Synthesis of (2S,3S)-3′-fluoroisoleucine

Article abstract of DOI:10.1039/b316219b

The fluorinated amino acid, (2S,3S)-3′-fluoroisoleucine, 3, has been synthesised by a route involving stereoselective cuprate or photochemical addition to the compound 4, derived in turn from (2S)-pyroglutamic acid. This provides a reporter group for the hydrophobic amino acid (2S)-isoleucine for use in protein studies.

Full text of DOI:10.1039/b316219b

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Synthesis of (2S,3S )-3Ј-fluoroisoleucine
Jean-Damien Charrier, David S. Hadfield, 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 12th December 2003, Accepted 26th January 2004
First published as an Advance Article on the web 11th February 2004
The fluorinated amino acid, (2S,3S)-3Ј-fluoroisoleucine, 3, has been synthesised by a route involving stereoselective
cuprate or photochemical addition to the compound 4, derived in turn from (2S)-pyroglutamic acid. This provides a
reporter group for the hydrophobic amino acid (2S)-isoleucine for use in protein studies.
fluorinated amino acids in studying a variety of protein
interactions to be expanded. We therefore decided to initiate a
Introduction
Our interest in the use of fluorinated amino acids as reporter
groups for protein studies was stimulated by our work on the
binding of substrates and inhibitors to the anti-cancer drug
target enzyme, dihydrofolate reductase.1 As part of these
studies, we prepared a sample of the enzyme in which all
thirteen leucine residues were substituted by (2S,4S)-5-fluoro-
leucine, 1.2 The biological methods used gave protein in which
every leucine residue was labelled and there was incomplete
incorporation at each position. Use of fluorinated amino acids
as reporter groups in studies of protein interactions would
ideally require incorporation at specific strategically relevant
positions and with complete incorporation at these positions.
We therefore turned to solid-state protein synthesis to achieve
this goal. The small, highly conserved protein, ubiquitin,
important in a number of cell regulatory mechanisms including
cell cycle control, was chosen as our template, and we com-
pleted a synthesis of a ubiquitin “mutant” in which the leucine
residues 50 and 67, close neighbours in the hydrophobic core,
were replaced by (2S,4S)-5-fluoroleucine, 1.3 This allowed us to
complete a variety of studies on the mutant and to show its
promise for the study of protein folding.3 After leucine, the
most common hydrophobic aliphatic amino acid in ubiquitin is
isoleucine 24 and the availability of a sample of this amino acid
containing a fluorine reporter group will enable the use of
synthesis of (2S,3S)-3Ј-fluoroisoleucine, 3, having the stereo-
chemistry found in the natural amino acid.
Results and discussion
The starting point for our synthesis was (5S)-N-tert-butoxy-
carbonyl-1H-5-tert-butyldiphenylsiloxymethylpyrrol-2(5H )-one
4 which had been shown to react with alkyl cuprates at C-4
entirely from the less hindered face.5 We therefore prepared
(5S)-N-tert-butoxycarbonyl-5-tert-butyldiphenylsiloxymethyl-
pyrrol-2(5H )-one 4 from (2S)-pyroglutamic acid by the method
of Shimamoto6 and reacted it with 2-methyl-1-propenyl-
magnesium bromide and copper bromide–dimethyl sulfide
complex to obtain the adduct 5 as a single diastereoisomer in
68% yield as shown in Scheme 1. Ozonolysis to the aldehyde 6
in 84% yield followed by reduction using sodium borohydride in
methanol gave the alcohol 7 in 75% yield. On one occasion
reduction of the aldehyde using sodium borohydride in
methanol gave a product in 45% yield which was evidently not
the alcohol 7 but which had an exchangeable one proton doub-
let at 4.78 ppm characteristic of ring opening of the lactam.
The other spectra were consistent with the lactone structure 8
which would have been formed by intramolecular nucleophilic
reaction of the alcohol 7 with the reactive amide/urethane
Scheme 1 Reagents and conditions (i) Me2C᎐CHMgBr/CuBrؒSMe2/THF/Ϫ78 ЊC, 68%; (ii) (a) O3/CH2Cl2/Ϫ78 ЊC, (b) Zn/HOAc, 84%; (iii) NaBH4/
᎐
MeOH, 75%; (iv) NaBH4/MeOH, 45%; (v) Bu4NF/THF, quant; (vi) hν/Ph2C᎐O/MeOH, 68%; (vii) Et2NSF3/CH2Cl2, 61%.
᎐
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
O r g . B i o m o l . C h e m . , 2 0 0 4 , 2, 7 9 7 – 8 0 2
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Scheme 2 Reagents and conditions (i) ref. 7.
system in the basic conditions of the reaction. This irreproduc-
ibility and the γ-lactone side-product were reminiscent of our
attempts to reduce the aldehyde 9 consistently to the alcohol 10
with NaB(CN)H3 when the lactone 11 was obtained in some
instances (Scheme 2).7 In both reactions, the alcohol group of
the expected product was separated from the electrophilic ring
carbonyl atom by three carbon atoms making γ-lactone form-
ation possible. We confirmed the structure of the oily product 8
by deprotection using tetrabutylammonium fluoride to obtain
the crystalline alcohol 12 in quantitative yield (Scheme 1).
The X-ray crystal structure (Fig. 1) of this compound was as
expected.
the chloride 14 in 87% yield. The stereochemistry of the fluor-
ide 13 was now ascertained using X-ray crystallography (Fig. 2),
thus confirming the stereochemistry both of the cuprate addi-
tion and of the photochemical addition of methanol to the
enone 3.
Fig. 2 X-ray structure of compound 13.
Hydrolysis of the lactam-urethane 13 with aqueous lithium
hydroxide in tetrahydrofuran as shown in Scheme 3 gave the
acid 15 in quantitative yield, and this was reacted with iso-butyl
chloroformate to give the mixed anhydride which was reduced
in situ to the alcohol 16 in 83% yield. Barton, McCombie radi-
cal deoxygenation was now employed to complete the fluoro-
isoleucine side chain. The imidazolethiocarbamate 17 was
prepared in quantitative yield and this was reduced with tri-n-
butyltin hydride and 2,2Ј-azobis(2-methylpropionitrile) to yield
the protected alcohol 18 in 65% yield.
It now remained to convert the protected alcohol to an acid.
