Interchange cyclic

Polyesters have been obtained in organic medium by polyesterification of hydroxy acids,328,329 hydroxy esters,330 stoichiometric mixtures of diols and diacids,331-333 diols and diesters,334-339 and diols and cyclic anhydrides.340 Lipases have also been reported to catalyze ester-ester interchanges in solution or in die bulk at moderate temperature.341 Since lipases obviously catalyze the reverse reaction (i.e., hydrolysis or alcoholysis of polyester), lipase-catalyzed polyesterifications can be regarded as equilibrium polycondensations taking place in mild conditions (Scheme 2.35). 

The polymerization of a ring compound usually proceeds by an interchange reaction, induced either catalytically or by the presence of small amounts of end-group-producing substances. For example, the polymerization of lactide, the cyclic dimer of lactic acid, is accelerated by small amounts of water. The water undoubtedly hydrolyzes the lactide to lactyllactic acid, which may then react with other lactide molecules by ester interchange. This reaction scheme can be represented as follows ... 

The formation of the adduct between 86 and the nitrile, i.e., 89, occurs more readily than that between 86 and ketones, since an activated nitrile is a better nucleophile than a ketone. Since the ce-proton in the adduct 89 is more acidic than the ce-proton in the ketonic adduct, also the ring opening will occur more easily. The interchange of a nitrile carbon with the ring carbon of a pyrimidine ring was also observed with the 3-benzyloxymethyl-l-ribosyl-5-cyanouracil. With a series of activated nitriles, the protected bi-cyclic nucleosides are formed. After deprotection, the corresponding bi-cyclic nucleosides are obtained (Scheme IV.35). 

Acidolysis of cyclic sulfites with sulfuric acid and ester interchange with dimethyl sulfate produce cyclic sulfates (104). 

We first note that all types of A orbitals (in D3h) have the same symmetry properties with respect to the rotations constituting the subgroup C3 also, both and " orbitals have the same properties with respect to these rotations. Thus we can use the group C3 to set up some linear combinations that will be correct to this extent. Since these rotations about the C3 axis do not interchange any of the orbitals 0, 02, 03 with those of the set 4, 05, 6, we can, temporarily, treat the two sets separately. We thus first write down linear combinations corresponding to the A and representations of C3. As shown in Section 7.3 for such cyclic systems, the characters are the correct coefficients, and we can thus write, by inspection of the character table for the group C3 ... 

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