An unusual one-component sealant based on a moisture cure has been developed. The chemistry involves the reaction of a 1,3-dipole (a difunctional nitrile oxide) with a dipolarophile (an unsaturated polymer); a difunctional hydroximoyl chloride, which forms a nitrile oxide on reaction with a latent base, reacts with the polymer on exposure to moisture by forming isoxazoline crosslinks. The hydroximoyl chloride was prepared from a bis(chloroketone) (e.g., 4,4′-bis(chloroacetyl)diphenyl ether), by reaction with nitrous acid; this avoided the use of expensive dialdehydes, as well as hydroxylamine. The preferred latent base was barium oxide, and almost any unsaturated polymer could be used. Such a system, stable indefinitely when dry, crosslinked rapidly when exposed to atmospheric moisture. Since water is generated when the initially formed barium hydroxide reacts with the hydroximoyl chloride, only catalytic amounts are required, and thick beads cure completely; skinning causes problems with other moisturecurable systems.