Glycocluster Synthesis by Native Chemical Ligation

Abstract

Serine-and mannoside-derived thioesters and a mannosidic cysteine derivative were prepared to test native chemical ligation (NCL) for protecting-group-free synthesis of small glycopeptide clusters, which are valuable tools in the glycosciences. The glycopeptides and glycopeptide clusters, respectively, which were obtained via NCL, bear the potential for dimerization and other derivatization of the thiol functionality.

S-Fluorenylmethyl protection of the cysteine side chain upon Nα-Fmoc deprotection

“…The O -acetylated mannopyranside 4 can be prepared by Staudinger ligation of O -acetyl-protected 2-azidoethyl α-D-mannopyranoside [11] and the cysteine derivative Fmoc-Cys(Trt)-OH as described earlier [1]. Then, a sequence of Fmoc-deprotection, leading to 5 and acidic removal of the trityl group by using TFA and triethylsilane (TES) as cation scavenger [12] yields the O -acetylated glycoamino acid derivative 6 together with its respective disulfide (not shown in Scheme 2), and de- O -acetylation under Zemplén conditions [13] furnishes the unprotected compound 3-dimer after oxidation in air, as reported previously [3]. However, when the trityl group in 4 was removed first, thiol 7 was obtained as expected, but the following Fmoc deprotection under standard conditions [14], employing 6 equiv of morpholine in DMF, unexpectedly led to the S -fluorenylmethyl (Fm)-protected glycoamino acid 8 in 77% yield (Scheme 2).…”

“…Glycoamino acid derivatives 1 [1], 2 [2], 3 [3], 4 [2], 5 [2] and 6 [3] were prepared according to the literature. Anhydrous DMF was purchased; other solvents were dried for reactions or distilled for chromatography.…”

“…The crude product was subjected to size-exclusion chromatography (LH-20, methanol) yielding 3-dimer (377 mg, 580 µmol, 66%) as a colorless lyophylisate. Analytical data are according to the literature [3]. …”

“…In the course of our work on the synthesis of glycoamino acids, we have recently used L-cysteine as a scaffold for the synthesis of various glycoclusters [1–3]. This is an attractive concept, because it can be combined with solid-phase peptide synthesis (SPPS) [4–5], as well as with native chemical ligation (NCL) utilizing an S → N acyl shift [3,6–10].…”

“…This is an attractive concept, because it can be combined with solid-phase peptide synthesis (SPPS) [4–5], as well as with native chemical ligation (NCL) utilizing an S → N acyl shift [3,6–10]. In addition, glycocysteine derivatives can be easily converted into the corresponding dimers by oxidiation of the cysteine moiety into the respective cystine form.…”