Synthesis of (2S,3R)-[3′,3′,3′-2H 3]-valine and (2S,3S)-4-fluorovaline

Article abstract of DOI:10.1039/b401646g

A new synthesis of (2S,3R)-[3′,3′,3′-²H ₃]-valine 2 has been completed and (2S,3S)-4-fluorovaline 4 has been synthesised for the first time. Both compounds have been prepared by routes involving stereoselective addition to the (S)-pyroglutamate derivative 5 and are available for studies in several areas of bio-organic chemistry.

Full text of DOI:10.1039/b401646g

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Synthesis of (2S,3R)-[3Ј,3Ј,3Ј-2H3]-valine and
(2S,3S )-4-fluorovaline†
Jean-Damien Charrier, Peter B. Hitchcock and Douglas W. Young*
Sussex Centre for Biomolecular Design and Drug Discovery, Department of Chemistry,
University of Sussex, Falmer, Brighton, UK BN1 9QJ
Received 4th February 2004, Accepted 5th March 2004
First published as an Advance Article on the web 29th March 2004
A new synthesis of (2S,3R)-[3Ј,3Ј,3Ј-2H3]-valine 2 has been completed and (2S,3S)-4-fluorovaline 4 has been
synthesised for the first time. Both compounds have been prepared by routes involving stereoselective addition
to the (S)-pyroglutamate derivative 5 and are available for studies in several areas of bio-organic chemistry.
Introduction
The synthesis of amino acids in which prochiral atoms or
groups are stereospecifically labelled has enabled the mech-
anisms of metabolic reactions to be studied in great detail.1
Further, subtle aspects of protein structure and molecular
recognition may be studied using such compounds. Thus we
have incorporated a sample of (2S,4R)-[5,5,5-2H3]-leucine, 1,
Fig. 1 NOE enhancements on compound 6.
specifically labelled in the pro-R methyl group with deuterium,
into the protein dihydrofolate reductase. This allowed assign-
ment of the diastereotopic methyl groups of all but one of the
thirteen leucine residues in the 1H-NMR spectrum of this
enzyme bound to the cancer drug, methotrexate.2 In order
that studies in these areas may be extended, we now report a
new synthesis of (2S,3R)-[3Ј,3Ј,3Ј-2H3]-valine 2. Samples of
(2RS,3S)-,3 (2R,3S)-,4 (2S,3R)-5 and (2S,3S)-5a,6 [3Ј,3Ј,3Ј-2H3]-
valines have already been synthesised by a variety of methods
but this new and relatively high yielding method should be
useful for incorporation into proteins.
enhancements to H-4 at 1.82 ppm (6.4%), H-3R (28.8%) and
H-6B (0.5%). Irradiation of H-4 at 1.82 ppm caused enhance-
ment to H-3S (2.9%) and H-6A (1.8%). This stereochemical
assignment was confirmed by an X-ray crystal structure of the
deuteriated compound (Fig. 2), the structure of the unlabelled
compound having previously been reported.11
The possibility of using fluorine as a reporter group in
protein folding studies encouraged us to synthesise the epimeric
(2S)-5-fluoroleucines7 and we have incorporated the (2S,4S)-
epimer 3 into all of the leucine residues of dihydrofolate
reductase.7 We have also achieved selective strategic incorpor-
ation of this fluorinated amino acid into the leucine residues
50 and 67 of the protein ubiquitin, which are close neighbours
in the hydrophobic core.8 In order that these studies may
be extended, we now report the stereoselective synthesis of
(2S,3S)-4-fluorovaline 4.
Results and discussion
The starting point for our synthesis of (2S,3R)-[3Ј,3Ј,3Ј-2H3]-
valine
2
was (5S)-N-tert-butoxycarbonyl-1H-5-tert-butyl-
diphenylsiloxymethyl-2(5H )-pyrrol-2-one 59 which has been
shown to react with methylcuprate entirely from the less
hindered face at C-4.10 We therefore reacted this compound
with [2H3]-methylmagnesium iodide and copper bromide–
dimethyl sulfide in tetrahydrofuran, as shown in Scheme 1, to
give
diphenylsilyloxymethyl-4-methylpyrrolidin-2-one
(4S,5S)-[4,4,4-2H3]-N-tert-butoxycarbonyl-5-tert-butyl-
6
in 89%
Fig. 2 X-ray structure of compound 6.
yield. The stereochemistry was defined by the NOE experiments
shown in Fig. 1 since irradiation of H-6A at 3.4 ppm caused
enhancement of H-4 at 1.82 ppm (3.4%). Irradiation of H-6B at
3.88 ppm caused enhancement of H-4 at 1.82 ppm (0.4%) and
to H-3S at 2.63 ppm (0.7%), thus defining the stereochemistry
of the latter peak. Irradiation of H-3S at 2.63 ppm caused
Hydrolysis of the lactam 6 was achieved using aqueous
lithium hydroxide in tetrahydrofuran, giving the acid 7 in 98%
yield.
