Asymmetric synthesis of N,O-diprotected (2S,3S)-N-methyl-δ- hydroxyisoleucine, noncoded amino acid of halipeptin A

Article abstract of DOI:10.1016/j.tetasy.2004.02.008

The first enantioselective synthesis of N,O-diprotected (2S,3S)-N-methyl-δ-hydroxyisoleucine 4, starting from readily available (tert-butyldimethylsilyl)-2-butyn-1-ol 5, is described. The key steps of this stereochemical flexible synthetic route involve a

Full text of DOI:10.1016/j.tetasy.2004.02.008

Tetrahedron:
Asymmetry
Tetrahedron: Asymmetry 15 (2004) 1181–1186
Asymmetric synthesis of N,O-diprotected (2S,3S)-N-methyl-
d-hydroxyisoleucine, noncoded amino acid of halipeptin A
Irene Izzo,* Elvira Avallone, Luca Della Corte, Nakia Maulucci and
Francesco De Riccardis*
Department of Chemistry, University of Salerno, via S. Allende, Baronissi I-84081 (SA), Italy
Received 2 February 2004; accepted 5 February 2004
Dedicated to Prof. Adolfo Zambelli on the occasion of his 70th birthday
Abstract—The first enantioselective synthesis of N,O-diprotected (2S,3S)-N-methyl-d-hydroxyisoleucine 4, starting from readily
available (tert-butyldimethylsilyl)-2-butyn-1-ol 5, is described. The key steps of this stereochemical flexible synthetic route involve a
silyl-assisted [3,3]-sigmatropic rearrangement, for the establishment of the correct stereoisomeric pattern, and a triethylsilane–TFA
induced reduction of an oxazolidinone intermediate, to yield the requested N-methylation.
ꢀ 2004Elsevier Ltd. All rights reserved.
1. Introduction
(2S,3S) considering that halipeptin C 3 contained an
N-methyl-valine belonging to the
L-series [(S)-configu-
N-Alkyl amino acids are quite common in bioactive
natural peptides1 and show important influence on
membrane permeability, proteolitic stability and con-
formational rigidity.2 Two of these N-methyl-d-
ration, Marfey analysis]4 and that the 1H NMR of
halipeptin B3 2 and C4 3 were ꢁvirtually superimposable,
except for few signalꢀ.4
hydroxyisoleucine and N-methyl-
L
-valine have recently
As a part of our investigation towards halipeptin A total
synthesis and with the aim of developing a stereo-
chemically flexible synthetic route useful for the elabo-
ration of a solid-phase compatible, protected version of
N-methyl-d-hydroxyisoleucine, herein we report the
first5 asymmetric synthesis of the N,O-diprotected
(2S,3S)-N-methyl-d-hydroxyisoleucine 4.
been found in the naturally occurring potent anti-
inflammatory cyclic depsipeptides halipeptin A3 1, B3 2
and C4 3, where they contribute to the 17-membered
macrolactone ring.
N
O
N
HN
S
OR
O
O
R'
4
3
O
HN
BnO
OH
O
N
O
Fmoc
1 halipeptin A, R = Me, R' = CH2CH2OH
2 halipeptin B, R = H, R' = CH2CH2OH
3, halipeptin C, R = H, R' = CH3
4
The C-2/C-3 relative configuration of the N-methyl-d-
hydroxyisoleucine was established to be erythro [(2R,3R)
or (2S,3S)] by applying the J-based Murataꢀs method.3
The absolute configurations were then assigned as
2. Results and discussion
Our approach to the key intermediate (2S,3S)-2-amino-
3-methyl-4-pentenoic acid 11 was realized through a
silyl-assisted [3,3]-sigmatropic rearrangement6 of the
* Corresponding authors. Tel.: +39-089-965230; fax: +39-089-965296;
e-mail addresses: iizzo@unisa.it; dericca@unisa.it
0957-4166/$ - see front matter ꢀ 2004Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2004.02.008

1182
I. Izzo et al. / Tetrahedron: Asymmetry 15 (2004)1181–1186
optically active (S)-(Z)-1-(tert-butyldimethylsilyl)-2-
butenyl N-tert-butyloxycarbonylglycinate 9 (Scheme 1).
This last compound was prepared starting from the
fluorenylmethyl chloroformate, followed by cyclization
with paraformaldehyde.13 Side-chain functionalization
proved quite difficult to achieve because of the inher-
ently unstable protected heterocycle 13.14 A satisfying
hydroboration–oxidation reaction was eventually ob-
tained using BH3ÆSMe2 (3 equiv) at 0 ꢁC followed by the
use of the mild oxidation reagent sodium perborate,15
compatible with the base labile fluorenylmethoxycar-
bonyl functionality. Under these conditions, primary
alcohol 14 was obtained in 55% yield (based on the
recovered starting material) with no evidence of regio-
isomeric secondary alcohol formation. Protection of the
hydroxyl group was then attempted with TBS-Cl,
(1.0–4.0 equiv) in the presence of triethylamine (1.0–
5.0 equiv) in CH2Cl2 at 0 ꢁC or rt; or with acetic anhy-
dride (2.0 equiv)/pyridine (2.1 equiv) in CH2Cl2 at 0 ꢁC
or rt. In both cases the reactions proceeded very slowly
and resulted in partial loss of the Fmoc protecting group
or complete decomposition of the starting material.
