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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

The applicability of an intramolecular Nicholas reaction for the preparation of 10-membered O- and N-enediynes fused to indole, 1,2,3-triazole, and isocoumarin was investigated. The general approach to acyclic enediyne precursors fused to heterocycles includes inter- and intramolecular buta-1,3-diyne cyclizations with the formation of iodoethynylheterocycles, followed by Sonogashira coupling. The nature of both a heterocycle and a nucleophilic group affects the possibility of a 10-membered ring closure by the Nicholas reaction. Among oxacycles, an isocoumarin-fused enediyne was obtained. In the case of O-enediyne annulated with indole, instead of the formation of a 10-membered cycle, BF3-promoted addition of an OH-group to the proximal triple bond at the C3 position afforded dihydrofuryl-substituted indole. For 1,2,3-triazole-fused analogues, using NH-Ts as a nucleophilic functional group allowed obtaining 10-membered azaenediyne, while the substrate with a hydroxyl group gave only traces of the desired 10-membered oxacycle. An improved method for the deprotection of Co-complexes of cyclic enediynes using tetrabutylammonium fluoride in an acetone/water mixture and the investigation of the 10-membered enediynes' reactivity in the Bergman cyclization are also reported. In the solid state, all synthesized iodoethynylheterocycles were found to be involved in halogen bond (XB) formation with either O or N atoms as XB acceptors.

The Nicholas-type macrocyclization through NH-tosyl functional group has been found to be an efficient technique for the synthesis of a 10-membered azaenediyne system annulated with a benzothiophene. To compare the activity of azaenediyne synthesized with similar oxa- and carbocyclic enediynes the Bergman cyclization activation energies and the ability of enediynes to cleave DNA (pBR322 plasmid) were investigated. The order of reactivity predicted by DFT calculations (N-enediyne < C-enediyne < O-enediyne) was confirmed by DSC analysis data. Surprisingly azaenediyne was found to be more active in the DNA cleavage assay than the C-analogue.

To find promising analogues of naturally occurring enediyne antibiotics with a sufficient reactivity in the Bergman cyclization and moderately stable under isolation and storage, a scale of relative enediynes reactivity was created on the basis of calculated free activation energies for the Bergman cyclization within 12 known and new benozothiophene, benzene, and cinnoline annulated 9- and 10-membered enediynes. To verify the predicted reactivity/stability balance, three new carbocyclic enediynes fused to a benzothiophene core bearing 3,4,5-trimethoxybenzene, fluoroisopropyl, and isopropenyl substituents were synthesized using the Nicholas-type macrocyclization. It was confirmed that annulation of a 3,4,5-trimethoxybenzene moiety to a 10-membered enediyne macrocycle imparts high reactivity to an enediyne while also conferring instability under ambient temperature. Fluoroisopropyl-substituted 10-membered enediyne from the opposite end of the scale was found to be stable while moderately reactive in the Bergman cyclization. Along with the experimentally confirmed moderate reactivity (DSC kinetic studies), (fluoroisopropyl)enediyne showed a significant DNA damaging activity in plasmid cleavage assays comparable with the known anticancer drug Zeocin.

Opportunities were outlined for application of available KF as a source of fluoride ions and a base in desilylation/the Sonogashira coupling reactions under conditions of a one-pot process.

The Nicholas-type macrocyclization has been used for the first time for the synthesis of 10- and 11-membered oxaenediynes fused to a benzothiophene. The acyclic starting materials were easily synthesized by electrophilic cyclization of o-(buta-1,3-diynyl)thioanisoles followed by a Sonogashira coupling of the resulting 2-ethynyl-3-iodobenzothiophenes with functionalized alkynes. A high reactivity of the 10-membered oxacycles in the Bergman cyclization was predicted by DFT calculations and confirmed by differential scanning calorimetry. The ability of enediynes to induce single-strand PM2 DNA scissions was also found.

The synthesis of enediynes fused to an isocoumarin core was achieved through the electrophilic cyclization of o-(buta-1,3-diynyl)benzoates as a key step, followed by the Sonogashira coupling of the resulting 3-ethynyl-4-iodoisocoumarins with acetylenes. This approach allowed different substituents to be introduced to both ethynyl moieties with complete regiocontrol. The ability of the isocoumarin-fused enediynes to undergo Bergman cyclization was studied by differential scanning calorimetry.

