Formation of Macrocyclic Ethers by Free Radical Cyclization: Effects of Chain Length, Substituents, and Solvents.

Free radical reduction by tributylstannane of omega-iodopolyoxaalkyl acrylates derived from tri-, tetra-, penta-, hexa-, and heptaethylene glycols gives mixtures of uncyclized reduction products and macrocyclic ethers formed by endo cyclization. The rate constants for cyclization of the intermediate radicals at 80 degrees C in benzene were determined under carefully defined conditions to be 15 x 10(4), 13 x 10(4), 5.1 x 10(4), 10 x 10(4) and 3.6 x 10(4) s(-) (1), for formation of the 12-, 15-, 18-, 21- and 24-membered rings, respectively. These values indicate that the presence of oxygen atoms in the chains increases the rate by a factor of 10-30 by comparison with the previously reported cyclization of alkenyl species. The rate constants at 80 degrees C in benzene and the endo/exo ratio for reductive cyclizations of the methacrylate, crotonate, cinnamate, maleate, and fumarate esters of 8-iodo-3,6-dioxaoctanol have been determined. The reduction of the 8-iodo-3,6-dioxaoctyl acrylate in solvents of varying polarity indicated that the cyclization rate has a relatively low solvent dependence.