A simple and efficient method for the resolution of all four diastereomers of 4,4,4-trifluorovaline and 5,5,5-trifluoroleucine

Full text of DOI:10.1021/jo011097q

1722
J . Org. Chem. 2002, 67, 1722-1725
trifluoromethyl-containing amino acids are highly desir-
A Sim p le a n d Efficien t Meth od for th e
Resolu tion of All F ou r Dia ster eom er s of
4,4,4-Tr iflu or ova lin e a n d
able for peptide synthesis to avoid compositional hetero-
geneity of samples and for exploration of stereoelectronic9
and packing effects.
5,5,5-Tr iflu or oleu cin e
There has been no lack of effort in attempts to prepare
enantiomerically pure TFL and TFV.10 However, practi-
cal methods for enantioselective synthesis and optical
resolution have been limited by low efficiency or com-
promised enantiomeric purity of the final products. Re-
ported here is an efficient resolution of all four diaster-
eomers of TFL and TFV. The method as outlined in
Scheme 1 is simple and practical. Appropriately deriva-
tized TFL and TFV could be separated into two enan-
tiomeric pairs by flash column chromatography. Subse-
quent enzymatic deacylation of the N-acetyl enantiomeric
pairs of amino acids with porcine kidney acylase I de-
livers all four diastereomers in an optically pure form.11
In the course of our study on the synthesis of enan-
tiomerically pure TFV, we found that Boc-protected 4,4,4-
trifluorovalinol R-S diastereomers could be easily sepa-
rated by column chromatography on silica gel.12 This
finding encouraged us to develop a resolution scheme for
racemic TFV and TFL. As shown in Scheme 2, Boc-TFV
1 was first converted to Boc-trifluorovalinol 2 via esteri-
fication of 1 with methyl iodide, followed by reduction of
the methyl ester with sodium borohydride in methanol
in 73% overall yield for the two steps. The racemic
mixture of trifluorovalinols was easily separated into the
two enantiomeric pairs 2a [(2S,3R) + (2R,3S)] and b
[(2S,3S) + (2R,3R)] by column chromatography on silica
gel using n-pentane/ethyl ether (1:1) as the eluant.
Although the methyl esters of Boc-TFV 1 are also
separable, they are not stable toward racemization in the
subsequent reduction step. Oxidation of the hydroxyl
groups of 2a and b with PDC in DMF and removal of
the Boc-protecting group with 30% trifluoroacetic acid in
methylene chloride followed by acylation of the free
amino group afforded the N-acetyl amino acids 3a and
b, respectively. Finally, enzymatic deacylation of 3a and
b with porcine kidney acylase I afforded the four dia-
stereomers 4a -d . Only those diastereomers that had an
S configuration at CR were deacylated by the enzyme.
Removal of the acetyl group from the two CR-R diaster-
eomers was realized by refluxing with 3 N HCl.
Xuechao Xing,† Alfio Fichera,† and Krishna Kumar*
Department of Chemistry, Tufts University,
62 Talbot Avenue, Medford, Massachusetts 02155
kkumar01@tufts.edu
Received November 26, 2001
Fluorinated amino acids have recently emerged as a
valuable set of pharmacologically active agents.1 Fluorine-
containing amino acids have been shown to act as
inhibitors of enzymes2 and function as antitumor and
antibacterial agents.1d,e There has been a special interest
in trifluoromethyl-containing amino acids in peptide and
protein design3,4 because the replacement of methyl with
trifluoromethyl groups is accompanied by a substantial
increase in hydrophobicity, owing to the low polarizibility
of the fluorine atoms.5,6 It is therefore anticipated that
introduction of trifluoromethyl groups into the hydro-
phobic core of proteins would impose minor structural
perturbation but would result in enhanced stability of
the folded form in aqueous solution. We have recently
reported a peptide system based on the coiled coil region
of the transcriptional activator GCN4 with an unnatural
hydrophobic core, prepared by substituting four leucine
and three valine residues with 5,5,5-trifluoroleucine
(TFL) and 4,4,4-tifluorovaline (TFV), respectively. The
resulting coiled coil structure was more resistant to
thermal- and denaturant-induced unfolding than was its
hydrocarbon counterpart.7 Similar studies from other
laboratories have reported the incorporation of CF3-
containing amino acids into peptides by in vivo methods
with enhanced thermal and chemical stability.8 It must
be emphasized, however, that in these studies both TFL
and TFV were incorporated into peptides as a mixture
of two CR-S diastereomers.7,8 Enantiomerically pure
* To whom all correspondence should be addressed. Tel: (617) 627-
3441. Fax: (617) 627-3443.
† X.X. and A.F. contributed equally to this work.
(1) (a) Filler, R.; Kabayashi, Y. Biomedical Aspects of Fluorine
Chemistry; Elsevier Biomedical: Amsterdam, 1982. (b) Williams, R.
