(+)- and (-)-2-aminocyclobutane-1-carboxylic acids and their incorporation into highly rigid β-peptides: Stereoselective synthesis and a structural study

Article abstract of DOI:10.1021/jo0510843

Several derivatives of (+)- and (-)-2-aminocyclobutane-1-carboxylic acid, 1, have been prepared through enantiodivergent synthetic sequences. The stereoselective synthesis of free amino acid (+)-1 has been achieved, and this product has been fully charact

Full text of DOI:10.1021/jo0510843

(+)- and (-)-2-Aminocyclobutane-1-carboxylic Acids and Their
Incorporation into Highly Rigid â-Peptides: Stereoselective
Synthesis and a Structural Study
Sandra Izquierdo,† Federico Ru´a,† Abdelouahid Sbai,†,| Teodor Parella,‡
AÄ ngel AÄ lvarez-Larena,§ Vicenc¸ Branchadell,† and Rosa M. Ortun˜o*,†
Departament de Quı´mica, Servei de RMN, and Unitat de Cristal‚lografia, Universitat Auto`noma de
Barcelona, 08193 Bellaterra, Barcelona, Spain
rosa.ortuno@uab.es
Received May 30, 2005
Several derivatives of (+)- and (-)-2-aminocyclobutane-1-carboxylic acid, 1, have been prepared
through enantiodivergent synthetic sequences. The stereoselective synthesis of free amino acid
(+)-1 has been achieved, and this product has been fully characterized for the first time.
Stereocontrolled alternative synthetic methodologies have been developed for the preparation of
bis(cyclobutane) â-dipeptides in high yields. Among them, enantio and diastereomers have been
synthesized. â,â- and â,δ-Dimers resulting from the coupling of a cyclobutane residue and a linear
amino acid have also been prepared. The ability of the cyclobutane ring as a structure-promoting
unit both in the monomers and in the dimers has been manifested. The NMR structural study and
DFT theoretical calculations evidence the formation of strong intramolecular hydrogen bonds giving
rise to cis-fused [4.2.0]octane structural units that confer high rigidity on these molecules both in
solution and in the gas phase. The contribution of a cis-trans conformational equilibrium derived
from the rotation around the carbamate N-C(O) bond has also been observed, the trans form being
the major conformer. In the solid state, this equilibrium does not exist, and moreover, intermolecular
hydrogen bonds are present.
Introduction
tuftsine.2 Small oligomers of cycloalkane â-amino acids
have shown a great propensity to adopt preferred con-
formations leading to secondary structures.1b,3 Regarding
cyclobutane-containing â-peptides,4 we have recently
shown that tetrapeptide 3, formed by two units of (-)-
2-aminocyclobutane-1-carboxylic acid, (-)-1, and two
â-alanine residues joined in alternance, presents a 14-
helical folding in solution.5 In contrast, oligomers of
â-alanine do not show any defined conformational bias,
neither in the crystal state6a nor in aqueous solution.6b
Cyclobutane amino acids (CBAAs) have been found in
nature isolated or incorporated into peptides.1a,b This is
the case, for instance, of the antibiotic X-1092 produced
by the Streptomyces species X-1092.1c,d Furthermore, the
incorporation of the cyclobutane ring into conformation-
ally restricted peptidomimetics sometimes results in the
preparation of biologically active products such as the
tuftsine analogue Thr-[Mom2]-Pro-Arg with increased
resistance toward enzimatic hydrolysis with respect to
(2) Gershonov, E.; Granoth, R.; Tzehoval, E.; Gaoni, Y.; Fridkin, M.
J. Med. Chem. 1996, 30, 4833.
† Departament de Qu´ımica.
‡ Servei de RMN.
(3) For reviews on the â-peptide secondary structures, see: (a)
Gellman, S. H. Acc. Chem. Res. 1998, 31, 173. (b) DeGrado, W. F.;
Schneider, J. P.; Hamuro, Y. J. Pept. Res. 1999, 54, 206. (c) Cheng, R.
P.; Gellman, S. H.; DeGrado, W. F. Chem. Rev. 2001, 101, 3219. (d)
Martinek, T. A.; Fu¨lop, F. Eur. J. Biochem. 2003, 270, 3657.
(4) For a review on the synthesis of cyclobutane amino acids and
peptides, see: Ortun˜o, R. M.; Moglioni, A. G.; Moltrasio, G. Y. Curr.
Org. Chem. 2005, 9, 237.
§ Unitat de Cristal‚lografia.
| Present address: De´partement de Chimie, Faculte´ des Sciences
de Mekne`s, Beni M’hamed, Mekne`s, Morocco.
(1) (a) Bell, E. A.; Qureshi, M. Y.; Pryce, R. J.; Janzen, D. H.; Lemke,
P.; Clardy, J. J. Am. Chem. Soc. 1980, 102, 1409. (b) Seebach, D.; Bech,
A. K.; Bierbaum, D. J. Chem. Biodiversity 2004, 1, 1111. (c) Adlington,
R. M.; Baldwin, J. E.; Jones, R. H.; Murphy, J. A.; Parisi, M. F. J.
Chem. Soc., Chem. Commun. 1983, 1479. (d) Baldwin, J. E.; Adlington,
R. M.; Parisi, M. F.; Ting, H.-H. Tetrahedron 1986, 42, 2575.
(5) Izquierdo, S.; Kogan, M. J.; Parella, T.; Moglioni, A.; Branchadell,
V.; Giralt, E.; Ortun˜o, R. M. J. Org. Chem. 2004, 69, 5093.
10.1021/jo0510843 CCC: $30.25 © 2005 American Chemical Society
Published on Web 09/08/2005
J. Org. Chem. 2005, 70, 7963-7971
7963

Izquierdo et al.
