1,3-cycloaddition of nitrile oxides to cyclic and open-chain 4-oxo-2-sulfinylbut-2-enoic acid derivatives

The reactions of tert-butyl (E)-4,4-diethoxy-2-p-tolylsulfinylbut-2-enoate with benzonitrile, acetonitrile, and bromoformonitrile oxides yield isoxazoles. However, (S-5,S-S)- and (R-5,S-S)-5-ethoxy-3-p-tolylsulfinylfuranones with benzonitrile oxide afford isoxazolines. The reactivity of the double bond as a dipolarophile is strongly increased by the sulfinyl group. This is also the case of the regioselectivity, which usually is opposite to that exhibited by dipolarophiles lacking the sulfinyl group. Electrostatic interactions seemingly explain this behaviour. The pi-facial selectivity of the reactions with the cyclic dipolarophiles is dependent on steric effects, but some role seems to be exerted by electrostatic attraction. (C) 1999 Elsevier Science Ltd. All rights reserved.