Synthesis and conformational studies of peptides from new C-linked carbo-β-amino acids (β-Caas) with anomeric methylamino- and difluorophenyl moieties

Article abstract of DOI:10.1039/b810817j

New C-linked carbo-β-amino acids (β-Caas), Cbz-(S)-β-Caa- (NHBoc)-OMe (1) and Cbz-(R)-β-Caa-(NHBoc)-OMe (2), with an additional amine group (methylamino group of NHBoc) at the C-1 position of the lyxofuranoside side chain and Boc-(S)-β-Caa-(diFP)-OMe (3) and Boc-(R)-β-Caa-(diFP)- OMe (4), with a C-difluorophenyl (diFP) moiety at the anomeric position of the lyxofuranoside side chain were prepared from d-mannose. β-Peptides [tetra- and hexapeptides] were synthesized from these β-Caas, 'epimeric' [at the amine stereocentre (C_β)], using the concept of 'alternating chirality' to carry out their conformational studies [NMR (CDCl₃), CD and MD]. In the monomer design, it was envisaged that the presence of an additional amine group in 1 or 2 would help in solubilizing the peptides in water, while, the C-difluorophenyl (diFP) moiety of 3 and 4 is expected to enhance the biological activity. The peptides having 1 and 2, though could not retain their 12-10-mixed helices in water, have shown moderate activity against Gram positive and Gram negative bacterial strains. The peptides prepared from 3 and 4, much against our expectations, did not display any biological activity.

Full text of DOI:10.1039/b810817j

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PAPER
www.rsc.org/obc | Organic & Biomolecular Chemistry
Synthesis and conformational studies of peptides from new C-linked
carbo-b-amino acids (b-Caas) with anomeric methylamino- and
difluorophenyl moieties†
Gangavaram V. M. Sharma,*a Velaparthi Subash,a Nelli Yella Reddy,a Kongari Narsimulu,b Rapolu Ravi,b
Vivekanand B. Jadhav,a Upadhyayula S. N. Murthy,c Kankipati Hara Kishorec and Ajit C. Kunwar*b
Received 25th June 2008, Accepted 13th August 2008
First published as an Advance Article on the web 18th September 2008
DOI: 10.1039/b810817j
New C-linked carbo-b-amino acids (b-Caas), Cbz-(S)-b-Caa-(NHBoc)-OMe (1) and
Cbz-(R)-b-Caa-(NHBoc)-OMe (2), with an additional amine group (methylamino group of NHBoc) at
the C-1 position of the lyxofuranoside side chain and Boc-(S)-b-Caa-(diFP)-OMe (3) and
Boc-(R)-b-Caa-(diFP)-OMe (4), with a C-difluorophenyl (diFP) moiety at the anomeric position of the
lyxofuranoside side chain were prepared from D-mannose. b-Peptides [tetra- and hexapeptides] were
synthesized from these b-Caas, ‘epimeric’ [at the amine stereocentre (Cb)], using the concept of
‘alternating chirality’ to carry out their conformational studies [NMR (CDCl3), CD and MD]. In the
monomer design, it was envisaged that the presence of an additional amine group in 1 or 2 would help
in solubilizing the peptides in water, while, the C-difluorophenyl (diFP) moiety of 3 and 4 is expected to
enhance the biological activity. The peptides having 1 and 2, though could not retain their 12–10-mixed
helices in water, have shown moderate activity against Gram positive and Gram negative bacterial
strains. The peptides prepared from 3 and 4, much against our expectations, did not display any
biological activity.
our earlier studies, we designed ‘new motifs’ using C-linked carbo-
b-amino acids14 (b-Caa; ‘epimeric’ at the amine stereocentre),
Introduction
having carbohydrate moieties as side chains,15–17 and demonstrated
that peptides derived with ‘alternating chirality’ generate very
robust and stable right handed 10–12-mixed helices.18a Similarly,
peptides made from the dipeptide repeats of b-Caa–b-hGly (b-
homo glycine), resulted in both right- and left-handed mixed
helices,18b where the ‘switch’ in the handedness was governed by
the stereochemistry at the Cb of the b-Caa used in the design.
Our further designs with dipeptide repeats to enhance the skeletal
diversity resulted in a–g-hybrid peptides,19a which generated a
10–12-helix devoid of b-amino acid. Yet another new motif for
the realization of 10–12-b-helices from peptides with a-aminoxy
acid (Ama) and (R)-b-Caa alternating was recently reported.19b
Therapeutic applications of cationic b-peptides or their
biomimetic analogues, as antibacterial agents are promising.20,21
Their direct action on the bacterial cell membrane is likely to pre-
vent rapid development of bacterial resistance. The designs used by
different research groups22 for imparting antibacterial activity in
b-peptides had helical structures with charged hydrophilic residues
on one face of the helix and hydrophobic residue on its other face,
providing amphiphilicity to the helix.
In our studies, the design of b-Caas was based on the C-
linked ribosyl glycine moiety that is present in nikkomycins,23
the dipeptide antibiotics. The b-peptides investigated by us
however could not be explored for therapeutic applications due
to their inherent limitation of insolubility in water. To address the
problems associated with water solubility and biological activity,
we designed new b-Caa monomers. In the first simple design on
bifunctional b-Caas, an additional amine group (NHBoc) was
introduced at the C-1 position of a carbohydrate side chain, which
Designing non-biological polymers, which fold into predictable
secondary and tertiary structures, referred to as ‘foldamers’,1 has
been an area of intense research activity.2 b-Peptides, obtained
from b-amino acids,3 the higher homologues of a-amino acids,
display a wide variety of secondary structures,4–6 thereby pro-
viding an unparalleled opportunity to understand the factors
governing protein structure and folding.7 The wide variety of
biological activities8 of b-peptides, besides their resistance to
enzyme degradation9 and enhancing protein binding ability,10
has made them attractive targets in the area of peptidomimetics
and drug discovery. From extensive theoretical and molecular
dynamics simulation studies11 it was found that there is an
intrinsic preference for the formation of 10–12-mixed helices in
these b-peptides. The mixed 10–12-helices (b-helices), containing
intertwined 10- and 12-membered (mr) H-bond rings, unique to b-
peptides, were first reported by Seebach et al.12 by using alternating
2
3
b –b -amino acids as a motif, while, Kessler et al.13 realized them
using furanose amino acid (FAA)–b-hGly (b-homo glycine). In
aD-211, Discovery Laboratory, Organic Chemistry Division III, Hyderabad,
500 007, India. E-mail: esmvee@iict.res.in
bCentre for Nuclear Magnetic Resonance, Hyderabad, 500 007, India.
E-mail: kunwar@iict.res.in
cBiology Group, Indian Institute of Chemical Technology, Hyderabad, 500
007, India
† Electronic supplementary information (ESI) available: Experimental
details and spectral data for compounds 21, 2, 28, 29, 5, 6, 31, 32, 35,
38, 40, 41, 10, 11, 9, 12, 47, 51, 52, 3, 4, 55, 56, 60, 62, 63, 15, 64, 16,
NMR spectra, solvent titration plots and distance constraints used in MD
calculations for selected compounds. See DOI: 10.1039/b810817j
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is expected to help enhancing solubility of derived peptides in
water.
reduction with Ph3P in methanol for 1 h, followed by protection
with (Boc)2O for 5 h furnished 20 (75%).
In yet another novel design, we envisaged b-Caas having
C-difluorophenyl (diFP) glycoside as side chain, to promote bi-
ological activity. The rationale behind such a design was two fold:
firstly, C-aryl glycoside moieties24 are present in several bio-active
compounds. The difluorophenyl moiety is part of the structure
of the drugs such as fluconazole and diflunisal. Secondly, the
highly electronegative character of fluorine imparts very different
properties to the molecule,25 allowing exploration by replacing
hydrogen with fluorine in drug design and discovery.26 Further,
it is well documented that fluorinated amino acids enhance
protein stability and support in other applications.27 Based on
the above observations, the preparation of a new class of b-Caas,
having difluorophenyl (diFP) C-glycofuranoside as side chain, was
considered to improve the biological activity of the b-peptides
thus derived. The present work reports the synthesis of ‘epimeric’
(at the amine stereocentre, Cb), bifunctional b-Caas 1 and 2 and
b-Caa (diFP) 3 and 4 (Fig. 1), with NHBoc and C-difluorophenyl
(diFP) groups respectively at the anomeric position (C-1) of
lyxofuranoside side chain from D-mannose, conversion of these
monomers into the corresponding tetra- and hexapeptides 7–16
(Fig. 2 and 3) and their structural studies by extensive NMR, MD
and CD analysis. Thus, the structural features incorporated in the
newly designed b-Caas 1–4 in the present study: a) would help
in solubilizing the peptides in water due to the additional amine
group (NHBoc) and b) the C-difluorophenyl (diFP) glycoside side
chain might provide desirable biological activity to the b-peptides
13–16.
Further, hydrolysis of the 5,6-acetonide in 20, on reaction
with PTSA in aq. MeOH at room temperature for 8 h gave the
diol 21 in 87% yield. Oxidative cleavage of the diol in 21 with
NaIO4 in a solution of MeOH–H2O at room temperature for 2 h
afforded aldehyde 22, which on subsequent Wittig olefination with
(methoxycarbonylmethylene)triphenylphosphorane in benzene at
reflux for 5 h furnished a,b-unsaturated ester 23 as a cis–
trans mixture of isomers (81%). Michael addition on 23 with
benzylamine29 at room temperature for 12 h gave an ‘epimeric’ (at
the amine stereocentre) mixture of esters 24 (43%) and 25 (25%)
separable by column chromatography. Hydrogenolysis of esters 24
and 25 in the presence of 10% Pd–C in methanol under hydrogen
atmosphere afforded the respective amines 26 and 27, which were
treated with DIPEA and Cbz-Cl in CH2Cl2 at room temperature
for 2 h to furnish 1 (87%) and 2 (90%) respectively. Base hydrolysis
(aq. NaOH in methanol) of 1 and 2 gave the corresponding acids
28 (83%) and 29 (86%) respectively. The overall yield was 14%.
2. Synthesis of amino acids 5 and 6
Reaction of the ester 30 with benzylamine19,29 (Scheme 2) at room
temperature for 12 h gave a separable mixture of esters 31 (38%)
and 32 (26%), which on reaction with 10% Pd–C in methanol and
subsequent treatment of the resulting amines 33 and 34 with Et3N
and (Boc)2O in THF furnished 5 (92%) and 6 (88%) respectively.
3. Synthesis of peptides 7–12
The synthesis of the peptides 7–12 is outlined in Schemes 3 and
4. Accordingly, Boc-(R)-b-Caa-OMe 6 (Scheme 3) was saponified
with aq. 4 N NaOH to give acid 35 (90%), which on coupling with
33 in the presence of EDCI, HOBt and DIPEA in CH2Cl2 afforded
the dipeptide 36 (83%). Ester 36 on exposure to CF3COOH in
CH2Cl2 was converted into the salt 37, which on condensation with
acid 28 in the presence of EDCI, HOBt and DIPEA in CH2Cl2
furnished the tripeptide 7 in 60% yield.
Base hydrolysis of tripeptide 7 gave the corresponding acid 38,
which on coupling with 27 in the presence of EDCI, HOBt and
DIPEA in CH2Cl2 afforded the tetrapeptide 8 in 49% yield. Peptide
8 on sequential treatment with base, followed by Cbz deprotection
with 10% Pd–C in MeOH at room temperature under hydrogen
Results and discussions
1. Synthesis of amino acids 1 and 2
The main strategy in the synthesis of 1 and 2, from D-mannose
lies in the conversion of a) the anomeric (C-1) carbon into a Boc
protected aminomethyl group and b) to install the amino acid side
chain at the C-4 centre using the C-5 carbon. Accordingly, the
known alcohol 1728 was treated with p-TsCl, Et3N and DMAP
(catalytic) in CH2Cl2 at room temperature for 3 h to give 18 in
83% yield (Scheme 1). Tosylate 18 was further reacted with NaN3
in DMF at 70 ◦C for 6 h to afford azide 19 (65%), which on
Fig. 1 Structures of C-linked carbo b-amino acids 1–6.
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Fig. 2 Structures of peptides 7–12 containing b-Caas 1, 2, 5 and 6.
Fig. 3 Structures of peptides 13–16 derived from b-Caas 3 and 4.
atmosphere and subsequent exposure to CF3COOH gave the acid
salt 8a. Similarly, peptide 8 on exposure to CF3COOH underwent
Boc deprotection to furnish the salt 8b.
while, exposure to CF3COOH furnished the salt 44. Peptide 43 on
further Cbz and Boc deprotection with 10% Pd–C and CF3COOH
respectively gave the salt 10a.
Likewise, Boc-(S)-b-Caa-OMe 5 (Scheme 3) was saponified with
aq. 4 N NaOH to give acid 40, which on coupling with 34 in the
presence of EDCI, HOBt and DIPEA in CH2Cl2 afforded the
dipeptide 41 (84%). Coupling of the salt 42 (obtained from 41 by
the exposure to CF3COOH) with acid 29 gave peptide 10 (67%).
