https://pubs.rsc.org/en/content/articlepdf/2019/NP/C8NP00092A

Marine natural products.

The Huisgen 1,3-dipolar cycloaddition reaction of organic azides and alkynes has gained considerable attention in recent years due to the introduction in 2001 of Cu(1) catalysis by Tornoe and Meldal, leading to a major improvement in both rate and regioselectivity of the reaction, as realized independently by the Meldal and the Sharpless laboratories. The great success of the Cu(1) catalyzed reaction is rooted in the fact that it is a virtually quantitative, very robust, insensitive, general, and orthogonal ligation reaction, suitable for even biomolecular ligation and in vivo tagging or as a polymerization reaction for synthesis of long linear polymers. The triazole formed is essentially chemically inert to reactive conditions, e.g. oxidation, reduction, and hydrolysis, and has an intermediate polarity with a dipolar moment of ∼5 D. The basis for the unique properties and rate enhancement for triazole formation under Cu(1) catalysis should be found in the high ∆G of the reaction in combination with the low character of polarity of the dipole of the noncatalyzed thermal reaction, which leads to a considerable activation barrier. In order to understand the reaction in detail, it therefore seems important to spend a moment to consider the structural and mechanistic aspects of the catalysis. The reaction is quite insensitive to reaction conditions as long as Cu(1) is present and may be performed in an aqueous or organic environment both in solution and on solid support.

Cu-catalyzed azide-alkyne cycloaddition.

Multicomponent reactions (MCRs) are fundamentally different from two-component reactions in several aspects. Among the MCRs, those with isocyanides have developed into popular organic-chemical reactions in the pharmaceutical industry for the preparation of compound libraries of low-molecular druglike compounds. With a small set of starting materials, very large libraries can be built up within a short time, which can then be used for research on medicinal substances. Due to the intensive research of the last few years, many new backbone types have become accessible. MCRs are also increasingly being employed in the total synthesis of natural products. MCRs and especially MCRs with isocyanides offer many opportunities to attain new reactions and basic structures. However, this requires that the chemist learns the “language” of MCRs, something that this review wishes to stimulate.

Multicomponent Reactions with Isocyanides.

Controlled/living radical polymerization: Features, developments, and perspectives

This paper presents a review of living radical polymerization achieved with thiocarbonylthio compounds [ZC(=S)SR] by a mechanism of reversible addition–fragmentation chain transfer (RAFT). Since we first introduced the technique in 1998, the number of papers and patents on the RAFT process has increased exponentially as the technique has proved to be one of the most versatile for the provision of polymers of well defined architecture. The factors influencing the effectiveness of RAFT agents and outcome of RAFT polymerization are detailed. With this insight, guidelines are presented on how to conduct RAFT and choose RAFT agents to achieve particular structures. A survey is provided of the current scope and applications of the RAFT process in the synthesis of well defined homo-, gradient, diblock, triblock, and star polymers, as well as more complex architectures including microgels and polymer brushes.

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Living radical polymerization by the RAFT process

S-Propargyl xanthates undergo upon heating a [3,3] sigmatropic rearrangement followed by a reversible ring closure into a novel betaine. This betaine can be implicated in carbon–carbon bond forming processes, in the synthesis of esters, in the inversion of secondary alcohols, in the formation of alkenes, for the generation of rigid, cisoid dienes that are highly reactive in both inter- and intra-molecular Diels–Alder cycloadditions, and in various other synthetically useful transformations. 1 	Introduction 2 	An Unexpected Transformation 3 	Evidence for the Betaine Intermediate 4 	A Method for the Synthesis of Esters and for the Inversion of Secondary Alcohols 5 	A General Alkylation Process 6 	The Case of Carbon Acids 7 	A Synthesis of Alkenes 8 	Further Trapping Experiments. Concerted or Not Concerted? 9 	Rigid Cisoid Dienes 10 	Propargyl Radicals 11 	Concluding Remarks

https://www.thieme-connect.com/products/ejournals/pdf/10.1055/s-0037-1611638.pdf

Sulfur Betaines from S-Propargyl Xanthates. Unusual Chemistry from a Simple Functional Group

A concise access to a new class of selective anti-inflammatory steroid derivatives

Ring Opening Induced by Iminyl Radicals Derived from Cyclobutanones: New Aspects of Tin Hydride Cleavage of S-Phenyl Sulfenylimines.

Reduction of 3β-benzoyloxycholest-5-en-4-one (3) with sodium borohydride under various conditions affords only minor amounts of 3β-benzoyloxycholest-5-en-4β-ol (2a). The major product is the 4α-isomer (6). Using sodium borodeuteride it has been shown that the 3β-benzoyloxycholest-5-en-4β-ol obtained is deuteriated at the 3α-position in relatively alkaline conditions, but at 4α- under conditions near neutrality. A plausible mechanism has been proposed for this interesting result.

Stereochemistry of the reduction of 3β-benzoyloxycholest-5-en-4-one with sodium borohydride

N-Hydroxy-2-thiopyridone gives hydroxyl radicals on irradiation with visible light and these can be incorporated in useful radical chain processes involving hydrogen abstraction.

Samir Z. Zard

Papers

Sulfur Betaines from S-Propargyl Xanthates. Unusual Chemistry from a Simple Functional Group

A concise access to a new class of selective anti-inflammatory steroid derivatives

Ring Opening Induced by Iminyl Radicals Derived from Cyclobutanones: New Aspects of Tin Hydride Cleavage of S-Phenyl Sulfenylimines.

Stereochemistry of the reduction of 3β-benzoyloxycholest-5-en-4-one with sodium borohydride

N‐Hydroxy‐2‐pyridinethione: A Mild and Convenient Source of Hydroxyl Radicals.