Pyrimidine-2-thiones, desulfurization

The thiones are readily desulfurized with Raney nickel to give the corresponding unsubstituted compounds in bicyclic systems in the 2-, 4- and 7-positions, and in tricyclic systems such as (95). The 2-methylthio derivatives may be similarly desulfurized. Thione groups in the 4-position, but not the 2-position, in pyrido-[2,3- f ]- and -[3,2- f]-pyrimidines may be replaced directly with ammonia or amines. 

Pyrimidine-4(3H)-thione, 6-methoxy-5-nitro-reduction, 3, 88 Pyrimidinethiones acidic pK, 3, 60 S-acylation, 3, 95 N-alkylated synthesis, 3, 139 aminolysis, 3, 94 desulfurization, 3, 93 electrophilic reactions, 3, 69 hydrolysis, 3, 94 oxidation, 3, 94, 138 pyrimidinone synthesis from, 3, 133 reactions... 

Surprisingly, Kashima et al. (83TL209) reported the formation of individual 1,4-dihydro- and 1,6-dihydropyrimidines on desulfurization of the corresponding pyrimidine-2-thiones with Raney Ni and claimed that no tautomerization occurs under the reaction conditions (heating under reflux in MeOH). 

Sulfur-containing pyrimidines are frequent intermediates in substitution reactions. Several methods exist for exchange of the thiol group with hydrogen, either by hydrogenolysis, especially by the use of Raney Nickel, or by oxidative desulfurization which involves a sulfinic acid intermediate. Oxidation may also lead to a sulfonic acid. The 2- and 4-/6-thiones are considerably more stable to air oxidation than 5-thiol derivatives. Disulfides are conveniently formed by oxidation with bromine. Oxidation with chlorine at low temperature can be used to prepare sulfonyl chlorides. These reactions have been summarized [Pg.187]

Oxidative desulfurization can be effected by ozonolysis (Scheme 74). 2-Thiol-4(3//)-quinazolinone on ozonolysis in dry dichloromethane yields the disulfide (448) which is resistant to further oxidation under the reaction conditions. In acetic acid, desulfurization results with hydrogen substitution (449). Rationalization of the reaction in acetic acid involves formation of an unstable sulfinic acid which loses SO2 with replacement by hydrogen. In dichloromethane containing ethanol, the 2-ethoxy product (450) formed, corresponding to nucleophilic substitution of the reactive sulfinic acid from the oxidation. Similarly, ozonolysis of pyrimidine-2-thione acid gave bis-2-pyrimidinyl disulfide in dry dichloromethane and 2-ethoxypyrimidine in the presence of ethanol <93TL1631>. 

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