Pectinatone, a new antibiotic from the mollusc siphonaria pectinata

Hidden interactions in the intertidal rocky shore: variation in pedal mucus microbiota among marine grazers that feed on epilithic biofilm communities

In marine ecosystems, most invertebrates possess diverse microbiomes on their external surfaces, such as those found in the pedal mucus of grazing gastropods and chitons that aids displacement on different surfaces. The microbes are then transported around and placed in contact with free-living microbial communities of micro and other macro-organisms, potentially exchanging species and homogenizing microbial composition and structure among grazer hosts. Here, we characterize the microbiota of the pedal mucus of five distantly related mollusk grazers, quantify differences in microbial community structure, mucus protein and carbohydrate content, and, through a simple laboratory experiment, assess their effects on integrated measures of biofilm abundance. Over 665 Amplicon Sequence Variants (ASVs) were found across grazers, with significant differences in abundance and composition among grazer species and epilithic biofilms. The pulmonate limpet Siphonaria lessonii and the periwinkle Echinolittorina peruviana shared similar microbiota. The microbiota of the chiton Chiton granosus, keyhole limpet Fissurella crassa, and scurrinid limpet Scurria araucana differed markedly from one another, and form those of the pulmonate limpet and periwinkle. Flavobacteriaceae (Bacteroidia) and Colwelliaceae (Gammaproteobacteria) were the most common among microbial taxa. Microbial strict specialists were found in only one grazer species. The pedal mucus pH was similar among grazers, but carbohydrate and protein concentrations differed significantly. Yet, differences in mucus composition were not reflected in microbial community structure. Only the pedal mucus of F. crassa and S. lessonii negatively affected the abundance of photosynthetic microorganisms in the biofilm, demonstrating the specificity of the pedal mucus effects on biofilm communities. Thus, the pedal mucus microbiota are distinct among grazer hosts and can affect and interact non-trophically with the epilithic biofilms on which grazers feed, potentially leading to microbial community coalescence mediated by grazer movement. Further studies are needed to unravel the myriad of non-trophic interactions and their reciprocal impacts between macro- and microbial communities.

Bioactive Compounds from Marine Heterobranchs

The natural products of heterobranch molluscs display a huge variability both in structure and in their bioactivity. Despite the considerable lack of information, it can be observed from the recent literature that this group of animals possesses an astonishing arsenal of molecules from different origins that provide the molluscs with potent chemicals that are ecologically and pharmacologically relevant. In this review, we analyze the bioactivity of more than 450 compounds from ca. 400 species of heterobranch molluscs that are useful for the snails to protect themselves in different ways and/or that may be useful to us because of their pharmacological activities. Their ecological activities include predator avoidance, toxicity, antimicrobials, antifouling, trail-following and alarm pheromones, sunscreens and UV protection, tissue regeneration, and others. The most studied ecological activity is predation avoidance, followed by toxicity. Their pharmacological activities consist of cytotoxicity and antitumoral activity; antibiotic, antiparasitic, antiviral, and anti-inflammatory activity; and activity against neurodegenerative diseases and others. The most studied pharmacological activities are cytotoxicity and anticancer activities, followed by antibiotic activity. Overall, it can be observed that heterobranch molluscs are extremely interesting in regard to the study of marine natural products in terms of both chemical ecology and biotechnology studies, providing many leads for further detailed research in these fields in the near future.

Polypropionate Derivatives with Mycobacterium tuberculosis Protein Tyrosine Phosphatase B Inhibitory Activities from the Deep-Sea-Derived Fungus Aspergillus fischeri FS452

Fiscpropionates A-F (1-6), six new polypropionate derivatives featuring an unusual long hydrophobic chain, were isolated from the deep-sea-derived fungus Aspergillus fischeri FS452. Their structures were elucidated on the basis of spectroscopic analysis, and the absolute configurations were determined by J-HMBC analysis, electronic circular dichroism (ECD) calculations, and the modified Mosher's method. This is the first discovery of polypropionates from marine-derived fungi, and compounds 4 and 5 represent the first examples of polypropionate derivatives containing a 3-hydroxypiperidin-2-one as part of an imide linkage. In addition, compounds 1-4 exhibited significant inhibitory activities against Mycobacterium tuberculosis protein tyrosine phosphatase B (MptpB) with the IC₅₀ values of 5.1, 12, 4.0, and 11 μM, respectively. Enzyme kinetic experiments suggested that they all acted through a noncompetitive mechanism. A preliminary structure-activity relationship is discussed.

Hyafurones, hyapyrrolines, and hyapyrones: polyketides from Hyalangium minutum

Seven new polyketides, for which the trivial names hyafurones A1-B (1-3), hyapyrrolines A (4) and B (5), and hyapyrones A (6) and B (7) are proposed, were isolated from the fermentation broth of the myxobacteria Hyalangium minutum, strains NOCB-2(T) and Hym-3. Their structures were elucidated from NMR and HRESIMS data, and their geometric configuration was assigned based on NOE and vicinal (1)H coupling data. Both hyafurone B (3) and hyapyrone B (7) inhibited growth of the Gram-positive bacterium Nocardia flava, while 7 showed antifungal activity against Mucor hiemalis.

