1.2.4- Triazine-3,5-dione, bromination

On the other hand, a proton directly bound to the 1,2,4-triazine ring can be replaced by halogen. This is an electrophilic substitution reaction at carbon, and, as expected for such a heavily aza-substituted ring system, it needs considerable activation by electron-donating substituents. The halogenation reaction is best known in bromination at the 6-position. From the published data it seems that either an amino group in the 3-position or an oxo group in the 5-position is necessary. The formation of 6-halo-substituted compounds has been reported for l,2,4-triazin-5-ones, l,2,4-triazine-3,5-diones, 3-amino-l,2,4-triazines and 3-amino-l,2,4-triazin-5-ones. 

A halogen directly attached to the 1,2,4-triazine ring is very reactive and can be replaced by most other nucleophiles as outlined in Scheme 9. Tlie reactivity toward neutral nucleophiles decreases from the 5-position to the 3- and 6-positions, while that towards anionic nucleophiles decreases from the 5-position to the 6- and 3-positions. Among nucleophiles which displace halogen atoms are water, alcohols, amines, hydrazine, hydroxyl-amine, thiols and hydrogen sulfide. It is also reported that the bromine in 6-bromo-1,2,4-triazine-3,5-dione can be replaced by fluorine (323) or by a cyano group (324). 

Most dihydro-l,2,4-triazinones can be converted into the corresponding thiones, and into chloro-dihydrotriazines, and they can be alkylated by the usual methods. These reactions are also known for the diones. As was mentioned earlier, in these reactions the 5-position is the most reactive. Treatment of 1,2,4-triazine-3,5-diones (258) with phosphorus pentasulfide leads firstly to the 5-thioxo-4,5-dihydro-l,2,4-triazin-3-ones (259), and then to the l,2,4-triazine-3,5-dithiones (260). Further reactions reported for l,2,4-triazine-3,5-diones (258) are bromination in the 6-position to give (261), replacement of one (262) or both oxygens by chlorine (263) on reaction with SOCI2,... 

The chemistry of l-methyl-truns-cyclo-octene has been studied. In particular it reacts with acidic methanol to give 1-methoxy-l-methylcyclo-octane, with bromine to give a complex mixture, and with peracid to give its epoxide. On warming in acetic acid it is isomerized to 1-methyl-cis-cyclo-octene, and it reacts with diazomethane and phenyl azide to give cycloaddition products. With 4-phenyl-1,2,4-triazine-3,5-dione, the ene product (121) is obtained. The major difference in reactivity between trans-cyclo-octene and its 1-methyl analogue was found for electrophilic addition reactions where the stability of the incipient 3° cation made the 1-methyl compound more reactive.Transannular products were obtained from the reaction between trans-... 

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