M. Synthesis of Optically Active R-Amino Acids; Pergamon Press:
Oxford, 1989. (c) Welch, T.; Eswarakrishnan, S. Fluorine in Bioorganic
Chemistry; Wiley-Interscience: New York, 1991. (d) Fluorine-Contain-
ing Amino Acids; Kukhar’, V. P., Soloshonok, V. A., Eds.; Wiley &
Sons: Chichester, 1994. (e) Tsushima, T.; Kawada, K.; Ishihara, S.;
Uchida, N.; Shiratori, O.; Higaki, J .; Hirata, M. Tetrahedron 1988, 44,
5375-5387. (f) Ojima, I.; Kato, K.; Nakahashi, K.; Fuchikami, T.;
Fujita, M. J . Org. Chem. 1989, 54, 4511-4522. (g) Eberle, M. K.; Keese,
R.; Stoeckli-Evans, H. Helv. Chim. Acta 1998, 81, 182-186.
(2) Kollonitsch, J .; Patchett, A. A.; Marburg, S.; Maycock, A. L.;
Perkins, L. M.; Doldouras, G. A.; Duggan, D. E.; Aster, S. D. Nature
1978, 274, 906-908.
This strategy was also applied to the resolution of TFL
(Scheme 3). Initially, Boc-TFL 5 was also converted to
the corresponding alcohols following the procedure used
for Boc-TFV 1, but we found that the trifluoroleucinols
were not separable by column chromatography on silica
(8) (a) Tang, Y.; Ghirlanda, G.; Petka, W. A.; Nakajima, T.; DeGrado,
W. F.; Tirrell, D. A. Angew. Chem., Int. Ed. 2001, 40, 1494-1496. (b)
Tang, Y.; Ghirlanda, G.; Vaidehi, N.; Kua, J .; Mainz, D. T.; Goddard,
W. A., III; DeGrado, W. F.; Tirrell, D. A. Biochemistry 2001, 40, 2790-
2796.
(9) (a) Bretscher, L. E.; J enkins, C. L.; Taylor, K. M.; Derider, M.
L.; Raines, R. T. J . Am. Chem. Soc. 2001, 123, 777-778. (b) Holmgren,
S. K.; Bretscher, L. E.; Taylor, K. M.; Raines, R. T. Chem. Biol. 1999,
6, 63-70.
(3) Xing, X.; Fichera, A.; Kumar, K. Org. Lett. 2001, 3, 1285-1286
and references therein.
(4) Bilgic¸er, B.; Xing, X.; Kumar, K. J . Am. Chem. Soc. 2001, 123,
11815-11816.
(10) (a) Matsumura, Y.; Urushihara, M.; Tanaka, H.; Uchida, K.;
Yasuda, A. Chem. Lett. 1993, 1255-1256. (b) Vlasacova, V.; Tolman,
V.; Zivny, K. J . Chromatogr. 1993, 639, 273-279. (c) Kitazume, T.;
Lin, J . T.; Yamazaki, T. Tetrahedron: Asymmetry 1991, 2, 235-237.
(d) Weinges, K.; Kromm, E.; Liebigs Ann. Chem. 1985, 90-102.
(11) Chenault, H. K.; Dahmer, J .; Whitesides, G. M. J . Am. Chem.
Soc. 1989, 111, 6354-6364.
(5) (a) Seebach, D.; Renaud, P.; Schweizer, W. B.; Zueger, M. F. Helv.
Chim. Acta 1984, 67, 1843. (b) Hagan, D. O.; Rzepa, H. S. Chem.
Commun. 1997, 645.
(6) (a) Seebach, D. Angew. Chem., Int. Ed. Engl. 1990, 29, 1320. (b)
Resnati, G. Tetrahedron 1993, 49, 9385-9445.
(7) Bilgic¸er, B.; Fichera, A.; Kumar, K. J . Am. Chem. Soc. 2001, 123,
4393-4399.
(12) Xing, X.; Fichera, A.; Kumar, K. Unpublished observations.
10.1021/jo011097q CCC: $22.00 © 2002 American Chemical Society
Published on Web 02/08/2002

Notes
J . Org. Chem., Vol. 67, No. 5, 2002 1723
Sch em e 1
The N-acetyl amino acids 7a and b were obtained from
6a and b, respectively, by straightforward functional-
group transformations that included reduction of the
methyl ester group to hydroxyl, oxidation of the hydroxyl
to acid, and replacement of the Boc protecting group with
an acetyl group. In the final step, enzymatic deacylation
was applied to 7a and b to give diastereomerically pure
compounds 8a -d .