CHART 1
both (+)- and (-)-1 by [2 + 2]-photocycloaddition of
ethylene to a chiral uracyl derivative to afford a 60:40
mixture of diastereomers. Isolation of each isomer by
chromatography allowed the pursuit of the synthesis of
both enantiomeric amino acids.13 Previously to these
works, we reported on the stereodivergent synthesis of
the orthogonally protected derivatives (-)-6a7 and 1114
with opposite chirality. These compounds were prepared
from chiral half-ester 4 (Scheme 1), which results from
the enzymatic desymmetrization of meso cyclobutane 1,2-
dicarboxylic acid methyl ester,15 and some aspects on
their use in the preparation of bis(cyclobutane) â-dipep-
tides (bisCBDPs) were described in a preliminary com-
munication.14
Now in this article, we show the efficiency and the
versatility of our stereodivergent synthetic approach to
cyclobutane â-amino acids and â-peptides. Thus, we
describe full details on the synthesis of both enantiomeric
CBAAs (+)- and (-)-1 and several useful derivatives for
the preparation of oligomers. â,â- and â,δ-Dipeptides
containing a cyclobutane residue and a linear fragment
are representative instances. We also provide and discuss
improved methodology for the preparation of the bisCB-
DPs formally resultant from self-coupling of the conve-
niently protected enantiomeric amino acids, as well as
one with another. Finally, we present a structural study
made on the basis of X-ray diffraction analysis, NMR
experiments, and DFT theoretical calculations, all of
them accounting for the ability of the cyclobutane ring
to induce secondary structures both in the monomers and
in different types of cyclobutane-â-peptides.
Otherwise, dipeptide (-)-2 (Chart 1) adopts a hairpin-
like conformation in the solid state.7 In addition, Claridge
et al. reported that a hexamer formed by oxetane residues
forms a 10-helix.8 Moreover, interesting biological activi-
ties have been displayed by â-foldamers. This finding has
prompted the research in the synthesis and study of
conformationally constrained â-amino acids9 and â-oli-
gomers10 allowing the production of novel antibiotics and
nonhaemolytic agents11a-d as well as mimics of somato-
statin11e among other pharmaceutically active products.1b,11f
(-)-2-Aminocyclobutane-1-carboxylic acid, (-)-1, has
been very recently synthesized by Bolm et al. via the
desymmetrization of a cyclic meso anhydride through its
quinine-mediated opening in the presence of benzyl
alcohol.12 Gauzy et al. have reported on the synthesis of
Results and Discussion
1. Synthesis of Amino Acids and Peptides. The
general strategy to synthesize the enantiomers (+)- and
(-)-1 and some conveniently protected derivatives is
depicted in Scheme 1. Half-ester 4 could be transformed
into (-)-6a or 6b through a Curtius rearrangement in
the presence of benzyl alcohol or tert-butyl alcohol,
respectively. Saponification of the methyl ester and
subsequent hydrogenolysis of the benzyl carbamate (-)-
7a allowed us to achieve the first synthesis of the free
amino acid (-)-1.7 The synthetic way to CBAA (+)-1
passes through the asymmetric diester 8 prepared in 90%
yield by treatment of 4 with tert-butyl trichloroacetimi-
date.16 Selective saponification of the methyl ester under
mild conditions (0.25 M NaOH, 0 °C, 1 h) to avoid
epimerization led quantitatively to 9, which is a pseu-
doenantiomer of 4. The acyl azide 10 was prepared in
91% yield by reaction of 9 with ethyl chloroformate,
followed by treatment with sodium azide. Subsequent
Curtius rearrangement was accomplished by heating 10
in the presence of benzyl alcohol in refluxing toluene to
afford fully protected â-amino acid 11. The intermediate
carboxylic acid (+)-7a was quantitatively prepared by
reaction of 11 with trifluoroacetic acid and triethylsi-
(6) (a) Pavone, V.; Di Blasio, B.; Lombardi, A.; Isernia, C.; Pedone,
C.; Benedetti, E.; Valle, G.; Crisma, M.; Toniolo, C.; Kishore, R. J.
Chem. Soc., Perkin Trans. 2 1992, 1233. (b) Glickson, J. D.; Applequist,
J. J. Am. Chem. Soc. 1971, 93, 3276.
(7) Mart´ın-Vila`, M.; Muray, E.; Aguado, G. P.; AÄ lvarez-Larena, AÄ .;
Branchadell, V.; Minguillo´n, C.; Giralt, E.; Ortun˜o, R. M. Tetrahe-
dron: Asymmetry 2000, 11, 3569.
(8) Claridge, T. D. W.; Goodman, J. M.; Moreno, A.; Angus, D.;
Barker, S. F.; Taillefumier, C.; Watterson, M. P.; Fleet, G. W.
Tetrahedron Lett. 2001, 42, 4251.
(9) Ortun˜o, R. M. In Enantioselective Synthesis of â-Amino Acids,
2nd ed.; Juaristi, E., Soloshonok, V. A., Eds.; John Wiley and Sons:
New Jersey, 2005; pp 117-137.
(10) Gelman, M. A.; Gellman, S. H. In Enantioselective Synthesis of
â-Amino Acids, 2nd ed.; Juaristi, E., Soloshonok, V. A., Eds; John Wiley
and Sons: New Jersey, 2005; pp 527-585.
(11) (a) Werder, M.; Hausre, H.; Abele, S.; Seebach, D. Helv. Chim.
Acta 1999, 82, 1774. (b) Hamuro, Y.; Schneider, J. P.; DeGrado, W. F.
J. Am. Chem. Soc. 1999, 121, 12300. (c) Porter, E. A.; Weisblum, B.;
Gellman, S. H. J. Am. Chem. Soc. 2002, 124, 7324. (d) Raguse, T. L.;
Porter, E. A.; Weisblum, B.; Gellman, S. H. J. Am. Chem. Soc. 2002,
124, 12774. (e) Gademann, K.; Kimmerlin, T.; Hoyer, D.; Seebach, D.
