The acetonesensitized photocycloaddition reactions of uracil, thymine, 5-fluorouracil, and & (trimethylsilyl) uracil with alkynes afford 2, 4-diazabicyclo [4.2. 0] oct-7-ene-3, 5-diones. It is noteworthy that 5-(trimethylsily1) uracil gives very high yields in these reactions. High regioselectivity is exhibited for the reaction of uracil, 5-fluorouracil, and & (trimethylsiiyl) uracil with 1-alkynes, the 8-alkyl-substituted product being formed nearly exclusively. ...
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