the term ‘ c 1 - 6 alkyl ’ as used herein as a group or a part of the group refers to a linear or branched saturated hydrocarbon group containing from 1 to 6 carbon atoms . examples of such groups include methyl , ethyl , n - propyl , isopropyl , n - butyl , isobutyl , sec - butyl , tert butyl , n - pentyl , isopentyl , neopentyl or hexyl and the like . the term ‘ c 1 - 6 alkoxy ’ as used herein refers to an — o — c 1 - 6 alkyl group wherein c 1 - 6 alkyl is as defined herein . examples of such groups include methoxy , ethoxy , propoxy , butoxy , pentoxy or hexoxy and the like . the term ‘ c 1 - 6 alkanol ’ as used herein refers to a c 1 - 6 alkyl group substituted by one or more hydroxy groups . examples of such groups include methoxy , ethoxy , propoxy , butoxy , pentoxy or hexoxy and the like . the term ‘ c 3 - 8 cycloalkyl ’ as used herein refers to a saturated monocyclic hydrocarbon ring of 3 to 8 carbon atoms . examples of such groups include cyclopropyl , cyclobutyl , cyclopentyl , cyclohexyl , cycloheptyl or cyclooctyl and the like . the term ‘ halogen ’ as used herein refers to a fluorine , chlorine , bromine or iodine atom . the term ‘ haloc 1 - 6 alkyl ’ as used herein refers to a c 1 - 6 alkyl group as defined herein wherein at least one hydrogen atom is replaced with halogen . examples of such groups include fluoroethyl , trifluoromethyl or trifluoroethyl and the like . the term ‘ haloc 1 - 6 alkoxy ’ as used herein refers to a c 1 - 6 alkoxy group as herein defined wherein at least one hydrogen atom is replaced with halogen . examples of such groups include difluoromethoxy or trifluoromethoxy and the like . the term ‘ aryl ’ as used herein refers to a c 6 - 12 monocyclic or bicyclic hydrocarbon ring wherein at least one ring is aromatic . examples of such groups include phenyl , naphthyl or tetrahydronaphthalenyl and the like . the term ‘ heterocyclyl ’ as used herein refers to a 5 - 7 membered monocyclic aromatic ring , a fused 8 - 10 membered bicyclic aromatic ring , a 4 - 7 membered saturated or partially saturated monocyclic ring or a fused 8 - 12 membered saturated or partially saturated bicyclic ring containing 1 to 4 heteroatoms selected from oxygen , nitrogen and sulphur . examples of such monocyclic aromatic rings include thienyl , furyl , furazanyl , pyrrolyl , triazolyl , tetrazolyl , imidazolyl , oxazolyl , thiazolyl , oxadiazolyl , isothiazolyl , isoxazolyl , thiadiazolyl , pyranyl , pyrazolyl , pyrimidyl , pyridazinyl , pyrazinyl , pyridyl , triazinyl , tetrazinyl and the like . examples of such fused aromatic rings include quinolinyl , isoquinolinyl , quinazolinyl , quinoxalinyl , pteridinyl , cinnolinyl , phthalazinyl , naphthyridinyl , indolyl , isoindolyl , azaindolyl , indolizinyl , indazolyl , purinyl , pyrrolopyridinyl , furopyridinyl , benzofuranyl , isobenzofuranyl , benzothienyl , benzoimidazolyl , benzoxazolyl , benzoisoxazolyl , benzothiazolyl , benzoisothiazolyl , benzoxadiazolyl , benzothiadiazolyl and the like . examples of such saturated or partially saturated monocyclic rings include pyrrolidinyl , azetidinyl , pyrazolidinyl , oxazolidinyl , piperidinyl , piperazinyl , morpholinyl , thiomorpholinyl , thiazolidinyl , hydantoinyl , valerolactamyl , oxiranyl , oxetanyl , dioxolanyl , dioxanyl , oxathiolanyl , oxathianyl , dithianyl , dihydrofuranyl , tetrahydrofuranyl , dihydropyranyl , tetrahydropyranyl , tetrahydropyridinyl , tetrahydropyrimidinyl , tetrahydrothiophenyl , tetrahydrothiopyranyl , diazepanyl , azepanyl and the like . examples of such saturated or partially saturated bicyclic rings include indolinyl , isoindolinyl , benzopyranyl , quinuclidinyl , 2 , 3 , 4 , 5 - tetrahydro - 1h - 3 - benzazepine , tetrahydroisoquinolinyl and the like . it will be appreciated that compounds of formula ( i ) may exist in a variety of differing optical configurations . for example , in one embodiment , the compound of formula ( i ) has the