Reaction of thiosalicylic acid with 1,4-quinones and cyclization of the resulting arylsulfanylbenzoic acids

Abstract

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

Amination of thioxanthene-1,4,9-trione and its chloro derivatives

“…IR spectrum, ν, cm -1 : 1682 (C=O); 1624, 1611 (C=C). 1 The yellow fraction contained 0.04 g (33%) of dihydroxythioxanthenone II which was identified by comparison with an authentic sample [10] by the IR spectrum and TLC data.…”

“…A mixture of 0.15 g (0.5 mmol) of quinone III [10] and 0.1 ml (1 mmol) of piperidine in 13 ml of DMSO was kept for 1 h at ~20°C, 20 ml of water was added, and the precipitate was filtered off. Yield 0.13 g (76%).…”

“…The 1 H NMR spectra of amino derivatives IV characteristically contained a signal at δ ~6 ppm due to 2-H. Analogous signal in the spectra of monochloro-substituted quinones VI and VII, as well as of unsubstituted quinone I, is displaced downfield to the aromatic region (cf. [10]). A specific property of amino-and chloro-substituted thioxanthene-1,4,9-triones is their ability to readily undergo reduction to the corresponding hydroquinones in protic medium (e.g., in ethanol).…”

“…Known quinones of the thioxanthene series are thioxanthene-1,4,9-trione (I), which is available via oxidation of 1,4-dihydroxythioxanthen-9-one (II) with potassium bromate [9], ammonium cerium(IV) nitrate [10], or moist iodous acid [11], and its 2,3-dichloro derivative III obtained by oxidative chlorination of thioxanthenone II with molecular chlorine [12] thionyl chloride [13]. In the present work reactions of quinones I and III with amines were examined with a view to elucidate regioselectivity of the process.…”

Syntesis of 2- and 3-chloro-1,4-dihydroxythioxanthen-9-ones

“…A known approach to the chlorinated hydroquinones is the hydrochlorination of the corresponding quinones [8]. In this connection we studied the reaction with hydrochloric acid of thioxanthen-1,4,9-trione (IV) obtained by the oxidation of dihydroxythioxanthenone I [9] (Scheme 2).…”