Successful protection of the hydroxy group was finally
achieved with benzyl bromide in the presence of Ag2O.16
The etherification was quenched after 3 days and fur-
nished the desired product 15 in 45% yield, based on the
recovered starting material (36%). Compound 12 was
then submitted to the reduction step to cleave the
endocyclic C–O bond and liberate the N-methyl group.
Unfortunately the mixture Et3SiH/CF3CO2H/CHCl3
proved quite aggressive, giving the target compound 4 in
modest yield (21%).17
readily available chiral nonracemic (S)-alcohol
7
(ee >95%)7 through esterification with commercially
available N-Boc-glycine and a subsequent stereoselective
reduction of the alkyne moiety. The chelate–enolate
Claisen rearrangement was performed under Kazmaierꢀs
conditions8 and gave the expected (E)-anti-vinylsilane 10
in quantitative yield and with complete chirality trans-
fer.9 Quantitative removal of both tert-butyldimethyl-
silyl and tert-butyloxycarbonyl groups was possible
using an excess of HBF4 (80.0 equiv). Smaller amounts
of HBF4, even for prolonged reaction times at higher
temperatures, induced N-deprotection and only partial
desilylation.
OH
O
b
a
c
TBS
TBS
6
5
O
H
N
OH
O
Boc
d
TBS
TBS
8
7
O
H
N
O
O
Boc
e
R
OH
NHR1
TBS
O
9
O
10, R = TBS, R1 = Boc
11, R = TBS, R1 = H
b
OH
f
O
NHR
N
Fmoc
ꢀ
Scheme 1. Reagents and conditions: (a) PDC, 4 A m.s., CH2Cl2, 3 h,
84%; (b) (S)-Me-CBS reagent, BH3ÆTHF, THF, 30 min, 94%, ee >95%;
(c) N-Boc-glycine, EDC, DMAP, CH2Cl2, 2 h; (d) Pd/5% wt. on
CaCO3, H2, 3 h, 45 min, 75% (two steps); (e) LDA, ZnCl2, THF,
ꢀ78 ꢁC, 1 h and 30 min (quant.); (f) HBF4 (48% in water), 1,4-dioxane,
70 ꢁC, 22 h, 75%.
11, R = H
13
a
12, R = Fmoc
O
e
c
RO
4
O
N
Fmoc
Attention was then turned towards N-methylation and
regioisomeric hydration of the terminal double bond.
There are a number of methods useful for preparing
N-methyl amino acids; however the basic conditions
often necessary would induce partial racemization.10
Freidinger et al.11 reported a method in which N-Fmoc
amino acids were reacted with paraformaldehyde in the
presence of catalytic amounts of p-TsOH to form oxa-
zolidinones. These can be subsequently reduced to
N-methylated amino acids once treated with a few
equivalents of triethylsilane, using trifluoroacetic acid as
co-solvent. A unified approach to the synthesis of
14, R = H
d
15, R = Bn
Scheme 2. Reagents and conditions: (a) Fmoc-Cl, Na2CO3, 1,4-diox-
ane, 0 ꢁC, 18 h, 76%; (b) (CH2O)n, p-TsOH, toluene, reflux, 1 h, 62%;
(c) BH3ÆSMe2, 0 ꢁC, 5 h then, EtOH, H2O, NaBO3Æ4H2O, rt, 3 h, 55%;
(d) BnBr, Ag2O, CH2Cl2, 3 days, 45%; (e) Et3SiH, TFA/CHCl3, rt,
3 days, 21%.
3. Conclusion
N-methyl derivatives of the 20 common L-amino acids,
through 5-oxazolidinones, has since been reported by
Aurelio et al.12 showing the reliability of this synthetic
methodology.
In summary, we have accomplished the first asymmetric
synthesis of the N,O-diprotected (2S,3S)-N-methyl-d-
hydroxyisoleucine 4 through a stereospecific chelate–
enolate Claisen rearrangement as key step. Further
studies to improve the yield of the triethylsilane–TFA
induced reduction on labile oxazolidinone intermediate
We thus converted amino acid 11 to N-Fmoc-protected
5-oxazolidinone 13 (Scheme 2) by treatment with 9-

I. Izzo et al. / Tetrahedron: Asymmetry 15 (2004)1181–1186
1183
and towards the total synthesis of 1 are currently
underway.
4.2. 1-(tert-Butyldimethylsilyl)-2-butyn-1-one, 6
To a solution of 1-(tert-butyldimethylsilyl)-2-butyn-1-ol
ꢀ
(7.33 g, 39.8 mmol) in dry CH2Cl2 (300 mL), 4 A
molecular sieves (14.7 g) and PDC (24.1 g, 64.1 mmol)
were added. The mixture was stirred at room tempera-
ture for 3 h, then diluted with diethyl ether (300 mL) and
allowed to stir for an additional 30 min. Filtration
through a short pad of silica gel (particle size 0.063–
0.200 mm) and CaSO4 (10% in weight) afforded a solu-
tion, which was concentrated in vacuo. The residue was
flash-chromatographed (25% diethyl ether in petroleum
ether) to afford 6 (6.10 g, 84%) as a pale yellow oil.
