Large rings, definition

Remind yourself of our definition of smalt, normal, medium, and large rings, and what ring strain means, by rereading p. 454, We will deal with what happens in large rings a. little later,... 

Turning to the ring current definition of aromaticity, the chemical shift calculations for different structures for [18]annulene can now be carried out in a straightforward manner by quantum mechanics. These calculations predict quite a large ring current, and the chemical shifts are substantially larger for the inner protons if there is bond alternation than if the bonds were to be equivalent. (By equivalent here is meant similar to those in benzene and to each other, and the quantum mechanical stationary... 

The Upase-catalyzed ROP of small-, medium- and large-ring lactones has often been reported. By definition. Upases hydrolyze triglycerides in an aqueous medium at one or more of the three ester bonds, and have been characterized by their drastically increased activity at the Hpid-water interface of a micellar or emulsified substrate. In a nonpolar medium, these enzymes are capable of initiating the ROP of cychc lactones, and this properly has been exploited for the synthesis of polyesters, especially from macrolides. 

Another interesting feature of such rings is that there is no definite conformational relationship between the cis and trans configurations. The reason is that we have a large number of degrees of freedom in such rings and they become more similar to open chains. So the cis substituents can be so rotated on adjacent carbons that they become trans to each other. 

The effective molarity (EM) is formally the concentration of the catalytic group (RCOO- in [5]) required to make the intermolecular reaction go at the observed rate of the intramolecular process. In practice many measured EM s represent physically unattainable concentrations, and the formal definition is probably relevant only in reactions (which will generally involve very large cyclic transition states) where the formation of the ring or cyclic transition state per se is enthalpically neutral, or in diffusion-controlled processes. For the formation of small and medium-sized rings and cyclic transition states the EM as defined above contains, and may indeed be dominated by, the enthalpy of formation of the cyclic form. This topic has been discussed briefly by Illuminati et al. (1977) and will be treated at greater length in a future volume in this series. 

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