compound 1 ( 3.90 
 g , 15.7 mmol ) , triphenylphosphine ( ph3p ) ( 7.40 g , 28.3 
 mmol ) , and p - nitrobenzoic 
 acid ( 3.10 g , 18.8 mmol ) were dissolved in anhydrous thf ( 157 ml ) . 

diisopropylazodicarboxylate ( diad ) ( 6.35 
 g , 6.20 ml , 31.4 mmol ) was added dropwise over 30 min .
the solution 
 was removed from the ice bath , allowed to warm to room temperature , 
 and stirred for an additional 6 h. the solvent was removed under reduced 
 pressure .
the crude product was dissolved in ethyl acetate ( 100 ml ) 
 and washed with brine ( 2  200 ml ) .
the solvent was removed , 
 and the crude product was redissolved in ch2cl2 ( 75 ml ) .
the crude product was purified via column chromatography 
 ( ch2cl2 ) to yield compound 2 ( 4.7 
 g , 12.0 mmol ) as a colorless oil in 76% yield .
compound 2 ( 2.0 
 g , 5.0 mmol ) was dissolved in acetone ( 50 ml ) .
sodium azide ( 0.49 
 g , 7.5 mmol ) was added , and the solution was heated at reflux for 
 14 h. the solution was allowed to cool to room temperature , and the 
 solvent was removed under reduced pressure .
the crude product was 
 dissolved in ethyl acetate ( 50 ml ) and washed with distilled water 
 ( 2  50 ml ) .
the solvent was removed , and the crude product was 
 redissolved in ch2cl2 for purification .
compound 3 ( 0.80 g , 3.3 mmol ) was purified via column chromatography 
 ( 2% methanol in ch2cl2 v / v ) to obtain a colorless 
 oil in 65% yield .
compound 3 ( 2.23 g , 9.10 mmol ) and ph3p ( 2.86 g , 10.9 mmol ) 
 were dissolved in toluene ( 91 ml ) under 
 a nitrogen atmosphere .
the solution was cooled to 0 c and stirred 
 on ice for 10 min .
diad ( 2.20 g , 2.15 ml , 10.9 mmol ) was added dropwise 
 to the solution over 15 min .
( 4.30 g , 2.54 ml , 18.2 mmol ) and dipea ( 2.35 g , 3.16 ml , 18.2 mmol ) 
 were added to the solution , which was then stirred on ice for another 
 5 min .
the solution was removed from the ice bath , warmed to 45 c 
 in an oil bath , and stirred for 24 h. the solvent was removed under 
 reduced pressure , and the crude product was dissolved in ethyl acetate 
 ( 50 ml ) .
the crude product was washed with brine ( 2  75 ml ) 
 and dried over sodium sulfate .
the solvent was removed under reduced 
 pressure , and the crude product was redissolved in ch2cl2 ( 50 ml ) .
the crude product was purified via column chromatography 
 ( 07% ethyl acetate in hexanes v / v ) to obtain compound 5 ( 0.790 g , 2.28 mmol ) as a colorless oil in 25% yield .
h nmr ( 400 mhz , cdcl3 )  4.91 ( s , 1h ) , 4.494.47 
 ( dd , j = 8.6 , 6.4 hz , 0.4h , cis ) , 
 4.404.36 ( dd , j = 7.8 , 7.8 hz , 0.6h , trans ) , 3.843.75 ( m , 1h ) , 3.75 ( s , 3h ) , 3.693.66 
 ( d , j = 12.5 hz , 0.6h , trans ) , 3.603.58 
 ( d , j = 12.3 hz , 0.4h , cis ) , 2.472.44 
 ( m , 1h ) , 2.282.23 ( m , 1h ) , 1.46 ( s , 4h , cis ) , 1.42 ( s , 5h , trans ) .
c nmr ( 150.8 
 mhz , cdcl3 )  171.9 , 171.7 , 153.1 , 152.4 , 119.1 ( q , j = 293 hz ) , 79.8 , 77.1 , 56.5 , 56.1 , 51.5 , 51.4 , 51.2 , 36.6 , 
 35.6 , 27.2 .
f nmr ( 376.3 mhz , cdcl3 )  
 70.47 ( trans conformation ) , 70.53 
 ( cis conformation ) . hrms ( lifdi - tof )
compound 4 ( 0.36 g , 0.78 mmol ) and lioh ( 0.22 g , 0.93 mmol ) were dissolved 
 in a solution of water ( 4 ml ) and 1,4-dioxane ( 4 ml ) .
