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Synthesis 2008(22): 3569-3578  
DOI: 10.1055/s-0028-1083197
PAPER
© Georg Thieme Verlag Stuttgart ˙ New York

Efficient and Versatile Synthesis of 2,3-Dialkylimidazo[4,5-b]quinolin-9-ols by Microwave-Assisted One-Pot Beckmann Rearrangement

Bo Hyun Hwanga,b, Eun Bok Choia, Hyeon Kyu Leea, Hee Cheol Yanga, Bong Young Chungb, Chwang Siek Pak*a
a Korea Research Institute of Chemical Technology, Yusung-Ku Jang-Dong 100, Taejeon 305-600, Korea
Fax: +82(42)8600307; e-Mail: cspak@krict.re.kr;
b Department of Chemistry, Korea University, Anam-Dong Seongbuk-Gu, Seoul 136-701, Korea
Further Information

Publication History

Received 27 June 2008
Publication Date:
23 October 2008 (online)

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Abstract

A direct microwave-assisted one-pot Beckmann rearrangement of 3-acyl-2-(alkylamino)quinolin-4-(1H)-ones in ethanol-pyridine (2:1) provides 2,3-dialkylimidazo[4,5-b]quinolin-9-ols as major products along with 2-alkyl-N-alkyloxazolo[4,5-c]quinolin-4-amines and 3-alkyl-N-alkylisooxazolo[4,5-c]quinolin-4-amines as minor products. Reactions of 3-acylquinolin-4-(1H)-ones containing secondary amine substituent at C-2 give 2-alkyloxazolo[5,4-b]quinolin-9-ols as major products via elimination of the secondary amine group. A mechanism proposed for the formation of Beckmann rearrangement products involves initial generation of a nitrilium ion intermediate, which is trapped either by the adjacent nitrogen of the γ-amino group to form imidazoquinolinols or by the oxygen of γ-hydroxy group of the tautomeric form of the ketoxime to form oxazoloquinolinamines.

Key words

microwave - one-pot Beckmann rearrangement - cyclization - imidazoquinolinol - oxazoloquinoline

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X-ray crystal structure analysis of 2m, 3m, and 4m: atomic coordinates, bond lengths [Å] and angles [˚], anisotropic displacement parameters, hydrogen coordinates, torsion angles [˚] have been deposited at Cambridge Crystallographic Data Centre 12, Union Road, Cambridge CB2 1EZ, UK under deposition number CCDC 645836 for 2m, CCDC 646585 for 3m, and CCDC 647425 for 4m [fax: +44(1223)336 033, E-mail:deposit@ccdc.cam.ac.uk].