The 2-oxazolines, a sub-class of the azoles, consist of a 5-membered ring system consisting of one O- and one N-atom separated by a single carbon; these latter two atoms being formally sp² hybridised [1,2]. The group itself represents an important heterocyclic functionality as these molecules are routinely used as ligands in coordination chemistry [3-6] and catalysis [7-10], as directing groups in chemical modification strategies [11-14] and as a protecting group for carboxylic acids [15-18]. The ring is also found in a number of natural products, many of which have been the subject of total synthesis. Examples of these include complex molecules such as Basilibactin A down to very simple small molecules such as Oxytriphine (1 and 2, respectively: Figure 1) [19-21].

Sometime ago, Tohda and co-workers presented [22] a facile two-step strategy for the synthesis of enol-containing oxazolines initiating from 2,4,4-trimethyl-2-oxazoline (3). This protocol (Scheme 1) involves the (i) treatment of 3 with base (NEt₃) in the presence of two equiv. of a benzoyl- or alkoyl-chloride to yield an intermediate ethanone. This latter compound is then hydrolysed with excess alcoholic base (ii) to yield the desired enol-oxazoline product (5: Scheme 1). Materials such as 4 have found application in the production of other heterocycles [23] and as general synthetic intermediates [24-27]. A compound of general formula 4 (Scheme 1) has not previously been the subject of study via single crystal X-ray diffraction. In this report, we detail the solid-state properties of an example of crystalline 4 in which the R group (Scheme 1) is −C₆H₄OCH₃-p (i.e., 4: derived from 4-anisic acid).

As noted above, the title compound is the first crystallographically characterised example of this class of tertiary amides; solvent free crystals of 4 were obtained from equal volume mixtures of C₇H₈ and hexanes. A list of selected bond lengths and angles appears in Table 1. A noteworthy feature of the crystal form is the presence of two structurally distinct molecules of 4 (Molecule A and B) which are found in the unit cell (Figure 2).

These differ primarily in the torsion angles observed and the relative spacing of the two intramolecular aromatic rings (Table 1). Specifically for Molecule A, the O2A-C7A-C8A-C9A, C6A-C7A-C8A-C13A and N1A-C15A-C16A-C17A torsions are all narrower (20.6(5)°, 22.0(6)° and 33.9(5)°, respectively) than their respective angles observed for Molecule B (24.2(6)°, 28.8(5)° and 38.2(5)°, respectively). In the case of Molecule A, the relative spacing and twisting of the two aromatic rings results in a longer C22A•••O3A distance (6.66 Å) than that observed for the same spacing in B (6.27 Å). The heterocyclic rings in both molecules display deviations from planarity of 9.7° (Molecule A) and 8.0° (Molecule B) [21] as measured by their respective N1-C1-O1-C3 torsion angles. The bond lengths observed (Table 1) for the various functional groups are well within the expected ranges for such bonds and hence are otherwise unsurprising [28]. A crystal packing diagram is found in Figure 3. This molecule was also subjected to examination of its gas-phase structure using Density Functional Theory (DFT) at the b3lyp 6-311++G** level of sophistication. The calculated bond lengths and angles also appear in Table 1. The DFT measurements do a reasonable job at mimicking the solid-state structure (Table 1); interestingly rotation of the C6-C7 bond to a situation in which the C7-O2 ketone functionality is rotated approximately 180° relative to that of Molecules A or B is found to be a more stable gas-phase configuration by approximately 3.0 kJ/mol. The gas-phase form is also predicted, perhaps not surprisingly, to have a longer C22•••O3 distance (d_calc = 9.00 Å). Of the three torsion angles mentioned above, the gas-phase form is closer to that of Molecule A with the respective values of 15.8°, 20.1° and 22.0°. The calculated deviation from planarity of the heterocycle (estimated similarly via the N1-C1-O1-C3 torsion, vide supra) is 12.5°.

The crystal and molecular structure of the title compound, viz., (2Z)-2-[3-(4-methoxybenzoyl)-4,4-dimethyl-1,2,-oxazolidin-2-ylidene]-1-(4-methoxyphenyl)ethanone (4), has been reported. These data represent the first crystallographically characterised example of this class of tertiary amides. The gas phase structure of 4 has also been estimated by DFT and the calculated parameters are in good agreement with those observed in the solid-state form.