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

Nitroolefins or β-acetoxy-nitro compounds react with α-isocyano-esters in the presence of an organic base to give pyrroles in good yield.

A Useful Synthesis of Pyrroles from Nitroolefins.

S-Trifluoromethyl xanthates efficiently add to unactivated alkenes by a radical mechanism to give adducts with a trifluoromethyl group at the least hindered terminus of the olefin.

A Xanthate Transfer Radical Process for the Introduction of the Trifluoromethyl Group.

Three convergent routes to thiophenes are described, hinging on the radical addition of α-xanthyl ketones to ethyl vinyl sulfide or to vinyl pivalate. The latter route ultimately proved to be the most versatile and efficient (61–94%).

Flexible Routes to Thiophenes.

The radical addition of a-xanthyl ketones to vinyl pivalate gives adducts that are synthetic equivalents of 1,4-ketoaldehydes; treatment with ammonia or primary amines leads to the corresponding pyrroles in high yield.

A Practical Variation on the Paal-Knorr Pyrrole Synthesis

Reaction of the cholanic acid derivative (1) with thionyl chloride and pyridine gave, after quenching with methanol, the α-sulphine ester (3); treatment with acetic anhydride–sulphuric acid afforded the keto ester (5) which, on exposure to air in the presence of copper(II) ions and base, gave the 20-keto pregnane derivative (9) in excellent overall yield.

Samir Z. Zard

Papers

A Useful Synthesis of Pyrroles from Nitroolefins.

A Xanthate Transfer Radical Process for the Introduction of the Trifluoromethyl Group.

Flexible Routes to Thiophenes.

A Practical Variation on the Paal-Knorr Pyrrole Synthesis

A very short degradation of the bile acid side chain