Synthesis of a β-strand mimetic based on a pyridine scaffold

Article abstract of DOI:10.1016/j.tet.2006.08.080

A synthetic route to a 2,4-disubstituted pyridine as a potential β-strand mimetic has been developed and applied in the synthesis of a tripeptidomimetic of Leu-Gly-Gly. The pyridine scaffold replaces the central glycine, and is substituted with analogues

Full text of DOI:10.1016/j.tet.2006.08.080

Tetrahedron 62 (2006) 10937–10944
Synthesis of a b-strand mimetic based on a pyridine scaffold
David Blomberg,a Kay Brickmannb and Jan Kihlberga,b,
*
aOrganic Chemistry, Department of Chemistry, Umeꢀa University, SE-901 87 Umeꢀa, Sweden
bAstraZeneca R&D Mo€lndal, SE-431 83 Mo€lndal, Sweden
Received 25 May 2006; revised 3 August 2006; accepted 23 August 2006
Available online 25 September 2006
Abstract—A synthetic route to a 2,4-disubstituted pyridine as a potential b-strand mimetic has been developed and applied in the synthesis of
a tripeptidomimetic of Leu-Gly-Gly. The pyridine scaffold replaces the central glycine, and is substituted with analogues of leucine and gly-
cine in positions 4 and 2, respectively. 2-Fluoro-4-iodopyridine was chosen as the functionalized scaffold and was substituted with protected
leucinal in position 4 via a Grignard exchange reaction using iso-propyl magnesium chloride. The glycine moiety was introduced in position 2
via a nucleophilic aromatic substitution reaction (SNAr) facilitated by microwave irradiation. The synthetic sequence involved 12 steps with
an overall yield of 7%.
Ó 2006 Elsevier Ltd. All rights reserved.
1. Introduction
of a novel class of antibiotics targeting pilus assembly in
UPECs.6,7 The second system involves binding and presen-
tation of a glycopeptide from type II collagen by major
histocompatibility complex (MHC) molecules in an animal
model for rheumatoid arthritis (RA).8 This glycopeptide–
MHC interaction has been found to be essential for the
development of arthritis in mice, and further studies have
shown that vaccination with the glycopeptide epitope has
a protective effect.9 A recent study identified the minimal,
active glycopeptide epitope to consist of an octapeptide,10
thereby setting the stage for developing b-strand mimetics
as immunomodulators for treatment of RA.11
A b-strand is a saw-toothed arrangement where amino acid
side chains alternate above and below a linear peptide back-
bone.1 There are no intramolecular hydrogen bonds between
the amino acids that make up a b-strand. However, by revers-
ing the overall direction of the peptide backbone via a turn or
a loop, a second b-strand may hydrogen bond to the first one,
thereby initiating b-sheet formation. b-Strands are thus key
elements in b-sheet secondary structures2 and are also
known to be important in protein–protein and protein–ligand
interactions in various biological systems.2
Our research group is involved in studies of two systems
where interactions between b-strands and proteins are cru-
cial for the biological outcome. The molecular machinery
of pilus assembly in uropathogenic Escherichia coli
(UPEC) constitutes one system.3,4 Adhesive pili, which are
supramolecular protein appendages that anchor the UPEC
to host tissue, are required for the pathogenicity of the bac-
terium. Such pili are formed through a highly conserved pro-
cess called the chaperone/usher pathway, where interactions
between b-strands are required both in the folding of pilus
subunits and in the assembly of the subunits into functional
pili.3,4 Recently it was shown that peptides derived from
b-strands of pilus subunits can inhibit the protein–protein
interactions required for pilus assembly,5 suggesting that
b-strand mimetics may constitute leads for the development
The important biological functions of peptides, together
with their generally poor pharmacokinetic properties,
make the development of peptidomimetics highly desirable.
b-Strand mimetics have been developed by incorporation of
a wide range of amide bond bioisosters,2 including olefins12
in the peptide backbone. Introduction of cyclic systems13–17
to reduce flexibility and/or to induce extended conforma-
tions has also been used. Among cyclic systems, pyrroli-
nones have been particularly successful in retaining the
biological activity of the original peptide.15–17,18
In this study a synthetic route to b-strand mimetics 2, based
on a 2,4-disubstituted pyridine scaffold (Fig. 1), has been
developed. In mimetic 2, which was designed using semiem-
pirical and molecular mechanic calculations,19 the pyridine
scaffold replaces the central amino acid of a tripeptide
sequence. Residues corresponding to the N-terminal and
the C-terminal amino acids are attached at positions 4 and
2 of the pyridine ring, respectively.19 As reported previously
the scaffold permits introduction of a residue in position 3 of
Keywords: b-Strand mimetic; 2-Aminopyridine; Grignard exchange reac-
tion; Nucleophilic aromatic substitution.
* Corresponding author. Tel.: +46 90 7866890; fax: +46 90 138885; e-mail:
jan.kihlberg@chem.umu.se
0040–4020/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2006.08.080

10938
D. Blomberg et al. / Tetrahedron 62 (2006) 10937–10944
2-Fluoro-4-iodopyridine is a key building block and can be
synthesized in two steps from 2-fluoropyridine.21 The moi-
ety in mimetic 4, which corresponds to the leucine residue
was intended to be introduced at position 4 of the pyridine
scaffold via a Grignard exchange reaction22 of the iodine
atom with protected S-leucinal as electrophile.20 Introduc-
tion of the glycine equivalent, which corresponds to the third
amino acid of the tripeptide, was thereafter planned to be
achieved by displacement of the fluorine atom of the scaffold
via a nucleophilic aromatic substitution reaction (SNAr).
R1
O
R3
R1
N
R3
H
N
OH
N
H
N
H
H2N
N
H
O
R2
O
O
R2
O
2
1
Figure 1. b-Strand mimetic 2, which mimics tripeptide fragment 1, was
designed19 based on a 2,4-disubstituted pyridine scaffold. Mimetic 2 lacks
the two central amide bonds of tripeptide fragment 1, but retains some of
the hydrogen bonding capacity of 1.
The synthetic route started by reduction23 of Boc-protected
leucine 8 to alcohol 9 in 97% yield (Scheme 1). This was
achieved via activation of the carboxyl group of 8 as a mixed
anhydride using iso-butyl chloroformate, followed by reduc-
tion using sodium borohydride. Alcohol 9 was subsequently
oxidized24 to aldehyde 10 by treatment with Dess–Martin
periodinane (88%). By keeping the product cold during
work-up and continuing directly with the next step without
further purification, epimerization of this sensitive interme-
diate was avoided.25,26 In order to couple the central pyridine
scaffold to aldehyde 10, 2-fluoro-4-iodopyridine was treated
with iso-propyl magnesium chloride at room temperature
for 3 h to conduct a Grignard exchange reaction.20 Addition
of aldehyde 10 to the Grignard reagent then afforded alky-
lated pyridine 11 without affecting the fluorine atom in
position 2 of the pyridine ring. Purification of alkylated pyr-
idine 11 turned out to be more problematic than expected.
