Carboxamide bioisosteres

Patent applications from Pfizer disclosed 1,5-diaryl-pyrazoles bearing bioisosteric replacements for the 3-carboxamide moiety. One application showed that the amide could be replaced by a-aminoketones as exemplified by compound (416) [284]. The corresponding alcohols and their ethers were also described, including compounds that allowed the amine substituent and ether to form a ring system, such as a morpholine unit. This application also allowed for the replacement of the 1,5-diaryl-pyrazole by a 1,2-diaryl-imidazole bearing a 3-carbonyl substituent, as exemplified by compound (417). A further patent application from Pfizer claims compounds in which imidazoles replace the 3-carboxamide moiety in the 1,5-diaryl-pyrazole... 

The oldest example of the use of nonclassical isosteres involves the replacement of the carboxamide in foUc acid by sulfonamide, to give the sulfanilamides. Diaminopyrimidines, as antimalarial agents, are also based on folate isosterism, in addition to the exploitation of auxiliary binding sites on dihydrofolate reductase. This concept of nonclassical isosteres or bioisosteres — that is, moieties that do not have the same nnmber of atoms or identical electron structure — is really the classical structure modification approach. 

Biologically active molecules containing amide bonds suffer usually of pharmacokinetic liability. In order to increase their stability, bioisosteric transformation of the carboxamide have been performed and yielded a lot of successful examples especially in the area of petidomimetic. The isosteric replacements for peptidic bonds have been summarized by Spatola and by Fauchere. " The most used and well-established modihcations are iV-methylation, configuration change (o-conhguration at Ca), formation of a retroamide or an a-azapeptide, use of aminoisobutyric or dehydroamino acids, replacement of the amidic bond by an ester [depsipeptide], ketomethylene, hydroxyethyl-ene or thioamide functional group, carba replacement of the amidic carbonyl, and use of an olefinic double bond (Figure 15.33). 

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