The invention relates to an improved process for preparing aminocrotonylamino-substituted quinazoline derivatives of general formula (I) wherein the groups Ra, Rb, Rc and Rd have the meanings given in the claims, as well as sulphonyl derivatives of formula and the use thereof as synthesis components for preparing quinazolines of formula (I). The quinazoline derivatives of formula (I) are inhibitors of signal transduction mediated by tyrosinekinases and by the Epidermal Growth Factor-Receptor (EGF-R) and are therefore particularly suitable for the treatment of tumoral diseases.