stereochemistry shown in the following compound of formula ( i ) a : wherein r 1 , r 2 , r 3 and r 4 are as defined for compounds of formula ( i ). the stereochemistry demonstrated by compounds of formula ( i ) a provides the advantage of preparing the optimum diastereoisomer of atorvastatin which therefore results in the preparation of optically pure atorvastatin . in one embodiment , r 1 and r 2 independently represent hydrogen or c 1 - 6 alkyl . in a further embodiment , r 1 and r 2 both represent c 1 - 6 alkyl ( e . g . methyl ). in a yet further embodiment , r 1 and r 2 both represent methyl . in one embodiment , r 3 and r 4 do not both represent hydrogen . in one embodiment , r 3 represents c 1 - 6 alkyl optionally substituted by one or more ( e . g . 1 to 3 ) r 7 groups , c 2 - 6 alkenyl optionally substituted by one or more ( e . g . 1 to 3 ) r 7 groups , — x - heterocyclyl or — c ( r 5 )═ o . in one embodiment , r 3 represents c 1 - 6 alkyl optionally substituted by one or more r 7 groups ( e . g . — ch 2 — ch 2 — nh 2 , — ch 2 — ch 2 — nhcooch 3 , — ch 2 — ch 2 — conh 2 , — ch 2 — ch 2 — no 2 , — ch 2 — ch 2 — coo — ch 2 — ch 3 , — ch ( oh )— ch 2 — no , or — ch ( oh )— ch 2 oh ). in one embodiment , r 3 represents c 2 - 6 alkenyl optionally substituted by one or more r 7 groups ( e . g . — ch ═ ch — coo — ch 2 — ch 3 or — ch ═ ch — no 2 ). in one embodiment , r 3 represents — x - heterocyclyl optionally substituted by one or more c 1 - 6 alkyl groups , e . g . a group of formula ( i ): in one embodiment , r 3 represents — c ( r 5 )═ o ( e . g . — c ( h )═ o . in one embodiment , r 4 represents hydrogen , — or 5 , — x - heterocyclyl or — c (═ y )— o — r 5 . in one embodiment , r 4 represents — or 5 ( e . g . — oh ). in one embodiment , r 4 represents — x - heterocyclyl ( e . g . — o — so 2 - imidazole ). in a further embodiment , r 4 represents a group of formula ( ii ): in one embodiment , r 4 represents — c (═ y )— o — r 5 ( e . g . — c (═ s )— o — ch 3 ). according to a second aspect of the invention , there is provided a process for preparing a compound of formula ( ii ): wherein r 1 and r 2 are as defined for compounds of formula ( i ) and n represents an integer from 1 to 4 ; wherein r 1 , r 2 and n are as defined for compounds of formula ( ii ) and p 1 represents a suitable protecting group , such as carbobenzyloxy ( cbz ), t - butyloxycarbonyl ( boc ), 9 - fluorenylmethyloxycarbonyl ( fmoc ), benzyl , p - methoxyphenyl or an acetate ( e . g . — cooch 3 ) group ; or wherein r 1 , r 2 and n are as defined for compounds of formula ( ii ). step ( a ) typically comprises a hydrolysis reaction in the presence of a suitable base , such as sodium hydroxide . step ( b ) typically comprises a hydrogenation reaction , e . g . reaction of compound of formula ( iv ) with hydrogen in the presence of a suitable catalyst , such as raney nickel . in one embodiment , n represents an integer from 1 - 3 . in a further embodiment , n represents 2 . compounds of formula ( iii ) wherein n represents 2 and p 1 represents -— cooch 3 may be prepared in accordance with the following scheme 1 : wherein r 1 and r 2 are as defined above for compounds of formula ( ii ). step ( i ) typically comprises a wittig reaction by condensing a compound of formula ( v ) with a wittig reagent in the presence of a suitable base in a suitable solvent to afford a compound of formula ( vi ). in one embodiment , the wittig reagent is triethylphosphonoacetate and the solvent is 1 , 2 - dimethoxyethane . step ( ii ) typically comprises reacting the compound of formula ( vi ) with hydrogen in the presence of a suitable catalyst to afford a compound of formula ( vii ). in one embodiment , the catalyst is raney nickel and the solvent is ethanol . step ( iii ) typically comprises reacting the compound of formula ( vii ) with a suitable amine , such as ammonia , to give a compound of formula ( viii ). step ( iv ) typically comprises a hofmann rearrangement reaction by reacting the compound of formula ( viii ) with a suitable halogen atom , such as chlorine or bromine ( e . g . bromine ) and a suitable base ( e . g . sodium methoxide ) in the presence of a suitable solvent ( e . g . methanol ), followed by hydrolysis , to afford a compound of formula ( iii ) a . in one embodiment , the hofmann rearrangement reaction may additionally comprise an acetate salt , typically mercuric acetate , to afford a compound of formula ( iii ) a . compounds of formula ( iv ) wherein n represents 2 may be prepared in accordance with the following scheme 2 : wherein r 1 and r 2 are as defined above for compounds of formula ( ii ). step ( i ) typically comprises a henry ( nitroaldol ) reaction by reacting a compound of formula ( v ) with a suitable nitroalkane ( e . g . nitromethane ) in the presence of a suitable base ( e . g . sodium methoxide ) in a suitable solvent ( e . g . anhydrous methanol ) to afford a compound of formula ( ix ). in one embodiment , the nitroalkane is nitromethane , the base is sodium methoxide and the solvent is anhydrous methanol . step ( ii ) typically comprises reacting the compound of formula ( ix ) under eliminating conditions to afford a compound of formula ( x ). in one embodiment , the eliminating conditions comprise reacting the compound of formula ( ix ) with a dehydrating agent to yield a compound of formula ( x ). typical dehydrating agents include acid anhydrides and dicyclohexylcarbodiimide . in another embodiment , the nitroalcohol of formula ( ix ) is reacted with an anhydride , typically acetic anhydride , to form a nitroester . this nitroester may then be reacted with a mild base , for example acetate salts , e . g . sodium acetate , to form the compound of formula ( x ). step ( iii ) typically comprises reacting the compound of formula ( x ) with a reducing agent in a suitable solvent to afford a compound of formula ( iv ) a . in one embodiment , the reducing agent is sodium borohydride and the solvent is methanol . compounds of formula ( v ) may be prepared in accordance with the following scheme 3 : wherein r 1 and r 2 are as defined above for compounds of formula ( ii ). step ( i ) typically comprises reaction of a compound of formula ( xi ) with imidazole in the presence of sulfuryl chloride and a suitable solvent ( e . g . dichloromethane ) to yield a compound of formula ( xii ). step ( ii ) typically comprises reaction of a compound of formula ( xii ) with potassium thioacetate in the presence of a suitable solvent ( e . g . dimethylformamide ) to yield a compound of formula ( xiii ). step ( iii ) typically comprises a hydrogenation reaction in the presence of a suitable catalyst ( e . g . raney nickel ) in the presence of a suitable solvent ( e . g . methanol ) to yield a compound of formula ( xiv ). step ( iv ) typically comprises reacting a compound of formula ( xiv ) in the presence of a suitable acid ( e . g . aqueous hydrochloric acid ) to yield a compound of formula ( xv ). step ( v ) typically comprises an oxidisation reaction by reacting a compound of formula ( xv ) with an oxidising agent ( e . g . sodium periodate ) in a suitable solvent ( e . g . ethanol ) to yield a compound of formula ( v ). compounds of formula ( xi ) are either known or may be prepared in accordance with known procedures ( e . g . from d - glucose ). according to a further aspect of the invention , there is provided a compound selected from : according to a further aspect of the invention , there is provided a process for preparing atorvastatin which comprises the following steps : ( b ) reaction of a compound of formula ( xvii ) as defined above to yield a compound of formula ( xviii ) ( c ) reaction of a compound of formula ( xviii ) as defined above to yield atorvastatin . step ( a ) typically comprises reaction in a suitable solvent ( e . g . tetrahydrofuran and n - heptane ) and suitable agents ( e . g . pivalic acid and toluene ) at a suitable temperature ( e . g . room temperature ). step ( b ) typically comprises treatment of a compound of formula ( xvii ) with aqueous trifluoroacetic acid . step ( c ) typically comprises a wittig reaction involving one - carbon homologation . such a reaction will be readily apparent to the skilled person and is described in journal of american chemical society , ( 1977 ), 99 , 182 ; journal of organic chemistry , ( 1983 ), 48 , 3566 ; and tetrahedron letters , ( 1979 ), 26 , 2433 . compounds of formula ( ii ) a may be prepared in accordance with procedures described herein . compounds of formula ( xvi ) are known , for example , from compounds of formula ( xvii ) in u . s . pat . no . 5 , 003 , 080 . the invention will now be illustrated by the following non - limiting examples : to a stirred solution of 1 , 2 : 5 , 6 - di - o - isopropylidene - α - d - glucofuranose ( 250 g , 961 . 54 mmol ) in anhydrous dichloromethane ( 3 . 59 it ) imidazole ( 440 g , 646 mmole ) was added at 0 to − 5 ° c . over a period of 0 . 5 h . the reaction mixture was stirred for 1 h at this temperature and then a solution of sulfuryl chloride ( 149 ml ) in dichloromethane ( 20 ml ) was added drop wise to the mixture . after 5 h at 0 to − 5 ° c ., the reaction mixture was allowed to warm to room temperature and stirred at this temperature until the reaction is completed as indicated by thin layer chromatography ( tlc ). the precipitated salt was filtered and the filtrate was washed with water ( 3 × 1000 ml ). the organic layer was then washed successively with 10 % aqueous hydrochloric acid ( 10 × 1000 ml ), water ( 1 × 1000 ml ), saturated sodium bicarbonate solution ( 2 × 1000 ml ), water ( 1 × 1000 ml ) and finally with brine ( 2 × 1000 ml ). the organic layer was dried over sodium sulfate and concentrated under reduced pressure to give the title product ( e1 ) as a solid ; yield 320 g , yield 95 %. potassium thioacetate ( 7 . 3 g , 63 . 93 mmol ) was added to a stirred solution of 1 , 2 : 5 , 6 - di - o - isopropylidene - 3 - o -( imidazole - 1 - sulfonyl )- α - d - glucofuranose e1 ( 10 . 0 g , 25 . 64 mmol ) of in dimethylformamide ( 25 ml ) and the reaction mixture was heated at 75 - 85 ° c . for 5 h . after completion of the reaction as indicated by tlc , the reaction mixture was diluted with ethyl acetate ( 50 ml ) and water ( 25 ml ). charcoal ( 25 g ) was then added and the reaction was maintained at 55 - 60 ° c . for 0 . 5 h and then filtered . the filtrate was washed with water ( 10 × 25 ml ) and with brine ( 2 × 25 ml ), dried over sodium sulfate and concentrated under reduced pressure to give the title compound e2 ; yield : 7 . 5 g , 84 % which was subjected to the following step without further purification . the crude reaction mixture obtained by the procedure described in example 2 containing 3 - s - thioacetyl - 1 , 2 : 5 , 6 - di - o - isopropylidene - 3 - thio - α - d - allofuranose e2 ( 7 . 5 g , 23 . 58 mmole ) was dissolved in methanol and added to a slurry of freshly prepared raney nickel ( prepared from nickel - aluminium alloy , 60 g ) in 100 ml of methanol . the reaction mixture was stirred at room temperature for 15 h and filtered . the filtrate was reduced under reduced pressure to give the title product e3 ; yield : 5 g , 88 %, which was subjected to the following step without further purification . a slurry of 3 - deoxy - 1 , 2 : 5 , 6 - di - o - isopropylidene - α - d - ribo - hexofuranose e3 ( 5 . 0 g , 20 . 49 mmol ) in aqueous hydrochloric acid ( 200 ml , 0 . 01n ) was stirred at room temperature for 12 h . after the completion of reaction as indicated by tlc , the reaction mixture was neutralized by adding sodium bicarbonate and extracted with n - hexane ( 1 × 100 ml ) to remove non - polar organic impurities . the aqueous reaction mixture was then extracted with ethyl acetate ( 5 × 10 ml ). the organic layer was dried over sodium sulfate and concentrated under reduced pressure at 50 ° c . to give the title compound ( e4 ); yield : 4 g , 95 . 