合成路线1

A new process for the preparation of emtricitabine has been described: The esterification of 2-butene-1,4-diol (I) with butyryl chloride and DMAP in pyridine gives the diester (II), which by ozonolysis with O3 in methanol provides the hemiacetal (III). Cyclization of (III) with 2-mercaptoacetic acid in refluxing toluene affords racemic 2-(butyryloxymethyl)-1,3-oxathiolan-5-one (V), which is submitted to optical resolution to afford the (R)-enantiomer (VI). The reduction of (VI) with Li(t-BuO)3AlH, followed by acetylation with acetic anhydride gives 5-(acetoxy)-2(R)-(butyryloxy)-1,3-oxathiolane (VII), which by treatment with HCl in dichloroethane is converted into 2(R)-(butyryloxy)-5-chloro-1,3-oxathiolane (VIII). Condensation of (VIII) with 5-fluoro-bis(trimethylsilyl)cytosine (IX) ­ obtained by silylation of cytosine (X) with HMDS ­ by means of NaHCO3 in dichloroethane gives a diastereomeric mixture of the butyrate nucleosides (XI), which is hydrolyzed with butylamine in methanol yielding the corresponding diastereomeric mixture of the (2R,5S)-isomer, emtricitabine, and the (2R,5R)-isomer (XII). This mixture is separated by crystallization of the corresponding hydrochlorides obtained by treatment with HCl in methanol/dioxane.