()-3-Acetoxymethyl-7,8-difluoro-2,3-dihydro-4H-[1,4]benzoxazine (m.p. 73-
74°C) was synthesized by hydrogenation of a compound prepared from 2,3-
difluoro-6-nitrophenol, 1-acetoxy-3-chloro-2-propane and potassium iodide.
The hydrogenation was carried out on Raney nickel. The resulting compound
was dissolved in THF, and 3,5-dinitrobenzoyl chloride and pyridine were added
thereto, followed by heating at 60°C for 3 hours. The mixture was
concentrated, and the concentrate was dissolved in ethyl acetate, washed
successively with diluted hydrochloric acid, an aqueous solution of sodium
bicarbonate and water, dried over anhydrous sodium sulfate and concentrated.
Addition of n-hexane to the concentrate caused precipitation of yellow crystals
of a racemate. The yield of 3,5-dinitrobenzoyl derivative of the ()-3-
acetoxymethyl-7,8-difluoro-2,3-dihydro-4H-[1,4]benzoxazine 3.93 g.
To 2.0 ml of Amberlite XAD 7 was added 2.0 ml of a 0.05 M phosphoric acid
buffer (pH 7.0) having dissolved therein 20 mg of lipoprotein lipase, and the
system was allowed to stand at room temperature for 18 hours to thereby
adsorb the enzyme onto the resin. The resin was filtered. A solution of 250
mg of 3,5-dinitrobenzoyl derivative of ()-3-acetoxymethyl-7,8-difluoro-2,3-
dihydro-4H-[1,4]benzoxazine as a substrate in 25 ml of a mixed solvent of
benzene and n-hexane (4:1 by volume) was added to the resin, followed byallowing to react at 37°C for 4 hours. It was obtained 117 mg of a 3,5-
dinitrobenzoyl derivative of the (-)-3-acetoxymethyl-7,8-difluoro-2,3-dihydro-
4H-[1,4]benzoxazine and 65 mg of a derivative of the (-)-3-acetoxymethyl-
7,8-difluoro-2,3-dihydro-4H-[1,4]benzoxazine.
In 135 ml THF was dissolved 3.03 g of a 3,5-dinitrobenzoyl derivative of (-)-
3-acetoxymethyl-7,8-difluoro-2,3-dihydro-4H-[1,4]benzoxazine, and 135 ml of
ethanol and 30 ml of 1.0 N potassium hydroxide were added to the solution.
After 30 min 3 ml of acetic acid was added thereto for neutralization. The
mixture was concentrated. The solid was subjected to column chromatography
using 40 g of silica gel and eluted with chloroform/methanol to obtain 1.17 g
of (-)-7,8-difluoro-2,3-dihydro-3-hydroxymethyl-4H-[1,4]benzoxazine; [α]D22
= -14.1° (c = 1.80, CHCl3).
To 1.17 g of (-)-7,8-difluoro-2,3-dihydro-3-hydroxymethyl-4H-[1,4]
benzoxazine was added 2.77 g of thionyl chloride in pyridine. The reaction
mixture was concentrated and the concentrate was subjected to column
chromatography using 40 g of silica gel and eluted with chloroform to obtain
1.18 g of the reaction product as a colorless oily product. This product was
dissolved in 30 ml of dimethyl sulfoxide, and 0.41 g of sodium borohydride
was added thereto, followed by heating at 80-90°C for 1 hour. The reaction
mixture was dissolved in 500 ml of benzene, washed with water to remove
the dimethyl sulfoxide, dried over anhydrous sodium sulfate and concentrated
under reduced pressure. The concentrate was subjected to column
chromatography using 40 g of silica gel and eluted with benzene to obtain
0.80 g of (-)-7,8-difluoro-2,3-dihydro-3-methyl-4H-[1,4]benzoxazine as a
colorless oily product; [α]D25 = -9.6° (c = 2.17, CHCl3). Optical Purity: >99%
e.e.
To 1.13 g of (-)-7,8-difluoro-2,3-dihydro-3-methyl-4H-[1,4]benzoxazine was
added 1.58 g of diethyl ethoxymethylenemalonate, and the mixture was
stirred at 130-140°C for 70 min. The reaction mixture was subjected to
column chromatography using 50 g of silica gel and eluted with chloroform to
obtain 2.47 g of diethyl [(-)-7,8-difluoro-3-methyl-2,3-dihydro-4H-[1,4]
benzoxazin-4-yl]methylenemalonate. This product was dissolved in 5 ml of
acetic anhydride, and 10 ml of a mixture of acetic anhydride and concentrated
sulfuric acid (2/1 by volume) with stirring under ice-cooling, followed by
stirring at 50-60°C for 40 min. To the reaction mixture were added ice and an
aqueous solution of sodium bicarbonate, and the product was extracted three
times with 150 ml portions of chloroform. The combined extract was washed
with water, dried over anhydrous sodium sulfate and concentrated. The
precipitate was washed with a small amount of diethyl ether to yield 1.32 g of
(-)-ethyl 9,10-difluoro-3-methyl-7-oxo-2,3-dihydro-7H-pyrido[1,2,3-de][1,4]
benzoxazine-6-carboxylate.
In 12 ml of acetic acid was dissolved 1.20 g of the resulting compound, and
25 ml of concentrated hydrochloric acid was added, followed by refluxing at
120-130°C for 90 min. Upon allowing the reaction mixture to stand at room
temperature, colorless crystals were precipitated, which were collected by
filtration and washed successively with a small amount of water, ethanol and
diethyl ether to obtain 0.96 g of (-)-9,10-difluoro-3-methyl-7-oxo-2,3-dihydro-
7H-pyrido[1,2,3-de][1,4]benzoxazine-6-carboxylic acid.
In 30 ml of diethyl ether was suspended 324 mg of the resulting compound,and a large excess of boron trifluoride ethyl etherate was added thereto,
followed by stirring at room temperature for 30 min to form a chelate
compound. The product was collected by filtration and washed with a small
amount of diethyl ether to obtain 373 mg of a powder. The powder was
dissolved in 7 ml of dimethyl sulfoxide, and 136 mg of N-methylpiperazine
and 228 mg of triethylamine were added thereto, followed by stirring at room
temperature for 17 hours. The reaction mixture was concentrated to dryness
under reduced pressure, and to the solid were added 15 ml of 95% methanol
and 0.31 ml of triethylamine. The resulting mixture was refluxed for 3 hours.
The reaction mixture was concentrated under reduced pressure, and the
residue was filtered and washed successively with a small amount of ethanol
and diethyl ether to obtain 350 mg of a white powder. Recrystallization from a
mixed solvent of ethanol and thick aqueous ammonia gave 230 mg of S-(-)-
ofloxacin (Levofloxacin).
Melting Point: 225-227°C (with decomposition); [α]D23 = -76.9° (c = 0.39,
0.05 N NaOH).
