Giuseppe Cusmano

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.

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.

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.

Rearrangement of 3-(N-Heteroarylamino)-1,2,5-oxadiazoles: Triazolo[1,5-a]quinolines and Triazolo[1,5-a]pyridines

The rearrangement reaction of 1,2,5-oxadiazole derivatives bearing quinoline and pyridine heterocycles in a N-C-N side chain sequence was investigated. Triazolo[1,5-a]quinoline and triazolo[1,5-a]pyridine oximes were obtained in good yield

A synthesis of 1,2,4-triazolo-[1,5-f]phenanthridines by rearrangements of 1,2,5-oxadiazoles involving an NCN sequence with the imine nitrogen in an aromatic heterocyclic ring

A synthetic pathway to 1,2,4-triazolo[1,5-f]phenanthridine system by a base catalyzed rearrangement of 3-(6-phenanthridine)amino-1,2,5-oxadiazoles (5a,b) has been investigated. This ring transformation is the first example of the applicability of the mononuclear heterocyclic rearrangement involving an NCN sequence to the synthesis of bridged nitrogen systems

Rarrangements of N-ureidopyrroles : a case of mononuclear heterocyclic rearrangement involving an aromatic heterocycle as carbon-nitrogen-nitrogen sequence

The rearrangement, under basic conditions, of the N-ureidopyrroles (8) directly led to the pyrrolylalkenes (14). The reaction was explained in terms of initial mononuclear heterocyclic rearrangement leading to the pyrrolo[1,2-c] [1,2,3]-triazoles (12) followed by their decomposition to the final products under the reaction conditions. Attempts to thermally rearrange compound (8b) only gave products of decomposition of the ureido moiety

Mononuclear heterocyclic rearrangements. Part 10. Rearrangements in the 1,2,5-oxadiazole series

By studying the chemical behaviour of N-(4-methyl-1,2,5-oxadiazol-3-yl)-N′-phenylurea and of N-(4-methyl- or phenyl-1,2,5-oxadiazol-3-yl)-N′-arylformamidines in mononuclear heterocyclic rearrangements and comparing these results with those already obtained in the 1,2,4-oxadiazole and isoxazole series, relative tendencies towards mononuclear heterocyclic rearrangement have been estimated; viz. 1,2,4-oxadiazole > isoxazole > 1,2,5-oxadiazole.

Mononuclear heterocyclic rearrangements. Part 11. Rearrangements of 1,2,4-oxadiazoles, isoxazoles, and 1,2,5-oxadiazoles involving a sulphur atom

The reactions of 3-amino-5-methyl- and 3-amino-5-phenyl-1,2,4-oxadiazoles, 3-amino-5-methylisoxazole, and 3-amino-4-methyl- and 3-amino-4-phenyl-1,2,5-oxadiazoles with phenyl isothiocyanate have been investigated, and the reactivity of phenylthioureido-derivatives (3) of these ring systems towards rearrangement have been studied. The presence of a sulphur atom in the side-chain sequence (1; XYZ = NCS) greatly enhances the reactivity of the systems under consideration towards rearrangement. The tendency of the three heterocycles to rearrange decreases in the order 1,2,4-oxadiazole > isoxazole > 1,2,5-oxadiazole.

Steric factors in heterocyclic rearrangements. N-Methyl-N-phenylhydrazones of 3-benzoyl-5-phenyl-1,2,4-oxadiazole

In acetic acid at room temperature, the (E)- and (Z)-N-methyl-N-phenylhydrazones of 3-benzoyl-5-phenyl-1,2,4-oxadiazole rearranged into 4-benzoylamino-2,5-diphenyl-1,2,3-triazole via demethylation of an intermediate triazolium salt. When refluxed in benzene, the (E)-N-methyl-N-phenylhydrazone (8E) gave 3-benzoylamino-1-methylindazole (17) through prior isomerisation to the unisolated Z-isomer (8Z) and thence via a carbodi-imide intermediate derived from cleavage of the ring O–N bond.

Cyclization reactions of arylhydrazone adducts - synthesis of 1-aryl-1H-1,2,4-triazolo[4,3-b]indazoles

The 1,3-addition reaction of nitrilimines to 3-indazolinone and the acid assisted 6 π heteroelectrocyclic reaction of hydrazone adducts 13 have been investigated. Synthesis of the 1,3-subsituted 1H-1,2,4-triazolo [4,3-b]-indazole system via cyclization of 13 is described