The reactions of aromatic methylenedioxy compounds containing electron-withdrawing groups with sodium methoxide-thiols in dimethyl sulfoxide gave 3- and 4-hydroxybenzene derivatives in good yield by regioselective attack of the thiolate ions on the methylenedioxy ring. The formation mechanism and the reactivity of thiolate ions in the cleavage reaction of the methylenedioxy ring are discussed. Various biologically active compounds, 32a, 32d, 36b, 38b, 41b and 44-47, were prepared from the 4-hydroxybenzene derivatives and their Ca2+ antagonistic activities were evaluated. Among these compounds, 2-(2-bromophenylthiomethoxy)-10-(2-diethylaminoacetyl)-3- methoxyphenothiazine (46) showed the most potent Ca2+ antagonistic activity. Biological activity could be conveniently evaluated by measurement of the peak height of the vanadyl ion (+4 oxidation ion) signal produced by redox reaction between the phenothiazine derivatives and vanadate ion +5 oxidation ion) with ESR spectroscopy.