2-Azidoethoxy derivatives of 2-aminocyclohexanecarboxylic acids (ACHC): Interesting building blocks for the synthesis of cyclic β-peptide conjugates

Article abstract of DOI:10.1016/j.tet.2011.05.131

Oligomers of cyclic-β-aminoacids possess a high resistance to peptidase-catalyzed hydrolysis and display a high intrinsic tendency to adopt regular secondary structures. These characteristics are attractive to develop new biologically active substances. H

Full text of DOI:10.1016/j.tet.2011.05.131

Tetrahedron 67 (2011) 5770e5775
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2-Azidoethoxy derivatives of 2-aminocyclohexanecarboxylic acids (ACHC):
interesting building blocks for the synthesis of cyclic b-peptide conjugates
*
ꢀ
Jose J. Reina, Anna Bernardi
Universita’ degli Studi di Milano, Dipartimento di Chimica Organica e Industriale and ISTM-CNR, via Venezian 21, 20133 Milano, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
Oligomers of cyclic-
a high intrinsic tendency to adopt regular secondary structures. These characteristics are attractive to
develop new biologically active substances. However, cyclic- -peptides often show limited solubility in
water and cannot be conjugated to biologically relevant fragments, such as oligosaccharides, which are
often essential for full biological activity of natural -peptides. In this article, we report the synthesis of
one trans- and one cis-2-aminocyclohexanecarboxylic acid (ACHC), both functionalized with a hydroxy
b-aminoacids possess a high resistance to peptidase-catalyzed hydrolysis and display
Received 10 March 2011
Received in revised form 12 May 2011
Accepted 31 May 2011
b
Available online 12 June 2011
a
Keywords:
group, to increase the solubility in water, and an azidoethoxy group to allow the synthesis of cyclic-b-
peptide conjugates by a ‘click reaction’.
b-Aminoacids
Conjugates
Epoxidation
Ó 2011 Elsevier Ltd. All rights reserved.
Stereoselective synthesis
1. Introduction
to allow conjugation of
molecules to additional elements. For instance,
b
-peptides or other
b
-aminoacid-containing
-peptides could be
b
In the past few years, oligomers of
b
-aminoacids have become
connected to biologically relevant fragments, such as oligosaccha-
rides, which are often essential for full biological activity of natural
a subject of considerable interest.1e5 These unnatural peptides
possess a high resistance to peptidase-catalyzed hydrolysis6,7 and
often present discrete and predictable folding propensities
a-peptides. In our glycomimetic research program, functionalized
cyclic -aminoacids are required to achieve polyvalent presentation
b
(foldamers).8e11 Usually,
b
-peptides with cyclic
b-aminoacid resi-
of derived oligosaccharide mimics on dendrimers and other poly-
meric scaffolds, a topic of current high interest in the field of car-
bohydrate mimicry.19,20
dues display a higher intrinsic tendency to adopt a regular sec-
ondary structure (helix, sheet and turn) than acyclic residues
do.12e15 However, up to now, most of these results have been
Herein, we report the practical synthesis of enantiomerically
pure and orthogonally protected derivatives of trans- and cis-2-
aminocyclohexanecarboxylic acids (ACHC) 2 and 3, functionalized
in the cyclohexane ring with a hydroxy group, to increase solubility
in water, and with a functional 2-azidoethyl linker to be used as
a conjugation handle. Azides give access to various efficient ap-
proaches for bioconjugation,21 including the 1,3-dipolar cycload-
dition known as the ‘click reaction’.22
obtained in organic solvents, because cyclic
resulting peptides often show a limited solubility in water. Only
some examples of -peptides with a helical conformation in
b-aminoacids and the
b
aqueous solution have been obtained by modification of the cyclic
framework with appropriate hydrophilic substituents.12
Cyclic-b-aminoacids have also been exploited as scaffolds in the
synthesis of glycomimetic compounds. Our group demonstrated
the use of (1S,2R)-2-aminocyclohexanecarboxylic acid as a scaffold
to prepare a mimic of Lewis-x trisaccharide where sugars or sugar-
like fragments are connected avoiding glycosidic bonds.16 These
modifications of the oligosaccharide structure are used to improve
metabolic stability and activity and to simplify the synthesis of the
final glycomimetic compounds.
2. Results and discussion
Compounds 2 and 3 were prepared in a few synthetic steps from
commercially available tetrahydrophthalic anhydride 1 through the
known protected b
-cyclohexenecarboxylic acids 423 and 6.24 Key to
Introduction of additional substituents on cyclic b-aminoacid
our strategy was the stereoselective synthesis of epoxides 5 and 7
and their regio- and stereoselective opening by metal-catalyzed
alcoholysis (Scheme 1).
frameworks has also been actively sought after.17,18 Beside im-
proving (water) solubility, additional substituents can be exploited
€
The synthesis of 2 (Scheme 2) began with Bolm desymmetri-
zation25,26 of 1 in the presence of quinidine. The resulting cis-hemi-
ester 8 (93% ee) was epimerized to the trans-isomer 9 using
* Corresponding author. Tel.: þ39 02 5031 4092; fax: þ39 02 5031 4072; e-mail
address: anna.bernardi@unimi.it (A. Bernardi).
0040-4020/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2011.05.131

J.J. Reina, A. Bernardi / Tetrahedron 67 (2011) 5770e5775
5771
Scheme 1. General strategy for the synthesis of functionalized cyclic b-aminoacids 2 and 3.
