An Overview of Bioactive 1,3-Oxazole-Containing Alkaloids from Marine Organisms

1,3-Oxazole chemicals are a unique class of five-membered monocyclic heteroarenes, containing a nitrogen atom and an oxygen. These alkaloids have attracted extensive attention from medicinal chemists and pharmacologists owing to their diverse arrays of chemical structures and biological activities, and a series of 1,3-oxazole derivatives has been developed into therapeutic agents (e.g., almoxatone, befloxatone, cabotegravir, delpazolid, fenpipalone, haloxazolam, inavolisib). A growing amount of evidence indicates that marine organisms are one of important sources of 1,3-oxazole-containing alkaloids. To improve our knowledge regarding these marine-derived substances, as many as 285 compounds are summarized in this review, which, for the first time, highlights their sources, structural features and biological properties, as well as their biosynthesis and chemical synthesis. Perspective for the future discovery of new 1,3-oxazole compounds from marine organisms is also provided.


Introduction
Natural products and their structural analogs are an invaluable source of inspiration in drug design and development. 1,3-Oxazole chemicals are a unique class of five-membered monocyclic heteroarenes, containing a nitrogen atom and an oxygen atom, and usually possess a 2,4-or 2,5-substitution pattern with diverse chemical structures. It is noteworthy that these alkaloids display a wide range of biological properties, such as antibacterial, antifungal, anti-inflammatory, antimalarial, antioxidant, antiviral, cytotoxic, insecticidal, repellent activity, and an inhibitory effect on kinase, showcasing great therapeutic potential [1,2]. A growing amount of evidence indicates that 1,3-oxazole heterocycle is a fundamental structural motif found in marine natural products, which is putatively biosynthesized by the cyclodehydration and dehydrogenation of two amino acids ( Figure 1) [3]. By an extensive and in-depth literature search using DNP (dictionary of natural products) and SciFinder tools, a total of 285 marine-derived 1,3-oxazole-containing compounds  were isolated and reported until 2020. Almost 80% of these substances were produced by animals (sponges, ascidian, nudibranch, hydrozoan, coral, sea hare and plume) followed by plants (cyanobacteria and red algae) and microorganisms (bacteria and fungi) ( Figure 2). To better understand these marine-derived 1,3-oxazole substances and promote marine drugs, here, a systematic and comprehensive review is summarized for the first time. It highlights their biological sources, structural features and bioactivities, as well as biosynthesis and chemical synthesis by comparing with two recent review works, in which one mainly focuses on the biological activities of synthetic and natural oxazoles [4], and the other puts more emphasis on the sources, biological activities, structural features and chemical synthesis of natural oxazole-containing peptides [5]. On the basis of chemical structures, these 1,3-oxazole-based alkaloids are grouped into four major types including peptide, macrolide, polyketide and benzoxazole, and are respectively introduced herein. Detailed information for these natural products is supplied in the Supplementary Materials.

