STRUCTURAL STUDY OF THE SOLID-STATE RING-CLOSURE REACTION OF PHENYLTHIO)-2-HYDROXY-3-(4-METHOXYPHENYL)PROPIONIC ACID

The crystal structure of the phenylthio)-2-hydroxy-3-(4-methoxyphenyl) propionic acid has been determined from single-crystal X-ray diffracti on data in order to study the solid state ring closure reaction betwee n the carboxylic acid and the amino substituent. This reaction occurs only in the optically active crystals, whereas the racemate reacts onl y in melt or solution. phenylthio)-2-hydroxy-3-(4-methoxyphenyl)propio nic acid (C16H17NO4S) crystallizes in the monoclinic space group P2(1) with a = 14.240(2), b = 5.3154(11), c = 19.535(6) Angstrom, beta = 91 .93(2)degrees, V = 1477.8(6) Angstrom(3), Z = 4, D-x = 1.435 g cm(-3), mu = 20.63 cm(-1). The two independent molecules differ in their prot onization: one is a cation and the other is an anion. A very short hyd rogen bond [O ... O = 2.480(6) Angstrom] links the carboxylic acid and the carboxylate groups. The two molecules adopt different conformatio ns: one has an extended structure whereas the other has the amino grou p close to the carboxylate group. The latter molecule requires only mi nor changes for the solid-state reaction to take place forming the sev en-membered ring. From the analysis of the hydrogen-bonding pattern a reaction mechanism for the ring closure reaction is proposed.