Synthesis of 4,4-disubstituted 2-aminocyclopentanecarboxylic acid derivatives and their incorporation into 12-helical β-peptides

Article abstract of DOI:10.1021/ol0483293

(Chemical equation presented) An enantioselective synthetic route is reported for trans-2-aminocyclopentanecarboxylic acids (ACPC) bearing geminal side chain pairs at the 4-position. β-Peptides containing the 4,4-disubstituted ACPC residues adopt the 12-helical conformation, as demonstrated by 2D NMR analysis in aqueous solution. These 4,4-disubstituted ACPC residues display functional groups, including acidic and hydrogen bond donating groups, in a geometrically defined fashion, which should be useful for the design of β-peptides for specific applications.

Full text of DOI:10.1021/ol0483293

ORGANIC
LETTERS
2004
Vol. 6, No. 24
4411-4414
Synthesis of 4,4-Disubstituted
2-Aminocyclopentanecarboxylic Acid
Derivatives and Their Incorporation into
12-Helical
â-Peptides
Timothy J. Peelen,† Yonggui Chi,† Emily Payne English, and Samuel H. Gellman*
Department of Chemistry, UniVersity of Wisconsin, Madison, Wisconsin 53706
gellman@chem.wisc.edu
Received August 20, 2004
ABSTRACT
An enantioselective synthetic route is reported for trans-2-aminocyclopentanecarboxylic acids (ACPC) bearing geminal side chain pairs at the
4-position. -Peptides containing the 4,4-disubstituted ACPC residues adopt the 12-helical conformation, as demonstrated by 2D NMR analysis
in aqueous solution. These 4,4-disubstituted ACPC residues display functional groups, including acidic and hydrogen bond donating groups,
â
in a geometrically defined fashion, which should be useful for the design of
â-peptides for specific applications.
Unnatural oligomers with discrete folding propensities (“fol-
damers”) have received much attention in recent years.1,2
Oligomers consisting of â-amino acids constrained with five-
membered rings (Figure 1), such as trans-2-aminocyclopen-
CdO(i) f N-H(i + 3) (“12-helix”).3 A 12-helical structure
can be observed with as few as six residues in aqueous
solution.4 Properly designed 12-helical â-peptides mimic the
antimicrobial activity of natural host-defense peptides.5
Biological applications of non-12-helical â-peptides have also
been reported.6
The synthesis of a diverse set of â-amino acid residues is
vital for developing â-peptides with useful functions.7
Previously we showed that side chains can be introduced
into 12-helical â-peptides by sulfonylation of the ring
nitrogen of APC8 or by introduction of a few acyclic â-amino
acids among ACPC and/or APC residues.9 Diversity can be
achieved also by introducing side chains on the cyclopentane
ring. Recently, we reported the synthesis of ACPC residues
(2) For a review, see: Hill, D. J.; Mio, M. J.; Prince, R. B.; Hughes, T.
S.; Moore, J. S. Chem. ReV. 2001, 101, 3893-4011. For selected recent
examples, see: Jiang, H.; Le´ger, J.-M.; Huc, I. J. Am. Chem. Soc. 2003,
125, 3448-3449. Levins, C. G.; Schafmeister, C. E. J. Am. Chem. Soc.
2003, 125, 4702-4703.
(3) (a) Gellman, S. H. Acc. Chem. Res. 1998, 31, 173-180. (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.
(4) Wang, X.; Espinosa, J. F.; Gellman, S. H. J. Am. Chem. Soc. 2000,
122, 4821-4822.
(5) (a) Porter, E. A.; Wang, X.; Lee, H.-S.; Weisblum, B.; Gellman, S.
H. Nature 2000, 404, 565. (b) Porter, E. A.; Weisblum, B.; Gellman, S. H.
J. Am. Chem. Soc. 2002, 124, 7324-7330.
Figure 1. â-Amino acids with five-membered ring constraint.
tanecarboxylic acid (ACPC) and trans-3-aminopyrrolidine-
4-carboxylic acid (APC), have been shown to adopt a helical
conformation defined by 12-membered-ring hydrogen bonds,
† These authors contributed equally to the work.
(1) Cheng, R. P.; Gellman, S. H.; DeGrado, W. F. Chem. ReV. 2001,
101, 3219-3232.
10.1021/ol0483293 CCC: $27.50
© 2004 American Chemical Society
Published on Web 11/05/2004

