Cyclic alkynes (e.g., cyclooctynes such as dibenzocyclooctynes) can be photochemically generated from cyclopropenones as disclosed herein. The cyclic alkynes can be reacted (e.g., in situ) with materials having alkyne-reactive groups (e.g., azide groups in a “click” reaction). In preferred embodiments, the generation and reaction of the cyclic alkyne can proceed in the absence of a catalyst (e.g., Cu(I)). These reactions can be useful, for example, for the selective labeling of living cells that are metabolically modified with azido-containing surface monosaccharides, or for light-directed surface patterning.