C-Glycosyl Tyrosines. Synthesis and Incorporation into C-Glycopeptides.

The synthesis of the Fmoc-protected C-glycosyl tyrosines 1 and 2, together with two other related C-glycosyl tyrosines, has been achieved. Key reactions involved (i) the reaction of a glycal with an organozinc reagent (carrying an aryl iodide function) in the presence of a Lewis acid to establish the C-glycosyl linkage and (ii) subsequent cross coupling of the aryl iodide to an alanyl zinc reagent (in the presence of a Pd(0) catalyst) to complete the construction of the alpha-amino acid moiety. Using solid-phase peptide synthesis methods, two units of the mannosyl derivative 1 (shown as L-Tyr[C-Ac(4)-alpha-D-Man]) have been incorporated (with four units of glycine) into the linear hexapeptide 3 which was then converted to the C(2)-symmetric cyclic oligopeptide 4.