Scheme 2. Stereoselective synthesis of 2.
potassium t-amylate at ꢀ15 ꢁC, as described by Yue et al.,23 to afford
a 5.5:1 trans:cis mixture. Finally, the trans-isomer 9 was selectively
crystallized (Et2O/hexane) as its (R)-a-methylbenzylamine salt 10,
which improved the trans:cis ratio to 11:1, and the trans-hemi-ester
9 was liberated from salt 10 by treatment with HCl (1 N) and
extracted into AcOEt. The entire sequence could be performed
without any chromatographic purifications and yielded 9 with 80%
overall yields from 1. Curtius rearrangement of 9 and conversion of
the resulting isocyanate to a tert-butylcarbamate via reaction with
For the synthesis of the (1S,2R) cis isomer 3 (Scheme 3), Curtius
rearrangement of hemi-ester 8 under the conditions described
above afforded the protected cis amino acid 6,24 which was oxi-
dized with MCPBA to give epoxide 4 in 63% yield, as a single
isomer. The structure of epoxide 4 could not be fully determined
by NMR spectroscopic analysis, but the formation of a single iso-
mer likely results from H-bonding interaction of MCPBA with the
carbamate functionality.28 Thus, the structure of 7 was tentatively
attributed, and later confirmed upon analysis of the ring-opening
products.
tert-butyl alcohol and CuCl gave the protected
b-amino-
cyclohexenecarboxylic acid derivative 4 (65%).24e27 Remaining
traces of the 1,2-cis isomer were chromatographically removed at
this stage. MCPBA oxidation of
4 proceeded to afford the
1. DPPA, Et3N
Toluene, reflux
(1R,2R,4S,5R) epoxide 5. A single isomer was isolated, featuring the
epoxide ring cis to the carbamate moiety, as expected28 and pre-
viously described for the corresponding ethyl ester.17 No trace of
the 4R,5S isomer could be identified by 1H NMR spectroscopy.
Regioselective alcoholysis of 5 occurred uneventfully, using 20%
Cu(OTf)2 in chloroethanol,29 and afforded 11 in quantitativeyield. The
position of substituents and the relative configuration of the stereo-
centers in 11 could be unequivocally established by NMR spectro-
scopic analysis and are those expected by trans-diaxial opening of
a single chair-like conformation of 5, featuring carbomethoxy and
amino groups in a trans-diequatorial disposition. Protection of the
hydroxyl group is not necessary at this point, but it was performed
(TBDMSOTf, lutidine, 97%) for the sake of obtaining the final com-
pound in a fully orthogonally protected form. Finally, treatment of 12
COOMe
COOH
COOMe
NHBoc
2. tert-BuOH, CuCl
Toluene, 80ºC
65%
6
8
MCPBA
CH2Cl2
63%
COOMe
NHBoc
O
7
with NaN3 afforded the required functionalized (1R,2R) trans-
b-
Scheme 3. Synthesis of epoxide 7.
aminoacid synthon 2 in quantitative yield (Scheme 2).

5772
J.J. Reina, A. Bernardi / Tetrahedron 67 (2011) 5770e5775
Table 1
Alcoholysis of 7 is complicated by the conformational flexibility
Alcoholysis of 7 in chloroethanola
of the six-membered ring, which can attain two conformations of
similar energy, A and B (Fig. 1). Based on the trans-diaxial re-
quirement of epoxide opening reactions, these conformers are
expected to react with opposite regioselectivity: conformer A
should undergo C5-opening yielding the chloroethyl ether 13 and
conformer B should react preferentially at C4 affording ether 14
(Fig. 1 and Scheme 4). Molecular mechanics (MM2*) calculations
predict that the C5-opening product would exist as an equilibrium
mixture of two chair conformations 13-c1 and 13-c2. On the con-
trary, the C4-opening compound 14 is expected to adopt mainly
conformation 14-c2, featuring the carbomethoxy group in equa-
torial position. Although somewhat unexpectedly on the basis of A
parameters,30 a marked conformational bias for equatorial carbal-
koxy group over equatorial N-carbamate was previously observed
in cis-2-aminocarbalkoxy-cyclohexanecarboxylic acid esters.16
Entry
Catalyst
% Cat
T (ꢁC)
14/13 Ratiob
14 (%)c
1
2
3
4
5
6
7
BF3$Et2O
La(OTf)3
Zn(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
1 equiv
10
10
5
10
25
25
25
25
25
25
40
d
No rxn
Not isol.
Not isol.
55
60
65
1.2/1
1/1.3
1.4/1
1.5/1
2.2/1
2.3/1
20
20
74
a
Reactions were performed in chloroethanol, for 3 h at the temperature
indicated.
b
c
By 1H NMR spectroscopy of crude reaction mixtures.
Isolated yield.
The selectivity appears to slightly increase by increasing the
amount of catalyst from 5% to 20%. The best results were achieved
using 20% Cu(OTf)2 at 40 ꢁC (Table 1, entry 7): under these condi-
tions, the global yield of chloroethyl ethers was almost quantitative,
which gave 14 in a satisfactory 74% yields after chromatography. No
further selectivity increase was observed with higher catalyst
loading.
1H NMR spectroscopic analysis of the chloroethyl ether products
allowed to validate the computational predictions concerning the
conformational behavior of these cyclic b-aminoacids. Diagnostic
data were obtained from the coupling constants of proton H1,
which appeared as a quartet with J 4.4 Hz in 13 and as a doublet of
triplets with J 9.3 Hz and 4.1 Hz in 14. This is consistent with the
prediction that 13 exists as a pair of interconverting chairs, while 14
populates one main chair conformation with the carbomethoxy
group in equatorial position.
As for the 1,2-trans isomer, the free hydroxyl group of 14 was
protected as its silyl ether (Scheme 5, 95% yield) and, finally, chloro-
azide exchange in the linker was achieved using excess NaN3 in
DMF at 50 ꢁC, to obtain 3 in quantitative yield. The same sequence
on the minor isomer 13 afforded in similar yield the regioisomeric
derivative 16 (Scheme 5).
Fig. 1. Conformations A and B of 7, as calculated by molecular mechanics (MM2*), their
preferred opening pathways and conformational equilibria of the C5- and C4-opening
products 13 and 14.
