Abstract The alcohol XIa, obtained by reaction of dibenzo [b, e] thiepin-11 (6H)-one with vinylmagnesium bromide, was transformed by treatment with hydrogen bromide in acetic acid to the bromo compound XIIa which was converted via the nitrile XIIIa to the title acid VIIIa. The pure (E)-isomer was prepared and correlated via the dimethylamide XVIIIa with (E)- prothiadene (Ia). Similar procedures in the 2-methyl-6, 11-dihydrodibenzo [b, e] thiepin ...