Use of the SPhos Ligand to Suppress Racemization in Arylpinacolboronate Ester Suzuki Couplings Involving α-Amino Acids. Synthesis of Biaryl Derivatives of 4-Hydroxyphenylglycine, Tyrosine, and Tryptophan

alpha-Amino acid derivatives, particularly those of phenylglycine, can suffer significant racemization in Suzuki Couplings. When arylpinacolboronate esters are used as Coupling partners this unwanted side reaction can be Suppressed by the use of Pd(OAC)(2) as Pd(0) source, in the presence of Buchwald's SPhos ligand. The syntheses of biaryl amino acids of tyrosine, phenylglycine, and tryptophan, including Phg-Trp units similar to those found in the chloropeptin Family of natural products, are reported.