the solution 
 was stirred at room temperature for 14 h. the mixture was acidified 
 to ph 2 with dilute hcl and extracted with ethyl acetate ( 2  
 10 ml ) .
the crude product was purified via 
 column chromatography ( 02% methanol in ch2cl2 v / v ) to obtain compound 5 ( 0.22 g , 0.49 mmol ) 
 as an off - white solid in 63% yield .
h ( 600 mhz , cdcl3 )  4.92 ( s , 1h ) , 4.534.51 ( dd , j = 7.7 , 7.0 hz , 0.6h ) , 4.424.39 ( dd , j = 
 7.4 , 7.4 hz , 0.4h ) , 3.833.80 ( m , 0.4h ) , 3.743.66 ( m , 
 1.6h ) , 2.512.48 ( m , 1h ) , 2.442.40 ( m , 0.6h ) , 2.352.32 
 ( m , 0.4h ) , 1.48 ( s , 5h ) , 1.43 ( s , 4h ) . c nmr ( 150.8 mhz , 
 cdcl3) 177.7 , 174.7 , 155.9 , 153.4 , 120.1 ( q , j = 293 hz ) , 82.3 , 81.3 , 57.4 , 52.6 , 37.4 , 35.7 , 28.2 .
f nmr ( 564.5 mhz , cdcl3 )  70.49 
 ( trans conformation ) , 70.55 ( cis conformation ) .
compound 4 ( 1.3 
 g , 2.2 mmol ) was dissolved 
 in a solution of 1,4-dioxane ( 15 ml ) and 4 m hcl ( 15 ml ) .
the solution 
 was allowed to stir at reflux for 6 h. the solvent was removed under 
 reduced pressure .
compound 6 ( 0.78 g , 2.2 mmol ) was used 
 as a crude reagent in the next step without purification .
alternatively , 
 compound 5 could be subjected to identical conditions 
 to yield compound 6 .
h nmr ( 600 mhz , meod - d4 )  5.22 ( br s , 1h ) , 4.554.51 
 ( dd , j = 9.6 , 8.8 hz , 1h ) , 3.813.79 ( dd , j = 13.5 , 5.0 hz , 1h ) , 3.463.44 ( d , j = 13.5 hz , 1h ) , 2.532.52 ( m , 1h ) , 2.512.50 ( m , 1h ) . c nmr ( 150.8 mhz , meod - d4 )  
 168.7 , 120.0 ( j = 293 hz ) , 79.0 , 58.1 , 51.8 , 35.7 .
f nmr ( 376.3 mhz , meod - d4 )  
 71.56 . hrms ( lifdi - tof ) m / z : [ m ] calcd for c9h9f9no3 350.0439 , found 350.0420 .
crude 
 compound 6 ( 1.01 g , 2.90 mmol ) was dissolved in a solution 
 of 1,4-dioxane ( 15 ml ) and h2o ( 15 ml ) .
fmoc - osu ( 1.17 
 g , 3.48 mmol ) and k2co3 ( 0.80 g , 5.80 mmol ) 
 were added to the solution , and the resultant mixture was stirred 
 for 14 h at room temperature .
the 1,4-dioxane was removed under reduced 
 pressure , and the crude product was acidified with 2 m hcl ( 10 ml ) . 
 the crude product was extracted with ethyl acetate ( 2  20 ml )
. 
 the solvent was removed , and the crude product was redissolved in 
 ch2cl2 .
the product was purified via column 
 chromatography ( 04% methanol in ch2cl2 v / v ) to obtain the compound 8 as a white solid ( 0.83 
 g , 1.45 mmol ) in 50% yield .
h ( 600 mhz , cdcl3 )  7.777.76 ( d , j = 7.5 hz , 1.3h , trans ) , 7.747.73 ( d , j = 7.6 hz , 
 0.7h , cis ) 7.557.52 ( dd , j = 12.4 , 7.6 hz , 2h ) , 7.427.38 ( m , 2h ) , 7.327.29 
 ( dd , j = 7.4 , 7.3 hz , 2h ) , 4.92 ( s , 0.7h , trans ) , 4.89 ( s , 0.3h , cis ) , 4.594.57 
 ( dd , j = 7.8 , 7.4 hz , 0.7h , trans ) , 4.524.43 ( m , 2h ) , 4.384.37 ( dd , j = 7.4 , 7.4 hz , 0.3h , cis ) , 4.304.27 ( dd , j = 7.0 , 6.6 hz , 0.7h , trans ) , 4.174.16 
 ( dd , j = 6.6 , 6.6 hz , 0.3h , cis ) , 
 3.833.73 ( m , 1h ) , 3.71 ( s , 1h ) , 2.492.45 ( m , 1.7h , 
 mixture of cis and trans ) , 2.322.29 
 ( m , 0.3h , cis ) .
