Process for the preparation of a nitric oxide donating prostaglandin analogue

1 . A process for the preparation of hexanoic acid, 6-(nitrooxy)-(1S,2E)-3-[(1R,2R,3S,5R)-2-[(2Z)-7-(ethylamino)-7-oxo-2-hepten-1-yl]-3,5-dihydroxycyclopentyl]-1-(2-phenylethyl)-2-propen-1-yl ester of formula (I):

said process comprising the following steps:

1) reacting a compound of formula (II):

with a 6-(nitrooxy)hexanoyl chloride of formula (IVa)

in the presence of 4-dimethylaminopyridine in free form, to obtain a compound of formula (III) having a boronate protective group

2) removing the boronate protective group of the compound of formula (III) to obtain a compound of formula (I); and

3) purifying the compound of formula (I);

wherein the 6-(nitrooxy)hexanoyl chloride (IVa) of step 1) is prepared by a process comprising:

4) reacting a 2-caprolactone of formula (V)

with an inorganic base selected from KOH, NaOH and LiOH to obtain a 6-hydroxyhexanoic acid salt of formula (VI)

wherein M is K, Na or Li;

5) nitrating a compound of formula (VI) with a mixture of HNO 3 and H 2 SO 4 to obtain a 6-(nitrooxy)hexanoic acid of formula (VIIa);

6) purifying the 6-(nitrooxy)hexanoic acid of formula (VIIa) to provide a pure 6-(nitrooxy)hexanoic acid of formula (VIIa) by reverse phase chromatography using formic acid in water (H 2 O+HCOOH) and acetonitrile as eluent to obtain a plurality of chromatographic fractions, wherein the pure 6-(nitrooxy)hexanoic acid of formula (VIIa) has a content of 6-{[6-(nitrooxy)hexanoyl]oxy}hexanoic acid (a compound of formula (VIIb))

below an HPLC detection limit of 0.05%; and

7) reacting the pure 6-(nitrooxy)hexanoic acid of formula (VIIa) with a chlorinating agent to obtain the 6-(nitrooxy)hexanoyl chloride (IVa).

2 . The process according to claim 1 wherein step 1) is carried out at a temperature ranging from 0° C. to room temperature in an aprotic organic solvent.

3 . The process according to claim 2 wherein the aprotic organic solvent is methyltertbutyl ether.

4 . The process according to claim 1 wherein in step 1) the molar ratio of compound (II) to 6-(nitrooxy) hexanoyl chloride (IVa) is from 1:1.4 to 1:1.6 and the molar ratio of compound (II) to 4-dimethylaminopyridine is from 1:2.0 to 1:2.4.

5 . The process according to claim 1 wherein in step 4) the inorganic base is potassium hydroxide.

6 . The process according to claim 1 wherein step 4) is carried out in a solvent selected from methanol, ethanol or isopropanol.

7 . The process according to claim 6 wherein the solvent is methanol.

8 . The process according to claim 1 wherein step 5) is carried out in dichloromethane.

9 . The process according to claim 1 wherein in step 6) the concentration of the formic acid in water (H 2 O+HCOOH) is 0.1% w/w.

10 . The process according to claim 1 wherein in step 6), one or more chromatographic fractions of the plurality of chromatographic fractions are extracted with a solvent comprising CH 2 Cl 2 , dried and the solvent is evaporated, the one or more one or more chromatographic fractions containing the compound of formula (VIIa).

11 . The process according to claim 1 wherein step 7) is carried out in dichloromethane and the chlorinating agent is oxalyl chloride.