Iterative synthesis of deoxypropionate units: the inductor effect in acyclic conformation design

Conjugate addition of lithium dimethylcuprate to acyclic alpha,beta-unsaturated esters of varying lengths bearing terminal alkyl or phenyl groups leads to a preponderance of syn 1,3-adducts when one methyl is already present. Conversion to enoates, and iteration of cuprate additions also favors syn adducts to give contiguous deoxypropionate units in a growing chain. The effect of end-group variation (Me, i-Pr, phenyl, tert-butyl) in conjunction with the nature of the ester group (Me, tert-butyl, etc.) on the diastereoselectivity of syn and anti products was studied. The results are rationalized in terms of inductor effects related to the minimization of the 1,5-pentane interactions in energetically favored folded conformations and corroborated by homodecoupling NMR studies.

Iterative deoxypropionate synthesis based on a copper-mediated directed allylic substitution: formal total synthesis of borrelidin (C3-C11 fragment)

A new iterative strategy for the flexible preparation of any oligodeoxypropionate stereoisomer is presented which relies on an o-DPPB-directed copper mediated allylic substitution employing enantiomerically pure Grignard reagents; the reaction is working with perfect control over all aspects of the reaction selectivity. This key C--C bond-forming step features reversed polarity compared with established enolate alkylation methodology. It thus avoids existing problems of enolate alkylation strategies such as enolate reactivity as well as costs and problems associated with the chiral auxiliary. Practicability of this new method is demonstrated through application in natural product syntheses. Thus, an efficient synthesis of the northern part of the angiogenesis inhibitor borrelidin (28), the deoxypropionate building block 27, could be devised, representing a formal total synthesis.

Total Synthesis and Biological Evaluation of (−)-Pectinatone Employing a Methyl-Branched Wax Ester as Key Building Block

Unnatural (-)-pectinatone ((-)-3) was prepared in five steps starting from the highly methyl-branched wax ester 4, employing bromination of the ester enolate and subsequent base-induced elimination to the enoate 6 as the key step. Both (-)-3 and the amides 8b and 8c, which were isolated as by-products in the reaction sequence, displayed antimicrobial activity and cytotoxicity.

The chemistry of marine pulmonate gastropods

Secondary metabolites from pulmonate molluscs of the genera Siphonaria, Onchidium, and Trimusculus are described. Siphonaria and Onchidium biosynthesize mostly propionate-based metabolites whereas Trimusculus yields diterpene derivatives with a single type of labdane skeleton. The 42 regular polypropionates reported to date from Siphonaria are divided into two classes (class I, class II), based on their observed structural and stereochemical analogy. The strong resemblance between class I and cephalaspidean metabolites and between class II and onchidiid metabolites as well as the structural features of Trimusculus, in relation to the other pulmonates, encourage speculation about their biosynthetic and phylogenetic relationship. Class I metabolites could be suitable material to evidence that type I PKS modules are perhaps used iteratively in their biosynthesis.

Marine mollusks from Australia and New Zealand: chemical and ecological studies

Marine mollusks contain structurally diverse terpenes, polyketides, polypropionates and nitrogenous metabolites that may confer an ecological advantage on the host organism. From a chemical perspective, the most studied Australian taxa include representatives of the nudibranchs and sea hares, which are characterised by terpenes acquired from their specialised diets of sponges and algae, respectively. In contrast, siphonariid limpets that are prevalent on temperate seashores carry out de novo biosynthesis of polypropionate metabolites. Nitrogenous compounds isolated from Australian marine mollusks include precursors to the first commercially significant marine bioproduct, Tyrian Purple, and metabolites that are characteristic of ingested cyanobacteria.

Total synthesis of the cytotoxic cyclodepsipeptide (-)-doliculide: the "ester" effect in acyclic 1,3-induction of deoxypropionates

The total synthesis of the marine cyclodepsipeptide (-)-doliculide is described. An acyclic (2S,4S,6R)-trimethyl alkanoic acid precursor to the "hydrocarbon" portion of doliculide was synthesized starting with l-ascorbic acid based on a stereocontrolled iterative conjugate addition of lithium dimethylcuprate to acyclic delta-Cmethyl alpha,beta-unsaturated ester derivatives. syn/syn selectivity was achieved by relying on 1,3-induction in preferred folded conformations that avoid 1,5-syn-pentane interactions in the transition states. The nature of the ester moiety seems to play an important role in determining the syn/anti ratios of C-methyl adducts. The 1-methyl-1-cyclopentyl ester group was found to confer the best syn selectivity to the cuprate addition products, especially in seven-carbon enoates.

Application of conformation design in acyclic stereoselection: total synthesis of borrelidin as the crystalline benzene solvate

The total synthesis of (-)-borrelidin (treponemycin), a structurally distinct 18-membered macrolide antibiotic, has been achieved. It was isolated as the crystalline benzene solvate, and its structure was confirmed by a single-crystal X-ray analysis. The deoxypropionate subunit consisting of four alternating C-methyl groups with a C(4)-C(10) syn/syn/anti orientation was elaborated by a new method of iterative cuprate additions to acyclic alpha,beta-unsaturated esters relying on two consecutive 1,3-inductions and starting with d-glyceraldehyde as the chiral progenitor. The unique Z/E cyanodiene unit was obtained as a single isomer by application of the Still-Gennari olefination protocol. The gamma-hydroxycyclopentane carboxylic acid subunit was prepared from L-malic acid utilizing a sequential introduction of C-vinyl and C-allyl groups, capitalizing on 1,2-induction in an acyclic alpha,beta-unsaturated ester and carbocyclization by a Grubbs ring closure metathesis reaction. The prevalence of 1,3-syn-disposed deoxypropionate triads in the cuprate additions is rationalized on the basis of minimized syn-pentane interactions in the transition states. A virtual diamond lattice was used as a visual tool to portray the low-energy conformations of the acyclic substrates, and corroborated by (1)H NMR homodecoupling studies.