Hydrolysis of 10 with base afforded the corresponding acid 43,
In a further study, acid 38 on coupling with 44 (Scheme 4) in
the presence of EDCI, HOBt and DIPEA in CH2Cl2 gave the
peptide 9 (54%) yield. Base hydrolysis of 9 and subsequent Cbz
and Boc deprotections on the resulting acid, using 10% Pd–C and
CF3COOH in CH2Cl2, respectively afforded the salt 9a. Similarly,
peptide 9 on reaction with CF3COOH furnished the salt 9b.
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Scheme 1 Synthesis of amino acids 1 and 2. Reagents and conditions: a) p-TsCl, Et3N, cat. DMAP, CH2Cl2, 0 ◦C–RT, 3 h; b) NaN3, DMF, 70 ◦C, 6 h;
=
c) Ph3P, CH3OH, 1 h, then (Boc)2O, RT, 5 h; d) PTSA, CH3OH–H2O, 5 : 1, RT, 8 h; e) NaIO4, MeOH–H2O (5 : 1), RT, 2 h; f) Ph3P CHCOOCH3,
benzene, reflux , 5 h; g) BnNH2, RT, 12 h; h) H2, 10% Pd–C, CH3OH, RT, 12 h; i) Cbz-Cl, DIPEA, CH2Cl2, 0 ◦C–RT, 2 h; j) aq. 4 N NaOH, CH3OH,
0
◦C–RT, 2 h.
Scheme 2 Synthesis of amino acids 5 and 6. Reagents and conditions: a) BnNH2, RT, 12 h; b) H2, 10% Pd–C, CH3OH, RT, 12 h; c) (Boc)2O, Et3N, THF,
0 ◦C–RT, 2 h.
Scheme 3 Synthesis of peptides 7, 8 and 10. Reagents and conditions: a) aq. 4 N NaOH, CH3OH, 0 ◦C-RT; b) HOBt (1.2 equiv.), EDCI (1.2 equiv.),
DIPEA (2 equiv.), dry CH2Cl2, 0 ◦C–RT; c) CF3COOH, dry CH2Cl2, 0 ◦C–RT; d) H2, 10% Pd–C, CH3OH, RT.
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Scheme 4 Synthesis of peptides 9, 11 and 12. Reagents and conditions: a) aq. 4 N NaOH, CH3OH, 0 ◦C–RT; b) HOBt (1.2 equiv.), EDCI (1.2 equiv.),
DIPEA (2 equiv.), dry CH2Cl2, 0 ◦C–RT; c) CF3COOH, dry CH2Cl2, 0 ◦C–RT; d) H2, 10% Pd–C, CH3OH, RT.
Acid 43 on condensation with 26 and 39 (Scheme 4) indepen-
dently in the presence of EDCI, HOBt and DIPEA in CH2Cl2
afforded the peptides 11 (53%) and 12 (56%) respectively. The
peptide 11 on Boc deprotection by exposure to CF3COOH gave
corresponding amine salt 11a. Similarly, peptides 11 and 12
on sequential treatment with aq. 4 N NaOH, 10% Pd–C (Cbz
deprotection) and CF3COOH (Boc deprotection) furnished the
salts 11b and 12a respectively.
4. Synthesis of amino acids 3 and 4
Amino acids 3 and 4 were prepared from the known lactone 45.30
Accordingly, lactone 45 (Scheme 5) on reaction with the aryl
lithium reagent generated fr◦om 1-bromo-2,4-difluorobenzene and
n-BuLi in dry THF at -78 C gave 46 (85%) as a diastereomeric
mixture. Hydrolysis of 5,6-acetonide in 46 with 60% aq. AcOH at
room temperature afforded the diol 47 (75%). Oxidative cleavage
Scheme 5 Synthesis of amino acids 3 and 4. Reagents and conditions: a) 1-bromo-2,4-diflurobenzene, n-BuLi, dry THF, -78 ◦C–RT, 5 h; b) 60% aq.
AcOH, RT, 6 h; c) NaIO4, aq. sat. NaHCO3, CH2Cl2, 0 ◦C–RT, 5 h; d) Ph3P CHCOOCH3, CH3OH, 0 ◦C–RT, 5 h; e) Et3SiH, BF3·Et2O, dry CH3CN,
=
-10 ◦C, 1 h; f) BnNH2, RT, 12 h; g) H2, 10% Pd–C, CH3OH, RT, 12 h; h) (Boc)2O, Et3N, CH2Cl2, 0 ◦C–RT, 3 h; i) 4 N NaOH, CH3OH 0 ◦C–RT, 2 h;
j) CF3COOH, CH2Cl2, 0 ◦C–RT.
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of 47 (NaIO4, aq. NaHCO3) in CH2Cl2 furnished aldehyde 48,
which on subsequent Wittig olefination in MeOH gave ester 49
in 78% yield as a cis-isomer, as adjudged from the NMR data.
Ester 49 on reductive deoxygenation with triethylsilane31 and
BF3·OEt2 in dry CH3CN furnished b-C-aryl glycoside 50 (86%)
as an exclusive product. The exclusive formation of b-glycoside
50 may be attributed to the presence of 2,3-acetonide group at
the b-face, which thereby directs the nucleophile attack from the
a-face.
Michael addition on ester 50 (Scheme 5) with benzylamine29 at
room temperature gave a mixture of esters 51 (41%) and 52 (20%)
as epimers at the amine stereocentre. Hydrogenolysis of 51 and 52
with 10% Pd–C in MeOH and subsequent reaction of 53 and 54
with (Boc)2O and Et3N in CH2Cl2 furnished 3 (89%) and 4 (94%),
respectively. Esters 3 and 4 on hydrolysis with aq. NaOH solution
gave the respective acids 55 and 56, while exposure to CF3COOH
afforded the TFA salts 57 and 58 respectively. The overall yield
was 24%.
The absolute stereochemistry of 3 was unambiguously obtained
from the spectral analysis (Fig. 4). From the 1H NMR spectrum
3
(500 MHz) of 3 at 303 K, the observed couplings, JC1H-C2H
=
3.7 Hz, 3JC2H-C3H = 6.2 Hz, 3JC3H-C4H = 3.5 Hz and NOEs such as:
C1H–C4H and C1H–C3H in the NOESY spectrum provide ample
proof, that the difluorophenyl ring at C1 is in the b-position and
the sugar pucker is ◦E. Very strong NOEs, CH3(pro-R)–C2H and
CH3(pro-R)–C3H, suggest an envelop conformation for isopropyli-
dine ring, where C2, C3 and both oxygens are in one plane.
Scheme 6 Synthesis of peptides 13–16. Reagents and conditions: a) HOBt
(1.2 equiv.), EDCI (1.2 equiv.), DIPEA (2 equiv.), dry CH2Cl2, 0 ◦C–RT,
4 h; b) 4 N NaOH, CH3OH, 0 ◦C–RT, 2 h; c) CF3COOH, dry CH2Cl2,
0
◦C–RT, 2 h.
6. Conformational analysis of peptides 7–16
Having obtained the first family of water soluble peptides derived
from bifunctional b-Caas, it was important to learn as to whether
the design sustains a stable helix in water. As a first step, therefore,
NMR studies on 5–10 mM solution of peptides 8b, 9b and 11a
were conducted in water and DMSO-d6 at 303 K.32 The spectra
in water (90% H2O + 10% D2O) did not display any distinct
signatures of a secondary structure. Most of the NH resonances
were grouped in the region of 7.70–8.40 ppm. The large value of
the temperature coefficients of the amide proton chemical shifts
(Dd/DT > 7.6 ppb ◦C-1) imply that they do not participate in H-
bonding. The absence of medium range NOEs further ruled out
well defined folds. The strong propensity to form intermolecular
H-bonds with water molecules appears to destabilize formation of
the secondary structure. The spectra of these peptides in DMSO-
d6 were, however, broad and lacked the desired resolution, leading
us to abandon our efforts for a detailed NMR study. The NMR
studies of 9b were also carried out in CD3OH. Limited dispersion
of amide chemical shift and resonance overlap of CbH and CaH
region did not permit the analysis. Though the destabilization
of helices in water is observed for most of the b-peptides,21b,33 it
has been demonstrated that these peptides may reorganize into
helices upon docking to their receptors.34 Since, a preorganized
structure is not a prerequisite for antimicrobial activity,21b it does
not preclude these peptides from displaying biological activity.
The lack of secondary structures in peptides 8b, 9b and 11a,
obtained by using the concept of ‘alternating chirality’, compelled
us to check the validity of the design principle. Therefore the NMR
studies of 5–10 mM solutions of peptides 7–16 in CDCl3 solutions
were undertaken in order to find the presence of 10–12-helical
structures.32
Fig. 4 Ester 3 (A) characteristic NOE correlations; (B) energy minimized
structure.
5. Synthesis of peptides 13–16
Condensation of acid 55 with the salt 58 (Scheme 6) in the presence
of EDCI, HOBt and DIPEA in CH2Cl2 afforded dipeptide 59
(73%), which on base (aq. 4 N NaOH) hydrolysis gave 60, while
exposure to CF3COOH afforded salt 61. Condensation of acid 60
with 61 gave the tetrapeptide 13 (45%). Hydrolysis of 13 with base
furnished the acid 62, which on reaction with 61 in the presence
of EDCI, HOBt and DIPEA in CH2Cl2 afforded the hexapeptide
14 (34%).
Likewise, condensation of acid 56 with salt 57 in the presence
of EDCI, HOBt and DIPEA in CH2Cl2 furnished the dipeptide
63 (64%) yield. Ester 63 on base hydrolysis with aq. 4 N NaOH
gave the corresponding acid 64, which on coupling with the salt
65 (obtained from 63 by the exposure to CF3COOH) afforded
tetrapeptide 15 (48%). Base hydrolysis of 15 and coupling of the
corresponding acid 66 with 65 in the presence of EDCI, HOBt
and DIPEA in CH2Cl2 resulted in the hexapeptide 16 (41%).
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The tripeptide 7 does not seem to have a folded structure. For
tetrapeptide 8, a highly dispersed spectrum in the amide and CaH
region, with chemical shift (d) dispersion of 1.68 and 0.64 ppm
respectively, indicate the presence of a well defined structure. The
amide protons, NH(1)–NH(4) resonated at 6.73, 7.23, 7.33 and
8.31 ppm respectively, suggesting involvement of several of them
in H-bonds. To confirm this observation, solvent titration studies35
were carried out by adding up to 50% v/v DMSO-d6 and small
variation in their d values (DdNH) confirmed, that except NH(2),
all amide protons participate in intramolecular H-bonding. The
3JNH-CbH = 8.6–9.3 Hz for all the four residues suggest that NH and
CbH protons are in antiperiplanar (ap) arrangement corresponding
to C(O)–N–Cb–Ca(f) ~ 120◦ (Fig. 5). Values of 3JCaH-CbH > 10 Hz
and < 5 Hz imply predominance of a single conformation about
N–Cb–Ca–C(O) (q).
provide compelling evidence for the presence of extended 10–12-
helix for the peptide 9.
The CDCl3 spectrum of 10 shows the appearance of two amide
signals with d > 7 ppm, which hints at their participation in
intramolecular H-bonds. Solvent titration studies35 confirmed that
NH(2) and NH(3) participate in H-bonds because of small DdNH
3
(< 0.51 ppm). The observation of the JCaH-CbH with values >
9 Hz and < 5 Hz, suggest predominantly a single conformation
3
about Ca-Cb. The JCaH-CbH values, along with various CaH–CbH
and strong sequential NOEs, CaH(i-1)–NH(i) confirm that the q
for the predominant conformation is ~ 60◦, which is found to be
consistent with a right handed helix. Additionally, NOEs, CbH(1)–
NH(3) and CbH(1)–CaH(pro-R)(3) demonstrate the presence of a
12–10-mixed helical structure. However, larger deviations from
extreme values of the couplings as well as weaker NOEs compared
to 8 and 9 indicate the presence of a sizeable fraction of disordered
structures in 10. In the present study 10 is the smallest peptide that
generated a well defined structure.
Peptide 11, a tetrapeptide, made by an elongation of 10 with
monomer 1 at the C-terminal end, again shows the features very
similar to those of 10. The participation of the NH(2) and NH(3)
in H-bonding was confirmed from their large d as well as small
DdNH shifts in the solvent titration studies.35 The indirect couplings
as well as the medium range NOEs, CbH(1)–NH(3) and CbH(1)–
CaH(pro-R)(3), are consistent with a 12–10-H-bonded arrangement.
For peptide 12, the extended structure of 11, all but the first
and the sixth amide protons are not involved in H-bonds.35 The
3JNH-CbH = 8.0–9.8 Hz, for all the residues corresponds to f ~
120◦, which is in accordance with the value for mixed helices.
