Formal Classification of Selectivity Problems

The question of the selectivity of a reaction is so critical to organic synthesis that a detailed discussion is most certainly warranted. We will start by examining some general aspects of this problem. 

The problems related to selectivity are diverse. Therefore, we will only examine a few typical cases to illustrate some of the main facets of this 

In contrast, the chemistry of the oxidation of a primary alcohol to an aldehyde differs sharply from the oxidation of an aldehyde to a carboxylic acid (case (b)). Advantage, in this case, must be taken of the difference in the mechanisms of these steps. Among the reagents which can effectively oxidize alcohols and remain rather inert toward aldehydes are pyridinium chlorochro-mate (a chromium trioxide-hydrogen chloride complex of pyridine) or dimethyl sulfoxide-Lewis acid. 

Ensuring the selective monoalkylation of ketones [case (c)] is of special importance in synthetic practice and numerous approaches are elaborated for this purpose. This problem deserves special comment and will be considered later (Section 2.13). 

The orientation of the approach of the nucleophile determines the ratio of cis and trans isomers in the resulting mixture, (147 + 149) (148 + 150). In actuality, electrophilic addition of Br is directed almost exclusively at C-2 and hence products 147 and 148 are formed preferentially. While the ratio of these isomers is very sensitive to the reaction conditions, it is rather difficult to achieve a high stereoselectivity in the reaction and hence this method cannot be recommended for the preparation of the pure stereoisomers 147 or 148. 

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