The valine side chain was now accessed by decarboxylation
of the acid 7 using methodology developed by Barton et al.12
Thus the acid 7 was converted into the mixed anhydride
using iso-butyl chloroformate, and this was reacted with
N-hydroxy-2-thiopyridone followed by addition of tert-butyl-
† Electronic supplementary information (ESI) available: copies of all
1H and 13C spectra, except for those already shown in Fig. 3. See http://
www.rsc.org/suppdata/ob/b4/b401646g/
O r g . B i o m o l . C h e m . , 2 0 0 4 , 2, 1 3 1 0 – 1 3 1 4
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4
1310

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Scheme 1 Reagents and conditions: (i) 2H3CMgI/CuBrؒSMe2/THF/Ϫ78 ЊC, 89%; (ii) aq. LiOH/THF, 98%; (iii) (a) iBuO2CCl/N-methylmorpholine/
THF, (b) N-hydroxythiopyridone/Et3N, (c) tBuSH/hν, 82%; (iv) Bu4NF/THF, 85%; (v) RuCl3/NaIO4/CH3CN/CCl4/H2O, 77%; (vi) 6 M aq. HCl, 99%.
Scheme 2 Reagents and conditions: (i) ref. 14; (ii) (a) iBuO2CCl/N-methylmorpholine/THF, (b) N-hydroxythiopyridone/Et3N, (c) tBuSH/hν, 73%;
(iii) Bu4NF/THF, 86%; (iv) RuCl3/NaIO4/CH3CN/CCl4/H2O, 74%; (v) 6 M aq. HCl, 95%.
thiol and irradiation with visible light. The product 8, which
was obtained in 82% yield, was deprotected using tetrabutyl-
ammonium fluoride to give the alcohol 9 in 85% yield.
Oxidation using ruthenium chloride and sodium periodate gave
the protected labeled valine in 77% yield. Deprotection using
6 M aqueous hydrochloric acid finally gave the target (2S,3R)-
[3Ј,3Ј,3Ј-2H3]-valine 2 as the hydrochloride. Literature values for
(2S,3S)-4-fluorovaline 4, thus widening the possibility of
strategic substitution of hydrophobic amino acids in proteins
by amino acids bearing a fluorine reporter group. This will
expand the use of 19F-NMR spectroscopy in the study of
protein interactions.
Experimental
1
the H-5 and 13C-13 NMR spectra were in keeping with those
found (Fig. 3), the pro-S methyl signal in 2H2O/NaO2H being to
higher field in each case.
Melting points were determined on a Kofler hot-stage
apparatus and are uncorrected. Optical rotation measurements
(in units of 10Ϫ1 deg cm2 gϪ1) were obtained on a Perkin Elmer
PE241 polarimeter using a 1 dm path length micro cell. IR
spectra were recorded on a Perkin Elmer 1720 Fourier Trans-
form spectrometer. 1H NMR spectra were recorded on a Bruker
DPX 300 (300 MHz) Fourier-transform instrument. 13C NMR
spectra were recorded on Bruker DPX 300 (75.5 MHz) and
AMX 500 (125.8 MHz) Fourier-transform instruments. DEPT
experiments were used to assign 13C NMR spectra where
necessary. 19F NMR spectra were recorded on a Bruker DPX
300 (282 MHz) Fourier-transform instrument and were refer-
enced to CFCl3 (0.00 ppm). All 1H- and 13C-NMR spectra were
recorded using TMS, DSS (0.00 ppm) or residual solvent peaks
as internal references. δ are in ppm and J in Hertz (Hz). Low-
resolution mass spectra were recorded on Kratos MS80F and
MS25 double focusing spectrometers. High-resolution mass
Our synthetic route to (2S,3S)-4-fluorovaline 4 has also
made use of stereoselective addition to the conjugated system
of the compound 5. In previous work,14 we converted this
compound into the acid 12 as shown in Scheme 2. Application
of the Barton methodology, used in our synthesis of the
labelled valine above, allowed us to convert this into the
fluoro-compound 13 in 73% yield. Deprotection using tetra-
butylammonium fluoride gave the alcohol 14 in 86% yield
and this was oxidised using ruthenium chloride and sodium
periodate to give the protected fluorovaline 15 in 74% yield.
Deprotection using 6 M hydrochloric acid gave (2S,3S)-
4-fluorovaline 4 in 95% yield.
