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

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-ammoacyloxymethyl)-3-nitrobenzoic acids (3), which were quantitatively incorporated onto aminomethylcopoly(styrene-1% 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 appropriate 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 with 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.