This was achieved by first deprotecting the silyl ether 18 in
quantitative yield using tetrabutylammonium fluoride in tetra-
hydrofuran. The resultant alcohol 19 was now oxidised with
Fig. 1 X-ray structure of compound 12.
Because of the problems associated with reduction of the
aldehyde 6, we investigated application of a direct photo-
chemical addition of methanol to the enone 4 discovered by
Mann et al.8 Irradiation of a solution of the dihydropyrrolone 4
in methanol containing benzophenone as photoexitant gave a
68% yield of the alcohol 7, identical in all respects with the
sample prepared by reduction of the aldehyde 6. The alcohol
was now treated with diethylaminosulfur trifluoride in di-
chloromethane which had been passed through a column of
basic alumina prior to use, to give the fluoride 13 in 61% yield.
Failure to use the basic alumina step resulted in production of
Scheme 3 Reagents and conditions (i) 1 M aq LiOH/THF/0 ЊC, quant.; (ii) (a) iBuO2CCl/Et3N/THF Ϫ40 ЊC, (b) NaBH4/THF/H2O, 83%; (iii) thio-
carbonyldiimidazole/CH2Cl2, quant.; (iv) Bu3SnH/AIBN/∆, 65%; (v) Bu4NF/THF, quant; (vi) RuCl3/NaIO4/CH3CN/CCl4/H2O, 84%; (vii) 6 M aq
HCl/40 ЊC, 94%.
O r g . B i o m o l . C h e m . , 2 0 0 4 , 2, 7 9 7 – 8 0 2
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ruthenium tetroxide, generated in situ from ruthenium tri-
chloride and sodium periodate, to give the acid 20 in 84% yield.
This was deprotected without further purification using 6 M
hydrochloric acid to yield (2S,3S)-3Ј-fluoroisoleucine 3 in 94%
yield.
We have therefore prepared the target amino acid (2S,3S)-
3Ј-fluoroisoleucine 3 making it available for incorporation
into strategic positions of proteins. This will allow 19F-NMR
spectroscopy to be used in the study of a variety of protein
interactions.
aromatics) and 7.70–7.72 (4H, m, aromatics); δC (75.5 MHz,
C6 H6) 17.9 (CH3), 19.4 [SiC(CH3)3], 25.4 (CH3), 27.0 [SiC-
(CH3)3], 28.1 [OC(CH3)3], 32.4 (C-4), 39.4 (C-3), 63.9 (C-6),
65.7 (C-5), 82.1 [OC(CH3)3], 127.2 (C-7), 128.1 and 130.1
(aromatics), 133.0 (C-8), 133.4, 133.5 and 136.0 (aromatics),
151.2 (urethane) and 172.1 (lactam).
2
(4S,5S )-N-tert-Butoxycarbonyl-5-tert-butyldiphenylsilyloxy-
methyl-2-oxopyrrolidine-4-carbaldehyde (6)
(4R,5S)-N-tert-Butoxycarbonyl-5-tert-butyldiphenylsilyloxy-
methyl-4-(2-methylprop-1-enyl)pyrrolidin-2-one
5
(4.20 g,
8.3 mmol) was dissolved in dichloromethane (100 ml) under
nitrogen and the solution was cooled to Ϫ78 ЊC. Nitrogen was
displaced by oxygen over 15 min and ozone was passed through
the solution until it became blue. Excess ozone was removed by
passing a stream of nitrogen through the solution. Zinc dust
(5.73 g, 87.6 mmol) and acetic acid (5.02 ml, 87.7 mmol) were
added and the mixture was allowed to warm to room temper-
ature and stirred for a further 1 h. The mixture was filtered
through a pad of Celite® and the solvents were removed
in vacuo. The residue was chromatographed on silica gel using
petroleum ether : ethyl acetate (7 : 3) as eluent to afford
(4S,5S)-N-tert-butoxycarbonyl-5-tert-butyldiphenylsilyloxy-
methyl-2-oxopyrrolidin-4-carbaldehyde 6 as a colourless oil
Experimental
Melting points were determined on a Kofler hot-stage appar-
atus 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-
1
form spectrometer. H-NMR spectra were recorded on Bruker
DPX 300 (300 MHz) and AMX 500 (500 MHz) Fourier-
transform instruments. 13C-NMR spectra were recorded on a
Bruker DPX 300 (75.5 MHz) Fourier-transform instrument.
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
(3.36 g, 84%), [α]22 ϩ1.49 (c 0.97, CHCl3); m/z [ϩve FAB,
D
referenced to CFCl3 (0.00 ppm). All H- and 13C-spectra were
1
NBA] 504 ([M ϩ Na]ϩ) and 404 ([M Ϫ Boc ϩ Na]ϩ); νmax
(KBr)/cmϪ1 1782 (urethane) and 1719 (aldehyde); δH (300 MHz,
recorded using TMS, 3-trimethylsilylpropanesulfonic acid
(DSS) (0.00 ppm) or residual solvent peaks as internal refer-
ences. δ are given in ppm and J in Hertz (Hz). Low-resolution
mass spectra were recorded on Kratos MS80F and MS25
double focusing spectrometers. High-resolution mass meas-
urements were recorded by the EPSRC National Mass Spec-
trometry Service (Swansea). Microanalyses were performed at
Medac Ltd. 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.
2
C6 H6) 1.08 [9H, s, SiC(CH3)3], 1.38 [9H, s, OC(CH3)3], 2.34
(1H, d, J4,3B 9.1, H-4), 2.50 (1H, d, J3A,3B 16.1, H-3A), 2.61 (1H,
dd, J3B,3A 16.1, J3B,4 9.1, H-3B), 3.41 (1H, d, J6A,6B 10.4, H-6A),
4.03 (1H, dd, J6B,6A 10.4, J6B,5 2.2, H-6B), 4.41 (1H, d, J5,6B 2.2,
H-5), 7.16–7.25 (6H, m, aromatics), 7.68–7.71 (4H, m, aro-
2
matics) and 8.86 (1H, s, H-7); δC (75.5 MHz, C6 H6) 19.3
[SiC(CH3)3], 26.9 [SiC(CH3)3], 28.0 [OC(CH3)3], 32.2 (C-3),
45.6 (C-4), 57.7 (C-5), 65.0 (C-6), 82.6 [OC(CH3)3], 128.0, 130.3,
133.0, 133.3 and 135.9 (aromatics), 150.6 (urethane), 170.4
(lactam) and 197.8 (aldehyde).
