Crystal structure of methyl 5-acetamido-2-t-butyl-2-methyl-1,3-benzodioxole-4-carboxylate.

1,3-benzodioxle-4-carboxylic acid] was designed as a novel class of chiral fluorometric agents.1 The chiral agent and the analogues have been conveniently applied to enantiomer analyses of various bio-molecules using HPLC and circular dichroism.2 We recently extended the molecular design of (S)TBMB carboxylic acid to chiral nitrobenzoic acids possessing the asymmetric 1,3-benzodioxole.3 The chiral nitrobenzoic acids are designed to function as ambidentate fluorescent ligands for constructing artificial βand γ-peptide linkages. The asymmetric nitrobenzoic acids were prepared via the nitration of racemic TBMB carboxylic acid, followed by optical resolution by chiral HPLC column chromatography. The nitration reaction produced two nitro-TBMB carboxylic acids, which were tentatively identified as oand m-nitro-derivatives. In order to establish the chemical structures, particularly of the o-nitro isomer, we studied the crystal structure of the titled compound, which was derived from methyl o-nitro-TBMB carboxylate via reduction of the nitro group followed by N-acetylation. A pale-yellow plate crystal of C16H21NO5 (Mw = 307.35) having an approximate dimension of 0.258 × 0.30 × 0.10 mm was obtained from hexane and mounted on a glass fiber. All measurements were made on a Rigaku AFC7R diffractometer with graphite-monochromated Cu-Kα radition and a rotating anode generator. The cell constants and an orientation matrix for data collection were obtained from a least-squares refinement for 25 reflections centered in the range 61.15 ̊ < 2θ < 78.69 ̊. The data matched with a primitive monoclinic cell with the dimensions listed in Table 1. The structure was solved by direct methods and expanded using Fourier techniques. All non-hydrogen atoms were analyzed with anisotropic temperature factors, in which a few atoms, such as O11 and O12, with low occupancy were also refined isotropically. All of the hydrogen atoms, except for an N–H, were calculated from the standard bond lengths and angles, and refined isotropically. The final cycle of the full-matrix least-squares refinement was based on 6620 all unique reflections and 451 variable parameters. The structure was finally converged with unweighted and weighted agreement factors of R(F) = 0.058 (I > 2σ(I)) and wR(F2) = 0.198. In one asymmetric unit, there existed two different molecules, I and II in 1:1 (Figs. 1 & 2), though little difference could be observed between the two concerning the bond length, the torsion angles, and the dihedral angles. The N–H hydrogen atom in both of I and II was involved in an intramolecular hydrogen bond to an ester –C=O with a distance of 1.88 and 1.87 Å, respectively. Though the phenyl ring was almost coplanar, the bond lengths of C2–C3 (1.430 Å) in I and C17–C18 (1.431 Å) in II were longer than the other bonds in the aromatic ring (Table 2). All of non-hydrogen atoms in the carboxyl and amide groups were located in a coplanar relation with the 685 ANALYTICAL SCIENCES MAY 2001, VOL. 17 2001 © The Japan Society for Analytical Chemistry