Synthesis of thiophenylalanine-containing peptides via Cu(I)-mediated cross-coupling

Abstract

Aryl thiolates have unique reactive, redox, electronic, and spectroscopic properties. A practical approach to synthesize peptides containing thiophenylalanine has been developed via a novel Cu(I)-mediated cross-coupling reaction between thiolacetic acid and iodophenylalanine-containing peptides in the solid phase. This approach is compatible with all canonical proteinogenic functional groups, providing general access to aryl thiolates in peptides. Peptides containing thiophenylalanine (pK(a) 6.4) were readily elaborated to contain methyl, allyl, and nitrobenzyl thioethers, disulfides, sulfoxides, sulfones, or sulfonates.

Figure 1

HPLC chromatograms of (a) Ac-T(4-I-Phe)PN-NH₂ peptide starting material (1); (b) peptide after cross-coupling reaction and peptide cleavage and deprotection, showing a mixture of thioacetyl (2) and disulfide (3) products; and (c) the result of treatment of the solution in (b) with DTT to effect thiolysis and reduction of disulfide, showing clean overall conversion to the 4-thiophenylalanine-containing product (4).

Figure 2

Crude HPLC chromatograms of cross-coupling reactions with different peptides. S.M. = residual starting material peptide.

Figure 3

Circular dichroism of native (16, red circles) and Y3(4-SH-Phe) (15, blue squares) trp cage at pH 4.0 (closed) and 7.0 or 8.5 (open). (a) CD data at 4 °C. (b) Thermal denaturation.

Figure 4

UV-Vis spectra of Ac-T(4-SH-Phe)PN-NH₂(4) at pH 4.0 (thiol, red squares) and pH 8.5 (thiolate, blue circles).

Scheme 1

Solution alkylation reactions on 4.^a

Scheme 2

Solid phase alkylation of Ac-TXPN-NH₂.

Scheme 3

Cross-metathesis on Ac-T(4-SAllyl-Phe)PN-NH₂.

Scheme 4

Solution phase peptide oxidation reactions.^a