Therefore, the alcohol functionality of crude 11 was directly
protected as a benzyl ether under phase transfer conditions,27
which allowed facile purification to give ether 12 (41%
from 10).
the pyridine ring, which corresponds to the side chain of the
central amino acid of the tripeptide.19,20 The two amide
bonds have been replaced by a keto functionality at position
4 of the pyridine scaffold and by an amine at position 2,
which thus serve as amide bioisosters. Additionally, the pyr-
idine nitrogen atom is positioned with potential to mimic the
carbonyl oxygen atom in the amide bond between the second
and third residues of the original tripeptide. As a conse-
quence of this modified hydrogen bonding pattern the b-
strand mimetic maintains the ability to form hydrogen bonds
with a complementary b-strand in one, but not in the other
direction (Fig. 1).
2. Results and discussion
In order to establish the synthetic conditions that allow the
synthesis of b-strand mimetics 2, we choose Leu-Gly-Gly
tripeptide mimetic 4 as our first target (Fig. 2). This requires
the central pyridine scaffold to be substituted with a leucine
and a glycine moiety in positions 4 and 2, respectively.
Model compound 4 was thus chosen so as to contain a
stereogenic center adjacent to the carbonyl group of the
N-terminal moiety, while the C-terminal residue was kept
simple at this stage. A retrosynthetic analysis revealed that
mimetic 4 could be prepared from protected leucinal 5,
2-fluoro-4-iodopyridine (6), and a glycine equivalent (7).
N
R1HN
F
iii
BocHN
R
OR2
11 R1 = Boc, R2 = H
12 R1 = Boc, R2 = Bn
13 R1 = H, R2 = Bn
14 R1 = Ac, R2 = Bn
iv 41%
v
8 R = CO2H
9 R = CH2OH
10 R = CHO
i 97%
ii 88%
R1
O
H
N
vi 86%
H2N
N
H
CO2H
O
3
N
vii or viii
14
AcHN
N
H
CO2R
OBn
N
OH
O
15 R = tBu
16 R = H
H2N
N
H
O
4
Scheme 1. Reagents and conditions: (i) NMM, iso-butyl chloroformate,
NaBH4, MeOH, THF, ꢀ15 ꢁC, 97%; (ii) Dess–Martin periodinane,
CH2Cl2, 88%; (iii) iso-PrMgCl, 2-fluoro-4-iodopyridine, THF; (iv) benzyl
bromide, QHSO4, 50% NaOH (aq), toluene, 41% from 10; (v) formic
acid; (vi) Ac2O, DMAP, CH2Cl2, 86% from 12; (vii) H2N-Gly-OtBu, pyri-
dine, 150/180 ꢁC; (viii) H2N-Gly-OH, satd NaHCO3 aq, 160 ꢁC.
N
I
F
+
6
+
H
OH
The C-terminal glycine moiety of the target b-strand mi-
metic was planned to be introduced by replacing the fluorine
atom of 12 in an SNAr reaction. In contrast to the substitution
of 2-fluoropyridine analogues of 12 with oxygen nucleo-
philes, which has been accomplished under relatively mild
conditions,19,20 substitution of 12 with amines turned out
PgHN
H2N
O
O
7
5
Figure 2. A retrosynthetic analysis suggests that b-strand mimetic 4 can be
prepared from protected leucinal 5, 2-fluoro-4-iodo-pyridine (6), and a
glycine equivalent (7).

D. Blomberg et al. / Tetrahedron 62 (2006) 10937–10944
10939
to be a significant challenge. Preferably the amino group of
a glycine derivative would serve as a nucleophile in the sub-
stitution reaction. Initial attempts to accomplish this substi-
tution resulted in partial cleavage of the Boc-group of 12.
The Boc-group was therefore removed using formic acid
to give 13 and replaced by an acetyl group by treatment
with acetic anhydride in dichloromethane to afford 14
(86% from 12). As revealed by LCMS analysis, microwave
irradiation of 14 at 150 ꢁC for 1 h with glycine tert-butyl es-
ter in pyridine gave the desired substitution product 15, but
only in trace amounts (appr. 1% yield). Raising the temper-
ature to 180 ꢁC did not increase the yield of 15, instead this
resulted in formation of a black solid in the reaction mixture,
almost certainly by decomposition and polymerization of
glycine tert-butyl ester. This was confirmed by running the
same experiment without 14 present, which also resulted
in a black solid. Based on the finding that the problems orig-
inated from the tert-butyl ester of glycine, substitution of 14
was attempted with unprotected glycine. In order to dissolve
glycine, aqueous sodium hydrogen carbonate was used as
solvent in the microwave assisted substitution reaction.
When carried out at 160 ꢁC for 1 h the desired product 16
was indeed obtained, but in an unsatisfactory yield (<10%
according to LCMS) and accompanied by equal amounts
of the product resulting from attack of water at position 2
of the pyridine ring.
or the tert-butyl ester of glycine with 2-fluoropyridine under
different conditions using microwave irradiation failed; no
reaction was observed with glycine while the tert-butyl ester
of glycine polymerized into an insoluble black solid.
Therefore, other glycine equivalents were explored as
nucleophiles. Microwave irradiation of ethanolamine and
2-fluoropyridine in pyridine at 210 ꢁC for 1 h gave derivative
18 (74%). Unfortunately, attempted oxidation of the alcohol
functionality in 18 with Dess–Martin periodinane to give the
corresponding aldehyde, or with ruthenium trichloride to the
corresponding acid was unsuccessful. In an attempt to cir-
cumvent the problematic oxidation step, 2-fluoropyridine
was converted to 2-aminopyridine28 (19) by heating in
25% aqueous ammonia in a sealed steel cylinder. Anisalde-
hyde was then used to investigate different conditions for re-
ductive amination of 19. At best, a modest 36% yield could
be obtained when sodium triacetoxyborohydride was used as
the reducing agent in 1,2-dichloroethane under basic con-
ditions.29 Disappointingly, when these conditions were
applied to reductive amination of 2-aminopyridine with
glyoxylic acid, or with the more soluble tert-butyl glyoxy-
late30 neither of the products were obtained.
Nucleophilic substitution of 2-fluoropyridine (17) was then
investigated using allylamine as nucleophile, with the alkene
part intended as a carboxylic acid precursor. Substitution
was achieved by microwave irradiation of 2-fluoropyridine
and allylamine in pyridine at 190 ꢁC for 1 h to give
substituted pyridine 20 (64%). Oxidation of the alkene
moiety of 20 was accomplished by a catalytic amount of
potassium osmate with N-methyl morpholine N-oxide as
co-oxidant in a solvent mixture of water, tetrahydrofuran,
and acetone to give diol 21 (51%). Further oxidation of
diol 21 was first attempted with lead tetraacetate in toluene
to give the corresponding aldehyde, and then with sodium
periodate and bromine in methanol to give an ester function-
ality,31 but neither of the desired products were obtained.
