An acid-labile anchoring linkage for solid-phase synthesis of C-terminal peptide amides under mild conditions

A series of polymer-supported benzylamides substituted with one to three alkoxy groups in the ring positions were prepared and shown to give carboxamides upon treatment with acid. Based on the initial screening, the bis(o-methoxy)-p-alkoxybenzylamide anchoring linkage was selected for a detailed evaluation of its suitability for solid-phase synthesis of C-terminal peptide amides. The handle derivative 5-[(2' or 4')-Fmoc-aminomethyl-3',5'-dimethoxyphenoxy]valeric acid (1) was prepared in seven facile steps [purification of intermediates unnecessary; overall yield 15% for crystalline product, which is a mixture of positional isomers], and was quantitatively coupled onto amino group-containing supports by use of N,N'-dicyclohexylcarbodiimide plus 1-hydroxybenzotriazole in N,N-dimethylformamide. Stepwise elaboration of peptide chains proceeded smoothly with both N alpha-9-fluorenyl-methyloxycarbonyl (Fmoc) and N alpha-dithiasuccinoyl (Dts) amino acids, and final cleavage of tert.-butyl side-chain protecting groups and of the anchoring linkage occurred readily in trifluoroacetic acid-dichloromethane (7:3) at 25 degrees. The methodology was demonstrated by the syntheses of H-Trp-Asp-Met-Phe-NH2 (tetragastrin) and H-Tyr-Gly-Gly-Phe-Met-NH2 (methionine-enkephalinamide), both with high yields and purities.