unless otherwise indicated , this disclosure uses definitions provided below . some of the definitions and formulae may include a “—” ( dash ) to indicate a bond between atoms or a point of attachment to a named or unnamed atom or group of atoms . other definitions and formulae may include an “═” to indicate a double bond . “ alkyl ” refers to straight chain and branched aliphatic hydrocarbon groups , generally having a specified number of carbon atoms ( i . e ., c 1 - 12 alkyl refers to an alkyl group having from 1 to 12 carbon atoms , inclusive ). examples of alkyl groups include , without limitation , methyl , ethyl , n - propyl , i - propyl , n - butyl , s - butyl , t - butyl , n - pentyl , s - pentyl , n - hexyl , and the like . “ halo ,” “ halogen ” and “ halogeno ” may be used interchangeably , and refer to fluoro , chloro , bromo , and iodo . the present disclosure describes materials and methods for preparing single enantiomers of 5 , 5 , 5 , 5 ′, 5 ′, 5 ′- hexafluoroleucine ( l - hfleu and d - hfleu ), and for preparing protected analogs of l - hfleu and d - hfleu , which are represented by formula i and formula i ′, respectively : in both formulas , r 1 and r 2 are n - terminal and c - terminal protecting groups , respectively . the n - terminal protecting group prevents undesirable reaction of the amino functional group during subsequent transformations , and includes , but is not limited to , benzyl , substituted benzyl , benzyloxycarbonyl ( cbz ), tert - butoxycarbonyl ( boc ), 9 - fluorenylmethoxycarbonyl ( fmoc ), and trityl . likewise , the c - terminal protecting group prevents undesirable reaction of the carboxy functional group and includes , but is not limited to , c 1 - 12 alkyl ( e . g ., tert - butyl ) and c 1 - 12 haloalkyl . particularly useful compounds of formula i thus include n - cbz - l - hfleu tert - butyl ester , n - cbz - l - hfleu , n - boc - l - hfleu , l - hfleu tert - butyl ester , n - fmoc - l - hfleu , n - fmoc - l - hfleu ten - butyl ester , and n - trityl - l - hfleu . likewise , particularly useful compounds of formula i ′ include n - cbz - d - hfleu tert - butyl ester , n - cbz - d - hfleu , n - boc - d - hfleu , d - hfleu tert - butyl ester , n - fmoc - d - hfleu , n - fmoc - d - hfleu tert - butyl ester , and n - trityl - d - hfleu . scheme i shows a general method for preparing l - hfleu and its protected analogs ( formula i ) from an l - serine derivative , and employs hexafluoroacetone as a fluorocarbon source . as shown in scheme i , the method includes providing a protected l - serine as a chiral building block ( formula ii ). in one useful embodiment , r 1 and r 2 of formula ii are cbz and tert - butyl , respectively , which can be prepared by treating cbz - l - serine with tert - butyl bromide in the presence of potassium carbonate and a phase transfer reagent , such as benzyltriethylammonium chloride . other c - terminal and n - terminal protecting groups may be obtained using commonly employed methods of amino acid chemistry ( see , e . g ., m . bodanszky & amp ; a . bodanszky , the practice of peptide synthesis ( 2d ed . 1994 ). as shown in scheme i , the compound of formula ii is reacted with an iodinating agent in an aprotic solvent , such as dimethylformamide ( dmf ), at a temperature between about 20 ° c . and about 100 ° c . for about 1 h to about 24 h , to yield a compound of formula iii . suitable iodinating agents include methyltriphenoxyphosphonium iodide , iodotriphenoxyphosphonium iodide , triphenylphosphine - n - iodosuccinimide , triphenylphosphine - tetraiodomethane , triphenylphosphine - 2 , 4 , 5 - triiodoimidazole , a mixture of triphenylphosphine , iodine , and imidazole , and the like . a particularly useful iodinating agent is methyltriphenoxyphosphonium iodide . following iodination , the compound of formula iii is reacted with zinc dust in an aprotic solvent ( e . g ., dmf ) and in the presence of a catalyst ( e . g ., cubr . sme 2 ) to form an organozinc reagent , which is subsequently reacted in - situ with hexafluoroacetone gas at a temperature between about − 30 ° c . and about − 25 ° c . to yield , upon work up , a compound having the requisite fluorinated carbon skeleton ( formula iv ). as shown in scheme i , the compound of formula iv includes a tertiary hydroxy group that is subsequently displaced by a hydrogen atom , yielding a protected l - hfleu compound ( formula i ). a useful technique for displacing the tertiary hydroxy group includes radical deoxygenation , in which the hydroxy group is converted to an oxalyl ester , a thionocarbonate , or a dithiocarbonate by treatment with an oxalyl chloride , a chlorothionoformate , or a chlorodithioformate , usually in the presence of base , such as pyridine . suitable oxalyl chlorides , chlorodithioformates , and chlorothionoformates include phenyl and methyl oxalyl chloride , phenyl chlorodithioformate , and phenoxy , 2 , 4 , 6 - trichlorophenoxy , 4 - fluorophenoxy , pentafluorophenoxy and thiophenoxy chlorothionoformate . the resulting oxalyl ester , thionocarbonate , or dithiocarbonate is subsequently treated with a source of hydrogen radicals in the presence of a radical initiator , to afford the compound of formula i . although trialkyltin hydrides , such as tributyltin hydride , are a good source of hydrogen radicals , other useful sources include triphenylsilane , diphenylsilane , dialkylphosphite , as well as salts of hypophosphorous acid . suitable radical initiators include azaisobutyronitrile ( aibn ). for a discussion of radical deoxygenation , see , e . g ., d . h . r . barton & amp ; h . w . mccarobie , 1 j . chem . soc . perkin trans . 1 , 1574 ( 1975 ); m . j . robins & amp ; j . s . wilson 103 j . am . chem . soc . 933 ( 1981 ); m . j . robins & amp ; j . s . wilson , 105 j . am . chem . soc . 4059 ( 1983 ); s . c . dolan and j . machillan , j . chem . soc . chem . comm . 1588 ( 1985 ). as shown in scheme i , the amino , carboxy or amino and carboxy groups may be de - protected by displacing r 1 ( formula vi ) or r 2 ( formula v ) or both r 1 and r 2 ( l - hfleu ) using conventional techniques . for example , if r 1 and r 2 are , respectively , cbz and tert - butyl , the carboxy functional group can be de - protected by treating the compound of formula i with trifluoroacetic acid ( tfa ) in an aprotic solvent ( e . g ., dichloromethane ). additionally , the amino functional group can be de - protected by treating the compound of formula i with hydrogen in the presence of a pd catalyst or by treating the compound of formula i with hbr and acetic acid . similarly , r 1 can be displaced from the compound of formula v to yield l - hfleu . though not shown in scheme i , r 1 can be displaced earlier in the process by treating the compound of formula iv with hydrogen . in addition to preparing l - hfleu and its protected analogs ( formula i ), the method depicted in scheme i can be used to prepare d - hfleu and protected analogs ( formula i ′) by utilizing starting materials having opposite stereochemistry ( i . e ., a d - serine derivative ). thus , for example , cbz - d - serine may be treated with tert - butyl bromide in the presence of potassium carbonate and a phase transfer reagent to yield a protected d - serine , which is subsequently reacted with an iodinating agent . the resulting iodide is reacted with zinc to form an organozinc reagent , which in turn is reacted with hexafluoroacetone to generate a compound having a tertiary hydroxy group that is subsequently displaced using radical deoxygenation to yield n - cbz - d - hfleu tert - butyl ester . de - protecting the amino and caroboxy functional groups gives d - hfleu . the following examples are intended as illustrative and non - limiting , and represent specific embodiments of the present invention . preparation of 2 - benzyloxycarbonylamino - 3 - hydroxy - propionic acid tert - butyl ester ( formula ii , r 1 = cbz , r 2 = tert - butyl ) a mixture of cbz - l - serine ( 10 . 