Self Cycloaddition of o‐Naphthoquinone Nitrosomethide to (±) Spiro{naphthalene(naphthopyranofurazan)}‐one Oxide: An Insight into its Formation.

2‐Hydroxy‐1‐naphthaldehyde oxime was oxidized by AgO (or Ag2O), in presence of N‐methyl morpholine N‐oxide (NMMO), to the title spiro adduct‐dimer (±)‐Spiro{naphthalene‐1(2H),4′‐(naphtho[2′,1′:2,3]pyrano[4,5‐c]furazan)}‐2‐one‐11′‐oxide by a Diels‐Alder(D−A) type self‐cycloaddition, through the agency of an o‐naphthoquinone nitrosomethide (o‐NQM). Moreover, 2‐hydroxy‐8‐methoxy‐1‐naphthaldehyde oxime was prepared and subjected to the same oxidation conditions. Its sterically guided result, 9‐methoxynaphtho[1,2‐d]isoxazole, was isolated, instead of the expected spiro adduct. The peri intramolecular H bonding in the oxime is considered to have a key contribution to the outcome. Geometry and energy features of the oxidant‐ and stereo‐guided selectivity of both oxidation outcomes have been explored by DFT, perturbation theory and coupled cluster calculations. The reaction free energy of the D−A intermolecular cycloaddition is calculated at −82.0 kcal/mol, indicating its predominance over the intramolecular cyclization of ca. −37.6 kcal/mol. The cycloaddition is facilitated by NMMO through dipolar interactions and hydrogen bonding with both metal complexes and o‐NQM. The 8(peri)‐OMe substitution of the reactant oxime sterically impedes formation of the spiro adduct, instead it undergoes a more facile cyclodehydration to the isoxazole structure by ca. 4.9 kcal/mol. [ABSTRACT FROM AUTHOR]

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