Nicolò Vivona

Ring Transformations of Five-Membered Heterocycles*

Publisher Summary This chapter reviews that ring transformations of heterocycles constitute an interesting area that continues to be of great importance in mechanistic studies and in synthetic design. With reference to five-membered heterocycles, an enormous variety of ring transformations are known and there has always been some difficulty in rationalizing these reactions systematically. A greatly significant classification can be recognized in molecular rearrangements of heteromonocycles that involve the participation of a given number of side-chain atoms in the new ring formation, including special rearrangements where interchanges of annular atoms simply occur. This review is concerned with the rearrangements of five-membered heterocycles represented by the generalized pattern 5→ 6, where W pivotal center is a nitrogen (W=N), sulfur (W=S), or carbon atom (W=C), and the side-chain contains three or four participating atoms.

Study on the thermotropic properties of highly fluorinated 1,2,4-oxadiazolylpyridinium salts and their perspective applications as ionic liquid crystals

A new series of fluorinated salts, iodides and trifluoromethanesulfonates, was synthesized from perfluoroalkylated 1,2,4-oxadiazolylpyridines. Their thermotropic properties were investigated by combined temperature resolved small angle and wide angle X-ray scattering, differential scanning calorimetry and polarised optical microscopy. The UV–visible and photoluminescence properties were studied for all compounds. The results showed for two compounds the existence of an enantiotropic mesomorphic smectic liquid crystal phase. All iodides showed thermochromism phenomena suggesting prospective applications in optoelectronics.

Addition reactions of heterocycles—IV : Indoles and nitrilimines

Abstract The reactivity of indole derivatives towards nitrilimines has been studied. Substituents at positions 1, 2 and 3 of the indole ring greatly affect the course of the reaction. 1,3-Dipolar cycloaddition products (3a,8a,dihydropyrazole-[3,4-b]-indole derivatives) and non-cyclic addition products (3-indolyl derivatives) were obtained depending on these substituents. The structures reported were assigned on the basis of satisfactory analytical, spectroscopic and chemical data.

THE SYNTHESIS OF FLUORINATED HETEROAROMATIC COMPOUNDS. PART 1. FIVE-MEMBERED RINGS WITH MORE THAN TWO HETEROATOMS. A REVIEW

INTRODUCTION 449 I. OXADIAZOLES 450 1. 1,2,4-Oxadiazoles 451 a) The Amidoxime Route 451 b) The Cycloaddition Route 456 c) The Ring-rearrangement Route 457 2. 1,3,4-Oxadiazoles 460 a) The Diacylhydrazine Route 461 b) The Acyl-tetrazole Rearrangement Route 462 c) The Photoinduced Ring-rearrangement Approach 464 3. 1,2,5-Oxadiazoles 465 II. THIADIAZOLES 466 1. 1,2,4-Thiadiazoles 466 2. 1,3,4-Thiadiazoles 468 3. 1,2,5-Thiadiazoles 471 III. TRIAZOLES 473 1. 1,2,4-Triazoles 473 a) Heterocyclization Reactions 474 b) Ring-rearrangement Reactions 479 2. 1,2,3-Triazoles 481 a) Heterocyclization Reactions 482 IV. TETRAZOLES 487 V. CONCLUDING REMARKS 492 REFERENCES 492 ORGANIC PREPARATIONS AND PROCEDURES INT., 37 (5), 447-506 (2005)

Mononuclear heterocyclic rearrangements—vi : Conversion of 1,2,4-oxadiazoles into imidazoles

Abstract The rearrangement of N - (1,2,4 - oxadiazol - 3 - yl)β - enamino ketones 3 a–d and N-(1,2,4 - oxadiazol - 3 - yl)β - enaminoesters 3 e–h into 2 - acylamino - imidazolyl derivatives 9 a–h by the action of sodium ethoxide in N,N-dimethylformamide is the first example of a mononuclear heterocyclic rearrangement involving a nucleophilic carbon. Compounds prepared by condensation of 3 - amino - 1,2,4 - oxadiazoles 1 a–b with β - dicarbonyl compounds 2 a–d, show spectroscopic properties in agreement with cis-chelated structures 4. By condensation between 1 a–b and benzoylacetic ester 2 d, N - (1,2,4 - oxadiazol - 3 - yl)β - ketoamides 6 a–b have been obtained as secondary products. In solution, these compounds are in equilibrium with the corresponding tautomers 7 a–b.

