N-Butoxycarbonyl-5-oxo-l-proline ethyl ester

The molecular structure of the title compound, C12H19NO5, may be visualized as made up of two nearly perpendicular planes [dihedral angle = 87.39 (12)°] and its crystal structure is a good example of C—H⋯O interactions assuming significance in optimizing supramolecular aggregation in crystals in a molecule which is severely imbalanced in terms of donors to acceptor atoms. The pyrrolidine ring adopts a (3 T 2) twist conformation with puckering parameters Q = 0.2630 (4) Å and ϕ = 59 (9)°. The crystal structure features R 2 4(10) and R 3 4(26) ring motifs formed by four weak C—H⋯O interactions, leading to supramolecular sheets lying parallel to the bc plane.

The molecular structure of the title compound, C 12 H 19 NO 5 , may be visualized as made up of two nearly perpendicular planes [dihedral angle = 87.39 (12) ] and its crystal structure is a good example of C-HÁ Á ÁO interactions assuming significance in optimizing supramolecular aggregation in crystals in a molecule which is severely imbalanced in terms of donors to acceptor atoms. The pyrrolidine ring adopts a ( 3 T 2 ) twist conformation with puckering parameters Q = 0.2630 (4) Å and ' = 59 (9) . The crystal structure features R 2 4 (10) and R 3 4 (26) ring motifs formed by four weak C-HÁ Á ÁO interactions, leading to supramolecular sheets lying parallel to the bc plane.
Data collection: APEX2 (Bruker, 2009); cell refinement: SAINT (Bruker, 2009); data reduction: SAINT; program(s) used to solve structure: SHELXTL (Sheldrick, 2008); program(s) used to refine structure: SHELXTL; molecular graphics: SHELXTL; software used to prepare material for publication: SHELXTL and PLATON (Spek, 2009). derivatives are known for their active role in biological functions. For instance, 5-oxo-proline or pyroglutamic acid, found in many proteins including bacteriorhodopsin, acts as a proton pump, captures light energy and uses it to move protons across the membrane out of the cell (Hayashi, et al., 2003;Nishikawa, et al., 2005). Also, N-boc-4-oxo-L-proline ethyl ester is a part of the starting material on stereoselective synthesis of peptide hormone cholecystokinin (Holladay, et al., 1991). The present paper describes the accurate description of the crystal structures of N-boc-5-oxo-L-proline ethyl ester (Fig.1).
The crystal structure of the title compound is a simple example which demonstrates the importance of weak C-H···O interactions assuming significance in optimizing supramolecular aggregation in crystals. The molecule may be thought of as highly imbalanced in terms of donors to acceptor atoms which has at least three carbonyl O atoms O3, O4 and O5, out of the total five O atoms (O1→O5), available for participation in intermolecular interactions. O1 and O2 of the respective ethyl ester and the tert-butyloxy groups do not participate in the hydrogen-bonding environment owing to unfavourable steric reasons. The intermolecular interaction patterns may be vizualized as molecular chains interconnected to each other to form a sheet. The ethyl C2 and ethyl C3 atoms act as donors to the carbonyl O3 which is a bifurcated acceptor at (x, y -1, z) and (-x + 2, y -1/2, -z + 1/2), respectively. The associated graph-set motif (Bernstein et al., 1995) is a R 4 2 (10) ring through C2-H2A···O3 and C3-H3A···O3 hydrogen bond leading to chains parallel to the b-axis (Fig.2). The ethyl C6 and ethyl C12 atoms act as donors to the bifurcated acceptor (carbonyl) O5 at (x, y, z + 1) and (x, y -1, z), respectively, forming a R 4 3 (26) ring motif through a C6-H6B···O5 and C12-H12A···O5 hydrogen bonds forming sheets parallel to the bc-plane (Fig.3).
The resulting yellow solution was stirred at room temperature for 2 h. The reaction mixture was concentrated. The residue was purified by column chromatography to afford boc-oxo-L-proline ethylester (0.8 g, 95%). Crystals of the title compound were grown from its solution in ethanol by slow evaporation at room temperature.

Refinement
All the hydrogen atoms were placed at geometrically calculated positions. They were allowed to ride on respective parent atoms with U iso values constrained to 1.2 times U eq (1.5 times for ethyl H atoms) and the target C-H distance fixed at 0.96 Å for ethyl hydrogen atoms and 0.93 Å for all others.

Computing details
Data collection: APEX2 (Bruker, 2009); cell refinement: SAINT (Bruker, 2009); data reduction: SAINT (Bruker, 2009); program(s) used to solve structure: SHELXTL (Sheldrick, 2008); program(s) used to refine structure: SHELXTL (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXTL (Sheldrick, 2008) and PLATON (Spek, 2009).   Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.