Incorporation of β-Silicon-β3-Amino Acids in the Antimicrobial Peptide Alamethicin Provides a 20-Fold Increase in Membrane Permeabilization

1,2-Additions on Chiral N-Sulfinylketimines: An Easy Access to Chiral α-Tertiary Amines

Chiral α-tertiary amines, a motif present in α,α-disubstituted α-amino acids, in a wide range of natural products, and many drugs and drug candidates, are important targets in organic chemistry. Among the possible strategies, 1,2-addition to chiral N-sulfinyl­ketimines is one of the best routes to form chiral α-tertiary amines with a high level of stereoselectivity. In this review, we focus first on the addition of organometallic reagents or other nucleophiles as enols or ylides to chiral N-sulfinylketimines. Then secondly we cover a selection of applications of these additions in the synthesis of valuable biologically active compounds.

1 Introduction

2 1,2-Addition Reaction Methodologies

2.1 Organolithium Reagent Additions

2.2 Grignard Additions

2.3 Organozinc Reagent Additions

2.4 Organoindium Reagent Additions

2.5 Organoboron Reagent Additions

2.6 Strecker Reactions

2.7 Palladium-Catalyzed Reactions

2.8 Enols, Enolates, and Other Deprotonated Reagent Additions

2.9 Ylide Additions

2.10 Heteroatom Nucleophiles

2.11 Miscellaneous Reactions

3 Applications to the Synthesis of Biologically Active Molecules

4 Conclusions

Silylated Sugars – Synthesis and Properties

Silicon has been used in carbohydrate chemistry for half a century, but mostly as a protective group for sugar alcohols. Recently, the use of silicon has expanded to functionalization via C–H activation, conformational arming of glycosyl donors, and conformational alteration of carbohydrates. Silicon has proven useful as more than a protective group and during the last one and a half decades we have demonstrated how it influences both the reactivity of glycosyl donors and stereochemical outcome of glycosylations. Silicon can also be attached directly to the sugar C-backbone, which has even more pronounced effects on the chemistry and properties of the molecules. In this Account, we will give a tour through our work involving silicon and carbohydrates.

1 Introduction

2 Conformational Arming of Glycosyl Donors with Silyl Groups

3 Silyl Protective Groups for Tethering Glycosyl Donors

4. Si–C Glycosides via C–H Activation

4.1 C–H Activation and Oxidation of Methyl 6-Deoxy-l-glycosides

4.2 Synthesis of All Eight 6-Deoxy-l-sugars

4.3 Synthesis of All Eight l-Sugars by C–H Activation

4.4 Modification of the Oxasilolane Ring

5 C–Si in Glycosyl Donors – Activating or Not?

6 Si–C-Substituted Pyranosides

7 Perspective

Direct Access to α-Aminosilanes Enabled by Visible-Light-Mediated Multicomponent Radical Cross-Coupling

α-Aminosilanes are an important class of organic compounds that show biological activity. In this communication, a new approach to α-aminosilanes that utilizes photoredox catalysis to enable three-component coupling of organo(tristrimethylsilyl)silanes with feedstock alkylamines and aldehydes is presented. A wide range of highly functionalized α-aminosilanes can be obtained in good yields under mild conditions. Both primary amines and secondary amines are compatible with this transformation. Moreover, optically pure α-aminosilanes are accessible by using chiral amines. Mechanistic studies indicate that reactions proceed through radical/radical cross-coupling of silyl radicals with α-amino alkyl radicals.

Characteristics and therapeutic applications of antimicrobial peptides

The demand for novel antimicrobial compounds is rapidly growing due to the phenomenon of antibiotic resistance in bacteria. In response, numerous alternative approaches are being taken including use of polymers, metals, combinatorial approaches, and antimicrobial peptides (AMPs). AMPs are a naturally occurring part of the immune system of all higher organisms and display remarkable broad-spectrum activity and high selectivity for bacterial cells over host cells. However, despite good activity and safety profiles, AMPs have struggled to find success in the clinic. In this review, we outline the fundamental properties of AMPs that make them effective antimicrobials and extend this into three main approaches being used to help AMPs become viable clinical options. These three approaches are the incorporation of non-natural amino acids into the AMP sequence to impart better pharmacological properties, the incorporation of AMPs in hydrogels, and the chemical modification of surfaces with AMPs for device applications. These approaches are being developed to enhance the biocompatibility, stability, and/or bioavailability of AMPs as clinical options.