Also, when direct oxidation32 of the olefin in 20 to a methyl
ester was attempted by ozonolytic cleavage in a mixture of
methanolic sodium hydroxide and dichloromethane, all
starting materials were consumed but no product was
formed. To eliminate the possibility that the anilinic proton
of 20 interferes during oxidation, aminopyridine 20 was pro-
tected33 using Boc-anhydride and a catalytic amount of 4-
dimethylaminopyridine to give derivative 22 (99%). Just as
for 20 oxidation of 22 to diol 23 (88%) was successful, but
again further oxidation of the diol failed. However, when
Boc-protected aminopyridine 22 was treated with ozone in
methanolic sodium hydroxide and dichloromethane,34 the
olefin was oxidized to give the desired ester 24 (65%).
In view of the difficulties encountered in the nucleophilic
substitutions of 14 it was decided to study the reactions
between 2-fluoropyridine (17) and various amines as model
systems (Scheme 2). Just as for 14, attempts to react glycine
N
Oxidation
OH
N
H
18
i 74%
Reductive
amination
N
N
ii 51%
F
NH2
17
19
iii 64%
N
N
Oxidation
Oxidation
iv 51%
N
H
N
H
OH
OH
20
21
v 99%
N
N
N
vi 88%
N
Boc
N
OH
Synthesis of the Leu-Gly-Gly b-strand mimetic from build-
ing block 14 was then brought to completion based on the
learnings from the model study. Consequently, 14 was sub-
jected to microwave irradiation in neat allylamine (2.5 h,
17 bar,w150 ꢁC) to give substituted pyridine 25 which, after
aqueous work-up, was protected33 with a Boc-group to
afford protected 2-aminopyridine 26 (86% from 14, Scheme
3). In order to investigate if more sterically demanding
amino acid derivatives than the glycine equivalents ethanol-
amine and allylamine could be employed in the critical
aromatic substitution of 14, leucinol and iso-leucinol
were chosen as nucleophiles. Building block 14 was first
Boc OH
vii 65%
22
23
OMe
N
24
Boc
O
Scheme 2. Reagents and conditions: (i) ethanolamine, 2-fluoropyridine,
pyridine, 210 ꢁC, 74%; (ii) 25% NH3 in H2O, w140 ꢁC, 51%; (iii) allyl-
amine, 2-fluoropyridine, pyridine, 190 ꢁC, 64%; (iv) potassium osmate,
NMO, H2O, THF, acetone, 51%; (v) Boc2O, DMAP, CH2Cl2, 99%; (vi)
potassium osmate, NMO, H2O, THF, acetone, 88%; (vii) NaOH (2 M in
MeOH), CH2Cl2, O3, ꢀ78 ꢁC/rt, 65%.

10940
D. Blomberg et al. / Tetrahedron 62 (2006) 10937–10944
32. Finally, oxidation of the alcohol moiety of 32 using
N
N
R
Dess–Martin periodinane gave b-strand mimetic 33 (66%
from 30) in enantiomerically pure form according to chiral
chromatography. In conclusion, the synthesis of b-strand
mimetic 33 was accomplished in a 12-step synthetic se-
quence with an overall yield of 7%.
27 R = i-Bu (86%)
28 R = sec-Bu (80%)
OH
AcHN
N
H
OBn
OBn
i
N
iv
58%
ii
3. Experimental
3.1. General
AcHN
N
R
AcHN
F
OBn
25 R = H
26 R = Boc
iii 86%
14
1H NMR and 13C NMR were recorded in CDCl3 or in
CD3OD at 298 K. 1H NMR and 13C NMR signals are
assigned with support from appropriate 2D-NMR and are
presented in Supplementary data. For compounds that con-
tain an uneven mixture of diastereomers (12–14 and 25–
30), only signals for the major diastereomer are assigned.
All microwave irradiations were performed in a Smithcreator
with EmrysÔ process vials (2–5 mL for compounds 18, 20,
and 25, or 0.5–1.5 mL for compounds 27 and 28), tempera-
ture and pressure measurements were performed by infrared
detection. Chiral HPLC was run on a Pirkle covalent (S,S)
whelk-O1 10/100 Krom FCC, with heptane/CH2Cl2/2-prop-
anol 48:48:4 for compounds 33 and 33rac, 49:49:2 for com-
pounds 31 and 31rac as eluent. Chromatograms of both 33
and 33rac are presented in Supplementary data.
N
N
N
vi
OMe
OMe
AcHN
N
AcHN
OR1
R2
O
O
Boc O
31
R1 = Bn, R2 = Boc
29
30
v 75%
R
1 = H, R2 = Boc
vii
72%
N
viii
N
ix
OMe
OMe
66%
AcHN
N
H
AcHN
N
H
OH
O
O
O
32
33
Scheme 3. Reagents and conditions: (i) (a) leucinol, microwave irradiation
200 ꢁC, to give 27, 86%; (b) iso-leucinol, microwave irradiation 200–
210 ꢁC, to give 28, 80%; (ii) allylamine, microwave irradiation (17 bar,
w150 ꢁC); (iii) Boc2O, DMAP, CH2Cl2, 86% from 14; (iv) NaOH (2 M in
MeOH), CH2Cl2, O3, ꢀ78 ꢁC/rt, 58%; (v) Pd/C, H2 (1 atm), MeOH,
AcOH, 75%; (vi) Dess–Martin periodinane, CH2Cl2; (vii) formic acid,
72% from 30; (viii) 25% TFA in CH2Cl2; (ix) Dess–Martin periodinane,
CH2Cl2, 66% from 30.
3.2. Procedures
3.2.1. tert-Butyl [(1S)-1-(hydroxymethyl)-3-methylbu-
tyl]carbamate (9). Boc-Leu-OH$H2O (8, 7.0 g, 28 mmol)
was evaporated from toluene and dissolved in THF
(80 mL). N-Methyl morpholine (3.3 mL, 29 mmol) was
added and the reaction was cooled to ꢀ20 ꢁC. The reaction
was treated with iso-butyl chloroformate (4.4 mL, 29 mmol)
and stirred for 30 min. The formed precipitate was removed
by filtration and rinsed with THF (30 mL). To the clear fil-
trate NaBH4 (3.2 g, 84 mmol) was added in one portion fol-
lowed by careful addition of methanol (200 mL) at ꢀ20 ꢁC.
