Ring isomerization

A ring isomerization giving the by-product 158a trapped as its 5,5-dioxide 158b was observed to compete with processes 506 - 152c(Ar = Ph, 87KGS614). 

The effect of lactone ring-size on the inhibition was studied, for N-acetyl-)3-D-glucosaminidase from bovine epididymis, with lactones and lactone derivatives unable to undeigo ring-isomerization, by Pokomy and co-workers. From a comparison of Kj values for 2-acetamido-2-deoxy-D-glu-cono-1,5-lactone (0.45 nM) with the 1,4-lactone (4.5 fiM) and of Kj for the methyl ) -furanoside with that for the pyranose (4 mM), it was concluded that the 1,4-lactone has an 10-fold lower inhibitory potency than the 1,5-lactone. The weak inhibition by the 5,6-O-isopropylidene derivative of the... 

The acyl group of an intermediate acyl-HP (e.g., 17) in principle may attack at either nitrogen atom of the pyrimidine ring. Therefore, in addition to ring isomerism another type of isomerism exists for synthesis B, caused by the position of substituents (positional isomerism). 

For example, in the ring isomerization reaction, methylcyclopentane forms a methylcyclopentene intermediate in its reaction sequence to cyclohexane. The intermediate can also further dehydrogenate to form methylcyclo-pentadiene, a coke precursor. Bakulen et al. (4) states that methylcyclo-pentadiene can undergo a Diels-Alder reaction to form large polynuclear aromatic coke species. Once any olefinic intermediate is formed, it can either go to desired product or dehydrogenate further and polymerize to coke precursors. This results in a selectivity relationship between the desired products and coke formation as shown on the next page. 

Thus, the number of ring isomerization sites available for reaction is the total number of active sites minus those sites blocked by the formation of coke. Since the metal and acid sites can be used for reactions other than isomerization, intermediates from these other reactants adsorbed on the site also affect the ring isomerization reaction by reducing the number of active sites for the ring isomerization reaction. Since isomerization involves both metal and acid sites in a unique way, all intermediates from other reactions that utilize metal and acid sites in this manner have the potential to block these active sites. This is illustrated in Fig. 13. 

In our start-of-cycle (SOC) model, the start-of-cycle rate constants for ring isomerization implicitly contained the total active ring isomerization site concentration ... 

The parameter, a, defined in the following equation, relates the concentration of active ring isomerization sites at any time to the initial site... 

Fig. 13. C6 aging mechanism ring isomerization site deactivation.

The start-of-cycle rate constant for ring isomerization X° is modified to include deactivation by multiplying its kinetic equations by a, ... 

As discussed previously, the rate of coke buildup can be related to the concentration of components in the gas phase [Eq. (26)]. An example for the coking of ring isomerization sites by cyclohexane intermediates is... 

Identical equations can be written for the coke formed from the other adsorbed intermediates in the C6 system of Eq. (24). Thus, the total rate of decrease in active ring isomerization site concentration can be solved by putting the solutions for the individual coke formation steps [similar to Eq. (33)] into Eq. (31). This can be expressed as... 

Following the same arguments, the ring isomerization deactivation rate expression can be directly extended to include the effects of all kinetic lumps (defined in Table VII) ... 

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