Linear Peptides
Marine algae and microorganisms are the major sources of linear peptides with the 1,3-oxazole motif. The chemical investigation of a red seaweed of Delesseriaceae from the coasts of north Dakar (Dakar, Senegal) successively led to the isolation of four new linear dipeptides almazoles A-D (1-4) (Scheme 1), in which each compound bears an unusual 2,5-disubstituted oxazole moiety [6][7][8]. Absolute structures of these algal metabolites were originally determined by biomimetic synthesis from L-tryptophan with N,N-dimethyl-Lphenylalanin, for which Robinson-Gabriel cyclization served as the key oxazole forming step [6]. Lately, almazole C (3) has been well prepared by a four-step synthesis, with a 32% overall yield through the Aza-Wittig reaction [9], while compound 4 was the first bioactive member of the almazole family, possessing potent antibacterial against Serratia marcescens and Salmonella typhi XLD [8], and its original structure was revised as 5-(3-indolyl)oxazole-4-carboxylic acid by Horne and his coworkers [10]. Streptochlorin (5) and martefragin A (6) are another two novel 5-(3-indolyl)oxazole-containing alkaloids, respectively produced by marine strain Streptomyces sp. 04DH110 from the east sea of Korea [11] and the red algae Martensia fragilis collected off the coast of Uozu (Uozu, Japan) [12]. In addition to potent fungicidal activity against Pythium, Botrytis cinerea, Septoria tritici, Pyricularia Oryzae, Fusarium culmorum and Rhizoctonia solani, compound 5 displayed a strong inhibitory effect on angiogenesis by blocking NF-κB signaling, as well as human leukemia and normal liver cell lines with IC 50 values of 1.05 and 10.9 µg/mL, respectively [11,13,14]. It is noteworthy that one three-step synthesis of compound 5 had been readily achieved by sequential asmicester condensations and sulfanyl-Lithium exchange-trapping [15]. As a promising inhibitor of lipid peroxidation, compound 6 was firstly synthesized by Nishida and coworkers in 1998, and its absolute configuration was unambiguously determined as S, S [16]. Almazolone (7) had similar biogenetic features to compounds 1-4 and was purified from Senegal red alga Haraldiophyllum sp. [17]. Co-culture of Serratia sp. and Shewanella sp. resulted in the production of serratiochelin A (8), which displayed antiproliferative activity against human melanoma cell lines and non-malignant lung fibroblasts, as well as antimicrobial activity toward Staphylococcus aureus [18]. Two new indole-containing peptides, JBIR-34 (9) and JBIR-35 (10), were produced by one symbiotic strain Streptomyces sp. Sp080513GE-23 from Haliclona sp., and exhibited a potent DPPH radical scavenging capability with IC 50 values of 1.0 and 2.5 mM, respectively [19]. The unique 4-methyloxazoline moiety was formed by a α-methyl-L-serine from D-alanine and a 5,10-methylene-tetrahydrofolate by FmoH [20]. Nigribactin (11) was isolated as a new siderophore from marine Vibrio nigripulchritudo and was shown to enhance the expression of the spa encoding protein A [21]. Phorbazoles A-D (12-15) were a novel class of marine alkaloids, containing chlorinated pyrrole and 1,3-oxazole moieties from the Indo-Pacific sponge Phorbas aff. clathrata (Levi) [22]. Recently, the chemical preparation of phorbazole B (13) has been achieved by simple catalytic chlorination and iodization to protect the oxazole ring [23]. The first chemical analysis of the marine mollusc Aldisa andersoni, collected off Muttom coast, (Kanyakumari, India) afforded two new cytotoxic phorbazole analogs, 9-chloro-phorbazole D (16) and N1-methyl-phorbazole A (17), as well as phorbazoles A (2), B (13) and D (15) [24].
Two novel linear and achiral polyketide-peptides, ariakemicins A (18) and B (19), were obtained as an inseparable mixture from one marine gliding bacterium Rapidithrix sp., collected off the muddy land alongside the Ariake Inland Sea, and displayed selectively antimicrobial activities against Gram-positive bacteria (Brevibacterium sp., S. aureus, and Bacillus subtilis), and weak cytotoxicity against cancer cell lines A549 and BHK [25]. Breitfussins A-H (20)(21)(22)(23)(24)(25)(26)(27) were the first marine natural products containing an indole-oxazolepyrrole framework from hydrozoan Thuiaria breitfussi inhabits in the Arctic ocean, and were found to excellently inhibit PIM1 and DRAK1 kinases [26,27]. Furthermore, breitfussin C (22) strongly exhibited a cytotoxic effect on cancer cell lines (MCF-7, HT-29, MOLT-4, MV-4-11 and MRC-5). One concise total synthesis of the halogen-rich dipeptides 20 and 21 was created by Bayer and his coworkers in 2015, which consists of the palladium-catalyzed cross-coupling of indole and pyrrole on an oxazole core and selective lithiation/iodination of a common indole-oxazole fragment [28]. Mechercharmycin B (28) was a new linear peptide containing four 1,3-oxazole rings produced by the marine strain Thermoactinomyces sp. YM3-251 from Mecherchar (Palau) [29]. Siphonazole (29) and dimethoxy analog (30) were the first naturally occurring substances that incorporated oxazole subunits connected by a two-carbon tether from a marine microbe Herpetosiphon sp., and exhibited selective cytotoxicity to human breast cancer HTB-129 and acute T cell leukemia TIB-152 [30]. The first chemical synthesis of 29 was achieved in 2007 through the preparation of an oxazole ring using rhodium carbene, and the installation of the pentadienyl amino side-chain [31].

