The present invention provides a simple, cost effective and industrially useful process for the preparation of 7-amino-3-vinyl cephalosporanic acid. The present invention further provides a crystalline form of 7-amino-3-vinyl cephalosporanic acid.
7-amino-3-vinyl cephalosporanic acid of Formula I (herein after 7-AVCA) is an important synthetic intermediate in the preparation of cephalosporin antiobiotics for example cefdinir.
(Formula Removed)
Process for preparation of 7-AVCA and synthetic intermediates thereof is disclosed by Sigeru et al. Tetrahdron Letters, 23 (21), 2187-2188 (1982) and in US Patent No 4,705,851, European Patent No 122002.
Additionally, European Patent No 503453 (herein after the '453 patent) discloses a process for preparation of 7-AVCA and its derivatives wherein silyl protected 7-amino-3-haloalkyl cephalosporanic acid is used as the starting material in a Witting type reaction in the presence of a strong base. It is mentioned that in order to maintain a degree of silylation the base used must be free of moisture and should itself not contain any moiety that could be silylated.
US Patent No 6,401,841 discloses a process for preparation of 7-AVCA and its derivatives which is a modification of the process of the '453 patent wherein the Wittig reaction using silyl protected 7-aminocephalosporanic acid is carried out in the presence of a weak base.
US Patent No 6,835,829 (herein after the '829 patent) discloses a process for purification for 7-AVCA. As per the disclosure of the '829 patent, 7-AVCA obtained
by following the process of the '453 patent is contaminated with 7-amino-3-methylcephalosporanic acid of Formula A (herein after 7-ADCA),
(Formula Removed)
FORMULA A
which is the 3-methyl derivative of 7-AVCA. The process reported in the '829 patent involves further purification of 7-AVCA prepared by methods known in the art, by chromatographic purification or by forming quaternary ammonium salts of the 4-carboxyl group followed by basification to get 7-AVCA or its derivative. 7-AVCA thus purified has 7-ADCA content ranging from 0.1-0.8%.
Use of silyl protected 7-aminocephalosporanic acid is not favourable firstly because the silylating agents are costly and secondly the silyl protecting groups are prone to hydrolysis and require careful control of the reaction conditions so as to ensure moisture free environment. Use of chromatographic techniques renders the process costly, tedious and time consuming whereas purification by quaternary salt formation is an added process step requiring use of additional reagents. The processes for preparation of 3-vinyl cephalosporanic acid known in the art involve multistep synthesis and contaminated with undesired compounds, which makes them costly and industrially unfavourable.
(Formula Removed)
A high content of 7-ADCA in 7-AVCA is undesirable because the former leads to formation of by products when 7-AVCA is further subjected a series of reactions for the preparation of cephalosporin antibiotics. The source of 7-ADCA could be the 7-protected amino-3-haloalkylcephalosporanic carboxylate of Formula II,
(Formula Removed)
wherein R1 and R2 independently represent hydrogen or suitable amino protecting group used in cephalosporin chemistry, R3 represents suitable carboxyl protecting group used in cephalosporin chemistry and X represents halogen, which is the starting material in the preparation of 7-AVCA via Wittig reaction.
The present inventors have surprisingly found that 7-AVCA of Formula I can be prepared without the use of silylating agents or purification by chromatography or quaternary salt formation. The present process enables significant depletion of 7-ADCA during the course of the synthesis. 7-AVCA prepared by the present process is found to have 0.2% or less of 7-ADCA. The present process is simple, cost effective, and is industrially viable as it allows preparation of pure 7-AVCA (having 0.2% or less of 7-ADCA) without any additional purification steps like salt formation or chromatography. The present process also enables preparation of 7-AVCA in crystalline form.
A first aspect of the present invention provides a process for the preparation of pure 7-AVCA of Formula I,