The purity of the intermediates and of the final
diastereomers was ascertained using 1H, 13C, and 19F
NMR spectroscopy. The 19F NMR technique is particu-
larly useful in this case for purity control because of its
high sensitivity and the large chemical-shift dispersion
observed for these compounds. The enantiomeric pairs
exhibited baseline-separated 19F NMR spectra in each
case. Contamination by the other enantiomeric pair or
racemization during chemical transformation could be
easily detected. The optical purity of the products was
also verified by NMR analysis of dipeptides formed by
coupling with a side chain-protected methyl ester of
L-serine.3 The 19F NMR spectra clearly showed four peaks
for dipeptides derived from the racemic mixture, two
peaks for dipeptides derived from enantiomeric pairs, and
only one peak for the diastereomerically pure dipeptide.
In summary, we have developed a simple method for
efficient resolution of all four diastereomers of TFL and
TFV.
Sch em e 2a
Exp er im en ta l Section
Gen er a l P r oced u r es. Flash column chromatography was
performed on Kieselgel 60 silica gel (230-240 mesh, EM Science)
using standard literature procedures.13 Analytical thin-layer
chromatography was performed using E. Merck silica gel Kie-
selgel 60 F254 (0.25 mm) plates. Compounds were made visible
by using UV light, by exposure to iodine vapor, or by staining
with a ninhydrin solution followed by heating. Reagents and
solvents were of reagent grade or better and were obtained from
Aldrich Chemical Co., Fluka Chemie AG, Fluorochem USA,
Lancaster Synthesis, or Novabiochem Corp. Deuterated solvents
were obtained from Cambridge Isotope Laboratories.
Infrared spectra were obtained on a Mattson 1000 FT-IR
instrument with a 4 cm-1 band-pass. Spectra of solid samples
were recorded with either KBr pressed plates or thin layers of
organic solutions between NaCl plates. Mass spectra were
obtained on a Hewlett-Packard GC-MS (model 5988A) with a
dip-probe using conditions as indicated. Nuclear magnetic
resonance spectra were recorded on a Bruker AM-300 or a
Bruker DPX-300 instrument using standard deuterated solvents.
19F NMR spectra were measured using CFCl3 (δ ) 0) for organic
solvents and CF3CO2H (δ ) -76.50) for D2O as the internal
standards. Optical rotations were measured using an AUTOPOL
IV digital polarimeter (Rudolph Research Analytical, NJ ).
N-Boc-4,4,4-tr iflu or ova lin ol (2). To a suspension of Boc-
DL-trifluorovaline (1.30 g, 4.79 mmol) and NaHCO3 (1.21 g, 14.37
mmol) in 20 mL of dry DMF was added 0.33 mL of CH3I (5.27
mmol) at room temperature under argon. The resulting mixture
was stirred for 5 h and then partitioned between 75 mL of ethyl
acetate and 50 mL of water. The organic layer was washed with
water (3 × 50 mL), dried over MgSO4, and concentrated to yield
1.36 g (95%) of the Boc-DL-trifluorovaline methyl ester as a pale-
yellow oil.
a
Reagents and conditions: (a) NaHCO3, CH3I, DMF, rt, 95%;
(b) NaBH4, CH3OH, 77%; flash column chromatography, n-pen-
tane/Et2O (1:1), silica gel/2 (300:1); (c) PDC, DMF, rt, 65%; (d) 30%
CF3CO2H/CH2Cl2, 100%; (e) NaOH/H2O, Ac2O, 0 °C, 95%; (f)
porcine kidney acylase I, pH 7.50, 25 °C, 95%; (g) 3 N HCl, 98%.
Sch em e 3a
a
Reagents and conditions: (a) NaHCO3, CH3I, DMF, rt, 95%;
The Boc-TFV methyl ester (855 mg, 3 mmol) was dissolved
in 20 mL of methanol, and NaBH4 (681 mg, 18 mmol) was added
in small portions at 0 °C. The reaction mixture was stirred
overnight at room temperature and then diluted with 80 mL of
ethyl acetate, washed with water (3 × 50 mL), and dried over
MgSO4. After removal of the solvent, the crude product (Boc-
trifluorovalinol) was chromatographed on a silica gel column
(silica gel, 300 g) using n-pentane/Et2O (1:1) as the eluant to
flash column chromatography, n-pentane/Et2O (4:1), silica gel/6
(400:1); (b) NaBH4, CH3OH, 94%; (c) PDC, DMF, rt, 60%; (d) 30%
CF3CO2H/CH2Cl2, 100%; (e) NaOH/H2O, Ac2O, 0 °C, 95%; (f)
porcine kidney acylase I, pH 7.5, 25 °C, 95%; (g) 3 N HCl, 96%.
gel. Interestingly, the methyl esters of 5 were readily
separated into two enantiomeric pairs, 6a and b, on silica
gel using n-pentane/ethyl ether (3:1) as the eluant and
were stable toward racemization in the reduction step.
(13) Still, W. C.; Kahn, M.; Mitra, A. J . Org. Chem. 1978, 43, 2923.

1724 J . Org. Chem., Vol. 67, No. 5, 2002
Notes
give 452 mg of 2a as a pale-yellow solid (58%) and 214 mg of 2b
as a white solid (28%).