O
COOMe
NHBoc
Cl
HO
3. Conclusions
13
Cl
COOMe
NHBoc
OH
Cu(OTf)2
In conclusion, in this work we have established a practical
synthesis of enantiomerically pure 1,2-trans and 1,2-cis-2-
aminocyclohexanecarboxylic acid (ACHC) derivates 2 and 3 fea-
turing a hydroxyl group and a versatile 2-azidoethyl linker. The
regioisomeric 1,2-cis compound 16 was also prepared, as a minor
isomer of 3. All compounds were prepared in the 2R series using
O
7
HO
O
COOMe
NHBoc
Cl
€
Bolm desymmetrization of tetrahydrophthalic anhydride 1 with
14
quinidine. The enantiomeric 2S series can be prepared using qui-
nine in the initial step.25 We have also shown that both 2 and 3 are
conformationally well-defined structures, populating a single (2) or
a major (3) chair conformation. The conformational equilibrium of
3 appears dominated by an apparent bias of the carbomethoxy
group to occupy the equatorial position preferentially over the N-
carbamate group.16 All the compounds prepared are orthogonally
protected and can be used in combination with other ACHC de-
Scheme 4. Alcoholysis of 7.
The alcoholysis of epoxides can be catalyzed by a number of
Lewis acids and some of these were examined in order to achieve
maximum yield and regioselectivity in the 2-chloroethanol open-
ing reaction of 7 (Table 1). We focused our effort on optimizing the
C-4 opening process, since it is predicted to afford a product (14)
with reduced conformational freedom.
No reaction was obtained without a catalyst or using BF3$Et2O as
the promoter (entry 1). In contrast, good reactivity was obtained in
chloroethanol at room temperature using various metal triflates
(entries 2e6), which are known catalysts for alcoholysis of epox-
ides.29 The reaction showed little regioselectivity, however the two
ether products proved easily separable by flash chromatography
and could be fully characterized, based on the different 13C chem-
ical shifts of C4 and C5, which are found, respectively, at 71.1 and
79.8 in 13 and 79.3 and 70.2 in 14. This set of experiments allowed
to select Cu(OTf)2 as the promoter of choice for the synthesis of 14.
rivatives, as building blocks to construct
water solubility. The azido group can also be used as a tether to
conjugate different residues to the -peptides like in natural pro-
-aminoacids.
b-peptides with improved
b
teins or peptides composed of
a
4. Experimental section
4.1. Material and methods
Solvents were dried by standard procedures: dichloromethane,
methanol, N,N-diisopropylethylamine and triethylamine were

J.J. Reina, A. Bernardi / Tetrahedron 67 (2011) 5770e5775
5773
Scheme 5. Synthesis of the 1,2-cis functionalized b-aminoacids.
dried with calcium hydride; chloroform and pyridine were dried
with activated molecular sieves. Reactions requiring anhydrous
conditions were performed under nitrogen.1H and 13C NMR spectra
were recorded at 400 MHz with a Bruker AVANCE-400 instrument.
J 6.6 Hz, NHBoc) 4.03 (br s, 1H, H2), 3.69 (s, 3H, OCH3), 3.27 (t, 1H, J
3.6 Hz, H5), 3.18 (t, 1H, J 3.1 Hz, H4), 2.65 (q, 1H, J 5.9 Hz, H1), 2.35
(td, 1H, J 15.7, 4.0 Hz, H6eq), 2.26e2.14 (m, 2H, H3eq, H6ax), 1.92 (dd,
1H, J 15.7, 5.7 Hz, H3ax), 1.43 (s, 9H, CH3Boc); 13C NMR (100 MHz,
Chemical shifts (
d
) for the 1H and 13C NMR spectra are expressed in
CDCl3) d (ppm) 173.6 (C]O), 154.9 (C]O), 52.0 (CH3O), 51.7 (C4),
parts per million relative to internal Me4Si as standard. Signal
multiplicities are abbreviated as follows: s, singlet; br. s, broad
singlet; d, doublet; t, triplet; q, quartet; m, multiplet. Mass spectra
were obtained with a Bruker ion-trap Esquire 3000 (ESI ionization)
or Autospec Fission spectrometer (FAB ionization) and FT-ICR Mass
Spectrometer APEX II & Xmass 4.7 Magnet software (Bruker Dal-
tonics). Thin-layer chromatography (TLC) was carried out on pre-
coated Merck F254 silica gel plates. Flash chromatography (FC) was
carried out on MachereyeNagel silica gel 60 (230e400 mesh).
51.1 (C5), 45.9 (C2), 41.2 (C1), 28.5 (C3), 28.3 (CH3Boc), 23.8 (C6); ESI-
MS for C13H21NO5 calcd Mþ 271.1 exptl 270.9 Mþ & 294.2 (MþNa)þ;
HRMS (ESI): (MþNa)þ, found 294.13142 C13H21NO5Na requires
294.13119.
4.2.3. Methyl (1R,2R,4R,5R)-2-(N-tert-butoxycarbonylamine)-4-(2-
chloroethoxy)-5-hydroxycyclohexanecarboxylate (11). To a solution
of 5 (130 mg, 0.48 mmol) in 2-chloroethanol (2 mL) was added
a catalytic amount of Cu(OTf)2 (40 mg, 0.01 mmol) and the solution
was stirred at room temperature under N2 atmosphere for 3 h. The
reaction was monitored by TLC (hexane/AcOEt, 2:1). A solution of
NH4Cl/NH3 aq (1:1) (30 mL) was added to the reaction mixture.