We have therefore succeeded in completing a new synthesis of
(2S,3R)-[3Ј,3Ј,3Ј-2H3]-valine 2, making it available for a variety
of studies in bio-organic chemistry. We have also prepared
O r g . B i o m o l . C h e m . , 2 0 0 4 , 2, 1 3 1 0 – 1 3 1 4
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was extracted with ethyl acetate. The organic layer was washed
with aqueous ammonium chloride until the aqueous layer was
no longer blue. The organic layer was dried (MgSO4) and the
solvents were removed in vacuo. The residue was chromato-
graphed on silica gel using petroleum ether : ethyl acetate (9 : 1)
as eluent to afford (4S,5S)-[4,4,4-2H3]-N-tert-butoxycarbonyl-
5-tert-butyldiphenylsilyloxymethyl-4-methylpyrrolidin-2-one 6 as
a colourless oil which was crystallised from ethanol/water as a
white solid (4.21 g, 89%), mp 77 ЊC; [α]22 Ϫ15.16 (c 1.0,
D
CHCl3); m/z [FAB, PEG/NBA] Found 493.2580 ([MϩNa]ϩ),
[C27H342H3NO4Si ϩ Na] requires 493.2578; m/z [ϩve FAB,
NBA] 963 ([2MϩNa]ϩ) and 493 ([MϩNa]ϩ); νmax (KBr)/cmϪ1
2
1748 and 1711; δH (300 MHz, C6 H6) 0.98 [9H, s, SiC(CH3)3],
1.33 [9H, s, OC(CH3)3], 1.60 (1H, dd, J3R,3S 17.3, J3R,4 1.8, H-
3R), 1.82 (1H, dd, J4,3S 8.6, J4,3R 1.8, H-4), 2.63 (1H, dd, J3S,3R
17.3, J3S,4 8.6, H-3S), 3.40 (1H, dd, J6A,6B 10.3, J6A,5 2.0, H-6A),
3.53 (1H, m, H-5), 3.88 (1H, dd, J6B,6A 10.4, J6B,5 4.1, H-6B),
7.08–7.15 (6H, m, aromatics) and 7.54–7.59 (4H, m, aromatics);
2
δC (75.5 MHz, C6 H6) 19.4 [SiC(CH3)3], 20.0 (m, C2H3), 27.0
[SiC(CH3)3], 28.2 [OC(CH3)3ϩ C-4], 40.5 (C-3), 64.5 (C-6),
65.7 (C-5), 81.9 [OC(CH3)3], 128.1, 130.1, 133.3, 133.6 and
135.9 (aromatics), 151.3 (urethane) and 172.1 (C-2).
Crystal data – compound 6‡
C27H342H3NO4Si, M = 470.7, monoclinic, space group P21
(No. 4), a = 9.767(5), b = 13.052(4), c = 11.178(4) Å, β =
105.26(3)Њ, V = 1375(1) Å3, Z = 2, Dcalc = 1.14 Mg mϪ3, F(000) =
504, µ(Cu-Kα) = 0.99 mmϪ1, T = 293(2) K, 2272 total
reflections measured, 2145 independent reflections collected
on a Nonius CAD4 diffractometer (Rint = 0.040) using
Cu-Kα radiation (λ = 1.5418 Å). Refinement using SHELXL-
97. Final residues were R1 = 0.048, wR2 = 0.117 (for 1819
reflections with I > 2σ(I )), R1 = 0.060, wR2 = 0.128 for all
reflections.
(3S,4S )-[3Ј,3Ј,3Ј-2H3]-4-tert-Butoxycarbonylamino-5-tert-
butyldiphenylsilyloxy-3-methylpentanoic acid (7)
1 M Aqueous lithium hydroxide (6.38 ml, 6.38 mmol)
was added to a solution of (4S,5S)-[4Ј,4Ј,4Ј-2H3]-N-tert-
butoxycarbonyl-5-tert-butyldiphenylsilyloxymethyl-4-methyl-
pyrrolidin-2-one 6 (1 g, 2.13 mmol) in tetrahydrofuran (10 ml)
at room temperature. The mixture was stirred overnight and
extracted with ethyl acetate and aqueous sodium bicarbonate.
The combined aqueous phases were acidified to pH 4–4.5 with
10% aqueous citric acid and extracted with ethyl acetate. The
combined organic layers were dried (MgSO4) and the solvents
were removed in vacuo. The residue was chromatographed
on silica gel using petroleum ether : ethyl acetate (3 : 1) as eluent
to afford (3S,4S)-[3Ј,3Ј,3Ј-2H3]-4-tert-butoxycarbonylamino-5-
tert-butyldiphenylsilyloxy-3-methylpentanoic acid 7 as a pale yel-
low oil (1.02, 98%) which was used directly in further reactions,
2
1
Fig. 3 NMR spectra in H2O/NaO2H: (a) H; (b) 13C; of (i) (2S,3R)-
[3Ј,3Ј,3Ј-2H3]-valine, 2, and (ii) (2S)-valine.
measurements were recorded by the EPSRC National Mass
Spectrometry Service (Swansea). Solvents were freshly distilled
before use. Column chromatography was performed using
Merck Kieselgel 60 (230–400 mesh) – Art 9385 and Sorbisil C60
40/60A. Petroleum ether refers to that fraction of hexanes of bp
60–80 ЊC.