J. Med. Chem. 2001, 2460. (f) Arvidsson, P. I.; Ryder, N. S.; Weiss, H.
M.; Hook, D. F.; Escalante, J.; Seebach, D. Chem. Biodiversity 2005,
2, 401.
(13) Gauzy, C.; Pereira, E.; Faure, S.; Aitken, D. J. Tetrahedron Lett.
2004, 45, 7095. Optical rotation values for each enantiomer are not
provided.
(14) Izquierdo, S.; Mart´ın-Vila`, M.; Moglioni, A. G.; Branchadell, V.;
Ortun˜o, R. M. Tetrahedron: Asymmetry 2002, 13, 2403.
(15) Sabbioni, G.; Jones, J. B. J. Org. Chem. 1987, 52, 4565.
(16) Thierry, J.; Yue, C.; Potier, P. Tetrahedron Lett. 1998, 39, 1557.
(12) Bolm, C.; Schiffers, I.; Atodiresei, I.; Hackenberger, P. R.
Tetrahedron: Asymmetry 2003, 14, 3455.
7964 J. Org. Chem., Vol. 70, No. 20, 2005

(+)- and (-)-2-Aminocyclobutane-1-carboxylic Acids
SCHEME 1a
a Reagents and conditions. (a) (i) ClCO2Et, Et3N, acetone, 0 °C, 3 h. (ii) NaN3, H2O, rt, 1.5 h. (b) BnOH, refluxing toluene, 3.5 h. (c)
refluxing t-BuOH, overnight. (d) 0.25 M NaOH, 1:10 THF-H2O, 0 °C, 1 h. (e) H2, 10% Pd/C, MeOH, 1 atm, rt, 1.5 h. (f) Cl3C-C(NH)OtBu,
CH2Cl2, rt, overnight. (g) TFA, Et3SiH, CH2Cl2, rt, 2 h. (h) CH2N2, ether. (i) H2N(CH2)2CO2Me, DEC, HOBt, DMF, rt, 20 h.
SCHEME 2a
lane.17 Esterification with diazomethane provided the
methyl ester (+)-6a whose specific rotation magnitude
compared well with that of the enantiomer (-)-6a show-
ing that optical purity is respected throughout the
synthetic sequence.
Moreover, catalytic hydrogenolysis (Pd/C) of the benzyl
carbamate in (+)-7a led quantitatively to the enantio-
meric free amino acid (+)-1 whose melting point, specific
rotation, and 1H NMR spectrum compared well with
those corresponding to (-)-1.
Dipeptide (+)-2 could also be prepared, in 71% yield,
by coupling acid (+)-7a with â-Ala-OMe.
The synthesis of bisCBDPs was first tried by coupling
acids (+)- or (-)-7a with the amines resulting from the
hydrogenolysis of benzyl carbamates (+)-or (-)-6a. Revis-
ing the results of several batches, we detected a serious
problem derived from the instability of the cyclobutane
ring under the hydrogenation conditions. Amino ester 12,
resulting from the hydrogenation of (-)-7a (Scheme 2),
is a captodative system that can evolve to an open-chain
iminium salt, which is reduced in situ to the achiral
δ-amino ester 13.
Then, we studied the hydrogenation under several
conditions related to the catalyst (Pd/C, Pd(OH)2) and
reaction time. We found that conditions needed to
produce the free amine without ring-opening were dif-
ficult to reproduce. Reactivity was slightly increased
when Pd(OH)2 was used. With this catalyst, at short
reaction times, a mixture of starting material, cyclobu-
a Reagents and conditions. (a) H2, catalyst. (b) TFA, Et3SiH,
CH2Cl2, rt, 2 h. (c) H3O+. (d) DEC, HOBt, DMF, rt, 20 h.
tylamine 12 and linear amino ester 13, was detected on
some occasions showing that reduction of the iminium
salt competes with the hydrogenolysis of the benzyl
(17) Mehta, A.; Jaouhari, R.; Benson, T. J.; Douglas, K. T. Tetra-
hedron Lett. 1992, 33, 5441.
J. Org. Chem, Vol. 70, No. 20, 2005 7965

Izquierdo et al.
SCHEME 3a
carbamate. Obviously, longer reaction times favored the
production of 13. This amino acid was condensed with
(+)- and (-)-7a to provide dipeptides (+)- and (-)-14,
respectively, which are constituted of a â- and a δ-residue
(Scheme 2).18
To avoid this difficulty, we then prepared the N-Boc
derivative 6b as described above (Scheme 1). Deprotec-
tion of the amine was carried out under treatment with
trifluoroacetic acid and triethylsilane affording 12. When
this product was submitted to lyophilization before
peptide coupling to remove acid traces that diminish the
reaction yield, the aldehyde shown in Scheme 2 was
1
detected by H NMR along with amine 12.
Other authors have also given accounts on the ease of
some cyclobutane derivatives to give open-chain products
under several conditions.19 Fortunately, the ring-opening
is not observed when a less electrophilic amido group,
instead of an ester, is placed in a vicinal position with
respect to a free cyclobutylamine, allowing us to synthe-
size peptides such as 3 (Chart 1).
To circumvent this difficulty and, at the same time, to
shorten the synthetic sequence, we performed the in situ
reactions of carboxylic acids (+)- and (-)-7a and 7b,
respectively, with the isocyanates resulting from Curtius
rearrangement of acyl azides 5 or 10, in refluxing toluene
for 6 h (Scheme 3). Under these conditions, dipeptides
(-)-15a, 15b, 16, and 17 resulted in 40-50% yield, after
purification by flash chromatography.
