tert-Butyl (2S)-2-{3-[(R)-bis(tert-butoxycarbonyl)amino]-2-oxopiperidin-1-yl}-3-methylbutanoate1

The title compound, C24H42N2O7, is a chiral lactam-constrained amino acid with a six-membered ring backbone and isopropyl and tert-butyl ester side chains. The conformation of the six-membered ring can be described as a half chair, with two CH2 C atoms lying 0.443 (1) and −0.310 (1) Å out of the best plane of the other four atoms (mean deviation = 0.042 Å). Both N atoms are sp 2 hybridized, lying 0.0413 (9) and 0.067 (1) Å out of the planes defined by the three C atoms bonded to them. The absolute configuration was determined, based on resonant scattering of light atoms in Cu Kα radiation.

The title compound, C 24 H 42 N 2 O 7 , is a chiral lactamconstrained amino acid with a six-membered ring backbone and isopropyl and tert-butyl ester side chains. The conformation of the six-membered ring can be described as a half chair, with two CH 2 C atoms lying 0.443 (1) and À0.310 (1) Å out of the best plane of the other four atoms (mean deviation = 0.042 Å ). Both N atoms are sp 2 hybridized, lying 0.0413 (9) and 0.067 (1) Å out of the planes defined by the three C atoms bonded to them. The absolute configuration was determined, based on resonant scattering of light atoms in Cu K radiation.

Related literature
For synthesis and chemical interest, see: Oguz (2003); Oguz et al. (2001). For a similar structure, see: Valle et al. (1989). For absolute configuration parameters, see: Hooft et al. (2008). We are grateful to Dr Umut Oguz for providing the crystalline sample.

Crystal data
In the title molecule ( Fig. 1), the central six-membered ring adopts a conformation close to the C 2 half chair, with the diad axis bisecting the N1-C1 and C3-C4 bonds. Atoms C1, C2, C5 and N1 are coplanar to within a mean deviation 0.042 Å (maximum 0.0588 (10) Å for N1), the other two atoms lying alternately above and below this plane, C3 by -0.310 (1) and C4 by 0.443 (1) Å. The C5-N1-C1-C2 torsion angle, which would be zero for an ideal half chair, is -13.36 (18)°. This conformation is similar to that seen in a similar lactam-restricted analog of Boc-L-Pro-L-Leu-Gly-NH 2 , which has torsion angle somewhat closer to zero, 5.6 (11)° and smaller mean deviation for these four atoms, 0.014 Å (Valle et al., 1989).

Experimental
The synthesis of the title compound is detailed by Oguz (2003), who prepared a suitable single-crystal by recrystallization from hexanes.

Refinement
Hydrogen atoms were located from difference maps and included in the refinement in riding mode with C-H distances = 0.98 -1.00 Å and U iso (H) = 1.5 U eq (methyl C) or 1.2 U eq (non-methyl C). Refinement of the Flack (1983) parameter was used to determine the absolute configurations of the two asymmetric centers in the molecule. Fig. 1. View of the title molecule (50% probability displacement ellipsoids); H atoms are not shown for clarity.

Refinement
Refinement on F 2 Hydrogen site location: inferred from neighbouring sites Least-squares matrix: full H-atom parameters constrained  (17)