REACTIONS OF THE QUINONES 439 



Groups elsewhere on the naphthoquinone rings influence the addition; 

 thus juglone reacts with thioglycolate to form the 3-thiojuglone whereas 

 juglone acetate forms the 2-thiojuglone derivative, the acetyl group making 

 the ketone groups asymmetrical (Thomson, 1951). 



2-Substituted 1,4-naphthoquinones react readily with glutathione to 

 yield derivatives containing the glutathionyl group bound at the 3-position 

 via the S atom of the glutathione (Nickerson et al., 1963). The rates and 

 yields depend on the electronegativity of the substituted groups, the reac- 

 tion mechanism being a nucleophilic substitution. The product of the 

 reaction of menadione and glutathione was isolated and named thiodione, 

 the structure of which is perhaps stabilized by hydrogen bonds between 

 glutamyl and glycyl carboxylate groups and the quinone atoms. The 

 redox potential (Eq) is some 13 mv higher for thiodione than for menadione. 

 Thiodione is photolabile and visible light causes the appearance of the bur- 

 gundy-colored 3-mercaptomenadione and some dihydrothiodione (Strauss 

 et al, 1963). 



Reaction of quinones with cysteine may be complicated by subsequent 

 intramolecular rearrangements. Kuhn and Beinert (1944) studied the ad- 

 dition of cysteine to p-benzoquinone in order to understand better the 

 inhibition of pyruvate decarboxylase they had previously observed. Mix- 

 ture of the reactants in equimolar concentrations at 20° and pH 5-6 leads 

 to a loss of the color and odor of the quinone, and the nitroprusside reaction 

 becomes negative. Further addition of j)-benzoquinone gives a brown color- 

 ation, and adding up to 2 moles of quinone per mole of cysteine yields a 

 violet precipitate and the simultaneous appearance of 1 mole of hydro- 

 quinone. The violet precipitate was thought to be a benzo-p-dihydrothiazine 

 derivative resulting from ring closure through the amino group of the cys- 

 teine: 



COO 



H20 



since reaction of jj-benzoquinone with the ethyl ester of cysteine yields a 

 yellow precipitate of comparable structure. Burton and David (1952) 

 state that excess quinone dehydrogenates the newly closed ring to give a 

 true benzo-p-thiazine. Thus the simple 1,4 addition may be complicated 

 by the presence of an amino group at the proper distance. One is reminded 

 of the report of Sullivan (1926) that the reaction of l,2-naphthoquinone-4- 



