The present invention relates to a method for site-selective coupling of a first agent to a second agent, comprising the steps of: contacting a first agent comprising at least one furan moiety with an activation signal and with a second agent comprising at least one hydrazine moiety or at least one hydroxylamine moiety, thereby activating said furan moiety to an activated furan moiety; and reacting said activated furan moiety with the hydrazine moiety or the hydroxylamine moiety, thereby site-selectively coupling said first agent to said second agent.