The single step reactions of N, N'-substituted/-unsubstituted barbituric acids with various alkyl dihalides under phase transfer catalytic conditions using DMF–K2CO3(base), TBAHSO4 (catalyst) provide spirobarbituric acids in moderate to high yields. Irrespective of the existence of C5-monoalkylated compounds in the enolic form (confirmed by the isolation of some of its analogues), the second alkylation predominantly takes place at C5. The ...
[McKeown, Robert H.; Prankerd, Richard J. Journal of the Chemical Society, Perkin Transactions 2: Physical Organic Chemistry (1972-1999), 1981 , p. 481 - 487]