3JCaH-CbH values of > 10 Hz and < 5 Hz for all the residues suggest
predominance of a single conformation about Ca-Cb, which along
with some sequential NH–CaH NOEs confirm a value of ~ 60◦
for q. The distinct signatures for the 12–10–12–10-H-bonds in the
ROESY spectrum are very clearly noticeable. The medium range
NOEs like CbH(1)–NH(3), CbH(1)–CaH(pro-R)(3), CbH(3)–NH(5)
andCbH(3)–CaH(pro-R)(5) strongly support the presence of 12-mr H-
bonds between NH(3)–CO(Boc) and NH(5)–CO(2) and medium
NOEs NH(2)–NH(3) and NH(4)–NH(5) show the presence of
10-mr H bonds exist between NH(2)–CO(3) and NH(4)–CO(5).
1H NMR study on tetrapeptide 13, in CDCl3 showed a well
dispersed spectrum in both amide and alpha regions. Most
of the amide protons appear at low field. Solvent titration
studies35 confirm that except NH(2) all of them participate in
Fig. 5 Schematic representation of backbone torsion angles in b-peptides.
The NOEs such as: NH(2)–CaH(pro-R)(1), NH(3)–CaH(pro-S)(2),
NH(4)–CaH(pro-R)(3) support a value of q ~ 60◦. These observations
along with the presence of medium range NOEs, CbH(2)–NH(4)
and CbH(2)–CaH(pro-R)(4) and weak NOEs, NH(1)–NH(2) and
NH(3)–NH(4), in the ROESY experiments (Fig. 6) confirm the
10–12–10-H-bonded arrangement in tetrapeptide 8. The structure
is stabilized by one 12-mr H-bond NH(4)–CO(1) and two 10-mr
H-bonds [NH(1)–CO(2) and NH(3)–CO(4)]. The exchange peaks
in the ROESY spectrum indicate the presence of a very small
amount of other isomer.
Hexapeptide 9, an extension of 8 at the C-terminal, again
displayed all the signatures of a well defined 10–12-mixed
helix. All the amide protons, excluding NH(2) participate in
intramolecular H-bonding, which was confirmed by the solvent
titration studies.35 The distinctive NOEs, CbH(2)–NH(4), CbH(2)–
CaH(pro-R)(4), CbH(4)–NH(6) and CbH(4)–CaH(pro-R)(6), confirm
the 12-mr H-bonds between NH(4)–CO(1) and NH(6)–CO(3),
whereas 10-mr H-bonds between NH(1)–CO(2), NH(3)–CO(4)
and NH(5)–CO(6) were confirmed by the NOEs, NH(1)–NH(2),
NH(3)–NH(4) and NH(5)–NH(6) respectively. The above data
along with the coupling constants and the H-bonding information,
3
intramolecular H-bonding. The JNH-CbH = 8.1–9.5 Hz for all
Fig. 6 ROESY spectrum of 8: The characteristic NOE interactions CbH(2)–NH(4), CbH(2)–CaH(pro-R)(4), NH(1)–NH(2) and NH(3)–NH(4) are marked
as 1, 2, 3 and 4 respectively.
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Fig. 7 ROESY spectrum of 13: The characteristic NOE interactions CbH(2)–NH(4), CbH(2)–CaH(pro-R)(4), NH(1)–NH(2) and NH(3)–NH(4) are marked
as 1, 2, 3 and 4 respectively.
the four residues, suggest that NH and CbH protons are in
ap arrangement, corresponding to C(O)-N-Cb-Ca(f) ~ 120◦.
Observation of 3JCaH-CbH > 10 Hz and < 5 Hz, clearly demonstrates
the predominance of a single conformation around Ca-Cb, while
the characteristic NOEs (Fig. 7) like, CbH(2)–NH(4), CbH(2)–
CaH(pro-R)(4), qualify 12-mr H-bond, NH(4)–CO(1), while 10-mr
H-bonds, NH(1)–CO(2) and NH(3)–CO(4), are confirmed by
the NOEs, NH(1)–NH(2) and NH(3)–NH(4). These observations
fully support a 10–12–10-H-bonded arrangement in peptide 13.
For hexapeptide 14, the participation of all amide protons,
excluding NH(2), was confirmed by their low field ds as well
Fig. 8 CD spectra of 8–12.
as solvent titration studies (DdNH < 0.34 ppm).35 A single
conformation around Ca-Cb is indicated by the values of JCaH-CbH
(> 10 Hz and < 5 Hz), while, the characteristic NOEs, such
as: CbH(2)–NH(4), CbH(2)–CaH(pro-R)(4), CbH(4)–NH(6), CbH(4)–
CaH(pro-R)(6), NH(1)–NH(2), NH(3)–NH(4) and NH(5)–NH(6)
qualified the extended 10–12-mixed helix.
1H NMR spectra of tetrapeptide 15 and hexapeptide 16
distinctly show the presence of a 12–10-mixed helix, as confirmed
by the low field amide resonances, solvent titration studies,
coupling constants and characteristic NOEs.35 In tetrapeptide
15, the presence of a very small amount of the other rotamer
is indicated by the exchange peaks in the ROESY spectrum.
Except for the second residue in 8, 9, 13 and 14, and the first
3
Fig. 9 CD spectra of 13–16.
residues in 10–12, 15 and 16, for all b-Caa residues JCbH-C4H
>
9 Hz implies c1(CbH–Cb–C4–C4H) ~ 180◦. However, like earlier
observations,15 the second residue in 8, 9, 13 and 14 and the first
3
residue in 10–12, 15 and 16 have JCbH-C4H ~ 6 Hz, suggesting
predominance of structures with |c1| ~ 60◦. The sugar ring
3
3
3
couplings of JC1H-C2H ~ 0 Hz, JC2H-C3H ~ 5.8 Hz and JC3H-C4H
~
3.2 Hz are in conformity with the 2T3 sugar pucker for the furanose
rings.36
The CD spectra (100 mM solution in methanol) of the peptides
8, 9, 11 and 12 (Fig. 8) show diagnostic signatures of a right handed
12–10-mixed helix with a maxima at about 203 nm, with very little
excursion in the negative molar ellipticity. For 10 the maximum
has shifted to 197 nm, partly due to fraying at the termini and
contributions from other disordered structures.
On the other hand, 13–16 (Fig. 9) show distinctly different
signatures. Though the maxima around 203–208 nm are no-
ticeable, a shoulder at higher wavelength around 218 nm is
unmistakably present. For 15, the maximum has shifted to a lower
wavelength compared to 13, 14 and 16, possibly due to fraying and
contributions from other disordered structures.
Fig. 10 CD spectra of 9b and 9a.
For the restraint molecular dynamics (MD) studies (Fig. 11
and 12), constraints were derived from the volume integrals ob-
tained from the ROESY spectra using a two-spin-approximation.
Fig. 11A and B show the 20 lowest energy superimposed structures
of 8 and 9 respectively. The MD structures depict the features
already alluded to the heavy atom and the backbone RMSDs
˚
˚
˚
˚
respectively of 1.56 A and 0.74 A for 8 and 1.67 A and 1.10 A
for 9. These values are significantly larger than those observed for
the corresponding peptides studied earlier, which resulted in very
The peptides 9a and 9b display rather unusual CD spectra
(Fig. 10).
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good antibacterial activity against Chromobacterium violaceum
compared to all other bacterial strains. The MIC values for peptide
11b were better in Gram negative Pseudomonas oleovorans and
Chromobacterium violaceum bacteria when compared to Gram
positive bacteria. The activity of all these peptides however is very
much inferior to that observed for melittin.
Similarly, the peptides 13–16 were tested for their antimicrobial
activity against S. aureus, E. faecalis, E. faecium and E. coli and
found to show no activity.32
Fig. 11 Stereoviews of the superimposition of the 20 lowest energy
structures of (A): peptide 8 and (B): peptide 9 (sugars are replaced with
methyl groups after calculations).
8. Conclusions
New C-linked carbo-b-amino acids (b-Caas) have been prepared,
with a methylamino (NHBoc) group and a difluorophenyl (diFP)
moiety at the anomeric (C-1) position of the lyxofuranoside side
chain. The new ‘epimeric’ (at Cb) monomers were utilized for the
synthesis of b-peptides, using the design principle of ‘alternating
chirality’ to realize the 12–10-mixed helical patterns in them. The
helical structures in the new peptides were ascertained from the
NMR, CD and MD studies. No helical patterns were observed
in aqueous solutions for the peptides having bifunctional b-Caas,
after the removal of protecting groups. Though, some of the water
soluble peptides have shown moderate antibacterial activity, the
peptides derived from b-Caas (diFP) have shown no activity, much
against our expectations. Further, no effect of the bulky aromatic
ring, with fluorines, was observed on the helix formation and
stability. The present study thus creates new b-Caas with anomeric
substitutioninthesugarsidechain. Expandingtheconformational
space by using new b-amino acids opens up novel options and
enhances the diversity in synthetic peptides and proteins.
Fig. 12 Stereoviews of the superimposition of the 20 lowest energy
structures of (A): peptide 13 and (B): peptide 14 (sugars are replaced
with methyl groups after calculations).
robust mixed helices. Fig. 12A and B show the 20 lowest energy
superimposed structures of 13 and 14 respectively. The heavy atom
˚
˚
and the backbone RMSDs respectively are 1.86 A and 0.51 A for
˚
˚
13 and 1.71 A and 0.62 A for 14.
6. Evaluation of antibacterial activity
Experimental section
The b-peptides 9a, 10a, 11b and 12a were evaluated for their
antibacterial activity. The standard measurement of antibacterial
potency of a compound is the minimum inhibitory concentration
(MIC), required for complete inhibition of growth, were obtained
by the broth dilution method.37 The above peptides showed
antibacterial activity against a variety of bacterial strains whose
MIC values towards the six selected bacterial strains are shown in
Table 1. The MIC required for complete inhibition of growth was
6.3 mg mL-1 for 9a against Gram positive Bacillus sphaericus and
Gram negative Chromobacterium violaceum (standard: melittin).
The MIC values of peptide 12a (6.3 mg mL-1) also showed
(3aR,4R,6R,6aS)-6-[(4R)-2,2-Dimethyl-1,3-dioxolan-4-yl]-2,2-
dimethylperhydrofuro[3,4-d][1,3]dioxol-4-yl-methyl 4-methyl-1-
benzenesulfonate (18)
A solution of 17 (5 g, 18.2 mmol) and Et3N (4.9 mL, 36.4 mmol)
in CH2Cl2 (50 mL) containing DMAP (0.1 eq.) at 0 ◦C was
treated with p-TsCl (3.82 g, 20 mmol) and stirred at ambient
temperature for 3 h. The reaction mixture was diluted with CH2Cl2
(30 mL), washed with water (30 mL), brine (30 mL) and dried
(Na2SO4). Evaporation of solvent and purification of the residue
by column chromatography (silica gel, 20% EtOAc in petroleum
Table 1 Antibacterial activities of water soluble carbo-b-peptidesa
Microorganism
Gram positive
Gram negative
Bacillus
subtilis
Bacillus
sphaericus
Serratia
marcescens
Pseudomonas
oleovorans
Klebsiella
aerogenes
Chromobacterium
violaceum
Peptide
9a
25
50
25
25
0.8
6.3
25
25
12.5
0.4
12.5
25
25
12.5
0.4
12.5
25
12.5
12.5
6.3
12.5
50
25
12.5
6.3
6.3
25
12.5
6.3
10a
11b
12a
Melittin
3.1
a Activities are expressed as minimum inhibitory concentration (MIC, mg mL-1) defined as lowest concentration required for inhibition of growth of
bacterial strain.
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ether) afforded 18 (6.48 g, 83%) as a colorless syrup; [a]D = +9.7
(c 1.0, CHCl3); IR (neat): 2950, 1580, 1210, 1175, 1005, 825 cm-1;
1H-NMR (CDCl3, 500 MHz): d 7.79 (d, 2H, J = 8.3 Hz, Ar-H),
7.37 (d, 2H, J = 8.3 Hz, Ar-H), 4.77 (dd, 1H, J = 3.7, 6.0 Hz,
C3H), 4.72 (dd, 1H, J = 1.3, 6.0 Hz, C2H), 4.30 (ddd, 1H, J = 4.5,
6.3, 7.4 Hz, C5H), 4.19 (ddd, 1H, J = 1.3, 4.6, 5.8 Hz, C1H), 4.08
(dd, 1H, J = 4.5, 10.5 Hz, C6H), 4.02 (dd, 1H, J = 5.8, 8.6 Hz,
CH2a), 4.01 (dd, 1H, J = 4.5, 10.5, Hz, C6¢H), 3.90 (dd, 1H, J =
4.6, 8.6 Hz, CH2b), 3.86 (dd, 1H, J = 3.7, 7.4 Hz, C4H), 2.46 (s,
3H, Ar-CH3), 1.47 (s, 3H, Me), 1.41 (s, 3H, Me), 1.36 (s, 3H, Me),
1.32 (s, 3H, Me); 13C NMR (CDCl3, 100 MHz): d 130.0, 127.9,
112.9, 109.1, 82.5, 82.4, 81.7, 80.9, 73.3, 69.6, 66.5, 26.8, 26.0, 25.0,
24.5, 21.6; HRMS (ESI): m/z calculated for C20H28O8S (M++H)
429.1583, found 429.1578.