The feasibility of ring-closing metathesis (RCM) as a synthetic entry to 10- and 11-membered dienediynes fused to a benzothiophene core was explored by experimental and theoretical investigations. An established sequence of iodocyclization of o-(buta-1,3-diynyl)thioanisoles followed by Sonogashira coupling to form diethynylbenzothiophenes was used to synthesize terminal benzothiophene-fused enediyne diolefins as substrates for RCM. Encountering an unexpected lack of reactivity of these substrates under standard RCM conditions, we applied DFT calculations to reveal that the underlying cause was a positive change in Gibbs free energy. The change in Gibbs free energy was also found to be positive for RCM of indole- and benzannulated terminal diolefins when affording smaller than 12-membered rings. We found that modification of the enediyne-diolefin substrate as the Co2(CO)6-alkyne complex allowed the target benzothiophene-fused 11-membered dienediyne to be obtained via RCM; the alkyne complexation strategy therefore provides one valid technique for overcoming challenges to macrocyclization of this kind.

Novel poly(arylene ethynylene)s comprising a cinnoline core were prepared in high yields via a three-step methodology. A Richter-type cyclization of 2-ethynyl- and 2-(buta-1,3-diynyl)aryltriazenes was used for cinnoline ring formation, followed by a Sonogashira coupling for the introduction of trimethylsilylethynyl moieties and a sila-Sonogashira coupling as the polycondensation technique. The fluorescence of the cinnoline-containing polymers in THF was highly sensitive to quenching by Pd2+ ions.

The synthesis of enediynes fused to an isocoumarin core was achieved through the electrophilic cyclization of o-(buta-1,3-diynyl)benzoates as a key step, followed by the Sonogashira coupling of the resulting 3-ethynyl-4-iodoisocoumarins with acetylenes. This approach allowed different substituents to be introduced to both ethynyl moieties with complete regiocontrol. The ability of the isocoumarin-fused enediynes to undergo Bergman cyclization was studied by differential scanning calorimetry.

The investigation of Richter cyclization of o-(1-alkynyl)- and o-(1,3-butadiynyl)arenediazonium salts generated by acid cleavage of triazenes was carried out by the methods of NMR and ESI MS (Electrospray Ionization Mass Spectrometry). The effect was shown of the substituents at the carbon atom of the triple bond and in the aromatic ring, of the solvent, temperature, and the reagents ratio on the rate of the cyclization and the yield of its products, 4-bromo-3-alkyl- and 4-bromo-3-ethynylcinnolines; the conditions of their synthesis were optimized. The possibility to use 4-bromo-3-ethynylcinnolines as substrates of electrophilic cyclization was demonstrated.

For the first time, electrophilic cyclization and ring-closing metathesis were used as key steps in the synthesis of a macrocyclic enediyne system, which allowed a 12-membered dienediyne containing a thiophene-fused ring to be obtained. The macrocycle synthesized represents a class of

Examples of intramolecular cyclizations of functionalized diacetylenes are considered in the review. Cyclizations occur with the participation of one of the triple bonds that leads to ethynyl-substituted heterocycles, or of two triple bonds that is used in the synthesis of polycondensed heterocyclic systems and ensembles.

The reactions of dimethyl acetylenecarboxylate with 3,4,5,6-tetrahydro-2H-1,2,4-triazepine-3-thiones and 4,5-dihydro-1H-pyrazole-1-carbothioamides are convenient methods for the synthesis of 7,8-dihydrothiazolo[3,2-b][1,2,4]triazepin-3-ones derivatives and methyl esters of (2Z)-[2-(4,5-dihydro-1H-pyrazol-1-yl)-4-oxo-1,3-thiazol-5(4H)-ylidene]acetic acids, respectively. The reaction of methyl propynoates with 4,5-dihydro-1H-pyrazole-1-carbothioamides or with 5,5,7-trimethyl-2,4,5,6-tetrahydro-3H-1,2,4-triazepine-3-thione gives 2-(4,5-dihydro-1H-pyrazol-1-yl)-4H-1,3-thiazin-4-ones.

A simple, convenient, and promising strategy for the synthesis of 2-ethynyl-3-iodo-benzothiophenes, -benzofurans, and -indoles based on electrophilic cyclization of easily available ortho- functionalized (buta-1,3-diynyl)arenes was developed. The unique potential of using these compounds as starting materials for the syn- thesis of enediyne systems, containing thiophene, furan, and pyrrole units is demonstrated.

Synthesis of 2-(buta-1,3-diynyl)-N,N-dimethylanilines based on reductive methylation of ortho-iodoanilines using CH2O–NaBH3CN and coupling with terminal diacetylens was developed. Altered sequence including dimethylation of 2-(buta-1,3-diynyl)anilines was also successfully achieved and gave higher overall yields in the case of anilines without acceptor substituent in the ring.

