curvularin derivatives isolated for the first time from natural sources [55]. Compounds **44**−**46** displayed

cytotoxic activities against Du145, HeLa, Huh 7, MCF-7, NCI-H460, SGC-7901, and SW1990 cell lines with IC50 values ranging from 3.8 *μ*M to 10 *μ*M. Compound **44** is likely formed via Michael's addition of 3-mercaptolactate to the double bond *Δ*10,11 of dehydrocurvularin. Esterification or acylation of **45** probably leads to the biosynthesis of **44** and **46** [55].

**Figure 14.** Structures of compounds **44**–**46** and dehydrocurvularin.

Ansamycins are characterized by an aromatic nucleus connected with a polyketide chain back to a nonadjacent position through an amide bond. Hertweck et al. isolated four unusual ansa macrolides, compounds **47**–**50** (Figure 15), from *Streptomyces* sp. HKI0576, a bacterial endophyte separated from the stem of mangrove *Bruguiera gymnorrhiza* [56]. This was the first report on discovering ansamycins from a plant endophyte. In addition, the degree of "in-built diversification" of these four compounds is unprecedented for complex polyketides. Among them, divergolide A (**47**) represents an unusual type of ansa macrolide with an unusual branched side chain and a disrupted polyketide backbone. Furthermore, the tricyclic chromophore is unprecedented for macrolides, and related *O*-heterocyclic substructures are only known from aromatic polyketides, such as the nogalamycin aglycone [57] and chaetoxanthone [58]. Divergolide B (**48**) represents another unusual type of ansa macrolide featuring a novel benzopyran/chromene core as the first congener of **47**. In addition, compounds **49** and **50** share substructures with **47** and **48** but feature structurally intriguing tetracyclic scaffolds. Furthermore, the ansa macrolides display significant antimicrobial and cytotoxic activities, probably regulating the immunity of the mangrove tree. Compounds **47**–**50** are biosynthesized from a common linear polyketide using 3-amino-5- hydroxybenzoic acid (AHBA) as a primer unit. Various reactions, including an optional acyl migration, generate the diverse multicyclic structures [56,59] (Scheme 6).

**Figure 15.** Structures of compounds **47**–**51**.

In 2014, Shen et al., cloned the biosynthetic gene cluster involved in the biosynthesis of the divergolides from the endophytic *Streptomyces* sp. W112 isolated from *Camptotheca acuminata*. Following gene disruption, gene overexpression, and bioinformatics analysis, they laid the foundation for further elucidation of the biosynthetic pathway as well as titer improvement [60]. In addition, Zhong et al. [61] conducted genome sequencing, bioinformatics analysis, and further isolations of four new divergolide congeners with a similar endophytic bacteria, *Streptomyces* sp. from *Bruguiera gymnorrhiza.* They showed that specialized acyltransferase domains are for selecting extender units, and the branched isobutylmalonyl-CoA is involved.

**Scheme 6.** Proposed biosynthetic pathway for compounds **46**–**49** [56].

The total synthesis of divergolide A using the ring-closing metathesis (RCM) approach was published by Dai et al., in 2012 [62]. Subsequently, Rasapalli et al. synthesized the western section of divergolides C (**49**) and D (**50**) and demonstrated the robustness of C4-C5 as an appropriate approach for the further total synthesis of divergolides C and D in 2013 [63]. This chemical method was also conducted for divergolides A and B. Studies on the total synthesis of divergolides A-D using inexpensive, readily available starting materials and simple operations have also been constantly reported in recent years [64–66].

A macrocyclic polyketide with an unusual carbon skeleton, namely hainanmycin A (**51**) (Figure 15), was isolated from *Streptomyces* sp. 219807 (from mangrove soil collected in Sanya) [67]. Compound **51** featured an unprecedented structural skeleton of a 17-membered carbocyclic framework. The cyclo-heptadeca framework containing a cyclopentenone ring substituted with a naturally occurring bridgehead enol motif is unique among NPs. It represents a new subgroup, a minor family of carbocyclic polyketide macrolides. Hong et al. [67] proposed a plausible biosynthetic pathway for **51** based on the biosynthesis of akaeolide [68], an analogue of **51**. Shortly, the PKS condenses acetyl-CoA and other building units (e.g., methylmalonyl-CoA and malonyl-CoA) to a linearized polyketide backbone. A thioesterase (TE) then releases the backbone with the formation of a *δ*-lactone ring. Further construction of the C-C bonds of C-16/C-12 and C-18/C-2 generate the structural core. Notably, a C-18 aldehyde intermediate (**S1**) might be involved in the C-18/C-12 carbon bond formation (Scheme 7).

**Scheme 7.** Proposed biosynthetic pathway for compound **51** [67].