c nmr ( 150.8 mhz , cdcl3 )  173.1 , 156.1 , 143.5 , 143.4 , 141.4 , 127.9 , 127.2 , 
 124.9 , 124.8 , 120.12 ( q , j = 293 hz ) , 120.09 , 77.9 , 
 68.6 , 67.6 , 57.6 , 56.7 , 53.0 , 52.7 , 47.2 , 47.0 , 37.6 , 35.8 .
f nmr ( 564.5 mhz , cdcl3 )  70.38 ( trans conformation ) , 70.43 ( cis conformation ) .
hrms ( ci - tof ) m / z : [ m ] calcd for c24h18f9no5 571.1041 , found 571.1027 .
3.96 g , 15.1 mmol ) were dissolved 
 in toluene ( 126 ml ) under 
 a nitrogen atmosphere .
the solution was cooled to 0 c and stirred 
 on ice for 10 min .
diad ( 3.05 g , 2.98 ml , 15.1 mmol ) was added dropwise 
 to the solution over 15 min .
perfluoro - tert - butanol 
 ( 5.95 g , 3.52 ml , 25.2 mmol ) and dipea ( 3.18 g , 4.38 ml , 25.2 mmol ) 
 were added to the solution , which was stirred on ice for another 5 
 min .
the solution was removed from the ice bath , warmed to 45 c 
 in an oil bath , and stirred for 24 h. the solvent was removed under 
 reduced pressure , and the crude product was dissolved in ethyl acetate 
 ( 50 ml ) .
the crude product was washed with brine ( 2  75 ml ) 
 and dried over sodium sulfate .
the solvent was removed under reduced 
 pressure , and the crude product was redissolved in ch2cl2 ( 50 ml ) .
the crude product was purified via column chromatography 
 ( 07% ethyl acetate in hexanes v / v ) to obtain compound 5 ( 2.07 g , 4.47 mmol ) as a colorless oil in 36% yield .
h nmr ( 400 mhz , cdcl3 )  4.84 ( br s , 1h ) , 
 4.524.50 ( dd , j = 8.3 , 3.7 hz , 0.4h , minor ) , 
 4.414.38 ( dd , j = 9.0 , 3.1 hz , 0.6h , major ) , 
 3.843.78 ( dd , j = 12.5 , 5.6 hz , 0.6h , major ) , 
 3.763.75 ( d , j = 6.5 hz , 0.4h , minor ) , 3.72 
 ( s , 3h ) , 3.683.65 ( d , j = 13.0 hz , 0.6h , 
 major ) , 3.583.55 ( d , j = 13.0 hz , 0.4h , minor ) , 
 2.532.38 ( m , 2h ) , 1.48 ( s , 4h , major ) , 1.43 ( s , 5h , minor ) .
c nmr ( 150.8 mhz , cdcl3 )  172.9 , 172.8 , 
 154.2 , 154.0 , 129.4 , 129.3 , 124.8 , 124.6 , 124.5 , 120.1 ( q , j = 293 hz ) , 105.1 , 80.9 , 80.8 , 66.6 , 66.2 , 57.5 , 57.1 , 
 52.3 , 37.6 , 36.6 , 28.3 , 28.2 .
f nmr ( 376.3 mhz , cdcl3 )  70.42 ( minor ) , 70.44 ( major ) .
hrms 
 ( lifdi - tof ) m / z : [ m ] calcd for c15h18f9no5 463.1041 , found 463.1042 .
compound 8 ( 1.75 g , 3.78 mmol ) 
 and lioh ( 0.1086 g , 4.53 mmol ) were 
 dissolved in a solution of water ( 20 ml ) and 1,4-dioxane ( 20 ml ) . 