After 1 h the reaction was quenched with satd NH4Cl aq fol-
lowed by addition of EtOAc. The two phases were separated
and the organic layer was washed with brine, dried over
Na2SO4, and concentrated under reduced pressure. The
residue was purified by flash chromatography (silica gel
pretreated with triethylamine) EtOAc/heptane 1:4/1:2 to
give alcohol 9 (5.9 g, 97%) as a clear oil; [a]2D0 ꢀ25.8 (c 1,
dissolved in leucinol (appr. 20 equiv) and heated to 200 ꢁC
by microwave irradiation, which afforded substituted pyri-
dine 27 (86%). Encouraged by this result, the even more
sterically hindered iso-leucinol was used as nucleophile
and gave the desired compound 28 (80%) when heating to
210 ꢁC. Further attempts to convert derivatives 27 and 28,
or analogues thereof, into more complex b-strand mimetics
will be the subject of future studies. Instead the synthetic se-
quence continued with oxidation of the olefinic part of 26 to
methyl ester 29 (58%) by ozonolytic cleavage in basic meth-
anolic solution.32 Careful adjustment of the reaction time
was necessary to avoid oxidation of the benzyl ether in 29
to an undesired benzoyl ester. Thereafter the benzyl ether
of 29 was removed by hydrogenolysis in a mixture of meth-
anol and acetic acid to give 30 (75%). Oxidation to ketone 31
using Dess–Martin periodinane followed by removal of the
Boc-protective group afforded the desired b-strand mimetic
33 (72% from 30). Somewhat surprisingly, chiral chromato-
graphy of mimetic 33 on a silica based column, revealed that
partial epimerization (w60% ee) of the chiral center of 33
had occurred. However, ketone 31, the direct precursor of
33, was found to be enantiomerically pure as determined
by chiral chromatography. It was therefore concluded that
cleavage of the Boc-group of pure 31, under acidic condi-
tions had caused the epimerization via enolization of the
ketone. To circumvent this problem, acidic removal of the
Boc-group was performed already on alcohol 30 using tri-
fluoroacetic acid (25%) in dichloromethane to give amine
1
CHCl3); H NMR (400 MHz, CDCl3) d 4.62 (d, J¼8.0 Hz,
1H), 3.76–3.59 (m, 2H), 3.53–3.44 (m, 1H), 2.72 (s, 1H),
1.71–1.59 (m, 1H), 1.43 (s, 9H), 1.34–1.25 (m, 2H), 0.92
(d, J¼1.7 Hz, 3H), 0.91 (d, J¼1.7 Hz, 3H); 13C NMR
(100 MHz, CDCl3) d 156.5, 79.6, 66.5, 51.0, 40.5, 28.4,
24.8, 23.0, 22.2; IR (neat): 3590–3138, 1688, 1529 cmꢀ1
;
FABHRMS calcd for C11H24NO3 (M+H): 218.1756, found:
218.1756.
3.2.2. tert-Butyl [(1S)-1-formyl-3-methylbutyl]carba-
mate (10). Alcohol 9 (0.11 g, 0.51 mmol) was dissolved in
CH2Cl2 (3 mL) and treated with Dess–Martin periodinane
in CH2Cl2 (1.6 mL, 15 wt % in CH2Cl2, 0.76 mmol). After
1 h a white precipitate was formed and sodium bisulfite
(1.0 g, 5.3 mmol) in satd NaHCO3 aq was added. The
organic layer was washed with satd NaHCO3 aq, dried

D. Blomberg et al. / Tetrahedron 62 (2006) 10937–10944
10941
over Na2SO4, and concentrated under reduced pressure at
0 ꢁC to give aldehyde 10 (96 mg, 88%) as a clear oil, which
was used without further purification for the next step.
IR (neat): 1652, 1552 cmꢀ1
C20H26FN2O2 (M+H): 345.1978, found: 345.1977.
;
FABHRMS calcd for
3.2.5. 2-(Pyridin-2-ylamino)ethanol (18). 2-Fluoropyri-
dine (0.3 mL, 3.5 mmol) was dissolved in pyridine (1 mL)
and ethanolamine (2.1 mL, 35 mmol). The reaction was sub-
jected to microwave irradiation at 210 ꢁC for 1 h. Satd
NaHCO3 aq and EtOAc were added to the reaction and the
two phases were separated. The aqueous layer was extracted
with EtOAc and CH2Cl2. The combined organic phases were
dried over Na2SO4 and concentrated under reduced pressure
to give alcohol 18 (0.36 g, 74%) as a colorless amorphous
3.2.3. tert-Butyl {(1S)-1-[(RS)-(benzyloxy)(2-fluoropyri-
din-4-yl)methyl]-3-methylbutyl}carbamate (12). 2-Fluo-
ro-4-iodopyridine (1.2 g, 5.4 mmol) and iso-propyl
magnesium chloride (2.6 mL, 5.5 mmol) was stirred in
THF (2 mL) for 3 h. To this solution aldehyde 10 (0.56 g,
2.6 mmol) dissolved in THF (2 mL) was added and the mix-
ture was stirred for another 15 h. The reaction was quenched
with satd NH4Cl aq followed by addition of satd NaHCO3 aq
and brine and extraction with EtOAc. The combined organic
layers were dried over Na2SO4 and concentrated under
reduced pressure. To the residue toluene (40 mL) and
NaOH aq (50%, 30 mL) were added. The vigorously stirred
two phase system was treated with benzyl bromide
(0.31 mL, 2.8 mmol) and tetrabutylammonium hydrogen
sulfate (0.10 g, 0.30 mmol). After 3 h water was added fol-
lowed by extraction with Et2O. The combined organic layers
were dried over Na2SO4 and concentrated under reduced
pressure. The residue was purified by flash chromatography
EtOAc/heptane 1:9/1:4 to give 12 (0.43 g, 41%) as a clear
oil; 1H NMR (400 MHz, CDCl3) d 8.18 (d, J¼5.1 Hz, 1H),
7.40–7.27 (m, 5H), 7.12 (d, J¼5.1 Hz, 1H), 6.92 (s, 1H),
4.60 (d, J¼9.9 Hz, 1H), 4.56 (d, J¼12 Hz, 1H), 4.43 (d,
J¼1.7 Hz, 1H), 4.32 (d, J¼12 Hz, 1H), 3.94–3.85 (m, 1H),
1.64–1.51 (m, 1H), 1.44–1.36 (m, 2H), 1.30 (s, 9H), 0.91
(d, J¼6.6 Hz, 3H), 0.89 (d, J¼6.6 Hz, 3H); 13C NMR
(100 MHz, CDCl3) d 164.0 (d, JC–F¼239 Hz), 155.1, 147.5
(d, JC–F¼15 Hz), 137.1, 128.5, 128.1, 128.0, 119.9,
107.7 (d, JC–F¼37 Hz), 80.5, 79.3, 71.8, 53.2, 41.2,
1
solid; H NMR (400 MHz, CDCl3) d 8.02–7.97 (m, 1H),
7.34 (ddd, J¼9.2, 7.1, and 1.9 Hz, 1H), 6.57–6.52 (m, 1H),
6.42 (d, J¼8.4 Hz, 1H), 5.06 (br s, 1H), 4.92 (br s, 1H),
3.77 (t, J¼4.8 Hz, 2H), 3.49–3.42 (m, 2H); 13C NMR
(100 MHz, CDCl3) d 158.8, 147.1, 137.5, 112.9, 108.3,
63.0, 45.2; IR (neat): 3347–3132, 1607, 1524 cmꢀ1
;
FABHRMS calcd for C7H11N2O (M+H): 139.0871, found:
139.0878.
