Pyrimidines, 1,2-dihydro-2- -, formation rearrangement

A similar degenerate ring transformation was observed when 4-amino-6-chloro-l-methylpyrazolo[3,4-(f]pyrimidine was treated with dilute alkali. In this reaction as well, the expected product, 4-amino-6,7-dihydro-6-oxo-l-methylpyrazolo[3,4-(f]pyrimidine, was not obtained, but rather a rearranged isomer, i.e., 6-amino-4,5-dihydro-4-oxo-l-methylpyrazolo-[3,4- f] pyrimidine. The formation of this compound takes place according to the same mechanism as that postulated for the formation of 86 (Scheme 11.39) (54JOC1570). 

When 2,4-dioxopyrido[l,2-a]pyrimidines 391 were treated dropwise in acetonitrile with tetrachlorosilane in 1,2-dichloroethane in the presence of sodium iodide at room temperature, 2-acetamido-3,4-dihydro-2//-pyridopyrimidin-4-ones 396 were obtained after work-up [92MJ24 93IJC(B)637]. In the first step iodotrichlorosilane was formed, and 1,2-addition of iodotrichlorosilane to a 2-carbonyl group of pyridopyrimidine 391 led to the formation of a-iodosilyl esters 392, which reacted with acetonitrile. After the rearrangement of adduct 393 and a further reaction with a second mole of iodotrichlorosilane, the hydrolysis of disylylated intermediates 395 yielded 2-acetamido-3,4-dihydro-2//-pyridopyrimidin-4-ones 396 (see Scheme 24). 

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