Protection of Functional Groups as an Ultimate Tool in Selectivity Control

14 PROTECTION OF FUNCTIONAL GROUPS AS AN ULTIMATE TOOL IN SELECTIVITY CONTROL 

In the previous sections we examined options to achieve selectivity based upon variations in reaction conditions and/or reagents or even in the mechanism of the process. While this approach is an efficient way to achieve a goal, it does not come without a price tag. An adjustment of the basic and general method is required to meet the needs of a particular task. In practice, it frequently turns out to be more efficient to utilize a totally different approach to selectivity problems. We will explain this in terms of a generic example. 

The presence of an a, -conjugated double bond noticeably reduces the electrophilic activity of a carbonyl carbon. This effect allows one to protect, selectively, an isolated keto group as a ketal in the presence of a conjugated enone moiety. Thus, the selective transformations of conjugated enones, a situation frequently encountered in steroid chemistry, are achieved in this manner. 

A closer look at the above example might be instructive as to the general methodology in the use of protecting groups. The overall selectivity in this 

Protecting groups may serve more than the purposes illustrated in the above examples. Quite often the conversion of the functional group into a certain protected derivative might be also aimed at the modification of its reactivity pattern. We feel it appropriate to illustrate this point with a few additional examples. 

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