Abstract
Benzoins of the general formula, R-CO-CHOH-R , readily combine with phthalic anhydride in the presence of a tert. base and the resultant esters form salts with alkaloids which are readily separable by fractional crystallisation into diastereoisomeric forms. Decomposition of these alkaloidal salts with ice-cold dilute hydrochloric acid liberates optically active benzoin hydrogen phthalates: these, in turn, by hydrolysis with very dilute aqueous alcoholic sulphuric acid yield the optically pure benzoins. 4-Methoxybenzoin was prepared by condensation of benzaldehyde with anisaldehyde in presence of potassium cyanide in aqueous ethanolic solution; 4-dimethylamino-benzoin was similarly prepared by condensation of benzaldehyde with dimethylamino benzaldehyde. Both these benzoins were also prepared by the "reversion" procedure. 4'-Methoxybenzoin was prepared from 4-methoxybenzoin as follows:-4-Methoxybenzoin was reduced by means of tin and hydrochloric acid to its desoxybenzoin, which was brominated in the presence of a 500 watt tungsten lamp and the resulting ?-bromodesoxybenzoin was hydrolysed by heating with water in presence of dioxan as solvent. Pivaloin (hexamethylacetoin) was obtained by the condensation of two molecular proportions of ethyl pivalate in the presence of sodium followed by hydrolysis. The pivalic acid was obtained by carbonation of tertiary butyl magnesium chloride. Whereas optically active hydrogen phthalates of secondary alcohols are generally hydrolysed by means of alkali to obtain the active carbinols; basic hydrolysis of optically active hydrogen phthalates of benzoins gives racemic benzoins instantaneously. The hydrogen phthalates of benzoins were therefore hydrolysed by means of very dilute aqueous ethanolic sulphuric acid (Acid hydrolysis with acyl-oxygen fisson). Attempts to obtain fractionally crystallisable aikaloidal salts of 4'-methoxybenzoin and pivaloin hydrogen phthalates proved to be fruitless.