Organic compounds -- part of the class 532-570 series – Organic compounds – Unsubstituted hydrocarbyl chain between the ring and the -c-...
Reexamination Certificate
2001-12-13
2003-09-09
Berch, Mark L. (Department: 1624)
Organic compounds -- part of the class 532-570 series
Organic compounds
Unsubstituted hydrocarbyl chain between the ring and the -c-...
C540S364000, C435S118000, C435S121000
Reexamination Certificate
active
06617451
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to chiral cis-azetidinones having an unsaturated side-chain prepared by enantioselective acylation of a monocyclic &bgr;-lactam with Penicillin G Amidase.
BACKGROUND OF THE INVENTION
Non-classical &bgr;-lactams such as the monobactams (e.g., aztreonam), carbapenems (e.g., thienamycin), and carbacephems (e.g., loracarbef) have been shown to be clinically effective anti-bacterial agents. Much attention has been focussed on the development of economical, large scale synthesis of these derivatives. Since loracarbef is available only via a total synthesis, improvements in the manufacturing process are of particular interest. One of the most challenging synthetic steps is the resolution of the chiral cis-&bgr;-lactam intermediates. U.S. Pat. No. 5,057,607 provides a enantiomerically selective biocatalyzed acylation of a racemic mixture of cis-3-amino-2-[2-(2-furanyl)ethyl]-4-oxo-1-azetidine acetic acid (or its alkyl ester) with methyl phenylacetate (or methyl phenoxyacetate) in the presence of a penicillin G amidase enzyme. The racemic materials serve as the substrate for the enzyme. Once acylated, the acylated active enantiomer is isolated and the inactive enantiomer is disposed of thus reducing the overall yield of the intermediate. Therefore, there is a need for a means of producing the desired enantiomeric cis-&bgr;-lactam intermediate with higher overall yields.
BRIEF SUMMARY OF THE INVENTION
The present invention provides a cis-&bgr;-lactam compound having structure II and a process for preparing the cis-&bgr;-lactam compound having structure II:
where R
1
is —SO
3
−
, —SO
3
(C
1
-C
4
)alkyl, —CH
2
CO
2
H, or —CH
2
CO
2
R
1′
, where R
1′
is (C
1
-C
4
)alkyl, benzyl, or substituted benzyl (e.g., p-nitrophenylmethylene); R
2
is benzyl or phenoxymethylene; and R
3
is 2-furyl, phenyl, or 2-methoxyphenyl. In a preferred embodiment, R
1
is —CH
2
CO
2
H or an ester thereof. Compound II is prepared by the steps of
(i) providing a racemic mixture of cis-azetidinones having structures Ia and Ib
where R
1
and R
3
have the same meaning as above, and (ii) reacting the racemic mixture with a (C
1
-C
4
)alkyl phenylacetate or a (C
1
-C
4
)alkyl phenoxyacetate in the presence of penicillin G amidase. Once acylated, Compound II may be further modified. For example, Compound II may be hydrogenated to form a compound having a saturated sidechain (Compound III below).
DEFINITIONS
As used herein, the term “(C
1
-C
4
)alkyl” refers to an alkyl group having one to four carbon atoms (e.g., methyl, ethyl, propyl and butyl). The alkyl group may be a straight or branched chain (e.g., n-propyl, i-propyl, n-butyl, i-butyl, s-butyl, and t-butyl)
DETAILED DESCRIPTION OF THE INVENTION
The use of Penicillin G Amidase for the enzymatic acylation of &bgr;-lactam antibiotics and resolution of cis racemic azetidinones has been previously reported. (see, e.g., U.S. Pat. No. 5,057,607, incorporated herein by reference; Zmijewski Jr., M. J., et al., “Enantioselective acylation of a Beta-Lactam Intermediate in the synthesis of Loracarbef using Penicillin G Amidase,”
Tetrahedron Letters
, 32(13), 1621-1622 (1991); and Briggs, B. S., et al., “Side Chain Selectivity and Kinetics of Penicillin G Amidase in Acylating a Cis-Racemic &bgr;-Lactam Intermediate in the Synthesis of Loracarbef,”
New J. Chem
., 18, 425-434 (1994).) However, none of the previous investigators realized the utility and advantages of enantioselectively acylating a cis-&bgr;-lactam intermediate having an unsaturation in the side chain. Since enzymes are very substrate sensitive, it was also unknown until the discovery by the Applicants that one could enantioselectively acylate a cis-&bgr;-lactam intermediate having an unsaturation in the side chain in the presence of Pencillin G Amidase.
