Further studies of alkaloids from Reniera sarai: Structures of saraine-3 and isosaraine-3; absolute stereochemistry of saraine-1 and saraine-2

Marine Organisms as Alkaloid Biosynthesizers of Potential Anti-Alzheimer Agents

The incidence of neurodegenerative diseases, such as Alzheimer's disease (AD), increases continuously demanding the urgent development of anti-Alzheimer's agents. Marine organisms (MO) have to create their own defenses due to the adverse environment where they live and so synthesize several classes of compounds, such as akaloids, to defend themselves. Therefore, the identification of marine natural products with neuroprotective effects is a necessity. Being that AD is not only a genetic but also an environmental complex disease, a treatment for AD remains to discover. As the major clinical indications (CI) of AD are extracellular plaques formed by β-amyloid (Aβ) protein, intracellular neurofibrillary tangles (NFTs) formed by hyper phosphorylated τ-protein, uncommon inflammatory response and neuron apoptosis and death caused by oxidative stress, alkaloids that may decrease CI, might be used against AD. Most of the alkalolids with those properties are derivatives of the amino acid tryptophan mainly with a planar indole scaffold. Certainly, alkaloids targeting more than one CI, multitarget-directed ligands (MTDL), have the potential to become a lead in AD treatment. Alkaloids to have a maximum of activity against CI, should be planar and contain halogens and amine quaternization.

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.

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.

Natural products with anti-Bredt and bridgehead double bonds

Well over a hundred years ago, Professor Julius Bredt embarked on a career pursuing and critiquing bridged bicyclic systems that contained ring strain induced by the presence of a bridgehead olefin. These endeavors founded what we now know as Bredt's rule (Bredtsche Regel). Physical, theoretical, and synthetic organic chemists have intensely studied this premise, pushing the boundaries of such systems to arrive at a better understood physical phenomenon. Mother nature has also seen fit to construct molecules containing bridgehead double bonds that encompass Bredt's rule. For the first time, this topic is reviewed in a natural product context.

Natural products from marine organisms with neuroprotective activity in the experimental models of Alzheimer's disease, Parkinson's disease and ischemic brain stroke: their molecular targets and action mechanisms

Continuous increases in the incidence of neurodegenerative diseases, such as Alzheimer's disease (AD), Parkinson's disease (PD), and brain stroke demand the urgent development of therapeutics. Marine organisms are well-known producers of natural products with diverse structures and pharmacological activities. Therefore, researchers have endeavored to identify marine natural products with neuroprotective effects. In this regard, this review summarizes therapeutic targets for AD, PD, and ischemic brain stroke and marine natural products with pharmacological activities on the targets according to taxonomies of marine organisms. Furthermore, several marine natural products on the clinical trials for the treatment of neurological disorders are discussed.

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.

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.