(ESI): m/z calculated for C18H31NO7 (M+ + Na) 396.1998, found
396.2007.
Methyl (E–Z)-3-((3aS,4R,6R,6aR)-6-[(tert-butoxycarbonyl)-
amino]methyl-2,2-dimethylperhydrofuro[3,4-d][1,3]dioxol-4-yl)-2-
propenoate (23)
To a solution of 21 (2.66 g, 8.0 mmol) in MeOH–H2O (30 mL; 5 : 1),
NaIO4 (1.71 g, 8.0 mmol) was added and the reaction mixture was
stirred at room temperature for 2 h. MeOH was removed, the
residue extracted with CH2Cl2 (3 ¥ 30 mL), dried (Na2SO4) and
evaporated to give aldehyde 22 as a yellow liquid, which was used
as such for the next reaction.
A solution of 22 (1.90 g, 6.31 mmol) in benzene (10 mL)
was added to a stirred solution of (methoxycarbonylmethy-
lene)triphenylphosphorane (2.52 g, 7.54 mmol) in benzene (10 mL)
and heated at reflux for 5 h. Benzene was evaporated and residue
purified by flash column chromatography (silica gel, 15% EtOAc
in petroleum ether) to give a cis–trans-mixture of 23 (1.82 g, 81%)
as a pale yellow syrup.
(3aR,4R,6R,6aS)-6-[(4R)-2,2-Dimethyl-1,3-dioxolan-4-yl]-2,2-
dimethylperhydrofuro[3,4-d][1,3]dioxol-4-yl-methyl azide (19)
To a solution of 18 (7.03 g, 16.42 mmol) in dry DMF (15 mL),
NaN3 (3.18 g, 49.0 mmol) was added and stirred at 70 ◦C for
6 h. The reaction mixture was diluted with EtOAc (20 mL),
washed with water (20 mL), brine (20 mL) and dried (Na2SO4).
Evaporation of solvent and purification of the residue by column
chromatography (silica gel, 10% EtOAc in petroleum ether)
afforded 19 (3.19 g, 65%) as a white semi solid; mp 65–67 ◦C;
[a]D = -4.9 (c 0.5, CHCl3); IR (neat): 2988, 2951, 2168, 2092,
trans 23. [a]D = -19.0 (c 2.0, CHCl3); IR (neat): 3370, 2979,
1791, 1521, 1438, 1368, 1270, 1252, 1165, 1098, 860 cm-1; 1H NMR
(CDCl3, 500 MHz): d 6.96 (dd, 1H, J = 5.2, 15.7 Hz, C5H), 6.12
(dd, 1H, J = 1.6, 15.7 Hz, C6H), 4.78 (dd, 1H, J = 4.3, 5.9 Hz,
C3H), 4.73 (br s, 1H, NH), 4.61 (dd, 1H, J = 1.3, 5.9 Hz, C2H),
4.51 (ddd, 1H, J = 1.6, 4.3, 5.2 Hz, C4H), 4.16 (m, 1H, C1H),
3.75 (s, 3H, COOMe), 3.33 (m, 1H, CH2a), 3.11 (ddd, 1H, J =
4.8, 8.7, 13.9 Hz, CH2b), 1.45 (s, 3H, Me), 1.44 (s, 9H, Boc), 1.31
(s, 3H, Me); 13C NMR (CDCl3, 100 MHz): d 166.3, 155.9, 141.8,
122.6, 113.4, 83.3, 83.2, 82.2, 79.4, 51.6, 40.2, 29.6, 28.3, 26.2, 25.1;
HRMS (ESI): m/z calculated for C17H27NO7 (M+ + Na) 380.1685,
found 380.1680.
1
1455, 1248, 1207, 1163, 1088, 887 cm-1; H-NMR (CDCl3, 500
MHz): d 4.81 (dd, 1H, J = 3.8, 6.0 Hz, C3H), 4.63 (dd, 1H, J =
1.5, 6.0 Hz, C2H), 4.38 (ddd, 1H, J = 4.7, 6.3, 7.4 Hz, C5H), 4.22
(ddd, 1H, J = 1.5, 4.7, 6.6 Hz, C1H), 4.10 (dd, J = 6.3, 8.8 Hz,
C6H), 4.05 (dd, 1H, J = 4.7, 8.8 Hz, C6¢H), 3.94 (dd, 1H, J = 3.8,
7.4 Hz, C4H), 3.41 (dd, 1H, J = 6.6, 13.0 Hz, CH2a), 3.24 (dd, 1H,
J = 4.7, 13.0 Hz, CH2b), 1.50 (s, 3H, Me), 1.45 (s, 3H, Me), 1.38 (s,
3H, Me), 1.35 (s, 3H, Me); 13C NMR (CDCl3, 100 MHz): d 113.0,
109.2, 83.5, 83.1, 81.6, 80.9, 73.3, 66.7, 51.3, 26.8, 26.1, 25.0, 24.6;
HRMS (ESI): m/z calculated for C13H21N3O5 (M++Na) 322.1378,
found 322.1386.
cis 23. [a]D = -126.3 (c 0.3, CHCl3); IR (neat): 3374, 2967,
1799, 1535, 1429, 1373, 1265, 1241, 1171, 1092, 845 cm-1; 1H NMR
(CDCl3, 500 MHz): d 6.33 (dd, 1H, J = 6.7, 11.7 Hz, C5H), 5.97
(dd, 1H, J = 1.6, 11.7 Hz, C6H), 5.33 (ddd, 1H, J = 1.6, 4.0,
6.7 Hz, C4H), 5.01 (dd, 1H, J = 4.0, 5.9 Hz, C3H), 4.77 (br s, 1H,
NH), 4.59 (d, 1H, J = 5.9 Hz, C2H), 4.18 (dd, 1H, J = 5.4, 9.2 Hz,
C1H), 3.73 (s, 3H, COOMe), 3.35 (m, 1H, CH2a), 3.04 (ddd, 1H,
J = 3.8, 9.2, 14.0 Hz, CH2b), 1.47 (s, 3H, Me), 1.44 (s, 9H, Boc),
1.30 (s, 3H, Me); 13C NMR (CDCl3, 100 MHz): d 165.9, 155.6,
145.3, 120.5, 112.6, 83.3, 83.1, 82.5, 79.6, 51.5, 40.1, 29.7, 28.4,
tert-Butyl N-((3aR,4R,6R,6aS)-6-[(4R)-2,2-dimethyl-1,3-
dioxolan-4-yl]-2,2-dimethylperhydrofuro[3,4-d][1,3]dioxol-4-
ylmethyl)carbamate (20)
A solution of 19 (2.5 g, 8.36 mmol) and Ph3P (4.37 g, 16.7 mmol)
in methanol (10 mL) was stirred below 20 ◦C for 1 h and then
treated with (Boc)2O (2.22 mL, 8.36 mmol) and stirred at ambient
temperature for 5 h. Methanol was evaporated and the residue
purified by column chromatography (silica gel, 25% EtOAc in
petroleum ether) to give 20 (2.33 g, 75%) as a pale yellow syrup,
[a]D =+4.8 (c 0.5, CHCl3); IR (neat): 3365, 2981, 2936, 1701, 1522,
26.2, 24.8; HRMS (ESI): m/z calculated for C17H27NO7 (M+
+
Na) 380.1685, found 380.1672.
Methyl (3S)-3-((3aS,4R,6R,6aR)-6-[(tert-butoxycarbonyl)amino]-
methyl-2,2-dimethylperhydrofuro[3,4-d][1,3]dioxol-4-yl)-3-
(benzylamino)propanoate (24) and methyl (3R)-3-((3aS,4R,6R,
6aR)-6-[(tert-butoxycarbonyl)amino]methyl-2,2-dimethylper-
hydrofuro[3,4-d][1,3]dioxol-4-yl)-3-(benzylamino)propanoate (25)
1
1455, 1369, 1253, 1210, 1118, 850 cm-1; H-NMR (CDCl3, 500
MHz): d 4.78 (dd, 1H, J = 3.7, 6.0 Hz, C3H), 4.67 (br s, 1H, NH),
4.57 (d, 1H, J = 6.0 Hz, C2H), 4.39 (ddd, 1H, J = 4.7, 6.4, 7.4 Hz,
C5H), 4.09 (dd, J = 6.4, 8.7 Hz, C6H), 4.07 (m, 1H, C1H), 4.03
(dd, 1H, J = 4.7, 8.7 Hz, C6¢H), 3.85 (dd, 1H, J = 3.7, 7.4 Hz,
C4H), 3.26 (m, 1H, CH2a), 3.07 (ddd, 1H, J = 4.6, 8.9, 14.0 Hz,
CH2b), 3.24 (dd, 1H, J = 4.7, 13.0 Hz, CH2b), 1.49 (s, 3H, Me),
1.45 (s, 9H, Boc), 1.44 (s, 3H, Me), 1.37 (s, 3H, Me), 1.33 (s, 3H,
Me); 13C NMR (CDCl3, 100 MHz): d 155.8, 112.8, 109.1, 83.6,
83.1, 80.7, 80.5, 73.3, 66.8, 40.0, 28.3, 26.9, 26.1, 25.1, 24.6; HRMS
A mixture of 23 (1.84 g, 5.15 mmol) and benzylamine (1.38 mL,
12.8 mmol) was stirred at room temperature. After 12 h, the
reaction mixture was directly purified by column chromatography.
First eluted (silica gel 25% EtOAc in petroleum ether) was 25
(0.59 g, 25%) as a pale yellow syrup; [a]D = +37.25 (c 1.3, CHCl3);
IR (neat): 3366, 2980, 2939, 2397, 1714, 1561, 1446, 1264, 1168,
1
1103, 754 cm-1; H NMR (500 MHz, CDCl3): d 7.36–7.20 (m,
5H, Ar-H), 4.79 (dd, 1H, J = 3.8, 6.1 Hz, C3H), 4.76 (br s,
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1H, NH), 4.51 (d, 1H, J = 6.1 Hz, C2H), 4.04 (dd, 1H, J =
5.3, 9.7 Hz, C1H), 3.88 (d, 1H, J = 13.1 Hz, BnCH2a), 3.85
(d, 1H, J = 13.1 Hz, BnCH2b), 3.78 (dd, 1H, J = 3.6, 8.4 Hz,
C4H), 3.68 (s, 3H, COOMe), 3.47 (ddd, 1H, J = 4.7, 6.5, 8.4 Hz,
CbH), 3.29 (m, 1H, CH2a), 2.99 (ddd, 1H, J = 3.9, 9.7, 13.6 Hz,
CH2b), 2.72 (dd, 1H, J = 4.7, 15.2 Hz, CaH), 2.60 (dd, 1H, J =
6.5, 15.2 Hz, Ca‘H), 1.45 (s, 3H, Me), 1.44 (s, 9H, Boc), 1.32 (s,
3H, Me); 13C NMR (CDCl3, 150 MHz): d 172.8, 155.8, 140.3,
128.9, 128.8, 128.2, 128.1, 126.8, 112.5, 83.0, 82.4, 81.2, 80.7,
79.4, 79.1, 53.2, 51.4, 51.0, 39.6, 36.0, 28.3, 26.2, 24.9; HRMS
(ESI): m/z calculated for C24H37N2O7 (M+ + H) 465.2600, found
465.2598.
Second eluted was (30% EtOAc in petroleum ether) 24 (1.02 g,
42.6%) yield as a yellow solid; mp 84–86 ◦C; [a]D = +21.5 (c 0.5,
CHCl3); IR (neat): 3392, 2976, 2871, 2358, 1739, 1685, 1531, 1274,
1164, 1040, 862, 701 cm-1; 1H NMR (CDCl3, 500 MHz): d 7.35–
7.21 (m, 5H, Ar-H), 4.75 (dd, 1H, J = 3.7, 6.0 Hz, C3H), 4.73 (br s,
1H, BocNH), 4.55 (d, 1H, J = 6.0 Hz, C2H), 4.08 (dd, 1H, J = 5.4,
9.2 Hz, C1H), 3.97 (dd, 1H, J = 3.7, 8.4 Hz, C4H), 3.91 (d, 1H, J =
12.9 Hz, PhCH2a), 3.82 (d, 1H, J = 12.9 Hz, PhCH2b), 3.69 (s,
3H, COOMe), 3.44 (dt, 1H, J = 10.8, 5.4 Hz, CbH), 3.27 (m, 1H,
CH2a), 3.02 (ddd, 1H, J = 4.3, 9.2, 13.8 Hz, CH2b), 2.74 (dd, 1H,
J = 5.1, 15.1 Hz, CaH), 2.57 (dd, 1H, J = 5.7, 15.1 Hz, Ca‘H), 1.44
(s, 3H, Me), 1.42 (s, 9H, Boc), 1.30 (s, 3H, Me); 13C NMR (CDCl3,
150 MHz): d 172.4, 158.8, 140.0, 128.4, 128.3, 127.0, 112.5, 83.1,
82.8, 81.2, 80.6, 79.5, 54.3, 51.7, 51.2, 39.7, 35.3, 28.2, 26.1, 24.8;
HRMS (ESI): m/z calculated for C24H37N2O7 (M+ + H) 465.2600,
found 465.2607.
