The reported [EP 0 240 226 (1986) and EP 0 282 236 (1987)] procedure was modified so as to avoid the step of cyclization of the carbamate to dibenzothiazepinone in polyphosphoric acid. The new route is based on a condensation of carbamate with piperazine, followed by cyclization of the amide obtained in the phosphoryl chloride - phosphorus pentoxide mixture. The resulted piperazinyl-dibenzothiazepine is then condensed with chloroethoxyethanol to afford the final quethiapine.