Oligomers of enantiopure bicyclic γ/δ-amino acids (BTAa). 1. Synthesis and conformational analysis of 3-Aza-6,8-dioxabicyclo[3.2.1]octane-7-carboxylic acid oligomers (PolyBTG)

Article abstract of DOI:10.1021/ol006548s

(Matrix Presented) A series of dimeric through pentameric oligomers of a bicyclic γ/δ-amino acid (BTG) were synthesized using peptide coupling methods in solution with PyBroP or HATU. The analysis of ¹H NMR and CD spectra suggests that these ol

Full text of DOI:10.1021/ol006548s

ORGANIC
LETTERS
2000
Vol. 2, No. 25
3987-3990
Oligomers of Enantiopure Bicyclic
γ/δ-Amino Acids (BTAa). 1. Synthesis
and Conformational Analysis of
3-Aza-6,8-dioxabicyclo[3.2.1]octane-7-carboxylic
Acid Oligomers (PolyBTG)
Fabrizio Machetti, Alessandro Ferrali, Gloria Menchi, Ernesto G. Occhiato, and
Antonio Guarna*
Dipartimento di Chimica Organica “U. Schiff” and Centro di Studio sulla Chimica e
la Struttura dei Composti Eterociclici e loro Applicazioni, C.N.R., UniVersita` di
Firenze, Via G. Capponi 9, I-50121 Firenze, Italy
guarna@chimorg.unifi.it
Received September 5, 2000
ABSTRACT
A series of dimeric through pentameric oligomers of a bicyclic γ/δ-amino acid (BTG) were synthesized using peptide coupling methods in
solution with PyBroP or HATU. The analysis of 1H NMR and CD spectra suggests that these oligomers could have a partially ordered structure
in alcohol solutions.
Several homooligomers of noncoded R,1 â,2 γ,3 and δ4 amino
acids have been synthesized in recent years, and their
conformational proprieties have been extensively studied. It
has been demonstrated that these oligomers (named “fol-
damers” by Gellman)5 can adopt a variety of secondary
structures, including helices, sheets, and turns, depending
either on the number of units and the amino acid structure
(side chain substitution, configuration of the stereocenters,
etc.) or the external conditions (solvent, temperature, solid
state, etc.). Furthermore, the possibility that â-peptides could
adopt a tertiary structure has been recently suggested.6 The
great interest in the synthesis of these peptides has been also
stimulated by the observation that, similar to the proteins
and RNA, these oligomers can catalyze a wide range of
processes. For example, the synthetic application of polypep-
tides, with adopted folded structures, as catalyst in transes-
terification reactions has recently been reported.7
(1) For example: Yang, D.; Qu, J.; Li, B.; Ng, F.-F.; Wang, X.-C.;
Cheung, K.-K.; Wang, D.-P.; Wu, Y.-D. J. Am. Chem. Soc. 1999, 121,
589-590.
(2) (a) Seebach, D.; Abele, S.; Gademann, K.; Guichard, G.; Hintermann,
T.; Jaun, B.; Matthews, J. L.; Schreiber, J. HelV. Chim. Acta 1998, 81,
932-976 and references therein. (b) Appella, D. H.; Christianson, L. A.;
Klein, D. A.; Richards, M. R.; Powell, D. R.; Gellman, S. H. J. Am. Chem.
Soc. 1999, 121, 7574-7581 and references therein.
(3) (a) Hintermann, T.; Gademann, K.; Jaun, B.; Seebach, D. HelV. Chim.
Acta 1998, 81, 983-1002. (b) Hanessian, S.; Luo, X.; Schaum, R.;
Michnick, S. J. Am. Chem. Soc. 1998, 120, 8569-8570. (c) Hanessian, S.;
Luo, X.; Schaum, R. Tetrahedron Lett. 1999, 40, 4925-4929.
(4) (a) Szabo, L.; Smith, B. L.; McReynolds, K. D.; Parril, A. L.; Morris,
E. R.; Gervay, J. J. Org. Chem. 1998, 63, 1074-1078. (b) Smith, M. D.;
Claridge, T. D. Tranter, G. E.; Sansom, M. S. P.; W.; Fleet, G. W. J. J.
Chem. Soc., Chem. Commun. 1998, 2041-2042. (c) Long, D. D.; Hunger-
ford, N. L.; Smith, M. D.; Brittain, D. E. A.; Marquess, D. G.; Claridge, T.
D. W.; Fleet, G. W. J. Tetrahedron Lett. 1999, 40, 2195-2198. (d) Claridge,
T. D. W.; Long, D. D.; Hungerford, N. L.; Aplin, R. T.; Smith, M. D.;
Marquess, D. G.; Fleet, G. W. J. Tetrahedron Lett. 1999, 40, 2199-2202.
(e) Schwalbe, H.; Wermuth, J.; Richter, C.; Szalma, S.; Eschenmoser, A.;
Quinkert, G. HelV. Chim. Acta 2000, 83, 1079-1107.
(5) Gellman, S. H. Acc. Chem. Res. 1998, 31, 173-180.
(6) Appella, D. H.; Christianson, L. A.; Karle, L. I.; Powell, D. R.;
Gellman, S. H. J. Am. Chem. Soc. 1999, 121, 6206-6212.
(7) Rossi, P.; Felluga, F.; Tecilla, P.; Formaggio, F.; Crisma, M.; Toniolo,
C.; Scrimin, P. J. Am. Chem. Soc. 1999, 121, 6948-6948 and references
therein.
10.1021/ol006548s CCC: $19.00 © 2000 American Chemical Society
Published on Web 11/11/2000

Thus, the design and synthesis of new oligomers which
possess the specific structural properties of the natural
biopolymers such as catalysis and molecular recognition is
of current interest for producing artificial enzymes and
receptors.
Thus, starting from the monomer Bn-BTG-OMe (1), a
strategy for oligomerization has been developed leading to
the synthesis of a dimer (Bn-(BTG)2-OMe, 2), a trimer
(Bn-(BTG)3-OMe, 3), a tetramer (Bn-(BTG)4-OMe, 4),
and a pentamer (Bn-(BTG)5-OMe, 5) (Scheme 1).
Herein, we describe the synthesis and a preliminary
conformational study of new γ/δ-peptide oligomers (po-
lyBTG), constructed from the enantiopure (1R,7R)-3-aza-
6,8-dioxabicyclo[3.2.1]octane-7-carboxylic acid (BTG). This
compound, recently prepared by us,8 belongs to a new series
of amino acids (BTAa)9 containing the 3-aza-6,8-dioxabicyclo-
[3.2.1]octane-7-carboxylic acid skeleton. The bicyclic lactams
[BTAa(O)] can be easily synthesized from L- or D-R-amino
aldehyde derivatives (in turn prepared from R-amino acids)
and (R,R), (S,S), or (R,S) tartaric acid mono methyl esters
through amide bond formation and carbonyl ketalization. The
amide group is then selectively reduced to give the final
amino acids BTAa (Figure 1).
Scheme 1a
Figure 1.
These BTAa can be described as conformationally rigidi-
fied γ- or δ-amino acids according to the Seebach10 and
Gellman11 subdivisions (see Figure 1).
A large panel of these new γ/δ-amino acids, different
either in the type of substitution or in the configuration of
the stereocenters, have been prepared by simply changing
the starting R-amino acids and tartaric acids. Owing to the
unique features of the new BTAa molecular scaffoldswhich
has a rigid skeleton, several points of substitution and
coordination, a proline-like secondary amino function, a
carboxylic group which can be oriented in exo or endo
positionswe were intrigued by the possibility of producing
BTAa homooligomers and studying their structural proper-
ties.
a Reagents and conditions: (i) H2 (1 atm), 20% Pd(OH)2/C,
MeOH, 21 h, 100%; (ii) HCl 4 M, 21 h, 100%; (iii) PyBrOP,
DIPEA, CH2Cl2, 97%; (iv) LiOH, MeOH-THF, 100%; (v)
PyBrOP, DMF, DIPEA, 68%; (vi) HATU, DIPEA, DMF, 50%.
The orthogonally protected monomer Bn-BTG-OMe (1)-
was synthesized as previously reported.8 Briefly, the coupling
of (R,R)-2,3-di-O-isopropylidentartaric acid mono methyl
ester with 2-(N-benzylamino)acetaldehyde, followed by a
treatment in refluxing toluene with H2SO4 adsorbed on SiO2,
gave (1R,5S,7R)-3-benzyl-2-oxo-6,8-dioxa-3-azabicyclo-
[3.2.1]octane-7-exo-carboxylic acid methyl ester (Bn-BTG-
(O)-OMe), which was selectively reduced with BH3‚Me2S
to give 1. The deprotection of the amino group by hydro-
genation over 20% Pd(OH)2/C gave 1b whereas treatment
with 4 M HCl furnished 1a. Amide bond formation between
1b and 1a to give dimer 212 was accomplished using N,N′-
dicyclohexylcarbodiimide/1-hydroxybenzotriazole (DCC/
(8) Guarna, A.; Guidi, A.; Machetti, F.; Menchi, G.; Occhiato, E. G.;
Scarpi, D.; Sisi, S.; Trabocchi, A. J. Org. Chem. 1999, 64, 7347-7364.
(9) We named these compounds BTAa (i.e., Bicycles from Tartaric Acid
and Amino acid) using the single letter code of the R-amino acids from
which the various type of BTAa’s are derived (see ref 8).
(10) Seebach, D.; Overhand, M.; Ku¨nle, F. N. M.; Martinoni, B.; Obere,
L.; Hommel, U.; Widmer, H. HelV. Chim. Acta 1996, 79, 913-941. See
also: Ondetti, M. A.; Pluscec, J.; Weaver, E. R.; Williams, N.; Sabo, E.
F.; Kocy, O. Chemistry and Biology of Peptides; Ann Arbor Science
Publishers: Ann Arbor, 1972; p 525.
(11) Dado, G. P.; Gellman, S. H. J. Am. Chem. Soc. 1994, 116, 1054-
1062.
3988
Org. Lett., Vol. 2, No. 25, 2000