Urea derivatives such as 18 were often obtained in 30-
35% yield along with the peptides. Similar results were
found by Hibbs et al. in the preparation of peptides
incorporating an endo-5-norbornene residue following a
similar methodology.20 The formation of these byproducts
could be avoided by using a molecular sieve pad, inter-
calated between the condenser and the reaction flask, to
secure a water-free medium. In this way, pure dipeptides
(-)-15a and 15b were obtained in 66-70% yield and no
urea trace was detected in any case. The structures of
the peptides and the ureas were elucidated by X-ray
diffraction analysis of compounds (-)-15a, 16, 17, and
18. The structure of the already known acid (-)-7a7 has
also been elucidated by X-ray diffraction analysis in this
work. Dipeptide 17 was converted into (+)-15a to asses
the enantiomeric relationship with (-)-15a by removal
of the tert-butyl ester as usual, followed by methylation
with diazomethane. All compounds 15-17 are suitably
protected for their later incorporation into longer oligo-
mers.
a Reagents and conditions. (a) Et3N, toluene, 110 °C, 6 h. (b) (i)
TFA, Et3SiH, rt, 4 h. (ii) CH2N2.
groups. Every chain comprises two antiparallel ‚‚‚(H-
N-CdO‚‚‚)n patterns (Figure 1, Table 1) where all amide
groups adopt a trans disposition. This packing description
is common to the three dipeptides and can be related to
a similar biased conformation. In this way, a hairpin-
like folding can be considered, where the second residue
is placed in the middle of the hairpin (Figure 2). The
amide planes are orthogonal to the mean plane of the
hairpin in order to form the above-mentioned intermo-
lecular hydrogen bonds. Conformational differences are
mainly involved in the disposition of terminal groups. For
example, the bulky tert-butyl ester of compound 17 is
placed outside of the hairpin, whereas the smaller methyl
ester can be inside (16) or outside (15a). Moreover, some
structural disorder is present in the phenyl and tert-butyl
groups (not shown in the figures, see the Supporting
Information). Crystal structures of (-)-7a, (-)-15a, 16,
17, and 18 have been deposited with the Cambridge
Crystallographic Data Center and allocated deposition
numbers CCDC 272745, CCDC 272746, CCDC 272747,
CCDC 272748, and CCDC 272749, respectively.
3. NMR Studies. Complete 1H and 13C resonance
assignments were performed using 1D and 2D NMR
techniques (COSY, HSQC, and HMBC experiments).
Moreover, variable-temperature 2D NOESY spectra were
recorded at several temperatures, and temperature coef-
ficients (Table 2) were measured on the synthesized
molecules in order to state the conformational bias in
1
solution. Although the H NMR spectra were recorded
in several deuterated solvents such as CDCl3, acetoni-
trile, acetone, and methanol, most experiments were
preferably carried out in CDCl3 solution. As a general
trend, it can be stated that in all studied compounds the
presence of two different conformational structures was
observed due to the cis-trans amide-bond isomerization
in the benzyl carbamate fragment (Figure 3). The major
conformation was assigned to the trans rotamer based
on the above X-ray crystal structures and DFT theoretical
calculations (vide infra). The amount of the minor cis
rotamer was in the range of 5-10% as previously
reported for tetrapeptide 3.5 The determination of the
exact temperature and the energy barrier involved in the
cis-trans interconversion is hampered due to peak
broadening and chemical-shift dependence on tempera-
ture.
2. X-ray Diffraction Analysis. The conformation and
interactions in the solid state for the bisCBDPs (-)-15a,
16, and 17 have been determined from single-crystal
X-ray data. Suitable crystals were obtained from metha-
nol-pentane or from ethyl acetate-pentane solutions.
Crystal structures of the three dipeptides have, as a
common feature, the existence of infinite chains of
molecules linked by hydrogen bonds involving amide
(18) Latter to publication, we have realized that measures of specific
rotations described for dipeptides (+)- and (-)-15, and 17 in ref 14
were made on contaminated samples and are, therefore, wrong.
(19) Aitken, D. J.; Gauzy, C.; Pereira, E. Tetrahedron Lett. 2004,
45, 2359.
(20) Hibbs, D. E.; Hursthouse, M. B.; Jones, I. G.; Jones, W.; Malik,
K. M. A.; North, M. J. Org. Chem. 1998, 63, 1496.
(21) (a) Jeffrey, G. A.; Lewis, L. Carbohydr. Res. 1978, 60, 179. (b)
Taylor, R.; Kennard, O. Acta Crystallogr.1983, B39, 133.
7966 J. Org. Chem., Vol. 70, No. 20, 2005

(+)- and (-)-2-Aminocyclobutane-1-carboxylic Acids
FIGURE 1. Intermolecular hydrogen bonds forming infinite chains in the crystal structures of dipeptides (-)-15a, 16, and 17
(from left to right).
TABLE 1. Geometries of the Intermolecular Hydrogen Bonds Forming Infinite Chains in the Crystal Structures of
Dipeptides (-)-15a, 16, and 17a
compound
N11‚‚‚O10(s)b
H11‚‚‚O10(s)b
N11-H11‚‚‚O10(s)c
s
15a
16
17
2.982 (2)
3.049 (4)
3.019 (6)
2.014
2.049
2.031
155.41
163.07
159.71
x, y + 1, z
x + 1, y, z
x + 1, y, z
N18‚‚‚O17(s)b
H18‚‚‚O17(s)b
N18-H18‚‚‚O17(s)c
s
15a
16
17
3.035 (2)
2.997 (4)
3.164 (6)
2.038
2.009
2.145
162.25
159.76
169.96
x, y - 1, z
x - 1, y, z
x - 1, y, z
a N11-H11 and N18-H18 (1.030 Å) and the rest of parameters involving H11 and H18 have been normalized;21 s is the symmetry
code of the neighbor molecule. b In angstroms. c In degrees.
