合成路线1

The acylation of 2-butene-1,4-diol (XXII) with butyryl chloride and DMAP in pyridine gives the dibutyrate (XXIII), which is ozonolyzed with O3 in dichloromethane yielding the aldehyde (XXIV). The cyclization of (XXIV) with mercaptoacetic acid (XXV) in refluxing toluene affords 2-(butyryloxymethyl)-1,3-oxathiolane-5-one (XXVI), which is reduced with lithium tri-tert-butoxyaluminum hydride and acetylated with acetic anhydride in THF to give 5-acetoxy-2-(butyryloxymethyl)-1,3-oxathiolane (XXVII). The condensation of (XXVII) with disilylated 5-fluorocytosine (XV) (obtained from cytosine (XVI) as described) by means of SnCl4 in dichloromethane yields the racemic butyryl nucleoside rac-(cis)-(XXVIII). The biological resolution of this racemic mixture has been performed by digestion with the enzyme pig liver esterase (PLE-A) providing a mixture of unreacted (-)-(cis)-(2R,5S)-(XXVIII) butyrate and hydrolyzed (+)-(cis)-(2S,5R)-(XXIX) that are separated by fractional extraction. The desired (-)-(cis)-(2R,5S)-enantiomer is finally hydrolyzed to the target compound with NaOMe in methanol. The biological resolution of rac-(cis)-(XXVIII) can also be performed with the enzyme Amano PS-800 yielding a mixture of unreacted (+)-(cis)-(2S,5R)-(XXVIII) butyrate and the desired (-)-(cis)-(2R,5S) target nuceloside that are separated by fractional extraction.