A novel organocatalyticformal [3 + 2] cycloaddition reaction with in situ generation of N-carbamoyl nitrones is presented. For the first time, N-Boc- and N-Cbz-protected isoxazolidines have been directly obtained as single diastereoisomers in generally high yields and enantiomeric excesses using mild reaction conditions and inexpensive, readily available Cinchona alkaloid quaternary ammonium salts