{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,7,30]],"date-time":"2026-07-30T09:36:37Z","timestamp":1785404197394,"version":"3.56.0"},"reference-count":19,"publisher":"MDPI AG","issue":"10","license":[{"start":{"date-parts":[[2026,5,15]],"date-time":"2026-05-15T00:00:00Z","timestamp":1778803200000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"name":"Ministry of Education and Science","award":["BG16RFPR002-1.016-0001"],"award-info":[{"award-number":["BG16RFPR002-1.016-0001"]}]},{"award":["BG16RFPR002-1.016-0001"],"award-info":[{"award-number":["BG16RFPR002-1.016-0001"]}],"id":[{"id":"https:\/\/ror.org\/053pcb396","id-type":"ROR","asserted-by":"publisher"}]}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Molecules"],"abstract":"<jats:p>An operationally simple procedure for the synthesis of 2-alkyl-4-quinolone N-oxides, relying on controlled platinum-catalyzed partial hydrogenation of 2-nitrobenzoyl enamines, has been developed. The Pseudomonas aeruginosa metabolite 2-heptyl-4-quinolone-N-oxide (HQNO) and four analogous products have been prepared in good yield and high purity by this method. All products showed ampholytic properties, with a tendency to form isolable organic-soluble hydrochlorides by switching from the N-hydroxy-4-quinolone to 4-hydroxyquinoline-N-oxide tautomeric form upon partitioning between dichloromethane and 1M aqueous HCl. In basic medium, on the other hand, water-soluble salts of the N-hydroxy-4-quinolone tautomers were formed. NMR measurements indicate pH-dependent equilibrium with fast exchange between the 4-quinolone and the protonated 4-quinolinol tautomer.<\/jats:p>","DOI":"10.3390\/molecules31101680","type":"journal-article","created":{"date-parts":[[2026,5,15]],"date-time":"2026-05-15T14:19:29Z","timestamp":1778854769000},"page":"1680","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":0,"title":["A Synthesis of 4-Quinolone N-Oxides and NMR Evidence of Their Protonation-Assisted Enolisation"],"prefix":"10.3390","volume":"31","author":[{"ORCID":"https:\/\/orcid.org\/0000-0002-3625-9933","authenticated-orcid":false,"given":"Plamen","family":"Angelov","sequence":"first","affiliation":[{"name":"University Centre on Tautomeric Research and Education in Science and Technology (ERA Chair UCTREST), University of Plovdiv Paisii Hilendarski, 24 Tsar Asen Str., 4000 Plovdiv, Bulgaria"},{"name":"Department of Organic Chemistry, University of Plovdiv Paisii Hilendarski, 24 Tsar Asen Str., 4000 Plovdiv, Bulgaria"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Yordanka","family":"Sapundzhieva","sequence":"additional","affiliation":[{"name":"University Centre on Tautomeric Research and Education in Science and Technology (ERA Chair UCTREST), University of Plovdiv Paisii Hilendarski, 24 Tsar Asen Str., 4000 Plovdiv, Bulgaria"},{"name":"Department of Organic Chemistry, University of Plovdiv Paisii Hilendarski, 24 Tsar Asen Str., 4000 Plovdiv, Bulgaria"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-6680-3856","authenticated-orcid":false,"given":"Francisco","family":"Alonso","sequence":"additional","affiliation":[{"name":"Instituto de S\u00edntesis Org\u00e1nica and Departamento de Qu\u00edmica Org\u00e1nica, Facultad de Ciencias, Universidad de Alicante, Apdo. 99, 03080 Alicante, Spain"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-5640-6120","authenticated-orcid":false,"given":"Paraskev","family":"Nedialkov","sequence":"additional","affiliation":[{"name":"Department of Pharmacognosy, Faculty of Pharmacy, Medical University of Sofia, 2 Dunav Str., 1000 Sofia, Bulgaria"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"1968","published-online":{"date-parts":[[2026,5,15]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"152","DOI":"10.1021\/cr100063u","article-title":"4-Quinolones: Smart Phones of the Microbial World","volume":"111","author":"Huse","year":"2011","journal-title":"Chem. 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