The chemistry of heterocyclic compounds has traditionally been and remains a bright area of chemical science in Russia. This is due to the fact that many heterocycles find the widest application. These compounds are the key structural fragments of most drugs, plant protection agents. Many natural compounds are also derivatives of heterocycles. At present, more than half of the hundreds of millions of known chemical compounds are heterocycles. This collective review is devoted to the achievements of Russian chemists in this field over the last 15–20 years. The review presents the achievements of leading heterocyclists representing both RAS institutes and university science. It is worth noting the wide scope of the review, both in terms of the geography of author teams, covering the whole of our large country, and in terms of the diversity of research areas. Practically all major types of heterocycles are represented in the review. The special attention is focused on the practical applications of heterocycles in the design of new drugs and biologically active compounds, high-energy molecules, materials for organic electronics and photovoltaics, new ligands for coordination chemistry, and many other rapidly developing areas. These practical advances would not be possible without the development of new fundamental transformations in heterocyclic chemistry. Bibliography — 2237 references.

In the search for fundamentally new, active, stable, and readily synthetically accessible cycloalkynes as strain-promoted azide–alkyne cycloaddition (SPAAC) reagents for bioorthogonal bioconjugation, we integrated two common approaches: the reagent destabilization by the increase of a ring strain and the transition state stabilization through electronic effects. As a result new SPAAC reagents, heterocyclononynes fused to a heterocyclic core, were created. These compounds can be obtained through a general synthetic route based on four crucial steps: the electrophile-promoted cyclization, Sonogashira coupling, Nicholas reaction, and final deprotection of Co-complexes of cycloalkynes from cobalt. Varying the natures of the heterocycle and heteroatom allows for reaching the optimal stability-reactivity balance for new strained systems. Computational and experimental studies revealed similar SPAAC reactivities for stable 9-membered isocoumarin- and benzothiophene-fused heterocycloalkynes and their unstable 8-membered homologues. We discovered that close reactivity is a result of the interplay of two electronic effects, which stabilize SPAAC transition states (π_in* → σ* and π* → π_in*) with structural effects such as conformational changes from eclipsed to staggered conformations in the cycloalkyne scaffold, that noticeably impact alkyne bending and reactivity. The concerted influence of a heterocycle and a heteroatom on the polarization of a triple bond in highly strained cycles along with a low HOMO–LUMO gap was assumed to be the reason for the unpredictable kinetic instability of all the cyclooctynes and the benzothiophene-fused oxacyclononyne. The applicability of stable isocoumarin-fused azacyclononyne IC9N-BDP-FL for in vitro bioconjugation was exemplified by labeling and visualization of HEK293 cells carrying azido-DNA and azido-glycans.

Copper-catalyzed azide–alkyne cycloaddition is a useful tool for the synthesis of both 1,2,3-triazoles and 5-iodo-1H-1,2,3-triazoles starting from either terminal alkynes or iodoalkynes. 5-Iodotriazoles have been recognized as very useful building blocks for the synthesis of diverse 1,4,5-trisubstituted 1,2,3-triazoles. Synthetic application of 5-iodo-1,2,3-triazoles through the creation of a new C–C, C–heteroatom, or C–D(T) bond along with the application areas of both iodotriazoles and products of their modification including radiolabeled compounds are discussed.1 Introduction2 Synthetic Approaches to 5-Iodo-1H-1,2,3-triazoles3 5-Iodotriazoles in C–C Bond Formation3.1 Intermolecular C–C Cross-Coupling3.2 Intramolecular Cross-Coupling: Direct Arylation and C–I/C–I Homocoupling­3.3 Other Transformations4 5-Iodotriazoles in Radiolabeling, Halogen Exchange, and Heterocoupling Reactions5 Summary