Mechanistic aspects of selective formation of 10-, 20-, and 25-membered macrocyclic oligoesters in the cationic polymerization of 6,8-dioxabicyclo[3.2.1]octan-7-one

Abstract

complex can be prepared by the salt linkage formations of the acidic and basic groups in Hb with KPVS and PDDA ions if the complexation is carried at an appropriate pH. In the basic region, however, the cleavage of the salt linkage between the -OSOS and basic groups in the R b I I I I KPVS-Hb complex is observed in the process of the com-ABSTRACT Mechanisms of the selective formation of lo-, 20-, and 25-membered macrocyclic oligoesters (cyclic dimer, tetramer, and pentamer, respectively) from 6,8-dioxabicyclo[3.2.l]octan-7-one (1) are discussed on the basis of product distributions in the oligomerization of racemic monomer 1, optically active (+)-(1R,5R)-6,8-dioxabicyclo[3.2.1]octan-7-one (lR), and enantiomerically unbalanced monomer mixtures. Optically active cyclic tetramer (4R) and cyclic pentamer (5R) of 1R are predominantly formed by a tail-biting reaction of a growing oligomer chain, while racemic cyclic tetramer (4) and cyclic pentamer (5) are mainly produced by a back-biting reaction of an initially formed polymer of 1. Cyclic dimer (2) is formed primarily by intramolecular reaction of unsymmetrical oligomen which are formed from 4 and 5 by the reaction with monomer. All these macrocyclic oligomers are formed via an SN2-type mechanism involving the exclusive alkyl-oxygen fiision of 1. The selective formation of 2,4,5,4R, and 5R is remarkably dependent upon the reaction conditions, especially temperature, time, solvent, and optical purity of the monomer. Solubility and molecular symmetry of cyclic oligomers, interactions between a cyclic oligomer and ita opposite enantiomer or a solvent molecule, and conformation of a growing chain are important factors controlling the selective formation of the cyclic oligomers of specific ring sizes. It is not an unusual but rather common phenomenon that cyclic oligomers of various ring sizes are formed in the 0024-9297/81/2214-ll80$01.25/0 cationic ring-opening polymerization of a variety of cyclic monomers.' These cyclic oligomers are often in equilib-

Synthesis and ring-opening polymerization of bicyclic lactones containing a tetrahydropyran ring. 2,6-Dioxabicyclo[2.2.2]octan-3-one

“…There are three possible modes of bond cleavage when the bicyclic lactone 1 is polymerized (Scheme II). The first is the acyl-oxygen scission (a) of the ester linkage, leading to a cis-disubstituted tetrahydropyran ring (9). The second is the alkyloxygen scission (b) of the ester linkage, which gives a trans-disubstituted tetrahydropyran ring (SN2 type reaction) or a mixture of cis-and trans-disubstituted tetrahydropyran rings (SN1 type reaction) (10).…”

Synthesis of 20- and 25-membered optically pure macrolides by cationic ring-opening oligomerization of 6,8-dioxabicyclo[3.2.1]octan-7-one of lower optical purities

“…Therefore, the macrodilide 2 should be produced by a mechanism completely different from the mechanism of the formation of the macrolides 4 and 5. In a previous paper,26 we proposed a possible mechanism for the formation of 2, involving the reaction of the oxonium ion of the chiral macrolides (for instance 4R+) and the monomer of the opposite chirality (IS), followed by intramolecular reaction of the resulting unsymmetrical oligomer ion (Figure 7 in ref 26).…”

Cationic Ring-Opening Oligomerization of the Two Stereoisomers of 4-Bromo-6,8-dioxabicyclo[3.2.1]octan-7-one

“…Probably a small amount of cyclic dimer of Se as detected in the 1 H NMR spectrum in Figure 3 is produced by a back-biting mechanism similar to that proposed previously for the formation of 2 in the oligomerization of 1. 5 Such neighboring group participation of the axially oriented bromine atom was also postulated to explain the different behavior of the two stereoisomers of 4-bromo-6,8-dioxabicyclo[3.2. l]octane in their cationic polymerization. 19 In summary, the stereoisomers Sa and Se displayed significantly different behavior in their cationic oligomerization at -40 and 0°C.…”