1.Identification and suppression of decomposition during carbodiimide-mediated reactions of Boc-amino acids with phenols, hydroxylamines and amino acid ester hydrochlorides.
Benoiton NL1, Lee YC, Chen FM. Int J Pept Protein Res. 1993 Jun;41(6):587-94.
N-tert-Butoxycarbonylamino acids (Boc-Xaa-OH) were coupled with p-nitrophenol (HONp) in dichloromethane using N,N'-dicyclohexylcarbodiimide (DCC) and N-ethyl-N'(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC), and the products were identified and quantitated by high-performance liquid chromatography. Boc-Xaa-OH with Xaa = Val was coupled also with pentafluorophenol (HOPf) and hydroxy-containing additives (HOR), and the products were similarly determined as the methylamides. EDC-mediated reactions of Boc-Xaa-OH gave 8-25% of Boc-Xaa-Xaa-OR as well as Boc-Xaa-OR for R = Ph, Np, Pf, benzotriazole (Bt) and 5-norbornene-endo-2,3-dicarboxamide; DCC-mediated couplings, 5-7% for R = Np and Bt. No dimer was formed in couplings with N-hydroxysuccinimide or 3-hydroxy-4-oxo-3,4-dihydro-1,2,3-benzotriazine. Dimerization was eliminated from DCC-mediated reactions by the addition of 1 equiv. of N-methylmorpholine, from the EDC-mediated reactions by carrying them out in pyridine.