Rf ¼ 0:67 (30% diethyl ether in petroleum ether). 1H
NMR (CDCl3, 400 MHz): d 0.22 (6H, s, (CH3)2–Si),
0.97 (9H, s, –((CH3)3–C)Si), 2.09 (3H, s, H-4). 13C NMR
(CDCl3, 100 MHz): d ꢀ7.5 (·2), 4.5, 16.9, 26.9 (·3),
85.0, 98.5, 177.2. HR-EIMS m=z 182.1120 (calcd
182.1127 for C10H18OSi).
4. Experimental
All reactions were carried out under a dry argon
atmosphere using freshly distilled solvents, unless
otherwise noted. Tetrahydrofuran (THF) was distilled
from LiAlH4. Toluene and methylene chloride were
distilled from calcium hydride. Glassware was flame-
dried (0.05 Torr) prior to use. When necessary, com-
pounds were dried in vacuo over P2O5 or by azeotropic
removal of water with toluene under reduced pressure.
Starting materials and reagents purchased from com-
mercial suppliers were generally used without purifica-
tion. Reaction temperatures were measured externally;
reactions were monitored by TLC on Merck silica gel
plates (0.25 mm) and visualized by UV light, I2, or
spraying with KMnO4, p-anisaldehyde or phosphomo-
lybdic acid solutions and drying.
4.3. (S)-1-(tert-Butyldimethylsilyl)-2-butyn-1-ol, 7
Flash chromatography was performed on Merck silica
gel 60 (particle size: 0.040–0.063 mm). Yields refer to
chromatographically and spectroscopically (1H and 13C
NMR) pure materials. The NMR spectra were recorded
at room temperature or when indicated at 80 or 100 ꢁC
on a Bruker DRX 400, a Bruker DRX 300 and a Bruker
AM 250 spectrometers. Chemical shifts are reported
relative to the residual solvent peak (CHCl3 : d ¼ 7:26,
13CDCl3: d ¼ 77:0; C2H2Cl4: d ¼ 5:80, 13C2D2Cl4:
d ¼ 72:1; HOD: d ¼ 4:79). High resolution spectra (EI-
and ES-MS) were performed on a VG 70-250SE and a
Q-Star Applied Biosystem mass spectrometers. Optical
rotations were measured with a JASCO DIP-1000
polarimeter.
(S)-Me-CBS reagent (1 M in toluene, 11.0 mL,
11.0 mmol) and BH3ÆTHF (1 M in THF, 27.4mL,
27.4mmol) were mixed, under N 2 and at ꢀ78 ꢁC, and
allowed to stir for 30 min. A solution of 1-(tert-butyl-
dimethylsilyl)-2-butyn-1-one (1.00 g, 5.48 mmol) dis-
solved in dry THF (18.0 mL) was then added via
cannula. The reaction mixture was stirred for 30 min.
Methanol (17.0 mL) was added and the solution allowed
to stir for an additional 30 min at ꢀ78 ꢁC, diluted with
diethyl ether and allowed to warm to room temperature.
The mixture was washed with 5% solution of NaHCO3,
brine, dried over MgSO4 and concentrated in vacuo.
The residue was flash-chromatographed (20% diethyl
ether in petroleum ether) to give 7 (0.95 g; 94%,
ee >95%). Rf ¼ 0:74(30% diethyl ether in petroleum
ether). 1H NMR (CDCl3, 400 MHz): d 0.07 (3H, s,
(CH3)–Si), 0.10 (3H, s, (CH3)–Si), 0.97 (9H, s, –((CH3)3–
C)Si), 1.87 (3H, d, J ¼ 2:6 Hz, H-4), 4.18 (1H, q,
J ¼ 2:6 Hz, H-1). 13C NMR (CDCl3, 100 MHz): d ꢀ8.6,
4.1. 1-tert-Butyldimethylsilyl)-2-butyn-1-ol, 5
To a solution of 2-butyn-1-ol (2.53 g, 42.8 mmol) in dry
THF (45.0 mL), n-BuLi (2.5 M in hexane, 18.0 mL,
44.9 mmol) was added at ꢀ78 ꢁC. The mixture was
stirred at 0 ꢁC for 30 min. To the mixture was added a
solution of TBS-Cl (6.77 g, 44.9 mmol) in THF (9.0 mL)
at ꢀ78 ꢁC. After stirring at room temperature for 16 h,
n-BuLi (2.5 M in hexane, 20.5 mL, 51.4mmol) was
added and the reaction mixture stirred at ꢀ45 ꢁC for 3 h.
The reaction was then quenched by addition of 10%
AcOH in THF at ꢀ78 ꢁC and extracted with diethyl
ether. The organic phase was washed with saturated
NaHCO3, brine, dried over MgSO4 and concentrated in
vacuo. The residue was purified by flash-chromatogra-
phy (5–10% diethyl ether in petroleum ether) to afford 5
(6.62 g, 84%) as a pale yellow oil. Rf ¼ 0:74(30% diethyl
ꢀ8.0, 3.8, 16.9, 26.8 (·3), 55.0, 80.1, 84.0. HR-EIMS
25
m=z 184.1285 (calcd 184.1283 for C10H20OSi). ½aŠ
ꢀ73.8 (c 1.2, CHCl3).
