An Epoxide Ring-Opening Reaction via Hypervalent Silicate Intermediate: Synthesis of Statine

Article abstract of DOI:10.1055/s-2003-41049

The azide- and cyanide-opening reaction of epoxide with TBAF and TMSN ₃ in THF or TBAF and TMSCN in MeCN occurred regioselectively to afford β-hydroxy azides and cyanides in good yield. These hypervalent silicates have been shown to be highly effective as nucleophilic azide and cyanide donors under mild conditions. This methodology has been applied to the preparation of statine.

Full text of DOI:10.1055/s-2003-41049

PAPER
2161
An Epoxide Ring-Opening Reaction via Hypervalent Silicate Intermediate:
Synthesis of Statine
E
poxide Ring-Ope
i
ning vi
r
a
H
yperv
o
alent Silicat
y
e
I
ntermedia
u
te ki Konno,* Emi Toshiro, Naoyuki Hinoda
Department of Biological Science and Technology, Faculty of Engineering, University of Tokushima, 2-1 Minamijosanjima-cho,
Tokushima, 770-8506, Japan
Fax +81(88)6569213; E-mail: konno@bio.tokushima-u.ac.jp
Received 16 May 2003; revised 30 June 2003
not without problems, such as difficulty in preparation
Abstract: The azide- and cyanide-opening reaction of epoxide with
and/or storage of reagents or catalyst, work-up and isola-
tion of products.
TBAF and TMSN3 in THF or TBAF and TMSCN in MeCN oc-
curred regioselectively to afford -hydroxy azides and cyanides in
good yield. These hypervalent silicates have been shown to be high-
In 1995 DeShong reported an application of hypervalent
ly effective as nucleophilic azide and cyanide donors under mild
silicate derivatives for nucleophilic displacement by fluo-
conditions. This methodology has been applied to the preparation of
ride.15 Takaya also published a report of azide displace-
statine.
ment using hypervalent silicates at the same time.16 Since
Key words: hypervalent silicates, azido anion, cyano anion, amino
these reports, this and related hypervalent silicates have
acids, statine
been shown to be versatile reagents with numerous syn-
thetic applications.17 It seems that hypervalent silicates
have the ability to assist other reactions under mild condi-
-Hydroxy azides are an important class of organic com-
tions.
pounds, and they serve as precursors in the synthesis of
On the other hand, the structural class of -hydroxy- -
aziridines, -amino alcohols, and amino sugars. -Hy-
amino acids has in recent years been the object of much
droxy cyanides are also important compounds in organic
attention, especially in connection with the development
synthesis. These are aldol-type adducts and useful precur-
of new pharmaceutical science based on protease inhibi-
sors in the synthesis of -hydroxy carboxylic acid deriva-
tors.18 Statine, (3S,4S)-4-amino-3-hydroxy-6-methylhep-
tives and -amino alcohols. These compounds are useful
tanoic acid, is an essential component of pepstatine19 and
building blocks for the syntheses of natural products,
miraziridine A20 natural antibiotics, which act as an inhib-
itor of aspartic proteases such as renin, pepsin and cath-
epsin D. Due to these reasons, a practical, highly
stereoselective synthesis of statine has been desired.21
complex molecules, and peptidomimitics.1 The usual syn-
thetic methods to serve compounds involve ring-opening
reaction of 1,2-epoxides with azide and cyanide reagents.
The reactions often require harsh conditions with high re-
action temperatures, polar solvents such as DMSO,
HMPA, and DMF and a large excess of the nucleophilic
azide and cyanide. Furthermore, in recent years several
novel methods have been introduced for this reaction. For
example, sodium azide impregnated on a calcium cation
exchange Y-type zeolite (CaY) induced the nucleophilic
ring-opening of epoxides in aprotic solvents.2 Other re-
ported reagents are tributyltin azide without a solvent and
promoter,3 dibutyltin azide in DMF,4 triethylaluminum/
hydrogen azide,5 diethylaluminum azide,6 trimethylsilyl
azide in the presence of Lewis acid7 and also Cr(salen).8
Also, acetone cyanohydrin in basic media9 or KCN/
LiClO4 in acetonitrile10 were used. Sugita et al. reported
ring-opening of epoxide with TMSCN on solid bases,
which reaction in the presence of TBAF proceeded in
good yield.11 In most cases, the presence of a cation or
Lewis acid seems to be necessary to facilitate the cleavage
of epoxides.12 On the other hand, use of a phase-transfer
catalyst13 and polymer-supported azide systems14 has also
been reported. However, the reported methodologies are
In this paper, we report the application of hypervalent sil-
icates to an epoxide ring-opening reaction and the prepa-
ration of Boc-statine (1).
