Theoretical considerations on the extended Boulton—Katritzky scheme

The First Kinetic Evidence for Acid Catalysis in a Monocyclic Rearrangement of Heterocycles: Conversion of the Z-Phenylhydrazone of 5-Amino-3-benzoyl-1,2,4-oxadiazole into N,5-Diphenyl-2H-1,2,3-triazol-4-ylurea

“…This reaction represents an interesting example of the “monocyclic rearrangements of heterocycles” (MRHs) recognized by Boulton and Katritzky 2a-c as a general class of rearrangements (Scheme ) reportedly occurring 1,2a-c when D = O. This statement has received strong support from considerations on the aromaticity of five-member heterocycles 3 and on its dependence on the nature of the heteroatom(s). − The interest of MRHs derives from both their intriguing mechanistic aspects ,,, and their wide synthetic applicability, 1a,b,, also considering, at this regard, that several azoles show interesting pharmacological activity or can be used as masked functionalities 2 …”

“…The nitrogen-onto-nitrogen nucleophilic attack and the oxygen−nitrogen bond rupture represent (Scheme ) an intramolecular concerted process, whose intermolecular counterpart is not known. 8b,c In dioxane/water the process is usually characterized by the presence of two different reaction pathways: a base-catalyzed one, occurring at higher p S + values, and an uncatalyzed, p S + -independent one, showing up at lower p S + values. In some instances (e.g., when one or more nitrogroups are present in the arylhydrazono moiety), the second pathway could not be evidenced (see below) 1d…”

1,2,4-Oxadiazole Rearrangements Involving an NNC Side-Chain Sequence

“…The reaction has been the subject of numerous synthetic applications ,, and has intriguing mechanistic aspects …”

“…This rearrangement, also classified as an internal nucleophilic substitution, ,,− , typically occurs on 1-oxa-2-azoles (D = O) that are O−N bond-containing heterocycles: isoxazoles, , 1,2,4-oxadiazoles, 1,2,5-oxadiazoles, , and 1,2,5-oxadiazole-2-oxides. , …”

Ring transformation of 1-[(1,2,4-oxadiazol-3-yl)methyl]isoquinolines into 2-acylaminopyrazolo[5,1-a]isoquinolines

“…Compounds (1 3) were also obtained by reaction of the isoquinolinylacetamide oxime (1 1 ) with esters (6). A comparision of ring transformations in the series of compounds ( 8) and (12) indicated that both followed the same mechanism.…”