Elimination, direction functional groups

In the synthesis of molecules without functional groups the application of the usual polar synthetic reactions may be cumbersome, since the final elimination of hetero atoms can be difficult. Two solutions for this problem have been given in the previous sections, namely alkylation with nucleophilic carbanions and alkenylation with ylides. Another direct approach is to combine radical synthons in a non-polar reaction. Carbon radicals are. however, inherently short-lived and tend to undergo complex secondary reactions. Escheirmoser s principle (p. 34f) again provides a way out. If one connects both carbon atoms via a metal atom which (i) forms and stabilizes the carbon radicals and (ii) can be easily eliminated, the intermolecular reaction is made intramolecular, and good yields may be obtained. 

First we need to find the functional group and make sure that the functional group is connected directly to the parent chain. Remember from the last section that if there are two functional groups, one of them gets priority. The functional group that gets priority is the one that needs to be connected to our parent chain. Of the three possibilities shown above, this rule eliminates the last possibility, because the functional group (OH) is not connected directly to the parent chain. 

Synthetic efficiency. In many organic syntheses, it may be possible to eliminate the need for the protection and deprotection of functional groups, and save many synthetic steps. Water-soluble substrates can be used directly. This will be especially useful in carbohydrate and protein chemistry. 

Some disconnections must have -in the retrosynthetic direction- high priority. For instance, the elimination (or modification) of protecting groups (especially, hydrolytic cleavages) or particularly unstable functional groups. 

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