containing side chains at the 3-position of ACPC.10 Here
we disclose a route to cyclopentane-based â-amino acids that
are symmetrically disubstituted at the 4-position (Figure 1).
We demonstrate that these side chains, including the bulky
phenyl substituent, are compatible with folding into the 12-
helical conformation. Thus, the new â-amino acids allow
the incorporation of a variety of functional groups along the
outside of the 12-helical scaffold, including acidic and
hydrogen bond donating groups.
Precursors for monomers 1c and 1d were prepared by the
oxidation of disubstituted cyclohexenes (Scheme 2). Disub-
Scheme 2
â-Amino acids 1a-d were prepared enantioselectively
from 3,3-disubstituted hexanedioic acids. The dicarboxylic
acid precursors for monomers 1a (R ) Me) and 1b (R )
Ph) could be accessed by oxidation of known cyclohexanones
(Scheme 1). 4,4-Dimethylcyclohexanone (2) readily under-
Scheme 1
stituted cyclohexenes 6 and 7 were prepared from the Diels-
Alder reaction between butadiene and methylidene malonate
generated in situ from the corresponding malonate ester and
formaldehyde.14 Cyclohexene 6 was reduced to diol 8 and
protected as the bis-tert-butyl ether 9. Disubstituted cyclo-
hexenes 7 and 9 were oxidized to the corresponding diacids
4c and 4d in aqueous KMnO4.
Diacids 4a-d were converted to enantiopure aminoesters
12a-d and 13a-d through an auxiliary-based synthesis
(Scheme 3). The diacids 4a-d were first converted to the
corresponding diesters 10a-d, which were cyclized to the
â-keto esters 11a-d. The 4,4-disubstituted regioisomeric
product was predominant in each case, presumably because
of steric effects. The â-keto esters were converted to
diastereomeric trans-amino esters 12a-d and 13a-d, in one-
pot operations, by enamine formation with R-methylbenzyl-
amine and subsequent reduction using NaBH3CN.15 In each
case the reduction was highly trans-selective, albeit with low
selectivity between the two trans diastereomers. These
isomers could be readily separated from each other and from
the minor cis isomers by column chromatography, allowing
access to both enantiomers of the â-amino acids.16 The
stereochemistry of the diastereomeric â-amino esters was
assigned by X-ray crystallography of salts derived from 12a-
c.17,18 Saponification of the ester, hydogenolysis of the
auxiliary, and Fmoc protection yielded the protected â-amino
acids 14a-d and ent-14a-d.19
went oxidative C-C bond cleavage in aqueous KMnO4 to
furnish the desired diacid 4a.11 The more hydrophobic 4,4-
diphenylcyclohexanone could not be cleaved using aqueous
conditions. Instead, the ketone was converted to TMS enol
ether 5,12 which was oxidized to diacid 4b in organic
