title: Formation of Macrocyclic Ethers by Free Radical Cyclization: Effects of Chain Length, Substituents, and Solvents.
abstract: 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.
authors.affiliation: Laboratoire de Chimie Organique et Organométallique, Associé au CNRS, URA 35, Université Bordeaux 1, 351 Cours de la Libération, F-33405 Talence-Cedex, France, and Research School of Chemistry, The Australian National University, Canberra, ACT 0200, Australia.
authors.forename: Annie
authors.initials: A
authors.lastname: Philippon
authors.forename: Marie
authors.initials: M
authors.lastname: Degueil-Castaing
authors.forename: Athelstan L. J.
authors.initials: AL
authors.lastname: Beckwith
authors.forename: Bernard
authors.initials: B
authors.lastname: Maillard