it will be appreciated that the compound provided by the present invention is an isomer . this isomer may exist and be isolated in enantiomerically pure form , or in admixture with one or more of its other isomers . the present invention provides the isomer in any mixture of isomers other than a racemic mixture , which is described in gb 1 , 415 , 256 . in certain embodiments , the isomer is substantially free of the ( r , r )- enantiomer , which can exhibit a different potency , resulting in significant variations in the potency of admixtures . it may exist as a 1 : 1 diastereomeric mixture with the ( r , s )- isomer , but is most preferably enantiomerically pure ( i . e . substantially free of all other isomers ). for example , the isomer may comprise at least 50 % by weight of all 3 - acetylamino - 4 - hydroxy - α - phenylethyl ) aminomethyl benzyl alcohol present , preferably at least 75 %, such as at least 90 %, at least 95 % or at least 99 %. as used herein , the term “ pharmaceutically acceptable salt ” refers to a salt prepared from a pharmaceutically acceptable , relatively non - toxic acid , including inorganic acids and organic acids . suitable acids include acetic , benzenesulfonic , benzoic , camphorsulfonic , carbonic , citric , dihydrogenphosphoric , ethenesulfonic , fumaric , galactunoric , gluconic , glucuronic , glutamic , hydrobromic , hydrochloric , hydriodic , isobutyric , isethionic , lactic , maleic , malic , malonic , mandelic , methanesulfonic , monohydrogencarbonic , monohydrogenphosphoric , monohydrogensulfuric , mucic , nitric , pamoic , pantothenic , phosphoric , phthalic , propionic , suberic , succinic , sulfuric , tartaric , toluenesulfonic , including p - toluenesulfonic m - toluenesulfonic and o - toluenesulfonic acids , and the like ( see , e . g ., berge et al ., j . pharm . sci ., 66 : 1 - 19 ( 1977 ); stahl and wermuth , handbook of pharmaceutical salts , wiley vch , ( 2002 )). also included are salts of other relatively non - toxic compounds that possess acidic character , including amino acids , such as arginine and the like , and other compounds , such as aspirin , ibuprofen , saccharin , and the like . acid addition salts can be obtained by contacting the neutral form of such compounds with a sufficient amount of the desired acid , either neat or in a suitable inert solvent . as solids , salts can exist in crystalline or amorphous modifications . the compounds of the present invention may also be prepared in deuterated form , i . e ., in which one or more hydrogen atoms , for example on the acetyl group , are replaced with deuterium . it is also contemplated that the acetyl group in the compound of formula ( i ) may be replaced with a fluoroacetyl group ( i . e . a group in which one , two or three of the acetyl hydrogen atoms is replaced with a fluorine atom ). such compounds may be prepared by a process analogous to that described herein for the preparation of the acetyl compound . the acetamide isomer and its pharmaceutically acceptable salts can be prepared by a process , which comprises reacting a compound of general formula ( ii ) in which p 1 represents a hydrogen atom or a hydroxyl protecting group , with a compound of general formula ( iii ) in which p 2 represents a hydrogen atom or a hydroxyl protecting group and p 3 represents a benzylic amine protecting group , to afford a compound of general formula ( iv ) or a salt thereof , followed by removing any protecting groups p 1 , p 2 and p 3 and , if desired , forming a pharmaceutically acceptable acid addition salt . the protecting groups may be any suitable protecting group , for example as described in green et al ., “ protective groups in organic chemistry ,” ( wiley , 2 nd ed . 1991 ). examples of hydroxyl protecting groups include aralkyl groups , such as benzyl , and trialkylsilyl groups , such as t - butyl - dimethylsilyl ( tbdms ). examples of a benzylic amine protecting group are benzyl groups optionally substituted on the benzene ring by one or more , for example 1 , 2 or 3 optional substituents , for example selected from halo , ( 1 - 4c ) alkyl and ( 1 - 4c ) alkoxy ; for example unsubstituted benzyl . the reaction between the compounds of formula ( ii ) and ( iii ) is conveniently performed by melting the two compounds together , for example by heating in the range of from 110 to 130 ° c . any protecting groups represented by p 1 , p 2 and p 3 may be removed using a conventional procedure . for example , a benzyl group can be removed by catalytic hydrogenation in the presence of palladium on carbon , and a trialkylsilyl group by treatment with tetrabutylammonium fluoride . compounds of formula ( ii ) can be prepared by reacting a compound of formula ( v ) in which z represents a leaving atom or group , such as a bromine atom , with a base , for example an alkali metal carbonate such as potassium carbonate . compounds of formula ( v ) can be prepared by stereoselective reduction of a compound of formula ( vi ) using , for example , borane in the presence of a chiral auxiliary , such as ( 1s , 2r )- 1 - amino - 2 - indanol , followed by reduction of the nitro group to an amino group and acetylation of the resultant amino group . compounds of general formula ( iii ) can be prepared by reacting a compound of general formula ( vii ) the hydroxyl group may then be protected , for example by reaction with a trialkylsilyl halide , such as t - butyldimethylsilyl chloride . it will be appreciated that the percentage by weight comprised by the compound of formula ( i ) of all 3 - acetylamino - 4 - hydroxy - α - phenylethyl ) aminomethyl benzyl alcohol present in the final product of the process will depend upon the enantiomeric purity of the starting materials used and any enantiomeric purification steps taken , such as chiral liquid chromatography . the intermediates of general formula ( iv ) are believed to be novel and are provided as a further aspect of the present invention . according to another aspect , therefore , the present invention provides a pharmaceutical composition , which comprises a compound of formula ( i ) or a pharmaceutically acceptable salt thereof , as described herein , together with a pharmaceutically acceptable carrier . the pharmaceutical composition according to the invention may be adapted for administration to patients by any convenient route , such as by oral , mucosal ( e . g . nasal , sublingual , vaginal , buccal or rectal ), parenteral or transdermal administration . it may be in the form of , for example , a solution , suspension , powder , tablet , aerosol formulation , lozenge , suppository , emulsion , hard or soft gelatin capsule or syrup . the compound of formula ( i ) may be dissolved in the carrier , diluted by the carrier or supported by the carrier . thus the carrier may be a support for the compound of formula ( i ), such as a capsule , sachet , paper or other pharmaceutical container . in one embodiment , the pharmaceutical composition is an aqueous solution adapted for administration using a nebuliser . the aqueous formulation may be isotonic and buffered at an optimal ph for stability . the aqueous formulation for nebulization could also be a suspension of nanoparticles or a micronized suspension of free base or an insoluble salt or a cyclodextrin adduct . in another embodiment , the pharmaceutical composition is an aerosol formulation adapted for administration using a metered dose inhaler , the aerosol formulation comprising the acetamide isomer in crystalline form and a propellant or in solution with an appropriate propellant , combination of propellants or combination of propellant ( s ) and an acceptable co - solvent or other solubilizing agent . the propellant may be any suitable propellant used in aerosol formulations , for example , a hydrofluoroalkane ( hfa ), such as 1 , 1 , 1 , 2 - tetrafluoroethane ( hfa134 ) or 1 , 1 , 1 , 2 , 3 , 3 , 3 - heptafluoropropane ( hfa227 ) or a combination of propellants . hfa134 is preferred . the propellant may comprise at least 90 % by weight of the aerosol formulation , which may also include , inter alia , inert gases to aide in aerosol formation . the aerosol formulation may further comprise a surfactant . the surfactant serves to stabilize and disperse the acetamide isomer in a suspension , and may also serve as a valve lubricant in the metered dose inhaler . it may be any suitable surfactant used in aerosol formulations . examples of surfactants used in aerosol formulations are described in u . s . pat . no . 