Boc-(R)-b-Caa-(S)-b-Caa-OCH3 (36)
A solution of 35 (0.8 g, 2.2 mmol), HOBt (0.35 g, 2.65 mmol),
EDCI (0.50 g, 2.6 mmol) in CH2Cl2 (10 mL) was stirred at 0 C
◦
under N2 atmosphere for 15 min and treated sequentially with the
amine 33, DIPEA (0.7 mL, 2.24 mmol) and stirred for 8 h. The
reaction mixture was quenched at 0 ◦C with sat. NH4Cl (10 mL)
solution. After 10 min, the reaction mixture was diluted with
CHCl3 (10 mL), washed with 1 N HCl (10 mL), water (10 mL),
aq. sat. NaHCO3 (10 mL) solution and brine (10 mL) solution.
The organic layer was dried (Na2SO4) and evaporated to give the
residue, which was purified by column chromatography (silica gel,
55% EtOAc in petroleum ether) to give 36 (1.12 g, 83%) as a white
◦
solid; mp 143–145 C; [a]D = +63.4 (c 0.25, CHCl3); IR (KBr):
3431, 3343, 2927, 1744, 1689, 1508, 1375, 1170, 1093, 1021, 996,
872 cm-1; 1H NMR (CDCl3, 500 MHz): d 6.55 (br s, 1H, NH-2),
5.68 (br s, 1H, NH-1), 4.88 (s, 2H, C1H-1, 2), 4.77 (dd, 1H, J = 3.4,
5.9 Hz, C3H-2), 4.77 (dd, 1H, J = 3.4, 5.9 Hz, C3H-1), 4.59 (m, 1H,
CbH-2), 4.54 (d, 1H, J = 5.9 Hz, C2H-2), 4.52 (d, 1H, J = 5.9 Hz,
C2H-1), 4.31 (m, 1H, CbH-1), 4.18 (dd, 1H, J = 3.3, 8.0 Hz, C4H-2),
4.13 (dd, 1H, J = 3.4, 6.7 Hz, C4H-1), 3.68 (s, 3H, COOMe), 3.31
(s, 3H, OMe), 3.30 (s, 3H, OMe), 2.79 (dd, 1H, J = 5.3, 15.9 Hz,
CaH(pro-R)-1), 2.77 (dd, 1H, J = 5.6, 15.9 Hz, CaH(pro-S)-1), 2.72 (m,
1H, CaH(pro-R)-2), 2.56 (dd, 1H, J = 5.6, 15.0 Hz, CaH(pro-S)-2), 1.48
(s, 3H, Me), 1.46 (s, 3H, Me), 1.43 (s, 9H, Boc), 1.30 (s, 3H, Me),
1.29 (s, 3H, Me); 13C NMR (CDCl3, 100 MHz): d 172.0, 170.2,
155.8, 112.8, 112.6, 106.9, 106.8, 85.2, 84.9, 79.9, 79.5, 78.9(2),
54.7, 54.6, 51.7, 47.6, 46.2, 38.6, 35.6, 28.3, 26.0, 25.9, 24.7, 24.5;
FABMS: m/z calculated for C28H46N2O13 619.9 [52, (M + H)+],
519.9 [100, (M + H - Boc)+], 345.9 (6), 276.8 (14), 115.5 (18), 57.3
(36); HRMS (ESI): m/z calculated for C28H46N2O13 (M+ + Na)
641.2897, found 641.2897.
Cbz-(S)-b-Caa(NHBoc)-OCH3 (1)
A solution of 24 (0.162 g, 3.49 mmol) in methanol (20 mL) was
treated with 10% Pd–C (cat.) and stirred at room temperature
under hydrogen atmosphere for 12 h. The reaction mixture
was filtered and filtrate evaporated to give methyl (3S)-3-
((3aS,4R,6R,6aR)-6-[(tert-butoxycarbonyl)amino]methyl-2,2-
dimethylperhydrofuro[3,4-d][1,3]dioxol-4-yl)-3-aminopropanoate
(26) as a yellow liquid, which was used as such for the next
reaction.
Cbz-(S)-b-Caa(NHBoc)-(R)-b-Caa-(S)-b-Caa-OCH3 (7)
A solution of 36 (0.87 g, 1.41 mmol) and TFA (0.8 mL) in CH2Cl2
was stirred at 0 ◦C to room temperature for 2 h. The solvent was
evaporated under reduced pressure, resulting salt 37 was dried
under high vacuum and used as such for further reaction.
A solution of 26 (1.50 g, 4.01 mmol) and DIPEA (1.40 mL,
8.02 mmol) in CH2Cl2 (15 mL) at 0 ◦C was treated with Cbz-
Cl (0.82 g, 4.82 mmol) and stirred at room temperature for
2 h. Solvent was evaporated and purified the residue by column
chromatography (silica gel, 30% EtOAc in petroleum ether) to give
1 (1.76 g, 86%) as a pale yellow syrup; [a]D = +16.9 (c 0.5, CHCl3);
IR (neat): 3354, 2980, 2941, 2284, 1669, 1523, 1226, 1168, 753 cm-1;
1H NMR (CDCl3, 500 MHz); d 7.37–7.28 (m, 5H, Ar-H), 5.40 (d,
1H, J = 8.6 Hz, NH), 5.12 (d, 1H, J = 12.4 Hz, PhCH2a), 5.10
(d, 1H, J = 12.4 Hz, PhCH2b), 4.68 (br m, 1H, NH), 4.68 (dd,
1H, J = 3.9, 6.0 Hz, C3H), 4.54 (d, 1H, J = 6.0 Hz, C2H), 4.37
(m, 1H, CbH), 4.09 (dd, 1H, J = 3.9, 9.5 Hz, C4H), 4.08 (dd, 1H,
J = 5.3, 9.3 Hz, C1H), 3.67 (s, 3H, COOMe), 3.21 (ddd, 1H, J =
5.3, 7.1, 14.1 Hz, CH2a), 3.05 (ddd, 1H, J = 4.9, 9.3, 14.1 Hz,
CH2b), 2.80 (dd, 1H, J = 5.4, 16.2 Hz, CaH), 2.73 (dd, 1H, J =
6.0, 16.2 Hz, 1H, Ca‘H), 1.45 (s, 3H, Me), 1.42 (s, 9H, Boc), 1.29
(s, 3H, Me); 13C NMR (CDCl3, 100 MHz): d 171.9, 155.9, 155.8,
136.4, 128.4, 128.1, 128.0, 113.0, 83.0, 80.6, 79.6, 79.1, 66.6, 51.7,
47.9, 39.7, 36.3, 28.3, 26.0, 24.7; HRMS (ESI): m/z calculated for
C25H36N2O9 (M+ + H) 509.2499, found 509.2486.
A mixture of acid 28 (0.70 g, 1.41 mmol), HOBt (0.22 g,
1.70 mmol), EDCI (0.32 g, 1.70 mmol) in CH2Cl2 (10 mL) was
stirred at 0 ◦C under N2 atmosphere for 15 min and treated
sequentially with the amine 37 and DIPEA (0.5 mL, 2.11 mmol)
and stirred for 8 h. Workup as described for 36 and purification
by column chromatography (silica gel, 70% EtOAc in petroleum
ether) gave 7 (0.84 g, 60%) as a white solid; mp 100–102 ◦C; [a]D =
+58.0 (c 0.25, CHCl3); IR (KBr): 3370, 2986, 2942, 1717, 1666,
1520, 1337, 1100, 972, 864 cm-1; 1H NMR (CDCl3, 500 MHz): d
7.35–7.28 (m, 5H, Ar-H), 7.28 (br s, 1H, NH-3), 7.03 (d, 1H, J =
9.2 Hz, NH-2), 6.12 (br s, 1H, NH-1), 5.11 (d, 1H, J = 12.4 Hz,
PhCH2a), 5.00 (d, 1H, J = 12.4 Hz, PhCH2b), 5.05 (t, 1H, J =
12.4 Hz, NHBoc), 4.89 (s, 1H, C1H-3), 4.88 (s, 1H, C1H-1), 4.82
(dd, 1H, J = 3.6, 6.1 Hz, C3H-3), 4.79 (dd, 1H, J = 3.6, 6.1 Hz,
C3H-2), 4.77 (dd, 1H, J = 3.5, 6.1 Hz, C3H-1), 4.56 (d, 1H, J =
6.1 Hz, C2H-3), 4.53 (d, 1H, J = 6.1 Hz, C2H-2), 4.51 (d, 1H, J =
6.1 Hz, C2H-1), 4.67 (m, 1H, CbH-2), 4.43 (m, 1H, CbH-3), 4.27
(dd, 1H, J = 3.6, 8.8 Hz, C4H-3), 4.24 (m, 1H, CbH-1), 4.05 (dd,
1H, J = 3.5, 6.3 Hz, C4H-2), 3.95 (dd, 1H, J = 3.7, 6.0 Hz, C4H-
1), 3.61 (s, 3H, COOMe), 3.36 (s, 3H, OMe), 3.30 (s, 3H, OMe),
4152 | Org. Biomol. Chem., 2008, 6, 4142–4156
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3.29–2.97 (m, 1H, CH2a), 3.01 (dd, 1H, J = 5.0, 16.8 Hz, CaH(pro-S)
-
319 (82), 212 (100); HRMS (ESI): (M+ + Na) 1359.6322, found
1359.6333.
3), 2.97 (m, 1H, CH2b), 2.76 (dd, 1H, J = 5.3, 14.7, Hz CaH(pro-S)-1),
2.73 (dd, 1H, J = 5.4, 16.8 Hz, CaH(pro-R)-3), 2.61 (dd, 1H, J = 7.6,
15.7 Hz, CaH(pro-R)-2), 2.58 (dd, 1H, J = 4.0, 14.7 Hz, CaH(pro-R)-1),
2.50 (dd, 1H, J = 4.7, 15.7 Hz, CaH(pro-S)-2), 1.50 (s, 3H, Me), 1.48
(s, 3H, Me), 1.46 (s, 3H, Me), 1.40 (s, 9H, Boc), 1.29 (s, 6H, Me),
1.27 (s, 3H, Me); 13C NMR (CDCl3, 150 MHz): d 173.2, 171.3,
169.9, 156.1, 155.9, 136.5, 128.5, 128.1, 128.0, 112.8, 107.2, 106.7,
96.1, 85.1, 84.8, 83.1, 82.9, 80.5, 79.9, 79.4, 79.3, 79.0, 78.9, 78.1,
66.5, 55.0, 54.7, 51.9, 48.9, 47.2, 46.0, 39.5, 39.2, 38.1, 34.5, 29.7,
28.3, 26.2, 26.1, 25.9, 24.9, 24.8, 24.1; FABMS: m/z calculated
for C47H70N4O19 1017 [14, (M + Na)+], 995 [40, (M + H)+], 895
[16, (M + H - Boc)+], 519 (30), 470 (29), 377 (28), 276 (100), 227
(30); HRMS (ESI): m/z calculated for C47H70N4O19 (M+ + Na)
1017.4531, found 1017.4497.
(3aS,4R,6R,6aS)-4-(2,4-Difluorophenyl)-6-[(4R)-2,2-dimethyl-
1,3-dioxolan-4-yl]-2,2-dimethylperhydrofuro[3,4-d][1,3]dioxol-
4-ol (46)
A stirred solution of 1-bromo-2,4-difluorobenzene (14.96 g,
◦
77.5 mmol) in dry THF (100 mL) at -78 0 C was treated with
n-BuLi (48.4 mL, 77.5 mmol) dropwise over a period of 15 min.
A solution of lactone 45 (10 g, 38.75 mmol) in dry THF (100 mL)
was added dropwise at the same temperature and the reaction
mixture allowed to reach room temperature and stirred for 5 h.