At room temperature, the NMR signals appeared as
broad multiplets confirming the contribution of dynamic
processes experimentally evidenced by the presence of
strong negative exchange cross peaks in selective 1D
NOESY and 2D NOESY spectra. When the temperature
between +0.05/+0.12 ppm confirming a minor variation
of its surroundings. In the case of the t-Bu-derivative 17,
the NH11 proton shows a milder hydrogen bonding than
NH18 clearly evidenced by the broad resonance peaks of
NH11, whereas NH18 displays a well-resolved doublet
at room temperature. The strong NH11-residual HDO
exchange cross peak analysis in the NOESY spectra and
temperature coefficients also account for the exchange
differences between both NH protons (Table 2, Figure 4).
Moreover, the trans disposition of the N-H18 bond is
assessed by the strong NOE observed between NH18 and
Hb.
1
is lowered, all H resonances became sharper and the
particular variation of the NH and Ha,a′ chemical shifts
could be used to monitor hydrogen-bonding properties by
measurement of temperature chemical-shift coefficients.
These data are valuable to establish, in a qualitative way,
the apparent strength of possible intermolecular hydro-
gen bonding with carbonyl groups. For the monomers,
the NH chemical shift usually shows a considerable
upfield effect of about -0.25/-0.35 ppm in the minor
conformation, probably due to the anisotropic properties
of the carbonyl group.
However, a complicating factor for the proper analysis
of this minor conformation is that tiny chemical exchange
cross peaks are also visible even at 230-250 K. The
carbamate NH resonance in the major conformation
always shows a large coupling-constant value with the
vicinal Ha proton (between 8 and 9 Hz) confirming its
preferred anti disposition.
Finally, for dimer 2c5 (Figure 3c), the ethylene frag-
ment offers a more flexible motion allowing more free
rotations around the peptide CO-N bond. Thus, although
in acetonitrile and acetone this molecule displays the
same behavior as that described above, more flexibility
for the CO-NH18 bond is evidenced in CDCl3 by simul-
taneous NOE between Hb and both the NH18 and
diastereotopic R-NCH2 protons. This effect is also high-
lighted from the analysis of the minor conformation in
which the NH11 proton shows a different reversal
downfield shift of +0.18 ppm, whereas a very strong
upfield shift of -0.62 ppm is observed for NH18.
4. Theoretical Calculations. DFT theoretical calcu-
lations were done to account for the conformational
features of (-)-6a, (-)-15a, and 16 as representative
instances. The corresponding structures for the cis-trans
rotamers related to the carbamate group in (-)-6a are
For the highly rigid dicyclobutane derivatives 16 and
17, similar trends are also observed despite of their
diastereomeric relationship (Figure 3b). Whereas NH11
protons experience a general upfield effect in the minor
conformation around -0.3 ppm, the other amide NH18
proton feels a smaller but observable downfield effect
J. Org. Chem, Vol. 70, No. 20, 2005 7967

Izquierdo et al.
FIGURE 3. Intramolecular hydrogen bonds, cis-trans rota-
mers, and significant NOEs for (a) monomer (-)-6a, (b) dimer
17, and (c) dimer 2c.
a ∆G0 of 2.3 kcal mol-1 and a ∆Gq of 17.9 kcal mol-1 in
the gas phase. In chloroform solution these values are
2.6 and 17.6 kcal mol-1, respectively.
In both structures, we can observe the presence of an
intramolecular hydrogen bond. Other type of conformers
due to the flexibility of the benzyl carbamate also emerge.
Thus, the C(carbonyl)-O-C-C(phenyl) dihedral angle
of the absolute minimum is -93°, but another conformer
with a dihedral angle of -172° has been located as being
only 0.3 kcal mol-1 higher in Gibbs energy. The chemical
shifts for the NH proton in both conformers have been
computed, the obtained values being δ 5.8 ppm (trans)
and 5.4 ppm (cis). The difference ∆δ of 0.4 ppm is in
excellent agreement with the observed differences be-
tween the major and minor conformers.
FIGURE 2. Molecular conformations of dipeptides (-)-15a,
16, and 17 (from top to bottom) in the solid state.
Taking into account the general conformational bias
of the bisCBDPs, Figure 6 shows the most stable struc-
tures obtained for (-)-15a and 16.
TABLE 2. Temperature Coefficientsa and, in
Parentheses, Chemical Shiftsb Measured for Several
Cyclobutane Derivatives in CDCl3 Solution
These structures can be compared with those obtained
from X-ray diffraction (Figure 2). The main difference
between the solid state and the gas-phase optimized
structures is that, in the latter, there are intramolecular
CO‚‚‚HN hydrogen bonds. The comparison with the
structure optimized for the monomer (-)-6a (Figure 5)
shows that dimerization strengthens the CO‚‚‚HN hy-
drogen bonds. Moreover, NH18 displays a stronger
hydrogen bonding than NH11 in good accordance with
NMR results.
compound
NH11
NH18
2c
-1.3 (5.77)
-0.8 (5.74)
-1.1 (5.79)
-1.3 (5.82)
-3.3 (6.74)
(-)-6a
16
-0.7 (6.42)
-0.6 (6.58)
17
a In ppb/°C. b In ppm at 298 K.
shown in Figure 5. The trans rotamer is the most stable
one, and its rearrangement toward the cis one involves
7968 J. Org. Chem., Vol. 70, No. 20, 2005

(+)- and (-)-2-Aminocyclobutane-1-carboxylic Acids
FIGURE 4. Expansions of the 1H NMR spectra for dimer 17
(in CDCl3) at several temperatures: (A) 295 K; (B) 280 K; (C)
260 K.
FIGURE 6. Structures of dipeptides (-)-15a and 16. Selected
interatomic distances in Å.
