Heterocyclic Nucleosede Analogues by Cycloaddition Reactions of 1-Vinylthymine with 1,3-Depoles

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Abstract
1,3-Dipolar cycloaddition of 1-vinylthymine to azides, nitrile oxides, nitrones and nitronates has been investigated as a route to heterocyclic nucleoside analogues in which the nucleoside ribose moiety has been replaced by an alternative heterocycle. Reaction of 1-vinylthymine with highly reactive nitrile oxides affords 1-(isoxazolin-5-yl)thymine products in excellent yield at room temperature. The less reactive nitrone dipoles undergo cycloaddition to 1-vinylthymine at elevated temperature to afford l-(isoxazolidin-5-yl)thymine cycloadducts in good-to-moderate yields, but show a tendency to eliminate thymine from the cycloaddition products over long reaction times. Azide cycloadditions to 1-vinylthymine proceed only under forcing conditions to which the fragile triazollne products are unstable.