Vinylogous principle

First review covering the vinylogous principle Fuson RC (1935) Chem Rev 16 1... 

The electrophilicity of C=C double bonds conjugated with electron withdrawing groupings leads to a -synthons. This is an important example of the vinylogous principle ... 

Fig. 1.3 The vinylogy principle applied to the catechol O-methyltransferase inhibitor, tolcapone.

The vinylogy principle was first formulated by Claisen in 1926, who observed for formylacetone acidic properties similar to that of acetic acid. The vinyl goup plays the role of an electron-conducting channel between the carbonyl and the hydroxyl group. The same effect explains the acidity of ascorbic acid (Figure 14.19). 

Today the vinylogy principle is explained by the meso-meric effect and it applies to all conjugated systems imine and ethynyl groups, phenyl rings, and aromatic heterocycles (Figure 14.20). For a review of the chemical aspects of the vinylogy principle see Krishnamurthy. ... 

FIGURE 14.31 Application of the vinylogy principle to the design of a fenbendazole bioisostere. ... 

However, the vinylogy principle is sometimes applied to the design of bioisosteres. Thus the guanidinic group of the benzimidazole fenbendazole can be compared with its vinylogue in the corresponding imidazo[l,2-a]pyridine (Fig. 12.31). Both compounds are anthelminthics of similar potency. 

Fig. 4.4 Examples of molecules where the vinylogy principle gives similar C-H acidity to methyl groups...

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