Hydroxy-, derivatives thiones, oxidation

Tautomerism, particularly keto-enol, thione-thiol and amino-imino, was discussed in CHEC-I <84CHEC-l(3B)l>. In general, hydroxy substituents at all positions of the heterocyclic ring of pyridazines, phthalazines, and cinnolines exist in the keto form, with the exception (noted in CHEC-I) of the 3- and 5-hydroxy derivatives of pyridazine 1-oxide. In cases where two hydroxy or thiol substituents are present, the normal situation is for only one to be in the keto or thione form. In contrast, amino derivatives usually exist as such. 

Sulfonic acids are conveniently made by oxidation of the corresponding thiones or disulfides. Direct sulfonation is best effected with chlorosulfonic acid. Direct thiation can be achieved with sulfenyl chlotides to give 5-thiopyrimi-dines in excellent yields. For example, the reaction of 6-hydroxy-4(l//)-pytimidinone [4,6(1//,5//)-pyrimidinedione] 72 with benzylsulfenyl chloride or 2-methoxyphenylsulfenyl chloride gave the respective 5-thio derivatives 73 in 96% and 97% yields <1993SC2363, 2001JME3355>. 

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