(4R,5S )-N-tert-Butoxycarbonyl-5-tert-butyldiphenylsilyloxy-
methyl-4-(2-methylprop-1-enyl)pyrrolidin-2-one (5)
(4S,5S )-N-tert-Butoxycarbonyl-5-tert-butyldiphenylsilyloxy-
methyl-4-hydroxymethylpyrrolidin-2-one (7)
2-Methyl-1-propenylmagnesium bromide (0.5 M in tetrahydro-
furan, 44.4 ml, 22.2 mmol) was added to a solution of copper
bromide–dimethyl sulfide complex (2.28 g, 11.1 mmol) in dry
tetrahydrofuran (20 ml) at 15 ЊC. After 15 min the mixture was
cooled to Ϫ78 ЊC. A solution of (5S)-N-tert-butoxycarbonyl-
1H-5-tert-butyldiphenylsiloxymethylpyrrol-2(5H )-one 46 (1 g,
2.22 mmol) and trimethylsilyl chloride in (1 M in tetrahydro-
furan, 4.43 ml, 4.43 mmol) in tetrahydrofuran (10 ml) was
added dropwise over 15 min. After 1 h, aqueous ammonium
chloride (20 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 was extracted with
ethyl acetate. The organic layers were 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 chromatographed on silica
gel using petroleum ether : ethyl acetate (9 : 1) as eluent to
afford (4R,5S)-N-tert-butoxycarbonyl-5-tert-butyldiphenylsilyl-
Method A. A solution of (4S,5S)-N-tert-butoxycarbonyl-5-tert-
butyldiphenylsilyloxymethyl-2-oxopyrrolidine-4-carbaldehyde
6 (3.658 g, 7.60 mmol) in methanol (100 ml) was cooled to
0 ЊC and sodium borohydride (316 mg, 8.36 mmol) was
added in small portions. The solution was stirred for 25 min at
room temperature. The solvent was removed in vacuo, the resi-
due was extracted with ethyl acetate and the extracts were
washed with aqueous ammonium chloride. 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 (6 : 4) as eluent to afford (4S,5S)-N-tert-
butoxycarbonyl-5-tert-butyldiphenylsilyloxymethyl-4-hydroxy-
methylpyrrolidin-2-one 7 as a colourless oil which crystallised
as a white solid on standing (2.77 g, 75%), mp 132–134 ЊC;
[α]22 Ϫ21.3 (c 1.0, CHCl3); m/z [FAB, PEG/NBA] Found:
D
506.2331 ([M
ϩ ϩ Na] requires:
Na]ϩ), [C27H37NO5Si
506.2339); m/z [ϩve FAB, NBA] 506 ([M ϩ Na]ϩ) and 384
([M Ϫ Boc ϩ H]ϩ); νmax (KBr)/cmϪ1 3472 (OH), 1775 and 1736
2
oxymethyl-4-(2-methylprop-1-enyl)pyrrolidin-2-one
5
as
a
(urethane) and 1697; δH (300 MHz, C6 H6) 1.11 [9H, s,
colourless oil (757 mg, 68%), [α]22 Ϫ42.76 (c 0.72, CHCl3);
SiC(CH3)3], 1.42 [9H, s, OC(CH3)3], 1.78 (1H, br s, OH),
2.12 (1H, dd, J3A,3B 17.8, J3A,4 1.7, H-3A), 2.20 (1H, m, H-4),
2.82 (1H, dd, J3B,3A 17.6, J3B,4 9.2, H-3B), 3.07 (2H, d, J7,4 6.5,
H-7), 3.59 (1H, dd, J6A,6B 11.3, J6A,5 3.2, H-6A), 4.11 (2H, m,
H-6B ϩ H-5), 7.16–7.29 (6H, m, aromatics) and 7.67–7.72 (4H,
D
m/z [FAB, PEG/NBA] Found: 530.2720 ([M
ϩ
Na]ϩ),
[C30H41NO4Si ϩ Na] requires: 530.2703; m/z [ϩve FAB, NBA]
530 ([M ϩ Na]ϩ); νmax (KBr)/cmϪ1 1782 and 1752 (urethane)
2
and 1719; δH (300 MHz, C6 H6) 1.13 [9H, s, SiC(CH3)3], 1.36
2
(3H, s, CH3), 1.43 (3H, s, CH3), 1.45 [9H, s, OC(CH3)3], 2.00
(1H, dd, J3A,3B 17.2, J3A,4 3.1, H-3A), 2.78 (1H, dd, J3B,3A 17.2,
J3B,4 8.8, H-3B), 3.03 (1H, m, H-4), 3.69 (1H, dd, J6A,6B 10.3,
J6A,5 2.1, H-6A), 3.93 (1H, m, H-5), 4.06 (1H, dd, J6B,6A 10.3,
J6B,5 4.7, H-6B), 4.92 (1H, d, J7,4 9.4, H-7), 7.16–7.26 (6H, m,
m, aromatics); δC (75.5 MHz, C6 H6) 19.4 [SiC(CH3)3], 27.0
[SiC(CH3)3], 28.1 [OC(CH3)3], 36.0 (C-3), 36.2 (C-4), 61.6
(C-5), 64.7 (C-6), 65.4 (C-7), 82.2 [OC(CH3)3], 128.2, 130.1,
133.3, 133.6 and 135.9 (aromatics), 151.0 (urethane) and 173.4
(lactam).
O r g . B i o m o l . C h e m . , 2 0 0 4 , 2, 7 9 7 – 8 0 2
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Method B. (5S)-N-tert-Butoxycarbonyl-1H-5-tert-butyl-
diphenylsilyloxymethylpyrrol-2(5H )-one 4 (2.50 g, 5.54 mmol)
and benzophenone (1.01 g, 5.52 mmol) were dissolved in meth-
anol (2 l) in a Pyrex vessel. The solution was degassed by pass-
ing nitrogen through it for 1.5 h and irradiated for 24 h, at room
temperature, using a medium pressure immersion mercury
vapour lamp fitted with a Pyrex filter. The solvents were
removed in vacuo to yield a clear yellow oil. Flash column
chromatography on silica gel using petroleum ether : ethyl acet-
ate (6 : 4) as eluent gave (4S,5S)-N-tert-butoxycarbonyl-5-tert-
butyldiphenylsilyloxymethyl-4-hydroxymethylpyrrolidin-2-one 7
as a clear colourless oil which crystallised on standing (1.82 g,
68%); mp 133–134 ЊC; [α]D24 Ϫ22.0 (c 1.0, CHCl3) with identical
spectra to those for the sample prepared by method A above.
