4-hydroxy-2-quinolones 125. Ethyl 3-bromo-2,4-dioxo-1,2,3,4-tetrahydroquinoline-3-carboxylates as potential brominating agents

Abstract

The spatial structural features of 3-bromo-3-ethoxycarbonyl-2,4-dioxo-1,2,3,4-tetrahydroquinolines have been studied by X-ray analysis. It has been experimentally confirmed that these compounds can be regarded as potential brominating agents.The bromination route of ethyl 1R-4-hydroxy-2-oxo-1,2-dihydroquinoline-3-carboxylic acids using molecular bromine in acetic acid depends on the presence of water in the reaction mixture. Its presence predetermines the formation of 1-R-3-bromo-3-ethoxycarbonyl-2,4-dioxo-1,2,3,4-tetrahydroquinolines. This reaction occurs instantly upon addition of bromine. On the other hand, after thorough dehydration of the reagents and solvent an electrophilic attack occurs only at position 6 of the quinolone ring and in this case completion of the bromination requires several hours [2]. In practice, carrying out the reaction with a strictly stoichiometric ratio of the reacting material and in absolutely anhydrous conditions could not always be achieved. As a consequence there was sometimes observed a partial formation of the 3-bromo-substituted isomers which, in contrast to the colorless 6-bromoquinolines, are bright yellow in color. This process does not, however, cause a significant problem since the high solubility in most organic solvents allows a ready removal of the unwanted admixture from the target 6-bromo derivatives.This formation of the 6-bromoquinolines has been confirmed by us via a counter synthesis using intermediate products with a well known position for the bromine atom in the molecule [3], e.g. from X-ray analysis. At the same time, the steric structural features of their 3-bromo-substituted isomers has remained obscure for a long time.In one of the bromination experiments [2] of ethyl 4-hydroxy-2-oxo-1-propyl-1,2-dihydroquinoline-3-carboxylate (1), prolonged cooling of the filtrate remaining after separation of the basic 6-bromoquinoline 2 at -25ºC caused the growth of several yellow single crystals which were subjected to X-ray analysis.

Use of Bromine and Bromo-Organic Compounds in Organic Synthesis

“…Ukrainets and co-workers found that 3-bromo-3-(ethoxycarbonyl)-2,4-dioxo-1,2,3,4-tetrahydroquinones could be regarded as potential brominating agents. 1128 They studied the reaction product of this reagent with cyclohexene in anhydrous carbon tetrachloride in the presence of a catalytic amount of peroxide. The product is a mixture, the main components of which are 3bromocyclohexene (11%) and trans-1,2-dibromocyclohexane (78%) (Scheme 383).…”

6-Ethoxycarbonyl-5,7-dihydroxy-2,3-dihydro-1H-pyrido[3,2,1-ij]quinolinium tribromide

“…For general background, see: Ukrainets et al (2004Ukrainets et al ( , 2007. For chemical bond lengths, see: Bü rgi & Dunitz (1994).…”