Thiosalicylic acid reacts with 1,4-benzoquinone to give 2-(1,4-dihydroxyphenylsulfanyl)benzoic acid which undergoes intramolecular cyclization to 1,4-dihydroxythioxanthen-9-one in the presence of dehydrating agents. Cyclization of arylsulfanylbenzoic acids obtained by reaction of thiosalicylic acid with 1,4-naphtho-and anthraquinones leads to 1,4-quinone derivatives, benzo-and naphthothioxanthenetriones.Dihydroxythioxanthene derivatives were found to exhibit biological activity [1-4] and initiate photochemical polymerization [5]. The most widely known representative of this series of compounds is 1,4-dihydroxythioxanthen-9-one (I) which was synthesized about 100 years ago [6] by cyclization of 2-[(2,5-dihydroxyphenyl)sulfanyl]benzoic acid (II); the latter was prepared from benzoquinone III and thiosalicylic acid (IV) in acetic acid (Scheme 1).1,4-naphthoquinone (VII) in ethanol gave 2-(1,4-dioxo-1,4-dihydronaphthalen-2-ylsulfanyl)benzoic acid (V) and benzothioxanthene VI (Scheme 2). However, the parameters of the products given in [4] are not fully consistent with the assumed structures. In particular, the 1 H NMR spectrum of acid V lacks a signal typical of quinoid compounds which should be located in a stronger field than signals from aromatic protons [7]. Therefore, the assignment of structures of compounds I and II made in [4] seems to be insufficiently substantiated. The goal of the present work was to elucidate the structure of compounds formed by reaction of thiosalicylic acid (IV) with 1,4-benzo-, 1,4-naphtho-, and 1,4-anthraquinones and of their cyclization products, thioxanthen-9-one derivatives.We have found that acid IV reacts with 1,4-benzoquinone (III) at room temperature not only in acetic acid [6] but also in other solvents (such as diethyl ether and ethanol) and that the reaction occurs as 1,4-addition of the nucleophile at the conjugated bond system of quinone III with participation of one carbonyl group (which is typical of quinones), followed by rearrangement of the adduct to produce dihydroxy-substituted diphenyl sulfide II. The 1 H NMR spectrum of the isolated product contained signals from aromatic protons and three signals assignable to hydroxy protons. A broadened downfield signal at δ 12-14 ppm was assigned to proton in the carboxy group, and the position of the two other OH signals (2′-OH and 5′-OH; δ 8.93 and 9.03 ppm) indicated equal contributions of the corresponding protons to exchange processes. In the IR