¼
D
4.4. (S)-1-(tert-Butyldimethylsilyl)-2-butynyl N-tert-
butyloxycarbonyl-glycinate, 8
To a solution of (S)-1-(tert-butyldimethylsilyl)-2-butyn-
1-ol (1.33 g, 7.22 mmol) in dry CH2Cl2 (23.0 mL) N-Boc-
glycine (3.86 g, 22.0 mmol), 1-ethyl-3-(3-dimethylamino-
propyl)-carbodiimide hydrochloride (EDC, 2.31 g,
12.1 mmol) and N,N-dimethylaminopyridine (DMAP,
0.044 g, 0.360 mmol) were added at 0 ꢁC. The reaction
mixture was stirred at 0 ꢁC for 2 h, then concentrated in
vacuo, diluted with diethyl ether and washed with sat-
urated aq NaHCO3 and brine. The combined organic
phases were dried over MgSO4, filtered and concen-
trated in vacuo. Crude 8 was used in the next step
without further purification. Rf ¼ 0:31 (20% diethyl
1
ether in petroleum ether). H NMR (CDCl3, 400 MHz):
d 0.07 (3H, s, (CH3)–Si), 0.10 (3H, s, (CH3)–Si), 0.97
(9H, s, –((CH3)3–C)Si), 1.87 (3H, d, J ¼ 2:6 Hz, H-4),
4.18 (1H, q, J ¼ 2:6 Hz, H-1). 13C NMR (CDCl3,
100 MHz): d ꢀ8.6, ꢀ8.0. 3.8, 16.9, 26.8 (·3), 55.0, 80.1,
84.0 HR-EIMS m=z 184.1287 (calcd 184.1283 for
C10H20OSi).
1
ether in petroleum ether). H NMR (CDCl3, 400 MHz):

1184
I. Izzo et al. / Tetrahedron: Asymmetry 15 (2004)1181–1186
d 0.09 (3H, s, (CH3)–Si), 0.10 (3H, s, (CH3)–Si), 0.94
(9H, s, –((CH3)3–C)Si), 1.45 (9H, s, –((CH3)3–CO) 1.85
(3H, d, J ¼ 2:6 Hz, H-4), 3.93 (2H, m, CO2CH2NH),
5.00 (1H, br s, NH), 5.29 (1H, m, H-1). 13C NMR
(CDCl3, 100 MHz): d ꢀ8.1, ꢀ7.7, 3.9, 16.9, 26.7 (·3),
28.3 (·3), 42.6, 58.4, 75.9, 80.1, 84.6, 155.5, 170.1. HR-
added. The mixture was stirred for 22 h at 70 ꢁC and
allowed to reach room temperature. Saturated aq
NaHCO3 was then added and the mixture washed with
diethyl ether. The pH of the mixture was adjusted to 1
with 1 M HCl, washed with diethyl ether and concen-
trated in vacuo. The residue was purified over Dowex 50
W (100–200 mesh, Hþ activated). H2O followed by 1 M
aqueous NH3 eluted 11 (0.085 g; 75%). Rf ¼ 0:38 (10%
EIMS m=z 341.2026 (calcd 341.2022 for C17H31NO4Si).
25
D
½aŠ ¼ ꢀ106.2 (c 0.7, CHCl3).
1
methanol in CHCl3). H NMR (D2O, 250 MHz): d 1.11
(3H, d, J ¼ 7:0 Hz, –CH3), 2.74(1H, m, H-3), 3.53 (1H,
d, J ¼ 5:6 Hz, H-2), 5.19 (1H, d, J ¼ 10:0 Hz, H-5), 5.20
(1H, d, J ¼ 17:1 Hz, H0-5), 5.71 (1H, ddd, J ¼ 17:1,
10.0, 7.7 Hz, H-4). 13C NMR (D2O, 1,4-dioxane, exter-
nal standard: d 66.7, 62.5 MHz): d 15.5, 38.7, 59.3, 118.1,
4.5. (S)-(Z)-1-(tert-Butyldimethylsilyl)-2-butenyl N-tert-
butyloxycarbonylglycinate, 9
To a solution of crude 8 (1.22 g, 3.58 mmol) in ethyl
acetate (9.0 mL), Lindlarꢀs catalyst (Pd/5% wt. on
CaCO3, 0.764g) was added and the mixture stirred
under H2 for 3.75 h. The suspension was filtered on a
pad of Celiteꢂ and washed with CH2Cl2. Solvents were
removed in vacuo and the residue flash-chromato-
graphed (10–20% diethyl ether in petroleum ether) to
give 9 (1.86 g, 75% for two steps). Rf ¼ 0:64(30% diethyl
137.0, 174.5. HR-ESMS m=z 130.0890 (calcd 130.0868
25
D
for C6H12NO2). ½aŠ ¼ ꢀ18.0 (c 1.1, H2O).