(2S,3S)-4,4,4-Tr iflu or ova lin e (4b)1f: 1H NMR (300 MHz,
D2O) δ 4.24 (dd, 1H, J ) 2.1, 3.9 Hz), 3.23 (m, 1H), 1.30 (d, 3H,
J ) 7.2 Hz); 19F NMR (282.6 MHz, D2O/CF3CO2H) δ -70.04 (d,
(2S,3R)-(2R,3S)-N-Boc-4,4,4-tr iflu or ovalin ol (2a): 1H NMR
(300 MHz, CDCl3) δ 5.04 (d, 1H, J ) 9.3 Hz), 4.02 (m, 1H), 3.62
3F, J ) 9.0 Hz); [R]23.3 ) +12.8° (c 0.5, 1 N HCl).
D
(m, 3H), 2.61 (m, 1H), 1.44 (s, 9H), 1.15 (d, 3H, J ) 7.2 Hz); 13
C
(2R,3S)-4,4,4-Tr iflu or ova lin e (4c): 1H NMR (300 MHz, D2O)
δ 4.24 (dd, 1H, J ) 2.1, 3.9 Hz), 3.23 (m, 1H), 1.30 (d, 3H, J )
7.2 Hz); 19F NMR (282.6 MHz, D2O/CF3CO2H) δ -70.04 (d, 3F,
J ) 9.0 Hz).
1
NMR (75.5 MHz, CDCl3) δ 156.20 (CdO), 127.83 (q, CF3, J CF
) 279.9 Hz), 80.26 (C), 62.78 (CH2), 51.09 (CH), 38.47 (q, CH,
2J CF ) 25.6 Hz), 28. 40 (3 × CH3), 8.76 (CH3); 19F NMR (282.6
MHz, CDCl3/CFCl3) δ -70.63 (d, 3F, J ) 9.0 Hz); FT-IR (KBr
pellet, ν˜max, cm-1) 3435s, 3300s, 2990s, 2979m, 2954m, 1691s,
1539s, 1537s, 1265s, 1172s, 1125; GC-MS (CI, CH4): 258 (14,
[M + 1]+), 242 (4), 202 (100), 158 (37), 57 (14); CI-HRMS m/z
258.1322, calcd for C10H19NO3F3 258.1317.
(2R,3R)-4,4,4-Tr iflu or ova lin e (4d ): 1H NMR (300 MHz,
D2O) δ 4.35 (t, 1H, J ) 2.7 Hz), 3.27 (m, 1H), 1.22 (d, 3H, J )
7.5 Hz); 19F NMR (282.6 MHz, D2O/CF3CO2H) δ -71.69 (d, 3F,
J ) 9.3 Hz).
N-Boc-5,5,5-tr iflu or oleu cin e m eth yl ester (6). A mixture
of Boc-DL-trifluoroleucine (1.25 g, 4.38 mmol), iodomethane (0.3
mL, 4.82 mmol), NaHCO3 (1.1 g, 13.15 mmol), and dry DMF
(20 mL) was stirred at room temperature under argon for 6 h
and then diluted with 200 mL of ethyl acetate and washed with
water (4 × 100 mL). The organic layer was dried over Na2SO4
and concentrated to give 1.25 g of the product as a pale-yellow
oil (95%). Column chromatography on silica gel (500 g) using
Et2O/n-pentane (1:4) as the eluant afforded 420 mg of (2S,4R)-,
(2R,4S)-N-Boc-5,5,5-trifluoroleucine methyl ester (6a ) (32%), 347
mg of (2S,4S)-, (2R,4R)-N-Boc-5,5,5-trifluoroleucine methyl ester
(6b) (27%), and 337 mg of the mixture of 6a and b (26%).
(2S,3S)-(2R,3R)-N-Boc-4,4,4-tr iflu or ovalin ol (2b): 1H NMR
(300 MHz, CDCl3) δ 5.11 (d, 1H, J ) 8.4 Hz), 3.80 (m, 1H), 3.66
(m, 2H), 3.45 (t, 1H, J ) 5.7 Hz), 2.53 (m, 1H), 1.42 (s, 9H), 1.15
(d, 3H, J ) 7.2 Hz); 13C NMR (75.5 MHz, CDCl3) δ 156.43 (Cd
O), 127.91 (q, CF3, 1J CF ) 280.2 Hz), 80.30 (C), 62.92 (CH2), 52.56
2
(CH), 38.89 (q, CH, J CF ) 24.8 Hz), 28. 40 (3 × CH3), 10.59
(CH3); 19F NMR (282.6 MHz, CDCl3/CFCl3) δ -68.76 (d, 3F, J
) 8.5 Hz); FT-IR (film, ν˜max, cm-1) 3436s, 3302s, 3012m, 2990m,
2954m, 1691s, 1532s, 1265s, 1172s, 1127s; GC-MS (CI, CH4):
258 (14, [M + 1]+), 242 (4), 202 (100), 182 (8), 57 (14); CI-HRMS
(NH3, 170 eV) m/z 258.1314, calcd for C10H19NO3F3 258.1317.