The reaction mixture was quenched with NH4Cl solution (10 mL),
water (100 mL) was added and the mixture was extracted with
EtOAc (2 ¥ 150 mL). The organic layer was washed with water
(100 mL), brine (100 mL) and dried (Na2SO4). It was evaporated
and purified by column chromatography (silica gel, 15% EtOAc in
petroleum ether) to afford 46 (12.26 g, 85%) as a white solid; mp
145–147 ◦C; IR (KBr): 3325, 3087, 2987, 2928, 1625, 1584, 1469,
1373, 1270, 1055, 846 cm-1; 1H NMR (CDCl3, 300 MHz); d 7.49
(m, 1H, Ar-H), 6.89–6.78 (m, 2H, Ar-H), 4.96 (m, 1H, C2H), 4.87
(m, 1H, C3H), 4.45 (m, 1H, C6H), 4.31–4.06 (m, 3H, C6¢H, C4H,
C5H), 1.30 (s, 3H, Me), 1.29 (s, 3H, Me), 1.28 (s, 3H, Me), 1.24 (s,
3H, Me); 13C NMR (CDCl3, 75 MHz): d 162.3, 158.9, 133.2–130.0
(4C), 112.9, 112.2, 111.9, 86.9, 79.3, 79.2, 73.4, 66.4, 26.5, 25.6,
Cbz-(S)-b-Caa(NHBoc)-(R)-b-Caa-(S)-b-Caa-(R)-
b-Caa(NHBoc)-OCH3 (8)
A mixture of 38 (0.18 g, 0.18 mmol), HOBt (0.03 g, 0.22 mmol),
EDCI (0.04 g, 0.22 mmol) and DIPEA (0.04 mL, 0.28 mmol) in
CH2Cl2 was stirred at 0 ◦C for 15 min and treated with amine 27
(0.07 g, 0.12 mmol) under nitrogen atmosphere for 8 h. Workup
as described for 36 and purification by column chromatography
(silica gel, 1.8% methanol i◦n CHCl3) afforded 8 (0.12 g, 49%) as
a white solid; mp 117–120 C; [a]D = +64.5 (c 0.25, CHCl3); IR
(KBr): 3412, 2939, 1720, 1657, 1546, 1378, 1209, 1101, 1026, 966,
879 cm-1; 1H NMR (CDCl3, 500 MHz): d 7.39–7.27 (m, 5H, Ar-H),
8.31 (d, 1H, J = 9.2 Hz, NH-4), 7.33 (d, 1H, J = 9.2 Hz, NH-3),
7.23 (d, 1H, J = 9.7 Hz, NH-2), 6.73 (d, 1H, J = 8.5 Hz, NH-1),
5.16 (br s, 1H, BocNH-1), 5.12 (d, 1H, J = 12.5 Hz, PhCH2a),
5.10 (d, 1H, J = 12.5 Hz, PhCH2b), 5.05 (br s, 1H, BocNH-4),
4.97 (dd, 1H, J = 3.6, 6.0 Hz, C3H-3), 4.89 (s, 1H, C1H-2), 4.85 (s,
1H, C1H-3), 4.85 (m, 1H, CbH-2), 4.77 (dd, 1H, J = 3.6, 6.0 Hz,
1H, C3H-1), 4.74 (dd, 1H, J = 3.7, 6.0 Hz, C3H-4), 4.73 (m, 1H,
CbH-4), 4.71 (dd, 1H, J = 3.5, 6.0 Hz, C3H-2), 4.56 (d, 1H, J =
6.0 Hz, C2H-4), 4.54 (d, 1H, J = 6.0 Hz, C2H-1), 4.53 (d, 1H,
J = 6.0 Hz, C2H-2), 4.52 (d, 1H, J = 6.0 Hz, C2H-3), 4.39 (m,
1H, CbH-3), 4.33 (m, 1H, CbH-1), 4.16 (dd, 1H, J = 4.9, 9.9 Hz,
C1H-1), 4.13 (dd, 1H, J = 5.5, 9.8 Hz, C1H-4), 4.07 (dd, 1H,
J = 3.6, 9.5 Hz, C4H-3), 4.04 (dd, 1H, J = 3.6, 7.8 Hz, C4H-
1), 3.98 (m, 1H, C4H-2), 3.82 (dd, 1H, J = 3.7, 7.8 Hz, C4H-4),
3.67 (s, 3H, COOMe), 3.30 (m, 1H, CH2a), 3.29 (m, 1H, CH2c),
3.28 (s, 3H, OMe), 3.27 (s, 3H, OMe), 2.96 (m, 1H, CH2d), 2.95
(m, 1H, CH2b), 2.94 (dd, 1H, J = 3.6, 12.7 Hz, CaH(pro-R)-4), 2.64
(dd, 1H, J = 2.7, 12.5 Hz, CaH(pro-S)-2), 2.58 (m, 1H, CaH(pro-S)-1,
CaH(pro-R)-1), 2.48 (dd, 1H, J = 4.0, 13.0 Hz, CaH(pro-R)-3), 2.41 (dd,
1H, J = 10.1, 12.7 Hz, CaH(pro-R)-4), 2.39 (t, 1H, J = 12.5 Hz,
CaH(pro-R)-2), 2.37 (dd, 1H, J = 4.6, 13.0 Hz, CaH(pro-S)-3), 1.49 (s,
6H, Me), 1.45 (s, 3H, Me), 1.44 (s, 3H, Me), 1.43 (s, 9H, Boc),
1.40 (s, 9H, Boc), 1.31 (s, 3H, Me), 1.30 (s, 3H, Me), 1.26 (s, 3H,
Me), 1.25 (s, 3H, Me); 13C NMR (CDCl3, 150 MHz): d 174.4,
171.1, 170.0, 169.4, 156.1, 155.9, 155.8, 136.6, 128.4, 127.8, 127.7,
113.1, 112.9, 112.7, 112.4, 106.9, 106.8, 85.0, 84.9, 83.4, 83.2, 83.1,
83.0, 80.7, 80.6, 80.5, 79.9, 79.7, 79.4, 79.3, 79.2, 78.4, 78.3, 66.3,
54.6, 54.2, 52.2, 48.6, 48.3, 46.6, 46.3, 46.2, 40.4, 40.1, 39.4, 37.9,
37.6, 28.3, 26.2, 25.9, 25.8, 25.1, 24.9, 24.8, 24.0; FABMS: m/z
calculated for C63H96N6O25 1360 [10, (M + Na)+], 1338 [39, (M +
H)+], 1238 [14, (M + H - Boc)+], 619 (23), 562 (44), 377 (46),
25.3, 24.8; HRMS (ESI): m/z calculated for C18H22O6F2 (M+
+
Na) 395.1282, found 395.1286.
(1R)-1-[(3aS,4R,6R,6aS)-6-(2,4-Difluorophenyl)-6-hydroxy-2,2-
dimethylperhydrofuro[3,4-d][1,3]dioxol-4-yl]ethane-1,2-diol (47)
A mixture of 46 (12.26 g, 32.9 mmol) and 60% aq. AcOH (85 mL)
was stirred at room temperature for 6 h. The reaction mixture was
neutralized with solid NaHCO3 and sat. aq. NaHCO3 solution
(pH = 7) and extracted with EtOAc (3 ¥ 300 mL). Organic layers
were dried (Na2SO4), evaporated and the residue purified the
by column chromatography (silica gel, 50% EtOAc in petroleum
ether) to give 47 (8.16 g, 75%) as a colorless syrup; IR (neat): 3390,
1
2986, 2939, 1618, 1466, 1378, 1212, 1035, 893 cm-1; H NMR
(CDCl3, 300 MHz); d 7.49 (m, 1H, Ar-H), 6.91–6.79 (m, 2H, Ar-
H), 4.96 (m, 2H, C2H, C3H), 4.30 (m, 1H, C6H), 3.90–3.87 (m, 3H,
C6¢H, C4H, C5H), 1.29 (s, 3H, Me), 1.24 (s, 3H, Me); 13C NMR
(CDCl3, 75 MHz): d 162.2, 161.3, 133.3 (4C), 113.1, 86.8, 79.6,
79.5, 78.8, 69.5, 63.9, 25.8, 25.0; HRMS (ESI): m/z calculated for
C15H18O6F2 (M+ + Na) 355.0969, found 355.0976.
Methyl (Z)-3-[(3aS,4R,6aS)-6-(2,4-difluorophenyl)-6-hydroxy-
2,2-dimethylperhydrofuro[3,4-d][1,3]dioxol-4-yl]-2-propenoate (49)
A stirred solution of 47 (8.16 g, 24.57 mmol) in CH2Cl2
(100 mL) was treated with NaIO4 (5.25 g, 24.57 mmol),
sat. aq. NaHCO3 solution (4.9 mL) at 0 ◦C and allowed
to stir at room temperature for 5 h. The reaction mix-
ture was filtered, dried (Na2SO4) and concentrated under re-
duced pressure to afford (3aS,4S,6aR)-6-(2,4-difluorophenyl)-2,2-
dimethylperhydrofuro[3,4-d][1,3] dioxole-4-carbaldehyde (48) as a
colorless syrup, which was used as such for the next reaction.
A solution of 48 (6.70 g, 22.3 mmol) and (methoxycarbonyl-
methylene)triphenylphosphorane (8.20 g, 24.56 mmol) in MeOH
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(70 mL) was stirred at 0 ◦C and allowed to stir at room temperature
for 5 h. Methanol was evaporated and the residue purified by
column chromatography (silica gel, 15% EtOAc in petroleum
ether) to give 49 (6.2 g, 78%) as a pale yellow syrup; IR (neat):
111.5 (4C), 81.7, 81.4, 80.9 (2C), 80.7, 80.3, 79.1, 76.6, 47.8, 45.8,
28.4 (3C), 25.2, 25.1, 24.2, 23.9; HRMS (ESI): m/z calculated for
C38H46N2O11F4 (M+ + Na) 805.2935, found 805.2926.
1
3434, 2988, 1719, 1617, 1467, 1209, 1033, 820 cm-1; H NMR
Boc-[(S)-b-Caa(diFP)-(R)-b-Caa(diFP)]2-OMe (13)
(CDCl3, 300 MHz); d 7.51 (m, 1H, Ar-H), 6.92–6.80 (m, 2H, Ar-
H), 6.45 (dd, 1H, J = 6.7, 11.7 Hz, C5H), 6.03 (dd, 1H, J = 1.7,
11.7 Hz, C6H), 5.70 (ddd, 1H, J = 1.7, 4.0, 6.7 Hz, C4H), 5.20
(dd, 1H, J = 4.0, 5.8 Hz, C3H), 5.01 (m, 1H, C2H), 3.75 (s, 3H,
OMe), 1.27 (s, 3H, Me), 1.25 (s, 3H, Me); 13C NMR (CDCl3, 75
MHz): d 166.2, 161.3, 158.3, 144.9, 133.3 (4C), 120.7, 113.4, 113.0,
86.9, 81.2, 77.3, 51.6, 25.9, 25.1; HRMS (ESI): m/z calculated for
C17H18O6F2 (M+ + Na) 379.0969, found 379.0976.
A mixture of 60 (0.24 g, 0.31 mmol), HOBt (0.05 g, 0.37 mmol)
and EDCI (0.07 g, 0.37 mmol) in CH2Cl2 (10 mL) was stirred at
0 ◦C for 15 min and treated with 61 (0.24 g, 0.31 mmol; obtained
from 59 on exposure to TFA) and DIPEA (0.10 mL, 0.62 mmol)
under nitrogen atmosphere for 8 h. Workup as described for 36 and
purification by column chromatography (silica gel 1.4% MeOH in
CHCl3) afforded 13 (0.201 g, 45%) as a white solid; mp 156–158 ◦C;
[a]D = +162.7 (c 0.27, CHCl3); IR(KBr): 3435, 3300, 2983, 1723,
1
1655, 1470, 1272, 1108, 995 cm-1; H NMR (CDCl3, 303 K, 500
Methyl (Z)-3-[(3aS,4R,6S,6aR)-6-(2,4-difluorophenyl)-2,2-
dimethylperhydrofuro[3,4-d][1,3]dioxol-4-yl]-2-propenoate (50)
MHz): d 8.08 (d, 1H, J = 8.8 Hz, NH-4), 7.06 (d, 1H, J = 9.5 Hz,
NH-2), 7.03 (d, 1H, J = 8.4 Hz, NH-3), 7.22–7.15 (m, 4H, Ar-H),
6.85–6.76 (m, 8H, Ar-H), 5.78 (d, 1H, J = 8.1 Hz, NH-1), 5.09
(d, 1H, J = 4.4 Hz, C1H-3), 5.04 (d, 1H, J = 4.4 Hz, C1H-1), 4.97
(m, 1H, CbH-2), 4.87 (d, 2H, J = 4.4 Hz, C1H-2, C1H-4), 4.86 (m,
1H, C3H-1), 4.85 (m, 1H, C3H-3), 4.80 (m, 2H, C2H-1, C3H-2),
4.85 (m, 1H, C3H-3), 4.77 (m, 2H, C1H-3, CbH-4), 4.76 (m, 2H,
C2H-4, C3H-4), 4.60 (m, 1H, CbH-3), 4.46 (m, 1H, CbH-1), 3.93
(m, 1H, C4H-1), 3.79 (dd, 1H, J = 3.8, 9.5 Hz, C4H-3), 3.70 (dd,
1H, J = 3.7, 9.5 Hz, C4H-4), 3.68 (s, 3H, COOMe), 3.67 (m, 1H,
C4H-2), 2.93 (dd, 1H, J = 4.4, 13.7 Hz, CaH(pro-S)-4), 2.70 (m, 1H,
CaH(pro-S)-1), 2.64 (dd, 1H, J = 3.8, 13.0 Hz, CaH(pro-S)-2), 2.58 (m,
1H, CaH(pro-S)-3), 2.57 (dd, 1H, J = 4.5, 12.3 Hz, CaH(pro-R)-1), 2.51
(m, 1H, CaH(pro-R)-4), 2.49 (dd, 1H, J = 5.1, 13.1 Hz, CaH(pro-R)-3),
2.46 (dd, 1H, J = 11.0, 13.0 Hz, CaH(pro-R)-2), 1.52 (s, 3H, Me),
1.47 (s, 3H, Me), 1.46 (s, 9H, Boc), 1.44 (s, 3H, Me), 1.43 (s, 3H,
Me), 1.26 (s, 3H, Me), 1.25 (s, 3H, Me), 1.25 (s, 3H, Me), 1.21
(s, 3H, Me); 13C NMR (CDCl3, 150 MHz): d 173.4, 170.8, 170.1,
169.9, 162.1 (4C), 160.4 (4C), 155.7, 129.6–129.4 (16C), 112.9–
112.7 (4C), 111.6–111.4 (8C), 81.8, 81.7, 81.5, 81.3, 81.2, 81.2,
80.9, 80.7, 80.5, 80.2, 80.0, 78.8, 76.5, 76.4, 47.3, 47.2, 46.5, 46.0,
28.2 (3C), 25.3 (3C), 25.1, 24.4, 24.3, 24.1, 23.8; HRMS (ESI):
m/z calculated for C70H80N4O19F8 (M+ + Na) 1455.5186, found
1455.5186.