[α]22 Ϫ12.30 (c 1.0, CHCl3); m/z [ϩve FAB, NBA] 511
D
([MϩNa]ϩ) and 489 ([MϩH]ϩ); νmax (film)/cmϪ1 3325 (NH) and
1713 (acid); δH (300 MHz, C2HCl3) 0.99 [9H, s, SiC(CH3)3], 1.35
[9H, s, OC(CH3)3], 2.07 (1H, dd, J2A,2B 15.1, J2A,3 8.1, H-2A),
2.21 (1H, dd, J3,2A 8.1, J3,2B 4.2, H-3), 2.48 (1H, dd, J2B,2A 15.1,
J2B,3 4.2, H-2B), 3.37 (1H, m, H-5A), 3.62 (2H, m, H-5B ϩ H-
4), 4.79 (1H, d, JNH,4 9.6, NH), 7.14–7.32 (6H, m, aromatics),
7.55–7.62 (4H, m, aromatics) and 9.59 (1H, br s, COOH);
δC (75.5 MHz, C2HCl3) 19.4 (m, C2H3), 19.3 [SiC(CH3)3],
26.9 [SiC(CH3)3], 28.3 [OC(CH3)3], 31.5 (C-3), 38.3 (C-2),
56.1 (C-4), 63.8 (C-5), 79.5 [OC(CH3)3], 127.7, 129.8, 133.0,
133.1 and 135.5 (aromatics), 155.9 (urethane) and 178.8
(acid).
(4S,5S )-[4Ј,4Ј,4Ј-2H3]-N-tert-Butoxycarbonyl-5-tert-butyl-
diphenylsilyloxymethyl-4-methylpyrrolidin-2-one (6)
[2H3]-Methylmagnesium iodide (1 M in diethyl ether, 100 ml,
100 mmol) was added to copper bromide–dimethyl sulfide
complex (10.28 g, 50 mmol) in dry tetrahydrofuran (90 ml) at
Ϫ15 ЊC. After 15 min the mixture was cooled to Ϫ78 ЊC. (5S)-
N-tert-Butoxycarbonyl-5-tert-butyldiphenylsilyloxymethyl-
2(5H )-pyrrol-2-one 59 (4.51 g, 10 mmol) and trimethylsilyl
chloride (1 M in tetrahydrofuran, 20 ml, 20 mmol) in tetra-
hydrofuran (45 ml) were added dropwise over 15 min. After 1 h,
aqueous ammonium chloride (100 ml) was added and the
mixture was allowed to warm to room temperature. The brown
mixture was filtered through a pad of Celite® and the filtrate
‡ CCDC reference number 232480. See http://www.rsc.org/suppdata/
ob/b4/b401646g/ for crystallographic data in.cif or other electronic
format.
O r g . B i o m o l . C h e m . , 2 0 0 4 , 2, 1 3 1 0 – 1 3 1 4
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(2S,3R)-[3Ј,3Ј,3Ј-2H3]-2-tert-Butoxycarbonylamino-O-tert-
butyldiphenylsilyl-3-methylbutan-1-ol (8)
were removed in vacuo to afford a colourless oil (111 mg, 77%).
Chromatography on silica gel using ethyl acetate as eluent gave
(2S,3R)-[3Ј,3Ј,3Ј-2H3]-N-tert-butoxycarbonylvaline 10 as
a
iso-Butyl chloroformate (0.487 ml, 3.76 mmol) was added at
Ϫ15 ЊC with stirring to a solution of (3S,4S)-[3Ј,3Ј,3Ј-2H3]-
4-tert-butoxycarbonylamino-5-tert-butyldiphenylsilyloxy-3-
methylpentanoic acid 7 (1.666 g, 3.41 mmol) in dry tetrahydro-
furan (15 ml) containing N-methylmorpholine (0.413 ml,
3.76 mmol). After 15 min, the flask was protected from light,
triethylamine (0.619 ml, 4.44 mmol) and N-hydroxy-2-thio-
pyridone (493 mg, 3.88 mmol) were added and the mixture was
stirred for 1 h in the dark. tert-Butylthiol (3.85 ml, 34.14 mmol)
was added and the mixture was irradiated with two desk lamps
(100 watt) for 1 h at room temperature. A slight decolourization
was observed. Ethyl acetate was added and the solution was
washed with 0.5 M aqueous hydrochloric acid, brine and water.
