Intramolecular 1,3-dipolar cycloaddition at the periphery of heterocyclic systems. Part 2. A mechanistic proposal for the facile oxime-nitrone isomerization at the periphery of pyridine and pyrido[1,2-a]pyrimidine systems

Intramolecular nitrone- and azomethine ylidecycloaddition reactions at the periphery of pyrido[1,2-a]pyrimidin-4(4H)-one system

Isoxazolo[4′,3′:4,5]pyrido- and pyrrolo[2′,3′:4,5]pyrido-[2,3-d]pyrido[1,2-a]-pyrimidines were obtained in the intramolecular cycloaddition reactions of the nitrone and azomethine ylide at the periphery of pyrido[1,2-a]pyrimidin-4(4H)-one system, respectively.

Revisiting oxime–nitrone tautomerism. Evidence of nitrone tautomer participation in oxime nucleophilic addition reactions

Oxime–nitrone tautomerism takes place through a biomolecular mechanism. Participation of nitrone tautomer in nucleophilic addition reactions is evidenced by the first time.

Intramolecular Cycloaddition of O-tert-Butyldimethylsilyloximes in the Presence of BF3·OEt2

[reaction: see text] Intramolecular cycloaddition of novel 1,3-dipoles, N-boranonitrones, was examined. Treatment of O-tert-butyldimethylsilyloximes 9-12 having olefin moieties with 2 equiv of BF3 x OEt2 generated N-boranonitrones, which underwent intramolecular cycloaddition to afford N-nonsubstituted cycloadducts 16 (and/or 18) after extractive workup. Despite the Lewis-acidic conditions, the olefin geometry of the substrates was retained in the cycloadducts in the present cycloaddition. The electronic nature of the N-boranonitrones appeared to be electrophilic. In the case of substrate 11c, having an electron-donating methyl group at an internal position of the olefin moiety, the cycloaddition gave the bridged cycloadduct 18b. The cycloaddition proceeded at relatively low temperature, and the diastereoselectivity was high.

1-Aryl-2-chloro-5-methoxy-1H-3-pyrrolecarbaldehyde as Synthons for Fused Heterocycles: Synthesis of Pyrazolo[3,4-d]pyrrolo[2,3-b]pyridine Derivatives

Pyrazolo[3,4- d]pyrrolo[2,3- b]pyridine derivatives (6a–f) were obtained by oxidation of tosylhydrazones of 2-allylamino-5-methoxypyrrole-3-carbaldehydes (3a–f) with lead tetraacetate. The nitrilimine intermediates (5a–f) underwent intramolecular 1,3-dipolar cycloaddition to yield the tricyclic heterocycles 6a–f in good yields.