The representative results for -hydroxy azide are sum-
marized in Table 1. As shown, the ring-opening reaction
of benzyl glycidol and methyl glycidol with TBAF and
TMSN3 occurred at the endo-position to give 3-azido-1-
benzyloxy- and 3-azido-1-methoxypropan-2-ols in 79%
and 35% yields, respectively (entries 1, 2). Similarly, the
reaction of benzoyl-protected glycidol also proceeded
cleanly to afford 3-azido-1-benzoyloxypropan-2-ol in
87% yield (entry 3). Though the reaction of epichlorohy-
drin contained two reaction positions under the same con-
dition to give a complex mixture, treatment of
epichlorohydrin with 3 equivalents of TBAF and TMSN3
in THF reflux afforded 1,3-diazidopropan-2-ol in 76%
yield (entry 4). On the other hand, treatment with one
equivalent of TBAF and TMSN3 in THF at room temper-
ature for 12 hours gave 1-azido-3-chloropropan-2-ol as a
single isomer in 66% yield. It is noteworthy that the ep-
oxide-opening of these reactions occurred with faster re-
gioselectivity than leaving of chloride ion and the
corresponding compound would be a very useful chiral
building block (entry 5). Treatment of styrene oxide with
Synthesis 2003, No. 14, Print: 02 10 2003. Web: 22 08 2003.
Art Id.1437-210X,E;2003,0,14,2161,2164,ftx,en;F04303SS.pdf.
DOI: 10.1055/s-2003-41049
© Georg Thieme Verlag Stuttgart · New York

2162
H. Konno et al.
PAPER
silicate gave the corresponding 1-azido-2-phenyl-2-etha- unfortunately the chemical yield was about 10% due to
nol in 75% yield as a major product. In this case, ring- poor reactivity (entry 10). In all cases, these yields were
opening occurs at the less hindered position (entry 6). On moderate because the water contained in TBAF solution
the other hand, the reaction of 2,3-epoxy-3-phenylpro- gave diols as by-products in ca. 2–20% yields. (Table 1)
panol afforded the corresponding 3-azide-3-phenylpro-
Next, we attempted the convertion of epoxides to -hy-
pane-1,2-diol in 58% yield (entry 7). Treatment of
droxy cyanides. As shown in Table 2, hypervalent silicate
cyclohexene oxide with silicate for 2 hours gave anti-azi-
cyanide was similarly effective for epoxy ring-opening re-
action. Benzyl and methyl glycidol with TBAF and
dohydrin in 96% yield. The same transformation was car-
ried out in the presence of TMSN3 in DMF, but it took 50
TMSCN in MeCN at 82 °C for 2 hours afforded -hydroxy
hours.3 The observed dramatic acceleration of the reaction
cyanides in 78 and 40% yields, respectively (entries 1, 2).
rate was presumed to be mostly due to combination with
Under the same conditions, benzoyl glycidol afforded the
TBAF. Also, that an SN2 process leading to inversion of
corresponding 3-benzoyloxy-2-hydroxy-1-propanenitrile
configuration is involved in this reagent is evident from
in poor yield (entry 3). Though the regioselectivity of ep-
the result (entry 8). In addition, reaction of the complex
ichlorohydrin with silicate cyanide failed to give a com-
compound with chiral centers proceeded smoothly to give
plex mixture, use of 3 equivalents of the reagent at 82 °C
the desired -hydroxy azide in 47% (entry 9). Further-
afforded the corresponding 1,3-dicyanopropan-2-ol in
more, reaction of 2,3-epoxy alcohol proceeded at the C-2
76% yield (entries 4, 5). As for styrene oxide, 2,3-epoxy-
position to give 2-azidoheptadecan-1,3-diol. However,
3-phenylpropanol, and cyclohexene oxide, these reactions
proceeded to give the desired compounds in the same
manner of azide displacement (entries 6–8) (Table 2).
Table 1 Reaction of Epoxides with Tetrabutylammonium Trimeth-
ylfluoroazidosilicates in THF
Table 2 Reaction of Epoxides with Tetrabutylammonium Trimeth-
ylfluorocyanidosilicates in THF
En Substrate
try
Producta
TempTimeYield
(°C) (h) (%)b
Entry Substrate
Producta
Temp Time Yield
(°C) (h) (%)b
1
67
67
67
67
2
2
2
2
79
35
87
76
66
75c
OH
O
BnO
BnO
MeO
BzO
N3
N3
N3
N3
1
82
82
82
82
r.t.