12 . A process for the preparation of hexanoic acid, 6-(nitrooxy) (1S,2E)-3-[(1R,2R,3S,5R)-2-[(2Z)-7-(ethylamino)-7-oxo-2-hepten-1-yl]-3,5-dihydroxycyclopentyl]-1-(2-phenylethyl)-2-propen-1-yl ester of formula (I);

comprising:

reacting a 2-caprolactone of formula (V)

with an inorganic base selected from KOH, NaOH and LiOH to obtain a 6-hydroxyhexanoic acid salt of formula (VI)

wherein M is K, Na or Li;

nitrating the 6-hydroxyhexanoic acid salt of formula (VI) with a mixture of HNO 3 and H 2 SO 4 to obtain a 6-(nitrooxy) hexanoic acid of formula (VIIa)

purifying the 6-(nitrooxy) hexanoic acid of formula (VIIa) to provide a pure 6-(nitrooxy) hexanoic acid of formula (VIIa) by reverse phase chromatography using formic acid in water (H 2 O+HCOOH) and acetonitrile as eluent to obtain a plurality of chromatographic fractions, wherein the pure 6-(nitrooxy) hexanoic acid of formula (VIIa) has a content of 6-{[6-(nitrooxy) hexanoyl]oxy}hexanoic acid (a compound of formula (VIIb))

below an HPLC detection limit of 0.05%;

reacting the pure 6-(nitrooxy) hexanoic acid of formula (VIIa) with a chlorinating agent to obtain a 6-(nitrooxy) hexanoyl chloride of formula (IVa)

without purification of the 6-(nitrooxy) hexanoyl chloride of formula (IVa), reacting the 6-(nitrooxy) hexanoyl chloride of formula (IVa) with a compound of formula (II);

in the presence of 4-dimethylaminopyridine in free form, to obtain a compound of formula (III) having a boronate protective group

 and

removing the boronate protective group of the compound of formula (III) to obtain a compound of formula (I); and

purifying the compound of formula (I).

13 . A process for the preparation of 6-(nitrooxy) hexanoic acid (compound (VIIa)) having a purity of 99% and a content of {[6-(nitrooxy) hexanoyl]oxy}hexanoic acid (compound (VIIb)) less than 0.05%, wherein said process comprises the following steps:

4) Reacting 2-caprolactone (V)

with KOH in methanol to obtain 6-hydroxyhexanoic acid salt of formula (VI)

wherein M is K;

5) Nitrating the compound of formula (VI) with a mixture of HNO 3 and H 2 SO 4 in dichloromethane at a temperature from about 5° C. to 10° C.;

6) Purifying the mixture by reverse phase chromatography using a solution of 0.1% formic acid in water and acetonitrile as eluents, followed by extraction of fractions with dichloromethane to obtain the 6-(nitrooxy) hexanoic acid (VIIa)

14 . The process of claim 13 , wherein nitrating the compound of formula (VI) with the mixture of HNO 3 and H 2 SO 4 in dichloromethane is performed at a temperature of 10° C.

15 . Hexanoic acid, 6-(nitrooxy)-, (1S,2E)-3-[(1R,2R,3S,5R)-2-[(2Z)-7-(ethylamino)-7-oxo-2-hepten-1-yl]-3,5-dihydroxycyclopentyl]-1-(2-phenylethyl)-2-propen-1-yl ester of formula (I) having a chemical purity above 98% and containing an amount of 15-(6-chlorohexanoyl) ester of bimatoprost (compound (X)) of about 0.12%

and an amount of 6-{[6-(nitrooxy) hexanoyl]oxy}hexanoic acid ester of bimatoprost (compound (XII)) below 0.05%

16 . A pharmaceutical formulation containing hexanoic acid, 6-(nitrooxy)-, (1S,2E)-3-[(1R,2R,3S,5R)-2-[(2Z)-7-(ethylamino)-7-oxo-2-hepten-1-yl]-3,5-dihydroxycyclopentyl]-1-(2-phenylethyl)-2-propen-1-yl ester of formula (I) and at least a pharmaceutically acceptable excipient, wherein hexanoic acid, 6-(nitrooxy)-, (1S,2E)-3-[(1R,2R,3S,5R)-2-[(2Z)-7-(ethylamino)-7-oxo-2-hepten-1-yl]-3,5-dihydroxycyclopentyl]-1-(2-phenylethyl)-2-propen-1-yl ester has a chemical purity above 98%, and contains an amount of 15-(6-chlorohexanoyl) ester of bimatoprost (compound (X)) about 0.12% and an amount of the dimeric impurity 6-{[6-(nitrooxy) hexanoyl]oxy}hexanoic acid ester of bimatoprost (compound (XII)) below 0.05%.