Then, the solution was extracted with AcOEt (2ꢂ30 mL). The or-
ganic phases were dried over Na2SO4 anhyd, and the solvent was
eliminated in vacuo to obtain 11 (160 mg, 95%) without further
4.2. Synthesis
4.2.1. Methyl (1R,2R)-2-(N-tert-butoxycarbonylamine)cyclohex-4-
enecarboxylate (4). A solution of 923 (600 mg, 3.26 mmol) in dry
toluene (6 mL) was treated with Et3N (550
mL, 2.20 mmol) and
DPPA (770
m
L, 3.42 mmol). The solution was heated slowly at 80 ꢁC,
purification as a transparent oil. [a 20
(400 MHz, CDCl3) d (ppm) 4.52 (br s, 1H, NHBoc) 3.99 (m, 1H, H2),
]
ꢀ16.7 (c 0.4, CHCl3); 1H NMR
D
kept at this temperature until evolution N2 ceased, and refluxed for
3 h. Then, the solution was cooled at room temperature and t-BuOH
(1.67 mL, 16.30 mmol) and CuCl (15 mg, 0.13 mmol) were added
and the reaction was stirred in reflux overnight. Then, the reaction
mixture was cooled and room temperature and washed with
NaHCO3 satd (2ꢂ50 mL). The aqueous phases were extracted with
Et2O (50 mL). The combined organic phases were dried over
Na2SO4 and concentrated under reduced pressure. The residue was
purified by flash silica gel column chromathography using as eluent
(hexane/AcOEt, 7:1) to obtain 4 (541 mg, 65%) as a transparent oil.
3.86e3.76 (m, 2H, CH2O, H5), 3.62 (s, 3H, OCH3), 3.65e3.53 (m, 3H,
CH2O, CH2Cl), 3.44e3.38 (dt, 1H, H4), 2.70 (br dt, 1H, J 8.8, 4.1 Hz,
0
H1), 2.24e2.12 (m, 1H, H6), 1.97e1.66 (m, 3H, 2H3, H6 ), 1.36 (s, 9H,
CH3Boc); 13C NMR (100 MHz, CDCl3)
d (ppm) 173.6 (C]O),154.9 (C]
O), 78.4 (C4), 69.3 (CH2O), 67.4 (C5), 52.0 (OCH3), 43.6 (C1), 43.2
(CH2Cl), 31.1 (C3), 30.0 (C6), 28.3 (CH3Boc); ESI-MS for C15H26ClNO6
calcd Mþ 351.1 exptl 374.2 (MþNa)þ; HRMS (ESI): (MþNa)þ, found
374.13380C15H26NO6ClNa requires 374.13409.
[
a 20
]
ꢀ26.6 (c 1.00, CH2Cl2); 1H NMR (400 MHz, CDCl3)
d
(ppm)
4.2.4. Methyl (1R,2R,4R,5R)-2-(N-tert-butoxycarbonylamine)-4-(2-
chloroethoxy)-5-(tert-butyldimethylsilanyloxy)cyclohexane carboxyl-
ate (12). To a solution of 11 (43 mg, 0.122 mmol) and 2,6-lutidine
D
5.71e5.52 (m, 2H, H4, H5), 4.61 (br s, 1H, NHBoc), 4.02(br s, 1H, H2),
3.68 (s, 1H, COOMe), 2.68 (dd, 1H, J 14.3, 8.3 Hz, H1), 2.56e2.40 (m,
0
0
2H, H3, H6), 2.33e2.23 (m, 1H, H6 ), 2.00e1.90 (m, 1H, H3 ), 1.43 (s,
(28
(42
m
L, 0.245 mmol) in CH2Cl2 (300
mL) was added TBDMSOTf
9H, CH3Boc); 13C NMR (100 MHz, CDCl3)
d
(ppm) 174.0 (C]O), 155.0
mL, 0.183 mmol) and the solution was stirred at room temper-
(C]O), 125.0 (C3 or C4), 124.2 (C3 or C4), 51.9(COOMe), 47.25 (C2),
44.7 (C1), 31.2 (C3), 28.3 (CH3Boc), 26.6 (C6); ESI-MS for C13H21NO5
calcd Mþ 255.1 exptl 278.3 (MþNa)þ; HRMS (ESI): (MþNa)þ, found
278.13623C13H21NO4Na requires 278.13628.
ature under N2 atmosphere for 3 h. Then, H2O (5 mL) was added to
the reaction. The reaction mixture was extracted with AcOEt
(15 mL). The organic phase was dried over Na2SO4 anhyd, and the
solvent was eliminated in vacuo. The residue was purified by flash
silica gel column chromathography using as eluent (hexane/AcOEt,
4.2.2. Methyl (1R,2R,4S,5R)-2-(N-tert-butoxycarbonylamine)-4,5-
11:1) to obtain 12 (58 mg, 97%) as a transparent oil. [a 20
]
ꢀ8.2 (c
D
epoxycyclohexanecarboxylate (5). To
a
solution of
4
(170 mg,
0.95, CHCl3); 1H NMR (400 MHz, CDCl3)
d (ppm) 4.47 (br s, 1H,
0.67 mmol) in dry CH2Cl2 (3 mL) was added MCPBA (160 mg,
0.94 mmol) at 0 ꢁC. Then, the reaction mixture was warmed until
room temperature and stirred for 3 h. The reaction was monitored
by TLC (hexane/AcOEt, 2:1). The solution was diluted with CH2Cl2
(20 mL), washed with NaHCO3 satd (3ꢂ20 mL), dried over Na2SO4
anhyd and concentrated under reduced pressure. The residue was
purified by flash silica gel column chromatography using as eluent
NHBoc), 3.95 (br s, 1H, H2), 3.92e3.80 (m, 2H, CH2O, H5), 3.69 (s, 3H,
OCH3), 3.69e3.59 (m, 3H, CH2O, CH2Cl), 3.47e3.42 (m, 1H, H4), 2.61
(dt, 1H, J 11.9, 3.6 Hz, H1), 2.16e2.11 (m, H, H6), 2.07 (td, 1H, J 13.5,
3.7 Hz, H3),1.66 (td,1H, J 14.1, 3.5 Hz, H6), 1.63e1.55 (m, 1H, H3),1.37
(s, 9H, CH3Boc), 0.83 (s, 9H, SiC(CH3)3 t-Bu), 0.01 (s, 3H, SiCH3), 0.00 (s,
3H, SiCH3); 13C NMR (100 MHz, CDCl3)
d (ppm) 174.2 (C]O), 154.9
(C]O), 78.2 (C4), 69.2 (CH2O), 66.7 (C5), 51.9 (CH3O), 45.5 (C2), 44.1
(C1), 43.2 (CH2Cl), 31.3 (C6 or C3), 30.9 (C6 or C3), 28.4 (CH3Boc), 25.7
(SiC(CH3)3), ꢀ4.9 (SiCH3), ꢀ5.0 (SiCH3); ESI-MS for C21H40ClNO6Si
(hexane/AcOEt, 5:1) to obtain 5 (140 mg, 77%) as a white solid. [a 20
]
D
ꢀ35.5 (c 1.05, CHCl3); 1H NMR (400 MHz, CDCl3)
d
(ppm) 5.11 (d,1H,

