Total syntheses of rivularins D1 and D3

Dynamic Kinetic Resolution of Indole-Based Sulfenylated Heterobiaryls by Rhodium-Catalyzed Atroposelective Reductive Aldol Reaction

A highly enantio- and diastereoselective dynamic kinetic resolution (DKR) of configurationally labile 3-aryl indole-2-carbaldehydes is described. The DKR proceeds via a Rh-catalyzed intermolecular asymmetric reductive aldol reaction with acrylate esters, with simultaneous generation of three stereogenic elements. The strategy relies on the labilization of the stereogenic axis that takes place thanks to a transient Lewis acid-base interaction (LABI) between the formyl group and a thioether moiety strategically located at the ortho' position. The atropisomeric indole products present a high degree of functionalization and can be further converted to a series of axially chiral derivatives, thereby expanding their potential application in drug discovery and asymmetric catalysis.

Cationic Peptidomimetic Amphiphiles Having a N-Aryl- or N-Naphthyl-1,2,3-Triazole Core Structure Targeting Clostridioides (Clostridium) difficile: Synthesis, Antibacterial Evaluation, and an In Vivo C. difficile Infection Model

Clostridioides (also known as Clostridium) difficile is a Gram-positive anaerobic, spore producing bacterial pathogen that causes severe gastrointestinal infection in humans. The current chemotherapeutic options are inadequate, expensive, and limited, and thus inexpensive drug treatments for C. difficile infection (CDI) with improved efficacy and specificity are urgently needed. To improve the solubility of our cationic amphiphilic 1,1′-binaphthylpeptidomimetics developed earlier that showed promise in an in vivo murine CDI model we have synthesized related compounds with an N-arytriazole or N-naphthyltriazole moiety instead of the 1,1′-biphenyl or 1,1′-binaphthyl moiety. This modification was made to increase the polarity and thus water solubility of the overall peptidomimetics, while maintaining the aromatic character. The dicationic N-naphthyltriazole derivative 40 was identified as a C. difficile-selective antibacterial with MIC values of 8 µg/mL against C. difficile strains ATCC 700057 and 132 (both ribotype 027). This compound displayed increased water solubility and reduced hemolytic activity (32 µg/mL) in an in vitro hemolysis assay and reduced cytotoxicity (CC₅₀ 32 µg/mL against HEK293 cells) relative to lead compound 2. Compound 40 exhibited mild efficacy (with 80% survival observed after 24 h compared to the DMSO control of 40%) in an in vivo murine model of C. difficile infection by reducing the severity and slowing the onset of disease.

Marine Bi-, Bis-, and Trisindole Alkaloids

This chapter, covering the chemistry literature up until June 2013 and comprising 142 references, records the chemical structures of 130 bi-, bis-, and trisindole alkaloids isolated from a plethora of marine phyla including bacteria, algae, bryozoans, sponges, mollusks, hard corals, and ascidians. While the vast majority of bisindoles have been isolated from marine sponges, biindoles are more commonly found in red algae species than sponges. Trisindoles are far less common than bisindoles in the marine environment and have been limited to two species of sponge and a single species of marine microbe. Antimicrobial activity and cytotoxicity dominate the bioactivities explored for selected members of this family of alkaloids. Synthetic approaches to 28 natural products are presented in 33 schemes, and in the absence of any in vivo biosynthetic studies, the putative biosyntheses of eight bisindole metabolites are presented.

Design and synthesis of substrate mimetics based on an indole scaffold: potential inhibitors of 17β-HSD type 1

BACKGROUND

Human 17β-hydroxysteroid dehydrogenase type 1 (17β-HSD1) acts at a pre-receptor level. It catalyzes NADPH-dependent reduction of the weak estrogen estrone into the most potent estrogen estradiol, which exerts its proliferative effects via estrogen receptors. Overexpression of 17β-HSD1 in estrogen-responsive tissues is related to the development of hormone-dependent diseases, such as breast cancer and endometriosis. 17β-HSD1 thus represents an attractive target for development of new drugs.

METHODS

We designed and synthesized a series of 3-, 5- and 6-phenyl indole derivatives as mimetics of the steroid substrate estrone. All of these compounds were evaluated for inhibition of recombinant human 17β-HSD1 from Escherichia coli, at concentrations of 0.6 μM and 6.0 μM.

RESULTS

Among 14 indole derivatives, compound 9 was an initial hit inhibitor of 17β-HSD1, with moderate inhibition (64% at 6 μM). Molecular docking into the crystal structure of 17β-HSD1 (1A27) revealed that this 5-phenyl indole derivative binds to 17β-HSD1 similarly to co-crystalized E2. Compound 9 forms two H-bonds with 17β-HSD1: one between the indole nitrogen and His222, and the second between the phenolic OH group and catalytic Tyr155.

CONCLUSIONS

The indole scaffold is one of the possible starting points for the design of substrate mimetics of the steroid substrate estrone. Our study shows that these 6- and, especially, 5-phenol indole derivatives can act as moderate inhibitors of 17β-HSD1. Based on inhibition assays and docking simulations, we can infer further improvements of the 5-phenol indole derivatives that might result in better inhibition profiles.

Total synthesis of (+/-)-herbindole A, (+/-)-herbindole B, and (+/-)-cis-trikentrin A

Herein we describe a divergent total synthesis of the title compounds utilizing Diels-Alder reactions of monoimine quinoids, followed by cyclization of the aromatized adducts to generate the tricyclic skeletons. Elaboration to iodinated or triflated indole derivatives allows for installation of the requisite alkyl substitution via cross-coupling reactions.

Synthesis and cross-coupling reactions of substituted 5-triflyloxyindoles

N-arylsulfonyl quinone monoimines undergo smooth cycloadditions in a [4+2] sense to yield the expected cycloadducts. The crude cycloadducts, when subjected to a short series of simple transformations, produce synthetically useful quantities of 5-triflyloxyindoles in excellent overall yields. Such compounds are excellent participants in cross-coupling chemistry.

Hypervalent iodine-mediated vicinal syn diazidation: application to the total synthesis of (+/-)-dibromophakellstatin

[reaction: see text] The total synthesis of (+/-)-dibromophakellstatin is described. The molecule is constructed from a key syn-diazide, formed by a hypervalent iodine-mediated diazidation of a dihydrodipyrrolopyrazinone ring structure.

Total Synthesis of (±)-Herbindole B and (±)-cis-Trikentrin B

[reaction: see text] Herbindole B and cis-trikentrin B are naturally occurring indoles having the unusual and synthetically challenging pattern of carbon substitution at the 4-7 and 5-7 positions, respectively, with no substitution at the 1-3 positions. The total syntheses of these polyalkylated indoles have been achieved in 19 and 12 steps, respectively. The synthesis of herbindole B relies on two iterations of a quinine monoimine Diels-Alder reaction, while cis-trikentrin B uses a single cycloaddition of a suitable quinone monoimine. Indolization of the adducts provides suitably substituted benzopyrrole nuclei for elaboration to the natural products.