A Three-Dimensional Orthogonal Protection Scheme for Solid-Phase Peptide Synthesis under Mild Conditions

Article abstract of DOI:10.1021/ja00303a019

Several N^α-dithiasuccinoyl (Dts) amino acids (1) were esterified, by use of N,N'-dicyclohexylcarbodiimide (DCC) and without racemization, to tert-butyl 4-(hydroxymethyl)-3-nitrobenzoate (8).The resultant handle derivatives 4 were treated with trifluoroacetic acid to yield crystalline 4-(N^α-Dts-aminoacyloxymethyl)-3-nitrobenzoic acids (3), which were quantitatively incorporated onto aminomethylcopoly(styrene-1percent divinylbenzene)-resins by DCC-mediated couplings to give the starting point for stepwise solid-phase synthesis of peptides anchored as o-nitrobenzyl (ONb) esters.Assembly of the protected leucine-enkephalin-resin derivative Dts-Tyr(t-Bu)-Gly-Gly-Phe-Leu-ONb-resin (2) was achieved from the appropiate Dts-amino acids by a highly efficient protocol.By carrying out, in each conceivable order, either in solution or on the solid phase, one two, or all three of the following orthogonal treatments <(i) thiolytic removal of the Dts group; (ii) acidolytic cleavage of the tert-butyl ether; and (iii) photolytic cleavage at 350 nm of the ONb ester>, the common resin-bound intermediate 2 became the source of four partially or fully deblocked leucine-enkephalin derivatives.These four, namely Dts-Tyr(t-Bu)-Gly-Gly-Phe-Leu-OH, Dts-Tyr-Gly-Gly-Phe-Leu-OH, H-Tyr(t-Bu)-Gly-Gly-Phe-Leu-OH, and H-Tyr-Gly-Gly-Phe-Leu-OH, were each obtained pure in good yields and were characterized by amino acid composition, HPLC, 300-MHz 1H NMR, and fast atom bombardment mass spectrometry.The protected dipeptidyl sequence Prot-D-Val-L-Pro-ONb-resin was assembled with three different N^α-amino protecting groups and exposed to the recommended deblocking reagents.Loss of chains from the resin by diketopiperazine formation was very rapid with Prot=9-fluorenylmethoxycarbonyl (Fmoc) and also substantial with Prot=tert-butoxycarbonyl (Boc), but rather negligible wit h Prot=Dts.Thus, these experiments demonstrate the feasibility and benefits of a mild three-dimensional orthogonal protection scheme based on Dts for N^α-amino protection, tert-butyl derivatives for side chains, and o-nitrobenzyl esters for anchoring.