the solution was stirred at room temperature for 14 h. the reaction 
 mixture was acidified to ph 2 with dilute hcl and extracted with ethyl 
 acetate ( 2  75 ml ) .
the solvent was removed under reduced pressure , 
 and the crude product was redissolved in ch2cl2 .
the crude product was purified via column chromatography ( 02% 
 methanol in ch2cl2 v / v ) to obtain compound 9 ( 1.05 g , 2.34 mmol ) as an off - white solid in 62% yield .
h ( 600 mhz , cdcl3 )  4.86 ( s , 1h ) , 4.514.50 
 ( d , j = 6.7 hz , 0.4h , minor ) , 4.444.42 ( d , j = 7.8 hz , 0.6h , major ) , 3.823.76 ( m , 1h ) , 3.723.67 
 ( m , 0.6h , major ) , 3.563.55 ( m , 0.4h , minor ) , 2.522.44 
 ( m , 2h ) , 1.49 ( s , 3.6h ) , 1.44 ( s , 5.4h ) .
c nmr ( 150.8 
 mhz , cdcl3 )  177.4 , 175.5 , 154.7 , 153.5 , 120.1 ( q , j = 293 hz ) , 80.9 , 57.5 , 57.1 , 52.9 , 52.7 , 52.1 , 37.7 , 37.6 , 
 36.1 , 28.1 .
f nmr ( 564.5 mhz , cdcl3 )  
 70.38 ( major ) , 70.44 ( minor ) . hrms ( lifdi - tof )
m / z : [ m + h ] calcd for c14h17f9no5 450.0963 , 
 found 450.0958 .
compound 9 ( 2.0 g , 4.3 mmol ) was dissolved 
 in 1,4-dioxane ( 15 ml ) , and 4 m hcl ( 15 ml ) was added .
the solution 
 was allowed to stir at reflux for 6 h. the solvent was removed under 
 reduced pressure .
compound 10 ( 1.5 g , 4.3 mmol ) obtained 
 was used as a crude reagent in the next step .
alternatively , compound 8 could be subjected to the same conditions to yield compound 10 .
h nmr ( 600 mhz , meod - d4 )  5.20 ( s , 1h ) , 4.624.60 ( dd , j = 9.6 , 3.4 hz , 1h ) , 3.743.71 ( dd , j = 13.4 , 
 4.6 hz , 1h ) , 3.563.53 ( d , j = 13.9 hz , 1h ) , 
 2.742.68 ( ddd , j = 14.6 , 10.0 , 5.0 hz , 1h ) , 
 2.542.51 ( d , j = 14.6 hz , 1h ) .
c nmr ( 150.8 mhz , meod - d4 )  169.3 , 
 120.0 ( j = 293 hz ) , 78.9 , 58.3 , 52.1 , 36.1 .
hrms ( lifdi - tof ) m / z : [ m ] calcd for c9h9f9no3 350.0439 , found 350.0413 .
crude 
 compound 10 ( 0.50 g , 1.4 mmol ) was dissolved in 1,4-dioxane 
 ( 7 ml ) .
fmoc - osu ( 0.50 g , 1.4 mmol ) and k2co3 ( 0.39 g , 2.9 mmol ) were added , and the resultant solution was stirred 
 for 14 h at room temperature .
the 1,4-dioxane was removed under reduced 
 pressure , and the crude product was acidified with 2 m hcl ( 10 ml ) . 
 the crude product was extracted with ethyl acetate ( 2  20 ml )
. 
 the solvent was removed , and the crude product was redissolved in 
 ch2cl2 . the crude mixture ( 0.40 g , 0.72 mmol ) 
 was purified via column chromatography ( 04% methanol in ch2cl2 v / v ) to obtain compound 11 ( 0.38 
 g , 0.70 mmol ) as a white solid in 50% yield .
h ( 600 mhz , 
 cdcl3 )  7.777.69 ( d , j = 
 7.6 hz , 2h ) , 7.607.51 ( m , 2h ) , 7.427.28 ( m , 4h ) , 4.92 
 ( s , 0.5h ) , 4.88 ( s , 0.5h ) , 4.614.56 ( m , 1h ) , 4.534.49 
 ( m , 0.5h ) , 4.434.31 ( m , 2h ) , 4.164.13 ( dd , j = 6.3 , 6.1 hz , 0.5h ) , 3.903.86 ( m , 1h ) , 3.763.63 
 ( m , 1h ) , 3.74 ( s , 1h ) , 2.572.41 ( m , 2h ) .
c nmr 
 ( 150.8 mhz , cdcl3 )  176.3 , 175.7 , 154.7 , 154.3 , 
 143.9 , 143.8 , 143.60 , 143.58 , 141.4 , 141.3 , 127.8 , 127.74 , 127.70 , 
 127.13 , 127.10 , 127.0 , 125.1 , 125.0 , 124.8 , 121.1 , 119.97 ( j = 293 hz ) , 119.96 , 78.2 , 67.9 , 67.8 , 67.0 , 57.3 , 57.0 , 
 53.4 , 52.9 , 47.1 , 37.7 , 36.5 , 21.9 .