5 , 225 , 183 , which is hereby incorporated by reference . a preferred surfactant is oleic acid . the surfactant , when present , may generally be present in an amount of from 1 : 100 to 1 : 10 surfactant : acetamide isomer , preferably about 1 : 20 . the aerosol formulation may further comprise a co - solvent . a function of the co - solvent in the aerosol formulation is to facilitate dissolution of the surfactant , which may have poor solubility in the propellant . it may be any suitable carrier used in aerosol formulations . a co - solvent such as glycerol or ethanol may be used . a preferred co - solvent is ethanol , especially dehydrated ethanol . the content of ethanol may conveniently be up to 30 % by weight of the aerosol formulation , such as from 2 to 6 %. metered dose inhalers typically comprise a canister containing an aerosol formulation , a metering valve , a valve stem and an actuator which accepts the valve stem . in use , a patient depresses the canister into the actuator and inhales , causing a dose of the formulation to be administered and taken into the patient &# 39 ; s lungs . according to a further aspect , therefore , the present invention provides a metered dose inhaler comprising a canister containing an aerosol formulation as described herein , a metering valve and an actuator . preferably the interior surface of the canister is coated , for example with a protective polymer , or otherwise treated to minimize chemical or physical interaction between the formulation and the canister . the inhaler preferably has an aperture with a diameter in the range of from 0 . 2 to 0 . 60 mm . in yet another embodiment , the pharmaceutical composition is in the form of a dry powder suitable for inhalation or insufflation . the composition may comprise acetamide isomer crystals alone ( e . g . having a mass median aerodynamic diameter of from 1 to 10 microns , preferably from 2 to 7 microns ), or acetamide isomer blended , co - precipitated , co - crystallized or spray dried together with a suitable pharmaceutically acceptable carrier or carriers . suitable pharmaceutically acceptable carriers include , without limitation , solvates of one or more natural or synthetic carbohydrates , such as a monosaccharides , disaccharides , trisaccharides , oligosaccharides , polysaccharides , polyols , amino acids and proteins , and / or in the form of their pharmaceutically acceptable esters , acetals , or salts ( where such derivatives exist ). the carrier is preferably lactose , more preferably lactose monohydrate . the dry powder composition may be presented in unit dosage form in , for example , capsules or cartridges of e . g . gelatin , or blister packs from which the powder may be administered with the aid of an inhaler or insufflator . the dry powder composition may be presented in multi dose form metered with the aid of an inhaler or insufflator , or pre - metered into discrete doses within the device for serial administrations . the present invention therefore also provides a multidose dry powder inhaler , comprising a dry powder reservoir containing a dry powder aerosol formulation of acetamide isomer as described hereinabove , and a metering chamber . the compound of formula ( i ) according to the present invention may be co - administered with one of more other active ingredients , for example selected from steroids , such as beclomethasone , triamcinolone , funisolide , mometasone , budesonide or fluticasone , muscarinic receptor antagonists , such as ipratropium , tiatropium , or glycopyrrolate . accordingly , in one embodiment , the pharmaceutical composition in accordance with the present invention may further comprise a steroid and / or a muscarinic receptor antagonist and / or a controller agent or bronchodilator with a novel mechanism . in another embodiment , the pharmaceutical composition in accordance with the present invention may further comprise anti - inflammatory agents such as inhibitors of tumor necrosis factor alpha ( tnfα ), dipeptidyl peptidase iv , and antibodies to pro - inflammatory interleukins such as il4 and il13 . in another embodiment , the pharmaceutical composition in accordance with the present invention may further comprise mucolytic agents such as cromoglycate , acetylcysteine , arginine , or 2 - mercaptoethanesulphonate . according to another aspect , the present invention provides a method of treating bronchoconstrictive disease , which comprises administering to a patient in need of treatment an effective amount of a compound of formula ( i ) or a pharmaceutically acceptable salt thereof . the bronchoconstrictive disease may be , for example , chronic obstructive pulmonary disease ( such as emphysema or bronchitis ), cystic fibrosis , or asthma . the patient may be a human or a non - human mammal , such as a dog , cat , horse , cow , sheep or pig . preferably , the patient is a human . the amount of compound administered will depend upon many factors , such as the species , weight and age of the patient , and the severity of the condition to be treated . for example , a dose administered to a human may contain from 75 to 5 , 000 μg of the acetamide isomer ( calculated as the free base ). the dose may be administered , for example , once or twice per day . according to another aspect , the present invention provides a compound of formula ( i ) or a pharmaceutically acceptable salt thereof , for use in therapy . according to yet another aspect , the present invention provides the use of a compound of formula ( i ) or a pharmaceutically acceptable salt thereof in the manufacture of a medicament for the treatment of chronic obstructive pulmonary disease . according to a still further aspect , the present invention provides a pharmaceutical composition comprising a compound of formula ( i ) or a pharmaceutically acceptable salt thereof and a pharmaceutically acceptable carrier for the treatment of chronic obstructive pulmonary disease , or for use as a bronchodilator . although the foregoing invention has been described in some detail for purposes of illustration , it will be readily apparent to one skilled in the art that changes and modifications may be made without departing from the scope of the invention described herein . thf refers to tetrahydrofuran , etoac refers to ethyl acetate and et 2 o refers to diethyl ether . a cold ( 5 ° c .) solution of ( 1s , 2r )- 1 - amino - 2 - indanol ( 400 mg , 2 . 68 mmol ) in thf ( 160 ml ) was added dropwise to a cold ( 0 ° c .) solution of borane - diethylaniline complex ( 7 . 0 g , 43 mmol ) in thf ( 20 ml ). after complete addition , the resulting solution was stirred at ( 0 ° c .) for 30 min then 2 - bromo - 4 ′- benzyloxy - 3 ′- nitroacetophenone ( 20 . 0 g , 57 . 1 mmol ) was added in three portions over a 30 min period . the resulting solution was stirred at & lt ; 5 ° c . for 1 h , quenched by dropwise addition of acetone ( 17 ml ) then allowed to warm to ambient temperature overnight . the reaction mixture was concentrated in vacuo to a residue , which was dissolved in toluene ( 100 ml ) and washed in succession with 10 % h 2 so 4 ( 2 × 45 ml ), h 2 o ( 2 × 45 ml ) and sat . brine ( 1 × 40 ml ). the organic layer was dried over mgso 4 , clarified then concentrated in vacuo to a volume of ˜ 40 ml . heptane ( 45 ml ) was slowly added to give a thick slurry . the solid was collected on a filter and washed with heptane ( 2 × 5 ml ). this material was dissolved in warm toluene (˜ 50 ml ), the solution was clarified then diluted with heptane ( 50 ml ). the resulting mixture was stirred for 30 min , the solids were collected , washed with heptane ( 2 × 5 ml ) then dried to constant weight in vacuo to give 18 . 9 g ( 94 . 0 %) of the title compound . a solution of the product of step a ) ( 18 . 7 g , 53 . 1 mmol ) in toluene ( 40 ml ) and thf ( 40 ml ) was added to a parr shaker bottle containing pt 2 o ( 370 mg ). this mixture was shaken under h 2 ( 50 psi , 344 . 74 kpa ) until the reaction was complete ( 18 h ). the catalyst was removed by filtration , and the filtrate was concentrated to an oil . column chromatography ( 1 kg silica gel packed in and eluted with ch 2 cl 2 / meoh , 19 : 1 ) gave 11 . 9 g ( 69 . 6 %) of the title compound . a solution of the product in step b ( 10 . 0 g , 31 . 0 mmol ) in pyridine ( 100 ml ) was stirred at ambient temperature for 10 min . acetic anhydride ( 3 . 16 g , 30 . 9 mmol ) was added , and the reaction mixture was stirred at ambient temperature for 30 min then at 40 ° c . for 4 . 5 h . the reaction mixture was cooled to ambient temperature then concentrated in vacuo to a residue . this material was partitioned between ch 2 cl 2 ( 120 ml ) and 10 % aq hcl ( 50 ml ). the aqueous layer was extracted with ch 2 cl 2 ( 50 ml ). the combined organic layers were washed in succession with h 2 o ( 1 × 100 ml ) and brine ( 1 × 100 ml ), dried over mgso 4 , clarified then concentrated in vacuo to a thick slurry . after dilution with hexanes ( 60 ml ), the solid was collected on a filter , washed with hexanes ( 2 × 20 ml ) then dried to constant weight in vacuo to give 9 . 4 g ( 83 %) of material as a white solid . a solution of the product in step c ( 2 . 5 g , 6 . 9 mmol ) in meoh ( 15 ml ) and thf ( 15 ml ) was treated with k 2 co 3 ( 1 . 3 g , 9 . 