A R T I C L E S
Hodges and Raines
TLC and 1H NMR, and impurities persisted after a second flash
chromatographic step (3:1 hexane/EtOAc). The partially purified oil
crystallized upon standing and was recrystallized from hexane to afford
4 (2.37 g, 7.33 mmol, 36%) as colorless needles. 1H NMR δ 1.36 and
1.47 (s, 9H), 1.95-2.21 (m, 1H), 2.50-2.68 (m, 1H), 3.51-3.71 (m,
1H), 3.75-3.99 (m, 1H), 4.45 (dd, J ) 8, 8 Hz, 0.6 H), 4.54 (dd, J )
8, 8 Hz, 0.4H), 5.08-5.31 (m, 3H), 7.31-7.40 (m, 5H); 13C NMR δ
28.1 (28.3), 37.5 (JC-F ) 23.1 Hz) (36.5 (JC-F ) 23.3 Hz)), 52.9 (JC-F
) 23 Hz) (53.2 (JC-F ) 23 Hz)), 66.9, 80.6 (80.5), 91.0 (JC-F ) 178.6
Hz) (91.8 (JC-F ) 178.8 Hz)), 128.1, 128.2, 128.3, 128.5, 128.6, 135.2
(135.5), 153.5 (154.1), 172.4 (172.1); HRMS-ESI (m/z) [M + Na]+
calcd for C17H22FNO4Na 346.1431, found 346.1443.
each). The organic layer was dried over MgSO4(s) and concentrated
by rotary evaporation to a light yellow oil. Flash chromatography
(EtOAc) afforded 9 (0.30 g, 0.48 mmol, 79%) as a colorless oil. 13C
NMR δ 33.7 (JC-F ) 20.8 Hz), 33.9 (JC-F ) 21.3 Hz), 35.5 (JC-F
)
21.7 Hz), 36.5 (JC-F ) 21.2 Hz), 41.3, 47.1, 47.4, 52.8 (JC-F ) 23.7
Hz), 53.0 (JC-F ) 23.1 Hz), 58.0, 58.4, 60.3, 66.9, 67.1. 67.6, 91.5
(JC-F ) 181 Hz), 91.7 (JC-F ) 181 Hz), 119.8, 120.0, 124.8, 125.1,
127.1, 127.7, 128.2, 128.3, 128.4, 128.5, 135.3, 141.3, 143.6, 144.0,
144.4, 154.7, 162.5, 169.2, 170.5; HRMS-ESI (m/z) [M + Na]+ calcd
for C34H33F2N3O6Na 640.2235, found 640.2247.
N-9-Fluorenylmethoxycarbonyl-(2S,4S)-4-fluoroprolyl-(2S,4R)-
4-fluoroprolyl-glycine (1). Under an Ar(g) atmosphere, 9 (0.30 g,
0.49 mmol) was dissolved in a mixture of acetic acid (7 mL) and
methanol (7 mL). Palladium on carbon (10% w/w, 0.035 g, 0.033 mmol
Pd) was added to the solution, and the resulting black suspension was
stirred for 2 h under H2(g) delivered from a balloon. The mixture was
then filtered through Celite, and the filtrate was concentrated by rotary
evaporation. The oily residue was triturated with methanol, and the
resulting solid was recrystallized from methanol, providing 1 (0.10 g,
0.19 mmol, 38%) as colorless needles. 13C NMR (DMSO-d6) δ 34.9
(JC-F ) 20.9 Hz), 35.9 (JC-F ) 20.2 Hz), 36.0 (JC-F ) 21.8 Hz), 40.8,
46.8, 47.0, 53.3 (JC-F ) 23 Hz), 53.8 (JC-F ) 25 Hz), 57.2, 57.4, 58.2,
58.3, 67.0, 67.4, 90.9 (JC-F ) 176.9 Hz), 91.9 (JC-F ) 178.3 Hz), 92.9
(JC-F ) 175.9 Hz), 92.9 (JC-F ) 175.1 Hz), 120.3, 120.4, 125.4, 125.5,
127.4, 127.9, 140.8, 141.0, 143.8, 143.9, 144.0, 153.9, 154.0, 169.1,
169.4, 171.3, 171.4; HRMS-ESI (m/z) [M + Na]+ calcd for
C27H27F2N3O6Na 550.1766, found 550.1763.
