106 E. I. KLABUNOVSKII 



phone was crj'stallized from a solution ot (-f }-ethyl tartrate that partial optical 

 activation was observed [4-6]. 



By an improvement of the method it has been possible to resolve mandelic 

 acid (o-66°) imder the action of (+)-carvone [7]. A high degree of resolution 

 was achieved by using racemates with surface-active properties, such as cam- 

 phor, terpineol and pinene, by bubbling their vapours through aqueous solutions 

 of tannin, gelatin and leucine [8]. 



Better results are obtained if the resolution is carried out, not in the liquid 

 phase, but on the surface of a sohd adsorbent. 



It is known that diastereoisomers can be separated on optically inactive 

 adsorbents. On alimiinium oxide one may separate the (— )-menthyl ester of 

 DL-mandeUc acid [9] and the (— )-i5opropanolamide of DL-/5olysergic acid [10], 

 and Ar-benzoylcyc/oheptano-2 : 3-pyrrolidine, while on glucose one may separate 

 the brucine salt of DL-mandehc acid [11]. 



Similar behaviour is manifested by the superficial diastereoisomers formed by 

 the interaction of a racemate with a layer of optically active substance carried 

 on an inert adsorbent. Thus carbon, silica gel and aluminium oxide, when 

 carrying a layer of optically active alkaloid, are effective agents for the resolution 

 of mandelic acid [12]. 



Highly specific adsorbents of this typt have recently been made [13]. 



Curti & Colombo [14] used silica gel, precipitated in the presence of (+)- 

 camphorsulphonic acid. Using the artificial optically active adsorbent prepared 

 in this way, they succeeded in obtaining a 30% resolution of camphorsulphonic 

 acid and a 10% resolution of mandelic acid. 



Grubhofer & Schleith [15] increased the efficiency of such agents considerably 

 by using an ion-exchange resin, the carboxyhc acids of which had been esterified 

 with quinine. The quinine-containing resin (25% quinine) thus obtained would 

 resolve racemic acids almost completely. 



In this latter case, when the optically active agent is chemically associated with 

 the carrier, the mechanism of separation is based on the asymmetric adsorption 

 of the antipodes of the racemate; while in the previous cases, the resolution was 

 produced by selective solution of the antipodes in a layer of optically active 

 compound on an inert adsorbent. 



Natural dissymmetric adsorbents have also been studied in connection with 

 the resolution of racemates, for example, wool, silk, polysaccharides, colloidal 

 adsorbents [16] and quartz. 



The first attempts to demonstrate the differential adsorption of the antipodes 

 of tropacocaine, atropine and homatropine on the protein of wool or silk were 

 made by Willstatter [17] in 1904. 



The selective adsorption of antipodes on wool was discovered in the case of 

 racemic azo-dyes but the degree of resolution was insignificant and the findings 

 could not always be repeated (Ingersoll, Adams, Brode, 1922-26 [18]). Slight 

 selectivity is observed when the (-f )- and (— )-forms of an acid having the 

 formula 



(CH3)2N • C6H4 • N :N • C6H4 • CO • NH • CH(CgH5) • COOH 



are adsorbed on serum protein [19], and also when strains are adsorbed on 



