Synthesis and conformational analysis of α,β-difluoro-γ- amino acid derivatives

Article abstract of DOI:10.1002/chem.201003320

Conformationally restricted amino acids are of interest because they can be incorporated into shape-controlled peptides for applications in biotechnology and medicine.Within this context, β-and γ-amino acids have attracted considerable attention since their homologated carbon backbones are amenable to functionalization in ways that are not achievable with a-amino acids. For example, Seebach et al. have shown that the different diastereoisomers of α-fluoroβ-alanine lead to differently folded structures when they are incorporated into b-heptapeptides. These conformational differences are attributed to the preference of C-F bonds to align antiparallel to adjacent amide C=O bonds and gauche to vicinal C-N bonds. This result is a notable example of a growing body of research demonstrating that the C-F bond can be exploited as a conformational tool to optimize the properties of diverse functional molecules including pharmaceuticals, organocatalysts and liquid crystals.

Full text of DOI:10.1002/chem.201003320

DOI: 10.1002/chem.201003320
Synthesis and Conformational Analysis of a,b-Difluoro-g-amino Acid
Derivatives
Luke Hunter,* Katrina A. Jolliffe, Meredith J. T. Jordan, Paul Jensen, and
Rene B. Macquart[a]
Conformationally restricted amino acids are of interest
because they can be incorporated into shape-controlled pep-
tides for applications in biotechnology and medicine.[1]
Within this context, b- and g-amino acids have attracted
considerable attention since their homologated carbon back-
bones are amenable to functionalization in ways that are not
achievable with a-amino acids.[2] For example, Seebach et al.
have shown that the different diastereoisomers of a-fluoro-
b-alanine lead to differently folded structures when they are
incorporated into b-heptapeptides.[3] These conformational
butyric acid (3-fluoro-GABA) behave differently in biologi-
cal contexts due to conformational bias associated with N+
···FdÀ charge–dipole attraction.[8,9] However, to our knowl-
edge the a,b-difluoro-g-amino acid motif has never been
prepared in a stereoselective fashion, possibly reflecting an
associated synthetic challenge. In order to address this ques-
tion we describe here the stereoselective synthesis and con-
formational characterization of several a,b-difluoro-g-amino
acid derivatives.
The chosen synthetic route (Scheme 2) relies partly upon
methods developed by OꢀHagan et al. for the synthesis of
related fluorine-containing systems.[8,10] Thus, cinnamyl bro-
mide (1) underwent a Sharpless asymmetric dihydroxylation
reaction to furnish bromodiol 2,[11] which was treated with
potassium phthalimide to give the diol 3. Diol 3 was then
converted to the corresponding cyclic sulfate 4,[12] and subse-
quent ring-opening with tetrabutylammonium fluoride
(TBAF) gave the fluorohydrin 5a. Substitution of the re-
maining hydroxyl of 5a using the Deoxo-Fluor reagent[13]
furnished the difluoro compound 6a as a single stereoiso-
mer, although in modest yield due to a competing side-reac-
tion involving neighboring group participation and migra-
tion of the phenyl group.[14,15] The relative stereochemistry
of 6a was confirmed by single-crystal X-ray analysis.[14,16] To
complete the synthetic pathway, the phenyl group of 6a was
oxidized to the corresponding carboxylic acid[17] to give the
syn-a,b-difluoro-g-amino acid derivative 7a in good yield.
Alternatively, the intermediate diol 3 could be converted to
the epoxide 8 with high optical purity,[18] and subequent
ring-opening with HF-triethylamine[19] gave the fluorohydrin
5b. When the remaining hydroxyl group of 5b was substitut-
ed with fluorine using the Deoxo-Fluor reagent as before,
phenyl group migration was a less prominent side reaction,
and the difluoro compound 6b was isolated in significantly
higher yield. Finally, the phenyl group of 6b was oxidized as
before to give the anti-a,b-difluoro-g-amino acid derivative
7b in good yield. Although it was not performed in this
study, we note that the opposite enantiomers of 7a and 7b
should also be readily available by initiating the synthetic
sequence with the pseudoenantiomeric Sharpless AD
ligand.[20]
À
differences are attributed to the preference of C F bonds to
align antiparallel to adjacent amide C=O bonds[4] and
[5]
À
gauche to vicinal C N bonds. This result is a notable ex-
ample of a growing body of research demonstrating that the
À
C F bond can be exploited as a conformational tool to opti-
mize the properties of diverse functional molecules includ-
ing pharmaceuticals, organocatalysts and liquid crystals.[6]
In the context of shape-controlled peptides, it is intriguing
to conceptually extend the aforementioned studies of a-
fluoro-b-alanine by considering a hypothetical a,b-difluoro-
g-amino acid residue (Scheme 1), the conformation of which
Scheme 1. a,b-Difluoro-g-amino acid targets.
would be influenced by the fluorine gauche effect[7] in addi-
tion to the conformational effects observed in a-fluoro-b-
À
alanine. The presence of two vicinal C F bonds would also
give rise to different possible stereoisomers, potentially lead-
ing to useful differences in conformational behavior. Such
target molecules may also be viewed as a conceptual pro-
gression from the results of OꢀHagan et al., who demon-
strated that the different enantiomers of b-fluoro-g-amino-
[a] Dr. L. Hunter, Prof. K. A. Jolliffe, Dr. M. J. T. Jordan, Dr. P. Jensen,
Dr. R. B. Macquart
School of Chemistry
The University of Sydney
NSW 2006 (Australia)
Fax : (+)612-9351-3329
With both diastereoisomers of the target difluoro amino
acid (7a and 7b) in hand, the next objective was to investi-
gate whether they could be successfully incorporated into
short peptides. Accordingly, amino acid 7a was treated with
E-mail: luke.hunter@sydney.edu.au
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201003320.
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Chem. Eur. J. 2011, 17, 2340 – 2343

