Crystal conformations and molecular packing of (S)-2-methoxy-2-(9-phenanthryl)propanoic acid and a diastereomeric amide prepared from (R)-2-methoxy-2-(1-naphthyl)propanoic acid

This paper reports the first evidence of the contribution of T-shaped aromatic CH center dot center dot center dot pi interactions to the crystallization of 2-aryl-2-methoxypropanoic acid and amide. The crystal structures of (S)-2-methoxy-2-(9-phenanthryl)propanoic acid [(S)-M9PP acid, (S)-3] and a diastereomeric amide 6, prepared from (R)-2-methoxy-2-(1-naphthyl)propanoic acid [(R)-M alpha NP acid, (R)-1] and (S)-1-(1-naphthyl)ethylamine [(S)-4], were determined by X-ray crystallography. Like those of M9PP and M alpha NP esters, the carbonyl and methoxy groups of (S)-3 exhibited syn conformation. The crystal structure of (S)-3 was stabilized by consecutive intermolecular OH center dot center dot center dot O hydrogen bonds between the carboxy groups. In addition, the 9-phenanthryl groups formed a herringbone structure via aromatic CH center dot center dot center dot pi interactions. The anti conformation of the carbonyl and methoxy groups was observed in the crystal of amide 6, indicating an NH center dot center dot center dot O hydrogen bond. The two 1-naphthyl groups were parallel to one another and located on the same side of the M alpha NP plane, forming a herringbone structure with adjacent molecules. NMR analyses suggest that, in solution, intramolecular aromatic CH center dot center dot center dot pi interactions induce the 1-naphthyl groups to form a slant-parallel conformation. Namely, both the length and interactions between the substituents are important for crystallization. Normal-phase HPLC separation of diastereomeric amides is also discussed.