Other Monocyclic Peptides
In addition to mechercharmycin B (28), one monocyclic nonapeptide, mechercharmycin A (119) (Scheme 7), was purified from the marine strain Thermoactinomyces sp. YM3-251 and shown to have excellent cytotoxicity toward A549 and Jurkat cells, with IC 50 values of 40 and 46 nM, respectively [29]. Urukthapelstatin A (120) was detected in the fermentation broth of the marine strain Mechercharimyces asporophorigenens YM11-542, from Urukthapel Island (Palau), and displayed a broad spectrum of potential cytotoxicities [88]. By the macrocyclization of one macrothiolactone, and the azole formatted by the Aza-Wittig ring contraction, chemical synthesis of compound 120 was first achieved by Schwenk and coworkers [89]. Interestingly, the SAR study showed that the phenyl ring attached to the eastern oxazole, and the rigid lipophilic tripeptide section definitely affected its cytotoxicity [90]. One new thiopeptide TP-1161 (121) was isolated from one marine Nocardiopsis sp. and its biosynthetic gene cluster (BGC) was confirmed by targeted gene inactivation [91]. This substance exhibited no inhibitory effect on Gram-negative bacteria, but excellent antimicrobial activity against a panel of Gram-positive clinical isolates [92]. Wewakazole (122) and wewakazole B (123) were novel cyclic dodecapeptides produced by cyanobacteria Lyngbya majuscule from Papua New Guinea, and Moorea producens collected in the red sea, respectively [93]. Bioassay results suggested that compound 122 was active against nonsmall cell lung cancer H-460 cells (IC 50 10.1 µM) and that 123 had a stronger cytotoxicity against H460 cells (IC 50 1.0 µM) and MCF7 breast cancer cells (IC 50 0.58 µM). The first total synthesis of 123 was achieved through the careful choice of amino acid-protecting groups and the construction of three different substituted oxazoles [94].

Bicyclic Peptides
Chemical analysis of the ascidian Diazona angulate led to discovery of five new peptides with two 12-membered macrocycles connected by a quaternary carbon stereo center, substituted by triaryl groups in furan indole nuclei, diazonamides A-E (124-128) (Scheme 8).
In particular, compound 124 displayed an extraordinary antitumor effect on a HCT-116 human colon carcinoma and B-16 murine melanoma cancer cell lines, with IC 50 values less than 15 ng/mL [95,96], and was chemically synthesized by using the Witkop photocyclization reaction for the first time [97]. Lissoclinum patella is the prolific producer of the new sulfur-containing bicyclic peptides ulithiacyclamide (129)

Mono-Oxazole Macrolides
Until 2020, marine sponges were the only sources of macrolides containing a monooxazole motif. Two cytotoxic isomers, phorboxazoles A (138) (Scheme 10) and B (139), as well as the precursor (140) were detected in the Indian Ocean marine sponge Phorbas sp. [102,103]. Leiodolides A (141) and B (142) were the first members of a new class of mixed polyketide-nonribosomal peptide synthetase from the deep-water marine sponge Leiodermatium. They structurally possess a 19-membered ring and several unique functional groups, including a bromine substituent and an α-hydroxy-α-methyl carboxylic acid sidechain terminus. These substances had obvious cytotoxic effects against human colon cancer HCT-116 with IC 50 values of 2.5 and 5.6 µM, respectively [104]. Chemical investigation of the Madagascan sponge Fascaplysinopsis sp. afforded three macrolides with bis-epoxide motif salarins 143-145, which showed pronounced inhibitory on human leukemia cell lines UT-7 [105,106]. Theonezolides A-C (146-148) from the Okinawan Theonella sp. were novel oxazole-containing macrolides that compose of two main fatty acid chains, including a 37-membered macrolide ring with long side chains connected by amide bonds. They exhibited cytotoxicity on a murine lymphoma L1210 and human epidermoid carcinoma KB cells and induced a platelet morphology change and aggregation in rabbits [107][108][109].