A stirred solution of 49 (7.5 g, 21.0 mmol) and Et3SiH (6.70 mL,
42.1 mmol) in dry CH3CN (80 mL) was treated with BF3·OEt2
(2.64 mL, 21.0 mmol) dropwise at -10 ◦C and stirred for 1 h. The
reaction mixture was quenched with sat. aq. NaHCO3 solution
(10 mL), water (100 mL) and extracted with EtOAc (2 ¥ 150 mL).
The organic layers were washed with water (100 mL), brine
(100 mL), dried (Na2SO4) and evaporated under reduced pressure.
The residue was purified by column chromatography (silica gel, 8%
EtOAc in petroleum ether) to afford 50 (6.18 g, 86%) as a yellow
syrup; [a]D = +197.9 (c 0.75, CHCl3); IR (KBr): 2985, 1720, 1618,
1471, 1205, 1103, 996 cm-1; 1H NMR (CDCl3, 303K, 500 MHz):
d 7.47 (m, 1H, Ar-H), 6.91–6.78 (m, 2H, Ar-H), 6.52 (dd, 1H, J =
6.3, 11.6 Hz, C5H), 6.03 (dd, 1H, J = 1.6, 11.6, Hz, C6H), 5.16 (m,
2H, C3H, C4H), 5.0 (d, 1H, J = 4.0 Hz, C1H), 4.86 (dd, 1H, J =
4.0, 5.7, Hz, C2H), 3.75 (s, 3H, OMe), 1.48 (s, 3H, Me), 1.27 (s, 3H,
Me); 13C NMR (CDCl3, 75 MHz): d 166.2, 162.0, 159.0, 145.2,
132.7 (4C), 120.6, 112.9, 82.5, 81.6, 78.8, 77.1, 51.5, 25.2, 24.3;
HRMS (ESI): m/z calculated for C17H18O5F2 (M+ + Na) 363.1020
found 363.1037.
Boc-(S)-b-Caa(diFP)-(R)-b-Caa(diFP)-OMe (59)
A mixture of 55 (0.55 g, 1.24 mmol), HOBt (0.20 g, 1.48 mmol),
EDCI (0.28 g, 1.48 mmol) in CH2Cl2 (20 mL) was stirred at 0 ◦C
under N2 atmosphere for 15 min and treated with 58 (0.56 g,
1.24 mmol; obtained from 4 on exposure to TFA and DIPEA
(0.43 mL, 2.48 mmol) under nitrogen atmosphere for 8 h. Workup
as described for 36 and purification by column chromatography
(silica gel, 60% EtOAc in petroleum ether) afforded 59 (0.71 g,
73%) as a white solid; mp 95–97 ◦C; [a]D = +128.1 (c 0.28, CHCl3);
IR (KBr): 3440, 2929, 1736, 1629, 1471, 1207, 1108, 994 cm-1; 1H
NMR (CDCl3, 303K, 500 MHz): d 7.26–7.20 (m, 2H, Ar-H), 6.87–
6.84 (m, 4H, Ar-H), 6.76 (d, 1H, J = 8.6 Hz, NH-2), 5.37 (m, 1H,
NH-1), 4.96 (d, 1H, J = 3.7 Hz, C1H-1), 4.95 (m, 1H, C1H-2), 4.88
(m, 1H, C3H-1), 4.87 (m, 1H, C3H, C2H -2), 4.83 (m, 1H, CbH-2),
4.80 (m, 1H, C2H-2), 4.79 (m, 1H, C2H, C2H -1), 4.39 (m, 1H,
CbH-1), 3.94 (dd, 1H, C4H-1), 3.87 (dd, 1H, J = 3.6, 5.6 Hz, C4H-
2), 3.69 (s, 3H, COOMe), 2.86 (dd, 1H, J = 7.1, 16.1 Hz, CaH-2),
2.81 (dd, 1H, J = 6.4, 16.1 Hz, Ca’H-2), 2.70 (m, 2H, CaH, Ca’H-
1), 1.50 (s, 3H, Me), 1.48 (s, 3H, Me), 1.45 (s, 9H, Boc), 1.27 (s,
3H, Me), 1.26 (s, 3H, Me); 13C NMR (CDCl3, 150 MHz): d 172.0,
170.5, 162.1 (2C), 160.5 (2C), 155.8, 129.6 (8C), 113.0,112.8,111.6–
Boc-[(S)-b-Caa(diFP)-(R)-b-Caa(diFP)]3-OMe (14)
A mixture of 62 (0.1 g, 0.07 mmol), HOBt (0.01 g, 0.08 mmol)
and EDCI (0.016 g, 0.08 mmol) in CH2Cl2 (10 mL) was stirred
at 0 ◦C for 15 min and treated with 61 (0.055 g, 0.070 mmol),
and DIPEA (0.02 mL, 0.14 mmol) under nitrogen atmosphere
for 8 h. Workup as described for 36 and purification by column
chromatography (silica gel 1.9% MeOH in◦CHCl3) afforded 14
(0.05 g, 34%) as a white solid; mp 162–165 C; [a]D = +122.9 (c
0.15, CHCl3); IR(KBr) : 3434, 2932, 1722, 1653, 1471, 1378, 1272,
1
1109, 997 cm-1; H NMR (CDCl3, 303 K, 500 MHz): d 9.07 (d,
1H, J = 9.0 Hz, NH-4), 8.69 (d, 1H, J = 9.4 Hz, NH-6), 8.45 (d,
1H, J = 8.5 Hz, NH-3), 7.76 (d, 1H, J = 9.0 Hz, NH-5), 7.26–7.13
(m, 6H, Ar-H), 6.89–6.79 (m, 12H, Ar-H), 7.20 (d, 1H, J = 9.7 Hz,
NH-2), 5.88 (d, 1H, J = 9.1 Hz, NH-1), 5.25 (dd, 1H, J = 3.7,
6.0 Hz, C3H-5), 5.18 (dd, 1H, J = 3.7, 6.0 Hz, C3H-3), 5.14 (d, 1H,
J = 4.5 Hz, C1H-1), 5.09 (m, 1H, CbH-2), 5.02 (d, 1H, J = 4.5 Hz,
C1H-2), 4.92 (m, 3H, C1H-3, C1H-6, C2H-6), 4.91 (m, 4H, C3H-1,
C1H-4, C3H-4, CbH-6), 4.84 (m, 3H, C2H-1, C3H-2, C1H-5), 4.82
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(m, 1H, C2H-3), 4.79 (m, 1H, C2H-4), 4.76 (m, 1H, C2H-2), 4.75
(dd, 1H, J = 4.5, 6.0 Hz, C2H-5), 4.75 (m, 1H, CbH-3), 4.74 (m, 2H,
CbH-4, C2H-5), 4.59 (m, 2H, CbH-1, CbH-5), 4.03 (m, 1H, C4H-1),
3.91 (dd, 1H, J = 3.4, 9.7 Hz, C4H-3), 3.80 (m, 1H, C4H-5), 3.77
(m, 1H, C4H-2), 3.74 (m, 1H, C4H-6), 3.72 (s, 3H, COOMe), 3.63
(m, 1H, C4H-4), 3.06 (dd, 1H, J = 3.3, 12.5 Hz, CaH(pro-S)-6), 2.93
(dd, 1H, J = 3.4, 12.1 Hz, CaH(pro-S)-4), 2.86 (dd, 1H, J = 2.7,
12.1 Hz, CaH(pro-S)-2), 2.76 (dd, 1H, J = 5.4, 13.8 Hz, CaH(pro-S)-1),
2.74 (dd, 1H, J = 3.4, 12.7 Hz, CaH(pro-S)-3), 2.66 (dd, 1H, J = 3.2,
12.7 Hz, CaH(pro-S)-5), 2.65 (dd, 1H, J = 5.1, 13.8 Hz, CaH(pro-R)-1),
Ernst, D. Seebach and D. Hoyer, Helv. Chim. Acta, 2000, 83, 16; (f) K.
Gademann, T. Kimmerlin, D. Hoyer and D. Seebach, J. Med. Chem.,
2001, 44, 2460; (g) N. Umezawa, M. A. Gelman, M. C. Haigis, R. T.
Raines and S. H. Gellman, J. Am. Chem. Soc., 2002, 124, 368; (h) E. A.
Porter, X. F. Wang, H. -S. Lee, B. Weisblum and S. H. Gellman, Nature,
2000, 404, 565.
9 (a) T. Hintermann and D. Seebach, Chimia, 1997, 51, 244; (b) D.
Seebach, S. Abele, J. V. Schreiber, B. Martinoni, A. K. Nussbaum,
H. Schild, H. Schulz, H. Hennecke, R. Woessner and F. Bitsch, Chimia,
1998, 52, 734.
10 (a) S. Dedier, S. Krebs, J. R. Lamas, S. Poenaru, G. Folkers, J. A. L.
de Castro, D. Seebach and D. Rognan, J. Percept. Signal Transduct.
Res., 1999, 19, 645; (b) S. Krebs, J. R. Lamas, S. Poenaru, G. Folkers,
J. A. L. de Gastro, D. Seebach and D. Rognan, J. Biol. Chem., 1998,
273, 19072; (c) D. Seebach, S. Poenaru, G. Folkers and D. Rognan,
Helv. Chim. Acta, 1998, 81, 1181; (d) B. Gasslmaier, C. M. Krell, D.
Seebach and E. Holler, Eur. J. Biol., 2000, 267, 5101; (e) D. H. Liu and
W. F. DeGrado, J. Am. Chem. Soc., 2001, 123, 7553.
11 (a) Y. -D. Wu and D. -P. Wang, J. Am. Chem. Soc., 1998, 120, 13485;
(b) Y. -D. Wu and D. -P. Wang, J. Am. Chem. Soc., 1999, 121, 9352; (c) Y.
-D. Wu and D. -P. Wang, J. Chin. Chem. Soc., 2000, 47, 129; (d) Y. -D.
Wu, J. -Q. Lin and Y. -L. Zhao, Helv. Chim. Acta, 2002, 85, 3144; (e) K.
Mohle, R. Gunther, M. Thormann, N. Sewald and H. -J. Hofmann,
Biopolymers, 1999, 50, 167; (f) R. Gunther, H. -J. Hofmann and K.
Kuczera, J. Phys. Chem. B, 2001, 105, 5559; (g) C. Baldauf, R. Gunther
and H. -J. Hofmann, Angew. Chem., Int. Ed., 2004, 43, 1594; (h) W.
Damm and W. F. van Gunsteren, J. Comput. Chem., 2000, 21, 774;
(i) X. Daura, W. F. van Gunsteren and A. E. Mark, Proteins: Struct.,
Funct., Genet., 1999, 34, 269; (j) X. Daura, B. Jaun, D. Seebach, W. F.
van Gunsteren and A. E. Mark, J. Mol. Biol., 1998, 280, 925; (k) X.
Daura, W. F. van Gunsteren, D. Rigo, B. Jaun and D. Seebach, Chem.–
Eur. J., 1997, 3, 1410; (l) X. Daura, K. Gademann, H. Schafer, B. Jaun,
D. Seebach and W. F. van Gunsteren, J. Am. Chem. Soc., 2001, 123,
2393; (m) J. Aqvist, FEBS Lett., 1999, 457, 414; (n) L. A. Christianson,
M. J. Lucero, D. H. Appella, D. A. Klein and S. H. Gellman, J. Comput.
Chem., 2000, 21, 763.