The presence of the unsaturation in the side-chain provides several advantages that cannot be realized in the previously disclosed hydrogenated intermediate. For example, one may be able to recycle the undesired inactive cis-enantiomer by racemizing the unwanted cis-isomer. Scheme I below illustrates a potential means of providing this racemization and subsequent enantioselective acylation to increase the overall yield of the desired cis-enantiomer.
Applicants have observed that when the methyl ester of cis-3-amino-2-[trans-2-(2-furanyl)ethylene]-4-oxo-1-azetidineacetic acid is converted to its 3-p-nitrobenzyl imine derivative under basic conditions epimerization of the C-3 stereocenter occurs. Other substituents located alpha to the C-3 and/or C-2 stereocenters have also been shown to activate azetidinones towards racemization. (see, e.g., Alcaide, B., et al.,
Tetrahedron Letters
, 39, 5865-5866 (1998); Suarato, A., et al,
Tetrahedron Letters
, 42, 4059-4062 (1978); and Kametani, T., et al,
Heterocycles
, 16(4), 539-547 (1981)). Therefore, it is reasonable to believe that treatment with base will epimerize the two chiral centers thus providing a means for recovering additional desired enantiomer. Suitable bases include organic bases (e.g., trialkyl amines, pyridines, pyrimidines, quinolines, isoquinolines and derivatives thereof), metal alkoxides, diazabicyclo[5.4.0]undec-7-ene (DBU). Generally, the reaction is run in a non-nucleophilic solvent (e.g., methylene chloride, ethers, hydrocarbons, benzene and toluene) at about −35° C. to about 70° C.
The presence of the unsaturation in the azetidinone also provides a reactive site for derivatization. For example, the unsaturated side-chain may be converted to a methyl group. (see Scheme II) Cleavage of an unsaturation to produce an alkyl group is well known to those skilled in the art. The enantioselective acylation followed by conversion of the unsaturated side-chain could provide an alternative method for making enantiomerically pure intermediates in the synthesis of monobactams (e.g., Azetreonam).
Alternatively, one could oxidatively cleave the unsaturation to provide a carboxyl or aldehyde group at the 2 position of the &bgr;-lactam ring. The oxidized product could then be derivatized to provide a variety of intermediates that could be used in the development of new antibiotic agents.
The racemic mixture of cis-&bgr;-lactams can be obtained by methodology well-known to those skilled in the &bgr;-lactam art. For example, the cis-&bgr;-lactam racemates may be produced using the ketene/imine (2+2) cycloaddition reaction described in Hatanaka, et al.,
Tetrahedron Letters
, 24(44), pp. 4837-4838 (1983); Georg, F. I. and V. T. Ravikumar, The Organic Chemistry of &bgr;-Lactams, Chapter 6, G.I. Georg (Ed.), VCH, pp 295-368 (1993); and U.S. Pat. Nos. 4,260,743 and 5,159,073, both incorporated herein by reference. A preferred method utilizes the formation of a “Dane salts” which is formed by allowing a solution of a potassium salt of an &agr;-amino acid to react with a &bgr;-dicarbonyl compound. The Dane salt is then reacted with ethyl chloroformate and, triethylamine to form a mixed anhydride which is then reacted in situ with a Schiff base to afford the cis-isomer stereoselectively. For a more detailed description, see the preparation section of the Examples.
The term “penicillin G amidase” (or the alternative term “penicillin G acylase”) is well-known in the &bgr;-lactam art as an enzyme which catalyzes the hydrolysis of the penicillin G sidechain(phenylacetyl) from penicillin substrates. Penicillin G amidases suitable for use in the process of the present invention may be isolated by known methodology from many organisms, for example,
E. coli, B. megaterium, Ps. melanogenum, K. citrophila
, and
P. rettgei
. In this regard Schlwale and Sivarawan, Process Biochemistry, August., 1989, pp. 146-154 sets forth a review of the state of the art of penicillin G amidase (acylase) production and application. Penicillin G amidase isolated from
E. coli
is preferred.
Once isolated, the penicillin G amidase may be used in “free” form, i.e., solubilized in aqueous or substantially aqueous solutions, o
Crichfield Kathy Sue
Hart John Eric
Vaid Radhe Krishan
Verral Daniel Edward
Berch Mark L.
Eli Lilly and Company
Tucker Tina M.
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