4.8. (2S,3S)-2-(9H-Fluoren-9-ylmethoxycarbonylamino)-
3-methyl-4-pentenoic acid, 12
1
ether in petroleum ether). H NMR (CDCl3, 400 MHz):
To a suspension of 11 (0.113 g, 0.874mmol) in 1,4-
dioxane (1.2 mL), Na2CO3 (10% in water, 2.6 mL) and a
solution of 9-fluorenylmethoxycarbonyl chloride
(Fmoc-Cl, 0.226 g, 0.874mmol) in 1,4-dioxane (2.4mL)
were added at 0 ꢁC. The resulting mixture was stirred for
18 h at room temperature, poured in water (45.0 mL)
and extracted twice with diethyl ether to remove 9-flu-
orenylmethanol and dibenzofulvene. The pH of the
aqueous phase was adjusted to 1 with 1 M HCl and
the mixture extracted three times with ethyl acetate. The
residue was flash-chromatographed (2–10% methanol in
chloroform) to give 12 (0.233 g, 76%). Rf ¼ 0:5 (10%
d ꢀ0.02 (3H, s, (CH3)–Si), 0.04(3H, s, (C H3)–Si), 0.91
(9H, s, –((CH3)3–C)Si), 1.44 (9H, s, –((CH3)3–CO), 1.71
(3H, dd, J ¼ 6:9, 1.5 Hz, H-4), 3.88 (2H, m,
CO2CH2NH), 4.99 (1H, br s, NH), 5.39 (1H, dd,
J ¼ 10:7, 10.4Hz, H-2), 5.53, (1H, m, H-3), 5.65 (1H, br
d, J ¼ 10:4Hz, H-1). 13C NMR (CDCl3, 100 MHz): d
ꢀ8.2, ꢀ7.7, 13.6, 16.9, 26.8 (·3), 28.3 (·3), 42.7, 65.7,
79.8, 125.8, 127.2, 155.5, 170.2. HR-EIMS m=z 343.2175
25
(calcd 343.2179 for C17H33NO4Si). ½aŠ ¼ ꢀ9.2 (c 1.0,
D
CHCl3).
1
methanol in chloroform). H NMR (CDCl3, 400 MHz):
4.6. (2S,3S)-(E)-2-tert-Butoxycarbonylamino-5-(tert-
butyldimethylsilyl)-3-methyl-4-pentenoic acid, 10
d 1.12 (3H, d, J ¼ 6:7 Hz, –CH3), 2.86 (1H, m, H-3),
4.22 (1H, t-like, J ¼ 7:0 Hz, CH-Fmoc), 4.42 (3H, m, H-
2 and CH2-Fmoc overlapped), 5.09 (3H, m, H-5, H0-5
and NH overlapped), 5.63 (1H, ddd, J ¼ 16:0, 11.0,
7.4Hz, H-4), 7.31 (2H, t-like, J ¼ 7:3 Hz, Ar), 7.39 (2H,
t-like, J ¼ 7:3 Hz, Ar), 7.58 (2H, d-like, J ¼ 6:8 Hz, Ar),
7.76 (2H, d-like, J ¼ 6:8 Hz, Ar). 13C NMR (CDCl3,
100 MHz): d 16.0, 39.7, 47.0, 58.0, 67.2, 117.3, 120.1
(·2), 125.0 (·2), 127.0 (·2), 127.7 (·2), 137.2, 141.2 (·2),
To a solution of lithium diisopropylamide (LDA, 2 M,
in eptane/THF/ethylbenzene, 5.45 mL, 10.9 mmol) were
added a solution of 9 (0.931 g, 2.71 mmol) and ZnCl2
(1 M in ethyl ether, 3.25 mL, 3.25 mmol) in THF
(16.0 mL) at ꢀ78 ꢁC. The resulting mixture was stirred at
room temperature 1.5 h. The pH of the reaction mixture
was adjusted to 1 with 1 M HCl and the mixture was
extracted with diethyl ether. The combined organic
phases were washed with brine, dried over MgSO4, fil-
tered and concentrated in vacuo to give 10 (0.931 g,
143.8 (·2), 156.4, 176.1. HR-ESMS m=z 352.1567 (calcd
25
352.1549 for C21H22NO4). ½aŠ ¼ ꢀ3.8 (c 1.0, CHCl3).
D
1
quant.). Rf ¼ 0:42 (10% methanol in chloroform). H
4.9. (4S)-3-(9H-Fluoren-9-ylmethoxycarbonyl)-4-[(S)-3-
buten-2-yl]-oxazolidin-5-one, 13
NMR (CDCl3, 400 MHz): d 0.01 (6H, s, (CH3)2–Si),
0.85 (9H, s, –((CH3)3–C)Si), 1.12 (3H, d, J ¼ 6:8 Hz,
CH3–), 1.44 (9H, s, –((CH3)3–CO), 2.80 (1H, m, H-3),
4.22 (1H, m, H-2), 4.89 (1H, br d, J ¼ 8:2 Hz, NH), 5.77
(1H, d, J ¼ 18:6 Hz, H-5), 5.87 (1H, dd, J ¼ 18:6,
6.8 Hz, H-4). 13C NMR (CDCl3, 100 MHz): d ꢀ6.2 (·2),
16.2, 16.4, 26.4 (·3), 28.2 (·3), 42.5, 57.5, 80.2, 130.3,
To a solution of 12 (0.172 g, 0.490 mmol) in dry toluene
(11 mL), paraformaldehyde (0.011 g) and p-toluensulf-
onic acid (p-TsOH, 0.001 g) were added at 0 ꢁC. The
mixture was refluxed for 1 h with azeotropic water
removal. The solution was cooled, washed with 1 M
aqueous solution of NaHCO3, extracted with CH2Cl2,
dried over Na2SO4, filtered and concentrated in vacuo.