The organic layer was dried (MgSO4) and the solvents were
removed in vacuo. The residue was chromatographed on silica
gel using petroleum ether : ethyl acetate (19 : 1) as eluent to
colourless oil, [α]22 Ϫ5.63 (c 1.0, CHCl3); m/z [ϩve FAB
D
(glycerol, H2O)] 221 ([MϩH]ϩ); νmax (film)/cmϪ1 3320 (NH) and
1719 (acid ϩ urethane); δH (300 MHz, C2H3O2H) 0.82 (3H, d,
JMe,3 6.9, CH3), 1.35 [9H, s, OC(CH3)3], 2.00 (1H, m, H-3) and
3.90 (1H, d, J2,3 5.5, H-2); δC (75.5 MHz, C2H3O2H) 18.1 (C-4),
19.0 (m, C2H3), 28.7 [OC(CH3)3], 31.5 (C-3), 60.2 (C-2), 80.4
[OC(CH3)3], 158.2 (urethane) and 175.5 (C-1).
(2S,3R)-[3Ј,3Ј,3Ј-2H3]-Valine hydrochloride (2)
A solution of (2S,3R)-[3Ј,3Ј,3Ј-2H3]-N-tert-butoxycarbonyl-
valine 10 (56 mg, 0.254 mmol) in 6 M aqueous HCl (2.5 ml) was
stirred for 4 days at room temperature. The solvent was
removed in vacuo and the residue was azeotroped with diethyl
ether (3×) to eliminate residual HCl. (2S,3R)-[3Ј,3Ј,3Ј-2H3]-
Valine hydrochloride 2 was precipitated in diethyl ether as white
afford
(2S,3R)-[3Ј,3Ј,3Ј-2H3]-2-tert-butoxycarbonylamino-
O-tert-butyldiphenylsilyl-3-methylbutan-1-ol 8 as a colourless oil
crystals (39 mg, 99%), mp 238–240 ЊC; [α]22 ϩ10.35 (c 0.66,
(1.247 g, 82%), [α]22 Ϫ17.08 (c 0.1, CHCl3); m/z [FAB, PEG/
D
2
H2O); m/z [EIϩ] Found 75.1007 ([M Ϫ CO2H]ϩ), [C5H8 H3NO2
D
NBA] Found 445.2967 ([MϩH]ϩ), [C26H362H3NO3Si ϩ H]
requires 445.2966; m/z [ϩve FAB, NBA] 467 ([MϩNa]ϩ) and
445 ([MϩH]ϩ); νmax (film)/cmϪ1 3448 (NH) and 1702 (urethane);
Ϫ CO2H] requires 75.1002; m/z [ϩve FAB (glycerol, H2O)] 121
([MϩH]ϩ); νmax (KBr)/cmϪ1 3320 (NH) and 1731 (acid); δH (300
MHz, 2H2O, NaO2H) 0.97 (3H, d, JMe,3 6.8, CH3), 2.00 (1H, m,
2
δH (300 MHz, C6 H6) 0.76 (3H, d, JMe,3 6.6, CH3), 1.13 [9H, s,
2
H-3) and 3.16 (1H, d, J2,3 5.2, H-2); δC (125.9 MHz, H2O,
SiC(CH3)3], 1.47 [9H, s, OC(CH3)3], 1.73 (1H, m, H-3), 3.58
(1H, d, J1A,1B 10.0, H-1A), 3.60 (1H, d, J1B,1A 10.0, H-1B), 3.65
(1H, m, H-2), 4.55 (1H, d, JNH,2 8.7, NH), 7.20–7.31 (6H, m,
aromatics) and 7.64–7.79 (4H, m, aromatics); δC (75.5 MHz,
NaO2H) 17.7 (C-4), 19.2 (m, C2H3), 32.2 (C-3), 62.4 (C-2) and
183.9 (acid); δH (300 MHz, C2H3O2H) 0.88 (3H, d, JMe,3 7.0,
CH3), 2.08 (1H, m, H-3) and 3.64 (1H, d, J2,3 4.2, H-2); δC (74.5
MHz, C2H3O2H) 18.1 (C-4), 30.5 (C-3), 59.3 (C-2) and 171.2
(C-1).
C6 H6) 18.4 (C-4), 18.9 (m, C2H3), 19.5 [SiC(CH3)3], 27.1
2
[SiC(CH3)3], 28.6 [OC(CH3)3], 29.2 (C-3), 57.4 (C-2), 64.7 (C-1),
78.4 [OC(CH3)3], 128.1, 130.0, 133.8 and 136.0 (aromatics) and
155.8 (urethane).
