Compounds of Formula I: wherein: R1 and R2 are hydrogen or C1 to C12 linear or branched alkyl; R3 is hydrogen, C1 to C12 linear or branched alkyl, or nitrogen protecting groups; X is oxygen or nitrogen; and when X is oxygen, R4 is absent; and when X is nitrogen, R4 is C1 to C12 linear or branched alkyl or nitrogen protecting groups. The compounds are useful as reagents in 1,3-cycloaddition reactions.