Pyrazine ring 1,2,3,4-tetrahydro- from

Catalytic hydrogenation of the imide 24 (R = H) over palladium on charcoal results in partial reduction of the pyrazine ring to give the tetrahydro compound 37. The anomalous products 31 and 32 from the reaction of the anhydride of pyrazine-2,3-dicarboxylic acid and hydrazine were presumed to have been formed by reduction of the pyrazine ring by diimide. This could have been formed in situ by oxidation of hydrazine... 

Hydrogenolysis of 7-bromopyrido[2,3-fc]pyrazine over palladized strontium carbonate gave the parent heterocycle. Similar treatment provided the 2-methyl-3-oxo compound from its 7-bromo derivative, without reduction of the pyrazine ring. A variety of other 7-bromo compounds have also been dehalogenated by this technique. On the other hand, catalytic hydrogenation has also been used to reduce 2- and 3-0x0 compounds to the tetrahydro derivatives 117 and 118. Reduction of the... 

Cyclization of the alcohol 54 using hydrogen bromide was used in the unambiguous synthesis of 3-methylpyrido[3,4-b]pyrazine (56), obtained from the tetrahydro compound 55 by dehydrogenation over palladium on charcoal. Reduction of the nitro compound 57 resulted in cyclization to form the reduced derivative 58 of the ring system. 

The only examples of this synthesis employed isoxazolopyrazine substrates that were themselves made from pyrazines. Thus l-benzyl-5,6-dihydro-2(l//)-pyrazi-none 4-oxide (99) underwent addition by ethynylbenzene to give 5-benzyl-2-phemi-6,7-dihydro-3a//-isoxazolo[2,3-a]pyrazin-4(5//)-onc (100) (60%), which subsequently underwent ring cleavage by molybdenum hexacarbonyl in wet acetonitrile to afford l-benzyl-3-phenacyl-3,4,5,6-tetrahydro-2(l//)-pyrazinone (101) in 54% yield several analogues were made similarly.1539... 

Attempted oxidation of the 3-oxo compound 96 with potassium permanganate resulted only in ring breakdown. Nevertheless a solution of this reagent in acetone is commonly used to oxidize compounds such as the dihydro derivative 97 to the aromatic system without further oxidation occurring. Oxidation of the tetrahydro oxo compound 98 with potassium permanganate in 2M sodium hydroxide solution at room temperature gives the 2-oxo compound 99. This compound is sensitive to oxidation, and the use of iodine has been recommended for this conversion. If alkaline potassium ferricyanide is used, the product formed is the dioxo derivative 100. The dioxo compound is also obtained from l,2-dihydro-2-oxo- or 3,4-dihydro-3-oxopyrido[2,3-b]pyrazine under the same conditions. Recently ferric chloride has been used to aromatize a series of dihydropyrido[2,3-b]pyrazines. ... 

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