(2S,3S )-2-tert-Butoxycarbonylamino-O-tert-butyldiphenylsilyl-
3-fluoromethylbutan-1-ol (13)
(2S,3R)-[3Ј,3Ј,3Ј-2H3]-2-tert-Butoxycarbonylamino-3-methyl-
iso-Butyl chloroformate (0.332 ml, 2.56 mmol) was added
to a solution of (3S,4S)-4-tert-butoxycarbonylamino-5-tert-
butyldiphenylsilyloxy-3-fluoromethylpentanoic acid 1214 (1.169
g, 2.32 mmol) in dry tetrahydrofuran (10 ml) containing
N-methylmorpholine (0.281 ml, 2.56 mmol) at Ϫ15 ЊC with
stirring. After 15 min the flask was protected from light and
triethylamine (0.421 ml, 3.02 mmol) and N-hydroxy-2-thio-
pyridone (336 mg, 2.64 mmol) were added. The mixture was
stirred for 1 h in the dark and tert-butylthiol (2.62 ml,
23.2 mmol) was added. The mixture was irradiated with two
desk lamps (100 watt) for 1 h at room temperature. Slight
decolourization was observed. The mixture was diluted with
ethyl acetate, washed with 0.5 M aqueous hydrochloric acid,
brine and water. The organic layer was dried (MgSO4) and
the solvents were removed in vacuo. The residue was
chromatographed on silica gel using petroleum ether : ethyl
acetate (19 : 1) as eluent to afford (2S,3S)-2-tert-butoxycarb-
onylamino-O-tert-butyldiphenylsilyl-3-fluoromethylbutan-1-ol 13
as a colourless oil (783 mg, 73%), [α]22D Ϫ14.11 (c 1.0, CHCl3);
m/z [FAB, PEG/NBA] Found 460.2672 ([MϩH]ϩ),
[C26H382H3NO3SiF ϩ H] requires 460.2683; m/z [ϩve FAB,
NBA] 460 ([MϩH]ϩ); νmax (film)/cmϪ1 3369 (NH) and 1703
butan-1-ol (9)
Tetrabutylammonium fluoride (1 M in tetrahydrofuran, 10 ml,
10 mmol) was added to a solution of (2S,3R)-[3Ј,3Ј,3Ј-2H3]-2-
tert-butoxycarbonylamino-O-tert-butyldiphenylsilyl-3-methyl-
butan-1-ol 8 (1.12 g, 2.52 mmol) in tetrahydrofuran (40 ml).
The mixture was stirred for 5 days at room temperature when
TLC showed the reaction to be complete. The mixture was
diluted with ethyl acetate and washed with aqueous ammonium
chloride (2×). The organic layer was dried (MgSO4) and the
solvents were removed in vacuo. The residue was purified on
silica gel using petroleum ether : ethyl acetate (3 : 1) as eluent to
afford (2S,3R)-[3Ј,3Ј,3Ј-2H3]-2-tert-butoxycarbonylamino-3-
methylbutan-1-ol 9 as a colourless oil (443 mg, 85%), [α]22
D
Ϫ17.44 (c 1.0, CHCl3); m/z [FAB, PEG/NBA] Found 207.1795
([MϩH]ϩ), [C10H182H3NO3 ϩ H] requires 207.1788; m/z [ϩve
FAB, NBA] 207 ([MϩH]ϩ); νmax (film)/cmϪ1 3356 (NH) and
1686 (urethane); δH (300 MHz, C2H3O2H) 0.78 (3H, d, JMe,3 6.8,
CH3), 1.34 [9H, s, OC(CH3)3], 1.71 (1H, dq, J3,Me = J3,2 = 6.8,
H-3), 3.25 (1H, m, H-1A) and 3.43 (2H, m, H-1B ϩ H-2);
δC (75.5 MHz, C2H3O2H) 18.5 (C-4), 19.2 (m, C2H3), 28.8
[OC(CH3)3], 29.9 (C-3), 58.9 (C-2), 63.3 (C-1), 79.7 [OC(CH3)3]
and 158.5 (urethane).
2
(urethane); δH (300 MHz, C6 H6) 0.66 (3H, d, JMe,3 6.9, CH3),
1.10 [9H, s, SiC(CH3)3], 1.43 [9H, s, OC(CH3)3], 1.96 (1H, m, H-
3), 3.52 (2H, d, J1,2 5.0, H-1), 3.95 (1H, m, H-2), 4.05 (1H, ddd,
J4A,F 47.7, J4A,4B 8.8, J4A,3 6.2, H-4A), 4.18 (1H, ddd, J4B,F
48.7, J4B,4A 8.8, J4B,3 6.0, H-4B), 4.50 (1H, d, JNH,2 9.3, NH),
7.21–7.23 (6H, m, aromatics) and 7.68–7.71 (4H, m, aromatics);
(2S,3R)-[3Ј,3Ј,3Ј-2H3]-N-tert-Butoxycarbonylvaline (10)
(2S,3R)-[3Ј,3Ј,3Ј-2H3]-2-tert-Butoxycarbonylamino-3-methyl-
butan-1-ol 9 (136 mg, 0.66 mmol) was dissolved in a mixture of
acetonitrile (3 ml) and carbon tetrachloride (3 ml). Sodium
periodate (706 mg, 3.3 mmol), water (4.5 ml) and ruthenium
chloride hydrate (14 mg, 0.066 mmol) were added and the
mixture was stirred for 1.5 h at room temperature. Dichloro-
methane was added and the solution was filtered through a pad
of Celite®. The filtrate was dried (MgSO4) and the solvents
2
δC (75.5 MHz, C6 H6) 11.6 (d, 3JCF 6.2, CH3), 19.4 [SiC(CH3)3],
2
27.0 [SiC(CH3)3], 28.5 [OC(CH3)3], 35.9 (d, JCF 17.7, C-3),
3
53.1 (d, JCF 4.4, C-2), 64.4 (C-1), 78.9 [OC(CH3)3], 85.9
1
(d, JCF 169.4, C-4), 128.1, 130.1, 133.6 and 135.9 (aromatics)
2
and 155.7 (urethane); δF (282 MHz, C6 H6) Ϫ219.4 (td, JF, 4
47.6, JF, 3 19.5).