solvents.13
(6) (a) Werder, M.; Hauser, H.; Abele, S.; Seebach, D. HelV. Chim. Acta
1999, 82, 1774-1783. (b) Hamuro, Y.; Schneider, J. P.; DeGrado, W. F.
J. Am. Chem. Soc. 1999, 121, 12200-12201. (c) Liu, D.; DeGrado, W. F.
J. Am. Chem. Soc. 2001, 123, 7553-7559. (d) Gademann, K.; Kimmerlin,
T.; Hoyer, D.; Seebach, D. J. Med. Chem. 2001, 44, 2460-2468. (e) Raguse,
T. L.; Porter, E. A.; Weisblum, B.; Gellman, S. H. J. Am. Chem. Soc. 2002,
124, 12774-12785. (f) Gelman, M. A.; Richter, S.; Cao, H.; Umezawa,
N.; Gellman, S. H.; Rana, T. M. Org. Lett. 2003, 5, 3563-3565. (g)
Arvidsson, P. I., Ryder, N. S., Weiss, H. M., Gross, G., Kretz, O., Woessner,
R., Seebach, D. ChemBioChem 2003, 4, 1345-1347. (h) Potocky, T. B.;
Menon, A. K.; Gellman, S. H. J. Biol. Chem. 2003, 278, 50188-50194.
(7) For a recent review on â-amino acid synthesis, see: (a) Liu, M.;
Sibi, M. P. Tetrahedron 2002, 58, 7991-8035. For selected recent examples,
see: (b) Tang, W.; Wu, S.; Zhang, X. J. Am. Chem. Soc. 2003, 125, 9570-
9571. (c) Minter, A. R.; Fuller, A. A.; Mapp, A. K. J. Am. Chem. Soc.
2003, 125, 6846-6847. (d) Davis, F. A.; Prasad, K. R.; Nolt, M. B.; Wu,
Y. Org. Lett. 2003, 5, 925-927.
To determine whether 4,4-disubstituted ACPC residues are
compatible with the 12-helix, we prepared â-peptides 15 and
(8) Lee, H.-S.; Syud, F. A.; Wang, X.; Gellman, S. H. J. Am. Chem.
Soc. 2001, 123, 7721-7722.
(9) LePlae, P. R.; Fisk, J. D.; Porter, E. A.; Weisblum, B.; Gellman, S.
H. J. Am. Chem. Soc. 2002, 124, 6820-6821.
(13) Kaneda, K.; Kii, N.; Jitsukawa, K.; Teranishi, S. Tetrahedron Lett.
1981, 22, 2595-2598.
(10) (a) Woll, M. G.; Fisk, J. D.; LePlae, P. R.; Gellman, S. H. J. Am.
Chem. Soc. 2002, 124, 12447-12452. (b) Bunnage, M. E.; Chippindale,
A. M.; Davies, S. G.; Parkin, R. M.; Smith, A. D.; Withey, J. M. Org.
Biomol. Chem. 2003, 1, 3698-3707.
(14) (a) De Keyser, J. L.; De Cock, C. J. C.; Poupaert, J. H.; Dumont,
P. J. Org. Chem. 1988, 53, 4859-4862. (b) Nenajdenko, V. G.; Statsuk,
A. V.; Balenkova, E. S. Tetrahedron 2000, 56, 6549-6556.
(15) S)-R-Methylbenzylamine was used with â-ketoesters 11c and 11d,
not the (R)-enantiomer as shown.
(16) Compound 12c was purified from the other diastereomeric products
by selective recrystallization of the amine 12c‚HCl salt, not column
chromatography.
(11) Maryanoff, B. E.; Costanzo, M. J.; Nortey, S. O.; Greco, M. N.;
Shank, R. P.; Schupsky, J. J.; Ortegon, M. P.; Vaught, J. L. J. Med. Chem.
1998, 41, 1315-1343
(12) Heathcock, C. H.; Buse, C. T.; Kleschick, W. A.; Pirrung, M. C.;
Sohn, J. E.; Lampe, J. J. Org. Chem. 1980, 45, 1066-1081.
(17) See Supporting Information.
4412
Org. Lett., Vol. 6, No. 24, 2004