The residue was flash-chromatographed (10–20% ethyl
acetate in petroleum ether) to give 13 (0.110 g, 62%).
Rf ¼ 0:4(20% ethyl acetate in petroleum ether). 1H
NMR (C2D2Cl4, 100 ꢁC, 400 MHz): d 0.81 (3H, d,
J ¼ 6:7 Hz, –CH3), 2.54(1H, m, –C HCH3), 3.85 (1H, br
s, H-4), 4.08 (1H, t-like, J ¼ 6:7 Hz, CH-Fmoc), 4.53
146.5, 155.8, 176.5. HR-EIMS m=z 343.2171 (calcd
25
D
343.2179 for C17H33NO4Si). ½aŠ ¼ þ7.9 (1.0, CHCl3).
4.7. (2S,3S)-2-Amino-3-methyl-4-pentenoic acid, 11
To a solution of 10 (0.300 g, 0.874mmol) in 1,4-dioxane
(7.5 mL), HBF4 (48% in water, 9.7 mL, 69.9 mmol) was

I. Izzo et al. / Tetrahedron: Asymmetry 15 (2004)1181–1186
1185
(3H, d-like, J ¼ 6:7 Hz, CH2-Fmoc), 4.83, (1H, d,
J ¼ 4:6 Hz, H-2), 4.91 (1H, d, J ¼ 16:9 Hz,
–CH@CHH), 4.93 (1H, d, J ¼ 11:0 Hz, –CH@CHH),
5.19 (1H, d, J ¼ 4:6 Hz, H0-2), 5.55 (1H, ddd, J ¼ 16:9,
11.0, 7.8 Hz, CH@CH2), 7.18 (2H, t-like, J ¼ 7:3 Hz,
Ar), 7.26 (2H, t-like, J ¼ 7:3 Hz, Ar), 7.39 (2H, d-like,
J ¼ 6:8 Hz, Ar), 7.61 (2H, d-like, J ¼ 6:8 Hz, Ar). 13C
NMR (CDCl3, 100 MHz): d 15.0, 40.1, 47.2, 59.6, 67.1,
78.2, 117.3, 120.1 (·2), 124.5 (·2), 127.2 (·2), 127.9 (·2),
(1H, m, –CHHCH2O–), 1.66 (1H, m, –CHHCH2O–),
2.04(1H, m, –C HCH3), 3.28 (2H, m, –CH2CH2O–),
3.89 (1H, br s, H-4), 4.06 (1H, t-like, J ¼ 6:7 Hz, CH-
Fmoc), 4.27 (1H, d, J ¼ 12:0 Hz, –CHHPh), 4.31 (1H,
d, J ¼ 12:0 Hz, –CHHPh), 4.47 (3H, d-like, J ¼ 6:7 Hz,
CH2-Fmoc), 4.87 (1H, d, J ¼ 4:6 Hz, H-2), 5.18 (1H, d,
J ¼ 4:6 Hz, H0-2), 7.14(7H, m, Ar-Fmoc and Ar–Bn
overlapped), 7.24(2H, t-like, J ¼ 7:3 Hz, Ar-Fmoc),
7.37 (2H, d-like, J ¼ 6:8 Hz, Ar-Fmoc), 7.59 (2H, d-like,
J ¼ 6:8 Hz, Ar-Fmoc). 13C NMR (C2D2Cl4, 80 ꢁC,
100 MHz): d 13.0, 30.2, 31.4, 45.7, 57.2, 65.6, 66.4, 71.0,
76.3, 118.3 (ꢁ2), 122.8 (·2), 125.5 (·2), 126.2 (·3), 126.5
(·2), 127.1 (·2), 136.9, 139.6 (·2), 141.7 (·2), 151.2,
137.0, 141.4 (·2), 143.2 (·2), 153.4, 171.0. HR-ESMS
25
D
m=z 364.1571 (calcd 364.1549 for C22H22NO4). ½aŠ
¼
þ68.2 (c 1.0, CHCl3).
169.0. HR-ESMS m=z 472.2146 (calcd 472.2124 for
25
D
4.10. (4S)-3-(9H-Fluoren-9-ylmethoxycarbonyl)-4-[(S)-4-
hydroxybutan-2-yl]-oxazolidin-5-one, 14
C29H30NO5). ½aŠ ¼ þ25.6 (c 1.0, CHCl3).
To a solution of 13 (0.071 g, 0.19 mmol) in dry THF
(1.2 mL), BH3ÆSMe2 (2 M in THF, 0.29 mL, 0.59 mmol)
was added at 0 ꢁC. The reaction mixture was stirred at
0 ꢁC for 5 h and then EtOH (0.21 mL), H2O (0.19 mL)
and NaBO3Æ4H2O (0.09 g, 0.59 mmol) were added. The
resulting mixture was stirred for an additional 3 h and
then allowed to warm to room temperature. The mix-
ture was concentrated in vacuo to remove any excess of
THF, extracted with ethyl acetate and the combined
organic phases dried over Na2SO4, filtered and concen-
trated in vacuo. The residue was flash-chromatographed
(25–40% ethyl acetate in petroleum ether) to give 14
(0.034g, 55%, based on 15%––0.011 g––of recovered
starting material 13). Rf ¼ 0:5 (50% ethyl acetate in
petroleum ether). 1H NMR (C2D2Cl4, 100 ꢁC,
300 MHz): d 0.72 (3H, d, J ¼ 6:7 Hz, –CH3), 1.37 (1H,
m, –CHHCH2OH), 1.57 (1H, m, –CHHCH2OH), 2.00
(1H, m, –CHCH3), 3.40 (1H, m, –CH2CHHOH), 3.47
(1H, m, –CH2CHHOH), 3.91 (1H, d, J ¼ 4:0 Hz, H-4),
4.09 (1H, t-like, J ¼ 6:7 Hz, CH-Fmoc), 4.54 (3H, d-
like, J ¼ 6:7 Hz, CH2-Fmoc), 4.86 (1H, d, J ¼ 4:6 Hz,
H-2), 5.16 (1H, d, J ¼ 4:6 Hz, H0-2), 7.21 (2H, t-like,
J ¼ 7:3 Hz, Ar), 7.27 (2H, t-like, J ¼ 7:3 Hz, Ar), 7.40
(2H, d-like, J ¼ 6:8 Hz, Ar), 7.62 (2H, d-like,
J ¼ 6:8 Hz, Ar). 13C NMR (C2D2Cl4, 80 ꢁC, 100 MHz):
d 12.4, 31.1, 33.2, 45.7, 56.8, 58.7, 65.5, 76.3, 118.3 (·2),
123.0 (·2), 125.5 (·2), 126.2 (·2), 139.6 (·2), 141.7 (·2),
4.12. (2S,3S)-5-Benzyloxy-2-[(9H-fluoren-9-ylmethoxy-
carbonyl)-methylamino]-3-methyl-pentanoic acid, 4
To a solution of 15 (0.020 g, 0.042 mmol) in dry CHCl3
(0.5 mL), trifluoroacetic acid (0.5 mL) and triethylsilane
(0.02 mL, 0.13 mmol) were added at room temperature.
The mixture was stirred for 3 days. Any excess trifluo-
roacetic acid was eliminated by adding CH2Cl2 and
concentrating the mixture several times in vacuo. The
residue was flash-chromatographed (0–10% methanol in
chloroform) to afford 4 (0.004g, 21%). 1H NMR
(C2D2Cl4, 110 ꢁC, 400 MHz): d 0.86 (3H, d, J ¼ 6:7 Hz,
–CH3), 1.16 (1H, m, H-4), 1.56 (1H, m, H0-4), 2.74 (1H,
s, –NCH3), 3.37 (2H, m, H-5 and H0-5), 4.10 (1H, t-like,
J ¼ 6:7 Hz, CH-Fmoc), 4.16 (1H, m, H-2), 4.27 (2H, br
s, –CH2Ph), 4.39 (2H, d-like, J ¼ 6:7 Hz, CH2-Fmoc),
7.16 (7H, m, Ar-Fmoc and Ar–Bn overlapped), 7.24
(2H, t-like, J ¼ 7:3 Hz, Ar-Fmoc), 7.43 (2H, d-like,
J ¼ 6:8 Hz, Ar-Fmoc), 7.60 (2H, d-like, J ¼ 6:8 Hz, Ar-
Fmoc). 13C NMR (C2D2Cl4, 80 ꢁC, 100 MHz): d 14.7,
27.8, 27.9, 30.9, 45.7, 66.0, 66.1, 71.1, 71.9, 118.2 (·2),
123.1 (·2), 125.4( ·2), 125.7 (·3), 126.0 (·2), 126.5 (·2),
136.8, 139.5 (·2), 142.1 (·2), 155.2, 169.0. HR-ESMS
25
m=z 474.2271 (calcd 474.2280 for C29H32NO5). ½aŠ
¼
D
ꢀ52.2 (c 0.2, CHCl3).
151.2, 169.0. HR-ESMS m=z 382.1676 (calcd 382.1654
25
D
for C22H24NO5). ½aŠ ¼ þ14.8 (c 1.0, CHCl3).
Acknowledgements
This work has been supported by the MIUR (ꢁSostanze
4.11. (4S)-3-(9H-Fluoren-9-ylmethoxycarbonyl)-4-[(S)-4-
benzyloxybutan-2-yl]-oxazolidin-5-one, 15
ꢁ
naturali ed analoghi sintetici con attivita antitumoraleꢀ)
ꢁ
and by the Universita degli Studi di Salerno.
To a solution of 14 (0.026 g, 0.068 mmol) in dry CH2Cl2
(1.2 mL), Ag2O (0.044 g, 0.19 mmol), NaHCO3 (0.002 g)
and BnBr (0.025 mL, 0.200 mmol) were added at 0 ꢁC.
The mixture was stirred in the dark for 5 h at room
temperature. Ag2O (0.044 g, 0.19 mmol) and BnBr
(0.025 mL, 0.20 mmol) were again added. After 20 h at
room temperature in the dark, the mixture was filtered
and concentrated. The residue was flash-chromato-
graphed (20–30% diethyl ether in petroleum ether) to
afford 15 (0.009 g, 45%, based on 36%––0.009 g––of
recovered starting material 13). 1H NMR (C2D2Cl4,
80 ꢁC, 400 MHz): d 0.69 (3H, d, J ¼ 6:7 Hz, –CH3), 1.31
References and notes
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references cited therein.