Reactivity, Selectivity, and Synthesis of 4-C-Silylated Glycosyl Donors and 4-Deoxy Analogues

A method for introducing dimethylphenylsilyl at the 4-position in carbohydrates has been developed. Two C-silylated glycosyl donors were prepared via levoglucosenone, starting from cellulose. The glycosylation properties were studied using three glucoside acceptors, a 3-OH, 4-OH, and 6-OH. Compared with the 4-deoxy variant, it was found that the anomeric selectivity was influenced more by the C-2 substituents orientation than the silyl in the 4-position. In general, the reactivity of these donors was higher than the corresponding 4-deoxy-analogue, albeit a competition experiment showed that the introduction of a C-Si increases the relative reactivity by a modest factor of around two.

Asymmetric total synthesis and antidepressant activity of (−)-sila-mesembranol bearing a silicon stereocenter

Asymmetric total synthesis of (−)-sila-mesembranol, the silicon analog of the natural alkaloid (−)-mesembranol has been achieved in 3.3% yield over 11 steps. The synthetic (−)-sila-mesembranol in mice exhibits better antidepressant effects than its carbon counterpart.

Intersecting Xenobiology and Neometabolism To Bring Novel Chemistries to Life

The diversity of life relies on a handful of chemical elements (carbon, oxygen, hydrogen, nitrogen, sulfur and phosphorus) as part of essential building blocks; some other atoms are needed to a lesser extent, but most of the remaining elements are excluded from biology. This circumstance limits the scope of biochemical reactions in extant metabolism - yet it offers a phenomenal playground for synthetic biology. Xenobiology aims to bring novel bricks to life that could be exploited for (xeno)metabolite synthesis. In particular, the assembly of novel pathways engineered to handle nonbiological elements (neometabolism) will broaden chemical space beyond the reach of natural evolution. In this review, xeno-elements that could be blended into nature's biosynthetic portfolio are discussed together with their physicochemical properties and tools and strategies to incorporate them into biochemistry. We argue that current bioproduction methods can be revolutionized by bridging xenobiology and neometabolism for the synthesis of new-to-nature molecules, such as organohalides.

Accelerated Molecular Dynamics Applied to the Peptaibol Folding Problem

The use of enhanced sampling molecular dynamics simulations to facilitate the folding of proteins is a relatively new approach which has quickly gained momentum in recent years. Accelerated molecular dynamics (aMD) can elucidate the dynamic path from the unfolded state to the near-native state, “flattened” by introducing a non-negative boost to the potential. Alamethicin F30/3 (Alm F30/3), chosen in this study, belongs to the class of peptaibols that are 7–20 residue long, non-ribosomally synthesized, amphipathic molecules that show interesting membrane perturbing activity. The recent studies undertaken on the Alm molecules and their transmembrane channels have been reviewed. Three consecutive simulations of ~900 ns each were carried out where N-terminal folding could be observed within the first 100 ns, while C-terminal folding could only be achieved almost after 800 ns. It took ~1 μs to attain the near-native conformation with stronger potential boost which may take several μs worth of classical MD to produce the same results. The Alm F30/3 hexamer channel was also simulated in an E. coli mimicking membrane under an external electric field that correlates with previous experiments. It can be concluded that aMD simulation techniques are suited to elucidate peptaibol structures and to understand their folding dynamics.

The Road from Host-Defense Peptides to a New Generation of Antimicrobial Drugs

Host-defense peptides, also called antimicrobial peptides (AMPs), whose protective action has been used by animals for millions of years, fulfill many requirements of the pharmaceutical industry, such as: (1) broad spectrum of activity; (2) unlike classic antibiotics, they induce very little resistance; (3) they act synergically with conventional antibiotics; (4) they neutralize endotoxins and are active in animal models. However, it is considered that many natural peptides are not suitable for drug development due to stability and biodisponibility problems, or high production costs. This review describes the efforts to overcome these problems and develop new antimicrobial drugs from these peptides or inspired by them. The discovery process of natural AMPs is discussed, as well as the development of synthetic analogs with improved pharmacological properties. The production of these compounds at acceptable costs, using different chemical and biotechnological methods, is also commented. Once these challenges are overcome, a new generation of versatile, potent and long-lasting antimicrobial drugs is expected.

An efficient approach to trans-4-hydroxy-5-substituted 2-pyrrolidinones through a stereoselective tandem Barbier process: divergent syntheses of (3R,4S)-statines, (+)-preussin and (−)-hapalosin

Natural products of (+)-preussin and (−)-hapalosin have been asymmetrically synthesized through the stereoselective tandem Barbier process.