5774
J.J. Reina, A. Bernardi / Tetrahedron 67 (2011) 5770e5775
calcd Mþ 465.2 exptl 488.4 (MþNa)þ; HRMS (ESI): (MþNa)þ, found
CDCl3) d (ppm) 155.0 (C]O), 79.8 (C5), 71.1 (C4), 69.5 (CH2O), 52.0
488.22031 C21H40NO6ClSiNa requires 294.22056.
(OCH3), 47.3 (C2), 43.4 (CH2Cl), 42.7 (C1), 34.2 (C3), 28.3 (C6), 28.3
(CH3Boc); ESI-MS for C15H26ClNO6 calcd Mþ 351.1 exptl 374.2
(MþNa)þ; HRMS (ESI): (MþNa)þ, found 374.13385 C15H26NO6ClNa
requires 374.13409.
4.2.5. Methyl
(1R,2R,4R,5R)-4-(2-aminoethoxy)-2-(N-tert-butoxy-
carbonylamine)-5-(tert-butyldimethylsilanyloxy) cyclohexancarbox-
ylate (2). To a solution of 12 (34 mg, 0.073 mmol) in DMF (1 mL)
were added NaN3 (38 mg, 0.58 mmol) and a catalytic amount of I2,
the reaction was stirred for 72 h at 50 ꢁC. Then, the solution was
diluted in CH2Cl2 (10 mL), washed with water (3ꢂ10 mL) and dried
over Na2SO4 anhyd. The residue was purified by flash chromatog-
raphy using as eluent (hexane/AcOEt, 8:1) to yield 2 (35 mg, quant.)
Compound 14: [a 20
CDCl3) d (ppm) 4.90 (br s, 1H, NHBoc) 4.23 (br s, 1H, H2), 3.88 (td, 1H,
]
D
ꢀ3.2 (c 1.05, CHCl3); 1H NMR (400 MHz,
J 7.7, 3.3 Hz, CH2O), 3.69 (s, 3H, OCH3), 3.69e3.60 (m, 3H, CH2O,
CH2Cl, H5), 3.39 (dt, 1H, J 9.0, 3.8 Hz, H4), 2.76 (dt, 1H, J 9.3, 4.1 Hz,
H1), 2.29 (ddd, 1H, J 13.2, 5.7, 3.9 Hz, H3), 2.15 (td, 1H, J 14.2, 4.3 Hz,
H6), 1.85 (td, 1H, J 14.2, 9.3 Hz, H6), 1.55 (ddd, 1H, J 13.1, 9.1, 3.7 Hz,
as a transparent oil. [a 20
]
ꢀ11.4 (c 1.25, CHCl3); 1H NMR (400 MHz,
H3), 1.43 (s, 9H, CH3Boc); 13C NMR (100 MHz, CDCl3)
d (ppm) 155.0
D
CDCl3)
d
(ppm) 4.46 (br s, 1H, NH), 4.00e3.88 (m, 1H, H2), 3.88 (dd,
(C]O), 79.3 (C4), 70.2 (C5), 69.4 (CH2O), 52.0 (OCH3), 46.7 (C2), 43.4
(CH2Cl), 42.8 (C1), 29.7 (C3), 29.0 (C6), 28.3 (CH3Boc); ESI-MS for
C15H26ClNO6 calcd Mþ 351.1 exptl 374.2 (MþNa)þ; HRMS (ESI):
(MþNa)þ, found 374.13377 C15H26NO6ClNa requires 374.13409.
1H, J 6.0, 3.3 Hz, H5), 3.88e3.78 (m, 1H, CH2O), 3.67 (s, 3H, OCH3),
3.60e3.51 (m, 1H, CH2O), 3.48e3.38 (m, 2H, H4, CH2N3), 3.33e3.23
(m, 1H, CH2N3), 2.60 (dt, 1H, J 12.2, 3.5 Hz, H1), 2.17e2.05 (m, 2H, H3,
H6), 1.76 (td, 1H, J 13.4, 3.3 Hz, H6), 1.67e1.56 (m, 1H, H3), 1.42 (s, 9H,
CH3Boc), 0.88 (s, 9H, SiC(CH3)3), 0.06 & 0.05 (2s, 6H, SiCH3); 13C NMR
4.2.8. Methyl (1S,2R,4S,5S)-2-(N-tert-butoxycarbonylamine)-4-(2-
chloroethoxy)-5-(tert-butyldimethylsilanyloxy)cyclohexane carboxyl-
ate (15). To a solution of 13 (130 mg, 0.37 mmol) and imidazol
(38 mg, 0.56 mmol) in DMF (1 mL) was added TBDMS-Cl (83 mg,
0.56 mmol) and the reaction was for 6 h at room temperature. Then,
the solution was diluted in CH2Cl2 (15 mL), washed with water
(3ꢂ10 mL) and dried over Na2SO4 anhyd. The residue was purified by
flash chromatography using as eluent (hexane/AcOEt, 9:1) to yield 15
(100 MHz, CDCl3) d (ppm) 174.2 (C]O), 154.9 (C]O), 78.2 (C4), 68.2
(CH2O), 66.8 (C5), 51.9 (CH3O), 50.9 (CH2N3), 46.5 (C2), 44.1 (C1), 30.9
(C6, C3), 28.4 (CH3Boc), 25.8 (SiC(CH3)3), ꢀ4.8 (SiCH3), ꢀ5.0 (SiCH3);
ESI-MS for C21H40N4O6Si calcd Mþ 472.3 exptl 495.4 (MþNa)þ;
HRMS (ESI): (MþNa)þ, found 495.26080C21H40N4O6SiNa requires
495.26093.
