Abstract A series of novel 3-[5-ethyl-2-(2-phenoxy-ethyl)-pyridin]-5- substituted isoxazolines were synthesized via 1,3-dipolar cycloaddition of in situ generated nitrile oxide from 4-[2-(5-ethylpyridyl)-ethoxy]-benzaldoxime with alkenes to obtain new heterocyclic libraries containing a pyridine moiety in addition to the isoxazoline ring. The newly synthesized compounds were screened for their antimicrobial activity and were compared with standard drugs. The compounds demonstrated potent to weak antimicrobial activity. Of the compounds studied, compounds 3c and 3f showed significant antibacterial as well as antifungal activity. Compounds 3e and 3g showed moderate activity. The title compounds represent a novel class of potent antimicrobial agents. Keywords Isoxazolines Æ Antimicrobial activity Æ Chloramine-T Æ 1,3-dipolar cycloaddition Introduction Substituted isoxazolines represent an important class of heterocyclic com- pounds with a broad spectrum of biological activities. Substituted isoxazo- lines have revealed anti-influenza virus activities (Kai et al., 2001), antifungal properties (Basappa et al., 2003), group II phospholipase A 2 inhibition (Basappa et al., 2004), glycoprotein IIb/IIIa receptor antagonism (Sielecki S.L. Gaonkar Æ K.M. L. Rai (&) Æ B. Prabhuswamy Department of Studies in Chemistry, University of Mysore, Birkhauser logo, Mysore 570 006, India e-mail: kmlrai@yahoo.com, gaonkarslg@rediffmail.com Med Chem Res (2007) 15:407–417 DOI: 10.1007/s00044-006-0015-z ORIGINAL RESEARCH Synthesis of novel 3-[5-ethyl-2-(2-phenoxy-ethyl)- pyridin]-5-substituted isoxazoline libraries via 1,3-dipolar cycloaddition and evaluation of antimicrobial activities S.L. Gaonkar Æ K.M. Lokanatha Rai Æ B. Prabhuswamy Received: 5 July 2006 / Accepted: 2 October 2006 / Published online: 23 May 2007 Ó Birkha ¨ user Boston 2007