Protecting groups, functionalized Grignard reagents

With the alcohol protected, preparation of the Grignard reagent can proceed, and this can then react with the ketone carbonyl in a nucleophilic addition. The protecting group can then be removed by treatment with acid, to restore the hydroxyl function. This also involves a tertiary carbocation that is subsequently quenched with water. 

The cyanohydrin not only adds an additional C at the site of the C==0 but also introduces two new functional groups, OH and CN. which can be used to introduce other functional groups. The OH can be used to form an alkene (C=C), an ether (—RO), or a halogen compound (C—X) the C N can be reduced to an amine (CHjNHj), be hydrolyzed to a carboxyl (COOH) group, or react with Grignard reagents if the OH is protected. 

In aromatic systems, oxazolines can have three different functions (Fig. 4). Firstly, they can be used as protecting groups for carboxylic acids. Secondly, they activate even electron-rich aromatic systems for nucleophilic substitution. Fluorine or alkoxy groups in the ortho position can be substituted by strong nucleophiles such as Grignard reagents. Thirdly, when biaryl compounds with axial chirality are synthesized in these reactions, oxazolines can induce the formation of only one atropisomer with excellent selectivity. These three qualities were all used in the synthesis of 20, a precursor of the natural product isochizandrine [10]. 

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