4 mmol ), and the resulting mixture was stirred at ambient temperature for 2 . 5 h . the mixture was concentrated in vacuo to a residue , which was partitioned between h 2 o ( 50 ml ) and etoac ( 50 ml ). the aqueous layer was extracted with etoac ( 50 ml ). the combined organic extracts were washed with h 2 o ( 50 ml treated with a pinch of k 2 co 3 to make it basic ), dried over na 2 so 4 , clarified then concentrated in vacuo to a residue that was dried to constant weight in vacuo to give 1 . 84 g ( 94 %). this material was used in the next reaction step without further purification . to a solution of [( 1r )- n - benzyl - 2 -( 4 - methoxyphenyl )- 1 - methylethyl ] amine ( 5 . 30 g , 20 . 8 mmol ) in ch 2 cl 2 ( 25 ml ) was added a solution of bbr 3 in ch 2 cl 2 ( 25 . 0 ml , 1 . 0m , 25 . 0 mmol ) slowly over 0 . 5 h . after the addition , the mixture was stirred at ambient temperature for 22 h . water ( 125 ml ) was added , followed by the addition of 2 . 5m aq . naoh ( 15 ml ) to ph 6 . the mixture was extracted with etoac ( 4 × 200 ml ), and the organic layer was dried ( na 2 so 4 ) and concentrated . the residue ( 3 . 9 g ) was triturated with ch 2 cl 2 ( 120 ml ) and then concentrated to dryness to give the title compound ( 3 . 8 g , 76 %). a solution of the product of step e ) ( 3 . 20 g , 13 . 3 mmol ), tert - butyldimethylsilyl chloride ( 3 . 59 g , 23 . 8 mmol ), and imidazole ( 2 . 86 g , 42 . 0 mmol ) in dmf ( 30 . 0 ml ) was stirred at ambient temperature for 18 h . the mixture was concentrated to dryness , and the residue was partitioned between etoac ( 200 ml ) and sat . aq . nahco 3 ( 200 ml ). the aqueous layer was separated and again extracted with etoac ( 100 ml ). the combined organic layer was washed with brine ( 100 ml ), dried with na 2 so 4 , filtered and concentrated to give an oil . the oil was chromatographed on silica gel ( 100 g , eluted with 1 : 1 etoac : hexanes ) to give the title compound ( 4 . 0 g , 85 %) as a tan oil . a mixture of the products of step d ) ( 1 . 75 g , 6 . 18 mmol ) and f ) ( 2 . 20 g , 6 . 19 mmol ) was heated slowly to 110 ° c . to give a complete solution . the reaction solution was heated at 120 ° c . for 20 h . tlc ( etoac / hexanes , 1 : 1 ) showed an estimated 5 % of starting material remaining . heating was continued at 120 ° c . for 5 h then the solution was cooled to ambient temperature and chromatographed over a column of silica gel ( 200 g ) packed in and eluted with hexanes / etoac ( 2 : 1 ). fractions containing purified material were combined , clarified then concentrated in vacuo to afford the title compound as a yellow oil , 3 . 4 g ( 86 %). to a stirred solution of the product of step g ) ( 2 . 20 g , 3 . 44 mmol ) in thf ( 22 ml ) at 5 - 10 ° c ., a solution of tetra - n - butylammonium fluoride in thf ( 1 . 0 m , 4 . 50 ml , 4 . 50 mmol ) was added . the reaction mixture was stirred at ambient temperature for 1 h ( note 2 ), and tlc ( 1 : 1 etoac / hexanes ) showed complete consumption of 8 . the reaction mixture was diluted with etoac ( 300 ml ) and washed with deionized h 2 o ( 4 × 200 ml ), then dried ( mgso 4 ), filtered , and concentrated to give crude 9 ( 2 . 3 g ). the crude material was purified on a column of flash silica gel ( 82 g , 2 . 7 × 35 cm ), packed in and eluted with 1 : 1 etoac / hexanes . fractions containing the purified product were combined and concentrated to give a colorless , viscous oil . the oil was coevaporated with et 2 o ( 3 × 25 ml ) to give the title compound ( 919 mg , 51 %) as a white solid . a mixture of the product of step h ) ( 900 mg , 1 . 72 mmol ), palladium on carbon ( 500 mg of 10 wt % pd ) and etoh ( 45 ml ) was shaken under h 2 ( 50 psi , 344 . 74 kpa ) for 22 h . the mixture was clarified then concentrated in vacuo to an oily residue . the residue was co - evaporated with etoac ( 100 ml ) and etoac / ch 2 cl 2 ( 1 : 1 , 100 ml ) to give a white solid ( 536 mg ). tlc ( 4 : 1 ch 2 cl 2 / meoh ) of the solid showed some upper rf impurities . the solid was dissolved in ch 2 cl 2 ( 20 ml )/ meoh ( 1 ml ), then concentrated in the cold to ˜ 10 ml . hexanes (˜ 20 ml ) were added , and the mixture was again concentrated in the cold to give a suspension . a mixture of et 2 o ( 5 ml ), etoac ( 5 ml ) and hexanes ( 5 ml ) was added , and the mixture was concentrated to give a slurry . this slurry was diluted with hexanes ( 25 ml ), and the suspension was stirred vigorously for 0 . 