NMR Studies of Intramolecular Proton Exchange in Alkylated Naphthazarins

Article abstract of DOI:10.1002/mrc.1260250303

Three groups of alkyl-substituted naphthazarins were studied by 1H and 13C NMR in chloroform-d at ambient temperature.Preliminary 1H NMR studies were carried out on selected members of each group at low temperatures (down to-120 deg C) in dichloromethane-d₂ solutions.Monoalkylnaphthazarins were found to exist predominatly 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 exhange between the two minimum energy 1,4-dione tautomers (I and II). 13C NMR shifts were utilized to calculate tautomer populations of the unsymmetrical 2,6- and 2,7-(methyl, alkyl)naphthazarins.KEY WORDS 1H NMR 13C NMR Intramolecular proton exhange Alkylated naphthazarins