4.2.6. Methyl (1S,2R,4S,5R)-2-(N-tert-butoxycarbonylamine)-4,5-
(153 mg, 89%) as a transparent oil. [a 20
(400 MHz, CDCl3) d (ppm) 6.03 (d, 1H, J 9.4 Hz, NHBoc), 4.26 (dd, 1H, J
]
D
þ28.1 (c 0.65, CHCl3); 1H NMR
epoxycyclohexanecarboxylate (7). To
a
solution of
6
(400 mg,
1.57 mmol) in dry CH2Cl2 (4 mL) was added MCPBA (377 mg,
2.20 mmol) and the solution was stirred at room temperature for
3 h. The reaction was monitored by TLC (hexane/AcOEt, 2:1). Then,
the solution was diluted with CH2Cl2 (40 mL), washed with NaHCO3
satd solution (3ꢂ45 mL), dried over Na2SO4 anhyd and concen-
trated under reduced pressure. The residue was purified by flash
silica gel column chromatography using as eluent (hexane/AcOEt,
8.6, 3.2 Hz, H2), 3.94e3.89 (m, 1H, H4), 3.78e3.71 (m, 1H, CH2O),
3.69e3.63 (m, 1H, CH2O), 3.66 (s, 3H, OCH3), 3.57 (t, 2H, J 5.5 Hz,
CH2Cl), 3.46e3.42 (m, 1H, H5), 2.86 (td, 1H, J 12.4, 3.5 Hz, H1), 2.14
(ddd, 1H, J 14.4, 12.4, 2.1 Hz, H6), 2.00 (td, 1H, J 14.4, 3.6 Hz, H3), 1.79
(td, 1H, J 14.4, 3.1 Hz, H6), 1.71 (td, 1H, J 14.3, 3.6 Hz, H3), 1.38 (s, 9H,
CH3Boc), 0.92 (s, 9H, SiC(CH3)3), 0.10 (s, 3H, SiCH3), 0.09 (s, 3H, SiCH3);
13C NMR (100 MHz, CDCl3)
d (ppm) 173.9 (C]O), 155.0 (C]O), 77.4
4:1) to obtain 7 (265 mg, 63%) as a transparent oil. [a 20
]
þ25.9
(C5), 69.4 (CH2O), 68.7 (C4), 51.8 (CH3O), 46.7 (C2), 43.2 (CH2Cl), 40.6
(C1), 32.6 (C3), 28.3 (CH3Boc), 25.7 (SiC(CH3)3), 17.8 (C6), ꢀ5.0 (SiCH3),
ꢀ5.2 (SiCH3); ESI-MS for C21H40ClNO6Si calcd Mþ 465.2 exptl 488.4
(MþNa)þ & 504.3 (MþK)þ; HRMS (ESI): (MþNa)þ, found 488.22088
C21H40NO6ClSiNa requires 488.22056.
D
(c 0.51, CHCl3); 1H NMR (400 MHz, CDCl3)
d (ppm) 4.53 (d, 1H, J
10.1 Hz, NHBoc) 4.07 (dtd, 1H, J 9.9, 6.6, 3.3 Hz, H2), 3.72 (s, 3H,
OCH3), 3.23e3.19 (m, 2H, H4, H5), 2.65 (dd, 1H, J 15.6, 7.6 Hz, H6),
2.53e2.48 (m, 1H, H1), 2.23e2.19 (m, 2H, 2H3), 2.11 (ddd, 1H, J 15.6,
6.1 & 3.2 Hz, H6), 1.44 (s, 9H, CH3Boc); 13C NMR (100 MHz, CDCl3)
d
(ppm) 52.5 (CH3O), 52.3 (C4 o C5), 51.6 (C4 o C5), 46.0 (C2), 41.0 (C1),
4.2.9. Methyl (1S,2R,4R,5R)-2-(N-tert-butoxycarbonylamine)-4-(2-
chloroethoxy)-5-(tert-butyldimethylsilanyloxy)cyclohexane carboxyl-
ate (17). To a solution of 14 (360 mg, 1.03 mmol) and imidazol
(105 mg, 1.54 mmol) in DMF (3 mL) was added TBDMS-Cl (231 mg,
1.54 mmol) and the reaction was for 6 h at room temperature. Then,
the solution was diluted in CH2Cl2 (50 mL), washed with water
(3ꢂ25 mL) and dried over Na2SO4 anhyd. The residue was purified
by flash chromatography using as eluent (hexane/AcOEt, 9:1) to yield
30.4, 29.7 (C3), 29.0 (CH3Boc), 25.2 (C6); ESI-MS for C13H21NO5 calcd
Mþ 271.1 exptl 270.9 Mþ & 271.9 (MþH)þ; HRMS (ESI): (MþNa)þ,
found 294.13148C13H21NO5Na requires 294.13119.
4.2.7. Methyl (1S,2R,4S,SR)-2-(N-tert-butoxycarbonylamino)-4-(2-
chloroethoxy)-5-hydroxycyclohexanecarboxylate (13) and Methyl
(1S,2R,4R,5R)-2-(N-tert-butoxycarbonylamine)-4-(2-chloroethoxy)-
5-hydroxycyclohexanecarboxylate (14). To a solution of 7 (24 mg,
17 (397 mg, 86%) as a transparent oil. [a 20
]
D
ꢀ2.6 (c 1.00, CHCl3); 1H
0.088 mmol) in 2-chloroethanol (300
mL) was added a catalytic
NMR (400 MHz, CDCl3) d (ppm) 5.46 (br s, 1H, NHBoc), 4.11 (br s, 1H,
amount of Cu(OTf)2 (6.5 mg, 0.01 mmol) and the solution was
stirred at room temperature under N2 atmosphere for 3 h. The re-
action was monitored by TLC (hexane/AcOEt, 2:1). A solution of
NH4Cl/NH3 aq (1:1) (8 mL) was added to the reaction mixture.
