Intramolecular cyclization of β-nitroso-o-quinone methides. A theoretical endoscopy of a potentially useful innate 'reclusive' reaction. Issue 2 (14th January 2015)

• Record Type: Journal Article
• Authors: Tzeli, Demeter; Tsoungas, Petros G.; Petsalakis, Ioannis D.; Kozielewicz, Paweł; Zloh, Mire
• Is Part Of: Tetrahedron. Volume 71:Issue 2(2015)
• Page Start: 359
• Page End: 369
• Publication Date: 2015-01-14
• Subjects: β-Nitroso-o-quinone methides -- Intramolecular o- and peri-cyclization -- Innate reclusive reaction -- DFT and MP2 calculations; Chemistry, Organic -- Periodicals; 547.005
• DOI: 10.1016/j.tet.2014.11.020
• Languages: English
• ISSNs: 0040-4020

Abstract

Abstract: Oxidatively generated β-nitroso- o -quinone methides undergo an o - and/or peri -intramolecular cyclization to arene-fused 1, 2-oxazoles, 1, 2-oxazines or indoles. The reaction, found to be an innate process, has been scrutinized by DFT/B3LYP and MP2 calculations. Due to its rapidity, the process has been termed a 'reclusive' one. Competing o -(1, 5)- and peri -(1, 6)- or (1, 5)-cyclizations advance via successive transition states. Activation barriers are drastically lowered in AcOH, probably through H hopping or tunnelling whereas they are barely reduced in other solvents. Aromaticity indices, such as HOMA, I A and ABO, have been used to assess the stability of the end-heterocycles and the preponderance of any one of them. Thus, the preferred cyclization mode, that is, the prevalence or exclusive formation of one of the heterocycles, appears to be oxidant-directed rather than determined by the quinone methide geometry. The question of the peri -cyclization, being a primary or a secondary process, has been tackled. Graphical abstract: