Field of the Invention

The present invention relates to a process for the synthesis of (8S)-8- fluoroerythromycins which are well known as useful antibacterial agents and are described in U.S. Patents 4,673,736 and 4,514,562.

Background of the Invention

The current methods for preparing (8S)-8-fluoroerythromycins involve either treatment of 8,9-anhydroerythromycin 6,9-hemiacetals or their N-oxides with fluorinating agents such as perchloryl fluoride, fluoroxyperfluoroalkanes, fluoroxysulfϋrpentafluoride, molecular fluorine, lead tetracetate hydrogen fluoride or trifluoroacetylhypofluorite (U.S. Patent 4,514,562). More recently, the preparation of (8S)-8-fluoroerythromycins has been improved by exposure of erythromycin to perchloryl fluoride in an acetic acid buffer and an inert co-solvent (U.S. Patent 4,673,736). The aforementioned fluorinating agents have disadvantages; such as: hazards in handling, relatively high cost, the need for special reaction vessels, and chemical instability, which make them unsatisfactory for preparing commercial quantities of (8S)-8-fluoroerythromycins Perchloryl fluoride, in particular, is known to form unstable and explosive organoperchlorates in reactions with organic molecules. See Peet, H. H. J and Rocket, B W. J. Organometal. Chem. 82, C57 ( 1974) and Adcock. W and Khor, J. C. ibid. 91, C20 (1975)

N-F (electrophi c) fluorinating agents, characteπzed by a structure containing an N-F bond, are a well known class of reagents for introducing fluorine into organic molecules As a class, they have been shown to be stable due to their ability to be stored for long peπods of time and high melting points and are easilv handled reagents

The fluoπnation of double bonds in vaπous organic compounds with N-F fluorinating agents is reported in J Am Chem Soc 112, 8563 (1990) and J Org

Che 1993, 58, 2791-2796 The conditions repoπed for such N-F fluoπnations include use of an organic solvent such as acetonitπle, dichloromethane and tetrahvdrofjran and an oxygen containing nucleophilic reagent such as acetic acid

When these standard N-F fluoπnation conditions were used to attempt to fluoπnate 8- fluoroerythromycin or 8,9-anhydroerythromycin, poor yields of 8S-8- fluoroerythromycins were obtained

Summary of the Invention

This invention relates to a process for prepaπng (8S)-8-fluoroerythromycιns compπsing reacting 8,9-anhydroerythromycin 6,9-hemiacetals or an N-oxide thereof with a carboxylic acid and an N-F fluoπnating agent for a sufficient time and temperature to obtain an (8S)-8-fluoroerythromycιn An additional embodiment of the invention compπses prepaπng the 8,9-anhydroerythromycin 6,9-hemιacetal starting matenal from erythromycin or an N-oxide deπvative thereof and using same in situ for the reaction to form the (8S)-8-fluoroerythromycιn The reaction avoids the use of matenals which are characteπstically hazardous and/or expensive, while removing the need for a co-solvent

Detailed Description of the Invention

According to the invention, erythromycin, or an N-oxide thereof is reacted with an N-F fluorinating agent in the presence of a carboxylic acid.

N-F fluorinating agents are a well known class of organic compounds which are characterized by an N-F bond. These compounds can also be substituted with a variety of substituents which do not interefere with the fluorination reaction. Examples of substituents which may be present on the N-F fluorinating agents which do not interfere with the fluorination reactions include, for example, alkyl, alkoxy, Cl, Br, F, haloalkyl, phenyl, cycloalkyl and cyano. The N-F fluorinating agent can contain radioactive ^F in lieu of ^F to give radiolabelled products.