Then, the solution was extracted with AcOEt (2ꢂ20 mL). The or-
ganic phases were dried over Na2SO4 anhyd, and the solvent was
eliminated in vacuo. The final product was purified by silica gel
column chromatography using as eluent (hexane/AcOEt, 2:1) to
obtain 13 (9 mg, 30%) and 14 (20 mg, 65%) as a transparent oil.
H2), 3.85 (td, 1H, J 10.7, 5.6 Hz, CH2O), 3.75 (dd, 1H, J 8.8, 4.3 Hz, H5),
3.71e3.65 (m, 1H, CH2O), 3.69 (s, 3H, OCH3), 3.61 (t, 2H, J 5.4 Hz,
CH2Cl), 3.44e3.40 (m, 1H, H4), 2.68 (q,1H, J 5.1 Hz, H1), 2.31 (ddd,1H,
J 13.4, 10.0, 3.8.1 Hz, H3), 2.13e2.05 (m, 2H, 2H6), 1.70 (td, 1H, J 13.4,
4.4 Hz, H3), 1.45 (s, 9H, CH3Boc), 0.89 (s, 9H, SiC(CH3)3), 0.75 (s, 3H,
SiCH3), 0.65 (s, 3H, SiCH3); 13C NMR (100 MHz, CDCl3)
d (ppm) 174.0
(C]O), 155.1 (C]O), 79.2 (C4), 69.6 (CH2O), 68.8 (C5), 51.5 (CH3O),
44.8 (C2), 43.0 (CH2Cl), 41.4 (C1), 31.0 (C6), 29.7 (C3), 29.3 (CH3Boc),
25.8 (SiC(CH3)3), ꢀ4.8 (SiCH3), ꢀ5.1 (SiCH3); ESI-MS for
C21H48ClNO6Si calcd Mþ 465.2 exptl 488.4 (MþNa)þ; HRMS (ESI):
(MþNa)þ, found 488.22001 C21H40NO6ClSiNa requires 294.22056.
Compound 13: [a 20
]
þ36.2 (c 0.6, CHCl3); 1H NMR (400 MHz,
D
CDCl3)
d (ppm) 5.58 (br s, 1H, NHBoc), 4.03e3.90 (br s, 1H, H2), 3.90
(dd, 1H, J 9.6, 4.4 Hz, CH2O), 3.75 (s, 3H, OCH3), 3.74e3.63 (m, 4H,
CH2O, CH2Cl, H4), 3.33e3.26 (m, 1H, H5), 3.00 (q, 1H, J 4.4 Hz, H1),
2.55(br s, 1H, OH), 2.41 (td, 1H, J 13.9, 4.1 Hz, H6), 2.14 (dtd, 1H, J
12.9, 4.4, 1.1 Hz, H3), 1.83 (ddd, 1H, J 13.0, 10.6, 9.8 Hz, H3), 1.58 (ddd,
1H, J 14.0, 10.8, 4.6 Hz, H6), 1.46 (s, 9H, CH3Boc); 13C NMR (100 MHz,
4.2.10. Methyl (1S,2R,4S,5S)-4-(2-aminoethoxy)-2-(N-tert-butoxyc-
arbonylamino)-5-(tert-butyldimethylsilanyloxy) cyclohexancarbox-
ylate (16). To a solution of 15 (225 mg, 0.49 mmol) in DMF (5 mL)
were added NaN3 (452 mg, 6.90 mmol) and a catalytic amount of I2,

J.J. Reina, A. Bernardi / Tetrahedron 67 (2011) 5770e5775
5775
the reaction was stirred for 48 h at 50 ꢁC. Then, the solution was
References and notes
diluted in CH2Cl2 (50 mL), washed with water (3ꢂ50 mL) and dried
1. Enantioselective Synthesis of
b
-Aminoacids; Juaristi, E., Soloshonok, V. A., Eds.;
over Na2SO4 anhyd. The solvent was evaporated to yield 16 (231 mg,
Wiley-Interscience: New York, NY, 2005.
quant.) without further purification as a transparent oil. [a 20
]
D
þ13.3
€ €
2. Fulop, F. Chem. Rev. 2001, 101, 2181e2204.
(c 0.6, CHCl3); 1H NMR (400 MHz, CDCl3)
d
(ppm) 6.06 (d, 1H, J
3. Miller, J. A.; Nguyen, S. T. Mini-Rev. Org. Chem. 2005, 2, 39e45.
4. Ortuno, R. M.; Moglioni, A. G.; Moltrasio, G. Y. Curr. Org. Chem. 2005, 9,
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5. Davies, S. G.; Smith, A. D.; Price, P. D. Tetrahedron: Asymmetry 2005, 16,
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6. Frackenpohl, J.; Arvidsson, P. I.; Schreiber, J. V.; Seebach, D. ChemBioChem 2001,
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7. Weigand, H.; Wirz, B.; Schweitzer, A.; Camenisch, G. P.; Rodriguez-Perez, M. I.;
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9.4 Hz, NHBoc), 4.31 (td,1H, J 12.4, 3.2 Hz, H2), 3.98e3.94 (m,1H, H5),
3.73 (ddd, 1H, J 9.8, 5.6, 3.2 Hz, CH2O), 3.69 (s, 3H, OCH3), 3.62 (ddd,
1H, J 10.1, 5.7, 4.4 Hz, CH2O), 3.50e3.47 (m, 1H, H4), 3.35 (ddd, 1H, J
5.7, 4.2, 1.6 Hz, CH2N3), 2.87 (td, 1H, J 12.5, 3.5 Hz, H1), 2.18 (ddd, 1H,
J 14.9, 12.6, 2.2 Hz, H6), 2.04 (ddd, 1H, J 14.9, 4.0, 3.1 Hz, H3), 1.85 (td,
1H, J 14.9, 3.7 Hz, H6), 1.74 (td, 1H, J 14.0, 3.6 Hz, H3), 1.41 (s, 9H,
CH3Boc), 0.96 (s, 9H, SiC(CH3)3), 0.14 (s, 3H, SiCH3), 0.13 (s, 3H, SiCH3);
