Ferrocenoyl Amino Acids:  A Synthetic and Structural Study<sup>†</sup>

A series of ester-protected amino acids were coupled to ferrocenecarboxylic acid (<b>1</b>) using the DCC/HOBt protocol to give ferrocenoyl <i>N</i>-amino acids (amino acid = Glu(OBz)<sub>2</sub> (<b>2a</b>), Gly(OEt) (<b>2b</b>), Pro(OBz) (<b>2c</b>), Cys(SBz)OMe (<b>2d</b>), Ala(OBz) (<b>2e</b>), Tyr(OBz) (<b>2f</b>), Phe(OBz) (<b>2g</b>)). All products were fully characterized. The intermediate hydroxybenzotriazole active ester FcCOOBt (<b>3</b>) was isolated and fully characterized. The solid state structures of <b>2a</b>, <b>2d</b>, and <b>3</b> were determined by single-crystal X-ray diffraction. <b>2a</b>:  monoclinic <i>P</i>2<sub>1</sub> with <i>a</i> = 11.8142(5) Å, <i>b</i> = 9.7560(5) Å, <i>c</i> = 22.9456(10) Å, β = 90.246(5)°, <i>V</i> = 2644.7(2) Å<sup>3</sup>, <i>Z</i> = 2, <i>R</i> = 0.046. <b>2d</b>:  orthorhombic <i>P</i>2<sub>1</sub>2<sub>1</sub>2<sub>1</sub> with <i>a</i> = 9.957(2) Å, <i>b</i> = 11.680(2) Å, <i>c</i> = 36.452(2) Å, <i>V</i> = 4239.5(13) Å<sup>3</sup>, <i>Z</i> = 4, <i>R</i> = 0.065. The solid state structures of <b>2a</b> and <b>2d</b> show extensive CO···H−N hydrogen bonding. <b>3</b>:  triclinic <i>P</i>1̄ with <i>a</i> = 7.0391(5) Å, <i>b</i> = 10.7922(7) Å, <i>c</i> = 11.1690(7) Å, α = 108.071(5)°, β = 107.957(5)°, γ = 103.896(5)°, <i>V</i> = 712.5(2) Å<sup>3</sup>, <i>Z</i> = 2, <i>R</i> = 0.030. The long ester bond distance of 1.427(2) Å provides a rationale for its inherent reactivity toward primary and secondary amines.