Illustrative of the compounds included within this well known class of N-F fluorinating agents are N-fluorobenzenesulfonimide (NFSi) and derivatives (U.S.P. 5, 1 16,982), N-fluoromethanesulfonimide and derivatives, N-fluoropyridinium pyridine heptafluorodiborate (NFPy), l-chloromethyl-4-fluoro-l,4- diazoniabicyclo[2.2.2]octane bis(tetrafluoroborate) [F-TEDA] and other 1 -alkyl-4- fluoro-l,4-diazoniabicyclo[2.2.2]octane salts and fluorinated diazabicycloalkane derivatives thereof (U.S.P. 5,086,178), l-hydroxyl-4-fluoro-l,4- diazoniabicyclo[2.2.2]octane bis(tetrafluoroborate) [NFTh], and other fluorinated derivatives thereof (application Serial No. 173,297, filed December 23, 1993), N-alkyl- N-fluoro-p-toluenesulphonamides. N-fluoro-o-benzene-disulfonimide, N-fluoro-N- alkylsulfonamides, N-fluoropyridinium sulfonates (U.S.P. 5,081,249), perfluoro-N- fluoro-N-(4-pyridy!)methanesulphonamide and various substituted N-fluoropyridinium salts. Properties of these various species of N-F fluorinating agents have been reviewed by V. Murtagh [in "Further Progress on Electro-fluorination," Performance Chemicals, p 27, August/September 1992.]; G. G. Fuπn. [in L. German and S. Zemskov (eds.) New Fluorinating Agents in Organic Synthesis, Springer Verlag,

Berlin ( 1989) 35] and bv T Umemoto, et al [in "Power and Structure-Vaπable

Fluoπnatmg Agents The N-Fluoropyπdiruum Salt S\ stem". J Am Chem Soc 1 12.

8563 ( 1990)], "N-Fluorobenzenesulfonimide A Practical Reagent for Electrophilic

Fluoπnations" bv Differding, E , Ofner, H Synthetic Letters 187 ( 1991 ) N-fluoro-o- benzenedisulfonimide by Davis. F A . Han VV in "N-Fluoro-o-Benzenedisulfonimide

A Useful New Fluoπnating Reagent" Tetrahedron Letters 32, 1631 (1991 ), "N-

Fluoropyπdinium Salts Having Trichloro methyl Substituents" by Fung, A P et al

U S Patent 5,1 16,982 and l-alkyl-4-fluoro-l,4-dιazonιabιcyclo[2 2 2]octane salts by

Banks, R E , et al J Chem Soc , Chem Commun 595 ( 1992) A preferred class of N-F fluoπnating agent is represented by the broad-class of N-F fluoπnating agents defined which do not contain a pyπdine nucleus The preferred N-F fluoπnating agents are the fluoπnated diazabicycloalkane compounds descπbed in U S P 5,086, 178, the N-fluorobenzenesulfonimides descπbed in U S P 5,254,732, the 1- substιtuted-4-fluoro-l,4-dιazoruabιcyclo[2 2 2]octane salts descπbed in co-pending, commonly assigned application Seπal No 173,297, filed December 23, 1993, which application is incorporated by reference herein, and N-fluorodimethylsulfonimide and deπvatives as referred to in Journal of Fluoπne Chemistry, 58., page 141, 1992

The carboxylic acid may be any compound having a -COOH functionality, but is preferably a branched or straight chain carboxylic acid, which may be substituted with a vaπety of substituents such as halogens, electron donating or electron withdrawing groups, having a pKa which is less than or equal to that of acetic acid Still preferably, the carboxylic acid has from 2-8 carbon atoms and, most preferably, is acetic acid The concentration of the carboxylic acid in the reaction mixture is preferably about 0 05M to 1 5M and about 0 25M to 0 5M Preferably, the process is conducted in the absence of an men co-solvent, such as used in the pπor art N-F fluoπnating reactions. e g tetrahydroruran. acetonitπle or methylene chloπde The carboxylic acid is preferably buffered to a pH of about 4-7 and, more preferably, about 4-4 5 with any suitable buffering agent such as an alkali metal hydroxide, e g NaOH, KOH or LiOH

Good vields of the fluoroerythromv cin products are obtained wnen 8,9- ar.hvdroervthromvciπ 6.9-hemιacetals or the corresponding N-oxides are reacted with N ^' -F fluoπnating agents in accordance with the invention The anhydro reactant is reacted with at least about one molar equivalent of an N-F fluoπnating agent for about 1 to about 30 hours, preferably, about 10 to about 24 hours The reaction is preferably conducted at a temperature ranging from about -5 to about 50°C at atmospheπc pressure, still preferably, at about 10 to about 35°C , with about 23°C or room temperature being the temperature most preferred Preferably, the fluoπnation step is conducted at a pH of about 4 to about 7 and. more preferably, at a pH of about 4 to about 4 5