8. Gellman, S. H. Acc. Chem. Res. 1998, 31, 173e180.
9. Kirshenbaum, K.; Zuckermann, R. N.; Dill, K. A. Curr. Opin. Struct. Biol. 1999, 9,
13C NMR (100 MHz, CDCl3)
d (ppm) 173.9 (C]O), 155.0 (C]O), 77.6
(C4), 68.7 (C5), 68.2 (CH2O), 51.8 (CH3O), 50.9 (CH2N3), 46.7 (C2), 40.5
(C1), 32.5 (C3), 28.3 (CH3Boc), 25.7 (SiC(CH3)3), 17.8 (C6), ꢀ5.0 (SiCH3),
ꢀ5.1 (SiCH3); ESI-MS for C21H40N4O6Si calcd Mþ 472.3 exptl 495.4
(MþNa)þ; HRMS (ESI): (MþNa)þ, found 495.26058 C21H40N4O6SiNa
requires 495.26093.
530e535.
10. Stigers, K. D.; Soth, M. L.; Nowick, J. S. Curr. Opin. Chem. Biol. 1999, 3, 714e723.
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4.2.11. Methyl (1S,2R,4R,5R)-4-(2-aminoethoxy)-2-(N-tert-butoxycar-
bonylamine)-5-(tert-butyldimethylsilanyloxy) cyclohexancarboxylate
(3). To a solution of 17 (400 mg, 0.862 mmol) in DMF (9 mL) were
added NaN3 (448 mg, 6.90 mmol) and a catalytic amount of I2, the
reaction was stirred for 48 h at 50 ꢁC. Then, the solution was diluted
in CH2Cl2 (50 mL), washed with water (3ꢂ50 mL) and dried over
Na2SO4 anhyd. The solvent was evaporated to yield 3 (410 mg,
15. Appella, D. H.; Christianson, L. A.; Karle, I. L.; Powell, D. R.; Gellman, S. H. J. Am.
Chem. Soc. 1996, 118, 13071e13072.
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ꢀ
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18. (a) Fulop, F.; Palko, M.; Forro, E.; Dervarics, M.; Martinek, T. A.; Sillanpaa, R. Eur.
ꢀ
€€
€ €
J. Org. Chem. 2005, 15, 3214e3220; (b) Kiss, L.; Forro, E.; Sillanpaa, R.; Fulop, F.
ꢀ
J. Org. Chem. 2007, 72, 8786e8790; (c) Kiss, L.; Forro, E.; Martinek, T. A.;
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ꢀ
€ €
quant.) without further purification as a transparent oil. [a 20
]
ꢀ10.5
€ €
Fulop, F. Synlett 2010, 1302e1314.
D
(c 1.05, CHCl3); 1H NMR (400 MHz, CDCl3)
d (ppm) 5.52 (br s, 1H,
ꢀ
ꢀ
19. (a) Sattin, S.; Daghetti, A.; Thepaut, M.; Berzi, A.; Sanchez-Navarro, M.; Tab-
arani, G.; Rojo, J.; Fieschi, F.; Clerici, M.; Bernardi, A. ACS Chem. Biol. 2010, 3,
NHBoc), 4.10 (br s, 1H, H2), 3.87e3.70 (m, 2H, H5, CH2O), 3.70 (s, 3H,
OCH3), 3.62 (ddd, 1H, J 12.6, 7.1, 3.3 Hz, CH2O), 3.46e3.40 (m, 2H, H4,
CH2N3), 3.36e3.30 (m, 1H, CH2N3), 2.70 (q, 1H, J 4.8 Hz, H1), 2.31
(ddd, 1H, J 13.5, 10.5, 3.0 Hz, H3), 2.12 (dd, 2H, J 5.1, 4.3 Hz, 2H6), 1.76
(td, 1H, J 9.3, 4.5 Hz, H3), 1.46 (s, 9H, CH3Boc), 0.89 (s, 9H, SiC(CH3)3),
0.08 (s, 3H, SiCH3) 0.07 (s, 3H, SiCH3); 13C NMR (100 MHz, CDCl3)
ꢂ ꢃ _
ꢀ
301e312; (b) Luczkowiak, J.; Sattin, S.; Sutkeviciute, I.; Reina, J. J.; Sanchez-
ꢀ
Navarro, M.; Thepaut, M.; Martínez-Prats, L.; Daghetti, A.; Fieschi, F.; Delgado,
R.; Bernardi, A.; Rojo, J. Bioconjugate Chem. 2011, doi.org/10.1021/bc2000403.
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2006, 10, 262e271.
d
(ppm) 174.0 (C]O), 155.2 (C]O), 79.0 (C4), 68.7 (C5), 68.1 (CH2O),
51.4 (CH3O), 51.0 (CH2N3), 44.8 (C2), 41.3 (C1), 31.0 (C6), 28.8 (C3), 28.4
(CH3Boc), 25.8 (SiC(CH3)3), ꢀ4.8 (SiCH3), ꢀ5.1 (SiCH3); ESI-MS for
C21H40N4O6Si calcd Mþ 472.3 exptl 495.4 (MþNa)þ; HRMS (ESI):
(MþNa)þ, found 495.26055 C21H40N4O6SiNa requires 495.26055.
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Acknowledgements
Financial support by the EU ITN CARMUSYS (PITN-GA-2008-
213592) and Ministero dell Universita e della Ricerca (PRIN prot.
2008J4YNJY) is gratefully acknowledged.
0
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