<unk>  is a synthetic pyrimidine nucleoside prodrug—a nucleoside analog in which the hydrogen atoms on the 2' carbon of deoxycytidine are replaced by fluorine atoms. The synthesis described and pictured below is the original synthesis done in the Eli Lilly Company labs. Synthesis begins with enantiopure D-glyceraldehyde (R)-2 as the starting material which can made from D-mannitol in 2–7 steps. Then fluorine is introduced by a "building block" approach using ethyl bromodifluroacetate. Then, Reformatsky reaction under standard conditions will yield a 3:1 anti/syn diastereomeric mixture, with one major product. Separation of the diastereomers is carried out via HPLC, thus yielding the anti-3<unk>  in a 65% yield. At least two other full synthesis methods have also been developed by different<unk> roups. Illustration of the original synthesis process used and published by Hertel et al. in 1988 of Lilly laboratories.
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