(2S,3R)-(2R,3S)-N-Ac-4,4,4-tr iflu or ova lin e (3a ).1f A solu-
tion of alcohol 2a (257 mg, 1 mmol) in 4 mL of dry DMF was
treated with PDC (2.26 g, 6 mmol) at room temperature under
argon and stirred overnight. The reaction mixture was then
diluted with 20 mL of diethyl ether/30 mL of saturated NaHCO3
solution. The organic layer was washed with 10 mL of saturated
NaHCO3. The combined aqueous layers were acidified to pH 2
with 3 N HCl and extracted with diethyl ether (2 × 50 mL).
The combined organic layers were dried over MgSO4 and
concentrated to yield 176 mg of the corresponding Boc-trifluo-
rovaline7 (65%).
(2S,4R)-, (2R,4S)-N-Boc-5,5,5-tr iflu or oleu cin e Meth yl Es-
ter (6a ): 1H NMR (300 MHz, CDCl3) δ 5.29 (d, 1H, J ) 6.9 Hz),
4.32 (m, 1H), 3.70 (s, 3H), 2.31 (m, 1H), 2.12 (m, 1H), 1.58 (m,
1H), 1.37 (s, 9H), 1.11 (d, 3H, J ) 6.9 Hz); 13C NMR (75.5 MHz,
1
CDCl3) δ 172.72 (CdO), 155.29 (CdO), 128.09 (q, CF3, J CF
)
278.9 Hz), 80.27 (C), 52.54 (CH3), 51.70 (CH), 35.13 (q, CH, 2J CF
) 26.4 Hz), 32.98 (CH2), 28.30 (3 × CH3), 13.17 (CH3); 19F NMR
(282.6 MHz, CDCl3/CFCl3) δ -74.15 (d, 3F, J ) 8.2 Hz); FT-IR
(film, ν˜max, cm-1) 3360m, 2984m, 2938m, 1747s, 1716s, 1520s,
1368s, 1269s, 1168s, 1133m; GC-MS (CI, CH4): 300 (2, [M +
1]+), 284 (7), 244 (100), 200 (66), 82 (21), 57 (24); CI-HRMS m/z
300.1402, calcd for C12H21NO4F3 300.1423.
Boc-TFV (176 mg, 0.65 mmol) was treated with 4 mL of 40%
trifluoroacetic acid in CH2Cl2 for 10 min. After removal of the
solvent, the residue was dissolved in 2 mL of water and treated
with NaOH (260 mg, 6.5 mmol) at 0 °C, followed by dropwise
addition of acetic anhydride (0.13 mL, 1.3 mmol). The reaction
mixture was stirred at 0 °C for 30 min before it was allowed to
warm to room temperature. After stirring for another 1.5 h, the
mixture was diluted with 10 mL of water, acidified to pH 2 with
1 N HCl, and extracted with ethyl acetate (2 × 60 mL). The
combined organic layers were dried over MgSO4 and concen-
trated to give the desired product 3a as a white solid (132 mg,
(2S,4S)-, (2R,4R)-N-Boc-5,5,5-tr iflu or oleu cin e Meth yl Es-
ter (6b): 1H NMR (300 MHz, CDCl3) δ 5.02 (d, 1H, J ) 8.7 Hz),
4.38 (m, 1H), 3.76 (s, 3H), 2.32 (m, 1H), 1.91-1.74 (br m, 2H),
1.44 (s, 9H), 1.20 (d, 3H, J ) 6.9 Hz); 13C NMR (75.5 MHz,
1
CDCl3) δ 173.03 (CdO), 155.86 (CdO), 128.24 (q, CF3, J CF
)
278.9 Hz), 80.57 (C), 52.80 (CH3), 50.83 (CH), 35.02 (q, CH, 2J CF
) 26.9 Hz), 33.00 (CH2), 28.42 (3 × CH3), 12.28 (CH3); 19F NMR
(282.6 MHz, CDCl3/CFCl3) δ -74.03 (d, 3F, J ) 8.7 Hz); FT-IR
(KBr pellet, ν˜max, cm-1) 3368s, 3014m, 2983s, 2961m, 1763s,
1686s, 1527s, 1265s, 1214s, 1170s, 1053s, 1028s; GC-MS (CI,
CH4): 300 (2, [M + 1]+), 284 (7), 244 (100), 224 (30), 200 (66),
57 (24); CI-HRMS m/z 300.1410, calcd for C12H21NO4F3 300.1423.
