DEPBT

DEPBT is a coupling reagent that causes very little epimerization during coupling1 . It is especially useful for coupling easily epimerized amino acids such as arylglycines.2 DEPBT was also shown to be a superior reagent for head-to-tail cyclization of linear peptides.3 Solution Phase Coupling2,3 1. Dissolve the protected amino acid and amino acid ester in THF (tetrahydrofuran) (approximately 2 ml per mmole of amino acid ester). 2. Add 1.1 –1.2 equivalents of DEPBT and 2.0 equivalents of triethylamine (Et3N) or diisopropylethylamine (DIPEA). If the coupling involves amino acids highly suceptible to epimerization, cool the mixture to 0˚ C before adding the DEPBT and use sodium carbonate in place of triethylamine. 3. Stir for 1 to 2 hours. 4. Filter to remove any solid. Rinse solid with a little THF. 5. Evaporate in vacuo to obtain the crude peptide.

Solid Phase Coupling3

1. Suspend the resin in dichloromethane (DCM) or N,N-dimethylformamide (DMF) (approximately 10 ml per gram of resin). 2. Add 1.5 equivalents (based on resin substitution) of the the protected amino acid.

3. Add 3 equivalents of DIPEA or Et3N.

4. Add 1.5 equivalents of DEPBT and shake the mixture at room temperature for 1-2 hours.

5. Filter the resin.

6. Wash the resin 3 times with DMF then 3 times with DCM.

Cyclization

1. Prepare a 1x10-3 M to 2x10-3 M solution of the linear peptide in DMF.

2. Cool the solution in an ice bath and add 4 equivalents of Et3N and 2 equivalents of DEPBT.

3. Stir 30 minutes at 0˚ C, then warm to room temperature and stir for 24 hours.

4. Evaporate the DMF in vacuo.

5. Purify the residue by column chroatography to obtain the crude peptide.