82
2
2
2
2
78
40
11
76
O
O
O
OH
BnO
2
O
OH
OH
BnO
MeO
BzO
NC
CN
CN
CN
MeO
2
OH
OH
MeO
3
O
BzO
3
BzO
4
O
OH
Cl
N3
4
5
6
O
O
OH
Cl
Cl
5
O
r.t. 12
OH
CN
Cl
Cl
N3
12 <20
OH
6
67
67
67
2
6
2
O
OH
Cl
CN
N3
6
74
O
O
OH
CN
(84:16)
N3
7
58c
(80:20)
CN
O
54c
OH
7
8
82
82
12
OH
OH
OH
(98:2)
OH
OH
8
96
47
N3
(89:11)
O
6
85
CN
OH
O
9
OMOM
OEE
67 12
OH
N3
a Structures and regiochemical ratio were determined for the products
by 1H NMR spectroscopy.
C12H25
OH
O
OEE
O
b Yields refer to isolated products.
10
67 48 <10
OH
c Two regioisomers were obtained and the yield refers to both iso-
mers.
C14H29
OH
C14H29
OH
N3
a Structures and regiochemical ratio were determined for the products
by 1H NMR spectroscopy.
In addition, we attempted to synthesize (3S,4S)-Boc-sta-
tine (1) (Scheme 1). The synthesis began with the well-es-
tablished protection, amidation, and reduction procedure
for L-leucine to affored the Boc-leucinal (2) in three steps
b Yields refer to isolated products.
c Two regioisomers were obtained and the yield refers to both iso-
mers.
Synthesis 2003, No. 14, 2161–2164 © Thieme Stuttgart · New York

PAPER
Epoxide Ring-Opening via Hypervalent Silicate Intermediate
2163
and 65% yield. Epoxidation using Corey–Chaykovsky22
reaction to Boc-leucinal (2) afforded a 8:1 ratio of the de-
sired epoxy adduct 3 in 73% yield. An epoxy-ring opening
reaction of 3 with TMSCN/TBAF in THF condition pro-
ceeded smoothly to give regioselectively the desired hy-
droxy cyanide 4 in 90% yield. It is worth noting that the
epoxide-opening protocol (KCN/DMSO, TMSCN/Lewis
acids, furyllithium/Lewis acids, etc.) was not effective in
this case, and gave a mixture of unidentified products.
Acidic hydrolysis of 4 provided the desired Boc-statine
(1) with concomitant deprotected compound, statine. Sub-
sequently, the resulting statine was protected with di-tert-
butyl dicarbonate to give Boc-statine (1) in 93% yield.
with EtOAc. The EtOAc layer was washed with brine, dried
(MgSO4), and concentrated in vacuo to give the crude product. Pur-
ification of hydroxy azides or nitriles formed was effected by chro-
matography on silica gel (Tables 1 and 2). Product characterization
was performed by IR, 1H, 13C NMR, and MS spectroscopy.
-N-tert-Butyloxycarbonyl-l-leucinal (2)
To a stirred solution of L-leucine (15.0 g, 114 mmol) in H2O was
added (Boc)2O (49.9 g, 229 mmol) and NaOH (9.15 g, 229 mmol)
at 0 °C and then warmed to r.t. After stirring for 68 h, the mixture
was diluted with EtOAc and 1 N HCl, and extracted with EtOAc.
The EtOAc extract was washed with brine, dried (MgSO4), and con-
centrated in vacuo to give Boc-leucine (26.5 g, 96%), which was
immediately used in the next reaction. To a stirred solution of Boc-
leucine (5.77 g, 25.0 mmol) in CH2Cl2 was added Et3N (3.46 mL,
Spectral data of this compound were in good agreement 25.0 mmol), BOP-Cl [bis(2-oxo-3-oxazolidinyl)phosphinic chlo-
with those previously reported.21
ride, 10 g, 23.0 mol], and O,N-dimethylhydroxylamine hydrochlo-
ride (2.68 g, 25.0 mmol) at 0 °C. After stirring for 1 h, the mixture
was diluted with EtOAc.,The EtOAc extract was washed with 1 N
HCl, sat. aq NaHCO3 and brine, and dried (MgSO4). The solvent
was removed in vacuo and the residue was chromatographed on sil-
ica gel (4:1 hexane–EtOAc) to give the corresponding amide (5.00
g, 73%) as a colorless oil.