2.49 (1H, dd, J3B,3A 17.7, J3B,4 9.8, H-3B), 2.30 (1H, br s, OH)
and 1.45 (9H, s, C(CH3)3); δC (75.5 MHz, C2HCl3) 176.8 (lac-
tone), 156.3 (urethane), 80.4 (OC(CH3)3), 70.5 (C-5), 63.6 (C-7),
53.1 (C-6), 38.0 (C-4), 31.3 (C-3) and 28.3 (C(CH3)3).
Crystal data: compound 12†
C11H19NO5, M = 245.27, orthorhombic, space group P212121
(No. 19), a = 6.4668(5), b = 10.4584(4), c = 19.4553(12) Å,
V = 1315.8(1) Å3, Z = 4, Dcalc = 1.24 Mg mϪ3, F(000) = 528,
µ(Mo-Kα) = 0.10 mmϪ1, T = 173(2) K, 6633 total reflections
measured, 2280 independent reflections collected on a Nonius
Kappa CCD diffractometer (Rint = 0.060) using Mo-Kα radi-
ation (λ = 0.71073 Å). Refinement using SHELXL-97. Final
residues were R1 = 0.040, wR2 = 0.102 (for 2115 reflections with
I > 2σ(I )), R1 = 0.044, wR2 = 0.105 for all reflections.
(1ЈS,4S )-4-(1-N-tert-Butoxycarbonylamino-2-tert-butyl-
diphenylsilyloxyethyl)-dihydrofuran-2(3H)-one (8)
(4S,5S )-N-tert-Butoxycarbonyl-5-tert-butyldiphenylsilyloxy-
methyl-4-fluoromethylpyrrolidin-2-one (13)
Sodium borohydride (790 mg, 20.83 mmol) was added
to a solution of (4S,5S)-N-tert-butoxycarbonyl-5-tert-butyl-
Diethylaminosulfur trifluoride (1.09 ml, 8.28 mmol) was added
to a solution of (4S,5S)-N-tert-butoxycarbonyl-5-tert-butyl-
diphenylsilyloxymethyl-2-oxopyrrolidine-4-carbaldehyde
6
(5.01 g, 10.42 mmol) in methanol (100 ml) cooled to 0 ЊC.
The solution was stirred for 2 h at room temperature. The
solvent was removed in vacuo and the residue was dissolved
in ethyl acetate (100 ml) and washed with saturated aqueous
ammonium chloride (2 × 25 ml). The organic layer was dried
(MgSO4) and the solvents were removed in vacuo. The product
was purified by flash column chromatography on silica gel
using petroleum ether : ethyl acetate (3 : 1) as eluent to
afford (1ЈS,4S)-4-(1-N-tert-butoxycarbonylamino-2-tert-butyl-
diphenylsilyloxyethyl)-dihydrofuran-2(3H)-one 8 as a clear
colourless oil which crystallised on standing (2.26 g, 45%); mp
70–72 ЊC; [α]D27 ϩ14.1 (c 1.0, CHCl3); (Found: C, 67.05; H, 7.8;
N, 2.8. C27H37NO5Si requires: C, 67.1; H, 7.7; N, 2.9%); m/z
[ϩve FAB (3-NBA)] 506 [M ϩ Na]ϩ and 484 [M ϩ H]ϩ; νmax
(KBr) /cmϪ1 3520 (NH), 1779 (lactone) and 1675 (urethane);
δH (500 MHz, C2HCl3) 7.63–7.6 (4H, m, aromatics), 7.48–7.37
(6H, m, aromatics), 4.78 (1H, d, JNH,6 9.1, NH), 4.20 (1H, dd,
J5A,5B 8.6, J5A,4 8.5, H-5A), 3.94 (1H, dd, J5B,5A 8.6, J5B,4 8.5,
H-5B), 3.77 (1H, m, H-6), 3.67 (1H, dd, J7A,7B 10.6, J7A,5 4.3,
H-7A), 3.56 (1H, dd, J7B,7A 10.6, J7B,5 3.5, H-7B), 2.84 (1H, m,
H-4), 2.52 (1H, dd, J3A,3B 17.7, J3A,4 8.7, H-3A), 2.47 (1H, dd,
J3A,3B 17.7, J3B,4 9.8, H-3B), 1.44 (9H, s, C(CH3)3) and 1.07 (9H,
s, OSiC(CH3)3).
diphenylsilyloxymethyl-4-hydroxymethylpyrrolidin-2-one
7
(1.333 g, 2.76 mmol) in dichloromethane (35 ml) and the solu-
tion was stirred for 2 h at room temperature. Saturated aqueous
sodium bicarbonate was added and the mixture was extracted
with dichloromethane. The organic extracts were washed with
saturated aqueous sodium bicarbonate and dried (MgSO4). The
solvents were removed in vacuo at room temperature. The resi-
due was chromatographed on silica gel using petroleum ether :
ethyl acetate (4 : 1) as eluent to afford (4S,5S)-N-tert-butoxy-
carbonyl-5-tert-butyldiphenylsilyloxymethyl-4-fluoromethyl-
pyrrolidin-2-one 13 as a colourless oil which crystallised as a
white solid on standing (817 mg, 61%), mp 76–78 ЊC; [α]22
D
Ϫ32.77 (c 1.0, CHCl3); (Found : C, 66.5; H, 7.3; N, 2.8.