HOBt) or bromo-tris-pyrrolidino-phosphonium hexafluoro-
phosphate (PyBroP) as coupling reagents.
protons on C-2 in rotamers E and on protons on C-4 in
rotamers Z. An energy barrier (∆G#) of 18.7 kcal/mol in
DMSO-d6 for the rotation of the amide bond in 2 was also
Dimer 2 was obtained in pure form after filtration of the
reaction mixture, followed by FCC purification of the crude
residue obtained after evaporation of the solvent. The highest
yield (97%) was obtained using PyBrop. In both cases the
aqueous workup had to be avoided because extraction of
dimer 2 with organic solvents was particularly difficult and
lowered the final yield.
The tripeptide 3 could be obtained through two different
approaches: by elongating the oligomer from the N-terminal
or the C-terminal side. The latter approach resulted in the
best yield (68%) compared with a yield of 6% obtained by
the other procedure. In both cases couplings were performed
using PyBroP as activating reagent.
1
determined by recording a series of H NMR spectra with
increasing temperature until the signals for the two rotamers
were observed to coalesce.
1
Because of the high complexity of the H NMR spectra
for the longer oligomers, we were unable to gain information
on the possible preferred conformation in solution by this
means. Instead we found it more useful to record their CD
spectra.14
Circular dichroism (CD) spectra of monomer 1, dimer 2,
trimer 3, tetramer 4, and pentamer 5 were performed in
MeOH (Figure 3, top) and TFE solutions (Figure 3, bottom).
Hydrolysis of dimer 2 performed under basic condition
(LiOH in THF-MeOH) gave better yields than acid hy-
drolysis, wherein the formation of decomposition products
was observed by HPLC.
On the basis of the these results, the synthesis of tetramer
4 was performed by coupling of monomer 1b and acid trimer
3a, the latter obtained by hydrolysis of 3 with LiOH.
When the coupling was performed using PyBroP, the
yields of compound 4 were very low (8%). Thus, the reaction
was carried out using O-(7-azabenzotriazol-1-yl)-1,1,3,3-
tetramethyluroniumhexafluorophospate (HATU) as coupling
reagent to afford 4 in 30% yield after chromatography. In
this case the yield was lowered by the poor solubility of
tetramer 4 in organic solvents which affected the chromato-
graphic purification. However, the yield was improved up
to 50% when the isolation of 4 from the reaction mixture
was carried out by crystallization from MeOH. Alternative
synthesis of tetramer through a coupling between Bn-
BTG2-OH and H-BTG2-OMe gave lower yields using
either PyBroP or HATU (8% and 11%, respectively).
The tetramer 4 was hydrolyzed to 4a with LiOH, and the
pentamer 5 was obtained in 50% yield through the coupling
of 4a with 1b using HATU in DMF solution and purification
by crystallization from MeOH.
1
The H NMR spectra of dimer 2 in CDCl3, DMSO-d6,
and CD3OD showed two sets of signals of almost equal
intensity. This observation suggests the presence of two
rotamers related to E/Z conformations of the amide bond
(Figure 2). These are easily distinguished by the anisotropic
Figure 3. (Top) CD spectra of oligo-BTG in MeOH (100 µM):
monomer 1, dimer 2 trimer 3. (Bottom) CD spectra of oligo-BTG
in TFE (100 µM): monomer 1, dimer 2, trimer 3, tetramer 4,
pentamer 5.
Because of the low solubility of 4 and 5 in methanol, their
CD spectra were recorded only in TFE. As expected, the
oligomers gave a CD spectrum, both in MeOH and TFE,
distinctly different from those of monomer 1, due to the
presence of amide bond(s) which determines a positive band
and the crossover to negative values in MeOH (Figure 3,
Figure 2.
effect of the carbonyl group13 which causes a strong
downfield shift (∼1 ppm with respect to monomer 1) of the
(12) All new compounds were fully characterized by spectroscopic and
analytical means.
Org. Lett., Vol. 2, No. 25, 2000
3989