O
H
N
a, b, c
d
Boc
H
L-Leucine
65%
73%
2
Intermediate Boc-Leucine Amide
OH
H
N
O
H
IR (neat): 3323, 2956, 1712, 1661, 1390, 1366, 1171, 953 cm–1.
N
CN
e
Boc
Boc
1H NMR: = 0.92–0.97 (d, 6 H, J = 6.6 Hz), 1.43 (s, 9 H), 1.24–
1.59 (m, 2 H), 1.70–1.74 (m, 1 H), 3.20 (s, 3 H), 3.79 (s, 3 H), 4.72
(s, 1 H), 5.03–5.05 (d, 1 H, J = 8.3 Hz).
90%
3
4
To a stirred solution of the above amide (19.0 g, 69.2 mmol) in Et2O
was added LiAlH4 (3.28 g, 246 mmol) at 0 °C. After stirring for 1
h, the mixture was quenched with H2O, diluted with Et2O. The Et2O
layer was washed with aq sat. NaHCO3 solution and brine, dried
(MgSO4), concentrated in vacuo, and chromatographed on silica gel
(hexane–EtOAc, 10:1) to give Boc-leucinal (2) (13.8 g, 93%) as a
colorless oil.
OH
O
H
f
N
Boc
OH
93%
Boc-statine (1)
IR (neat): 2958, 2360, 1696, 1508, 1366, 1251, 1169, 1046, 500
cm–1.
1H NMR: = 0.93–0.98 (d, 6 H, J = 6.6 Hz), 1.45 (s, 9 H), 1.19–
1.77 (m, 3 H), 4.24 (s, 1 H), 4.90 (s, 1 H), 9.59 (s, 1 H).
13C NMR: = 22.0, 23.1, 28.3, 38.1, 65.8, 80.0, 155.6.
Scheme 1 Synthesis of Boc-statine (1). Reagents and conditions:
(a) (Boc)2O, aq NaOH, 1,4-dioxane; (b) BOP, NH(OMe)Me, Et3N,
CH2Cl2; (c) LiAlH4, THF, 0 °C; (d) Me3S+I–, NaH, DMSO; (e) TBAF,
TMSCN, THF, 60 °C, 12 h; (f) 6 N HCl, sealed tube, 120 °C, 8 h, then
(Boc)2O, NaHCO3, MeOH
HRMS: m/z calcd C11H22NO3 (M+ + H): 216.1600; found:
216.1604.
In conclusion, we have demonstrated hypervalent silicates
as an intermediate of azide and cyanide opening reaction
of epoxide under extremely mild conditions. In addition,
we synthesized Boc-statine (1) in six steps from L-leucine.
(2S,3S)-1,2-Epoxy-3-[(tert-butoxycarbonyl)amino]-5-methyl-
hexane (3)
To a suspension of NaH (60% in oil, 65 mg, 1.63 mmol) in DMSO
(2 mL) was added trimethyloxosulfonium iodide (333 mg, 1.63
mmol) at r.t. After stirring for 30 min, Boc-leucinal (2; 175 mg,
0.813 mmol) in DMSO was added to the mixture. After stirring for
1 h, the mixture was diluted with Et2O, the Et2O layer was washed
with H2O and brine, and dried (MgSO4). The solvent was evaporat-
ed in vacuo and the residue was purified by chromatography on sil-
ica gel (hexane–EtOAc, 6:1) to afford 3 (185 mg, 73%) as a
colorless oil; [ ]D26 –0.08 (c = 0.69, CHCl3).
1
All reactions were carried out under N2. H and 13C NMR spectra
were recorded at 400 and 100 MHz on a Jeol JNM-EX-400 spec-
trometers, respectively. The chemical shifts are reported in ppm ( )
relative to TMS in CDCl3. IR spectra were recorded in cm–1 on a
Jasco Valor-III or Perkin-Elmer Model 1600 spectrophotometer.
Mass spectra were recorded on a Jeol JMS-AX-500 or JMS-DX-
303 spectrometer. Optical rotations were measured with a Jasco-
DIP-370 digital polarimeter. C,H,N-analyses were recorded on a
Yanaco CHN Corder MT-5 analyzer.
IR (neat): 2948, 2917, 1715, 1692, 1516, 1465, 1450, 1388, 1362,
1243, 1166, 1053, 873 cm–1.
1H NMR: = 0.90–0.96 (d, 6 H, J = 6.5 Hz), 1.43 (s, 9 H), 1.35–
1.45 (m, 2 H), 1.70–1.77 (d, 1 H, J = 6.5 HZ), 2.59 (s, 1 H), 2.72–
2.76 (m, 1 H), 2.99 (s, 1 H), 3.97 (s, 1 H), 4.31 (br s, 1 H).
