N-(Fluoren-9-ylmethoxycarbonyl)-l-leucine

The title compound [systematic name: fluoren-9-yl N-(1-carboxy-3-methylbutyl)carbamate], C21H23NO4, exhibits torsion angles that vary from the typical values found in other Fmoc-protected amino acids, viz. the orientations of the fluorene and carboxyl groups [C—O—C—C = 93.8 (2) and N—C—C=O = −23.6 (2)°]. The crystal structure exhibits two intermolecular hydrogen bonds (O—H⋯O and N—H⋯O) that link the molecules into two-dimensional sheets parallel to the ab plane.

The bond distances and bond angles of (I, Fig. 1) are consistent with the typical values of Fmoc-protected amino acids found in the other crystal structures. Some torsion angles, however, are found to be quite different. The torsion angle of O2-C6-C1-N1, for example, is -23.6 (2)°, which is in disagreement with the previous observations in the Fmoc-protected amino acids in which the corresponding angles are 150.6° and 175.8° for (II) (Valle et al., 1984) and (III) (Yamada et al., 2008), respectively. Another example is that the torsion angle of C6-C1-N1-C7 in (I) is found to be -134.51 (15)°, which is in reasonable agreement with that of (II), -151.6°, but is inconsistent with that found in (II), -65.6°. Each angle between the fluorene ring and the NC(δb O)O plane is found to be different among the three Fmoc-protected amino acids.
Crystals of (I) contain two intermolecular hydrogen bonds (Table 1), which are formed between the carboxyl (O2-H2H) and amide oxygens (O3), and between the amide (N1-H1N) and the carbonyl (O1). The molecules are linked by O2-H2H···O3 hydrogen bonds to form a chain structure along the b axis. The linkage is supported by an additional C-H···O interaction (C8-H8A···O1). The chains are joined together by the N1-H1N···O1 hydrogen bonds to form a sheet structure parallel to the ab plane. The Fmoc and i-butyl moieties are packed between the sheets (Fig. 2).

Experimental
A powdered sample (I) was obtained from Wako Pure Chemical Industries, Ltd. (Osaka, Japan) and was used for crystallization without further purifications. Colourless needle like crystals of (I) were slowly grown from a saturated dichloromethane solution.

Refinement
All H atoms were found in difference maps an refined with isotropic thermal parameters.
supplementary materials sup-2 Figures Fig. 1. A view of the molecular structure of (I), showing the atom labeling scheme. Displacement ellipsoids are drawn at the 50% probability level. Fig. 2. A packing diagram of (I) viewed. The hydrogen atoms were omitted for clarity, except for those forming the hydrogen bonds. Broken lines indicate the hydrogen bonds.