Benzo quinone dehydrogenation

Oxidations of pyridopyrimidines are rare, but the covalent hydrates of the parent compounds undergo oxidation with hydrogen peroxide to yield the corresponding pyridopyrimidin-4(3 T)-ones. Dehydrogenation of dihydropyrido[2,3-(i]pyrimidines by means of palladized charcoal, rhodium on alumina, or 2,3-diehloro-5,6-dicyano-p-benzo-quinone (DDQ) to yield the aromatic derivatives have been reported. Thus, 7-amino-5,6-dihydro-1,3-diethylpyrido[2,3-d]-pyri-midine-2,4(lif,3f/)-dione (177) is aromatized (178) when treated with palladized charcoal in refluxing toluene for 24 hours. 

DEHYDROGENATION Palladium black. Potassium-Graphite. Tetrachloro-o-benzo-quinone. 

Oxidation of hydroquinone (1,4-benzenediol) produces a compound known as p-benzo-quinone. The oxidation can be brought about by mild oxidizing agents, and, overall, the oxidation amounts to the removal of a pair of electrons (2 e ) and two protons from hydroquinone. (Another way of visuahzing the oxidation is as the loss of a hydrogen molecule, H H, making it a dehydrogenation.)... 

Vinylbenzo[6]thiophene 90 reacted with benzoquinone to give a fully aromatized 1 1 cycloadduct 151 as result of a Diels-Alder reaction followed by dehydrogenation by excess quinone. Isolation of a dihydro derivative 152 was possible using a weaker solution of 3-vinylbenzo[b] thiophene, whereas a further excess of the vinyl compound and a very short reaction time afforded the terahydro derivative 153 (79AJC145). On the other hand, primary Diels-Alder adducts are isolated from 1-(3-benzo[6]thiophene)cyclohex-1 -ene and 1 -(3-benzo[fc]thiophene)-3,4-dihydronaphthalene on reaction with MA (50JA571). 

Their substrates are 2-phenylpyridines, imines, benzo[/i]quinones, phosphines, oxygen compounds, sulfur compounds, etc., as described in the previous sections or in Fig. 5.11, Tables 5.5 and 5.6. These reactions are arylations, alkenylations, alkylations, acylations, cyclizations, hydrogenations, oxidations, hydrosilylations, dehydrogenations, etc. 

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