Application of phase-trafficking methods to natural products research

The isolation of water-soluble natural products - challenges, strategies and perspectives

Covering period: up to 2019Water-soluble natural products constitute a relevant group of secondary metabolites notably known for presenting potent biological activities. Examples are aminoglycosides, β-lactam antibiotics, saponins of both terrestrial and marine origin, and marine toxins. Although extensively investigated in the past, particularly during the golden age of antibiotics, hydrophilic fractions have been less scrutinized during the last few decades. This review addresses the possible reasons on why water-soluble metabolites are now under investigated and describes approaches and strategies for the isolation of these natural compounds. It presents examples of several classes of hydrosoluble natural products and how they have been isolated. Novel stationary phases and chromatography techniques are also reviewed, providing a perspective towards a renaissance in the investigation of water-soluble natural products.

A microwave-assisted highly practical chemoselective esterification and amidation of carboxylic acids

The ubiquitousness of esters and amide functionalities makes their coupling reaction one of the most sought-after organic transformations.

Selective Monoesterification of Symmetrical Diols Using Resin-Bound Triphenylphosphine

Coupling reactions to make esters and amides are among the most widely used organic transformations. We report efficient procedures for amide bond formation and for the monoesterification of symmetrical diols in excellent yields without any requirement for high dilution or slow addition using resin-bound triarylphosphonium iodide. Easy purification, low moisture sensitivity, and good to excellent yields of the products are the major advantages of this protocol.

Taming of a superbase for selective phenol desilylation and natural product isolation

Hydroxyl moieties are highly prevalent in natural products. We previously reported a chemoselective strategy for enrichment of hydroxyl-functionalized molecules by formation of a silyl ether bond to a resin. To generate smaller pools of molecules for analysis, we developed cleavage conditions to promote stepwise release of phenolic silyl ethers followed by aliphatic silyl ethers with a "tamed" version of the superbase 1,1,3,3-tetramethylguanadine. We demonstrate this as a general strategy for selective deprotection of phenolic silyl ethers under neutral conditions at room temperature.

Isolation and characterization of bioactive compounds from plant resources: the role of analysis in the ethnopharmacological approach

The phytochemical research based on ethnopharmacology is considered an effective approach in the discovery of novel chemicals entities with potential as drug leads. Plants/plant extracts/decoctions, used by folklore traditions for treating several diseases, represent a source of chemical entities but no information are available on their nature. Starting from this viewpoint, the aim of this review is to address natural-products chemists to the choice of the best methodologies, which include the combination of extraction/sample preparation tools and analytical techniques, for isolating and characterizing bioactive secondary metabolites from plants, as potential lead compounds in the drug discovery process. The work is distributed according to the different steps involved in the ethnopharmacological approach (extraction, sample preparation, biological screening, etc.), discussing the analytical techniques employed for the isolation and identification of compound/s responsible for the biological activity claimed in the traditional use (separation, spectroscopic, hyphenated techniques, etc.). Particular emphasis will be on herbal medicines applications and developments achieved from 2010 up to date.

Ethyl acrylate conjugated polystyryl-diphenylphosphine — An extremely efficient catalyst for Henry reaction under solvent-free conditions (SolFC)

Over the last few decades, the fast-growing development of polymer supported reagents has led to the synthesis of a variety of reagents on solid support. Some of the major advantages of using such reagents are that they are less hygroscopic, easy to recover, and can be recycled. Here, we have demonstrated that in situ generated ethyl acrylate conjugated polystyryl-diphenylphosphine (PDPP–EA) derived from the reaction of a mixture of polystyryl-diphenylphosphine and ethyl acrylate in a stoichiometric ratio can efficiently catalyze the synthesis of β-nitroalcohols from the reaction of aldehydes and nitroalkanes under solvent-free conditions (SolFC).

Isolation of natural products by ion-exchange methods

The primary goal of many natural products chemists is to extract, isolate, and characterize specific analytes from complex plant, animal, microbial, and food matrices. To achieve this goal, they rely considerably on highly sophisticated and highly hyphenated modern instrumentation. Yet, the vast majority of modern instrumentation typically found in the laboratories of natural products chemists is founded on the simple principles of intermolecular forces to achieve separation. Ion-exchange chromatography (IEC) is, at heart, the most fundamental, and strongest, of these interactions and is considered a relatively inexpensive and effective medium in which to "clean-up" a sample. Additionally, IEC offers high recoveries of key analytes and offers the ability to modify the stationary and mobile phases in order to selectively "catch and release" compounds of interest.

Siloxyl ether functionalized resins for chemoselective enrichment of carboxylic acids

Although the carboxylic acid moiety is prevalent in many biologically produced molecules, including natural products and proteins, methods to chemoselectively target this functional group have remained elusive. Generally, strategies that utilize carboxylate nucleophilicity also promote reactions with other nucleophiles such as amines and hydroxyls. A reagent was sought to facilitate the selective isolation of carboxylic acid containing compounds from complex mixtures. Here, the development of siloxyl ether functionalized solid supports is described.

Identification of unprecedented purine-containing compounds, the zingerines, from ginger rhizomes (Zingiber officinale Roscoe) using a phase-trafficking approach

Three unprecedented purine-containing compounds, named [6]-, [8]-, and [10]-zingerines as they are 5-(6-amino-9H-purin-9-yl) analogs of [6]-, [8]-, and [10]-gingerols, respectively, were isolated from a methanolic extract of ginger rhizomes using a phase trafficking-based method that utilizes solid phase reagents allowing for fast and selective simultaneous separation of basic, acidic, and neutral components of natural products extracts.

Chemoselective enrichment for natural products discovery

Natural products account for a significant proportion of modern day therapeutic agents. However, the discovery of novel compounds is hindered by the isolation process, which often relies upon extraction and chromatographic separation techniques. These methods, which are dependent upon the physicochemical properties of the compounds, have a limited ability to both purify and concentrate the minor components of a biological extract. We have devised an isolation strategy based upon an orthogonal chemical feature, namely, functional group composition. Development of a functional group-targeted method is expected to achieve exceptional resolution given the large number of distinct moieties present in natural product extracts. Here, we describe the generation of controllably reversible covalent enrichment tags for the chemoselective isolation of alcohol-containing natural products from complex mixtures.