O r g . B i o m o l . C h e m . , 2 0 0 4 , 2, 1 3 1 0 – 1 3 1 4
1313

View Article Online
(2S,3S )-2-tert-Butoxycarbonylamino-3-fluoromethylbutan-1-ol
(14)
95%), mp 145–147 ЊC, [α]22 ϩ6.85 (c 1.2, MeOH); m/z [EIϩ]
D
Found 90.0720 ([M Ϫ CO2H]ϩ), [C5H10NO2F Ϫ CO2H] requires
90.0719; m/z (EI) 90 ([M Ϫ CO2H]ϩ); νmax (KBr)/cmϪ1 3392
(NH), 2972 (OH) and 1736 (acid); δH (300 MHz, C2H3O2H)
1.10 (3H, d, JMe,3 6.4, CH3), 2.65 (1H, m, H-3), 4.16 (1H, s,
H-2), 4.47 (1H, dd, J4A,F 47.0, J4A,4B 6.8, H-4A) and 4.62 (1H,
dd, J4B,F 46.4, J4B,4A 6.7, H-4B); δC (75.5 MHz, [2H4]-MeOH)
11.0 (d, 3JMeF 6.9, CH3), 36.4 (d, 2JCF 18.9, C-3), 57.3 (C-2), 85.6
Tetrabutylammonium fluoride (1 M in tetrahydrofuran, 5.84
ml, 5.84 mmol) was added to a solution of (2S,3S)-O-tert-
butyldiphenylsilyl-2-tert-butoxycarbonylamino-3-fluoromethyl-
butan-1-ol 13 (670 mg, 1.46 mmol) in tetrahydrofuran (20 ml).
The mixture was stirred for 5 days at room temperature until
the reaction was shown to be complete by TLC. Ethyl acetate
was added and the solution was washed with aqueous
ammonium chloride (2×). The organic layer was dried (MgSO4)
and the solvents were removed in vacuo. The residue was
chromatographed on silica gel using petroleum ether : ethyl
acetate (3 : 1) as eluent to afford (2S,3S)-2-tert-butoxycarb-
onylamino-3-fluoromethylbutan-1-ol 14 as a colourless oil (277
mg, 86%), [α]22D Ϫ14.00 (c 1.0, CHCl3); m/z [FAB, PEG/NBA]
Found 222.1499 ([MϩH]ϩ), [C10H20NO3F ϩ H] requires
222.1505; m/z [ϩve FAB, NBA] 244 ([MϩNa]ϩ) and 222
([MϩH]ϩ); νmax (film)/cmϪ1 3347 (NH) and 1694 (urethane);
δH (300 MHz, C2H3O2H) 0.94 (3H, d, JMe,3 6.9, CH3), 1.44 [9H,
s, OC(CH3)3], 2.10 (1H, m, H-3), 3.55 (2H, d, J1,2 5.8, H-1), 3.67
(1H, m, J2,1 = J2,3 = 5.8, H-2), 4.29 (1H, ddd, J4A,F 47.8, J4A,4B
9.1, J4A,3 6.0, H-4A) and 4.37 (1H, ddd, J4B,F 47.5, J4B,4A 9.1,
1
(d, JCF 168.6, C-4) and 171.3 (acid); δF (282 MHz, C2HCl3)
Ϫ224.9 (td, JF,4H 46.7, JF,3H 19.5).
Acknowledgements
We thank the BBSRC and EPSRC (BMS committee) for
funding, Dr A. G. Avent for NMR spectra, Dr A. Abdul-Sada
for mass spectra and the EPSRC Central Mass Spectroscopy
Service, Swansea for high resolution mass spectra.
References
1 D. W. Young, Top. Stereochem., 1994, 21, 381–465.
2 (a) R. A. August, J. A. Khan, C. M. Moody and D. W. Young,
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3 D. J. Aberhart and L. J. Lin, J. Chem. Soc., Perkin Trans. 1, 1974,
2320–2326.
4 J. E. Baldwin, R. M. Adlington, N. P. Crouch, L. C. Mellor,
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7857–7859; (b) N. M. Kelly, R. G. Reid, C. L. Willis and P. L.