Fluoro heterocycles. A photochemical methodology for the synthesis of 3-amino- and 3-(N-alkylamino)-5-perfluoroalkyl-1,2,4-oxadiazoles

Abstract A photochemical methodology for the synthesis of perfluoroalkyl-1,2,4-oxadiazoles has been described. 3-Amino- and 3-( N -alkylamino)-5-perfluoroalkyl-1,2,4-oxadiazoles have been prepared by irradiation of 3-perfluoroalkanoylamino-4-phenyl-1,2,5-oxadiazoles (furazans) at λ =313 nm in methanol and in the presence of ammonia or primary aliphatic amines.

Photoinduced molecular rearrangements. The photochemistry of 1,2,4-oxadiazoles in the presence of sulphur nucleophiles. Synthesis of 1,2,4-thiadiazoles

Abstract The photochemistry of some 1,2,4-oxadiazoles in the presence of sulphur nucleophiles has been investigated. Irradiation of the 5-amino-3-phenyl- and 3,5-diphenyl-1,2,4-oxadiazole at γ = 254 nm in methanol in the presence of sodium hydrogen sulphide or thiols gave a photo-induced redox reaction at the ring ON bond, leading to the corresponding N -substituted benzamidines. By contrast, irradiation of the 5-amino-3-phenyl-1,2,4-oxadiazole in the presence of thioureas or thiocarbamates, essentially gave 3-phenyl-5-substituted 1,2,4-thiadiazoles, which presume an NS bond formation between the ring-photolytic species and the sulphur nucleophile. In turn, irradiation of the same 5-amino-3-phenyl-1,2,4-oxadiazole in the presence of thioamides again afforded the redox reaction; in addition, amounts of 3-phenyl-5-substituted-1,2,4-thiadiazoles were also formed. Some mechanistic considerations are reported, and synthetic methodologies leading to 1,2,4-thiadiazoles are emphasized.

Mononuclear heterocyclic rearrangements. Part 11. Kinetic study of the rearrangement of (Z)-phenylhydrazones of some 5-alkyl-3-benzoyl-1,2,4-oxadiazoles into 4-acylamino-2,5-diphenyl-1,2,3-triazoles in benzene, dioxane–water, and acetonitrile

The kinetics of the title reactions have been measured at various catalyst concentrations. The 5-alkyl-substituted phenylhydrazones (alkyl = Me, Et, Pri, or But) rearrange more slowly than the 5-H parent compound; moreover the rearrangement rate is little affected by the structure of the 5-alkyl substituent. These facts are considered as evidence against a rearrangement mechanism involving catalyst addition to the C(5)–N(4) bond of the 1,2,4-oxadiazole ring. The observed reactivity pattern can be related to the influence of the 5-substituent on the leaving group ability of the substituent–C(5)–O system.

A GENERALIZED SYNTHESIS OF 3-AMINO-5-ARYL- , 3-AMINO-5- POLYFLUOROPHENYL-, AND 3-AMINO-5-ALKYL-1,2,4- OXADIAZOLES THROUGH RING-DEGENERATE REARRANGEMENTS

A generalized synthesis of 3-amino-5-aryl-, 3-amino-5-poly- fluorophenyl- and 3-amino-5-alkyl-1,2,4-oxadiazoles has been developed starting from the 3-amino-5-methyl-1,2,4-oxadiazole as a common synthon. Aroylation or alkanoylation of this aminooxadiazole, followed by thermally- induced ring-degenerate equilibration of resulting 3-acylamino compounds, and final acid hydrolysis of the 3-acetylamino-5-aryl- (or 5-polyfluorophenyl-), or 3- acetylamino-5-alkyl-1,2,4-oxadiazoles counterpart which is formed, gave the expected 3-amino-5-substituted 1,2,4-oxadiazoles. In the case of some 3- aroylamino compounds, yields of final 3-amino-5-aryloxadiazoles are higher than that expected on the basis of the thermally-induced equilibrium composition, since acid hydrolysis plays a significant role in the shift of the equilibrium itself. Satisfactory direct procedures have also been described. As expected, restrictions to the above methodology were found in the synthesis of 5-perfluoroalkyl derivatives.

Heterocyclic Rearrangements — The Rearrangement of 3-Aroylaminoisoxazoles into 2-Aroylaminooxazoles

When treated with t-BuOK in DMF at 110-120°C, 3-aroylamino-5-methylisoxazoles gave good yields of 2-aroylamino-5-methyloxazoles as the ring rearrangement products. The same to oxazole ring isomerization has been also observed in a photoinduced reaction