Exocyclic Oxazoles
Neopeltolide (193) (Scheme 14) was one of the new exocyclic oxazole-containing macrolides from a deep-water sponge of the family Neopeltidae, and was found to have potential antifungal effects against C. albicans and inhibitory activities against A-549, NCI-ADR-RES and P388 cell lines with IC 50 values of 1.2, 5.1 and 0.56 nM, respectively [120]. SAR analysis revealed that the substitution of the side chain and the stereochemistry of the macrolide carbon, especially C-11 and C-13, is vital for the overall biological property [121]. One doubly O-bridged 18-membered macrolide, leucascandrolide A (194), was detected in a calcareous sponge of a new genus Leucuscundra caveoluta from the Coral Sea [122]. Pharmacological study indicated that this metabolite was a novel inhibitor of cytochrome bc 1 [123]. Enigmazoles 195-197, from the sponge Cinachyrella enigmatica, were a new structural family of marine phosphomacrolides with an 18-membered macrocyclic ring, which consists of an embedded 2,6-cis-substituted tetrahydropyran ring, a 2,4-disubstituted oxazole ring in the side chain, and a phosphate group. These substances can inhibit cells with either a wild-type or mutant c-Kit. However, their actual cellular targets are still unknown [124]. The first chemical synthesis of 195 and 196 required a tandem olefin cross-metathesis/intramolecular oxa-Michael addition reaction [125].

Mono-Oxazole Polyketides
Calyculins (198-216) (Scheme 15) were novel polyketides containing a mono-oxazole and a phosphate group from the marine sponges Discodermia calyx, Lamellomorpha strongylat, Luffariella geometrica, Myriastra clavosa and Theonella swinhoei. These natural products exhibited extensive biological properties including cytotoxicity, antifungal activity and an inhibitory effect on protein phosphatases [126][127][128][129][130][131]. By OSMAC (one strain many compounds) strategy, inthomycin B (217) was produced by the marine sediment-derived Streptomyces YB104 and was found to have anti-oomycete, cytotoxic and herbicidal activities [132]. One concise method to synthesize 217 was developed by Webb and coworkers through the Stille coupling of a stannyl-diene with an oxazole vinyl iodide unit and a Kiyooka ketene acetal/amino acid-derived oxazaborolidine procedure as its cornerstones [133]. And the gene cluster (itm) responsible for biosynthesis of 217 was identified as a 95.3 kb trans-AT type I PKS system, of which the gene Itm15 is a cyclodehydrase to catalyze the formation of oxazole ring [132].

Conclusions
In recent decades, a tremendous number of 1,3-oxazole-containing alkaloids have been isolated and characterized from marine organisms, including marine invertebrates, nematodes, insects, vertebrates cyanobacteria, bacteria, and fungi. These substances possess unique chemical structures and exhibit a wide variety of biological properties. Many important marine-derived 1,3-oxazoles have great potential for the development of leading compounds in the search for new drugs and medicines. For example, mechercharmycin A (119) is a promising antitumor remedy, and (19Z)-halichondramide (168), neohalichondramide (175), and calyculin A-D (190, 200-202) have the potential to treat human leukemia cell line. Besides, plenty of these substances are potential antimicrobial agents, such as kocurin (118), TP-1161 (121) homopseudopteroxazole (272), and so on. However, (1) how to reduce cytotoxicity, (2) how to improve drug stability, and (3) how to make the drug work quickly and effectively in vivo still needs to be studied in further depth.
In recent years, however, the number of new 1,3-oxazole derivatives from marine organisms has greatly decreased, since almost all accessible macroorganisms have been collected and chemically analyzed. Simultaneously, marine microorganisms are shown to be one prolific and unexploited source of bioactive natural products, owing to their species richness and abundant secondary metabolite BGCs, especially symbiotic microbes of marine sponges, seaweeds, mangroves and tunicates [164][165][166][167]. In order to discover novel marine microbe-derived 1,3-oxazoles for new drug discovery, more efforts should be made to conduct strain isolation and chemical study using a combination of classical methods (e.g., cultivation and fermentation, bioassay-guided fractionation, structure elucidation) and advanced analytical techniques (e.g., metabolomics, higher-field NMR instruments, probe technology), genome mining and engineering and microbial cultivating systems (e.g., OSMAC approach) [168].

Conflicts of Interest:
The authors declare no conflict of interest.