C27H36NO4FSi requires: C, 66.8; H, 7.5; N, 2.9%); m/z [ϩve
FAB, NBA] 508 ([M ϩ Na]ϩ); νmax (KBr)/cmϪ1 1754 (urethane)
2
and 1707; δH (300 MHz, C6 H6) 1.09 [9H, s, SiC(CH3)3], 1.43
[9H, s, OC(CH3)3], 1.87 (1H, dd, J3A,3B 17.8, J3A,4 1.8, H-3A),
2.21 (1H, m, H-4), 2.69 (1H, dd, J3B,3A 17.8, J3B,4 9.3, H-3B),
3.49 (1H, dd, J6A,6B 10.3, J6A,5 1.3, H-6A), 3.62 (2H, dd, J7,F
47.0, J7,4 6.8, H-7), 4.02 (1H, m, H-5), 4.06 (1H, dd, J6B,6A 10.3,
J6B,5 3.3, H-6B), 7.16–7.29 (6H, m, aromatics) and 7.64–7.69
2
(4H, m, aromatics); δC (75.5 MHz, C6 H6) 19.3 [SiC(CH3)3],
2
27.0 [SiC(CH3)3], 28.1 [OC(CH3)3], 34.3 (d, JCF 26.5, C-4),
34.4 (C-3), 60.4 (d, 3J5,F 5.1, C-5), 65.0 (C-6), 82.3 [OC(CH3)3],
1
(1ЈS,4S )-4-(1-N-tert-Butoxycarbonylamino-2-hydroxyethyl)-
dihydrofuran-2(3H)-one (12)
84.2 (d, J7,F 172.3, C-7), 128.2, 130.2, 133.1, 133.4 and 135.9
(aromatics), 150.9 (urethane) and 171.2 (lactam); δF (282 MHz,
2
C6 H6) Ϫ221.6 (td, JF,H7 47.0, JF,H4 17.0).
Tetrabutylammonium fluoride (1.0 M in tetrahydrofuran, 0.10
ml, 0.10 mmol) was added to a solution of (1ЈS,4S)-4-(1-(N-
tert-butoxycarbonylamino-2-tert-butyldiphenylsilyloxyethyl)-
dihydrofuran-2(3H )-one 8 (50 mg, 0.10 mmol) in tetrahydro-
furan (1 ml) at room temperature. The reaction was stirred at
room temperature under nitrogen for 16 h. Saturated aqueous
ammonium chloride (1 ml) was carefully added and the crude
product was extracted into ethyl acetate (3 × 5 ml). The organic
extracts were washed with water (2 ml) and brine (2 ml). The
organic layer was dried (MgSO4) and the solvents were removed
in vacuo. The crude product was purified by flash column
chromatography on silica gel using petroleum ether : ethyl acet-
ate (1 : 1) as eluent, followed by recrystallisation from petrol-
eum ether to give (1ЈS,4S)-4-(1-N-tert-butoxycarbonylamino-2-
hydroxyethyl)-dihydrofuran-2(3H)-one 12 as a white crystalline
Crystal data: compound 13†
C27H36FNO4Si, M = 485.66, monoclinic, space group P21
(No. 4), a = 9.811(7), b = 13.067(6), c = 11.121(5) Å, β =
104.99(4)Њ, V = 1377(1) Å3, Z = 2, Dcalc = 1.17 Mg mϪ3, F(000) =
520, µ(Cu-Kα) = 1.06 mmϪ1, T = 293(2) K, 2277 total reflec-
tions measured, 2152 independent reflections collected on a
Nonius CAD4 diffractometer (Rint = 0.024) using Cu-Kα radi-
ation (λ = 1.5418 Å). Refinement using SHELXL-93. Final
residues were R1 = 0.064, wR2 = 0.163 (for 1848 reflections with
I > 2σ(I )), R1 = 0.074, wR2 = 0.175 for all reflections.
(4S,5S )-N-tert-Butoxycarbonyl-5-tert-butyldiphenylsiloxy-
methyl-4-chloromethylpyrrolidin-2-one (14)
22
solid (25 mg, quantitative), mp 76–77 ЊC, [α]D ϩ9.1 (c 1.0,
(4S,5S)-N-tert-Butoxycarbonyl-5-tert-butyldiphenylsilyloxy-
methyl-4-hydroxymethylpyrrolidin-2-one 7 (5.01 g, 10.4 mmol)
was dissolved in dichloromethane (75 ml) under nitrogen and
cooled to 0 ЊC. Diethylaminosulfur trifluoride (1.53 ml, 11.5
CHCl3); (Found: C, 53.6; H, 7.9; N, 5.7. C11H19NO5 requires:
C, 53.9; H, 7.8; N, 5.7%); m/z [EI]ϩ 246 [M ϩ H]ϩ and 146
[M Ϫ Boc]ϩ; νmax (KBr) /cmϪ1 3346 (OH), 1768 (lactone) and
1690 (urethane); δH (300 MHz, C2HCl3) 5.02 (1H, d, JNH,6 8.0,
NH), 4.44 (1H, dd, J5A,5B 9.0, J5A,4 8.5, H-5A), 4.11 (1H, dd,
J5B,5A 9.0, J5B,4 8.4, H-5B), 3.72 (1H, m, H-6), 3.65 (2H, m, H-7),
2.89 (1H, m, H-4), 2.59 (1H, dd, J3A,3B 17.7, J3A,4 8.7, H-3A),
† CCDC reference numbers 227306 (12) and 227307 (13). See http://
www.rsc.org/suppdata/ob/b3/b316219b/ 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, 7 9 7 – 8 0 2
800

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mmol) was added to the mixture which was stirred at room
temperature for 12 h. Methanol was added and the crude prod-
uct was extracted into ethyl acetate (3 × 50 ml). The organic
layers were washed with brine (25 ml) and water (25 ml) and
dried (MgSO4). The solvents were removed in vacuo to give a
clear colourless oil. Column chromatography on silica gel using
petroleum ether : ethyl acetate (3 : 1), followed by recrystallis-
ation from petroleum ether gave (4S,5S)-N-tert-butoxycarb-
onyl-5-tert-butyldiphenylsilyloxymethyl-4-chloromethylpyrrol-
idin-2-one 14 as a white crystalline solid (4.52 g, 87%), mp 76–
77 ЊC, [α]D27 ϩ30.8 (c 1.0, acetone); (Found: C, 64.5; H, 7.3; N,
2.8. C27H36NO4SiCl requires: C, 64.6; H, 7.2; N, 2.8%); m/z
[ϩve FAB (3-NBA)] 526 and 524 (1 : 3) [M ϩ Na]ϩ and 504 and
502 (1 : 3) [M ϩ H]ϩ; νmax (KBr)/cmϪ1 3482 (NH) and 1751
(urethane); δH (300 MHz, C2HCl3) 7.68–7.45 (4H, m, aro-
matics), 7.40–7.22 (6H, m, aromatics), 4.01 (1H, m, H-5), 3.86
(1H, dd, J6A,6B 10.6, J6A,5 3.6, H-6A), 3.65 (1H, dd, J6B,6A 10.6,
J6B,5 1.6, H-6B), 3.46 (2H, d, J7,4 7.1, H-7), 2.96 (1H, dd, J3A,3B
18.0, J3A,4 9.3, H-3A), 2.63 (1H, m, H-4), 2.27 (1H, m, J3B,3A
18.0, H-3B), 1.35 (9H, s, C(CH3)3) and 1.00 (9H, s, OSiC-
(CH3)3); δC (75.5 MHz, C2HCl3) 173.0 (lactam), 150.0
(urethane), 135.9, 133.2, 132.7, 130.4 and 128.3 (aromatics),
83.6 (OC(CH3)3), 65.1 (C-7), 62.6 (C-5), 47.7 (C-6), 37.3 (C-3),
36.5 (C-4), 28.4 (C(CH3)3), 27.2 (OSiC(CH3)3) and 19.6
(OSiC(CH3)3).
