Total syntheses of (+/-)-anchinopeptolide D and (+/-)-cycloanchinopeptolide D

Bioactive natural products from marine sponges belonging to family Hymedesmiidae

Natural products of marine origin exhibit extensive biological activities, and display a vital role in the exploration of new compounds for drug development. Marine sponges have been reported at the top with respect to the discovery of biologically active metabolites that have potential pharmaceutical applications. The family Hymedesmiidae belonging to the Demospongiae class includes ten accepted genera, of which four genera were explored for their bioactive metabolites, namely Phorbas, Hamigera, Hemimycale, and Kirkpatrickia. Genus Phorbas has received more attention due to the isolation of various classes of compounds with unique structures mainly diterpenes, alkaloids, sesterterpenes, and steroids that exhibited diverse biological activities including: antiviral, antimicrobial, and anti-inflammatory, whereas anticancer compounds predominated. This review focuses on the isolated secondary metabolites from family Hymedesmiidae with their biological potential and covers the literature from 1989 to 2020.

Natural products of marine origin exhibit extensive biological activities, and display a vital role in the exploration of new compounds for drug development.

Photoinduced Multicomponent Synthesis of α-Silyloxy Acrylamides, an Unexplored Class of Silyl Enol Ethers

The photoinduced, multicomponent reaction of α-diazoketones, silanols, and isocyanides affords α-silyloxy acrylamides, formally derived from α-keto amides. The presence of a secondary amido group makes classic preparative methods for silyl enol ethers unfeasible in this case, while the mild conditions required by this photochemical approach allow their synthesis in good yields; moreover, the general structure can be easily modified by varying each component of the multicomponent reaction. Fine-tuning of the reaction conditions (i.e., solvents, radiation, additives) can be exploited to obtain complete Z selectivity. The reactivity of this overlooked class of silyl enol ethers has been investigated, and features that could pave the way to new applications have been found.

Mechanistic elucidation of the mycofactocin-biosynthetic radical S-adenosylmethionine protein, MftC

Ribosomally synthesized and posttranslationally modified peptide (RiPP) pathways produce a diverse array of natural products. A subset of these pathways depends on radical S-adenosylmethionine proteins to modify the RiPP-produced peptide. Mycofactocin biosynthesis is one example of an S-adenosylmethionine protein-dependent RiPP pathway. Recently, it has been shown that MftC catalyzes the oxidative decarboxylation of the C-terminal tyrosine (Tyr-30) on the mycofactocin precursor peptide MftA; however, this product has not been verified by techniques other than MS. Herein, we provide a more detailed study of MftC catalysis and report a revised mechanism for MftC chemistry. We show that MftC catalyzes the formation of two isomeric products. Using a combination of MS, isotope labeling, and ¹H and ¹³C NMR techniques, we established that the major product, MftA*, is a tyramine-valine-cross-linked peptide formed by MftC through two S-adenosylmethionine-dependent turnovers. In addition, we show that the hydroxyl group on MftA Tyr-30 is required for MftC catalysis. Furthermore, we show that a substitution in the penultimate MftA Val-29 position causes the accumulation of an MftA** minor product. The ¹H NMR spectrum indicates that this minor product contains an αβ-unsaturated bond that likely arises from an aborted intermediate of MftA* synthesis. The finding that MftA* is the major product formed during MftC catalysis could have implications for the further elucidation of mycofactocin biosynthesis.

A Direct, Early Stage Guanidinylation Protocol for the Synthesis of Complex Aminoguanidine-containing Natural Products

The guanidine functional group, displayed most prominently in the amino acid arginine, one of the fundamental building blocks of life, is an important structural element found in many complex natural products and pharmaceuticals. Owing to the continual discovery of new guanidine-containing natural products and designed small molecules, rapid and efficient guanidinylation methods are of keen interest to synthetic and medicinal organic chemists. Because the nucleophilicity and basicity of guanidines can affect subsequent chemical transformations, traditional, indirect guanidinylation is typically pursued. Indirect methods commonly employ multiple protection steps involving a latent amine precursor, such as an azide, phthalimide, or carbamate. By circumventing these circuitous methods and employing a direct guanidinylation reaction early in the synthetic sequence, it was possible to forge the linear terminal guanidine containing backbone of clavatadine A to realize a short and streamlined synthesis of this potent factor XIa inhibitor. In practice, guanidine hydrochloride is elaborated with a carefully constructed protecting array that is optimized to survive the synthetic steps to come. In the preparation of clavatadine A, direct guanidinylation of a commercially available diamine eliminated two unnecessary steps from its synthesis. Coupled with the wide variety of known guanidine protecting groups, direct guanidinylation evinces a succinct and efficient practicality inherent to methods that find a home in a synthetic chemist's toolbox.