N-tert-Butoxycarbonyl-(2S,4R)-4-fluoroproline (5). Methanol was
added carefully to a mixture of 4 (2.29 g, 7.08 mmol) and Pd/C (10%
w/w, 0.74 g, 0.70 mmol Pd) under Ar(g). The resulting black suspension
was stirred for 18 h under H2(g) delivered from a balloon. The
suspension was filtered through Celite, and the filtrate was concentrated
by rotary evaporation to a colorless oil that slowly solidified into a
white powder. No further purification was required for 5 (1.65 g, 7.07
1
mmol, 100%). H NMR δ 1.44 and 1.49 (s, 9H), 2.06-2.74 (m, 2H),
3.44-3.77 (m, 1H), 3.83-4.00 (m, 1H), 4.43 (dd, J ) 8.5, 8.5 Hz,
0.5H), 4.54 (dd, J ) 8.5, 8.5 Hz, 0.5H), 5.10-5.34 (m, 1H), 10.0 (br
s, 1H); 13C NMR δ 28.1 (28.3), 37.4 (JC-F ) 22.1 Hz) (36.0 JC-F
)
22.1 Hz), 52.9 (JC-F ) 22.6 Hz) (53.4 (JC-F ) 22.8 Hz)), 57.5, 81.2
(81.8), 90.9 (JC-F ) 179.6 Hz) (91.4 (JC-F ) 179.3 Hz)), 153.6 (155.6),
178.1 (175.5); HRMS-ESI (m/z) [M - H]- calcd for C10H15FNO4
232.0985, found 232.0995.
N-tert-Butoxycarbonyl-(2S,4R)-4-fluoroprolyl-glycine Benzyl Es-
ter (6). To a solution of 5 (1.49 g, 6.39 mmol) in anhydrous CH2Cl2
(75 mL) at 0 °C under Ar(g) was added the tosyl salt of glycine benzyl
ester (2.15 g, 6.38 mmol), N,N′-diisopropylethylamine (DIEA; 2.3 mL,
13.2 mmol), and PyBOP (3.34 g, 6.42 mmol). The solution was stirred
overnight, and then washed with 10% w/v aqueous citric acid (3 × 25
mL), saturated aqueous NaHCO3 (3 × 25 mL), brine (25 mL), and
water (25 mL). The organic layer was dried over MgSO4(s) and
concentrated by rotary evaporation to a colorless oil. Flash chroma-
tography (1:1 hexane/EtOAc) afforded 1.92 g (5.05 mmol, 79%) of 6
N-tert-Butoxycarbonyl-(2S,3S)-3-hydroxyproline (12). To a sus-
pension of 11 (4.92 g, 37.5 mmol) in a mixture of THF (40 mL) and
H2O (20 mL) was added 10% w/v aqueous NaOH (20 mL) and a
solution of di-tert-butyl dicarbonate (12.3 g, 56.3 mmol) in THF (40
mL) and H2O (20 mL). The resulting biphasic mixture was stirred
overnight. Most of the THF was removed by rotary evaporation, and
the concentrate was washed with EtOAc (6 × 75 mL). The aqueous
layer was then acidified to pH 2-3 with 10% w/v aqueous KHSO4,
saturated with NaCl, and extracted with 2:1 CHCl3/EtOH (3 × 100
mL). The combined organic extracts were dried over MgSO4(s), filtered,
and concentrated by rotary evaporation to provide 12 (6.9 g, 30 mmol,
1
as an oily solid. H NMR δ 1.46 (s, 9H), 2.06-2.70 (m, 2H), 3.31-
1
3.63 (m, 1H), 3.81-4.18 (m, 3H), 4.31-4.59 (m, 1H), 5.06-5.30 (m,
3H), 7.25-7.45 (m, 5H); 13C NMR δ 20.2, 34.9 (JC-F ) 20 Hz), 41.4,
53.4 (JC-F ) 22.7 Hz), 58.1, 67.1, 81.1, 91.8 (JC-F ) 175 Hz), 128.3,
128.6, 135.1, 155.7, 169.4, 171.5; HRMS-ESI (m/z) [M + Na]+ calcd
for C19H25FN2O5Na 403.1645, found 403.1633.
80%) as an oily solid. H NMR δ 1.43 and 1.47 (s, 9H), 1.82-1.93
(m, 1H), 1.97-2.11 (m, 1H), 3.46-3.62 (m, 2H), 4.10 and 4.17 (s,
1H), 4.36-4.41 (m, 1H), 4.95 (s, 2H); 13C NMR δ 28.6 (28.7), 32.8
(33.4), 45.3 (45.8), 69.4 (69.1), 75.8 (75.0), 81.5 (81.3), 156.1 (156.4),
174.3 (174.0); HRMS-ESI (m/z) [M - H]- calcd for C10H16NO5
230.1028, found 230.1019.
