Deprotonation second order transformation

If kepi, k-epi are much larger than the rates k, ki of substitution, the enantiomeric ratio Hepi-1 is similar to kxjk (path C, dynamic kinetic resolution . Both mechanisms are performing when the rates of the two steps are similar. Since rates and equilibrium are temperature-dependent, enhancement of stereoselectivities can be achieved by sophisticated protocols (see Section m.E). The equilibrium 6/epi-6 is determined by the difference of free energy A AG. This effective energy difference is enlarged if it can be coupled with a second order transformation such as the selective crystallization of one diastere-omer dynamic thermodynamic resolution ). In fact, this applies to the first successful (—)-sparteine-mediated deprotonation (Section FV.C.l). 

The deprotonation of (E)-2-butenyl A/,A/-diisopropyl-carbamate leads to (lS,2E)-l-(A(ALdiisopropylcarbamoyloxy)-2-butenyllithium-(—)-sparteine with >90% de after crystallization, combined with a second-order asymmetric transformation (eq 3). It has been applied in the enantioselective synthesis of y-lactones, such as (+)-eldanolide (eq 3), dihydroavermectin Bib, and doubly branched sugar analogs. ... 

Order-of-magnitude generalizations concerning reactivity are often useful in developing models and making predictions about transformation and fate. Some reactions can be classified as fast relative to other pertinent physical and chemical processes, and can be treated as being instantaneous. Many protonation-deprotonation reactions, for example, occur at time scales significantly below one second, and can therefore be treated as pseudoequilibrium reaction steps. 

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