13C NMR: = 22.1, 23.0, 28.3, 42.3, 44.3, 47.2, 53.9, 79.2, 155.5.
HRMS: m/z calcd C12H24NO3 (M+ + H): 230.1757; found:
Epoxide Ring-Opening Reactions; General Procedure (Tables 1
and 2)
The appropriate epoxide (1.0 mmol) was dissolved in THF (5 mL)
at 25 °C. Trimethylsilyl azide (1.5 mmol, 173 mg) or trimethylsilyl
cyanide (1.5 mol, 149 mg) was added via a syringe followed by
TBAF (1.0 M solution in THF, 1.5 mmol, 1.5 mL). The solution was
stirred for 2 h at 67 °C. The reaction was cooled to r.t., extracted
230.1755.
Synthesis 2003, No. 14, 2161–2164 © Thieme Stuttgart · New York

2164
H. Konno et al.
PAPER
Anal. Calcd for C12H23NO3 (229.32): C, 62.85, H, 10.11, N, 6.11.
Found: C, 62.66, H, 9.96, N, 6.13.
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(3S,4S)-4-[(tert-Butoxycarbonyl)amino]-3-hydroxy-6-methyl-
heptanonitrile (4)
To a stirred solution of 3 (152 mg, 0.661 mmol) in THF (2 mL) was
added TMSCN (0.132 mL, 0.992 mmol) and TBAF (1.0 M solution
in THF, 0.289 mL, 0.992 mmol) at r.t. and then warmed to 60 °C.
After stirring for 12 h, the mixture was diluted with EtOAc, the
EtOAc layer was washed with brine, and dried (MgSO4). The or-
ganic phase was evaporated in vacuo and the residue was purified
by chromatography on silica gel (hexane–EtOAc, 2:1) to afford 4
(153 mg, 90%) as a colorless oil; [ ]D23 –19.6 (c = 0.72, CHCl3).
IR (neat): 3370, 2953, 2868, 2248, 1686, 1510, 1451, 1387, 1360,
1252, 1167, 1080, 1053, 873, 781, 570 cm–1.
1H NMR: = 0.85–0.88 (d, 6 H, J = 6.5 Hz), 1.17–1.29 (m 2 H),
1.37 (s, 9 H), 1.57–1.61 (t, 1 H, J = 6.5 Hz), 2.44–2.51 (m, 2 H),
3.55 (br s, 1 H), 3.66 (s, 1 H), 3.87 (s, 1 H), 4.71–4.73 (d, 1 H,
J = 8.1 Hz).
13C NMR: = 22.0, 23.1, 23.7, 24.9, 28.3, 40.8, 52.3, 69.6, 80.0,
118.2, 156.3.
HRMS: m/z calcd C13H25N2O3 (M+ + H): 257.1866; found:
257.1868.
Anal. Calcd for C13H24N2O3 (256.35): C, 60.91, H, 9.44, N, 10.93.
Found: C, 60.70, H, 9.23, N, 6.13.
(3S,4S)-4-[(tert-Butoxycarbonyl)amino]-3-hydroxy-6-methyl-
heptanoic Acid (Boc-Statine, 1)
To a stirred solution of 4 (153 mg, 0.597 mmol) in THF (1 mL) was
added 6 N HCl (2 mL) at r.t. and the mixture was heated to 120 °C.
After stirring for 12 h, the mixture was cooled to r.t., and then evap-
orated in vacuo. To a stirred solution of the crude residue in MeOH
(2 mL) was added (Boc)2O and aq 2 N NaHCO3 (2 mL) at r.t. After
stirring for 2 h, the mixture was diluted with EtOAc and 1 N HCl
and extracted with EtOAc. The organic phase was dried (MgSO4)
and evaporated in vacuo. The residue was purified by chromatogra-
phy on silica gel (hexane–EtOAc, 2:1) to afford Boc-statine (1) (146
mg, 93%) as a colorless oil. The spectral data were identical with
those reported previously.21
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Acknowledgments
We thank Prof. Hitoshi Hori (University of Tokushima) for useful
discussion and Prof. Takumichi Sugihara (Tokushima-bunri Uni-
versity) and Prof. Hiromasa Kiyota (Tohoku University) for measu-
rement of mass spectra. This work was supported by a Grant-in-Aid
for Scientific Research (14771242) from the Ministry of Education,
Culture, Sports, Science, and Technology, Japan and The Japan Sci-
ence Society (14-337) in Japan.
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