â,δ-dipeptides, some of them with an enantiomeric or
diastereomeric relationship. Two synthetic ways have
been explored for the preparation of bisCBDPs. The way
involving reaction of a carboxylic acid with an acyl azide
shortens the synthetic sequence and circumvents the
difficulties in the deprotection of amines derived from the
instability of the cyclobutane amino esters under several
conditions. From a structural point of view, the ability
of the cyclobutane ring to induce conformational bias has
been evidenced. The formation of intramolecular hydro-
gen bonds is a general trend both in solution and in the
gas phase (from DFT calculations), giving rise to the
formation of highly rigid bicyclic structural units both
in the monomers and in the dimers. For dipeptides 15-
17 in the solid state, both the amide C17(O)-N18 bond
and the carbamate group are disposed in a perpendicular
plane with respect to the mean cyclobutane ring of the
first residue, in such a way that infinite antiparallel
intermolecular hydrogen bond chains are formed. More-
over, a cis-trans equilibrium associated to the rotation
of the carbamate group is observed in solution, with the
trans conformer being the major one. In the solid state,
only the trans conformation is observed.
FIGURE 5. Structures of the trans and cis conformers of
amino acid (-)-6a. Selected interatomic distances in Å.
Concluding Remarks
The high versatility and efficiency of the enantiodi-
vergent synthetic approach to cyclobutane amino acids
and derivatives presented herein has been shown with
the synthesis of the enantiomeric CBAAs (+)- and (-)-1
and several types of oligomers. These include â,â- and
The potential of these and other related compounds in
the search of novel therapeutic agents is the object of
intense investigation in our laboratories.
J. Org. Chem, Vol. 70, No. 20, 2005 7969

Izquierdo et al.
removed at reduced pressure, and the residue was purified
by column chromatography (3:1 hexanes-EtOAc as eluent)
affording diester 8 (386 mg, 90% yield). Oil. [R]D +4.0 (c 0.50,
Experimental Section
Computational Details. All calculations have been done
using the Gaussian 03 program.22 A preliminary exploration
of the conformational space has been done using the semiem-
pirical AM1 method.23 The most stable structures have been
fully optimized using the B3LYP24 density functional method
with the 6-31G(d) basis set. Harmonic vibrational frequencies
of all the structures have been computed to verify that they
are energy minima or transition states. Gibbs energies of these
structures have been computed at 1 atm and 298.15 K. The
effect of solvation by chloroform (ꢀ ) 4.9) in the cis-trans
conformational rearrangement of (-)-6 has been taken into
account using the CPCM method.25 For the cis and trans
conformers of (-)-6 we have computed the 1H chemical shifts
using the gauge-independent atomic orbital (GIAO) method26
at the B3LYP level of calculation with the 6-31+G(d,p) basis
set.
MeOH). IR (film): 1733 cm-1 1H NMR (250 MHz, acetone-
.
d6): δ 1.42 (s, 9H), 2.16 (m, 2H), 2.24 (m, 2H), 3.36 (m, 2H),
3.64 (s, 3H). 13C NMR (62.5 MHz, acetone-d6): δ 21.5, 21.7,
28.3 (3C), 40.1, 41.4, 50.7, 79.6, 172.0, 173.1. Anal. Calcd for
C11H18O4: C, 61.60; H, 8.50. Found: C, 61.53; H, 8.48.
(1S,2R)-2-tert-Butyloxycarbonylcyclobutane-1-carbox-
ylic Acid, 9. NaOH (0.25 M, 7.5 mL) was added to a solution
of diester 8 (200 mg, 0.9 mmol) in 1:10 THF-H2O (17.6 mL).
The mixture was stirred at 0 °C for 1 h, and then the aqueous
solution was extracted with dichloromethane. After acidifica-
tion to pH 2 with 2 M HCl, the aqueous phase was extracted
with ethyl acetate. The solvent was evaporated under vacuo,
and the residue was chromatographed (EtOAc as eluent) to
give quantitatively (187 mg) half-ester 9. Oil. [R]D +9.3 (c 1.08,
MeOH). IR (film): 1798, 1690 cm-1 1H NMR (250 MHz,
.
Methyl-(1R,2S)-2-tert-butyloxycarbonylaminocyclobu-
tane-1-carboxylate, 6b. A solution of azide 5 (522 mg, 2.8
mmol) in t-BuOH (9 mL) was heated to reflux overnight. The
solvent was removed to give compound 6b as a solid (616 mg,
94% yield) that was purified by crystallization. Crystals, mp
76.1 °C (EtOAc-hexane). [R]D -131.0 (c 0.62, CH2Cl2). 1H
NMR (250 MHz, CDCl3): δ 1.38 (s, 9H), 1.87-1.97 (m, 2H),
2.25 (m, 2H), 3.35 (m, 1H), 3.67 (s, 3H), 4.41 (m, 1H), 5.33 (m,
1H). 13C NMR (62.5 MHz, CDCl3): δ 18.1, 27.8, (3C), 29.1,
44.9, 45.6, 51.3, 79.0, 154.4, 174.4. Anal. Calcd for C11H19-
NO4: C, 57.62; H, 8.35; N, 6.11. Found: C, 57.75; H, 8.44; N,
6.13.
(1R,2S)-2-tert-Butyloxycarbonylaminocyclobutane-1-
carboxylic Acid, 7b. A mixture of 6b (168 mg, 0.7 mmol) in
10:1 H2O-THF (7 mL) and 0.25 M NaOH (6 mL) was stirred
at 0 °C for 3 h. Then the reaction mixture was extracted with
CH2Cl2, and 4 M HCl was added to the aqueous layer to reach
pH 2. Then the acid aqueous phase was extracted with EtOAc,
and the solvent was removed at reduced pressure to afford
acid 2 as a solid (129 mg, 82% yield). Crystals, mp 117 °C
(ether-hexane). [R]D -48.6 (c 0.74, CH2Cl2). 1H NMR (250
MHz, CDCl3): δ 1.45 (s, 9H), 1.72 (m, 1H), 2.04 (m, 1H), 2.32
(m, 2H), 3.35 (m, 1H), 4.35 (m, 1H). 13C NMR (62.5 MHz,
CDCl3): δ 27.3, 27.8 (3C), 29.4, 45.6, 47.0, 81.0, 157.3, 176.9.
