[2-(4-Chlorophenyl)sulfonylethoxycarbonyl (Cps) amino-protecting group for the synthesis of peptides in a solution. Synthesis of [Leu]enkephalin and its [D-ala2]analog].

Solution syntheses of [Leu]enkephalin and its [D-Ala2]analogue were accomplished using a new 2-(4-chlorophenyl)sulfonylethoxycarbonyl (Cps) base-labile group for amino protection and a chromogenic acid-labile 4-(4-phenylazo)benzyloxybenzyl (Abz) group for carboxyl protection. The syntheses were performed by stepwise attachment of Cps-amino acids, which were introduced as pentachlorophenyl esters or as dicyclohexylammonium salts in the presence of tris(dimethylamino)chlorophosphonium perchlorate. To remove Cps-group, Abz-esters of Cps-peptides were treated with two molar equivalents of 1,8-diazabicyclo[5.4.0]undec-7-ene in dimethylformamide followed by neutralization of the base with an excess of 1-hydroxybenzotriazole; the deblocked amino components were then used without isolation. Fully deblocked pentapeptides were purified and characterized by HPLC, FAB mass spectra and amino acid composition.