COMMUNICATION
less, it was gratifying that despite the increased acidity of
the a-fluoromethine proton, no epimerization was observed
in these peptide coupling steps. The anti-difluoro amino acid
7b was also taken through the same peptide coupling se-
quence with similar results.
The dipeptide intermediates 9a and 9b are crystalline
solids that were amenable to single-crystal X-ray analysis
(Figure 1).[16] The X-ray structures reveal backbone confor-
Figure 1. X-ray crystal structures of dipeptides 9a and 9b. Fluorine atoms
are represented as spheres.
mations that are clearly different from one another, but
which are both in accordance with the known conformation-
Scheme 2. Reagents and conditions: i) AD-mix a, CH3SO2NH2, tBuOH,
H2O, 0 8C; ii) potassium phthalimide, phthalimide, DMF, 808C; iii)
SOCl2, pyridine, CH2Cl2, 08C then NaIO4, RuCl3, H2O, CH2Cl2, CH3CN,
08C; iv) TBAF, THF, CH3CN, 08C then H2SO4, H2O, THF, RT; v)
Deoxo-Fluor, 708C; vi) NaIO4, RuCl3, H2O, CH2Cl2, CH3CN, RT; vii)
al effects of fluorine.[6] In structure 9a, the a-C F bond
À
aligns antiparallel to the adjacent amide carbonyl (dihedral
angle 176.18), which is to be expected on the basis of dipole
relaxation.[4] The two vicinal C F bonds of 9a are gauche
(CF3SO2)2O, 1,4-diazabicyclo
RT; viii) Et3N.3HF, 1208C.
G
À
(68.18), consistent with the fluorine gauche effect,[7] and the
[5]
À
À
b-C F bond is gauche to the g-C N bond (63.08). These
constraints lead the carbon chain of 9a to adopt an extend-
ed zigzag conformation. In structure 9b (Figure 1), the a-
l-phenylalanine methyl ester under standard peptide cou-
pling conditions[21] to furnish the dipeptide 9a (Scheme 3).
Deprotection of the phthalimide group with hydrazine[22]
was followed by a second peptide coupling, this time with
Boc-l-alanine, to give the tripeptide 10a. The moderate
yield of this final sequence can be attributed to difficulties
associated with the phthalimide group removal. Neverthe-
À
C F bond again aligns antiparallel to the adjacent amide
carbonyl (174.78), and once again there are gauche relation-
À
ships between the vicinal C F bonds (59.48) and between
À
the b-C-F and g-C N bonds (57.88). However, because of
the different stereochemistry of 9b, these conformational
constraints lead in this case to a bent carbon chain.
To investigate whether the conformations observed in the
crystal structures (Figure 1) are intrinsically preferred by the
a,b-difluoro-g-amino acid motifs or are simply artifacts of
crystal packing forces, the NMR spectra of 9a/9b and 10a/
10b were analyzed.[14] Selected JHH and JHF values, which
are related to the corresponding molecular dihedral
angles,[23–25] are presented in Table 1. For the syn-isomers 9a
and 10a the coupling constants are generally either quite
large or quite small, suggesting that a single conformer dom-
inates in solution, featuring the same extended zigzag fluo-
roalkyl chain that was seen in the crystal structure of 9a. In
contrast, some coupling constants of the anti isomers 9b and
10b are more intermediate in magnitude; this can be seen
most clearly by comparing the aF-bH and aH-bF coupling
3
3
Scheme 3. Reagents and conditions: i) l-Phenylalanine methyl ester hy-
drochloride, EDC, HOBt, Et3N, DMF, RT; ii) H2NNH2.H2O, EtOH,
reflux; iii) N-Boc-l-alanine, EDC, HOBt, DMF, RT.
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L. Hunter et al.
Table 1. Selected coupling constants (Hz) from the NMR spectra of 9a/b
and 10a/b.[a]
X-ray and NMR findings. The next-higher energy conformer
À
of 11a is accessed by a rotation about the Cb Cg bond, but
3JHH
3JHF
this is considerably higher in energy and unlikely to be sub-
stantially populated. NMR spin–spin coupling constants
were calculated for the minimum-energy conformer of 11a