Anal. Calcd for C10H17NO4: C, 55.80; H, 7.96; N, 6.51. Found:
C, 55.45; H, 7.95; N, 6.35.
acetone-d6): δ 1.42 (s, 9H), 2.14 (m, 2H), 2.28 (m, 2H), 3.35
(m, 2H). 13C (NMR (62.5 MHz, acetone-d6): δ 21.3 (2C), 27.1
(3C), 40.0, 41.0, 79.2, 171.7, 173.5. Anal. Calcd for C10H16O4:
C, 60.00; H, 8.58. Found: C, 60.16; H, 8.43.
tert-Butyl-(1S,2R)-2-azidocarbonylcyclobutanecar-
boxylate, 10. A mixture of half-ester 9 (600 mg, 3.0 mmol),
ethyl chloroformate (0.6 mL, 3.8 mmol), freshly distilled Et3N
(0.8 mL, 3.5 mmol), and dry acetone (19 mL) was stirred at 0
°C for 3 h. Then a solution of sodium azide (429 mg, 6.6
mmmol) in water (10 mL) was added, and the resultant
mixture was stirred at room temperature for 1.5 h. The
reaction mixture was then extracted with dichloromethane.
The layers were separated, the organic phase was dried over
MgSO4, and the solvents were removed at reduced pressure
to afford azide 10 as a yellow oil that was used in the next
step without further purification (615 mg, 91% yield). IR
(film): 2134, 1714 cm-1 1H NMR (250 MHz, acetone-d6): δ
.
1.42 (s, 9H), 2.14 (m, 2H), 2.28 (m, 2H), 3.35 (m, 2H). 13C NMR
(62.5 MHz, acetone-d6): δ 22.0 (2C), 27.7 (3C), 42.4, 43.4, 80.5,
172.0, 180.2.
tert-Butyl-(1S,2R)-2-benzyloxycarbonylaminocyclo-
butane-1-carboxylate, 11. A solution of azide 10 (530 mg,
2.4 mmol) and benzyl alcohol (0.5 mL, 4.9 mmol) in dry toluene
(14 mL) was heated to reflux for 3.5 h. The solvent was
evaporated at reduced pressure, and the residue was chro-
matographed (dichloromethane as eluent) to provide pure 11
(590 mg, 82% yield). Oil. [R]D +41 (c 0.42, CHCl3). IR (film):
tert-Butyl-(1S,2R)-2-methoxycarbonylcyclobutane-1-
carboxylate, 8. tert-Butyl trichloroacetimidate (0.7 mL, 4.0
mmol) was added to a solution of half-ester 47 (316 mg, 2.0
mmol) in dry dichloromethane (20 mL), and the mixture was
stirred at room-temperature overnight. The solvent was
1
3330, 1738 cm-1. H NMR (250 MHz, CDCl3): δ 1.40 (s, 9H),
1.93 (m, 2H), 2.29 (m, 2H), 3.25 (m, 1H), 4.47 (m, 1H), 5.06 (s,
2H), 5.64 (broad s, 1H), 7.32 (m, 5H). 13C (NMR) (62.5 MHz,
CDCl3): δ 19.0 (2C), 26.6 (3C), 46.4, 46.6, 67.1, 81.5, 128.6,
129.0, 137.0, 155.8, 174.0.
Methyl-(1S,2R)-2-benzyloxycarbonylaminocyclobutane-
1-carboxylate, (+)-6a through Acid (+)-7a. Trifluoroacetic
acid (0.7 mL, 9.1 mmol) and Et3SiH (0.15 mL, 1.8 mmol) were
successively added to a solution of 11 (200 mg, 0.7 mmol) in
dry dichloromethane (1.5 mL). The mixture was stirred at
room temperature for 2 h, and the volatiles were removed
under reduced pressure to afford the crude acid (+)-7a as a
dense oil whose spectroscopic data compared well with those
described for its enantiomer (-)-7a.7 This product was used
in the next steps without further purification.
(22) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.;
Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.;
Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson,
G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.;
Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai,
H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo,
C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin,
A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma,
K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.;
Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.;
Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui,
Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.;
Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challa-
combe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.;
Gonzalez, G.; Pople, J. A. Gaussian 03, revision B.04; Gaussian, Inc.:
Pittsburgh, PA, 2003; http://www.gaussian.com.
An ethereal solution of excess diazomethane was distilled
onto acid (+)-7a (200 mg, 0.8 mmol) in 5 mL of ether at 0 °C,
and the resultant solution was stirred at room temperature
for 20 min. Excess diazomethane was destroyed by the addition
of CaCl2, and the solvent was removed to give (+)-6a quanti-
tatively (210 mg). Oil. [R]D +83 (c 0.70, CHCl3). IR (film): 3354,
(23) Dewar, M. J. S.; Zoebisch, E. G.; Healy, E. F.; Stewart, J. J. P.
J. Am. Chem. Soc. 1985, 107, 3902.
1
1732 cm-1. H NMR (250 MHz, CDCl3): δ 1.97 (m, 2H), 2.31
(24) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Lee, C.; Yang,
W.; Parr, R. G. Phys. Rev. B 1988, 37, 785.
(25) (a) Klamt, A.; Schu¨u¨rmann, G. J. Chem. Soc., Perkin Trans. 2
1993, 799. (b) Barone, V.; Cossi, M. J. Phys. Chem. A 1998, 102, 1995.
(26) Wolinski, K.; Hinton, J. F.; Pulay, P. J. Am. Chem. Soc. 1990,
112, 8251.
