合成路线1

Reductive cleavage of L-cystine (I) with sodium in liquid ammonia produced cysteine, which was subsequently alkylated with 2-(N-benzyloxycarbonylamino)ethyl tosylate (II) to afford thioether (III). After protection of (III) as the N-Boc derivative (IV), sulfur oxidation by means of Oxone(R) produced sulfone (V). The benzyloxycarbonyl protecting group of (V) was then cleaved by transfer hydrogenolysis employing formic acid and palladium catalyst, and the resulting amine (VI) was converted to amidine (VIII) by reaction thioacetimidate (VII). The Boc protecting group of (VIII) was finally removed by acidic treatment.