Reagents and Installable Synthetic Blocks

We have already employed in our text the term synthetic equivalents . Synthetic equivalency implies that the result of the actual reaction with a given reagent is equivalent to that of a virtual reaction employing the reactive intermediate of the corresponding structure. The concept of synthetic equivalency can be applied to reactions of any type and gives one the opportunity to describe the net structural outcome in general terms without the necessity to specify exact reagents. 

2 The Notion of Synthons. Trivial and Not-very-trivial C1-C4 Synthons and Reagents 

With the help of this synthon one can effect coupling of a carboxyl group with an electrophile, a reaction of the type  

Obviously the COOH species cannot exist, but nevertheless a conversion described by this formal equation has been known to organic chemistry for more than a century. A very simple and familiar reagent, the cyanide anion, serves as an equivalent of the described synthon in the reaction with electrophiles to give nitriles, E-CN. The latter can be hydrolysed to the corresponding carboxylic acids, E-COOH, as illustrated below  

This is also a non-existent entity, but here again a very trivial reagent, COj, can be utilized to do the job of the electrophilic carboxyl synthon to couple with organometallic nucleophiles to produce salts of carboxylic acids (see Section 2.4). 

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