0 g , 42 mmol ), benzyltriethylammonium chloride ( 9 . 5 g , 42 mmol ) and potassium carbonate ( 38 . 0 g , 275 mmol ) in ch 3 cn ( 80 ml ) was stirred vigorously for 5 h at rt . 2 - bromo - 2 - methyl propane was added and the reaction mixture was warmed to 45 - 50 ° c . and stirred rapidly . the reaction mixture became very thick after 2 - 3 h . additional ch 3 cn was added ( 50 ml ) to facilitate stirring and the resulting reaction mixture was stirred for 24 h . the reaction mixture was then cooled to rt and most of the ch 3 cn was removed by rotary evaporation . the reaction mixture was then partitioned between ethyl acetate ( 200 ml ) and water ( 100 ml ). the aqueous layer was extracted with ethyl acetate ( 3 × 100 ml ). the ethyl acetate layers were combined and washed with water ( 100 ml ), then with sat . aq . nacl solution ( 2 × 100 ml ), and then dried over na 2 so 4 , filtered and concentrated . if the product did not crystallize upon concentration , it was triturated with hexanes and filtered to give 2 - benzyloxycarbonylamino - 3 - hydroxy - propionic acid tert - butyl ester as a white solid ( 10 . 0 g , 81 % yield ). preparation of 2 - benzyloxycarbonylamino - 3 - iodo - propionic acid tert - butyl ester ( formula iii , r 1 = cbz , r 2 = tert - butyl ) methyltriphenoxyphosphonium iodide (( pho ) 3 pmei , 17 . 09 g , 40 mmol ) was added to a dmf ( 67 ml ) solution of 2 - benzyloxycarbonylamino - 3 - hydroxy - propionic acid tert - butyl ester at rt . the resulting solution was stirred at rt for 3 . 5 h . the reaction mixture was cooled to 0 ° c . ( ice bath ) and stirred vigorously . solid nahco 3 ( 10 g ) was added followed by the addition of water ( 20 ml ) over a period of 5 min . the reaction mixture was then partitioned between water ( 250 ml ) and 1 : 1 et 2 o - hexanes ( 250 ml ). the aqueous layer was extracted twice with 1 : 1 et 2 o - hexanes ( 100 ml ). the organic layers were combined and washed with aq . naoh ( 0 . 05 m , 200 ml portions ) until all of the phenol was removed ( as judged by tlc using 35 % etoac - hexanes ). the organic layer was further washed with sat . aq . nacl ( 100 ml ) and then dried over na 2 so 4 , filtered , and concentrated . the crude product was purified by column chromatography using 20 % etoac - hexanes to give 2 - benzyloxycarbonylamino - 3 - iodo - propionic acid tert - butyl ester as a clear oil that crystallized upon standing in the refrigerator ( 4 ° c .) into a white solid ( 9 . 19 g , 85 % yield ). r f 0 . 59 ( 30 % etoac - hexanes ); mp 34 - 36 ° c . ; 1 h - nmr ( 300 mhz , cdcl 3 ): δ 7 . 40 - 7 . 30 ( 5h ), 5 . 62 ( br d , j = 6 . 9 hz , 1h ), 5 . 19 - 5 . 05 ( 2h ), 4 . 42 ( ddd , j = 7 . 1 , 7 . 1 , 3 . 6 hz , 1h ), 3 . 59 ( d , j = 3 . 6 hz , 2h ), 1 . 50 ( s , 9h ); 13 c - nmr ( 100 mhz , cdcl 3 ): δ 168 . 1 , 155 . 4 , 136 . 0 , 128 . 5 , 128 . 2 , 128 . 1 , 83 . 5 , 67 . 1 , 54 . 0 , 27 . 9 , 8 . 4 . preparation of 2 - benzyloxycarbonylamino - 5 , 5 , 5 - trifluoro - 4 - hydroxy - 4 - trifluoromethyl - pentanoic acid tert - butyl ester ( formula iv , r 1 = cbz , r 2 = tert - butyl ) zinc dust ( 15 g , 230 mmol ) in a 3 - neck flask ( 250 ml ) was heated under vacuum ( 1 mmhg ) using a heat gun for 5 min ( the flask was hot to the touch ) and then allowed to cool to rt . the flask was then purged twice with dry nitrogen . dry dmf ( 38 ml ) was added and the resulting mixture was warmed to ca . 