69 %. a solution of sodium periodate ( 8 . 4 g , 39 . 27 mmol ) in water ( 64 ml ) was added drop wise to a cooled solution of 3 deoxy - 1 , 2 - o - isopropylidene - α - d - ribo - hexofuranose e4 ( 8 . 0 g , 39 . 22 mmol ) in ethanol ( 64 ml ) at 0 - 10 ° c . the reaction mixture was allowed to warm to room temperature and stirred for 1 h when the reaction completed as indicated by tlc . the precipitated mass was filtered and the filtrate was concentrated under reduced pressure at 75 ° c . to a thick syrup . ethyl acetate ( 100 ml ) was then added and the mixture was stirred at room temperature for 15 minutes until the thick syrup dissolved to give a homogenous solution . the solution was washed with brine ( 2 × 25 ml ), dried over sodium sulfate and filtered . the filtrate was concentrated under reduced pressure to give the title compound ( e5 ); yield : 6 . 5 g , 96 % which was subjected to the following step without further purification . triethylphosphonoacetate ( 11 g , 49 . 06 mmol ) 1 , 2 - dimethoxyethane ( 16 . 5 ml ) was added under an atmosphere of nitrogen to a slurry of sodium hydride ( 1 . 8 g , 45 mmol ; 60 % dispersion in mineral oil ) in 1 , 2 - dimethoxyethane ( 16 . 5 ml ) at 0 - 5 ° c . after 0 . 5 h at 0 - 5 ° c . a solution of 1 , 2 - o - isopropylidene - α - d - erythro - pentodialdo - 1 , 4 - furanose e5 ( 6 . 5 g , 37 . 79 mmol ) in 1 , 2 - dimethoxyethane ( 33 ml ) was added drop wise to the reaction mixture under an atmosphere of nitrogen at 0 - 5 ° c . the reaction mixture was allowed to warm to room temperature and stirred at this temperature for 1 h when the tlc of the reaction mixture indicated the completion of the reaction . the reaction was quenched by adding water ( 50 ml ). after 15 minutes , ethyl acetate ( 150 ml ) was added to the reaction mixture to facilitate the separation of layers . the organic layer was washed successively with water ( 1 × 50 ml ), brine ( 1 × 50 ml ), dried over sodium sulphate and filtered . the filtrate was concentrated under reduced pressure at 75 ° c . to give the title compound ( e6 ) as a thick syrup ; yield : 9 g , 94 %. a slurry of freshly prepared raney nickel ( 18 g of nickel - aluminium alloy gave approximately 9 . 0 g of raney nickel ) in ethanol ( 45 ml ) was added to ethyl 3 , 5 , 6 - trideoxy - 1 , 2 - o - isopropylidene - α - d - erythro - hept - 5 - enoato - 1 , 4 - furanose e6 ( 9 g , 37 . 19 mmole ) dissolved in ethanol ( 45 ml ). the reaction mixture was stirred under an atmosphere of hydrogen at 50 psi at room temperature until the uptake of hydrogen stopped and the reaction completed as indicated by tlc . the reaction mixture was then filtered and the raney nickel residue was washed with ethanol ( 18 ml ). the combined filtrate was concentrated under reduced pressure to give the title compound e7 ; yield : 9 . 0 g , 99 %. an aqueous solution of ammonia ( 90 ml ) was added to ethyl 3 , 5 , 6 - trideoxy - 1 , 2 - o - isopropylidene - α - d - erythro - heptanoato - 1 , 4 - furanose e7 ( 9 g , 36 . 89 mmole and the reaction mixture was stirred at room temperature for 15 h when the reaction completed as indicated by tlc . the reaction mixture was then extracted with ethyl acetate ( 4 × 100 ml ). the organic layer was dried over sodium sulfate and then filtered . the filtrate was concentrated under reduced pressure to give the title compound e8 ; yield : 6 . 8 g , 85 . 7 %). bromine ( 4 . 64 g , 29 . 03 mmole ) was added drop wise at − 45 ° c . to a cold solution of freshly prepared sodium methoxide ( prepared from 2 . 1 g of sodium ) in methanol ( 50 ml ). the reaction mixture was maintained at − 45 ° c . until the colour of bromine disappeared . a solution of 3 , 5 , 6 - trideoxy - 1 , 2 - o - isopropylidene - α - d - erythro - heptamido - 1 , 4 - furanose e8 ( 6 . 