3.2.6. Allyl-pyridin-2-yl-amine (20). Allylamine (0.79 mL,
10.5 mmol) and 2-fluoropyridine (0.3 mL, 3.5 mmol) were
dissolved in pyridine (2 mL) and subjected to microwave
irradiation to 190 ꢁC for 1 h. The reaction mixture was
concentrated under reduced pressure and purified by flash
chromatography EtOAc/heptane 2:1 to give aminopyridine
1
20 (0.3 g, 64%) as a colorless oil; H NMR (400 MHz,
CDCl3) d 8.09–8.05 (m, 1H), 7.39 (ddd, J¼8.8, 7.1, and
1.9 Hz, 1H), 6.55 (ddd, J¼7.1, 5.1, and 0.9 Hz, 1H), 6.37
(d, J¼8.8 Hz, 1H), 5.99–5.88 (m, 1H), 5.28–5.22 (m, 1H),
5.16–5.11 (m, 1H), 4.78 (br s, 1H), 3.94–3.89 (m, 2H); 13C
NMR (100 MHz, CDCl3) d 158.6, 148.1, 137.3, 135.0,
115.8, 112.9, 106.6, 44.6; IR (neat): 3371–3178, 1601,
1571, 1510 cmꢀ1; FABHRMS calcd for C8H11N2 (M+H):
135.0922, found: 135.0932.
28.2, 24.7, 23.0, 22.1; IR (neat): 1703, 1612 cmꢀ1
;
FABHRMS calcd for C23H32FN2O3 (M+H): 403.2397,
found: 403.2389.
3.2.4. N-{(1S)-1-[(RS)-(Benzyloxy)(2-fluoropyridin-4-yl)-
methyl]-3-methylbutyl}acetamide (14). Boc-protected
amine 12 (0.31 g, 0.77 mmol) was treated with formic acid
(12 mL) for 3 h. Formic acid was removed under reduced
pressure and the residue was dissolved in EtOAc and washed
with satd NaHCO3 aq and the aqueous phase was extracted
with EtOAc. The combined organic layers were dried over
Na2SO4 and concentrated under reduced pressure. The resi-
due was dissolved in CH2Cl2 (7.5 mL) followed by addition
of acetic anhydride (0.08 mL, 0.86 mmol) and 4-dimethyl-
aminopyridine (0.1 g, 0.82 mmol). After 2 h a 1:3 mixture
of satd NaHCO3 aq and brine was added and the two phases
were separated and the aqueous layer was extracted with
EtOAc. The combined organic phases were dried over
Na2SO4 and concentrated under reduced pressure. The resi-
due was purified by flash chromatography EtOAc/heptane
3.2.7. 3-(Pyridin-2-ylamino)propane-1,2-diol (21). Al-
kene 20 (0.12 g, 0.92 mmol) was dissolved in H2O
(5.5 mL), acetone (5.5 mL), and THF (5.5 mL). Potassium
osmate(VI) dihydrate (5 mg, 14 mmol) and N-methyl mor-
pholine N-oxide (0.23 g, 1.96 mmol) were added and the re-
action was stirred for 15 h. The solvents were removed under
reduced pressure with toluene as azeotrope. The residue was
purified by flash chromatography EtOH/toluene 1:6 to give
1
diol 21 (0.78 g, 51%) as a colorless amorphous solid; H
NMR (400 MHz, CD3OD) d 7.92–7.87 (m, 1H), 7.41 (ddd,
J¼9.2, 7.0, and 1.9 Hz, 1H), 6.58–6.51 (m, 2H), 3.80–3.72
(m, 1H), 3.53 (d, J¼5.6 Hz, 2H), 3.45 (dd, J¼14.1 and
4.6 Hz, 1H), 3.32 (dd, J¼14.1 and 6.3 Hz, 1H); 13C NMR
(100 MHz, CD3OD) d 160.6, 147.5, 138.8, 113.4, 110.2,
72.7, 64.8, 45.6; IR (neat): 3292, 1611, 1575 cmꢀ1
;
1
3:2 to give 14 (0.23 g, 86%) as a colorless oil; H NMR
FABHRMS calcd for C8H13N2O2 (M+H): 169.0977, found:
169.0984.
(400 MHz, CDCl3) d 8.17 (d, J¼5.2 Hz, 1H), 7.41–7.28
(m, 5H), 7.10 (d, J¼5.2 Hz, 1H), 6.88 (s, 1H), 5.53 (d,
J¼9.7 Hz, 1H), 4.57 (d, J¼11 Hz, 1H), 4.47 (d, J¼2.7 Hz,
1H), 4.34 (d, J¼11 Hz, 1H), 4.31–4.23 (m, 1H), 1.85 (s,
3H), 1.55–1.46 (m, 1H), 1.45–1.39 (m, 2H), 0.91 (d,
J¼6.5 Hz, 3H), 0.89 (d, J¼6.4 Hz, 3H); 13C NMR
(100 MHz, CDCl3) d 169.2, 164.0 (d, JC–F¼237 Hz), 154.9
(d, JC–F¼7 Hz), 147.6 (d, JC–F¼15 Hz), 136.9, 128.6,
3.2.8. tert-Butyl allyl(pyridin-2-yl)carbamate (22). Ami-
nopyridine 20 (55 mg, 0.41 mmol) was dissolved in
CH2Cl2 (2.5 mL) and treated with di-tert-butyl dicarbonate
(0.19 g, 0.86 mmol) and a catalytic amount of 4-dimethyl-
aminopyridine (5 mg, 41 mmol). After 15 h satd NaHCO3 aq
was added and the two phases were separated. The aqueous
phase was extracted with CH2Cl2 and the combined organic
phases were dried over Na2SO4 and concentrated under
128.3, 128.1, 119.7 (d, JC–F¼4 Hz), 107.5 (d, JC–F
¼
38 Hz), 80.0, 72.0, 51.7, 41.1, 24.8, 23.1, 23.0, 22.2;

10942
D. Blomberg et al. / Tetrahedron 62 (2006) 10937–10944
reduced pressure. The residue was purified by flash chroma-
tography EtOAc/heptane 1:7 to give Boc-protected amino-
pyridine 22 (96 mg, 99%) as a clear oil; 1H NMR
(400 MHz, CDCl3) d 8.38–8.34 (m, 1H), 7.67–7.57 (m,
2H), 7.01–6.96 (m, 1H), 6.00–5.89 (m, 1H), 5.18–5.06 (m,
2H), 4.58–4.53 (m, 2H), 1.50 (s, 9H); 13C NMR
(100 MHz, CDCl3) d 154.5, 154.2, 147.7, 136.9, 134.8,
119.6, 119.4, 115.8, 81.2, 49.2, 28.3; IR (neat): 1706,
1650, 1588, 1551 cmꢀ1; FABHRMS calcd for C13H19N2O2
(M+H): 235.1447, found: 235.1447.
under reduced pressure. The residue was dissolved in
CH2Cl2 (15 mL) and treated with di-tert-butyl dicarbonate
(0.36 g, 1.65 mmol) and 4-dimethylaminopyridine (0.01 g,
0.082 mmol). After 20 h a 1:3 mixture of satd NaHCO3 aq
and brine was added and the two phases were separated.