5 h at room temperature . the solid was collected and washed with hexanes ( 25 ml ) to give the title compound ( 318 mg , 54 %); m . p . 97 - 100 ° c ., with previous softening ( uncorrected ). ms m / z : [ m + h + ] 345 . 1 . 1 h nmr spectrum consistent with the assigned structure . the affinity of a test compound for adrenergic β 1 and β 2 receptors is investigated by evaluating the ability of the compound to displace specific binding of [ 125 i ]- cyanopidolol or [ 3 h ]- cgp - 12177 at human recombinant β 1 and β 2 receptors , respectively ( expressed in cho cells ). the ic 50 is defined as the concentration that inhibits 50 % of specific binding of the radioligand . the k i is calculated from the ic 50 and the known k d of the radioligand ( cheng and prusoff &# 39 ; s equation ). in this test , the compound of example 1 was found to afford a k i of & gt ; 20 μm with only 30 % inhibition of specific binding at a concentration of 20 μm for the β 1 receptor and 2 . 21 μm for the β 2 receptor . the β 1 / β 2 binding ratio was found to be & gt ; 9 . by way of comparison , the values found for arformoterol and the ( s , r ) isomer of formoterol were 0 . 155 μm ( β 1 ), 0 . 004 μm ( β 2 ) and 41 ( β 1 / β 2 ), and 2 . 50 μm ( β 1 ), 0 . 075 μm ( β 2 ) and 33 ( β 1 / β 2 ), respectively . the intrinsic activity of a test compound is assessed by evaluating its ability to increase camp production from human recombinant β 2 receptors expressed in cho cells . data are expressed as % response relative to a procaterol - induced camp increase . the compound of example 1 was found to have an intrinsic activity of 71 %. by way of comparison , arformoterol and ( s , r )- formoterol were found to have intrinsic activities of 98 % and 91 % respectively . functional agonism at adrenergic β 1 receptors is demonstrated by a positive chronotropic effect in isolated right atria from dunkin hartley guinea pigs . the concentration that gives 50 % maximal effect is the ec 50 . functional agonism at adrenergic β 2 receptors is demonstrated by relaxation of the spontaneous tone of isolated trachea from dunkin hartley guinea pigs . the concentration that gives 50 % maximal effect is the ec 50 . in these tests , an ec 50 could not be determined for the compound of example 1 for the β 1 functional assay as only a 32 % increase in heart rate was seen at a concentration of 30 μm . however , the compound of example 1 was found to have an ec 50 of 120 nm for the β 2 receptor . the β 1 / β 2 functional ratio was found to be & gt ; 250 . by way of comparison , the values found for arformoterol were 3 nm ( β 1 ), 0 . 041 nm ( β 2 ) and 75 . solution preparations : for each test compound , the following solutions are prepared . solution a is prepared from ˜ 30 mg of the test compound in 150 ml of 0 . 005 m citrate buffer , ph 5 . 0 (˜ 0 . 2 mg / ml ). solution b is prepared as follows : approximately 30 ml aliquot of solution a is transferred to a separated container and the ph of the solution is adjusted to ph 3 . 0 with 1 n hcl (˜ 0 . 2 ml ). solution c is prepared as follows : approximately 30 ml aliquot of solution a is transferred to a separated container and the ph of the solution is adjusted to ph ˜ 8 . 0 with 1 n naoh (˜ 0 . 2 ml ). note : because the volume of 1 n hcl or 1 n naoh used for adjusting ph was negligible , the concentration of test compound in solutions a , b and c were the same . as soon as the above solutions were prepared , aliquots of each solution were transferred into 11 vials , of which 9 vials are stored at − 20 ° c ., and one each is stored at 30 ° c . and 40 ° c ., respectively . at each interval listed below , two vials are removed from − 20 ° c . storage , and stored at 30 ° c . and 40 ° c ., respectively . the corresponding weeks under the storage condition ( 30 ° c . or 40 ° c .) are shown in the table below . on week 12 , the last vial stored at − 20 ° c . is removed and warmed up to the room temperature , which is the day 0 solution . sample analysis : on week 12 , all solutions are assayed by an hplc method using the day 0 solution at ph 5 as a standard solution . the test compounds were assayed by hplc with uv detection ( refer to table 1 for method conditions ). * these results were out of trend , but the cause was unknown . the high results at ph 5 might be due to evaporation at elevated temperature during storage . conclusion : at ph 3 or 5 , the compound of example 1 was found to be very stable for at least 12 weeks when stored at 30 ° c .