Total synthesis of bidensyneosides A2 and C: remarkable protecting group effects in glycosylation

A small-molecule degron with a phenylpropionic acid scaffold

Targeted protein degradation (TPD), employing proteolysis-targeting chimeras (PROTACs) composed of ligands for both a target protein and ubiquitin ligase (E3) to redirect the ubiquitin-proteasome system (UPS) to the target protein, has emerged as a promising strategy in drug discovery. However, despite the vast number of E3 ligases, the repertoire of E3 ligands utilized in PROTACs remains limited. Here, we report the discovery of a small-molecule degron with a phenylpropionic acid skeleton, derived from a known ligand of S-phase kinase-interacting protein 2 (Skp2), an E3 ligase. We used this degron to design PROTACs inducing proteasomal degradation of HaloTag-fused proteins, and identified key structural relationships. Surprisingly, our mechanistic studies excluded the involvement of Skp2, suggesting that this degron recruits other protein(s) within the UPS.

Polyacetylene Glycosides: Isolation, Biological Activities and Synthesis

Polyacetylene glycosides (PAGs) constitute a relatively small class of secondary metabolites characterized by the presence of a sugar unit anomerically connected to a polyacetylene. These compounds are found in fungi, seaweed, and more often in plants. PAGs exhibit a wide range of biological and pharmacological activities and, as a result, the literature of these compounds has grown exponentially in recent years.

The first total synthesis of cytopiloyne, an anti-diabetic, polyacetylenic glucoside

The first total synthesis of cytopiloyne 1, a novel bioactive polyacetylenic glucoside isolated from the extract of Bidens pilosa, is described. The structure of cytopiloyne was determined to be 2-β-D-glucopyranosyloxy-1-hydroxytrideca-5,7,9,11-tetrayne by using various spectroscopic methods, but the chirality of the polyyne moiety was unknown. Herein, the convergent synthesis of two diastereomers of cytopiloyne by starting from commercially available 4-(2-hydroxyethyl)-2,2-dimethyl-1,3-diozolane is described. The synthetic sequence involved two key steps: stereoselective glycosylation of the glucosyl trichloroacetimidate with 1-[(4-methoxybenzyl)oxy]hex-5-yn-2-ol to give the desired β-glycoside and the construction of the glucosyl tetrayne skeleton by using a palladium/silver-catalyzed cross-coupling reaction to form the alkyne-alkyne bond, the first such use of this reaction. Comparison between the observed and published characterization data showed the 2R isomer to be the natural product cytopiloyne.

An enantioselective total synthesis of natural antibiotic marasin

Synthetic studies directed toward the allenediyne antibiotic marasin are presented. Different approaches to the installation of the optically active chiral allenediyne motif were explored en route to the synthesis of the natural product. The stereoselectivity for the construction of the chiral allenediyne motif was dependent on not only the reaction employed but also the substrate structure.

Naturally occurring and synthetic polyyne glycosides

Polyyne glycosides are a class of natural products that have been found in terrestrial plants, fungi, and marine algae. Many display interesting and potentially useful biological activities, which in some cases has initiated synthetic efforts toward their formation. This review provides a comprehensive report of polyyne glycosides isolated from natural sources, as well as the synthesis of these molecules and their analogues.

A Short Synthesis of (S)-(+)-Siphonodiol

A short synthesis of the enantiomer of the polyacetylenic natural product siphonodiol is described. The synthesis is based on the strategy of taking advantage of the hidden symmetry of the target molecule and minimizing the use of protecting groups, thereby reducing the total number of steps and increasing the overall efficiency.

Synthesis of Naturally Occurring Polyynes

Over the past fifty years, hundreds of polyyne compounds have been isolated from nature. These often unstable molecules are found in sources as common as garden vegetables and as obscure as bacterial cultures. Naturally occurring polyynes feature a wide range of structural diversity and display an equally broad array of biological properties. Early synthetic efforts relied primarily on Cu-catalyzed, oxidative acetylenic homo- and heterocoupling reactions to assemble the polyyne framework. The past 25 years, however, have witnessed a renaissance in the field of polyyne natural product synthesis: transition-metal-catalyzed alkynylation reactions and asymmetric transformations have combined to substantially expand access to natural polyynes. This Review recounts these synthetic achievements and also highlights both the natural source(s) and biological relevance for many of these compounds.

Polyyne synthesis using carbene/carbenoid rearrangements

Rearrangement of a carbene/carbenoid intermediate to form an acetylene moiety, known as the Fritsch-Buttenberg-Wiechell (FBW) rearrangement, was developed for the formation of polyynes and polyyne frameworks within highly conjugated organic materials. Necessary precursors can be prepared through formation of an alkynyl ketone, followed by dibromoolefination under Corey-Fuchs conditions. The carbenoid rearrangement is brought about by treatment of the dibromoolefin with BuLi under mild conditions. The success of these FBW reactions is quite solvent-dependent, and nonpolar hydrocarbon solvents (e.g., hexanes, toluene, benzene) work quite well, while use of ethereal solvents such as diethyl ether and tetrahydrofuran (THF) does not provide the desired polyyne product. This protocol was successfully applied to the formation of silyl, alkyl, alkenyl, and aryl polyynes, including di-, tri-, and tetrayne products, as well as the construction of two-dimensional carbon-rich molecules. A one-pot variant of this procedure is being developed and is particularly applicable toward the synthesis of polyyne natural products. Formation of a series of triisopropylsilyl end-capped polyynes, from the triyne to decayne, was achieved. Third-order nonlinear optical properties of these polyynes were evaluated. This study shows that the molecular second hyperpolarizabilities for the polyynes as a function of length increase at a rate that is higher than all other nonaromatic organic oligomers.