合成路线1

2) The cyclization of 6-fluoro-3,4-dihydro-2H-1-benzopyran-2-carboxylic acid (IV) with KCN and ammonium carbonate in hot water (80-90 C) gives 6-fluoro-2',5'-dioxospiro[3,4-dihydro-2H-1-benzopyran-4,4'-imidazolidine]-2-carboxylic acid (X) as a (5:1)-mixture of the (RS,RS)- and (RS,SR)-diastereomers. This mixture is treated with SiCl4 and dry ammonia in pyridine, yielding the corresponding (5:1)-mixture of amides (XI), which is separated by recrystallization, affording the (RS,RS)-diastereomer (XII) as a pure compound. The optical resolution of (XII) with N-methylquininium hydroxide (XIII) and crystallization gives the (2S,4S)-isomer as the N-methylquininium salt (XIV), which is finally treated with 1N aqueous HCl in ethanol/water.