Site-selective modification of peptide backbones

Exciting developments in the site-selective modification of peptide backbones are allowing an outstanding fine-tuning of peptide conformation, folding ability, and physico-chemical and biological properties.

Modern tools for the chemical ligation and synthesis of modified peptides and proteins

The ability to improve nature's capacity by introducing modification of biological interest in proteins and peptides (P&P) is one of the modern challenges in synthetic chemistry. Due to the unfavorable pharmacokinetic properties, many native P&P are of little use as therapeutic agents. Today, few methods for the preparation of modified proteins are available. Initially introduced to realize the ligation between two standard peptidic sequences, and hence to afford native proteins, the modern chemical methodologies, in other words native chemical ligation, expressed ligation, Staudinger ligation, auxiliary mediated ligation, aldehyde capture, etc., can be virtually utilized to ligate a variety of peptidomimetic partners, allowing a systematic access to modified, unnatural large P&P.

Synthesis of indoles and tryptophan derivatives via photoinduced nitrene C–H insertion

Functionalized indoles and tryptophans can be obtained from stannylated alkenes and o-iodoanilines via Stille coupling.

Non-stabilized enolates – versatile nucleophiles in transition metal-catalysed allylic alkylations

This review covers new developments in the transition metal-catalyzed allylic alkylations of non-stabilized enolates, preferentially generated from ketone, esters or amides.

Total syntheses and biological evaluation of miuraenamides

The miuraenamides, relatively simple representatives of a class of cyclodepsipeptides with high antitumor activity, can be easily and flexibly obtained by the concept of peptide modification. A reaction sequence consisting of an aldol reaction, oxidation, and methylation of the glycine subunit of the cyclodepsipeptides allows the incorporation of the unusual α,β-unsaturated dehydroamino acid in one of the last steps of the synthesis.

[(p-Cymene)RuCl2 ]2 : an efficient catalyst for highly regioselective allylic alkylations of chelated amino acid ester enolates

Chelated amino acid ester enolates are excellent nucleophiles for ruthenium-catalyzed allylic alkylations. Although [Cp*Ru(MeCN)3 ]PF6 was found to be the most reactive catalyst investigated, with the resulting allyl complexes reacting at temperatures as low as -78 °C, unfortunately the process took place with only moderate regio- and diastereoselectivity. In contrast, [(p-cymene)RuCl2 ]2 allowed allylations to be performed with a high degree of regioretention. Secondary allyl carboxylates with a terminal double bond were found to be the most reactive substrates, giving rise to the branched amino acids with perfect regioretention and chirality transfer. In this case, no isomerization of the Ru-allyl complex formed in situ was observed, in contrast to the analogues palladium complexes. This isomerization-free protocol can also be used for the synthesis of (Z)-configured γ,δ-unsaturated amino acid derivatives, starting from (Z)-allylic substrates. Here, the more reactive phosphates were found to be superior to the carboxylates, providing the required amino acids in almost quantitative yield with perfect regio- and stereoretention. Therefore, the Ru-catalyzed allylation reactions are well positioned to overcome the drawbacks of Pd-catalyzed processes.

Creating diversity by site-selective peptide modification: a customizable unit affords amino acids with high optical purity

The development of peptide libraries by site-selective modification of a few parent peptides would save valuable time and materials in discovery processes, but still is a difficult synthetic challenge. Herein natural hydroxyproline is introduced as a "convertible" unit for the production of a variety of optically pure amino acids, including expensive N-alkyl amino acids, and to achieve the mild, efficient, and site-selective modification of peptides.

Highly stereoselective modifications of peptides via Pd-catalyzed allylic alkylation of internal peptide amide enolates

Pd-catalyzed allylations are excellent tools for stereoselective peptide modifications, showing several advantages compared to normal alkylations. Reactions of internal peptide amide enolates with Pd-allyl complexes proceed not only with high yields of up to 86%, they show also high regio- and diastereoselectivities (88-99%), giving rise to the trans-configured products. Therefore, this protocol is a powerful synthetic tool for the synthesis of natural product and drug molecules.

Zinc-mediated highly α-regioselective prenylation of imines with prenyl bromide

A highly α-regioselective prenylation of imines has been successfully developed. The efficiency of this approach is confirmed by a wide range of imines including N- and C-aryl aldimines, N-alkyl aldimines, C-alkyl aldimines, and N- and C-aryl ketimines. The approach uses prenyl bromide as the prenyl source and inexpensive and convenient zinc as the mediator as well as environmentally benign 1,3-dimethyl-2-imidazolidinone (DMI) as the solvent.

Stannylated allyl carbonates as versatile building blocks for the diversity oriented synthesis of allylic amines and amides

Stannylated allylic carbonates are suitable substrates for Pd-catalyzed allylic aminations. In DMF and with [allylPdCl](2) as catalyst the stannylated allyl amines formed can be directly coupled with electrophiles according to the Stille protocol, giving rise to highly functionalized building blocks in excellent yields.