and water (15 ml) was added. The reaction was stirred for a
further 1 h at 0 ЊC, quenched by careful addition of saturated
aqueous ammonium chloride (50 ml) and allowed to warm to
room temperature. The crude product was extracted into ethyl
acetate (3 × 50 ml) and washed with water (50 ml) and brine
(50 ml). The organic layer was dried (MgSO4) and the solvents
were removed in vacuo to give a clear pink oil. Purification by
column chromatography on silica gel using petroleum ether :
ethyl acetate (3 : 1) as eluent afforded (2S,3S)-2-N-tert-butoxy-
carbonylamino-1-tert-butyldiphenylsilyloxy-3-fluoromethyl-5-
25
hydroxypentane 16 as a clear colourless oil (2.99 g, 83%); [α]D
Ϫ0.1 (c 1.0, CHCl3); m/z (ESϩ) Found: 490.2799 ([M ϩ H]ϩ),
[C27H40NO4SiF ϩ H] requires: 490.2789; m/z [ϩve FAB
(3-NBA)] 490 [M ϩ H]ϩ and 390 [M Ϫ Boc]ϩ; νmax (film) /cmϪ1
3428 (OH) and 1694 (urethane); δH (500 MHz, C2HCl3) 7.66–
7.63 (4H, m, aromatics), 7.46–7.40 (6H, m, aromatics), 4.89
(1H, d, JNH,2 8.2, NH), 4.54 (1H, ddd, J6A,F 47.0, J6A,6B 9.7,
J6A,3 5.3, H-6A), 4.44 (1H, ddd, J6B,F 47.0, J6B,6A 9.7, J6B,3 5.3,
H-6B), 3.83 (1H, m, H-2), 3.74 (4H, m, H-5 and H-1), 2.20 (1H,
m, H-3), 1.99 (1H, br s, OH), 1.63 (2H, m, H-4), 1.44 (9H, s,
C(CH3)3) and 1.08 (9H, s, OSiC(CH3)3); δC (75.5 MHz, C2HCl3)
159.0 (urethane), 136.0, 133.4, 130.3 and 128.2 (aromatics),
83.0–81.5 (d, 1J6,F 164.4, C-6), 80.0 (OC(CH3)3), 64.6 (C-5), 61.0
2
(C-1), 55.0 (C-2), 35.8 (d, J3,F 19.0, C-3), 30.3 (C-4), 28.8
(C(CH3)3), 27.3 (C(CH3)3) and 19.6 (OSiC(CH3)3); δF (282
MHz) Ϫ225.15 (td, JF,H6 47.0, JF,H3 26.0).
(3S,4S )-4-tert-Butoxycarbonylamino-5-tert-butyldiphenyl-
silyloxy-3-fluoromethylpentanoic acid (15)
(3S,4S )-O-(4-tert-Butoxycarbonylamino-5-tert-butyldiphenyl-
silyloxy-3-fluoromethylpentyl) 1H-imidazole-1-carbothioate (17)
(4S,5S)-N-tert-Butoxycarbonyl-5-tert-butyldiphenylsilyloxy-
methyl-4-fluroromethylpyrrolidin-2-one 13 (97 mg, 0.20 mmol)
was dissolved in tetrahydrofuran (2 ml) and cooled to 0 ЊC. 1 M
Aqueous lithium hydroxide (0.24 ml, 0.24 mmol) was added
dropwise to the solution. The reaction was stirred for 1 h at 0 ЊC
and a further 2 h at room temperature. Ethyl acetate (5 ml) and
saturated aqueous sodium hydrogen carbonate (5 ml) were
added and the reaction was stirred for 1 h. The organic layer
was separated and the aqueous layer was further extracted with
ethyl acetate (3 × 5 ml). The aqueous layer was cooled to 0 ЊC
and acidified with 1 M hydrochloric acid. The acidified aqueous
layer was extracted with ethyl acetate (5 × 5 ml). The organic
layers were combined, washed with water (10 ml) and brine (10
ml) and dried (MgSO4). The residual colourless oil, (3S,4S)-4-
tert-butoxycarbonylamino-5-tert-butyldiphenylsilyloxy-3-fluoro-
methylpentanoic acid 15 was used for further reactions without
Thiocarbonyldiimidazole (1.20 g, 6.73 mmol) was added
to a solution of (2S,3S)-2-N-tert-butoxycarbonylamino-1-tert-
butyldiphenylsilyloxy-3-fluoromethyl-5-hydroxypentane
16
(2.99 g, 6.11 mmol) in dichloromethane (50 ml). The mixture
was stirred under nitrogen at room temperature for 12 h. The
solvents were removed in vacuo to give a clear orange oil. The
crude products were purified by flash column chromatography
on silica gel using petroleum ether : ethyl acetate (3 : 1)
as eluent. (3S,4S)-O-(4-tert-Butoxycarbonylamino-5-tert-butyl-
diphenylsilyloxy-3-fluoromethylpentyl)-1H-imidazole–1-carbo-
thioate 17 was obtained as a clear colourless oil (3.66 g,
quantitative); m/z (ESϩ) Found: 600.2731 ([M ϩ H]ϩ), [C31-
H42N3O4SiFS ϩ H] requires: 600.2727; m/z [ϩve FAB (3-NBA)]
622 [M ϩ Na]ϩ and 600 [M ϩ H]ϩ; νmax (film) /cmϪ1 3405 (NH)
and 1703 (C᎐O); δ (300 MHz, C2HCl3) 8.26 (1H, br s, imid-
᎐
H
23
further purification (100 mg quantitative), [α]D ϩ19.0 (c 1.0,
azole), 7.60–7.50 (4H, m, aromatics), 7.40–7.29 (6H, m, aro-
matics), 6.95 (1H, br s, imidazole), 4.77 (1H, d, JNH,5 8.5, NH),
4.64 (2H, t, J1,2 6.8, H-1), 4.55–4.25 (2H, dm, J6,F 47.4, H-6),
3.77–3.67 (1H, m, H-4), 3.66–3.59 (2H, m, H-5), 2.20–2.00 (1H,
m, H-3), 1.36 (9H, s, C(CH3)3) and 1.00 (9H, s, OSiC(CH3)3);
δC (75.5 MHz, C2HCl ) 184.4 (C᎐S), 156.0 (urethane), 135.9
MeOH); m/z [ϩve FAB (3-NBA)] 526 [M ϩ Na]ϩ, 504 [M ϩ
H]ϩ and 404 [M Ϫ Boc]ϩ; νmax (film) /cmϪ1 3325 (NH and OH)