50 ° c . using a heat gun and stirred vigorously . 1 , 2 - dibromoethane ( 1 . 0 ml , 12 mmol ) was added and the resulting mixture was allowed to cool to rt and stirred vigorously for 30 min . trimethylsilyl chloride ( tms - cl ) ( 0 . 3 ml , 2 mmol ) was then added and the resulting mixture stirred vigorously for another 30 min . a dry dmf solution ( 38 ml ) of 2 - benzyloxycarbonylamino - 3 - iodo - propionic acid tert - butyl ester ( 23 . 21 g , 57 mmol ) was then added over a period of 5 min and was consumed after 45 min ( tlc , 40 % et 2 o - hexanes , ninhydrin ). in another 3 - neck flask ( 250 ml ), cubr . sme 2 was heated under vacuum ( 1 mmhg ) using a heat gun until a light green color appeared ( ca . 5 min ). the light green solid was allowed to cool to rt , and then dry dmf ( 10 ml ) was added . the dmf solution of the organozinc reagent was transferred to the cubr / dmf mixture , leaving behind most of the unreacted zinc . the zinc was rinsed with dmf ( 10 ml ) and then the dmf portion was transferred as before making sure that most of the zinc was left behind . the resulting dmf solution of the organozinc reagent and cubr was cooled between − 20 ° c . and − 30 ° c . and stirred rapidly . hexafluoroacetone gas ( 15 . 2 g , 92 mmol ) was slowly bubbled into the reaction mixture ( ca . 10 - 15 min ) via a needle ( 6 ″, 20 ga .). other experiments have shown that only a stoichiometric amount of hexafluoroacetone is needed . the resulting mixture was stirred rapidly between − 20 ° c . and − 30 ° c . for 2 h and then warmed to rt overnight . water ( 100 ml ) was added followed by 1 m aq . hcl until ph = 4 . the reaction mixture was then poured into water ( 300 ml ) and etoac ( 200 ml ). the aq . layer was extracted with etoac ( 3 × 150 ml ). the etoac layers were combined and washed with 1 : 1 water - saturated aq . nacl ( 3 × 100 ml ), sat . aq . nacl ( 2 × 100 ml ), then dried over na 2 so 4 , filtered , and concentrated to give 2 - benzyloxycarbonylamino - 5 , 5 , 5 - trifluoro - 4 - hydroxy - 4 - trifluoromethyl - pentanoic acid tert - butyl ester as a white solid ( 23 . 96 g , 94 % yield ), which did not require purification . r f 0 . 49 ( 40 % et 2 o - hexanes ); mp 83 - 89 ° c . ; 1 h - nmr ( 400 mhz , cdcl 3 ): δ 7 . 50 ( br s , 1h ), 7 . 40 - 7 . 32 ( 5h ), 6 . 00 ( br s , 1h ), 5 . 13 ( dd , j = 16 . 1 , 12 . 1 hz , 2h ), 4 . 44 ( app dd , j = 4 . 8 , 4 . 4 hz , 1h ), 2 . 74 ( dd , j = 16 . 1 , 4 . 4 hz , 1h ), 2 . 24 ( ddd , j = 16 . 1 , 4 . 8 , 1 . 1 hz , 1h ), 1 . 1 ( s , 9h ); 13 c - nmr ( 100 mhz , cdcl 3 ): δ 169 . 9 , 157 . 1 , 135 . 3 , 128 . 62 , 128 . 58 , 128 . 4 , 128 . 3 , 123 . 1 ( q , j cf = 286 . 8 hz ), 84 . 6 , 74 . 9 ( quintet , j ccf = 29 . 4 hz ), 68 . 0 , 48 . 9 , 35 . 7 , 27 . 7 ; 19 f - nmr ( 376 . 3 mhz , cdcl 3 / c 6 f 6 ): δ 85 . 1 - 84 . 7 , 83 . 6 - 83 . 2 ; hrms - fab ( m - nitrobenzyl alcohol , m / z ): [ m + na ] + calculated for c 18 h 21 no 5 f 6 na , 468 . 1222 ; found , 468 . 1229 . preparation of n - cbz l - 5 , 5 , 5 , 5 ′, 5 ′, 5 ′- hexafluoroleucine tert - butyl ester ( formula i , r 1 = cbz , r 2 = tert - butyl ) dry pyridine ( 0 . 73 ml , 9 . 0 mmol ) was added drop wise to a rapidly stirred dry toluene ( 15 . 7 ml ) solution of 2 - benzyloxycarbonylamino - 5 , 5 , 5 - trifluoro - 4 - hydroxy - 4 - trifluoromethyl - pentanoic acid tert - butyl ester ( 3 . 493 g , 7 . 85 mmol ) and phenyl oxalyl chloride ( 1 . 738 g , 9 . 42 mmol ) at rt . the resulting mixture was stirred rapidly for 3 h , whereupon a white precipitate ( pyridinium chloride ) was formed . the reaction mixture was filtered through cotton to remove the precipitate . the filtrate was concentrated using rotary evaporation to azeotrope most of the excess pyridine . the resulting crude oxalate ester and aibn ( 0 . 