top) and the negative bands in TFE (Figure 3, bottom). Each
oligomer displays a negative Cotton effect at ca. 225-230
nm both in MeOH and TFE, with the intensity of the negative
maximum increasing with oligomer length.
the oligomers is possible. It is known in fact that the presence
of a band whose intensity increases with the oligomer chain
length is indicative of a secondary arrangement in solution.
Furthermore, the normalized length-dependent trend for poly-
BTG (Figure 4) is comparable to the length-dependent
behavior of proline oligomers in spite of the fact that poly-
BTG display lower mean residue ellipticity. Indeed, in both
the cases, when the number of units was increased, the
normalized CD spectra became quite similar, suggesting that
the extent of secondary structure formation should reach the
maximum.15c However, while the secondary structures are
stabilized by the formation of the hydrogen bonds in the
foldamers described previously by Seebach2a and Gellman,2b
in our poly-BTG a secondary structure could arise from a
conformationally restricted rotation around the amide bond.15
This latter phenomenon could be more dependent upon the
external conditions (i.e., temperature, solvent, concentration),
and this could explain the differences observed in the
Also the positive band, at ca. 210-215 nm, in the MeOH
spectra increases with the length of the chain, indicating an
additive contribute of each unit to the ellipticity value. The
presence of two bands in dimer 2 in TFE is noteworthy (a
maximum at 223 nm and a shoulder at 228 nm), while for
higher oligomers only one band is present (Figure 3). If we
normalize the data also for the number of amide groups,
which facilities the comparison among oligomers of different
length, we observe that the spectra for tetramer and pentamer
oligomers are nearly identical (Figure 4). Although not
1
complexity either between the CD and H NMR spectra or
the difference between the CD spectra in MeOH and TFE.
In conclusion, we have demonstrated that oligomers 2-5
can be easily prepared using standard solution coupling
procedures and have obtained preliminary evidences that
poly-BTAa’s could form secondary structures (foldamers)
in solution.
It is probable that elongation of the polypeptide chain and
introduction of a substituent at position 4 of BTAa monomer
could affect the E/Z amide bond equilibrium16 in the related
oligomers, thus further favoring the formation of secondary
structures in solution. Studies aimed at ascertaining this
possibility are in progress.
Figure 4. CD spectra of oligo-BTG in TFE (100 µM): dimer 2,
trimer 3, tetramer 4, pentamer 5. Molar ellipticity values for 2-5
have been normalized for oligomer concentration and the number
of amide groups.
Acknowledgment. This work was supported by MURST
Cofin 1998-2000. We thank Dr. L. Messori for helpful
assistance with the CD measurements.
OL006548S
(15) For non-hydrogen-bonded secondary structure, see: (a) Kirshen-
baum, K.; Barron, A. E.; Goldsmith, R. A.; Armand, P.; Bradley, E. K.;
Truong, K. T. V.; Dill, K. A.; Cohen, F. E.; Zuckermann, R. N. Proc. Natl.
Acad. Sci. U.S.A. 1998, 95, 4303-4308. (b) Armand, P.; Kirshenbaum,
K.; Goldsmith, R. A.; Farr-Jones, S.; Barron, A. E.; Truong, K. T. V.; Dill,
K. A.; Mierke, D. F.; Cohen, F. E.; Zuckermann, R. N. Proc, Natl. Acad.
Sci. U.S.A. 1998, 95, 4309-4314. (c) Huck, B. R.; Langenhan, J. M.;
Gellman, S. Org. Lett. 1999, 1, 1717-1720. (d) Abele, S.; Vo¨gtli, K.;
Seebach, D. HelV. Chim. Acta 1999, 82, 1539-1558.
(16) (a) Beausoleil, E.; Lubell, W. J. Am. Chem. Soc. 1996, 118, 12902-
12908. (b) Wu, W.-J.; Raleigh, P. J. Org. Chem. 1998, 63, 6689-6698. (c)
Swarbrick, M. E.; Gosselin, F.; Lubell, W. J. Org. Chem. 1999, 64, 1993-
2002 (d) Halab, L. Lubell, W. J. Org. Chem. 1999, 64, 3312-3321.
exhaustive, the analysis of the CD spectra of compounds
2-5 suggests that the formation of a secondary structure in
(13) Molecular modeling calculations (MacroModel v.6.5, Amber force-
field, Monte Carlo conformational search) showed that there is a free rotation
around the C(7′)sC(dO) bond. However, the conformations in which the
C(dO) bond forms dihedral angles of about 112° and 150° with the C(7′)s
O(6) bond are prevailing at room temperature.
(14) (a) Woody, R. W. Circular Dichroism: Principles and Applications;
VCH: Weinheim, 1994 (b) Mulkerrin, M. G. In Spectroscopic Methods
for Determining Protein Structure in Solution; Havel, H. A., Ed.; VCH:
New York, 1996.
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