The aqueous layer was extracted with EtOAc and the com-
bined organic phases were dried over Na2SO4. The solvent
was removed under reduced pressure and the residue was pu-
rified by flash chromatography EtOAc/heptane 1:1 to give 26
(0.27 g, 86%) as a colorless oil; 1H NMR (400 MHz, CDCl3)
d 8.32 (d, J¼5.1 Hz, 1H), 7.62 (s, 1H), 7.40–7.29 (m, 5H),
6.98 (dd, J¼5.1 and 1.1 Hz, 1H), 6.03–5.92 (m, 1H), 5.52
(d, J¼9.5 Hz, 1H), 5.16 (dd, J¼17 and 1.6 Hz, 1H), 5.10
(dd, J¼10 and 1.6 Hz, 1H), 4.59 (d, J¼12 Hz, 1H), 4.56–
4.52 (m, 2H), 4.47 (d, J¼2.4 Hz, 1H), 4.37 (d, J¼12 Hz,
1H), 4.35–4.27 (m, 1H), 1.91 (s, 3H), 1.51 (s, 9H), 1.49–
1.41 (m, 1H), 1.32–1.24 (m, 2H), 0.91 (d, J¼2.7 Hz, 3H),
0.89 (d, J¼2.7 Hz, 3H); 13C NMR (100 MHz, CDCl3)
d 169.5, 154.4, 154.1, 149.2, 147.4, 137.6, 134.8, 128.5,
128.1, 128.0, 117.6, 115.8, 81.1, 80.5, 71.7, 51.4, 49.1,
40.4, 28.3, 24.7, 23.2, 23.1, 22.2; IR (neat): 1704, 1650,
1601, 1555 cmꢀ1; FABHRMS calcd for C28H40N3O4
(M+H): 482.3019, found: 482.3023.
3.2.9. tert-Butyl (2,3-dihydroxypropyl)pyridin-2-ylcar-
bamate (23). Boc-protected aminopyridine 22 (96 mg,
0.41 mmol) was dissolved in H2O (2.5 mL), acetone
(2.5 mL), and THF (2.5 mL). Potassium osmate(VI) dihy-
drate (5 mg, 14 mmol) and N-methyl morpholine N-oxide
(0.10 g, 0.88 mmol) were added and the reaction was stirred
for 15 h. Brine, satd NaHCO3, and EtOAc were added and
the two phases were separated. The organic layer was dried
over Na2SO4 and concentrated under reduced pressure. The
residue was purified by flash chromatography EtOAc/
heptane 2:1 to give diol 23 (97 mg, 88%) as a colorless
oil; 1H NMR (400 MHz, CDCl3) d 8.29–8.25 (m, 1H),
7.69–7.63 (m, 1H), 7.59–7.54 (m, 1H), 7.08–7.03 (m, 1H),
4.01–3.91 (m, 2H), 3.87–3.79 (m, 1H), 3.64 (dd, J¼12 and
4.8 Hz, 1H), 3.58 (dd, J¼12 and 4.8 Hz, 1H), 1.48 (s, 9H);
13C NMR (100 MHz, CDCl3) d 154.5, 153.9, 146.4, 137.7,
120.3, 120.1, 82.2, 70.6, 64.1, 51.2, 28.1; IR (neat): 3605–
3064, 1705, 1594, 1572 cmꢀ1; FABHRMS calcd for
C13H21N2O4 (M+H): 269.1501, found: 269.1494.
3.2.12. {4-[(1RS,2S)-2-(Acetylamino)-1-(benzyloxy)-4-
methylpentyl]pyridin-2-ylamino}-S-leucinol (27). Fluoro-
pyridine 14 (0.18 g, 0.53 mmol) was dissolved in S-leucinol
(1.2 mL, 9.3 mmol) and heated by microwave irradiation
(200 ꢁC, 90 min). The reaction mixture was put on a silica
gel column and eluted with EtOH/toluene 1:15/1:8 to
give 27 (0.20 g, 86%) as a white foam; 1H NMR
(400 MHz, CDCl3) d 7.93 (d, J¼5.3 Hz, 1H), 7.38–7.26
(m, 5H), 6.49 (d, J¼5.3 Hz, 1H), 6.38 (s, 1H), 5.70 (d,
J¼9.4 Hz, 1H), 4.76 (d, J¼6.9 Hz, 1H), 4.55 (d, J¼
11.7 Hz, 1H), 4.31 (d, J¼11.7 Hz, 1H), 4.29 (d, J¼3.2 Hz,
1H), 4.28–4.20 (m, 1H), 3.88–3.80 (m, 1H), 3.71 (dd,
J¼11 and 3.2 Hz, 1H), 3.49 (dd, J¼11 and 6.6 Hz, 1H),
1.86 (s, 3H), 1.77–1.67 (m, 1H), 1.55–1.45 (m, 1H), 1.43–
1.36 (m, 4H), 0.97–0.85 (m, 12H); 13C NMR (100 MHz,
CDCl3) d 169.3, 158.9, 149.9, 147.0, 137.5, 128.3, 127.9,
127.8, 111.3, 106.1, 80.5, 71.5, 67.1, 52.7, 51.6, 41.0,
40.6, 24.9, 24.7, 23.1, 23.0, 22.9, 22.4, 22.1; IR (neat):
3371–3129, 1649, 1620, 1560 cmꢀ1; FABHRMS calcd for
C26H40N3O3 (M+H): 442.3070, found: 442.3069.
3.2.10. (tert-Butoxycarbonyl-pyridin-2-yl-amino)-acetic
acid methyl ester (24). Boc-protected aminopyridine 22
(0.10 g, 0.44 mmol) was dissolved in CH2Cl2 (3.5 mL)
and a 2 M solution of NaOH in methanol (0.90 mL) and
cooled to ꢀ78 ꢁC. O3 was passed through the solution,
which turned bright yellow at first and gradually decolor-
ized. A colorless precipitate was formed and the solution
turned blue and the excess of O3 was purged from the solu-
tion with a stream of oxygen. Water and Et2O were added
and the two phases were separated and the aqueous layer
was extracted with EtOAc. The combined organic phases
were dried over Na2SO4 and concentrated under reduced
pressure to give ester 24 (75 mg, 65%) as a colorless oil;
1H NMR (400 MHz, CDCl3) d 8.30–8.27 (m, 1H), 7.83
(br d, J¼8.6 Hz, 1H), 7.62 (ddd, J¼8.6, 7.3, and 1.9 Hz,
1H), 6.97 (ddd, J¼7.3, 4.9, and 0.9 Hz, 1H), 4.71 (s, 2H),
3.73 (s, 3H), 1.50 (s, 9H); 13C NMR (100 MHz, CDCl3)
d 170.6, 153.5, 153.5, 147.1, 137.0, 119.2, 118.2, 82.0,
3.2.13. {4-[(1RS,2S)-2-(Acetylamino)-1-(benzyloxy)-4-
methylpentyl]pyridin-2-ylamino}-S-iso-leucinol (28).