1
95%). H NMR (300 MHz, D2O) δ 4.96 (d, 1H, J ) 3.0 Hz), 3.07
(m, 1H), 2.04 (s, 3H), 1.15 (d, 3H, J ) 7.2 Hz); 19F NMR (282.6
MHz, D2O/CF3CO2H) δ -71.63 (d, 3F, J ) 8.8 Hz); FT-IR (KBr
pellet, ν˜max, cm-1) 3397s (br), 3253s, 3068m, 2981s, 2948m,
1686s, 1552s, 1369s, 1289s, 1174s, 1145s, 1055s; GC-MS (CI,
CH4): 214 (100, [M + 1]+), 196 (9), 172 (33), 82 (33), 57 (6).
(2S,3S)-(2R,3R)-N-Ac-4,4,4-tr iflu or ova lin e (3b):1f 1H NMR
(300 MHz, D2O) δ 4.67 (d, 1H, J ) 3.3 Hz), 3.07 (m, 1H), 2.04 (s,
3H), 1.17 (d, 3H, J ) 7.2 Hz); 19F NMR (282.6 MHz, D2O/CF3-
(2S,4R)-, (2R,4S)-N-Boc-5,5,5-tr iflu or oleu cin ol. To a solu-
tion of 6a (420 mg, 1.4 mmol) in methanol (10 mL) was added
NaBH4 (531 mg, 14.0 mmol) in small portions. The reaction
mixture was stirred at room temperature for 1 h before removal
of the solvent. The residue was partitioned between 100 mL of
ethyl acetate and 50 mL of water. The aqueous layer was
extracted with 100 mL of ethyl acetate. The combined organic
layers were dried over Na2SO4 and concentrated to yield 357
CO2H) δ -69.43 (d, 3F, J ) 8.8 Hz); FT-IR (KBr pellet, ν˜max
,
cm-1) 3397s (br), 3253s, 3068m, 2981s, 2948m, 1686s, 1552s,
1369s, 1289s, 1174s, 1145s, 1055s; GC-MS (CI, CH4): 214 (100,
[M + 1]+), 196 (9), 172 (33), 101 (10), 82 (33), 57 (6).
1
mg of the desired product as a white solid (94%). H NMR (300
MHz, CDCl3) δ 4.74 (m, 1H), 3.71 (m, 2H), 3.58 (m, 1H), 2.31
(m, 1H), 2.14 (m, 1H), 1.92 (m, 1H), 1.45 (s, 9H), 1.17 (d, 3H, J
) 7.0 Hz); 13C NMR (75.5 MHz, CDCl3) δ 156.26 (CdO), 128.41
(2S,3R)-4,4,4-Tr iflu or ova lin e (4a ).1f To a solution of 3a (107
mg, 0.5 mmol) in 1 mL of pH 7.9 aqueous LiOH/HOAc was added
porcine kidney acylase I (10 mg) at 25 °C. The mixture was
stirred at 25 °C for 48 h (pH was maintained at 7.5 by periodic
addition of 1 N LiOH). The reaction was then diluted with 5
mL of water, acidified to pH 5.0, heated to 60 °C with Norit,
and filtered. The filtrate was acidified to pH 1.5 and extracted
with ethyl acetate (2 × 10 mL). The aqueous layer was freeze-
dried to give 49 mg of 4a (95%). The combined organic layers
were concentrated, and the residue was refluxed in 3 N HCl for
6 h and then freeze-dried to yield 50 mg of 4c (98%).
1
(q, CF3, J CF ) 279.4 Hz), 80.14 (C), 64.78 (CH2), 50.73 (CH),
2
35.59 (q, CH, J CF ) 29.6 Hz), 31.74 (CH2), 28.52 (3 × CH3),
13.71 (CH3); 19F NMR (282.6 MHz, CDCl3/CFCl3) δ -73.84 (br
s, 3F); GC-MS (CI, CH4): 272 (100, [M + 1]+), 216 (68), 172
(26), 57 (11); CI-HRMS m/z 272.1474, calcd for C11H21NO3F3
272.1473.
1
(2S,4S)-, (2R,4R)-N-Boc-5,5,5-tr iflu or oleu cin ol: H NMR
(300 MHz, CDCl3) δ 4.58 (m, 1H), 3.79 (m, 1H), 3.68 (m, 1H),
3.58 (m, 1H), 2.27 (m, 1H), 2.05 (m, 1H), 1.80 (m, 1H), 1.45 (s,
9H), 1.18 (d, 3H, J ) 6.6 Hz); 13C NMR (75.5 MHz, CDCl3) δ
156.47 (CdO), 128.56 (q, CF3, 1J CF ) 278.7 Hz), 80.20 (C), 66.31
The other two diastereomers 4b and d were obtained from
3b using the same procedure as above.
