Synlett

The reliance on wasteful stoichiometric reagents to accomplish dehydration reactions such as esterification, amidation, and alcohol substitution is a longstanding challenge in synthetic chemistry. To address this problem, an electrochemical approach has been developed as a new conceptual platform for dehydration reactions. As a proof-of-concept demonstration, an electrochemical esterification protocol has been described that proceeds at room temperature, without acid or base additives, and without consuming stoichiometric reagents. This approach therefore overcomes key complications of esterification chemistry, and we envision that it will similarly enable improvements to a range of important, related transformations.1 Introduction2 An Electrochemical Design for Catalytic Dehydration3 Electrochemical Esterification4 Conclusions

Carbohydrates are synthetically challenging molecules with vital biological roles in all living systems. To better understand the biological functions of this fundamentally important class of molecules, novel methodologies are needed, including site-selective functionalization and glycosylation reactions. This account describes our efforts toward the development of novel methodologies for site-selective functionalization of carbohydrates and stereoselective glycosylation through various acylation reactions.

Synlett 2024; 35: V-DOI: 10.1055/s-0043-1763662Georg Thieme Verlag KG Rüdigerstraße 14, 70469 Stuttgart, GermanyArticle in Thieme eJournals:Table of contents

Polycations are an important class of water-soluble polymers because they form polyion complexes with DNA. Thus, the synthesis of polycations with controlled monomer sequences will be of increasing importance for the formation of well-defined polyion complexes. In this study, cationic homopolymer and alternating copolymer with the dense triazole backbone were synthesized by copper(I)-catalyzed azide–alkyne cycloaddition polymerization. The polycations obtained were characterized by potentiometric and turbidimetric titrations, and by complex formation with poly(acrylic acid).

A stereoselective synthesis of the C1–C13 segment of poecillastrin C has been achieved. The C1–C4 moiety was derived from diallyl l-tartrate, and the amide group at the C3 position was constructed by means of a traceless Staudinger reaction. The C1–C13 segment was submitted to model studies, including esterification with a bulky alcohol at the C1 position and Stille coupling with vinyl iodide at the C13 position. The reactivity of the C1 position was affected by the neighboring C2-protective group. When the C2 hydroxy group was protected as a TBS ether, the C1 carboxylic acid did not undergo esterification with a bulky secondary alcohol, whereas the p-methoxybenzylidene N,O-acetal afforded a 2,4-dimethyl-3-pentyl ester. Stille coupling of the C1–C13 segment with 1-iodohept-1-ene gave the southern part of the poecillastrin C macrolactam attached to simplified eastern and western parts.

We present a novel method for the chemoselective House–Meinwald rearrangement of trisubstituted epoxides under mild conditions with the use of simple perfluorinated disulfonimides as Brønsted acid catalysts. We isolated the α-quaternary aldehyde products in yields of 27–97% using catalyst loadings as low as 0.5 mol% on a scale of 1 mmol. In addition, we show the stereospecific rearrangement using an enantioenriched substrate, which makes this method suitable for applications in total synthesis of natural products.

A visible-light-mediated selective allylic C–H bond oxygenation of cyclic olefins is presented. Hence, the selective, mild monooxygenation of simple cycloalkenes has been achieved using an acridinium photoredox catalyst in combination with a phosphate base and a disulfide HAT reagent under air atmosphere at room temperature. The combination of both photocatalyst and HAT reagent, which can operate through a single or two different concurrent mechanistic pathways for the formation of the allyl radical, proved highly efficient, while the reaction with exclusively one or the other mediator performs in significantly lower yields. The formed allyl radical further reacts with a molecule of oxygen to build the corresponding peroxyradical that can abstract a hydrogen atom of another cycloalkene substrate, generating the known hydroperoxide intermediate in the formation of the ketone moiety. The advantages of this method rely on the easy use of air as oxygen source, as well as the selective monooxygenation of cycloalkenes without substitution in one of the allylic positions. Besides simple cyclic olefins, the method was also successfully applied in the oxidation of natural products such as the terpene valencene or cholesterol derivatives.

Herein, we report a regioselective cycloaddition strategy of N-aminopyridinium ylides with electron-deficient alkenes, in the presence of a hypervalent iodine reagent, PhI(OAc)2. A variety of multifunctionalized pyrazolo[1,5-a]pyridine architectures were smoothly afforded by the reactions of pyridine-, quinoline-, and isoquinoline-based N-ylides with diverse alkenes with or without a halogen atom adjacent to the electron-withdrawing group (EWG) under facile conditions.

We have revisited the imine-anion-mediated Smiles rearrangement for the synthesis of ortho-hydroxyphenyl arylketimines. Detailed examinations revealed that migration of various aromatic groups, previously considered to be unsuited to SNAr-type reactions, such as electron-rich or sterically hindered aromatic groups, can be accomplished by introducing bulky 9-anthryllithium as a nucleophile. Among the aromatic groups examined, naphthyl groups (1- and 2-naphthyl groups) exhibited an excellent performance, and their migration ability was well illustrated by the reaction with less bulky nucleophiles.

Control of radical reactivity is regarded as an important concern in the fields of catalysis and materials sciences. Radical species generated from monoruthenium acetylide complexes are, in general, highly reactive, and therefore structural characterization of these species has remained elusive. In this paper, a spectroscopic and structural characterization of the cationic radical species of a monoruthenium diacetylide bearing a Ru tetraphosphine fragment, [trans-(Ar–SC≡C)2Ru(dppe)2]SbCl6 ([1]+SbCl6) [Ar: p-t-BuC6H4; dppe: 1,2-bis(diphenylphosphino)ethane], is presented. The formation of the radical species [1]+ is supported by the vis-NIR, IR, and ESR studies. Furthermore, the solid-state structure of [1]+ reveals a significant contribution of the cumulenic Ru=C=C=S resonance structure. Remarkably, the thermal stability of [1]+ results from the incorporation of the electron-donating (arylsulfanyl)ethynyl ligands and the highly sterically demanding dppe ligands as compared with a monoruthenium complex with less-bulky and less-electron-rich derivatives.