Solution- and solid-state conformations of C(α)-alkyl analogues of methylphenidate (Ritalin) salts: avoidance of gauche(+)gauche(-) interactions

Preparation and structural analysis of (±)-threo-ritalinic acid

Hydrolysis of the methyl ester (±)-threo-methyl phenidate afforded the free acid in 40% yield, viz. (±)-threo-ritalinic acid, C13H17NO2. Hydrolysis and subsequent crystallization were accomplished at pH values between 5 and 7 to yield colourless prisms which were analysed by X-ray crystallography. Crystals of (±)-threo-ritalinic acid belong to the P21/n space group and form intermolecular hydrogen bonds. An antiperiplanar disposition of the H atoms of the (HOOC-)CH-CHpy group (py is pyridine) was found in both the solid (diffraction analysis) and solution state (NMR analysis). It was also determined that (±)-threo-ritalinic acid conforms to the minimization of negative gauche(+)-gauche(-) interactions.