A number of organoselenium reagents are introduced as efficient initiators of ring closures leading from unsaturated substrates to lactones, cyclic ethers, cyclic thioethers, N- heterocycles and carbocycles. These cyclizations often proceed with high ring selectivity and stereoselectivity and are accompanied by the incorporation of the phenylseleno group (PhSe) into the final product. Methods are described for the effective removal of this group ...