In another embodiment of the invention, the 8,9-anhydroeryihromycιn 6,9- hemiacetals or the corresponding N-oxides are prepared as unisolated reaction intermediates by the action of a carboxylic acid on erythromycin or the corresponding N-oxides A solution of erythromycin or the corresponding N-oxides dissolved in the carboxylic acid is allowed to mix as needed to form the anhydro deπvative Preferably, the reaction time is from about 1 minute to about 24 hours and, still preferably, from about 2 hours to about 4 hours The anhydro formation takes place at a reaction temperature of about -5 to about 50°C Preferably, the reaction temperature is about 20 to about 30°C The anhydro unisolated intermediate may then be used in situ to prepare the fluoroervthromycin with an N-F fluoπnating agent as descπbed above

Once the fluoroervthromycin is formed, conventional separation techniques can be employed to isolate the desired product These methods include phase separation. vacuum filtration and solv ent washing

Example 1

-!CCPe".ze"esu!fonιrιιde ^] τ buffered acetic acid (sodium salt ) Erythromycin A(5g, 6 813 mmole) was dissolved in glacial acetic acid (20 mL) at room temperature and stirred for 2 hours The pH of the mixture was adjusted to 4 3 with 6 N sodium hydroxide (approx 12 5mLϊ while the temperature was maintained below 20 ^C C N-fluorobenzenesulfonimide (2 2g, 6 813 mmole) was added and sttrnng continued for an additional 18 hours at 22°C Methylene chloπde ( 100 mL) was added, the solution was placed in an ice bath while the pH of the mixture was adjusted to 9 with 6 N sodium hydroxide (approx 85 mL) and the temperature was maintained below 20°C Next, water (100 mL) was added, the layers separated and the aqueous layer washed an additional two times with methylene chloπde ( 100 mL each) The methylene chloπde serves as an extractant for the fluπthromycin product The combined organic phases were washed 2 to 4 times with 5 percent sodium hydroxide ( 100 mL each) and dπed over magnesium sulfate .After filtration, the organic phase was concentrated under vacuum at room temperature to afford 4 91g of crude fluπthromycin The crude mateπal was dissolved in ethanol concentrated to 20 mL This process was repeated two more times After standing at 0°C for one night, the product was filtered under vacuum, washed with 5 mL cold ethanol and dπed under vacuum at 40°C to afford 3 48 g (68% yield) of fluπthromycm ((8S)-8- fluoroerythrornycin)

Example 2

Preparation of (3S)-8-fluoroervthromvαn A, from ervthromvcin λ with \- f uorobenzenesulfonimide in glacial acetic acid

Ervthromvcin A (5g, 6 813 mmole) and N-fluorobenzenesulfontmide (2 2g. 6.813 mmole) were dissolved in glacial acetic acid (20 mL) at room temperature and

stirred for 18 hours The reaction was worked-up in the same manner as Example 1 to a ford 4 96g of crude flurithromycin This material was recrystallized as described in Example 1 to yield 2.25g (44% yield) of fluorithromycin This example shows that reasonably good yields of flurithromycin may be obtained in the process of the invention without buffering.

Example 3

Preparation of (8S -8-fluoroervthromvcin A from ervthromvcin A with N- fluorobenzenesulfonimide in acetic acid

Erythromycin A (3g, 4. 1 mmoie) was dissolved in glacial acetic acid ( 12 mL) at room temperature and stirred for 2 hours. N-fluorobenzenesulfonimide ( 1.6g, 4 1 mmole) was added and stirring continued for an additional 4 hours at 22°C The reaction was worked-up in the same manner as Example 1 to afford 2 533g of crude flurithromycin This material was recrystallized as described in Example 1 to yield 0 8g (26% yield) of flurithromycin. This example shows the embodiment of the invention where the anydro derivative of erythromycin is prepared as an unisolated reaction intermediate and the N-F fluorination reaction is carried out without a buffer

Example 4

Preparation of (8S)-8-fluoroervthromycin A from erythromycin A with N-fluoro. N- chloromethyl triethylenediamine bis(tetrafluc'θborate) in (unbuffered) glacial acetic acid Erythromycin A ( l g, 1 363 mmoie) and N-fluoro. N-chloromethyl triethvlenediamine bis(tetrafluoroborate) (0 46g, 1 3 mmole) were dissolved in glacial acetic acid (4 mL) at room temperature and stirred for 5 hours The reaction was w orked up in the same manner as Example 1 to afford 1 16g (74% yield) of crude flurithromvcin.