(2S,4R)-4-Fluoroprolyl-glycine Benzyl Ester-HCl (7). Dipeptide
6 (1.9 g, 5.0 mmol) was dissolved in 38 mL of 4 M HCl in dioxane.
After 1 h, the solution was concentrated by rotary evaporation to a
white solid. Ether was added to the solid, forming a suspension that
was allowed to stand overnight. The suspension was filtered under
vacuum, affording 7 (1.6 g, 5.0 mmol, 100%) as a white solid. 1H NMR
(CD3OD) δ 2.24 (dddd, J ) 3.3, 10.4, 14.2, 38.8 Hz, 1H), 2.72-2.88
(m, 1H), 3.51-3.76 (m, 2H), 4.05 and 4.13 (d, J ) 17.7 Hz, 2H), 4.59
(dd, J ) 7.5, 10.5 Hz, 1H), 5.17 and 5.19 (d, J ) 12.3 Hz, 2H), 5.35-
N-tert-Butoxycarbonyl-(2S,3S)-3-hydroxyproline Methyl Ester
(13). A solution of diazomethane etherate was prepared in a Mini-
Diazald reactor (Aldrich) from 6.1 g (29 mmol) of Diazald. The solution
was added carefully behind a blast shield to a suspension of 12 (3.72
g, 16.1 mmol) in ether. Vigorous bubbling occurred. After the solution
was left to stand overnight, the solid had dissolved, and a yellow color
persisted. A few drops of acetic acid were added to quench and the
mixture was concentrated by rotary evaporation to a colorless viscous
oil. Residual acetic acid, indicated by 1H NMR spectroscopy, was
removed as an azeotrope with both benzene and hexanes. Further
concentration of the residue under reduced pressure did not remove all
traces of solvent, as benzene and hexanes were still present according
to 1H NMR spectroscopy. The mass of the remaining residue was 4.06
5.57 (m, 1H), 7.29-7.40 (m, 5H); 13C NMR (CD3OD) δ 37.9 (JC-F
)
21.8 Hz), 42.3, 53.4 (JC-F ) 24.4 Hz), 59.7, 68.1, 93.7 (JC-F ) 177.3
Hz), 129.4, 129.6, 137.0, 169.4, 170.6; ESI-MS (m/z) [M - Cl]+ 280.1.
N-9-Fluorenylmethoxycarbonyl-(2S,4S)-4-fluoroprolyl-(2S,4R)-
4-fluoroprolyl-glycine Benzyl Ester (9). To a solution of Fmoc-4S-
FlpOH42 (8, 0.22 g, 0.62 mmol) in anhydrous CH2Cl2 (1 mL) was added
0.20 g (0.63 mmol) of 7, which was insoluble. After cooling of the
suspension in a 0 °C bath, PyBroP (0.29 g, 0.62 mmol) and DIEA
(0.32 mL, 1.8 mmol) were added. The DIEA addition caused the
suspension to dissolve. After 5 min, the reaction was removed from
the cold bath and stirred for 90 min, at which point TLC (1:1:0.04
CHCl3/EtOAc/AcOH) showed no remaining 8. The reaction mixture
was diluted with CH2Cl2 (15 mL) and washed with 10% w/v aqueous
citric acid, saturated aqueous NaHCO3, brine, and water (15 mL of
1
g, of which greater than 90% appeared to be 13. H NMR δ 1.41 and
1.47 (s, 9H), 1.85-1.99 (m, 1H), 2.03-2.19 (m, 1H), 2.44 (br s, 1H),
3.54-3.70 (m, 2H), 3.75 (s, 3H), 4.18 (s, 0.6H), 4.30 (s, 0.4H), 4.44
(s, 1H); 13C NMR δ 28.2 (28.4), 31.2 (32.5), 44.2 (44.6), 52.1 (52.3),
68.0 (67.7), 75.1 (74.1), 80.2, 154.0 (154.7); ESI-MS (m/z) [M + Na]+
calcd for C11H19NO5Na 268.1, found 268.1.
N-tert-Butoxycarbonyl-(2S,3R)-3-hydroxyproline Benzyl Ester
(14). To a solution of 13 (7.7 g, 31 mmol) in anhydrous THF (250
mL) at 0 °C were added PPh3 (8.85 g, 32.9 mmol) and p-nitrobenzoic
9
15930 J. AM. CHEM. SOC. VOL. 127, NO. 45, 2005