and 1712 (C᎐O); δ (300 MHz, C2H3O2H) 7.70–7.65 (4H, m,
᎐
H
aromatics), 7.46–7.36 (6H, m, aromatics), 4.95 (1H, br s, NH),
4.42 (2H, dd, JH,F 47.4, J6,3 4.6, H-6), 3.83 (1H, m, H-4), 3.68
(2H, m, H-5), 2.54 (1H, m, H-3), 2.44 (1H, m, H-2A), 2.29 (1H,
dd, J2B,2A 11.6, J2B,3 8.9, H-2B), 1.44 (9H, s, C(CH3)3) and 1.05
(9H, s, OSiC(CH3)3); δC (75.5 MHz, C2HCl3) 176.1 (acid), 158.5
(urethane), 137.2, 134.6, 131.5 and 129.4 (aromatics), 86.2–84.0
᎐
3
and 133.1 (aromatics), 131.2 (imidazole), 130.4 and 128.3 (aro-
1
matics), 118.2 (imidazole), 84.1 (d, J6,F 166.2, C-6), 80.1
(OC(CH3)3), 72.2 (C-1), 64.0 (C-5), 52.9 (C-4), 38.8 (d, 2J3,F 18,
C-3), 28.8 (C(CH3)3), 27.3 (OSiC(CH3)3), 26.7 (C-2) and 19.7
(OSiC(CH3)3); δF (282 MHz) Ϫ225.22 (td, JF,H6 47.0, JF,H3 26.0).
1
(d, J6,F 159.8, C-6), 80.7 (OC(CH3)3), 65.6 (C-5), 54.5 (C-4),
39.2 (C-3), 33.4 (C-2), 29.4 (C(CH3)3), 27.9 (OSiC(CH3)3)
and 20.6 (OSiC(CH3)3); δF (282 MHz, C2HCl3) Ϫ228.2 (dt,
JF,H6 47.5, JF,H3 25.0).
(2S,3S )-1-(O-tert-Butyldiphenylsilyl)-2-N-tert-butoxycarbonyl-
amino-3-fluoromethylpentan-1-ol (18)
(3S,4S)-O-(4-tert-Butoxycarbonylamino-5-tert-butyldiphenyl-
silyloxy-3-fluoromethylpentyl) 1H-imidazole–1-carbothioate 17
(200 mg, 0.33 mmol) was dissolved in toluene (5 ml) in a two-
necked round bottomed flask. 2,2Ј-Azobis(2-methylpropio-
nitrile) (1 mg, 0.007 mmol) was added to the solution and the
mixture was heated to reflux under nitrogen. Tri-n-butyltin
hydride (1.0 M in toluene, 0.11 ml, 0.43 mmol) was added to the
refluxing mixture. The reaction was heated at reflux for a fur-
ther 2 h under nitrogen. The solvents were removed in vacuo to
give a pale yellow oil. Purification of the product by flash
column chromatography on silica gel using petroleum ether :
(2S,3S )-2-N-tert-Butoxycarbonylamino-1-tert-butyldiphenyl-
silyloxy-3-fluoromethyl-5-hydroxypentane (16)
(3S,4S)-4-tert-Butoxycarbonylamino-5-tert-butyldiphenyl-
silyloxy-3-fluoromethylpentanoic acid 15 (3.70 g, 7.37 mmol)
was dissolved in tetrahydrofuran (40 ml) under nitrogen and the
solution was cooled to Ϫ40 ЊC. Triethylamine (1.23 ml, 8.84
mmol) and iso-butyl chloroformate (1.11 ml, 8.48 mmol) were
added and the reaction was stirred at Ϫ 40 ЊC for 1 h. The
reaction was allowed to warm to 0 ЊC. Sodium borohydride
(840 mg, 22.1 mmol) in a mixture of tetrahydrofuran (60 ml)
O r g . B i o m o l . C h e m . , 2 0 0 4 , 2, 7 9 7 – 8 0 2
801

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ethyl acetate (3 : 1) as eluent gave (2S,3S)-1-(O-tert-butyl-
diphenylsilyl)-2-N-tert-butoxycarbonylamino-3-fluoromethyl-
pentan-1-ol 18 as a clear colourless oil (100 mg, 65%); [α]D
and the mixture was cooled to 0 ЊC. Sodium periodate (75 mg,
0.35 mmol) and ruthenium chloride hydrate (5 mg, 0.024 mmol)
were added sequentially and the mixture was stirred at room
temperature for 2 h. Diethyl ether (5 ml) was added and the
mixture was stirred for 30 min. The mixture was dried (MgSO4)
and filtered through a pad of Celite®. The solid residue was
washed with diethyl ether (3 × 5 ml). The organic layers were
combined and the solvents were removed in vacuo to give
(2S,3S)-2-N-tert-butoxycarbonylamino-3Ј-fluoroisoleucine 20 as
a clear colourless oil (18 mg, 84%) which was suspended in 6 M
aqueous hydrochloric acid (2 ml). The mixture was heated to
40 ЊC for 2 days. The solvents were removed in vacuo to afford a
white powder. Recrystallisation from methanol gave (2S,3S)-
3Ј-fluoroisoleucine 3 (10 mg, 94%; overall 82%); mp 163–165 ЊC;
27
Ϫ11.6 (c 1.0, CHCl3); m/z [ϩve FAB (3-NBA)] 496 [M ϩ
Na]ϩand 474 [M ϩ H]ϩ; νmax (film) /cmϪ1 3437 (NH) and 1703
(urethane); δH (300 MHz, C2HCl3) 7.55 (4H, m, aromatics),
7.40–7.25 (6H, m, aromatics), 4.73 (1H, d, JNH,2 8.0, NH), 4.60
(2H, dm, J6,F 48.0, H-6), 3.73 (1H, m, H-2), 3.61 (2H, m, H-1),
1.80 (1H, m, H-3), 1.39 (9H, s, C(CH3)3), 1.00 (9H, s, OSiC-
(CH3)3), 0.85 (3H, t, J5,4 7.4, H-5) and 0.75 (2H, m, H-4);
δC (75.5 MHz, C2HCl3) 156.0 (urethane), 136.0, 133.5, 130.2
1
and 128.2 (aromatics), 84.1 (d, J6,F 164.0, C-6), 79.6 (OC-
2
(CH3)3), 64.1 (C-1), 53.0 (C-2), 43.0 (d, J3,F 19.3, C-3), 29.8
(C-4), 28.8 (C(CH3)3), 27.3 (OSiC(CH3)3), 19.7 (OSiC(CH3)3)
and 12.3 (C-5); δF (282 MHz, C2HCl3) Ϫ226.7 (dt, JF,H6 47.5,
JF,H3 26.0).