Fluoropyridine 14 (0.10 g, 0.30 mmol) was dissolved in
S-iso-leucinol (1.1 g, 9.2 mmol) and heated by microwave
irradiation (200 ꢁC, 90 min and 210 ꢁC, 30 min). The reaction
mixture was put on a silica column and eluted with EtOH/
toluene 1:15/1:10 to give 28 (0.11 g, 80%) as a white
51.9, 47.7, 28.1; IR (neat): 1758, 1720, 1590 cmꢀ1
;
FABHRMS calcd for C13H19N2O4 (M+H): 267.1345,
found: 267.1344.
1
foam; H NMR (400 MHz, CDCl3) d 7.92 (d, J¼5.2 Hz,
3.2.11. tert-Butyl{4-[(1RS,2S)-2-(acetylamino)-1-(benzyl-
oxy)-4-methylpentyl]pyridin-2-yl}allylcarbamate (26).
Fluoropyridine 14 (0.23 g, 0.66 mmol) was dissolved in
allylamine (4 mL) and subjected to microwave irradiation
to 17 bar (w150 ꢁC) for 2.5 h. Allylamine was removed
under reduced pressure and the residue was dissolved in
CH2Cl2, followed by addition of a 1:3 mixture of satd
NaHCO3 aq and brine. The two phases were separated and
the aqueous layer was extracted with EtOAc. The combined
organic phases were dried over Na2SO4 and concentrated
1H), 7.37–7.27 (m, 5H), 6.49 (dd, J¼5.2 and 1.1 Hz, 1H),
6.38 (s, 1H), 5.73 (d, J¼9.7 Hz, 1H), 4.92 (d, J¼7.0 Hz,
1H), 4.55 (d, J¼12 Hz, 1H), 4.31 (d, J¼12 Hz, 1H), 4.29
(d, J¼3.2 Hz, 1H), 4.28–4.20 (m, 1H), 3.75 (d, J¼8.7 Hz,
1H), 3.63–3.55 (m, 2H), 1.86 (s, 3H), 1.74–1.64 (m, 1H),
1.58–1.46 (m, 2H), 1.42–1.35 (m, 2H), 1.26–1.15 (m, 1H),
0.97–0.86 (m, 12H); 13C NMR (100 MHz, CDCl3)
d 169.4, 159.1, 149.9, 147.1, 137.5, 128.3, 127.8, 127.8,
111.1, 106.1, 80.5, 71.4, 64.1, 58.8, 51.6, 40.5, 36.8, 25.8,
24.7, 23.0, 23.0, 22.1, 15.4, 11.6; IR (neat): 3474–3117,

D. Blomberg et al. / Tetrahedron 62 (2006) 10937–10944
10943
1649, 1609, 1561, 1516 cmꢀ1; FABHRMS calcd for
C26H40N3O3 (M+H): 442.3070, found: 442.3066.
ketone 31 (77 mg, 81%) as a colorless oil; [a]2D0 +4.4 (c 1,
1
CHCl3); H NMR (400 MHz, CDCl3) d 8.45 (dd, J¼5.1
and 0.6 Hz, 1H), 8.40 (s, 1H), 7.44 (dd, J¼5.1 and 1.5 Hz,
1H), 6.13 (d, J¼8.2 Hz, 1H), 5.61–5.54 (m, 1H), 4.74 (s,
2H), 3.75 (s, 3H), 2.05 (s, 3H), 1.79–1.59 (m, 2H), 1.53 (s,
9H), 1.46–1.36 (m, 1H), 1.07 (d, J¼6.5 Hz, 3H), 0.89 (d,
J¼6.5 Hz, 3H); 13C NMR (100 MHz, CDCl3) d 199.3,
170.3, 169.8, 154.9, 153.2, 148.3, 142.2, 116.5, 116.5,
82.7, 53.0, 52.1, 47.7, 42.1, 28.1, 25.2, 23.3, 23.2, 21.6; IR
(neat): 1753, 1709, 1652, 1598, 1554 cmꢀ1; FABHRMS
calcd for C21H32N3O6 (M+H): 422.2291, found: 422.2298.
3.2.14. Methyl N-{4-[(1RS,2S)-2-(acetylamino)-1-(benzyl-
oxy)-4-methylpentyl]pyridin-2-yl}-N-(tert-butoxycarbo-
nyl)glycinate (29). Olefin 26 (0.27 g, 0.57 mmol) was
dissolved in CH2Cl2 (10 mL) and a solution of 2 M NaOH
in methanol (0.56 mL) and cooled to ꢀ78 ꢁC. O3 was passed
through the solution, which turned bright yellow at first and
was decolorized gradually. A colorless solid was formed and
the solution turned light blue and the excess of O3 was purged
from the solution with a stream of oxygen. Water and Et2O
were added and the two phases were separated and the aque-
ous layer was extracted with EtOAc. The combined organic
phases were dried over Na2SO4, concentrated under reduced
pressure, and the residue was purified by flash chromato-
graphy EtOAc/heptane 3:2 to give 29 (0.17 g, 58%) as a col-
orless amorphous solid; 1H NMR (400 MHz, CDCl3) d 8.26
(d, J¼5.1 Hz, 1H), 7.80 (s, 1H), 7.40–7.28 (m, 5H), 6.98 (d,
J¼5.1 Hz, 1H), 5.53 (d, J¼9.8 Hz, 1H), 4.75 (d, J¼18 Hz,
1H), 4.67 (d, J¼18 Hz, 1H), 4.58 (d, J¼11 Hz, 1H), 4.47
(d, J¼2.7 Hz, 1H), 4.36 (d, J¼11 Hz, 1H), 4.34–4.25 (m,
1H), 3.75 (s, 3H), 1.92 (s, 3H), 1.51 (s, 9H), 1.49–1.41 (m,
1H), 1.34–1.23 (m, 2H), 0.93 (m, 6H); 13C NMR
(100 MHz, CDCl3) d 170.7, 169.6, 153.6, 153.5, 149.5,
147.1, 137.5, 128.4, 128.1, 127.9, 117.5, 116.5, 82.0, 80.5,
71.6, 52.0, 51.4, 47.8, 40.4, 28.1, 24.7, 23.2, 23.1, 22.1; IR
(neat): 1754, 1715, 1655, 1602 cmꢀ1; FABHRMS calcd for
C28H40N3O6 (M+H): 514.2917, found: 514.2917.