(m, 2H), 3.35 (m, 1H), 3.63 (s, 3H), 4.52 (m, 1H), 5.06 (s, 2H),
5.60 (broad s, 1H), 7.32 (m, 5H). 13C NMR (62.5 MHz, CDCl3):
δ 19.1, 28.3, 45.5, 46.3, 52.2, 67.2, 128.6, 129.0, 137.0, 155.8,
173.5. Anal. Calcd for C14H17NO4: C, 63.87; H, 6.51; N, 5.32.
Found: C, 64.04; H, 6.30; N, 5.31.
7970 J. Org. Chem., Vol. 70, No. 20, 2005

(+)- and (-)-2-Aminocyclobutane-1-carboxylic Acids
General Procedure for the Hydrogenation of Com-
pounds (+)- and (-)-7a. The synthesis of (-)-1 is described.
A solution of the N-protected amino acid (-)-7a (100 mg, 0.4
mmol) in dry methanol was hydrogenated over 10% Pd/C (9
mg) at room temperature and at 1 atm for 1.5 h. The catalyst
was removed by filtration through Celite, washed with dichlo-
romethane, and the filtrate was evaporated to afford quanti-
tatively (48 mg) the free amino acid (-)-1.
added, and the mixture was heated for 6 h. The solvent was
removed at reduced pressure, and the residue was purified
by column chromatography.
2-Benzyloxycarbonylamino-(1S,2R)-cyclobutane-1-car-
boxylic Acid N-Methoxycarbonylethyl Amide, (+)-2. This
was prepared according to Method A. 71% yield. Crystals, mp
94-95 °C (EtOAc) [R]D: +72 (c 0.8, MeOH), (lit.7 mp 91-93
°C, [R]D -67 (c 2.9, MeOH) for the enantiomer). 1H and 13C
NMR spectroscopic data are in good agreement with those
described for the enantiomer.7 Anal. Calcd for C17H22N2O5: C,
61.05; H, 6.64; N, 8.38. Found: C, 60.95; H, 6.82; N, 8.59.
Dipeptide (-)-15a. Method B. 529 mg, 70% yield. Crystals,
mp 79-80 °C (EtOAc-pentane). [R]D -167 (c 1.06, MeOH).
(1R,2S)-2-Aminocyclobutane-1-carboxylic Acid, (-)-1.
Crystals, mp 130 °C (dec) (from MeOH-H2O), [R]D: -83 (c 0.70,
H2O), (lit.12 mp 130 °C (dec), [R]D -80 (c 1.00, H2O). IR (film):
1
3354, 1732 cm-1. H NMR (250 MHz, D2O): δ 2.05 (m, 2H),
2.26 (m, 2H), 3.21 (m, 1H), 3.93 (m, 1H). 13C NMR (62.5 MHz,
D2O): δ 21.1, 25.0, 41.3, 45.5, 180.8.
IR (solid): 3304, 1714, 1655 cm-1 1H NMR (500 MHz,
.
(1S,2R)-2-Aminocyclobutane-1-carboxylic Acid, (+)-1.
Crystals, mp 131-133 °C (from MeOH-H2O), [R]D +80 (c 0.75,
H2O). Spectroscopic 1H and 13C NMR data are in total
agreement with those described above for the enantiomer.
General Procedures for the Synthesis of Pepetides.
Method A: Peptide Coupling. Hydroxybenzotriazole (HOBt)
(1.0 mmol) and 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide
(DEC) hydrochloride (3.4 mmol) were successively added to a
solution containing the acid (1.2 mmol) and the amine (1.1
mmol) in freshly distilled dry DMF (14 mL). The light-
protected mixture was stirred at room temperature under
argon atmosphere. Total conversion of the reaction was
monitored by TLC. After completion, ethyl acetate was added,
and the solution was washed with five portions of saturated
aqueous NaHCO3. The organic phase was dried over MgSO4,
and the solvent was removed at reduced pressure to afford
the corresponding peptide that was purified by column chro-
matography (3:2 hexanes-EtOAc).
CDCl3): δ 1.94 (m, 2H), 2.05 (m, 2H), 2.16 (m, 1H), 2.31 (m,
3H), 3.17 (m, 1H), 3.39 (m, 1H), 3.70 (s, 3H), 4.48 (q, J ) 8.7
Hz, 1H), 4.74 (q, J ) 8.5 Hz, 1H), 5.03 (d, J ) 12 Hz, 1H),
5.11(d, J ) 12 Hz, 1H), 5.90 (d, J ) 8.7 Hz, 1H), 6.54 (d, J )
8.7 Hz, 1H), 7.35 (m, 5H). 13C NMR (62.5 MHz, CDCl3): δ 18.5,
19.1, 29.2, 29.5, 44.0, 44.4, 46.1, 46.4, 51.7, 66.5, 127.9, 128.0,
128.4, 136.4, 155.6, 172.2, 174.6. Anal. Calcd for C19H24N2O5:
C, 63.30; H, 6.72; N, 7.77. Found: C, 63.41; H, 6.89; N, 7.49.
Acknowledgment. Financial support from MEC
through Project CTQ2004-01067 is gratefully acknowl-
edged.
Supporting Information Available: Full description of
products (+)-14, (-)-14, (+)-15a, 15b, 16, 17, and 18; 1H and
13C NMR spectra of (+)- and (-)-1, 10, 11, (+)- and (-)-14,
(+)-15a; NMR studies for 2c, 6a, 16, and 17; cartesian
coordinates and total energies for all computed structures;
drawings of models for the structural disorder found in the
crystals of dipeptides 16 and 17. This material is available
free of charge via the Internet at http://pubs.acs.org.
Method B: Reaction between a Carboxylic Acid and
an Azide. Carboxylic acids (-)-7a or 7b (5 mmol) were heated
in boiling anhydrous toluene (20 mL) for 40 min in such a way
that the refluxing solvent passed through a molecular-sieve
pad before returning to the flask. Azide 5 (4 mmol) was then
JO0510843
J. Org. Chem, Vol. 70, No. 20, 2005 7971