22
[α]D Ϫ3.0 (c 0.1, MeOH); m/z [FAB, CsI/glycerol] Found:
150.0926 ([M ϩ H]ϩ), [C6H11NO2F ϩ H] requires: 150.0930;
m/z [ϩve FAB (3-NBA)] 150 [M ϩ H]ϩ; νmax (KBr) /cmϪ1 3415
(NH and OH) and 1638 (acid); δH (500 MHz, 2H2O/2HCl
(20%)) 5.10–4.83 (2H, br m, J3Ј,F 48, H-3Ј), 4.66 (1H, m, H-2),
2.75 (1H, m, H-3), 1.79 (2H, m, H-4) and 1.24 (3H, m, H-5);
δC (75.5 MHz, C2H3O2H)) 170.0 (acid), 84.8 (d, 1J3Ј,F 164, C-3Ј),
(2S,3S )-2-N-tert-Butoxycarbonylamino-3-fluoromethylpentan-
1-ol (19)
Tetrabutylammonium fluoride (1 M in tetrahydrofuran, 0.25
ml, 0.25 mmol) was added to a solution of (2S,3S)-1-(O-tert-
butyldiphenylsilyl)-2-N-tert-butoxycarbonylamino-3-fluoro-
methylpentan-1-ol 18 (97 mg, 0.21 mmol) in tetrahydrofuran
(2 ml) at 0 ЊC. The mixture was allowed to warm to room tem-
perature and stirred for 12 h. A further portion of tetrabutyl-
ammonium fluoride (1 M in tetrahydrofuran, 0.11 ml, 0.11 mol)
was added and the reaction was stirred for a further 5 h at room
temperature. The reaction was quenched by careful addition of
saturated aqueous ammonium chloride (2 ml). The crude prod-
uct was extracted into ethyl acetate (3 × 5 ml) and washed with
water (2 ml) and brine (2 ml). The organic layer was dried
(MgSO4) and the solvents were removed in vacuo. The crude
product was purified using preparative thin layer chromato-
graphy using petroleum ether/diethyl ether (9 : 1) as eluent. The
desiredproduct(2S,3S)-2-N-tert-butoxycarbonylamino-3-fluoro-
methylpentan-1-ol 19 was obtained as a white crystalline solid
(48 mg, quantitative); mp 96–98 ЊC; [α]D27 Ϫ11.6 (c 1.0, CHCl3);
m/z (ESϩ) Found: 236.1667 ([M ϩ H]ϩ), [C11H21NO3F ϩ H]
requires: 236.1662; m/z [ϩve FAB (3-NBA)] 258 [M ϩ Na]ϩ and
236 [M ϩ H]ϩ; νmax (film) /cmϪ1 3430 (OH) and 1691 (urethane);
δH (300 MHz, C2HCl3) 5.00–4.90 (1H, br s, OH), 4.49 (1H, ddd,
J6A,F 47.3, J6A,6B 9.7, J6A,4 3.5, H-6A), 4.42 (1H, ddd, J6B,F 47.9,
J6B,6A 9.7, J6B,4 5.8, H-6B), 3.62 (3H, m, H-2 and H-1), 2.50 (1H,
br s, OH), 1.80 (1H, m, H-3), 1.30 (2H, m, H-4), 1.35 (9H, s,
C(CH3)3) and 0.92 (3H, t, J5,4 7.4, H-5); δC (75.5 MHz, C2HCl3)
2
3
55.6 (C-2), 42.0 (d, J3,F 16.5, C-3), 19.5 (d, J4,F 6.1, C-4) and
11.9 (C-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 National Mass Spectrometry
Service (Swansea) for high resolution mass spectra.
References
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1
156.7 (urethane), 84.0 (d, J6,F 164.0, C-6), 80.1 (OC(CH3)3),
63.3 (C-1), 54.1 (C-2), 43.0 (d, 2J3,F 22.0, C-3), 28.8 (C(CH3)3),
20.4 (C-4) and 12.2 (C-5); δF (282 MHz, C2HCl3) Ϫ226.0 (dt,
JF,H6 47.4, JF,H3 29.4).
4 P. L. Weber, S. C. Brown and L. Mueller, Biochemistry, 1987, 26,
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1213–1221.
6 K. Shimamoto, M. Ishida, H. Shinozaki and Y. Ofune, J. Org. Chem.,
1991, 56, 4167–4176.
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Perkin Trans. 1, 2002, 613–621.
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(2S,3S)-2-N-tert-Butoxycarbonylamino-3-fluoromethylpentan-
1-ol 19 (20 mg, 0.09 mmol) was dissolved in carbon tetrachlor-
ide (1 ml). Acetonitrile (1 ml) and water (1.5 ml) were added
O r g . B i o m o l . C h e m . , 2 0 0 4 , 2, 7 9 7 – 8 0 2
802