3.2.17. (2R,S)-{[4-(2-Acetylamino-4-methyl-pentanoyl)-
pyridin-2-yl]-tert-butoxycarbonyl-amino}-acetic acid
methyl ester (31rac). Prepared in the same way as 33rac
starting with compound 31 (4 mg, 9.5 mmol) to give 31rac
(3 mg, 75%) as determined by chiral HPLC; [a]2D0 0 (c
0.25, CHCl3); 1H NMR identical as for compound 31.
3.2.18. (2S)-[4-(2-Acetylamino-4-methyl-pentanoyl)-
pyridin-2-ylamino]-acetic acid methyl ester (33). Boc-
protected amine 30 (28 mg, 66 mmol) was dissolved in CH2Cl2
(6 mL) and treated with trifluoroacetic acid (2 mL) for
1.5 h. The reaction mixture was concentrated under reduced
pressure and coevaporated from CHCl3. The residue was dis-
solved in CH2Cl2 and treated with Dess–Martin periodinane
(0.22 mL, 15 wt % in CH2Cl2, 99 mmol). After 4 min sodium
bisulfite (0.19 g, 0.97 mmol) in satd NaHCO3 aq was added.
The organic layer was washed with satd NaHCO3 aq, dried
over Na2SO4, and concentrated under reduced pressure.
The residue was purified by flash chromatography EtOAc/
heptane 2:1/4:1 to give b-strand mimetic 33 (0.014 g,
3.2.15. Methyl N-{4-[(1RS,2S)-2-(acetylamino)-1-hy-
droxy-4-methylpentyl]pyridin-2-yl}-N-(tert-butoxycar-
bonyl)glycinate (30). Benzyl protected alcohol 29 (0.16 g,
0.30 mmol) and Pd/C (0.15 g) were added to a mixture of
MeOH (15 mL) and AcOH (0.15 mL). The reaction was
stirred vigorously under H2 atmosphere (1 atm) for 30 h.
Pd/C was removed by filtration through Celite and the
solvent was removed under reduced pressure. The residue
was purified by flash chromatography EtOAc/heptane
2:1/1:0 to give 30 (0.096 g, 75%) as a colorless amorphous
solid; 1H NMR (400 MHz, CDCl3) d 8.23 (d, J¼5.2 Hz, 1H),
7.75 (s, 1H), 7.01 (d, J¼5.2 Hz, 1H), 5.73 (d, J¼9.1 Hz, 1H),
4.74–4.67 (m, 1H), 4.69 (s, 2H), 4.16–4.06 (m, 1H), 3.92 (s,
1H), 3.74 (s, 3H), 1.94 (s, 3H), 1.67–1.57 (m, 1H), 1.51 (s,
9H), 1.45–1.37 (m, 2H), 0.92 (d, J¼2.3 Hz, 3H), 0.90 (d,
J¼2.3 Hz, 3H); 13C NMR (100 MHz, CDCl3) d 171.1,
170.7, 153.6, 153.4, 152.1, 147.1, 117.1, 115.7, 82.1, 74.7,
53.5, 52.0, 47.8, 39.5, 28.1, 24.8, 23.2, 23.1, 21.9; IR
1
66%) as a yellow oil. [a]2D0 +11.5 (c 1, CHCl3); H NMR
(400 MHz, CDCl3) d 8.24 (d, J¼5.2 Hz, 1H), 7.03 (d,
J¼5.2 Hz, 1H), 6.98 (s, 1H), 6.27 (d, J¼8.1 Hz, 1H), 5.55–
5.46 (m, 1H), 5.35 (t, J¼5.5 Hz, 1H), 4.19 (d, J¼5.5 Hz,
2H), 3.77 (s, 3H), 2.03 (s, 3H), 1.76–1.64 (m, 1H), 1.62–
1.52 (m, 1H), 1.44–1.35 (m, 1H), 1.01 (d, J¼6.5 Hz, 3H),
0.87 (d, J¼6.5 Hz, 3H); 13C NMR (100 MHz, CDCl3)
d 200.0, 171.4, 169.9, 158.4, 149.2, 142.6, 111.0, 107.0,
52.6, 52.3, 43.5, 41.9, 25.1, 23.3, 23.2, 21.8; IR (neat):
3447–3166, 1741, 1700, 1651, 1608 cmꢀ1; FABHRMS
calcd for C16H24N3O4 (M+H): 322.1767, found: 322.1768.
3.2.19. (2R,S)-[4-(2-Acetylamino-4-methyl-pentanoyl)-
pyridin-2-ylamino]-acetic acid methyl ester (33rac).
b-Strand mimetic 33 (5 mg, 16 mmol) was dissolved in
THF (2 mL) and 1,8-diazabicyclo[5.4.0]undec-7-ene (25 mL,
167 mmol) was added. The reaction was subjected to micro-
wave irradiation, 80 ꢁC for 0.5 h. The reaction was concen-
trated under reduced pressure and the residue was filtered
through a short path of silica gel with EtOAc as eluent to
give 33rac (4 mg, 80%) as determined by chiral HPLC;
(neat): 3280, 3254, 1713, 1607, 1603, 1529 cmꢀ1
;
FABHRMS calcd for C21H34N3O6 (M+H): 424.2448, found:
424.2457.
3.2.16. (2S)-{[4-(2-Acetylamino-4-methyl-pentanoyl)-
pyridin-2-yl]-tert-butoxycarbonyl-amino}-acetic acid
methyl ester (31). Alcohol 30 (96 mg, 0.23 mmol) was
dissolved in CH2Cl2 (3 mL) and treated with Dess–Martin
periodinane (0.80 mL, 15 wt % in CH2Cl2, 0.34 mmol) for
20 min. Sodium disulfite (0.49 g, 2.55 mmol) in satd
NaHCO3 aq was added and the two phases were separated
and the aqueous phase was extracted with EtOAc followed
by a wash with satd NaHCO3 aq. The organic layer was dried
over Na2SO4, concentrated under reduced pressure, and
purified by flash chromatography EtOAc/heptane 3:1 to give
1
[a]2D0 0 (c 0.25, CHCl3); H NMR identical as for b-strand
mimetic 33.
Acknowledgements
This work was funded by grants from the Swedish Research
€
€
Council, AstraZeneca R&D Molndal, and the Goran Gus-
tafsson Foundation for Research in Natural Sciences and
Medicine.

10944
D. Blomberg et al. / Tetrahedron 62 (2006) 10937–10944
16. Smith, A. B.; Guzman, M. C.; Sprengeler, P. A.; Keenan, T. P.;
Supplementary data
Holcomb, R. C.; Wood, J. L.; Carroll, P. J.; Hirschmann, R.
J. Am. Chem. Soc. 1994, 116, 9947–9962.
17. Smith, A. B.; Keenan, T. P.; Holcomb, R. C.; Sprengeler, P. A.;
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1H NMR and 13C NMR spectra for all new isolated com-
pounds as well as chiral chromatograms of compounds 33
and 33rac are included. This material is available free of
charge via the Internet at http://www.sciencedirect.com.
Supplementary data associated with this article can be found
in the online version, at doi:10.1016/j.tet.2006.08.080.
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