575 g , 2 . 35 mmol ) were dissolved in dry toluene ( 15 . 7 ml ) and then added drop wise to a rapidly stirred toluene ( 15 . 7 ml ) solution of bu 3 snh ( 3 . 22 ml , 11 . 9 mmol ) at 100 ° c . ( oil bath ) over a period of 5 min . the resulting solution was stirred at this temperature for 1 h and then cooled to rt . ether ( 20 ml ) was added followed by a kf - celite ® mixture ( ca . 10 g , to remove organotin compounds ) and the resulting mixture was stirred vigorously for 2 h . the solvent was decanted and the kf - celite ® residue was washed with ether ( 2 × 20 ml ). the solvent was removed and the resulting residue was purified by column chromatography using 13 % et 2 o - hexanes to give n - cbz l - 5 , 5 , 5 , 5 ′, 5 ′, 5 ′- hexafluoroleucine tert - butyl ester as a clear oil that solidified to a translucent , moist solid ( 2 . 829 g , 84 % yield ). r f 0 . 59 ( 40 % et 2 o - hexanes ); mp 43 - 45 ° c . ; 1 h - nmr ( 400 mhz , cdcl 3 ): δ 7 . 40 - 7 . 32 ( 5h ), 5 . 39 ( br d , j = 5 . 3 hz , 1h ), 5 . 13 ( s , 2h ), 4 . 45 - 4 . 38 ( 1h ), 3 . 38 - 3 . 22 ( 1h ), 2 . 40 - 2 . 28 ( 1h ); 13 c - nmr ( 100 mhz , cdcl 3 ): δ 169 . 6 , 156 . 2 , 135 . 9 , 128 . 5 , 128 . 3 , 128 . 2 , 83 . 6 , 67 . 3 , 52 . 4 , 44 . 5 ( quintet , j ccf = 28 . 4 hz ), 27 . 8 ; 19 f - nmr ( 376 . 3 mhz , cdcl 3 / c 6 f 6 ): δ 94 . 29 - 94 . 14 , 94 . 08 - 93 . 90 ; hrms - ci ( nh 3 , m / z ): [ m + nh 4 ] + calculated for c 18 h 25 n 2 o 4 f 6 , 447 . 1719 ; found , 447 . 1715 . tfa ( 16 ml , 210 mmol ) was added to a solution of n - cbz l - 5 , 5 , 5 , 5 ′, 5 ′, 5 ′- hexafluoroleucine tert - butyl ester ( 8 . 97 g , 21 mmol ) at 0 ° c . in ch 2 cl 2 ( 12 ml ) and the resulting mixture was stirred for 1 h and then allowed to warm to rt over a period of 3 h . the solvent was removed using rotary evaporation . the crude product was dissolved in toluene ( 50 ml ) and concentrated to remove most of the remaining tfa ; this process was repeated 3 times . the crude product ( formula v , r 1 = cbz ) was dissolved in meoh ( 100 ml ) and 10 % pd / c ( 0 . 8 g ) was added . the reaction mixture was stirred vigorously overnight under an atmosphere of h 2 . meoh ( 75 ml ) was added to dissolve the precipitated product and then n 2 was bubbled through the reaction mixture for 15 min . the reaction mixture was filtered through celite ® and concentrated . the crude product was washed with et 2 o ( 300 ml ) to give l - 5 , 5 , 5 , 5 ′, 5 ′, 5 ′- hexafluoroleucine as a light gray solid ( 4 . 62 g , 92 % yield , 96 % e . e ., mosher amide ). recrystallization from meoh gave compound l - 5 , 5 , 5 , 5 ′, 5 ′- hexafluoroleucine as white needles ( 99 % e . e ., mosher amide ). 1 h - nmr ( 400 mhz , cd 3 od ): δ 4 . 29 - 4 . 16 ( 1h ), 3 . 67 ( dd , j = 9 . 9 , 5 . 9 hz , 1h ), 2 . 37 - 2 . 28 ( 1h ), 2 . 24 - 2 . 15 ( 1h ); 19 f - nmr ( 376 mhz , cd 3 od / c 6 f 6 ): δ 96 . 6 ( quintet , j = 9 . 2 hz ), 95 . 9 ( quintet , j = 9 . 2 hz ); [ α ] d 22 =+ 9 . 3 ° ( c 0 . 7 , meoh ); helms - fab ( m - nitrobenzyl alcohol , m / z ): [ m + h ] + calculated for c 6 h 8 no 2 f 6 , 240 . 0459 ; found , 240 . 0461 . it is to be understood that the above description is intended to be illustrative and not restrictive . many embodiments will be apparent to those of skill in the art upon reading the above description . the scope of the invention should , therefore , be determined not with reference to the above description , but should instead be determined with reference to the appended claims , along with the full scope of equivalents to which such claims are entitled . the disclosures of all articles and reference , including patent applications , granted patents , and publications , are incorporated herein by reference in their entirety and for all purposes .