Resin-bound DCC

A drastically different method for identification of library members is possible when individual building blocks are bound to a solid-phase resin. This approach has been developed by Miller et al. who set up multiphase DCLs by mixing solution-phase building blocks with identical resin-bound ones and a fluorescent target [8, 16]. 

This is a consequence of competition between resin-bound and solution-phase disulfides for the limiting amount of fluorescent target. With 13 on the resin, immobilized 13-11 competes with solution-phase 11-11, while in the reverse experiment no 11-11 can be formed. Thus, the outcome suggested that 11-11 is a stronger binder than 13-11, which was subsequently confirmed by bioassays on isolated material. 

The resin-bound DCL strategy lends itself to further increases in Ubrary size, in particular when diversity is generated from binary combinations of monofunction-ahzed building blocks. Usage of multifunctionalized building blocks may also be possible, but will require more elaborate deconvolution experiments. 

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Keywords DNA G-quadruplex Nucleic acid recognition Resin-bound DCC RNA Transition metal complex... 

To CH2C12 (5mL) was added DCC (824 mg, 4 mmol) followed by the dropwise addition with stirring of formic acid (0.3 mL, 8 mmol). After 15 min the precipitate was removed by filtration and the filtrate used for resin-bound peptide formylation. 

To a soln of Boc amino acid (8 equiv) in CH2CI2 at 4 °C DCC (4 equiv) was added and the mixture left for 5-15 min at 4 C. The soln was filtered and the filtrate diluted with DMF and added to the resin-bound amino component. After 30 min, the resin was filtered and washed with DMF. 

Figure 2.2 Modern solid phase peptide synthesis. Process begins with a-N terminal Fmoc deprotection of resin bound C-terminal amino acid residue with piperidine (mechanism illustrated). Peptide link formation follows (typical solvent Al-methylpyrrolidone [NMP]) by carboxyl group activation with dicyclohexylcarbodiimide (DCC) (mechanism illustrated) in presence of hydroxybenzotriazole (HOBt). HOBt probably replaces DCC as an activated leaving group helping to reduce a-racemization during peptide link formation. Other effective coupling agents used in place of DCC/HOBt are HBTU 2-(lH-benzotriazol-l-yl)-l,l,3,3-tetramethyluronium hexafluorophosphate Py-BOP benzotriazole-l-yl-oxy-tns-pyrrolidino-phosphonium hexafluorophosphate. The Process of a-N deprotection, and peptide link formation, continues for as many times as required (n-times), prior to global deprotection and resin removal.

Oxime resin 125 in DMF was treated with 5 equiv of Boc-amino acid 126 and 5 equiv of DCC in DCM solution prepared at 0°C (Scheme 9.17) " and then shaking the mixture at room temperature for 24 h afforded resin-bound Boc-amino acid 127. After removal of the Boc group with 25% TFA in DCM for 30 min, the deprotected amino acid was swollen in anhydrous DCM and treated with 4 equiv of DIEA and t-butyl chlorosulfonylcarbamate that was prepared by adding t-butyl alcohol in DCM to chlorosulfonyl isocyanate (CSI) 128 in DCM at 0°C. Treatment of 129 with 25% TFA in DCM for deprotection followed by... 

Then N-Boc-O-benzylserine is coupled to the free amino group with DCC. This concludes one cycle (N° -deprotection, neutralization, coupling) in solid-phase synthesis. All three steps can be driven to very high total yields (< 99.5%) since excesses of Boc-amino acids and DCC (about fourfold) in CHjClj can be used and since side-reactions which lead to soluble products do not lower the yield of condensation product. One side-reaction in DCC-promoted condensations leads to N-acylated ureas. These products will remain in solution and not reaa with the polymer-bound amine. At the end of the reaction time, the polymer is filtered off and washed. The times consumed for 99% completion of condensation vary from 5 min for small amino acids to several hours for a bulky amino acid, e.g. Boc-Ile, with other bulky amino acids on a resin. A new cycle can begin without any workup problems (R.B. Merrifield, 1969 B.W. Erickson, 1976 M. Bodanszky, 1976). 

The details of the reaction mechanism with DCC were given in Chapter 43, p. 000, and can be shown more easily If we mark the polymer and spacer as P and the cyclohexyl groups as R . The DCC is protonated by the free carboxylic acid and is then attacked by the carboxylate anion. The intermediate is rather like an anhydride with a C=NR group replacing one of the carbonyl groups. It is attacked by the amino group of the polymer-bound amino acid. The by-product is dicyclohexy-lurea, which Is washed off the column of resin. 

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