2.61 (m, 1H, CaH(pro-R)-6), 2.55 (dd, 2H, J = 5.1, 12.7 Hz, CaH(pro-R)
-
3, CaH(pro-R)-5), 2.42 (dd, 1H, J = 12.1, 12.7 Hz, CaH(pro-R)-2), 2.20
(dd, 1H, J = 12.1, 12.7 Hz, CaH(pro-R)-4), 1.52 (s, 3H, Me), 1.51
(s, 3H, Me), 1.49 (s, 3H, Me), 1.45 (s, 9H, Boc), 1.44 (s, 3H, Me),
1.41 (s, 3H, Me), 1.41 (s, 3H, Me), 1.31 (s, 3H, Me), 1.28–1.26
(s, 12H, Me), 1.21 (s, 3H, Me); 13C NMR (CDCl3, 150 MHz):
d 173.9, 171.2, 170.9, 170.8, 169.9, 169.6, 162.2 (6C), 160.5 (6C),
155.7, 129.5 (24C), 113.0–112.4 (6C), 112.1–111.3 (12C), 84.4–76.4
(20C), 48.3, 47.3, 47.2, 46.8, 46.6, 46.2, 28.4 (3C), 25.4–24.9 (6C),
24.4–23.6 (6C); HRMS (ESI): m/z calculated for C102H114N6O27F12
(M+ + Na) 2105.7432, found 2105.7433.
Acknowledgements
Authors wish to thank DST, New Delhi for financial support. V. S.,
N. Y. R., K. K., V. B. J. and R. R. are thankful to CSIR and UGC,
New Delhi, for financial support.
12 (a) D. Seebach, K. Gademann, J. V. Schreiber, J. L. Matthews, T.
Hintermann, B. Jaun, L. Oberer, U. Hommel and H. Widmer, Helv.
Chim. Acta, 1997, 80, 2033; (b) D. Seebach, S. Abele, K. Gademann,
G. Guichard, T. Hintermann, B. Jaun, J. L. Matthews, J. V. Schreiber,
L. Oberer, U. Hommel and H. Widmer, Helv. Chim. Acta, 1998, 81,
932.
13 S. A. W. Gruner, V. Truffault, G. Voll, E. Locardi, M. Stockle and H.
Kessler, Chem.–Eur. J., 2002, 8, 4366.
14 G. V. M. Sharma, V. G. Reddy, A. S. Chander and K. R. Reddy,
Tetrahedron: Asymmetry, 2002, 13, 21.
15 C. Palomo, M. Oiarbide, A. Landa, G. -R. M. Concepcion, J. M.
Garcia, A. Gonzalez, J. M. Odriozola, M. Martin-Pastor and A.
Linden, J. Am. Chem. Soc., 2002, 124, 8637.
16 (a) F. Schweizer, Angew. Chem., Int. Ed., 2002, 41, 230; (b) S. A. W.
Gruner, E. Locardi, E. Lohof and H. Kessler, Chem. Rev., 2002, 102,
491; (c) T. K. Chakraborty, S. Ghosh and S. Jayaprakash, Curr. Med.
Chem., 2002, 9, 421.
17 (a) R. A. Dwek and T. D. Butters, Chem. Rev., 2002, 102(issue 2); (b) A.
Kobata, Acc. Chem. Res., 1993, 26, 319; (c) R. A. Dwek, Chem. Rev.,
1996, 96, 683; (d) A. P. Davis and R. S. Wareham, Angew. Chem., Int.
Ed., 1999, 38, 2979.
18 (a) G. V. M. Sharma, K. R. Reddy, P. R. Krishna, A. R. Sankar,
K. Narsimulu, S. K. Kumar, P. Jayaprakash, B. Jagannadh and
A. C. Kunwar, J. Am. Chem. Soc., 2003, 125, 13670; (b) G. V. M.
Sharma, K. R. Reddy, P. R. Krishna, A. R. Sankar, P. Jayaprakash,
B. Jagannadh and A. C. Kunwar, Angew. Chem., Int. Ed., 2004, 43,
3961.
References
1 S. H. Gellman, Acc. Chem. Res., 1998, 31, 173.
2 (a) D. Seebach and J. L. Matthews, Chem. Commun., 1997, 429; (b) R. P.
Cheng, S. H. Gellman and W. F. DeGrado, Chem. Rev., 2001, 101, 3219;
(c) D. Seebach, A. K. Beck and D. J. Bierbaum, Chem. Biodiversity,
2004, 1, 1111; (d) I. Huc, Eur. J. Org. Chem., 2004, 17; (e) Foldamers:
Structure, Properties, and Applications, ed. S. Hecht and I. Huc, Wiley-
VCH, Weinheim, 2007.
3 (a) M. A. Gelman and S. H. Gellman, in Enantioselective Synthesis
of b-Amino Acids, ed. E. Juaristi and V. A. Soloshonok, John Wiley
and Sons, Hoboken, NJ, 2nd edn, 2005, p. 527; (b) R. M. Ortuno,
in Enantioselective Synthesis of b-Amino Acids, ed. E. Juaristi and
V. A. Soloshonok, John Wiley and Sons, Hoboken, NJ, 2nd edn, 2005,
p. 117.
4 (a) J. Applequist, K. A. Bode, D. H. Appella, L. A. Christianson and
S. H. Gellman, J. Am. Chem. Soc., 1998, 120, 4891; (b) D. H. Appella,
L. A. Christianson, I. L. Karle, D. R. Powell and S. H. Gellman, J. Am.
Chem. Soc., 1996, 118, 13071; (c) D. H. Appella, J. J. Barchi, S. R.
Durell and S. H. Gellman, J. Am. Chem. Soc., 1999, 121, 2309; (d) R.
Threlfall, A. Davies, N. M. Howarth, J. Fisher and R. Cosstick, Chem.
Commun., 2008, 585.
5 (a) T. A. Martinek, G. Toth, E. Vass, M. Hollosi and F. Fulop, Angew.
Chem., Int. Ed., 2002, 41, 1718; (b) A. Hetenyi, I. M. Mandity, T. A.
Martinek, G. K. Toth and F. Fulop, J. Am. Chem. Soc., 2005, 127, 547;
(c) T. A. Martinek, I. M. Mandity, L. Fulop, G. K. Toth, E. Vass, M.
Hollosi, E. Forro and F. Fulop, J. Am. Chem. Soc., 2006, 128, 13539.
6 R. M. Ortuno, A. G. Moglioni and G. Y. Moltrasio, Curr. Org. Chem.,
2005, 9, 237.
7 (a) C. Branden and J. Tooze, Introduction to Protein Structure, Garland,
New York, 1991, p. 1; (b) D. J. Hill, M. J. Mio, R. B. Prince, T. S. Hughes
and J. S. Moore, Chem. Rev., 2001, 101, 3893; (c) C. M. Goodman, S.
Choi, S. Shandler and W. F. DeGrado, Nat. Chem. Biol., 2007, 3, 252.
8 (a) Y. Suhara, J. E. K. Hildreth and Y. Ichikawa, Tetrahedron Lett.,
1996, 37, 1575; (b) K. C. Nicolaou, H. Florke, M. G. Egan, T. A. Barth
and V. A. Estevez, Tetrahedron Lett., 1995, 36, 1775; (c) Y. Suhara,
M. Izumi, M. Ichikawa, M. B. Penno and Y. Ichikawa, Tetrahedron
Lett., 1997, 38, 7167; (d) K. Gademann, M. Ernst, D. Hoyer and D.
Seebach, Angew. Chem., Int. Ed., 1999, 38, 1223; (e) K. Gademann, M.
19 (a) G. V. M. Sharma, V. B. Jadhav, V. Subash, K. V. S. Ramakrishna,
P. Jayaprakash, K. Narsimulu and A. C. Kunwar, J. Am. Chem. Soc.,
2006, 128, 14657; (b) G. V. M. Sharma, V. Manohar, S. K. Dutta, V.
Subash and A. C. Kunwar, J. Org. Chem., 2008, 73, 3689.
20 (a) H. Wiegand, B. Wirz, A. Schweitzer, G. P. Camenisch, M. I. R.
Perez, G. Gross, R. Woessner, R. Voges, P. I. Arvidsson, J. Frackenpohl
and D. Seebach, Biopharm. Drug Dispos., 2002, 23, 251; (b) H. G.
Boman, Annu. Rev. Immunol., 1995, 13, 61; (c) P. Nicolas and A. Mor,
Annu. Rev. Microbiol., 1995, 49, 277; (d) A. Tossi, L. Sandri and A.
Giangaspero, Biopolymers, 2000, 55, 4.
21 (a) R. F. Epand, T. L. Raguse, S. H. Gellman and R. M. Epand,
Biochemistry, 2004, 43, 9527; (b) P. I. Arvidsson, N. S. Ryder, H. M.
This journal is
The Royal Society of Chemistry 2008
Org. Biomol. Chem., 2008, 6, 4142–4156 | 4155
©

View Article Online
Weiss, G. Gross, O. Kretz, R. Woessner and D. Seebach, ChemBioChem,
2003, 4, 1345.
28 C. Frechou, L. Dheilly, D. Beaupere, R. Uzan and G. Demailly,
Tetrahedron Lett., 1992, 33, 5067.
29 N. T. Patil, J. N. Tilekar and D. D. Dhavale, J. Org. Chem., 2001, 66,
1065.
22 (a) Y. Hamuro, J. P. Schneider and W. F. DeGrado, J. Am. Chem. Soc.,
1999, 121, 12200; (b) P. I. Arvidsson, J. Frackenpohl, N. S. Ryder, B.
Liechty, F. Petersen, H. Zimmermann, G. P. Camenisch, R. Woessner
and D. Seebach, ChemBioChem, 2001, 2, 771; (c) T. L. Raguse, E. A.
Porter, B. Weisblum and S. H. Gellman, J. Am. Chem. Soc., 2002, 124,
12774.
23 A. G. Barrett and S. Lebold, J. Org. Chem., 1991, 56, 4875.
24 (a) C. Jaramillo and S. Knapp, Synthesis, 1994, 1; (b) D. E. Levy and C.
Tang, in The Chemistry of C-Glycosides, Elsevier Science, Tarrytown,
New York, 1995; (c) M. H. D. Postema, in C-Glycoside Synthesis, ed.
C. W. Rees, CRC Press, Boca Raton, FL, 1995; (d) F. Nicotra, Top.
Curr. Chem., 1997, 187, 55; (e) Y. Du, R. J. Linhardt and I. R. Vlahov,
Tetrahedron, 1998, 54, 9913.
25 (a) J. A. Ma and D. Cahard, Chem. Rev., 2004, 104, 6119; (b) Biomed-
ical Frontiers of Fluorine Chemistry, ed. I. Ojima, J. R. McCarthy
and J. T. Welch, American Chemical Society, Washington, DC,
1996; (c) Organofluorine Chemistry: Principles and Applications, ed.
R. E. Banks, B. E. Smart and J. C. Tatlow, Plenum Press, New York,
1994.
30 G. V. M. Sharma, A. S. Chander, V. G. Reddy, K. Krishnudu,
M. H. V. R. Rao and A. C. Kunwar, Tetrahedron Lett., 2000, 41, 1997.
31 (a) S. Czernecki and G. Ville, J. Org. Chem., 1989, 54, 610; (b) G. V. M.
Sharma, A. S. Chander, K. Krishnudu and P. R. Krishna, Tetrahedron
Lett., 1997, 38, 9051.
32 See supporting information†.
33 (a) M. Rueping, B. Jaun and D. Seebach, Chem. Commun., 2000, 2267;
(b) P. I. Arvidsson, M. Rueping and D. Seebach, Chem. Commun., 2001,
649; (c) T. L. Raguse, J. R. Lia and S. H. Gellman, J. Am. Chem. Soc.,
2003, 125, 5592.
34 M. Werder, H. Hauser, S. Abele and D. Seebach, Helv. Chim. Acta,
1999, 82, 1774.
35 Solvent titration studies were carried out by sequentially adding up to
50% of DMSO-d6 to 600 uL CDCl3 solutions of the peptides. Small
changes in amide proton chemical shift (DdNH) has been used as an
indication for H-bonding.
36 G. V. M. Sharma, V. Subash, K. Narsimulu, A. R. Sankar and A. C.
Kunwar, Angew. Chem., Int. Ed., 2006, 45, 8207.
26 (a) F. D. M. Ismail, J. Fluorine Chem., 2002, 118, 27; (b) K. L. Kirk and
R. Filler, Biomed. Front. Fluorine Chem., 1996, 639, 1.
37 National Committee for Clinical Laboratory Standards [NCCL].
Standard method for dilution antimicrobial susceptibility tests for
bacteria, which grows aerobically, Natl. Comm. Clini. Lab. Stands.,
Villanova, 1982, p. 242.
27 (a) H. -P. Chiu, Y. Suzuki, D. Gullickson, R. Ahmad, B. Kokona, R.
Fairman and R. P. Cheng, J. Am. Chem. Soc., 2006, 128, 15556; (b) H.
Meng and K. Kumar, J. Am. Chem. Soc., 2007, 129, 15615.
4156 | Org. Biomol. Chem., 2008, 6, 4142–4156
This journal is
The Royal Society of Chemistry 2008
©