f nmr ( 564.5 mhz , 
 cdcl3 )  70.42 , 70.49 . hrms ( lifdi - tof ) m / z : [ m ] calcd for c24h18f9no5 571.1041 , found 571.1038 .
peptides were 
 synthesized on novagel peg - polystyrene graft rink amide resin ( emd 
 millipore ) by standard solid - phase peptide synthesis .
amide coupling 
 reactions with fmoc - perfluoro - tert - butyl 4-hydroxyproline 
 amino acids were conducted using 4 equiv each of amino acid and comu , 
 with the reaction allowed to proceed for 36 h. subsequent amide coupling 
 reactions in ac - gppxppgy - nh2 peptides were conducted 
 as double coupling reactions ( 2 h for the first coupling , 1 h for 
 the second coupling ) using hatu as a coupling reagent .
all peptides 
 were acetylated on the n - terminus ( 5% acetic anhydride in pyridine , 
 3 ml , 3  5 min ) and contained a c - terminal amide .
peptides were 
 cleaved from the resin and deprotected for 3 h using a 1 ml reaction 
 volume of 90% tfa with 5% h2o and 5% triisopropylsilane . 

tfa was removed by evaporation , and the peptides were dissolved in 
 1 ml of 500 mm phosphate buffer ( ph 7.2 ) .
crude peptide solutions 
 were filtered using a 0.45 m syringe filter before injection 
 on the hplc .
peptides were purified by reverse - phase hplc ( vydac semipreparative 
 c18 , 10  250 mm , 5 m particle size , 300  pore ) 
 using a 60 min linear gradient of 100% to 40% buffer a ( 98% h2o , 2% acetonitrile , 0.06% tfa ) in buffer b ( 20% h2o , 80% acetonitrile , 0.05% tfa ) .
analytical data for peptides : 
 ac - gpp((2s,4r)-hyp(c4f9))ppgy - nh2 [ tr , 37.8 min ; expected mass , 
 1056.4 , observed mass , 1078.4 ( m + na ) ] ; ac - gpp((2s,4s)-hyp(c4f9))ppgy - nh2 [ tr , 36.8 min ; expected mass , 
 1056.4 , observed mass , 1078.4 ( m + na ) ] ; ac-((2s,4r)-hyp(c4f9))kaaaakaaaakaagy - nh2 [ tr , 41.9 min ; expected mass , 
 1706.7 , observed mass , 853.5 ( m ) ] ; ac-((2s,4r)-hyp(c4f9))kaaaakaaaakaagy - nh2 [ tr , 40.8 min ; expected mass , 
 1706.7 , observed mass , 853.6 ( m ) ] . peptides ( 1050 m 
 final concentrations : ac-((2s,4r)-hyp(c4f9))kaaaakaaaakaagy - nh2 10 
 m ; ac-((2s,4s)-hyp(c4f9))kaaaakaaaakaagy - nh2 50 
 m ; ac - gpp((2s,4r)-hyp(c4f9))ppgy - nh2 20 m ; ac - gpp((2s,4s)-hyp(c4f9))ppgy - nh2 50 m ) were dissolved in water containing 5 mm phosphate buffer 
 ph 7.4 and 25 mm kf .
the temperatures for 
 each experiment were chosen to allow direct comparison to the analogous 
 peptides with proline .
experiments 
 were conducted 
 in 90% h2o/10% d2o containing 5 mm phosphate 
 buffer ph 4 , 25 mm nacl , and 100 m tsp as a reference for h nmr spectra .
f nmr experiments 
 were conducted without decoupling using a 1.5 s relaxation delay on 
 a brker 600 mhz ( 545.5 mhz for f ) nmr spectrometer 
 equipped with a brker smart probe .
 ( 2s,4r)- and ( 2s,4s)-perfluoro - tert - butyl 4-hydroxyproline 
 were synthesized ( as fmoc- , boc- , and free amino acids ) in 25 
 steps . 
 the key step of each synthesis was a mitsunobu reaction with 
 perfluoro - tert - butanol , which incorporated a perfluoro - tert - butyl group , with nine chemically equivalent fluorines . 
 
 both amino acids were incorporated in model -helical and polyproline 
 helix peptides . 
 each amino acid exhibited distinct conformational 
 preferences , with ( 2s,4r)-perfluoro - tert - butyl 4-hydroxyproline promoting polyproline helix . 
 
 peptides containing these amino acids were sensitively detected by 19f nmr , suggesting their use in probes and medicinal chemistry . 
Experimental Section