Example 5

Preparation of (8SV8-fluoroervthromvcin A from ervthromvcin A with 1-chloromethyl -^•fluoro-1. 4-diazoniabicvclo f 2 21 bis(tetrafluoroborate) in buffered acetic acid Erythromycin A (5g, 6 813 mmole) was dissolved in glacial acetic acid (20 mL) at room temperature and stirred for 2 hours. The pH of the mixture was adjusted to 4 3 with 6 N sodium hydroxide (approx. 12.5 mL) while the temperature was maintained below 20°C. l-Chloromethyl-4-fluoro-1.4-diazoniabicyclo [2.2.2] octane bis (tetrafluoroborate) (2 4g, 6 813 mmole) was added and stirring continued for an additional 3 hours at 22°C. The reaction was worked up in the same manner as Example 1 to afford 4 5g of crude flurithromycin This material was recrystallized as described in Example 1 to yield 2.6g (51% yield) of flurithromycin.

Example 6

Preparation of (8SV8-fluoroervthromvcin A from ervthromvcin A with N- fluoropyridinium pyridine heptafluorodiborate

Erythromycin A (0.5g, 0.681 mmole) was dissolved in glacial acetic acid (4 mL) at room temperature and stirred for 2 hours The pH of the mixture was adjusted to 4 3 with 6 N sodium hydroxide (approx 1.2 mL) while the temperature was maintained below 20°C. N-fluoropyridinium pvridine heptafluorodiborate (0 25g, 0 75 mmole) was added and stirring continued for an additional 2 hours at 22°C. The reaction was worked up in the same manner as Example 1 to afford an 11% yield of crude flurithromycin Although the yield was only 1 1%, this example demonstrates operation of the invention with N-F fluorinating agents derived from pvridine since with a co-solvent no flurithromycin is formed (See Comparative Example 4)

Example

fluoro- 1.4-dιazonιabιcvclo[2 2 2]octane bis(tetrafluoroborate). NFTh. in buffered acetic acid (sodium salt)

Erythromycin A (5 g, 6 813 mmole) was dissolved in glacial acetic acid (20 mL) at room temperature and stirred for 2 hours The pH of the mixture was adjusted to 4 3 with 6 N sodium hydroxide (approx 12 5 mL) while the temperature was maintained below 20 °C l -Hydroxyl-4-fluoro- l,4-dιazoruabιcyclo[2 2 2]octane bιs(tetrafluoroborate) (2 2 g, 6 813 mmole) was added and stimng continued for an additional 18 hours at 22°C Methylene chloπde ( 100 mL) was added, the solution placed in an ice bath while the pH of the mixture was adjusted to 9 with 6 N sodium hydroxide (approx 85 mL) and the temperature maintained below 20 °C Next, water ( 100 mL) was added, the layers separated and the aqueous layer washed an additional two times with methylene chloπde ( 100 mL each) The methylene chloπde serves as an extractant for the fluπthromycin product The combined organic phases were washed 2 to 4 times with 5% sodium hydroxide ( 100 mL each) and dπed over magnesium sulfate After filtration, the organic phase was concentrated under vacuum at room temperature to afford 4 6 g of crude fluπthromycin The crude matcπal was dissolved in ethanol concentrated to 20 mL This process was repeated two more times After standing at 0°C for one night, the product was filtered under vacuum, washed with 5 mL cold ethanol and dπed under vacuum at 40°C to afford 2 18 g (43% of fluπthromycin

Example 8

Preparation of (8S)-8-fJuoroervthromvcin A from anhvdroervthromycin A with N- fluorobenzenesulfoπimide in buffered acetic acid (sodium salt)

