Structural and stereochemical studies of saraines: Macrocyclic alkaloids of the sponge Reniera sarai

Natural Products from the Marine Sponge Subgenus Reniera

Marine sponges are one of the prolific producers of bioactive natural products with therapeutic potential. As an important subgenus of Haliclona, Reniera sponges are mainly distributed in the Mediterranean Sea and Atlantic area, and had been chemically investigated for over four decades. By an extensive literature search, this review first makes a comprehensive summary of all natural products from Reniera sponges and their endozoic microbes, as well as biological properties. Perspectives on strengthening the chemical study of Reniera sponges for new drug-lead discovery are provided in this work.

Marine-Derived Macrocyclic Alkaloids (MDMAs): Chemical and Biological Diversity

The curiosity and attention that researchers have devoted to alkaloids are due to their bioactivities, structural diversity, and intriguing chemistry. Marine-derived macrocyclic alkaloids (MDMAs) are considered to be a potential source of drugs. Trabectedin, a tetrahydroisoquinoline derivative, has been approved for the treatment of metastatic soft tissue sarcoma and ovarian cancers. MDMAs displayed potent activities that enabled them to be used as anticancer, anti-invasion, antimalarial, antiplasmodial, and antimicrobial. This review presents the reported chemical structures, biological activities, and structure-activity relationships of macrocyclic alkaloids from marine organisms that have been published since their discovery until May 2020. This includes 204 compounds that are categorized under eight subclasses: pyrroles, quinolines, bis-quinolizidines, bis-1-oxaquinolizidines, 3-alkylpiperidines, manzamines, 3-alkyl pyridinium salts, and motuporamines.

Recent progress on the total syntheses of macrocyclic diamine alkaloids

This review highlights the progress and challenges in the chemical synthesis of macrocyclic diamine alkaloids since 2006.

Application of (4+3) cycloaddition strategies in the synthesis of natural products

This review summarizes the applications of (4+3) cycloadditions, both classical and formal, in the syntheses of natural products in the last two decades.

Development and Application of a Novel SPE-Method for Bioassay-Guided Fractionation of Marine Extracts

The biological diversity of marine habitats is a unique source of chemical compounds with potential use as pharmaceuticals, cosmetics and dietary supplements. However, biological screening and chemical analysis of marine extracts pose specific technical constraints and require adequate sample preparation. Here we report an improved method on Solid Phase Extraction (SPE) to fractionate organic extracts containing high concentration of salt that hampers the recovery of secondary metabolites. The procedure uses a water suspension to load the extracts on a poly(styrene-divynylbenzene)-based support and a stepwise organic solvent elution to effectively desalt and fractionate the organic components. The novel protocol has been tested on MeOH-soluble material from three model organisms (Reniera sarai, Dendrilla membranosa and Amphidinium carterae) and was validated on a small panel of 47 marine samples, including sponges and protists, within discovery programs for identification of immuno-stimulatory and anti-infective natural products.

The Saraine Alkaloids

Saraines have been isolated by the group of Cimino from the marine sponge Reniera sarai collected in the Bay of Naples. These alkaloids can be classified into two categories, depending on the type of their structure. Saraines 1-3 and isosaraines 1-3 are characterized by a trans-2-oxoquinolizidine moiety bound directly to a tetrahydropyridine ring, both nuclei being also linked by two alkyl chains of different lengths. On the other hand, saraines A-C and misenine present a diazatricyclic central core, showing an unusual interaction between an aldehyde and a tertiary amine and giving them a zwitterionic character, and flanked with two chains forming macrocycles. In a first part of the chapter, the isolation, spectral data, and structure elucidation of saraines are presented. Biological properties and biogenetic proposals concerning these compounds are then described. The most important part is devoted to synthetic approaches and total synthesis of saraine A.

Marine natural products

This review covers the literature published in 2011 for marine natural products, with 870 citations (558 for the period January to December 2011) referring to compounds isolated from marine microorganisms and phytoplankton, green, brown and red algae, sponges, cnidarians, bryozoans, molluscs, tunicates, echinoderms, mangroves and other intertidal plants and microorganisms. The emphasis is on new compounds (1152 for 2011), together with the relevant biological activities, source organisms and country of origin. Biosynthetic studies, first syntheses, and syntheses that lead to the revision of structures or stereochemistries, have been included.