Scheme 3a
a Reagents and conditions: (a) MeOH, benzene, H2SO4 or MeI, K2CO3, DMF or BnBr, Cs2CO3, CH3CN, 70-100%; (b) KOtBu, THF,
51-70%; (c) (i) (R)-(+)-R-methylbenzylamine, AcOH, MeOH, (ii) NaBH3CN, 38-41% of 12a-d, 10-19% of 13a-d; (d) LiOH, 0 °C;
(e) 10% Pd/C/NH4HCO2, MeOH, reflux, 16 h; (f) Fmoc-OSu/NaHCO3, acetone/H2O, rt, 12 h, 63-76% over 3 steps.
16.20 Hexamers 15 and 16 were both suitable for two-
dimensional NMR analysis (Figure 2). The proton resonances
this type); four unambiguous CâH(i) to NH(i + 2) NOEs
were assigned (of five possible NOEs), and two unambiguous
CâH(i) to NH(i + 3) NOEs were assigned (of four possible
NOEs). These observations are fully consistent with a 12-
helical structure. Hexamer 15 displayed a similar set of NOEs
in water. Two of four CâH(i) to CRH(i + 2) NOEs, four of
five CâH(i) to NH(i + 2) NOEs, and two of four CâH(i) to
NH(i + 3) NOEs were unambiguously assigned.
Interestingly, a previously unobserved type of NOE was
observed for hexamer 15 in water. Two NOEs from CâH(i)
to CRH(i + 3) were identified. Crystal structure data for 12-
helical ACPC oligomers21,3b suggest that these two protons
would be too far apart for an NOE between them to be
observed.22 This contradiction suggests that the “12-helix”
may be a family of related conformers, perhaps in rapid
exchange with one another, rather than a single, rigid
conformation. The introduction of side chains at particular
positions on the five-membered rings of individual residues
may perturb the 12-helix conformational manifold in such a
way that new NOEs are detected.
Hexamer 16 also displayed NOEs consistent with 12-
helical structure in both methanol and water. In methanol,
one of four CâH(i) to CRH(i + 2) NOEs, four of five CâH(i)
Figure 2. NOEs between nonadjacent residues observed for
hexamers containing 4,4-disubstituted ACPC residues in 9:1 H2O/
D2O at pH 3.8. Dashed lines indicate NOEs that are ambiguous
because of resonance overlap. Data were obtained on a Varian 600
MHz spectrometer. Data for 15 was acquired at 14 °C at 5-10
mM concentration. Data for 16 was acquired at 4 °C at 5-10 mM
concentration.
(18) The stereochemistry of 12d and 13d could be determined by
incorporating 14d and ent-14d into short â-peptides containing cyclic
â-amino acid residues with known configuration, including (S,S)-ACPC.
The matched â-peptide, containing 14d, showed a 12-helical signature by
CD, whereas the mismatched â-peptide, containing ent-14d, showed a very
different CD signature.
(19) ent-14c could be incorporated into â-peptides and deprotected with
no observed decarboxylation of the â-dicarboxylic acids; see Supporting
Information.
(20) For CD characterization of these and other â-peptides containing
4,4-disubstituted ACPC residues, see Supporting Information.
(21) Appella, D. H.; Christianson, L. A.; Klein, D. A.; Powell, D. R.;
Huang, X.; Barchi, J. J., Jr.; Gellman, S. H. Nature 1997, 387, 381-384.
(22) The CHâ(i) to CHR(i + 3) NOEs are unlikely to arise from spin
diffusion because they were detected via ROESY. We attempted to probe
directly for spin diffusion by examining NOE intensity buildup as a function
of mixing time (NOESY), but poor signal-to-noise ratio hampered this
approach, presumably because the molecular weight of the hexa-â-peptides
falls in the range where the NOE is close to zero intensity. The origin and
significance of CHâ(i) to CHR(i + 3) NOEs will be examined with larger
â-peptides in the future.
of these â-peptides were well resolved, both in CD3OH and
in aqueous solution. NOEs are indicated as ambiguous where
resonance overlap precludes a definitive assignment (Figure
2).
Hexamer 15 displayed NOEs in methanol consistent with
the 12-helical conformation. Three unambiguous CâH(i) to
CRH(i + 2) NOEs were assigned (of four possible NOEs of
Org. Lett., Vol. 6, No. 24, 2004
4413

to NH(i + 2) NOEs, and one of four CâH(i) to NH(i + 3)
NOEs were unambiguously assigned. In water, the same
types of NOEs were identified and unambiguously as-
signed: three of four CâH(i) to CRH(i + 2) NOEs, four of
five CâH(i) to NH(i + 2) NOEs, and one of four CâH(i) to
NH(i + 3) NOEs. Only one fewer NOE was observed in
water for this peptide than in methanol, perhaps suggesting
that the structural differences between the two solvents are
minimal.
fellowship that is sponsored by the Department of Defense.
NMR spectrometers were purchased with partial support
from NIH and NSF. CD spectra were obtained in the NSF-
supported Biophysics Instrumentation Facility. Crystal-
lographic work was performed by Dr. Ilia A. Guzei.
Supporting Information Available: General procedures,
1
characterization data, H and 13C spectra of all numbered
intermediates in the schemes and crystallographic data in
CIF format. This material is available free of charge via the
Internet at http://pubs.acs.org.
Acknowledgment. This research was supported by the
NIH (GM56414). T.J.P was supported by a NIH postdoctoral
fellowship (GM065713). E.P.E. was supported by a NDSEG
OL0483293
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Org. Lett., Vol. 6, No. 24, 2004