Dibenzoxanthene method of producing compound

The present invention relates to a method for producing an oxygen-dibenzo hetero compound with a dehydrating agent of formula (I) (wherein, R1, R2 and R3 each independently represent an alkyl group of 1 to 2.) represented diphenyl oxa derivatives and process, (wherein, R1, R2 and R3 are the same meanings as described above) to give the formula (II) and dibenzo-oxa derivatives represented by formula (III) (wherein, R1, R2 and R3 mean the same as above) and dibenzo-oxa shown derivative mixture, followed by treatment with the mixture, to prepare a pharmaceutical useful as (Z) -11- (3- dimethylamino propylidene ) -6,11-dihydro-dibenzo [b, e] oxepin-2-acetic acid (common name: olopatadine) or an acid addition salt thereof.