3-4'-Bipiperidines via sequential [4 + 4]-[3,3]-retro-Mannich reactions

An approach to assembling unsymmetrically coupled piperidines is described, involving initial [4 + 4] photocycloaddition of 2-pyridones, followed by Cope rearrangement and retro-Mannich reaction. In these reactions, four stereogenic centers set during cycloaddition are relayed or erased during the subsequent steps. Two methods for retro-Mannich reaction are demonstrated.

Unexpected rearrangement of enantiomerically pure 3-aminoquinuclidine as a simple way of preparing diastereomeric octahydropyrrolo[2,3-c]pyridine derivatives

Reaction of (S)- or (R)-3-aminoquinuclidine with 2-chloropyrimidine or 2-bromopyrimidine led to an unexpected formation of both cis- and trans-octahydropyrrolo [2,3]pyridine derivatives. A single-step synthesis of two of the four stereoisomers of these octahydropyrrolo[2,3]pyridine derivatives provides a convenient way of generating stereochemically defined isomers. Optimization of reaction conditions was carried out by (1)H NMR monitoring. The relative and absolute stereochemistry of all four stereoisomers was determined by a combination of (1)H, (13)C, and (15)N NMR spectroscopy and vibrational circular dichroism spectroscopy.

Construction of an advanced tetracyclic intermediate for total synthesis of the marine alkaloid sarain A

In work directed toward a total synthesis of the marine alkaloid sarain A (1), the advanced intermediate 54, containing all the key elements and the seven stereogenic centers of sarain A, has been successfully synthesized from bicyclic lactam 4, previously prepared via an intramolecular stereospecific [3 + 2]-azomethine ylide dipolar cycloaddition. Intermediate lactam 4 could be efficiently converted to N-Boc derivative 12. Introduction of a two-carbon fragment into lactam 12 which eventually becomes the C-7',8' syn diol of the "eastern" ring was then achieved by C-acylation of the corresponding enolate with methoxyacetyl chloride followed by a highly stereoselective ketone reduction with Zn(BH4)2 to afford alcohol 16. Intermediate 16 has the incorrect C-7' relative stereochemistry for sarain A, but this problem was conveniently remedied by inverting the C-7' center via an intramolecular Ohfune-type cyclization of the silyl carbamate derived from Boc mesylate 27 to produce the key cyclic carbamate 28. It was then possible to convert acetal 28 to allylsilane 32 followed by cyclization to the alkaloid tricyclic core 33 via an allylsilane/N-sulfonyliminium ion cyclization. Formation of the "western" macrocyclic ring has been successfully addressed using functional group handles at C-3' and N-1' on the tricyclic core via a ring-closing olefin metathesis (RCM) strategy with the second-generation Grubbs ruthenium catalyst to produce intermediate macrolactam 47. A chelation-controlled addition of ethynylmagnesium bromide to advanced aldehyde 51 afforded a single diastereomeric adduct 53 which is tentatively assigned to have the correct C-7',8' syn-diol stereochemistry. This adduct could be rearranged to the conveniently protected amino carbonate 54 which is set up for construction of the remainder of the eastern ring of sarain A.

Synthetic studies toward sarain A. Formation of the western macrocyclic ring

Starting with the tricyclic core 2b, annulation to form the 13-membered western ring of sarain A has been achieved to afford the macrocycle 30a by initial construction of the sterically congested quaternary center at C-3, followed by elaboration of the C-3 side-chain and ring-closing olefin metathesis. Also included is a parallel conversion of tricycle 2c to macrocycle 30b containing a functionalized side-chain at N-1 suitable for attachment of the eastern macrocyclic ring.

Facile synthesis of the tricyclic core of sarain A. 3-Oxidopyridinium betaine cycloaddition approach

[formula: see text] A new approach to a suitably functionalized tricyclic core of sarains has been developed by means of Katritzky's cycloaddition using 3-oxidopyridinium betaines. A key step was the regioselective differentiation of the two nearly identical hydroxy groups derived from oxidative cleavage of the double bond in 8 to afford 14. A stereocontrolled construction of the tricyclic core 20 of sarains containing the requisite side chain at C-3' was achieved by an intramolecular conjugate addition.