using the SMD solvent model for water and chloroform,[14]
aH-bH bH-gH bH-gH’ aF-bH aH-bF bF-gH bF-gH’
9a
9b
10a 1.5
10b 1.6
1.2
1.6
7.8
3.2
7.7
6.1
4.7
8.7
4.1
6.1
27.0
23.0
26.9
21.3
28.9
21.4
28.6
23.8
13.6
29.4
15.7
24.9
24.2
14.4
29.0
15.8
3
3
and the calculated JHH and JHF values are in reasonably
close agreement with the experimental NMR data for 10a,
confirming that the linear conformation dominates in solu-
tion. In contrast, for the anti isomer 11b (Figure 2) there are
calculated to be two low-energy conformers separated by
only 1.6 kJmolÀ1, suggesting that a higher degree of confor-
mational disorder is likely for this isomer. The two low-
[a] Spectra were recorded in CDCl3 at 300 K.
constants between the a and b series (Table 1). This suggests
that significant averaging of g+/gÀ conformers about the
À
Ca Cb bond of the central amino acid residue is occurring
for 9b and 10b. When the 19F NMR spectrum of 10b is re-
corded at the lower temperature of 280 K two clear rotam-
ers are observed,[14] although the complexity of the resulting
peak shapes prevents a full analysis of the coupling con-
stants of each “frozen” conformer. Finally, the 1H–19F
HOESY spectra[14] of 9a/b and 10a/b reveal strong through-
space interactions between the a-fluorine atom and the
amide NH proton for all four compounds, consistent with a
energy conformers of 11b differ by a g+/gÀ rotation about
[14]
À
the Ca Cb bond, with the slightly higher-energy confor-
mer matching the X-ray result for 9b (Figure 1). The ob-
À
served NMR coupling constants about the Ca Cb bond of
10b are intermediate in magnitude between values calculat-
ed for the two low-energy conformers of 11b,[14] once again
consistent with an averaging of these conformers in the
NMR experiment. Overall, these modeling results clearly
support the X-ray and NMR data in illustrating the contrast-
ing conformational behavior of the syn- and anti-difluoro
diastereoisomeric series.
À
preferred antiparallel alignment of the a-C F bond and the
amide carbonyl in each case. Overall, the NMR evidence re-
inforces the solid-state result (Figure 1) and reveals a clear
contrast in conformational behavior between the syn and
anti diastereoisomeric series: the syn isomers have a strong
preference for a linear conformation while the anti isomers
exhibit g+/gÀ conformational disorder and a bent fluoroalkyl
chain.
In summary, synthetic access has been achieved to all pos-
sible stereoisomers of the a,b-difluoro-g-amino acid motif.
These building blocks can readily be coupled to other amino
acids under standard conditions with no detectable epimeri-
zation, and the resulting diastereoisomeric peptides exhibit
different, but predictable, conformational behavior. The a,b-
difluoro-g-amino acid motif described here should be of
relevance in the future design of shape-controlled bioactive
amino acids[27] and peptides,[28] and the results of our ongo-
ing investigations in these areas will be reported in due
course.
To further investigate the conformational behavior of
these compounds, calculations were performed with the
truncated analogues 11a and 11b (Figure 2) at the B3LYP/
6-31+G(d) level of theory using the SMD variant of the po-
larized continuum method for both water and chloroform
solvents.[14,26] For the syn isomer 11a, an extended zigzag
conformation clearly emerges as the minimum-energy geom-
etry in both water and chloroform, consistent with earlier
Acknowledgements
This work was funded by a University of Sydney Postdoctoral Research
Fellowship awarded to L.H.
Keywords: conformation
analysis
·
organofluorine
chemistry · peptides · stereochemistry
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Received: November 18, 2010
Published online: January 20, 2011
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