2
(CH2), 49.49 (CH), 35.15 (q, CH, J CF ) 26.7 Hz), 31.71 (CH2),
(2S,3R)-4,4,4-Tr iflu or ova lin e (4a ):1f 1H NMR (300 MHz,
D2O) δ 4.35 (t, 1H, J ) 2.7 Hz), 3.27 (m, 1H), 1.22 (d, 3H, J )
7.5 Hz); 19F NMR (282.6 MHz, D2O/CF3CO2H) δ -71.69 (d, 3F,
28.50 (3 × CH3), 12.56 (CH3); 19F NMR (282.6 MHz, CDCl3/
CFCl3) δ -73.98 (d, 3F, J ) 8.5 Hz); GC-MS (CI, CH4): 272
(100, [M + 1]+), 172 (26), 57 (11); CI-HRMS m/z 272.1466, calcd
for C11H21NO3F3 272.1473.
J ) 9.3 Hz); [R]23.7 ) +7.2° (c 0.75, 1 N HCl).
D

Notes
(2S,4R)-, (2R,4S)-N-Ac-5,5,5-tr iflu or oleu cin e (7a ).1e A mix-
J . Org. Chem., Vol. 67, No. 5, 2002 1725
(2S,4R)-5,5,5-Tr iflu or oleu cin e (8a ):14 19F NMR (282.6 MHz,
ture of (2S,4R)-, (2R,4S)-N-Boc-5,5,5-trifluoroleucinol (330 mg,
1.23 mmol), PDC (4.62 g, 12.3 mmol), and dry DMF (2.5 mL)
was stirred at room temperature under argon for 4 h and then
diluted with 50 mL of ethyl acetate and 50 mL of water. The
organic layer was washed with 30 mL of 1 N HCl and 2 × 30
mL of water, dried over MgSO4, and concentrated to give 198
mg of (2S,4R)-, (2R,4S)-N-Boc-5,5,5-trifluoroleucine as a pale-
brownish oil (60%).
D2O/CF3CO2H) δ -74.33 (d, 3F, J ) 9.0 Hz); [R]22.9 +21.6° (c
D
0.5, 1 N HCl).
(2S,4S)-5,5,5-Tr iflu or oleu cin e (8b):14 19F NMR (282.6 MHz,
D2O/CF3CO2H) δ -74.11 (d, 3F, J ) 8.2 Hz); [R]23.6 -4.0° (c
D
0.8, 1 N HCl).
(2R,4S)-5,5,5-Tr iflu or oleu cin e (8c): 19F NMR (282.6 MHz,
D2O/CF3CO2H) δ -74.33 (d, 3F, J ) 9.0 Hz).
(2R,4R)-5,5,5-Tr iflu or oleu cin e (8d ): 19F NMR (282.6 MHz,
D2O/CF3CO2H) δ -74.11 (d, 3F, J ) 8.2 Hz).
A solution of the above product (180 mg, 0.63 mmol) in 2 mL
of CH2Cl2 was treated with 0.5 mL of trifluoroacetic acid for 30
min at room temperature. After removal of the solvent, the
yellowish residue was dissolved in 2 mL of water and treated
with NaOH (126 mg, 3.15) at 0 °C, and acetic anhydride (0.12
mL, 1.26 mmol) was then added dropwise. The reaction mixture
was stirred at 0 °C for 30 min and then allowed to warm to room
temperature. After stirring for another hour, the mixture was
diluted with 30 mL of water, acidified to pH 2 with 3 N HCl,
and extracted with ethyl acetate (2 × 90 mL). The combined
organic layers were dried over Na2SO4 and concentrated to yield
136 mg of 7a as a white solid (95%). 1H NMR (300 MHz, D2O)
δ 4.48 (dd, 1H, J ) 6.1, 8.8 Hz), 2.51 (m, 1H), 2.27 (m, 1H), 2.06
(s, 3H), 1.79 (m, 1H), 1.18 (d, 3H, J ) 7.0 Hz); 13C NMR (75.5
Dip ep tid es. Boc-TF V-(2S,3S)-Ser (Ot-Bu )-OMe(2S). To a
stirred solution of (2S,4S)-5,5,5-trifluorovaline (4b ) (5 mg,
0.02 mmol) in DMF (1 mL) was added diisopropylethylamine
(DIEA, 0.01 mL, 0.06 mmol), O-(benzotriazol-1-yl)-N,N,N′,N′-tetra-
methyluronium hexafluorophosphate (HBTU, 8 mg, 0.02 mmol),
and the HCl salt of (2S)-H-Ser(Ot-Bu)-OMe (9 mg, 0.04 mmol),
sequentially. The mixture was stirred at room temperature for
20 min before dilution with water (5 mL) and extraction with
diethyl ether (15 mL). The organic layer was washed with 1 N
HCl (2 × 5 mL) and 5% NaHCO3 (2 × 8 mL), dried over MgSO4,
1
and concentrated to give 7 mg of the dipeptide (88%). H NMR
(300 MHz, CDCl3) δ 6.92 (d, 1H, J ) 7.8 Hz), 5.16 (d, 1H, J )
8.7 Hz), 4.65 (m, 1H), 4.39 (dd, 1H, J ) 5.1, 8.8 Hz), 3.81 (dd,
1H, J ) 2.7, 9.0 Hz), 3.74 (s, 3H), 3.56 (dd, 1H, J ) 3.0, 9.0 Hz),
3.04 (m, 1H), 1.46 (s, 9H), 1.23 (d, 3H, J ) 7.2 Hz), 1.14 (s,
9H);19F NMR (282.6 MHz, CDCl3/CFCl3) δ -68.57 (d, 3F, J )
8.7 Hz); HRMS (FAB) m/z 429.2223, calcd for C18H32O6N2F3
429.2212.
