NMR studies of intramolecular proton exchange in alkylated naphthazarins

Abstract

Three groups of alkyl-substituted naphthazarins were studied by 'H and "C NMR in chloroform-d at ambient temperature. Preliminary 'H NMR studies were carried out on selected members of each group at low temperatures (down to -120 "C) in dichloromethaue-d, solution. Monoalkylnaphthazarins were found to exist predominantly as the 2-alkyl tautomers ( I ) and the dimethylalkyl species as the 2,3-dimethyl-6-alkyl tautomers (I). However, both symmetrically and unsymmetrically substituted 2,6-and 2,7-dialkylnaphthazarins exhibited averaged chemical shifts and coupling constants indicating rapid proton exchange between the two minimum energy 174-dione tautomers ( I and ZZ). I3C NMR shifts were utilized to calculate tautomer populations of the unsymmetrical 2,6-and 2,7-(methyl, a1kyl)naphthazarins.

Novel Henna-Related Naphthazarine Photosensitizers for an Effective Photodynamic Therapy of Onychomycosis

“…Several procedures toward the synthesis of naphthazarin derivatives have been reported in the literature. − For the preparation of 2-/3-substituted naphthazarin derivatives in this study, we used a modified convergent method from Zahn and Ochwat (Scheme ). , Herein, the naphthazarin ring formation reaction involves 1,4-dimethoxybenzen and 2-/3-substituted fumaric/maleic acid derivatives as starting materials for a subsequent twin Friedel–Crafts acylation.…”

“…13 General Procedure for the Synthesis of Compounds 6−11. The synthesis for the target compounds 6−11 was carried out according to Elove et al 36 Following the procedure, AlCl 3 was submitted together with NaCl under a N 2 atmosphere and heated to 150 °C. Upon appearance of the melting process, the Fumar/maleic acid derivatives mixed with 1,4-dimethoxybenzene were added in portions.…”