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supplementary material; (c) J.-D. Charrier, D. S. Hadfield,
P. B. Hitchcock and D. W. Young, Org. Biomol. Chem., 2004,
DOI: 10.1039/b314933a.
8 D. Alexeev, P. N. Barlow, S. M. Bury, J.-D. Charrier, A. Cooper,
D. S. Hadfield, C. Jamieson, S. M. Kelly, R. Layfield, R. J. Mayer,
H. McSparron, N. C. Price, R. Ramage, L. Sawyer, B. A.
Starkmann, D. Uhrin, J. Wilken and D. W. Young, ChemBioChem,
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3
J4B,3 5.9, H-4B); δC (75.5 MHz, C2H3O2H) 11.6 (d, JCF 5.7,
2
3
CH3), 28.7 [OC(CH3)3], 36.4 (d, JCF 18.1, C-3), 54.1 (d, JCF
5.0, C-2), 63.0 (C-1), 79.9 [OC(CH3)3], 86.7 (d, 1JCF 167.8, C-4)
and 158.2 (urethane); δF (282 MHz, C2HCl3) Ϫ228.8 (td, JF,4H
47.7, JF,3H 21.5).
(2S,3S )-N-tert-Butoxycarbonyl-4-fluorovaline (15)
(2S,3S)-2-tert-Butoxycarbonylamino-3-fluoromethylbutan-1-ol
14 (68 mg, 0.31 mmol) was dissolved in a mixture of acetonitrile
(1.5 ml) and carbon tetrachloride (1.5 ml). Sodium periodate
(329 mg, 1.54 mmol), water (2 ml) and ruthenium chloride
hydrate (7 mg, 0.031 mmol) were added and the mixture was
stirred 1.5 h at room temperature. Dichloromethane was added
and the solution was filtered through a pad of Celite® and
dried (MgSO4). The solvents were removed in vacuo to afford
(2S,3S)-N-tert-butoxycarbonyl-4-fluorovaline 15 as a colourless
oil which crystallized on long standing as a white solid (53 mg,
74%), mp 69–71 ЊC; [α]22 Ϫ11.92 (c 0.36, MeOH); m/z [ϩve
D
FAB] 258 ([MϩNa]ϩ) and 236 ([MϩH]ϩ); νmax (film)/cmϪ1 3331
(NH) and 1719 (urethane/acid); δH (300 MHz, C2H3O2H) 0.90
(3H, dd, JMe,3 7.1, JMe,F 1.0, CH3), 1.43 [9H, s, OC(CH3)3], 2.45
(1H, m, H-3), 4.30 (2H, dt, J4,F 47.4, J4,3 7.8, H-4) and 4.36 (1H,
d, J2,3 3.6, H-2); δC (75.5 MHz, C2H3O2H) 9.9 (d, 3JCF 7.5, CH3),
2
27.7 [OC(CH3)3], 36.5 (d, JCF 18.7, C-3), 54.3 (C-2), 79.7
1
[OC(CH3)3], 84.7 (d, JCF 168.9, C-4), 157.3 (urethane) and
174.0 (acid); δF (282 MHz, C2HCl3) Ϫ227.0 (td, JF,4H 47.3, JF,3H
9 K. Shimamoto, M. Ishida, H. Shinozaki and Y. Ofune, J. Org.
Chem., 1991, 56, 4167.
10 C. Herdeis and H. P. Hubmann, Tetrahedron: Asymmetry, 1992, 3,
1213–1221.
11 C. Herdeis, H. P. Hubmann and H. Lotter, Tetrahedron: Asymmetry,
1994, 5, 351–354.
12 D. H. R. Barton, Y. Hervé, P. Potier and J. Thierry, Tetrahedron,
1988, 44, 5479–5486.
13 H. Kluender, C. H. Bradley, C. J. Sih, P. Fawcett and E. P. Abraham,
J. Am. Chem. Soc., 1973, 95, 6149–6150.
14 J.-D. Charrier, D. S. Hadfield, P. B. Hitchcock and D. W. Young,
Org. Biomol. Chem., 2004, DOI: 10.1039/b316219b.
15.4).
(2S,3S )-4-Fluorovaline hydrochloride (4)
A solution of (2S,3S)-N-tert-butoxycarbonyl-4-fluorovaline 15
(34 mg, 0.145 mmol) in 6 M aqueous HCl (1.5 ml) was stirred
for 4 days at room temperature. The solvent was removed
in vacuo and the residue was azeotroped with diethyl ether (3×)
to eliminate residual HCl. Precipitation using diethyl ether gave
(2S,3S)-4-fluorovaline hydrochloride 4 as white crystals (24 mg,
O r g . B i o m o l . C h e m . , 2 0 0 4 , 2, 1 3 1 0 – 1 3 1 4
1314