Boc-TF V-(2S,3R)-Ser (Ot-Bu )-OMe(2S): 19F NMR (282.6
MHz, CDCl3/CFCl3) δ -71.36 (d, 3F, J ) 7.9 Hz); HRMS (FAB)
m/z 429.2211, calcd for C18H32O6N2F3 429.2212.
Boc-TF V-(2R,3S)-Ser (Ot-Bu )-OMe(2S): 19F NMR (282.6
MHz, CDCl3/CFCl3) δ -71.48 (d, 3F, J ) 8.5 Hz); HRMS (FAB)
m/z 429.2222, calcd for C18H32O6N2F3 429.2212.
Boc-TF V-(2R,3R)-Ser (Ot-Bu )-OMe(2S): 19F NMR (282.6
MHz, CDCl3/CFCl3) δ -68.49 (d, 3F, J ) 9.0 Hz); HRMS (FAB)
m/z 429.2217, calcd for C18H32O6N2F3 429.2212.
1
MHz, D2O) δ 175.48 (CdO), 174.60 (CdO), 128.53 (q, CF3, J CF
2
) 278.9 Hz), 51.24 (CH), 34.88 (q, CH, J CF ) 26.6 Hz), 31.21
(CH2), 21.90 (CH3), 13.03 (CH3); 19F NMR (282.6 MHz, D2O/
CF3CO2H) δ -73.68 (d, 3F, J ) 9.0 Hz); FT-IR (KBr pellet, ν˜max
,
cm-1) 3343s, 3063-2487m (br), 2932m, 2894m, 1709s, 1613s,
1549s, 1266s, 1179s, 1137s; GC-MS (CI, CH4): 228 (100, [M +
1]+), 211 (47), 186 (26), 140 (16), 57 (11).
(2S,4S)-, (2R,4R)-N-Ac-5,5,5-tr iflu or oleu cin e (7b):1e 1H
NMR (300 MHz, D2O) δ 4.48 (dd, 1H, J ) 3.8, 11.6 Hz), 2.41 (m,
1H), 2.07 (s, 3H), 2.15-1.91 (br m, 2H), 1.16 (d, 3H, J ) 6.9
Hz); 13C NMR (75.5 MHz, D2O) δ 178.35 (CdO), 177.38 (CdO),
1
2
131.09 (q, CF3, J CF ) 278.3 Hz), 52.72 (CH), 37.31 (q, CH, J CF
) 26.6 Hz), 33.06 (CH2), 24.50 (CH3), 13.90 (CH3); 19F NMR
(282.6 MHz, D2O/CF3CO2H) δ -73.87 (d, 3F, J ) 8.5 Hz);
FT-IR (KBr pellet, ν˜max, cm-1) 3336s, 2977m, 2949m, 2897m,
2615m, 2473s, 1711s, 1628s, 1551s, 1276s, 1250s, 1127s, 1095s;
GC-MS (CI, CH4): 228 (100, [M + 1]+), 211 (47), 186 (26), 140
(16), 120 (3), 57 (11).
(2S,4R)-5,5,5-Tr iflu or oleu cin e (8a ).14 To a solution of 7a
(136 mg, 0.6 mmol) in 2 mL of pH 7.9 aqueous LiOH/HOAc was
added porcine kidney acylase I (18 mg) at 27 °C. The mixture
was stirred at 27 °C for 48 h (pH was maintained at 7.5 by
periodic addition of 1 N LiOH). It was further diluted with 5
mL of water, acidified to pH 5.0, heated to 60 °C with Norit,
and filtered. The filtrate was acidified to pH 1.5 and extracted
with ethyl acetate (2 × 50 mL). The aqueous layer was freeze-
dried to give 63 mg of 8a (95%). The combined organic layers
were concentrated, and the residue refluxed in 3 N HCl for 6 h
and was then freeze-dried to yield 64 mg of 8c (96%).
The other two diastereomers 8b and d were obtained from
7b using the same procedure as above.
Ack n ow led gm en t. We thank Tufts University for
support and Professor Marc d'Alarcao for helpful and
stimulating discussions.
Su p p or tin g In for m a tion Ava ila ble: NMR spectra (1H,
13C, or 19F) for all new compounds. This material is available
free of charge via the Internet at http://pubs.acs.org.
(14) Some data on 8a and
ref 10d.
b has been reported earlier: See
J O011097Q