Examples of masked functionalities in synthesis

A similar procedure using a-methoxylalkylphosphonium salts provides a route to ketones. Masked reactive functionalities have achieved fundamental importance in 

In designing a multistep synthesis, one must consider aspects of stereochemistry as well as functionality. In the chapters dealing with individual reactions, many examples were given in which the aspects of stereochemistry were a direct consequence of the reaction mechanism. For example, hydroboration-oxidation involves a syn addition followed by oxidation with retention of configuration. The generalization, widely but not universally correct, that reagents attack molecules from the sterically less hindered side was also illustrated on numerous occasions. 

We now wish to consider how these elements of stereochemistry come into play in synthesis. It is important to know how reaction stereochemistry is controlled by structural features of the reactant molecules. This topic can be broadly covered by the term asymmetric synthesis, which has been defined as a reaction in which an achiral unit in an ensemble of substrate molecules is converted by a reactant into a chiral unit in such a manner that the stereoisomeric products are produced in unequal amounts. Thus, we will be dealing with methods for controlling the configuration of newly formed chiral centers. As will be seen, these methods often depend on the fact that reagents attack molecules along the less hindered path. 

Morrison and H. S. Mosher, Asymmetric Organic Reactions, Prentice-Hall, Englewood Qiffs, NJ, 1971. 

Perhaps the simplest case of asymmetric synthesis to visualize is a reaction between an achiral molecule and a chiral reagent in such a way that a new chiral center is created at the reaction site. One such reaction is the Meerwein-Ponndorf-Verley reduction of ketones with aluminum salts of optically active alcohols. The 

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