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Structure of a new usnic acid derivative from a deacylating Mannich reaction: NMR studies supported by theoretical calculations of chemical shifts
Author(s) -
Nguyen Huong Giang Thi,
Nguyen Vinh Ngoc,
Kamounah Fadhil S.,
Hansen Poul Erik
Publication year - 2018
Publication title -
magnetic resonance in chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.483
H-Index - 72
eISSN - 1097-458X
pISSN - 0749-1581
DOI - 10.1002/mrc.4760
Subject(s) - chemistry , tautomer , morpholine , chemical shift , hydrogen bond , mannich base , enol , kinetic isotope effect , deuterium , usnic acid , computational chemistry , piperidine , solvent , medicinal chemistry , photochemistry , organic chemistry , molecule , lichen , physics , botany , quantum mechanics , biology , catalysis
In a conventional Mannich reaction using piperidine, hydroxypiperidines, morpholine, and N‐methylpiperazine with usnic acid, a deacetylation was observed resulting in a substitution at C‐2, a loss of an acetyl group, and a Mannich base with a stabilized enol. The enol has a hydrogen bond to the nitrogen of the secondary amine. The structure was investigated by nuclear magnetic resonance and deuterium isotope effects on 13 C chemical shifts as well as with density functional theory calculations to study the changed hydrogen bond pattern. It was found that the hydrogen bond involving the OH‐9 group in chloroform forms a strong hydrogen bond than in usnic acid itself and that this hydrogen bond becomes even stronger in the more polar solvent, dimethylsulfoxide. Tautomerism was observed in the Mannich base as demonstrated by deuterium isotope effects on chemical shifts. The position of the tautomeric equilibrium depends on the solvent, and the position of the equilibrium governs the strength of the OH‐9…O═C hydrogen bond.