One-pot synthesis of five- or six-membered carbocycles through intramolecular cycloadditions by the use of ethyl chloroformate

The Michael addition reactions of beta-nitrostyrenes 1 with 4-pentene-1-mag nesium bromide 2a or 3-butene-1-magnesium bromide 2b generated nitronates 3 or 4. Medium to high yields of isoxazolidine derivatives 9 and 10 were obt ained when nitronates 3 or 4 were treated with ethyl chloroformate and cata lytic amount of 4-dimethylaminopyridine (DMAP) at room temperature in one-p ot and the ratios of trans-9:cis-9 were from 1:3.00 to 1:4.06 and the ratio s of trans-1 0:cis-10 were >99:1. The formation of compounds 9 is proposed to proceed through intramolecular nitrile oxide-olefin cycloadditions (INOC ) because compounds 14a-d, obtained from the trapping of the nitrile oxides by ethyl chloroformate, could be isolated. The mechanism of the generation of compounds 1 0 is proposed to proceed through intramolecular alkoxycarbo nyl nitronate-olefin cycloadditions (IAOC) to form intermediates N-(ethoxyc arbonyl)isoxazolidines 13 and then eliminate EtOH and CO2 (or EtOCO2H) to y ield the final products. (C) 1999 Elsevier Science Ltd. All rights reserved .