A process for the preparation of hexanoic acid, 6-(nitrooxy)-, (1S,2E)-3-[(1R,2R,3S,5R)-2-[ -7-(ethylamino)-7-oxo-2-hepten-1-yl]-3,5-dihydroxycyclopentyl]-1-(2-phenylethyl)-2-propen-1-yl ester of formula comprising the following steps: 1) reacting bimatoprost with phenylboronic acid in toluene at the refluxing temperature and removing water by azeotropic distillation to obtain bimatoprost phenyl-boronate of formula 2) cooling down the solution to 25°C and adding N,N'-diisopropyl carbodiimide, a catalytic amount of dimethylaminopyridine and 6-(nitrooxy)hexanoic acid to obtain (1E,3S)-1-{(1S,5R,6R,7R)-7-[ -7-(ethylamino)-7-oxohept-2-en-1-yl]-3-phenyl-2,4-dioxa-3-borabicyclo[3.2.1]octan-6-yl}-5-phenylpent-1-en-3-yl 6-(nitrooxy)hexanoate of formula 3) removing the phenylboronate protecting group under basic conditions; 4) separating the organic phase and evaporating the solvent; 5) stirring the raw product in dichloromethane, filtering the mixture and evaporating the solvent to obtain the crude compound of formula; 6) purifying the crude compound of formula by applying a normal phase gravity silica gel column chromatography using an eluent mixture containing diisopropyl ether, acetone and water in 40:15:0.5 volume ratio; 7) collecting the fractions of appropriate purity and evaporating the solvent to provide pure compound of formula; 8) dissolving the pure compound of formula of step 7) in distilled methylene chloride and methanol, purifying the solution by gravity chromatography using an eluent mixture containing distilled methylene chloride and methanol in 30:1 volume ratio and collecting the fractions of appropriate purity and evaporating the solvent to obtain pure compound of formula; 9) dissolving the pure compound of formula of step 8) in a ethanol, treating the solution with activated charcoal, thereafter removing the activated charcoal by filtration and removing the solvent by evaporation under vacuum to yield pure hexanoic acid, 6-(nitrooxy)-, (1S,2E)-3-[(1R,2R,3S,5R)-2-[ -7-(ethylamino)-7-oxo-2-hepten-1-yl]-3,5-dihydroxycyclopentyl]-1-(2-phenylethyl)-2-propen-1-yl ester; said process being characterized in that the obtained hexanoic acid, 6-(nitrooxy)-, (1S,2E)-3-[(1R,2R,3S,5R)-2-[ -7-(ethylamino)-7-oxo-2-hepten-1-yl]-3,5-dihydroxycyclopentyl]-1-(2-phenylethyl)-2-propen-1-yl ester does not contain the 15-(6-chlorohexanoyl) ester of bimatoprost of formula, and contains not more than (≤) 0.20% (HPLC area %) of total impurities. 1) reacting bimatoprost with phenylboronic acid in toluene at the refluxing temperature and removing water by azeotropic distillation to obtain bimatoprost phenyl-boronate of formula 2) cooling down the solution to 25°C and adding N,N'-diisopropyl carbodiimide, a catalytic amount of dimethylaminopyridine and 6-(nitrooxy)hexanoic acid to obtain (1E,3S)-1-{(1S,5R,6R,7R)-7-[ -7-(ethylamino)-7-oxohept-2-en-1-yl]-3-phenyl-2,4-dioxa-3-borabicyclo[3.2.1]octan-6-yl}-5-phenylpent-1-en-3-yl 6-(nitrooxy)hexanoate of formula 3) removing the phenylboronate protecting group under basic conditions; 4) separating the organic phase and evaporating the solvent; 5) stirring the raw product in dichloromethane, filtering the mixture and evaporating the solvent to obtain the crude compound of formula; 6) purifying the crude compound of formula by applying a normal phase gravity silica gel column chromatography using an eluent mixture containing diisopropyl ether, acetone and water in 40:15:0.5 volume ratio; 7) collecting the fractions of appropriate purity and evaporating the solvent to provide pure compound of formula; 8) dissolving the pure compound of formula of step 7) in distilled methylene chloride and methanol, purifying the solution by gravity chromatography using an eluent mixture containing distilled methylene chloride and methanol in 30:1 volume ratio and collecting the fractions of appropriate purity and evaporating the solvent to obtain pure compound of formula; 9) dissolving the pure compound of formula of step 8) in a ethanol, treating the solution with activated charcoal, thereafter removing the activated charcoal by filtration and removing the solvent by evaporation under vacuum to yield pure hexanoic acid, 6-(nitrooxy)-, (1S,2E)-3-[(1R,2R,3S,5R)-2-[ -7-(ethylamino)-7-oxo-2-hepten-1-yl]-3,5-dihydroxycyclopentyl]-1-(2-phenylethyl)-2-propen-1-yl ester;