NOVELTY - Preparing lubiprostone, comprises: (a) reacting (4R)-4-(4,4-difluoro-3-oxo-octyl)-5-phenoxy-3,3a,4,5,6,6a-hexahydrocyclopenta(b)furan-2-one (II) with a reagent to provide a difluoro compound (III); (b) reacting (III) with dihydropyran to provide (4R)-4-(4,4-difluoro-3-oxo-octyl)-5-tetrahydropyran-4-yloxy-3,3a,4,5,6,6a-hexahydrocyclopenta(b)furan-2-one (IV); and (c) converting (IV) into (4R)-4-(4,4-difluoro-3-hydroxy-octyl)-5-tetrahydropyran-4-yloxy-3,3a,4,5,6,6a-hexahydrocyclopenta(b)furan-2-one (V).    USE - The method is useful for preparing lubiprostone (claimed), which is useful for treating chronic idiopathic constipation in adults, and irritable bowel syndrome with constipation in 18 years old women.    ADVANTAGE - The method provides lubiprostone and its intermediates in cost-effective and environment friendly manner.    DETAILED DESCRIPTION - Preparing lubiprostone, comprises: (a) reacting (4R)-4-(4,4-difluoro-3-oxo-octyl)-5-phenoxy-3,3a,4,5,6,6a-hexahydrocyclopenta(b)furan-2-one (II) with a reagent to provide a difluoro compound of formula (III); (b) reacting (III) with dihydropyran to provide (4R)-4-(4,4-difluoro-3-oxo-octyl)-5-tetrahydropyran-4-yloxy-3,3a,4,5,6,6a-hexahydrocyclopenta(b)furan-2-one (IV); (c) converting (IV) into (4R)-4-(4,4-difluoro-3-hydroxy-octyl)-5-tetrahydropyran-4-yloxy-3,3a,4,5,6,6a-hexahydrocyclopenta(b)furan-2-one (V); (d) converting (V) into (2R,4R)-4-(4,4-difluoro-3-hydroxy-octyl)-5-tetrahydropyran-4-yloxy-3,3a,4,5,6,6a-hexahydro-2H-cyclopenta(b)furan-2-ol (VI); (e) reacting (VI) with (4-carboxybutyl) triphenylphosphonium bromide to provide (Z)-7-((2R,5S)-2-((3S)-4,4-difluoro-3-hydroxy-octyl)-5-hydroxy-3-tetrahydropyran-4-yloxy-cyclopentyl)hept-5-enoic acid (VII); (f) converting (VII) into phenyl (Z)-7-((2R,5S)-2-((3S)-4,4-difluoro-3-hydroxy-octyl)-5-hydroxy-3-tetrahydropyran-4-yloxy-cyclopentyl)hept-5-enoate (VIII); (g) reacting (VIII) with oxidizing agent to provide phenyl (Z)-7-((2R)-2-(4,4-difluoro-3-oxo-octyl)-5-oxo-3-tetrahydropyran-4-yloxy-cyclopentyl)hept-5-enoate (IX); (h) reacting (IX) with an acid to provide phenyl (Z)-7-((2R,3aR)-2-(1,1-difluoropentyl)-2-hydroxy-5-oxo-3a,4,6,6a-tetrahydro-3H-cyclopenta(b)furan-4-yl)hept-5-enoate (X); and (i) converting (X) into lubiprostone.