Anhydroerythromvcin A ( 1 g. 1 4 mmole) was dissolved in glacial acetic acid (4-mL) at room temperature and the pH of the mixture was adjusted to 4 3 with 6 N

sodium hydroxide (approx. 2 5 mL) while the temperature was maintained below

20 ^C C N-fluorobenzenesulfonimide (0 44 g, 1 4 mmoie) was added and stimng continued for an additional 18 hours at 22°C Methylene chloride ( 10 mL) was added. the solution placed in an ice bath while the pH of the mixture was adjusted to 9 with 6 N sodium hydroxide (approx. 17 mL) and the temperature maintained below 20°C Next, water (lOmL) was added, the layers separated and the aqueous layer washed an additional two times with methylene chloride ( 10 mL each). The combined organic phases were washed 2 to 4 times with 5% sodium hydroxide (10 mL each) and dried over magnesium sulfate. After filtration, the organic phase was concentrated under vacuum at room temperature to afford 0.88 g of crude flurithromycin. The crude material was dissolved in ethanol concentrated to 2 mL. This process was repeated two more times. After standing at 0°C for one night, the product was filtered under vacuum, washed with 0.5 mL cold ethanol and dried under vacuum at 40°C to afford 0 53 g (51% yield) of flurithromycin.

Comparative Example 1 (See Example I)

Attempted Preparation of (8S)-8-fluoroervthromvcin A from ervthromvcin A with N-fluorobenzenesulfonimide in buffered acetic acid and tetrahvdrofuran as solvent Erythromycin A (5g, 6 813 mmole) was dissolved in glacial acetic acid (20 mL) at room temperature and stirred for 2 hours. N-fluorobenzenesulfonimide (2.2g, 6 813 mmole) dissolved in THF (4 mL) was added and stirring continued for an additional 18 hours at 22 ^C C The reaction was worked up in the same manner as Example 1 to afford only 0 93g (7% yield) of crude flurithromycin

Comparative Example 2 (See Example 4)

Attempted fluorination of S,9-anhvdroerythromvcin A 6.9-hemiacetal with 1 - chloromethyl-4-fluoro-1.4-diazoniabicvclo[2 2 2]octaπe bis(tetrafluoroborate) and acetic acid with acetonitrile as solvent

.An acetonuriie (4 2 mL) solution of 8,9-anhydroerythromycin A 6,9-hemiacetal

(0 3 g.O 42mmoie), l-chloromethyl-4-fluoro- l ,4-diazoniabicyclo[2 2 2]octane bis(tetrafluoroborate) (0 16 g, 0 46 mmole) and glacial acetic acid (0 04 mL. 0 84 mmole) was stirred at room temperature for 5 hours. The solution was evaporated,

5 diluted with methylene chloride (5 mL), washed twice with 5% NaOH (5 mL), dried through MgSO4 ^an d evaporated to afford 0.26 g of a solid which contained 8.2% flurithromycin by HPLC

Comparative Example 3 (See Example 4) Q

Attempted fluorination of ervthromvcin A N-oxide with I -chloromethyl-4-fluoro-1.4- diazoniabicvclo[2 2 2]octane bis(tetrafluoroborate) with acetonitrile as solvent .An acetonitrile (4 2 mL) solution of erythromycin A N-oxide (50 mg, 0 07 mmole), 1 -chloromethvi-4-fluoro- 1 ,4-diazoniabicyclo[2.2.2]octane bis(tetrafluoroborate) (26 5 mg, 0 07 mmole) and water (0.1 mL) was stirred at room temperature for 18 hours. The solution was diluted with methylene chloride (1 mL), washed twice with water (1 mL), dried through MgSO4 and evaporated to afford only starting material.

Comparative Example 4 (See Example 6)

Attempted fluorination of ervthromvcin A N-oxide with N-fluoropyridinium pyridine heptafluorodiborate with nitromethane as co-solvent

A nitromethane ( 1 mL) solution of er thromycin A N-oxide (50 mg, 0 407 mmole), N-fluoropyridinium pvridine heptafluorodiborate (25 mg, 0 07 mmole) was refluxed for 1 hour. The solution was diluted with methylene chloride (1 mL), washed twice with saturated NaHCO (1 mL), dried through MgSU4 and evaporated to afford only decomposition products.