14-Membered cyclic bis-semicarbazones: Stereoselective synthesis and structural featuresстатья
Информация о цитировании статьи получена из
Web of Science,
Scopus
Статья опубликована в журнале из списка Web of Science и/или Scopus
Дата последнего поиска статьи во внешних источниках: 23 ноября 2017 г.
Аннотация:Stereoselectivity of acid-catalyzed cyclization of 4-(1-aryl-3-oxobutyl)semicarbazide hydrazones into mixtures of trans- and cis-isomers of 14-membered cyclic bis-semicarbazones has been studied. The isomer ratio is dependent on solvent, catalyst acidity, substrate structure and concentration, reaction temperature and time, catalyst loading. The experimental data show that the macrocyclization of acyclic precursors initially formed from two molecules of substrates proceeds with low diastereoselectivity to give mixtures of trans- and cis-macrocycles followed by slow transformation of cis-isomers into transisomers
via ring opening by a retro-aza-Michael reaction resulting in high trans-selectivity. The driving force of this isomerization is shown to be a lower solubility of trans-isomers compared with cis-isomers. Some mechanistical aspects of the macrocycle formation were confirmed by calculations performed at the DFT B3LYP/6e311þþG(d,p) level of theory. Molecular structure of the obtained hexaaza macrocycles was established using single crystal and powder X-ray diffraction, IR, UVeVis, 1H and 13C NMR spectroscopy, and DFT calculations.