(Formula Removed)
said process comprises of,
a) crystallizing diprotected 7-amino-3-vinyl cephalosporanic acid (DPVC) of Formula III, wherein R1 and R2 independently represent hydrogen or suitable amino protecting group used in cephalosporin chemistry and R3 represents suitable
carboxyl protecting group used in cephalosporin chemistry, from a reaction mass thereof using an organic solvent,
(Formula Removed)
b) removing the carboxyl protecting group from DPVC of Formula III obtained in step a) to get monoprotected 7-amino-3-vinyl cephalosporanic acid (MPVC) of Formula IV,
(Formula Removed)
wherein R1, and R2 are as described above
c) removing the amino protecting group from MPVC of Formula IV obtained in step
b)
d) isolating 7-AVCA of Formula I from the reaction mass thereof.
Diprotected 7-amino-3-vinyl cephalosporanic acid (DPVC) of Formula IV used as the starting material can be prepared by methods known in the art. DPVC of Formula IV is crystallized from a reaction mixture thereof using an organic solvent. The organic solvent for crystallization of DPVC can be selected from the group comprising of C1.4 alkanols; C3.8 ketones; polar aprotic solvents and mixtures thereof. The acid used for hydrolysis of DPVC can be selected from organic or inorganic acids known to a person having ordinary skills in the art. The carboxyl protecting group from DPVC so obtained is removed in the presence of phenol or phenol ethers such as for example anisole, phenetol and the like to get monoprotected 7-amino-3-vinyl cephalosporanic acid (MPVC) of Formula V. The above said deprotection of DPVC can be effected at
a temperature of 20-80°C and optionally in the presence of an acid. The amino protecting group from MPVC of Formula V is then removed to afford 7-AVCA of Formula I. Removal of amino protecting group from MPVC of Formula V can be carried out using chemical reagents or by enzymatic hydrolysis using Penicillin-G amidase or other amidases (for example Penicillin-V amidase) capable of hydrolysing MPVC of Formula IV without affecting the cephem ring. 7-AVCA of Formula I thus obtained has 0.2% or less of the undesired 7-ADCA. 7-AVCA prepared by the present process does not require any further purification by chromatography or quaternary salt formation.
The amino and carboxyl protecting groups are known to a person having ordinary skills in the art. Few examples of amino protecting groups include but not limited to acyl groups such as benzoyl, benzyl carbonyl and the like. Few examples of carboxyl protecting groups include but not limited to aralkyl groups such as 4-methoxybenzyl or 2,4-dimethoxybenzyl, benzhydryl and the like.
A second aspect of the present invention provides crystalline 7-AVCA having characteristic X-ray diffraction (XRD) pattern as depicted in Figure 1.
A third aspect of the present invention provides crystalline 7-AVCA having X-ray diffraction (XRD) pattern wherein characteristic 29 values are obtained at 6.00, 7.36, 10.76, 12.02, 13.62, 13.88, 14.26, 14.42, 15.24, 15.88, 17.18, 18.04, 19.02, 21.24, 21.48, 21.94, 22.70, 23.36, 24.18, 25.58, 26.72, 28.44, 29.44, 29.66, 30.40, 30.62, 32.28, 33.52, 34.12, 34.54, 34.90, 35.42, 36.18, 36.78, 37.70, 38.02, 39.52 and 39.64.
A fourth aspect of the present invention provides crystalline 7-AVCA having characteristic Infrared spectrum as depicted in Figure 2.
A fifth aspect of the present invention provides crystalline 7-AVCA having moisture content of about 0.5 to 0.6% w/w as measured by Karl Fischer analysis.
While the present invention has been described in terms of its specific embodiments, certain modifications and equivalents will be apparent to those skilled in the art and are intended to be included within the scope of the present invention.
EXAMPLE 1
PREPARATION OF P-METHOXYBENZYL 7-PHENYLACETAMIDO-3-VINYL-3-CEPHEM -4-CARBOXYLATE (DPVC):
Acetone (1000 ml) was cooled to 10-15°C. To this 4-methoxybenzyl (6RJR)-3-(chloromethyl)-8-oxo-7-[(phenylacetyl)amino]-5-thia-1-azabicyclo[4.2.0]oct-2-ene-2-carboxylate (200 g), sodium iodide (62 g) and triphenyl phosphine (118 g) were added at 10-15°C. The reaction mixture was stirred maintaining the temperature at 20-25°C for 90 minutes and monitored by HPLC. The reaction mixture was subsequently cooled to 10-15°C and added dichloromethane (2000 ml) followed by formaldehyde (360.8 g) at 10 - 15°C. To the resultant mixture added sodium hydroxide solution (16.4 g in 400 ml deionized water) slowly at 10 - 15°C in 5-10 minutes and maintained the temperature at 10-15°C for 60-70 minutes. The reaction was monitored by HPLC. To this mixture was added deionized water (3600 ml) at 10-20°C and the pH adjusted to 5.5-6.0 by adding 5% aqueous hydrochloric acid. The layers were separated and dichloromethane was recovered from the organic layer at 35-40°C under vacuum. To the organic layer after removal of dichloromethane was added methanol and the crystals so obtained were filtered under suction and dried in air to afford the title compound.
Yield = 141 g (74%)
EXAMPLE 2 PREPARATION OF 7-AMINO-3-VINYL-3-CEPHEM - 4 - CARBOXYLIC ACID (7-AVCA)
To molten phenol (500 ml) at 45- 50°C was added 4-methoxybenzyl (6R,7R)-8-oxo-7-[(phenylacetyl)amino]-3-vinyl-5-thia-1-azabicyclo[4.2.0]oct-2-ene-2-carboxylate
(100 g) at 45 - 50°C. The mixture was stirred and maintained at 45-50°C for 7 hours (monitored by HPLC) followed by cooling to 35 - 40°C. To the cooled mixture was added n-butyl acetate (1000 ml) at 30-40°C followed by cooling to 10-15°C. To the resultant mixture added sodium bicarbonate solution (1500 ml, 1.5%) at 10-20°C and stirred for 10-15 minutes. The layers (aliquot-1) were separated and the organic layer was re-extracted with sodium bicarbonate solution (1000 ml, 1.5%) at 10-20°C. The layers were again separated (aliquot-2). The aliquots 1 and 2 were combined and washed twice with n-butyl acetate to remove phenol. The aqueous layer was subjected to enzymatic hydrolysis using wet Penicillin-G amidase (50 g) at 20-25°C. The reaction mixture was stirred and maintained at a pH of 7.5-7.9 by addition of sodium carbonate solution (5%) at 20- 30°C (monitored by HPLC). The enzyme was filtered and the filtrate washed with deionized water (200 ml) followed by cooling to 8-10°C. The pH of the cooled solution was adjusted to 2.4-2.5 by addition of hydrochloric acid (10%). The solid so obtained was filtered, washed with deionized water and acetone and dried under vacuum at 40-45°C till moisture content was not more than 1.0%.
Yield = 41.5 g (85%)
HPLC Purity: 99.42%
7-ADCA content: 0.17% (by HPLC)

