合成路线1

The condensation between methyl (R)-5-hydroxy-3-methylpentanoate (I) and 4-methoxybenzylamine (II) afforded amide (III). Swern oxidation of the alcohol function led to the corresponding aldehyde, which spontaneously evolved to the cyclic hemiaminal form (IV). Dehydration of (IV) to the tetrahydropyridinone (V) was accomplished by either heating in toluene, or by treatment with phosphoric acid in refluxing dimethyl carbonate. The bicyclic derivative (VI) was then obtained by addition of dichlorocarbene, generated from either ethyl trichloroacetate or chloroform and a base, to tetrahydropyridinone (V). Mono-dehalogenation of dichloro compound (VI) employing zinc dust and ethylenediamine provided a mixture of diastereomeric mono-chloro derivatives (VIIa-b) . After acid-promoted cleavage of the p-methoxybenzyl group, the major isomer (VIII) was isolated by selective crystallization from the reaction mixture. Alternatively, dichloro compound (VI) was first subjected to acidic p-methoxybenzyl group cleavage yielding (IX). Then, radical dehalogenation with triphenyltin hydride and AIBN produced a mixture of mono-chloro compounds, which were separated by column chromatography. Lactam (VIII) was converted to imidate (X) upon treatment with trimethyloxonium tetrafluoroborate. Optionally, the analogous ethyl imidate (XI) was prepared by a similar procedure. Treatment of imidates (X) or (XI) with either ammonia or ammonium acetate furnished the target amidine, which was finally isolated as the corresponding hydrochloride salt.