N-Allyl-N-( 2-nitrobenzenesulfonyl )-L-leucine methyl ester

# 2004 International Union of Crystallography Printed in Great Britain ± all rights reserved The structure of the title compound, C16H22N2O6S, has been determined as part of an ongoing investigation into the preparation of N-alkylated amino acid precursors for alkene cross-metathesis reactions for the generation of dynamic combinatorial libraries. The overall molecular conformation is stabilized by intramolecular CÐH O interactions.

The structure of the title compound, C 16 H 22 N 2 O 6 S, has been determined as part of an ongoing investigation into the preparation of N-alkylated amino acid precursors for alkene cross-metathesis reactions for the generation of dynamic combinatorial libraries. The overall molecular conformation is stabilized by intramolecular CÐHÁ Á ÁO interactions.

Comment
As part of our interest in the development of dynamic combinatorial libraries (Cousins et al., 1999;Ramstrom & Lehn, 2000;Bunyapaiboonsri et al., 2001;Lehn & Eliseev, 2001), we have synthesized a range of N-allyl-substituted amino acids as precursors for cross-metathesis of amino acids using Grubbs catalysts (Fu È rstner, 2000;Connon & Blechert, 2003). In this approach, the 2-nitrobenzenesulfonyl group (oNBS) is introduced prior to allylation in order to, ®rstly, protect the nitrogen, and secondly, increase the acidity of the NH proton such that the amide becomes more susceptible to allylation. We have previously reported the structure of N-allyl-N-(2-nitrobenzenesulfonyl)-l-phenylalanine methyl ester (Poulsen et al., 2003). In the present communication, we report the structure of the related compound N-allyl-N-(2nitrobenzenesulfonyl)-l-leucine methyl ester, (I).
The molecules of (I) are separated by normal van der Waals distances, with bond lengths in accord with conventional values (Allen et al., 1987) (Table 1). The conformational structure of (I) (Fig. 1) is very similar to that of the phenylalanine analog, with the shape determined by a number of intramolecular CÐHÁ Á ÁO interactions (Table 2) and thè spiralling' of the 2-nitrobenzenesulfonyl group above the plane of the carboxylate group to bring nitro atom O2 into close proximity to atom C7.

Experimental
Similar to the l-phenylalanine analog, (I) was prepared in accord with published procedures (Reichwein & Liskamp, 2000). To a solution of 2-nitrobenzenesulfonyl-l-leucine methyl ester (9.09 g, 27.5 mmol) were added K 2 CO 3 (7.6 g, 55 mmol) and allyl bromide (3.65 ml, 42 mmol) in anhydrous DMF (120 ml). The reaction mixture was stirred at room temperature for 18 h. Water (200 ml) was added and the mixture was extracted with diethyl ether (3 Â 150 ml). The combined extracts were washed with brine (2 Â 200 ml) and dried over MgSO 4 . The solvent was removed under reduced pressure to give an oily yellow residue. Crystals of (I) suitable for X-ray diffraction studies were obtained by crystallization from a mixture of hexane and ethyl acetate (yield 7.6 g, 83%; m.p. 342 K). 1   Hydrogen-bonding geometry (A Ê , ).  (Farrugia, 1997); software used to prepare material for publication: TEXSAN for Windows and PLATON.

Figure 1
View of the title compound, with the atom-numbering scheme. Displacement ellipsoids for non-H atoms are drawn at the 30% probability level. Molecular Structure Corporation (1999 supporting information . E60, o383-o385 [https://doi.org/10.1107/S1600536804003186] -N-(2-nitrobenzenesulfonyl) Geometry. Bond distances, angles etc. have been calculated using the rounded fractional coordinates. All e.s.d.'s are estimated from the variances of the (full) variance-covariance matrix. The cell e.s.d.'s are taken into account in the estimation of distances, angles and torsion angles 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 > 2σ(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.