5 g , 30 . 23 mmole ) in dioxane ( 32 . 5 ml ) and methanol ( 19 . 5 ml ) was added drop wise to the reaction mixture at − 45 ° c . the reaction mixture was then allowed to warm to the room temperature and then heated slowly until the bath temperature attained 55 - 60 ° c . after 1 h at 55 - 60 ° c ., the reaction mixture was neutralized with acetic acid and concentrated under reduced pressure below 50 ° c . to remove 1 , 4 - dioxane and methanol . ethyl acetate ( 150 ml ) was added to the mixture and extracted with water ( 1 × 50 ml ). the organic layer was washed with saturated sodium bicarbonate solution ( 3 × 25 ml ), water ( 3 × 50 ml ), brine ( 1 × 50 ml ), dried over sodium sulfate and filtered . the filtrate was concentrated under reduced pressure to give the rearranged title compound e9 ; yield : 4 . 5 g , 61 %. sodium hydroxide ( 35 ml ., 1n ) was added to 3 , 5 , 6 - trideoxy - 1 , 2 - o - isopropylidene - 6 - methoxycarbonylamido - α - d - erythro - hexofuranose e9 ( 3 . 5 g , 14 . 29 mmole ) at room temperature . the mixture was then heated slowly until the bath temperature attained 80 - 85 ° c . after 10 h at 80 - 85 ° c ., the reaction mixture was cool down to room temperature and then extracted with ethyl acetate ( 2 × 100 ml , 3 × 50 ml ). the combined ethyl acetate layers was dried over sodium sulfate and filtered . the filtrate was concentrated under reduced pressure to give the title compound e10 ; yield : 2 . 0 g , 74 %). to a solution of 3 , 5 , 6 - trideoxy - 1 , 2 - o - isopropylidene - 6 - amino - α - d - erythro - hexofuranose ( e10 ) ( 0 . 6 g , 3 . 21 mmol ) in tetrahydrofuran ( 5 ml ) was added at room temperature 1 , 4 - dicarbonyl compound ( compound xvii in u . s . pat . no . 5 , 003 , 080 ) ( 1 . 0 g , 2 . 5 mmole ), pivalic acid ( 0 . 32 g , 3 . 13 mmol ), toluene ( 5 ml ) and n - heptane ( 20 ml ). the reaction mixture was then heated slowly to reflux with azeotropic removal of water . after completion of the reaction as indicated by tlc ( usually takes 24 h - 30 h of reflux ), the reaction mixture was allowed to cool down to room temperature . ethylacetate ( 50 ml ) and water ( 25 ml ) were added to the reaction mixture to facilitate separation of layers . the organic layer was extracted with sodium hydrogencarbonate solution ( 3 × 50 ml ), water ( 3 × 50 ml ) and finally with brine ( 1 × 50 ml ); dried over sodium sulphate and filtered . the filtrate was concentrated under reduced pressure to give the title compound e11 ; yield : 1 . 5 g , 85 %. an aqueous solution of trifluoroacetic acid ( 2 ml ), ( tfa : water , 3 : 1 ) was added at 0 - 10 ° c . to 3 , 5 - dideoxy - 1 , 2 - o - isopropylidene - 6 -[ 5 -( 4 - fluorophenyl )- 2 -( 1 - methylethyl )- n , 4 - diphenyl - 1h - pyrrole - 3 - carboxamide ]- α - d - erythro - hexofuranose ( e11 ). on completion of the reaction as indicated by tlc ( 4 h at 0 - 10 ° c . ), the reaction mixture was diluted with cold water and then neutralized with solid sodium hydrogen carbonate . the precipitated solid was filtered , slurried with n - hexane and then filtered . the residue was dissolved in ethyl acetate and was extracted with water ( 2 × 25 ml ) and finally with brine ( 1 × 25 ml ); dried over sodium sulphate and filtered . the filtrate was concentrated under reduced pressure to give the title compound e12 ; yield : ( 0 . 4 g , 43 %). the title compound ( atorvastatin lactone ; e13 ) may be prepared by subjecting 3 , 5 - dideoxy - 6 -[ 5 -( 4 - fluorophenyl )- 2 -( 1 - methylethyl )- n , 4 - diphenyl - 1h - pyrrole - 3 - carboxamide ]-( α and β )- d - erythro - hexofuranose ( in e12 ) to a wittig reaction involving one - carbon homologation . such a reaction will be readily apparent to the skilled person and is described in journal of american chemical society , ( 1977 ), 99 , 182 ; journal of organic chemistry , ( 1983 ), 48 , 3566 ; and tetrahedron letters , ( 1979 ), 26 , 2433 .