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Debitus, C.; Cirino, G.; Gomez-Paloma, L. J. Am. Chem.
Soc. 2001, 123, 10870–10876. Halipeptins also show a
promising antitumor activity, being cytotoxic against

1186
I. Izzo et al. / Tetrahedron: Asymmetry 15 (2004)1181–1186
HCT-116 human colon carcinoma cells in nanomolar
range. William Fenical, personal communication.
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Aquino, M.; Izzo, I.; De Riccardis, F.; Gomez-Paloma, L.
Tetrahedron Lett. 2002, 43, 5707–5710.
5. A synthesis of a mixture of diastereoisomes of d-hydroxy-
isoleucine has been previously reported by Englisch-
Peters, S. Tetrahedron 1989, 45, 6127–6134.
6. Sakaguchi, K.; Suzuki, H.; Ohfune, Y. Chirality 2001, 13,
357–365.
7. Sakaguchi, K.; Suzuki, H.; Ohfune, Y. Tetrahedron Lett.
2000, 41, 6589–6592.
8. Kazmaier, U. Angew. Chem., Int. Ed. Engl. 1994, 33, 998–
999.
9. Morimoto, Y.; Takaishi, M.; Kinoshita, T.; Sakaguchi,
K.; Shibata, K. Chem. Commun. 2002, 42–43.
10. (a) Olsen, R. K. J. Org. Chem. 1970, 35, 1912–1915; (b)
Coggins, J. R.; Benoiton, N. L. Can. J. Chem. 1971, 49,
1968–1971; (c) McDermott, J. R.; Benoiton, N. L. Can. J.
Chem. 1973, 51, 1915–1919; (d) McDermott, J. R.;
Benoiton, N. L. Can. J. Chem. 1973, 51, 2555–2561; (e)
Cheung, S. T.; Benoiton, N. L. Can. J. Chem. 1977, 55,
906–910; (f) Cheung, S. T.; Benoiton, N. L. Can. J. Chem.
1977, 55, 916–921; (g) Chen, F. M. F.; Benoiton, N. L.
Can. J. Chem. 1977, 55, 1433–1435.
14. No substantial improvement of the yields were noted when
different reaction conditions were used: 9-BBN as hydro-
borating agent, excess of reagents (4.0–6.0 equiv), different
reaction times (3–12 h) and temperatures of reaction
(ꢀ10 ꢁC to rt).
15. Kabalka, W. G.; Shoup, T. M.; Goudgaon, N. M. J. Org.
Chem. 1989, 54, 5930–5933.
16. Van Hijfte, L.; Little, R. D. J. Org. Chem. 1985, 50, 3942–
3943.
17. To overcome the problem of the low yielding benzyl
protection and reductive oxazolidinone opening, we sub-
jected compound 14 to a direct reductive oxazolidinone
ring opening. Unfortunately, from the complex crude
reaction mixture, only compound 16 ((3S,4S)-3-[(9H-
fluoren-9-ylmethoxycarbonyl)-methylamino]-4-methyl-tet-
rahydro-2H-pyran-2-one) could be isolated in 30% yield.
1H NMR (C2D2Cl4, 110 ꢁC, 400 MHz): d 0.81 (3H, br,
–CH3), 1.16 (1H, br, H-5), 1.57 (1H, br, H0-5), 2.16 (1H, br,
H-4), 2.87 (1H, br, –NCH3), 3.37 (2H, m, H-5 and H0-5),
4.10–4.62 (5H, br, H-6, H0-6, CH-Fmoc, CH2-Fmoc, H-3),
7.16 (7H, br, Ar-Fmoc), 7.24(2H, br, Ar-Fmoc), 7.42 (2H,
br, Ar-Fmoc), 7.59 (2H, br, Ar-Fmoc). 13C NMR
(C2D2Cl4, 80 ꢁC, 100 MHz): d 14.0, 30.0, 33.2, 45.7, 65.0,
65.6, 72.1, 118.0 (·2), 123.0 (·2), 125.3 (·2), 125.7 (·3),
126.0 (·2), 126.5 (·2), 125.9, 139.5 (·2), 142.1 (·2), 154.2,
11. Freidinger, M. R.; Hinkle, J. S.; Perlow, D. S.; Arison, B.
H. J. Org. Chem. 1983, 48, 206–215.
168.4. HR-ESMS m=z 366.1699 (calcd 366.1705 for
25
D
C22H24NO4). ½aŠ ¼ ꢀ75.3 (c 0.5, CHCl3)
12. Aurelio, L.; Box, J. S.; Brownlee, R. T. C.; Hughes, A. B.;
Sleebs, M. M. J. Org. Chem. 2003, 48, 2652–2667.
13. It is interesting to note that complete spectral elucidation
(1H and 13C NMR) for compounds containing the 5-
oxazolidinone ring 13–15 was possible at 80 ꢁC or higher
temperatures (see Experimental section).
O
Fmoc
N
16
O