Anti-barnacle Activity of Isocyanides Derived from Amino Acids

A Chemical Toolbox to Unveil Synthetic Nature-Inspired Antifouling (NIAF) Compounds

The current scenario of antifouling (AF) strategies to prevent the natural process of marine biofouling is based in the use of antifouling paints containing different active ingredients, believed to be harmful to the marine environment. Compounds called booster biocides are being used with copper as an alternative to the traditionally used tributyltin (TBT); however, some of them were recently found to accumulate in coastal waters at levels that are deleterious for marine organisms. More ecological alternatives were pursued, some of them based on the marine organism mechanisms' production of specialized metabolites with AF activity. However, despite the investment in research on AF natural products and their synthetic analogues, many studies showed that natural AF alternatives do not perform as well as the traditional metal-based ones. In the search for AF agents with better performance and to understand which molecular motifs were responsible for the AF activity of natural compounds, synthetic analogues were produced and investigated for structure-AF activity relationship studies. This review is a comprehensive compilation of AF compounds synthesized in the last two decades with highlights on the data concerning their structure-activity relationship, providing a chemical toolbox for researchers to develop efficient nature-inspired AF agents.

Anti-Barnacle Activities of Isothiocyanates Derived from β-Citronellol and Their Structure-Activity Relationships

Antifouling agents with low toxicity are in high demand for sustaining marine industries and the environment. This study aimed to synthesize 15 isothiocyanates derived from β-citronellol and evaluate their antifouling activities and toxicities against cypris larvae of the barnacle Amphibalanus amphitrite. The synthesized isothiocyanates exhibited effective antifouling activities (EC₅₀ =0.10-3.33 μg mL^-1 ) with high therapeutic ratios (LC₅₀ /EC₅₀ >30). Four isothiocyanates with an amide or isocyano group showed great potential as effective antifouling agents (EC₅₀ =0.10-0.32 μg mL^-1 , LC₅₀ /EC₅₀ =104-833). The enantiomers of the isothiocyanates only slightly differed in their antifouling activities. These results may serve as a basis for further research and development of β-citronellol-derived isothiocyanates as effective low-toxic antifouling agents. To the best of our knowledge, this study is the first to report the antifouling activities of isothiocyanates derived from accessible natural products.

Effective Synthesis and Antifouling Activity of Dolastatin 16 Derivatives

Some derivatives of dolastatin 16, a depsipeptide natural product first obtained from the sea hare Dolabella auricularia, were synthesized through second-generation synthesis of two unusual amino acids, dolaphenvaline and dolamethylleuine. The second-generation synthesis enabled derivatizations such as functionalization of the aromatic ring in dolaphenvaline. The derivatives of fragments and whole structures were evaluated for antifouling activity against the cypris larvae of Amphibalanus amphitrite. Small fragments inhibited the settlement of the cypris larvae at potent to moderate concentrations (EC50 = 0.60-4.62 μg/mL), although dolastatin 16 with a substituent on the aromatic ring (24) was much less potent than dolastatin 16.

Medicinal Chemistry of Isocyanides

In eons of evolution, isocyanides carved out a niche in the ecological systems probably thanks to their metal coordinating properties. In 1859 the first isocyanide was synthesized by humans and in 1950 the first natural isocyanide was discovered. Now, at the beginning of XXI century, hundreds of isocyanides have been isolated both in prokaryotes and eukaryotes and thousands have been synthesized in the laboratory. For some of them their ecological role is known, and their potent biological activity as antibacterial, antifungal, antimalarial, antifouling, and antitumoral compounds has been described. Notwithstanding, the isocyanides have not gained a good reputation among medicinal chemists who have erroneously considered them either too reactive or metabolically unstable, and this has restricted their main use to technical applications as ligands in coordination chemistry. The aim of this review is therefore to show the richness in biological activity of the isocyanide-containing molecules, to support the idea of using the isocyanide functional group as an unconventional pharmacophore especially useful as a metal coordinating warhead. The unhidden hope is to convince the skeptical medicinal chemists of the isocyanide potential in many areas of drug discovery and considering them in the design of future drugs.

Isocyanides Derived from α,α-Disubstituted Amino Acids: Synthesis and Antifouling Activity Assessment

Herein, we contribute to the development of environmentally friendly antifoulants by synthesizing eighteen isocyanides derived from α,α-disubstituted amino acids and evaluating their antifouling activity/toxicity against the cypris larvae of the Balanus amphitrite barnacle. Almost all isocyanides showed good antifouling activity without significant toxicity and exhibited EC₅₀ values of 0.07 - 7.30 μg/mL after 120-h exposure. The lowest EC₅₀ values were observed for valine-, methionine-, and phenylalanine-derived isocyanides, which achieved > 95% cypris larvae settlement inhibition at concentrations of less than 30 μg/mL without exhibiting significant toxicity. Thus, the prepared isocyanides should be useful for further research focused on the development of environmentally friendly antifouling agents.

Design, Synthesis, and Antifouling Activity of Glucosamine-Based Isocyanides

Biofouling, an undesirable accumulation of organisms on sea-immersed structures such as ship hulls and fishing nets, is a serious economic issue whose effects include oil wastage and clogged nets. Organotin compounds were utilized since the 1960s as an antifouling material; however, the use of such compounds was later banned by the International Maritime Organization (IMO) due to their high toxicity toward marine organisms, resulting in masculinization and imposex. Since the ban, there have been extensive efforts to develop environmentally benign antifoulants. Natural antifouling products obtained from marine creatures have been the subject of considerable attention due to their potent antifouling activity and low toxicity. These antifouling compounds often contain isocyano groups, which are well known to have natural antifouling properties. On the basis of our previous total synthesis of natural isocyanoterpenoids, we envisaged the installation of an isocyano functional group onto glucosamine to produce an environmentally friendly antifouling material. This paper describes an effective synthetic method for various glucosamine-based isocyanides and evaluation of their antifouling activity and toxicity against cypris larvae of the barnacle Amphibalanus amphitrite. Glucosamine isocyanides with an ether functionality at the anomeric position exhibited potent antifouling activity, with EC₅₀ values below 1 μg/mL, without detectable toxicity even at a high concentration of 10 μg/mL. Two isocyanides had EC₅₀ values of 0.23 and 0.25 μg/mL, comparable to that of CuSO₄, which is used as a fouling inhibitor (EC₅₀ = 0.27 μg/mL).