合成路线1

The synthesis of pidotimod has been carried out using N-tert-butoxycarbonyl-L-pyroglutamic acid as starting material, in order to avoid the formation of diketopiperazine derivatives. L-Glutamic acid (I) was condensed with di-tert-butyl dicarbonate by means of triethylamine in DMF to give N-(tert-butoxycarbonyl)-L-glutamic acid (II), which is dissolved in THF and treated with dicyclohexylcarbodiimide (DCC) to obtain N-(tert-butoxycarbonyl)-L-glutamic anhydride (III). The treatment of anhydride (III) with dicyclohexylamine in THF-ethyl ether affords the dicyclohexylamine salt of N-(tert-butoxycarbonyl)-L-pyroglutamic acid (IV), which by acidification with aqueous citric acid yields the corresponding free acid (V). The condensation of equimolecular amounts of N-(tert-butoxycarbonyl)-L-pyroglutamic acid (V) with L-thiazolidine-4-carboxylic acid ethyl ester (VIII) by means of DCC in methylene chloride gives the coupled ester (IX), which is hydrolyzed with aqueous NaOH, and the corresponding sodium salt acidified to yield the N-tert-butoxycarbonyl derivative (X). Finally, this compound is deprotected with trifluoroacetic acid to obtain crystalline pidotimod (XI). The intermediate thiazolidine (VIII) has been obtained as follows: Esterification of L-thiazolidine-4-carboxylic acid (VI) with ethanol by means of SOCl2 gives the corresponding ethyl ester hydrochloride (VII), which by treatment with K2CO3 in water yields the free ester (VIII).