Reactions of 1-acylthiosemicarbazides with methyl 3-phenylprop-2-ynoate provide a new one-step procedure for the synthesis of 2-substituted 5-phenyl-7H-[1,2,4]triazolo[5,1-b][1,3]thiazin-7-ones. The product structure was determined on the basis of scalar spin-spin coupling constants 2 J(1H-15N) and 3 J(1H-15N) in the 1H and 15N NMR spectra and was proved by X-ray analysis. Methyl 3-phenylprop-2-ynoate reacted with 1-acetylthiosemicarbazide in ethanol in the presence of H2SO4 to give 3-methyl-7-phenyl[1,2,4]triazolo[3,4-b]-[1,3]thiazin-5-one as the only product.

Various diacetylene substrates (syntheses described) bearing heteroatom substituents at the ortho position can be used for the preparation of the target compounds.

The review summarizes published data on the reactions of cyclic and acyclic thioamides with derivatives of acetylenecarboxylic acids, which lead to the formation of both acyclic and heterocyclic systems, including thiazolidines and thiazines. Methods for determination of the structure of the condensation products and factors responsible for the reaction direction are discussed.

An efficient strategy for the synthesis of asymmetrically substituted enediynes fused to benzothiophene, benzofuran, and indole was developed. The proposed approach is based on the electrophilic cyclization of diacetylenes and Sonogashira coupling. Thus, iodocyclization of readily available ortho-functionalized (buta-1,3-diynyl)arenes was used as a direct way for the synthesis of 2-ethynyl-3-iodoheteroindenes. These substrates and their modified derivatives were easily converted by Sonogashira coupling with acetylenes to a variety of asymmetrically substituted acyclic enediynes fused to heterocycles. The tolerance of the developed methodology to a variety of functional groups is a great advantage in the synthesis of macrocyclic enediyne systems fused to a heterocyclic core. Synthesis of indole-fused 12-membered macrocyclic dienediyne was achieved using ring-closing metathesis as a key step.

Under the action of acids, title compounds (I) and (V) undergo cyclization to form cinnolines (II) and (VI), respectively.

It is known that 1,3-thiazines are most commonly synthesized by reactions of thioamides, thioureas inclusive, with acetylenecarboxylic acid derivatives that play the role of a three-carbon bulding block for constructing the thiazine ring [1 4]. According to [5], N-aminothiourea (thiosemicarbazide) reacts with methyl propynoate in methanol to give methyl 3-thiosemicarbazonopropanoate. We found [6] that in boiling ethanol thiosemicarbazide reacts with methyl phenylpropynoate, forming initially methyl 3-phenyl3-thiosemicarbazonopropanoate whose subsequent heterocyclization provides a monocyclic 1,3-thiazinone, methyl 3-[(4-oxo-6-phenyl-4H-1,3-thiazin-2-yl)hydrazono]-3-phenylpropanoate. Since thiosemicarbazide is easily acetylated in acetic acid, nucleophilic addition to its triple bond of 3-phenylpropynoate gets impossible and the reaction goes through the thioamide group, yielding the bicyclic 3-methyl-7-phenyl5H-[1,2,4]triazolo[3,4-b][1,3]thiazin-5-one [6]. Therefore, proceeding with searching for synthetic approaches to potentially biologically active 1,3-thioazinones and their fusion products, we turned to reactions of 1-(perfluoroacyl)thiosemicarbazides Ia, Ib and 5-(perfluoroalkyl)triazole-3-thiols IIa, IIb with methyl phenylpropynoate (III) in view of the increasing interest heterocyclic systems with perfluoroalkyl substituents have attracted over the past years [7]. This interest is explained by changes in chemical properties and biological activity produced by perfluoroalkyl substituents.

Examples of intramolecular cyclizations of functionalized diacetylenes are considered in the review. Cyclizations occur with the participation of one of the triple bonds that leads to ethynyl-substituted heterocycles, or of two triple bonds that is used in the synthesis of polycondensed heterocyclic systems and ensembles.

The reaction of thioamides (I) with mono- and diesters of acetylenecarboxylic acids proceeds differently leading to thiazines, e.g. (III), or thiazole derivatives such as (V).

The cyclocondensation of di- and monoacetylenic derivatives of o-phenylenediamine with different substituents at the triple bond with Ph-NCS or CS2 for studying the scope and limitations of the preparation of 2,3-dihydro[1,3]thiazolo and [1,3]thiazolo[3,2-a]benzimidazole derivatives is investigated.

Thiophenol, aniline and phenol derivatives are transformed to the corresponding 2-ethynyl-3-iodo-substituted heterocycles which are converted into the title systems.

Condensation of thiourea with acetylenecarboxylic acid esters is well known and is used in synthesis of 1,3-thiazin-4-ones [1 4]. It is also known that N-aminothiourea (thiosemicarbazide, I) reacts with methyl propynoate in methanol to form methyl 3-thiosemicarbazonopropanoate [5]. Therefore, proceeding with studies on synthetic routes to potentially bioactive 1,3-thiazinones and condensed systems based on them, we studied the reaction of I with methyl phenylpropynoate II.