Peptide trifluoromethyl ketone

This approach has been mainly applied to peptide-based inhibitors of proteases, where the inhibitory molecule is a peptide with a transition state isostere appended to it, and the cognate substrate is simply a peptide of the same amino acid sequence, but lacking the isostere functionality. Examples where good correlations between the free energy of inhibitor binding and the free energy of kcJKM have been found, include peptide-trifluoromethyl ketone inhibitors of human leukocyte elastase (Stein et al., 1987) and peptide-phosphonamidate inhibitors of the metalloprotease ther-molysin (Bartlett and Marlowe, 1983). 

Interest in linkers for carbonyl compounds has only slowly emerged in recent years. The main driving force for the development of such linkers was the need for methods to prepare peptide aldehydes and related compounds (e.g. peptide trifluoromethyl ketones), which can be highly specific and valuable enzyme inhibitors [700,701], and are potentially useful for the treatment of various diseases. 

The peptide aldehyde series demonstrated the potential for carbonyl derivatives to be potent inhibitors of serine proteinases. The 200-fold increase in values which was found in going from an aldehyde (7-1) Table 2.7) to its corresponding methyl ketone (7-2) Table 2.7) [129] implied that the physical properties of the carbonyl group were an important factor in determining the potency of the inhibitor. As a means of increasing both the stability and potency of peptide ketone derivatives Trainor and co-workers [130, 131] and Imperiali and Abeles [132] independently prepared the first peptide trifluoromethyl ketone (TFMK) inhibitors of serine proteinases in... 

Fluoromethyl ketones are one of the most widely used classes of peptidyl a-fluoroalkyl ketones, second only to trifluoromethyl ketones. Peptidyl fluoromethyl ketones are very effective as irreversible inhibitors of cysteine proteases the first reported use of a fluoromethyl ketone compound was the use of Z-Phe-Ala-CH2F as an irreversible inhibitor of cathepsin BJ2,31 Today, many lysine and arginine derivatives have been synthesized as potential inhibitors for trypsin and trypsin-like enzymesJ3 There are four basic methods for the synthesis of peptide fluoromethyl ketones (1) the reaction of HF with peptide diazomethyl ketones (Section 15.1.4.1.1), (2) a halogen-exchange reaction with a chloro-, bromo-, or iodomethyl ketone (Section 15.1.4.1.2), (3) a Henry nitro-aldol condensation reaction (Section 15.1.4.1.3), and (4) a modified Dakin-West acylation reaction (Section 15.1.4.1.4). 

In this section the synthesis of fluoroalkyl (Section 15.1.4.1.3), a,a-difluoroalkyl (Section 15.1.4.2.3), and trifluoromethyl- and perfluoroalkyl ketones are discussed collectively. The second most widely used method for synthesizing peptide fluoromethyl ketones is the Henry nitro-aldol condensation reaction, which involves the use of (3-nitro alcohols to build the fluoromethyl ketones. As with the modified Dakin-West procedure, the Henry reaction has also been used to synthesize mono-, di-, tri-, and extended fluoromethyl ketones, making it another extremely versatile synthetic method.19 12 19 27 29 33 341 However, similar to the Dakin-West procedure, the products of the Henry reaction are not chiral, since an achiral carbanion is involved in the crucial carbon bond forming step. 

Peptidyl fluoromethyl ketones can readily be synthesized by addition of organometallic reagents to peptide aldehydes and other carbonyl compounds 28 one variation of this reaction is suitable for the stereoselective synthesis of trifluoromethyl ketones. 23,31 ... 

The reaction of peptide aldehydes with ICF Zn has the obvious advantage that it allows the stereoselective synthesis of trifluoromethyl ketones since it avoids the use of an achiral carbanion in the carbon bond forming step, a requirement for the previously discussed synthetic procedures. In this method, a trifluoromethyl anion equivalent is reacted with a peptide aldehyde, which already contains the chiral a-carbon (Scheme 7). Provided the... 

Scheme 7 Synthesis of Trifluoromethyl Ketones by Reaction of Peptide Aldehydes with Trifluoroiodomethane/ ZincI31 ...

Table 9 Trifluoromethyl Ketones Synthesized by Reacting Peptide Aldehydes with Trifluoroiodo-methane/Zinc 31l...

Evidence for the tetrahedral intermediate includes a Hammett p constant of+2.1 for the deacylation reaction of substituted benzoyl-chymotrypsins and the formation of tetrahedral complexes with many inhibitors, such as boronates, sulfonyl fluorides, peptide aldehydes, and peptidyl trifluoromethyl ketones. In these last the chemical shift of the imidazole proton is 18.9 ppm, indicating a good low-barrier H-bond, and the pJQ of the imidazolium is 12.1, indicating that it is stabilized by 7.3 kcal mol 1 compared to substrate-free chymotrypsin. The imidazole in effect is a much stronger base, facilitating proton removal from the serine. 

Figure 13.9 Effect of peptide length on inhibition constant K, of trifluoromethyl ketones (Suckling, 1990).

S. Feeney, B. C. Gomes, B. J. Kosmider, G. B. Steelman, R. M. Thomas, E. P. Vacek, J. C. Williams, D. J, Wolanin, and S. A. Woolson, ]. Med. Chem., 38, 98 (1995). Non-Peptidic Inhibitors of Human Leukocyte Elastase. 5. Design, Synthesis, and X-Ray Crystallography of a Series of Orally Active 5-Aminopyrimidin-6-one-Containing Trifluoromethyl Ketones. 

Peptidyl fluoromethyl ketones are widely used as fairly potent inhibitors for a variety of proteases, including serine, cysteine, and aspartyl proteases. Unlike other halomethyl ketones (Section 15.1.3), fluoromethyl ketones are reversible transition-state mimics. The electron-withdrawing fluorine(s) next to the carbonyl group enhances the electrophilicity of the a-fluoroalkyl ketone functionality, thereby making the carbonyl more susceptible to nucleophilic attack. a-Fluoroalkyl ketones are good mimics of peptide bonds due to the small size of the fluorine and the stability of C F bonds. There are three general classes of peptidyl fluoromethyl ketones fluoromethyl ketones (irreversible inhibitors of cysteine proteases), difluoromethyl ketones (reversible inhibitors of both serine and aspartyl proteases), and trifluoromethyl/perfluoroalkyl ketones, which typically exist in hydrated forms and are excellent inhibitors of both serine and cysteine proteasesJ1 ... 

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