The chemistry of 5‐hydroxy‐6‐(hydroxyalkyl)uracils. Synthesis of spiro[pyrimidine‐4,2′‐pyrano[3,2‐ d ]pyrimidines]

Both 5‐acetoxy‐6‐(acetoxymethyl)‐1‐methyluracil 1 and its parent diol 11 are converted into the spiro[pyrimidine‐4,2′‐pyrano[3,2‐ d ]pyrimidine] 6 when treated, respectively, with hot methanolic pyridine and with one equivalent of acetic anhydride. The formation of 6 can be explained in terms of the generation and dimerization of the reactive 5‐oxo‐6‐methylene pyrimidine 2 . The structure of 6 was determined by 13 C nmr spectroscopy and by chemical transformations that lead to the pyrimidinylethylhydantoin 9 and the 6,6′‐[1,2‐ethanediyl]bispyrimidine 10 . The more complex 5‐hydroxy‐6‐(hydroxyalkyl)uracils represented by the 6,5′‐cyclourid‐ines 17 undergo an analogous dimerization when treated with acetic anhydride to give structures 22a and 22b . Dimer 22a was also prepared via the 5‐phosphate ester 18 . The stereochemistry of dimers 22a and 22b , which is apparent from their 1 H nmr spectra, indicates that two molecules of the enones 21a or 21b dimerize in a highly stereoselective manner.