Difficulties in coupling to conformationally constrained aromatic amino acids

Coupling of amino acids to 1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid (Tic) and 1,2,3,4-tetrahydro-7-hydroxyisoquinoline-3-carboxylic acid (HOTic) is difficult. In model experiments, use of 1-hydroxy-7-azabenzotriazole (HOAt) in combination with either N,N-diisopropylcarbodiimide (DIC) or O-(7-azabenzotriazol-1-yl)-1,1,3,3-Tetramethyluronium (HATU) for activation was effective in solving coupling difficulties. Based on this finding, HOTic was then incorporated into the 20-31 fragment of human epidermal growth factor (hEGF). [Abu^20,31,HOTic²²]hEGF(20-31)-NH₂ was shown to be a 'difficult sequence', but replacement of the Tyr at position 29 with HOTic facilitates the complete dodecapeptide synthesis. Abbreviations: Tic, 1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid; HOTic, 1,2,3,4-tetrahydro-7-hydroxyisoquinoline-3-carboxylic acid; HBTU, 2-(1H-benzotriazol-1-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate; BOP, benzotriazol-1-yl-oxy-tris(dimethylamino)phosphonium hexafluorophosphate; PyBrop, bromo-tris(pyrrolidino)phosphonium hexafluorophosphate; DCC, N,N-dicyclohexylcarbodiimide; DIC, N,N-diisopropylcarbodiimide; HOBt, 1-hydroxybenzotriazole; HOAt, 1-hydroxy-7-azabenzotriazole; HATU, O-(7-azabenzotriazol-1-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate; hEGF, human epidermal growth factor; Boc, t-butyloxycarbonyl; OtBu, t-butyloxy; Bzl, benzyl; OBzl, benzyloxy; 2ClZ, 2-chlorobenzyloxycarbonyl; Fmoc, fluorenylmethyloxycarbonyl; pMBHA, 4-methylbenzhydrylamine; NMP, N-methylpyrrollidinone; DCM, dichloromethane; DMF, N,N-dimethylformamide; DMSO, dimethyl sulfoxide; THF, tetrahydrofuran; DIEA, N,N-diisopropylethylamine; TFA, trifluoroacetic acid; TFMSA, trifluoromethanesulfonic acid; DMS, dimethyl sulfide; HPLC, high-performance liquid chromatography; ACN, acetonitrile.