Ring-Opening Polymerization of Bicyclic Lactones

1 Selective Cyclooligomerization of Bicyclic Oxalactones Having a Bicyclo[3.2.1]octane Skeleton 

The cyclic dimer 43 is a me so compound consisting of a pair of different enantiomeric units of 41. [45] All four substituents attached to the two tetra-hydropyran rings are located in the axial positions. This compound has a center of symmetry and is readily crystallized. The cyclic tetramer 44 is a racemic mixture of optically active enantiomers, that is, 44R and its enantiomer 44S. [46] Similarly, the cyclic pentamer 45 is a racemic mixture of 45R and its enantiomer 45S. [47] In these cyclic oligomers, every exo-cyclic acetal oxygen occupies the axial position and every carbonyl carbon occupies the equatorial position of the tetrahydropyran ring. The molecules of these oligomers are chiral but not asymmetric (gyrochiral 

Initiator, BF3OEt2, 1 mol% to monomer temp., —40 °C time, 24 h AN, acetonitrile CF, chloroform NP, 1-nitropropane By gel permeation chromatography 

By utilizing the specific oligomerization behavior of the bicyclic oxalactone described above, it was possible to prepare an optically pure cyclic tetramer and cyclic pentamer from a less optically pure monomer mixture, although the yield was low. [49] The results are presented in Table 3. Needless to say, 43 was concomitantly produced in larger amounts. 

The two stereoisomers of4-bromo-6,8-dioxabicyclo[3.2.1]-octan-7-one (47a and 47b) showed different behavior in their cationic oligomerizations at — 40 or 0 °C [50] The axial isomer 47 a showed a tendency to cyclodimerize, particularly at higher temperature, whereas the equatorial isomer 47 e was much less reactive and gave only a small amount of a cyclic dimer along with other oligomers. Reductive debromination using tri- -butylstannane converted the cyclic dimer 48 of 47 a to the cyclic dimer 43 of 41, indicating that the former also consisted of a pair of the enantiomeric monomeric units. 

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Finally the preparation of polyesters by ring-opening polymerization of bicyclic lactones will briefly be mentioned. This type of polymerization has previously been investigated. However, it is only recently that the importance of controlling the stereochemistry has been demonstrated. In this study the polymers obtained from 2-oxabicyclo[2.2.2]octan-3" one (19) were characterized by NMR. 

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