TFA Cleavage Reactions

Resins (19) ( 30 mg each) reacted with 5% TFA in DCM. Droplet of suspension was taken at various time intervals for single bead FTIR (Fig. 12.15) and kinetics analysis (Fig. 12.16). The data was also fitted to a first order reaction rate equation and rate constants were determined to be 4.8x10 (5% TFA). Cleavage of carbamides (18), (20), (21), ureas (22-25), amides (26-29), and sulfonamides (30-33) were studied in the same way. 

To compare the cleavage kinetics of various resin-bound compounds, the time course curves representing the best fit were plotted in Fig. 12.17. 

For resin-bound carbamates (18-21), indole linker was the most acid-labile linker for this class of compounds. Rink linker ranked second. Resin (20) was cleaved with 0.5% TFA in 20 min. Resin (21) was cleaved with 1% TFA in 2 min. Resins (18) and (19) required higher concentration of TFA. Resin (18) was cleaved with 1% TFA in 5 h. Alternatively, resin (19) was cleaved with 1% TFA in 12 h and with 5% TFA in only 16 min (not shown). 

For resin-bound ureas (22-25), Indole and Rink linkers, generally ranked 1 and 2 in cleavage kinetics, were still the most acid-labile linker for this class of compounds. Resins (24) and (25) were cleaved with 0.5% TFA in 2 and 23 min, respectively. Resins (22) and (23) were cleaved with 1% TFA in more than 10 h. The order of cleavage rates is similar to carbamate compounds. 

The resin-bound secondary amides (19-22) are the most difficult to cleave among the compounds studied. Resin (29) was cleaved with 1% TFA in 2 h. Resin (28) was cleaved with 1% TFA in 8 h. Resins (26) and (27) required 5% TFA and 8-15 h. This was the only occasion that linker III was not the most labile bond. Rink linker, the second best in all other series, became the most labile bond. 

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Tab. 12.3 l Resins studied in TFA cleavage reactions. Monitoring the Oeavage Completion 517... 

Fig. 12.15 Single bead IR spectra of resins (19) at various times during the TFA cleavage reaction (5% TFA).

Fig. 12.16 Time courses of TFA cleavage reactions for resin (19) obtained by integrations of the carbonyl bands. Lines represent the best fit obtained by PeakFit (Jandel Scientific, San Rafael, CA) analysis on a PC computer.

Fig. 12.17 Kinetic comparison of TFA cleavage reactions of all 16 resin-bound compounds (18-33). The kinetics of cleavage reaction was analyzed as in Figures 12.15 and 12.16. The curves represent the best fit and are displayed for each reaction. The first order reaction rate constants (s ) determined for these reactions are resins (18) (1%), 1.2x10 (19) (5%), 4.8x10 (20) (0.5%), 6.5x10 (21) (1%),...

The most useful phosphoramidites described to date are the di-r-butyl (29) and dibenzyl derivatives (30, 31), as their respective protecting groups are removed during the course of the normal TFA cleavage reaction. The utility of both these reagents in the global phosphorylation of Tyr, Ser, and Thr residues is now thoroughly established (3,4,24,32-35). 

TFA, anisole, 75% yield. " Thioanisole has been used in this cleavage reaction to scavenge the benzyl cation7 Its absence results in considerable alkylation of the indolocarbazole nucleus7 ... 

Final products are cleaved with TFA-H20 (19 1) (1 mL/50mg of resin) at 25°C for 1 h. The filtrate from the cleavage reaction is collected, combined with TFA washes (1 mL/50mg of resin) of the cleaved peptide-resin, and dried. Cleavage yields (>85%) are calculated by amino acid analysis. 

Numerous researchers have employed thiols as weak bases in the thioalkylation reaction to ligate unprotected peptides with a haloacetyl group to form thioethers at pH 7 8.5[90 91 131-133 or thioesters at acidic to basic conditions. 108"110 Of these two reactions, thioether formation is often the choice because thioesters suffer from instability in aqueous basic conditions. Haloacetyl derivatives, either as carboxylic acids or active esters, can be attached to either the N-terminal or side-chain amines during the stepwise solid-phase synthesis of either the peptide or the core and are stable to either HF or TFA cleavage conditions. Capping an amino group with a chloroacetyl group is compatible with Fmoc chemistry when used at a terminal step. 

The Suzuki coupling of arylboronic acids to the solid-supported arylha-lide 4 was initially performed by conventional heating methods, DMF at 85° in an oil bath for 16 h, to provide the desired products 6 after TFA cleavage. Due to the extended heating times necessary for the Suzuki reaction, full optimization of the reaction conditions were not investigated prior to compound library synthesis. Reaction conditions sufficient for the library synthesis were determined within a couple of weeks affording moderate yields of the desired oxazolidinones. Libraries of hundreds of... 

Reaction (414) is formally analogous to the oxidative addition of alkyl halides to noble metal complexes described earlier, and both homolytic and heterolytic processes can be envisaged. Heterolytic cleavage of C—H bonds represented in Eq. (415) is analogous to the interaction of the powerful oxidant Co3+ with alkanes in TFA in reaction (229). 

The cyclization of aspartic acid residues to form aspartimide is the most likely side-reaction observed in routine SPPS (Fig. 10). This is a sequence-dependent side-reaction that occurs either during chain elongation or during final TFA cleavage when Asp(OtBu)-AA sequence (AA = Ala, Gly, Ser, Asn(Trt)) is present in the peptide. Hydrolysis of the aspartimide ring leads to a mixture of both a- and P-peptides. Its reaction with piperidine used for Fmoc removal also leads to the formation of a- and p-piperidides. Normally, in Fmoc-based SPPS, Asp (OtBu) provides sufficient protection. However, for particular sequences such as Asp(OtBu)-Asn(Trt) particularly sensitive to aspartimide formation, addition of HOBt to the piperidine solution or protection of the aspartyl amide bond with the 2-hydroxy-4-methoxybenzyl (Hmb) group should be considered (36). 

TFA cleavage procedure is relatively easy to perform and uses standard laboratory glassware. Caution Perform this procedure in a fume hood. The following procedure is for 100 mg of peptide-resin and is carried out in a reaction vessel. Before the TFA cleavage, the amino terminal of the peptide must be deprotected from the N-a-Fmoc group. 

Stepwise analysis is an efficient and direct method to monitor the progress of side reactions in Fmoc chemistry. The analysis involves stepwise micro-scale TFA cleavage in conjunction with HPLC and MS. Micro-scale TFA cleavage can be conveniently carried out on a small amount of sample. MS analysis of peptide ladders provides a rapid method for monitoring and identification of side reactions in peptide synthesis. 

To the fully derivatized product on the solid support was added excess TFA/CH2CI2 (90 10) and the cleavage reaction was allowed to proceed for 0.5 h at rt. The cleavage solution was then removed by filtration through a cannula and the resin was rinsed with an appropriate solvent (e.g., MeOH/ CH2CI2). Concentration of the combined filtrates provided the crude product 12 in yields > 80% (56-67% isolated yields after purification by RP-HPLC). 

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