An unexpected Bromolactamization of Olefinic Amides Using a Three-Component Co-catalyst System

Reaction between (N,N-dimethylamino)pyridine and isocyanate unexpectedly produced a three-component mixture. By using this mixture as an unprecedented three-component catalyst system, a facile and selective bromolactamization of olefinic amides has been developed. The protocol confers enhanced selectivity of N- over O-cyclization, leading to the formation of a structurally diverse range of lactams including both small and medium ring sizes.

Synthesis of (+/-)-eusynstyelamide A

The synthesis of (+/-)-eusynstyelamide A has been accomplished in six steps in 13% overall yield from 6-bromoindole, methyl glycidate, and Boc-protected agmatine. If oxygen is carefully excluded from the reaction, the key NaOH-catalyzed aldol dimerization of the alpha-ketoamide proceeded efficiently to give Boc-protected eusynstyelamide A.

Ethyl 1-benzyl-4-hydroxy-2-methyl-5-oxopyrrolidine-3-carboxylate

In the title oxopyrrolidine, C₁₅H₁₉NO₄, the five-membered pyrrolidine ring is in a twist conformation and its mean plane makes an angle of 89.2 (3)° with the phenyl ring. In the crystal, mol­ecules pack as dimers via strong O—H⋯O [R₂²(10)] inter­actions cross-linked by weaker C—H⋯O and C—H⋯π inter­actions. Full synthetic and spectroscopic details are given for the title compound and related dicarboxyl­ates.

Synthesis of (+/-)-bistellettadine A

Esterification of the trienoic acid with o-xylylene dibromide gave the bis ester that underwent a templated Diels-Alder reaction to afford the macrodiolide stereospecifically in a single step. The synthesis of bistellettadine A was completed in four steps by hydrolysis and side chain elaboration.

Synthesis and structure-activity relationship of botryllamides that block the ABCG2 multidrug transporter

In previous work, botryllamides discovered from the marine ascidian Botryllus tyreus were characterized as selective inhibitors of the ABCG2 multidrug transporter. However, the structural basis for this activity could not be established. In this study, botryllamide F, the core botryllamide structure, and botryllamide G, the most potent botryllamide ABCG2 inhibitor, were synthesized along with a series of structural variants for evaluation of structure-activity relationships. The biological activity of synthetic botryllamide analogs implied that the 2-methoxy-p-coumaric acid portion, and the degree of double bond conjugation within this group, were critical for inhibition of ABCG2. However, variations in the substituents on the two aryl groups did not appear to significantly impact the potency or degree of inhibition.

Chemistry and biology of non-tetramic gamma-hydroxy-gamma-lactams and gamma-alkylidene-gamma-lactams from natural sources

Natural products containing non-tetramic gamma-hydroxy-gamma-lactams and gamma-alkylidene-gamma-lactams are usually but not exclusively derived from the mixed polyketide-non-ribosomal peptide biosynthetic pathway. Often they are fungal metabolites, although some plant- and marine-derived exceptions exist. Owing to their unique structures and biological properties, they have gained interest in the chemical and biological communities. In this review, we aim to emphasize the structural originality of these compounds, their biological properties and the synthetic efforts developed to reach them; 157 references are cited.

Eusynstyelamides A, B, and C, nNOS inhibitors, from the ascidian Eusynstyela latericius

Eusynstyelamides A-C (1-3) were isolated from the Great Barrier Reef ascidian Eusynstyela latericius, together with the known metabolites homarine and trigonelline. The structures of 1-3, with relative configurations, were elucidated by interpretation of their spectroscopic data (NMR, MS, UV, IR, and CD). The NMR data of 1 were found to be virtually identical to that reported for eusynstyelamide (4), isolated from E. misakiensis, indicating that a revision of the structure of 4 is needed. Eusynstyelamides A-C exhibited inhibitory activity against neuronal nitric oxide synthase (nNOS), with IC(50) values of 41.7, 4.3, and 5.8 microM, respectively, whereas they were found to be nontoxic toward the three human tumor cell lines MCF-7 (breast), SF-268 (CNS), and H-460 (lung). Compounds 1 and 2 displayed mild inhibitory activity toward Staphylococcus aureus (IC(50) 5.6 and 6.5 mM, respectively) and mild inhibitory activity toward the C(4) plant regulatory enzyme pyruvate phosphate dikinase (PPDK) (IC(50) values of 19 and 20 mM, respectively).

Bioactive cyclobutane-containing alkaloids

The present review describes research on novel natural cyclobutane-containing alkaloids and synthetic compounds isolated from terrestrial and marine species. More than 210 compounds have been confirmed to show antimicrobial, antibacterial, anticancer, and other activities. Structures, origins, biosynthesis, photodimerization, and biological activities of a selection of cyclobutane-containing alkaloids and selected synthetic analogs of natural alkaloids are reviewed.