WE CLAIM:
1. A process for the preparation of pure 7-AVCA of Formula I,

(Formula Removed)
said process comprises of,
a) crystallizing DPVC of Formula
(Formula Removed)
FORMULA III
wherein Ri and R2 represent hydrogen or suitable amino protecting group and R3 represents suitable carboxyl protecting group, from a reaction mass thereof using an organic solvent, b) removing the carboxyl protecting group from DPVC of Formula III obtained in step a) to get MPVC of Formula IV,
(Formula Removed)
wherein R1, and R2 are as described above,
c) removing the amino protecting group from MPVC of Formula IV obtained in step b)
d) isolating 7-AVCA of Formula I from the reaction mass thereof.

2. A process according to claim 1 wherein the organic solvent used in step a) is selected from the group comprising of C1-4 alkanols; C3.8 ketones; polar aprotic solvents and mixtures thereof.
3. A process according to claim 2 wherein the organic solvent is selected from methanol, ethanol, i-propanol, acetone, dimethyl sulphoxide, tetrahydrofuran.
4. A process according to claim 3 wherein the organic solvent is methanol.

5. A process according to claim 1 wherein 7-AVCA obtained as the product has 0.2%
or less of 7-ADCA.
6. A process according to claim 1 wherein R1 represents benzyl carbonyl, R2 represents hydrogen and R3 represents p-methoxybenzyl.
7. A process for preparation of 7-AVCA wherein the said process does not employ purification by chromatography or quaternary salt formation.
8. Crystalline 7-AVCA having characteristic X-ray diffraction (XRD) pattern as
depicted in Figure 1.
9. Crystalline 7-AVCA having X-ray diffraction (XRD) pattern wherein characteristic
29 values are obtained at 6.00, 7.36, 10.76, 12.02, 13.62, 13.88, 14.26, 14.42, 15.24,
15.88, 17.18, 18.04, 19.02, 21.24, 21.48, 21.94, 22.70, 23.36, 24.18, 25.58, 26.72,
28.44, 29.44, 29.66, 30.40, 30.62, 32.28, 33.52, 34.12, 34.54, 34.90, 35.42, 36.18,
36.78, 37.70, 38.02, 39.52 and 39.64.
10. Crystalline 7-AVCA having characteristic Infrared spectrum (IR) as depicted in
Figure 2.