Stereoselective synthesis of (-)-(1R,2S)-2-aminocyclobutane-1-carboxylic acid, a conformationally constrained β-amino acid

Article abstract of DOI:10.1016/S0957-4166(98)00462-5

The title compound as well as some derivatives have been synthesized for the first time in optically active form by means of a chemoenzymatic transformation used to induce asymmetry in achiral precursors. The enantio- and diastereomeric purity has been determined by HPLC and NMR techniques.

Full text of DOI:10.1016/S0957-4166(98)00462-5

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 9 (1998) 4291–4294
Stereoselective synthesis of (−)-(1R,2S)-2-aminocyclobutane-
1-carboxylic acid, a conformationally constrained β-amino acid
Marta Martín-Vilà,a Cristina Minguillónb and Rosa M. Ortuño a,∗
aDepartament de Química, Universitat Autònoma de Barcelona, 08193 Bellaterra, Barcelona, Spain
bLaboratori de Química Farmacèutica, Facultat de Farmàcia, Universitat de Barcelona, 08028 Barcelona, Spain
Received 30 October 1998; accepted 10 November 1998
Abstract
The title compound as well as some derivatives have been synthesized for the first time in optically active form
by means of a chemoenzymatic transformation used to induce asymmetry in achiral precursors. The enantio- and
diastereomeric purity has been determined by HPLC and NMR techniques. © 1998 Elsevier Science Ltd. All rights
reserved.
β-Amino acids are a widespread class of non-proteinogenic amino acids found in nature in free form
or as part of peptidic products with antibiotic, antifungal, cytotoxic and other pharmacological properties.
Among the non-peptidic products the β-lactams are prominent, including antibiotics and other medicinal
agents. β-Amino acids are also key structural components of important compounds such as the potent
enzyme inhibitors statins and the anticancer agent taxol (Paclitaxel®).1 These amino acids have recently
been the subject of renewed interest since the pioneering works of Seebach2 and Gellman3 showing that
polymers composed of β-amino acids (i.e. β-peptides), can fold into a stable helical secondary structure
analogous to the α-helix in proteins.4
Conformationally constrained α- and β-amino acids have been incorporated into peptides to be used
in structural and biomechanistic investigations as well as to obtain peptides with new or improved
properties.5 With this aim and as a part of our research program on the synthesis and structural study
of carbocyclic amino acids and related peptide surrogates,6 we envisaged the synthesis of cyclobutane β-
amino acids as an example of rigid molecules.7 Although several 2-aminocyclobutane-1-carboxylic acids
are known to be of interest as biologically active compounds or antioxidants,8 they have been synthesized
only in racemic form. In this paper we report the first synthesis of the optically active parent compound
9,9 with cis stereochemistry, both in free form and conveniently protected for its latter incorporation into
β-peptides. The enantio- and diastereomeric purity of the synthesized products has been determined by
NMR and by HPLC using new non-commercial chiral phases.
∗
Corresponding author. E-mail: rosa.ortuno@cc.uab.es
0957-4166/98/$ - see front matter © 1998 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(98)00462-5

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1. Synthesis of (−)-(1R,2S)-2-aminocyclobutane carboxylic acid 9 and related products
The synthesis of amino acid 9 and some derivatives was achieved by using a chemoenzymatic approach
to induce asymmetry in achiral precursors. Selective manipulation of the functional groups allowed the
free amino acid to be obtained, as well as the fully and the partially protected compounds 3, 7 and 8,
respectively, shown in Scheme 1.
Scheme 1.
The meso-diester 5 was obtained from diazomethane-promoted methylation of the commercial diacid
4 or by Fisher esterification of the bicyclic anhydride 1. Compound 1 is the [2+2] adduct of ethylene
and maleic anhydride10 and also provides the racemic hemiester 2 on heating in methanol without
acid catalysis. In turn, optically active (−)-(1R,2S)-2 was obtained through pig liver esterase-induced
chemoselective hydrolysis of 5, in >97% ee and 91% chemical yield following the procedure described
by Jones et al.11 In this work, the ee was determined by NMR.11 The parallel syntheses of both racemic
and (−)-(1R,2S)-3 were accomplished from these two intermediates. Racemic 3 was used in this work as
the reference standard in HPLC ee measurements, and (−)-3 is the key intermediate to prepare compounds
7, 8 and 9.
The attempted one-pot Curtius rearrangement from the acid 2 using diphenylphosphoryl azide was
unsuccessful. Therefore, the fully protected amino acid 3 was prepared stepwise by treatment of 2 with
ethyl chloroformate and triethylamine, followed by reaction with sodium azide. The resultant acyl azide
was decomposed by heating to reflux a toluene solution of 6 and benzylic alcohol, affording compound

M. Martín-Vilà et al. / Tetrahedron: Asymmetry 9 (1998) 4291–4294
4293
3 in 63% yield from 2. The product (−)-(1R,2S)-312 was determined to be 91% ee, by HPLC employing
two different chiral phases (vide infra).13
Saponification of (−)-3 to give the acid 7 was carried out by treatment with potassium carbonate in
methanol:water at room temperature for 4 h. The reaction time was crucial to avoid epimerization and
the use of stronger bases was precluded. Thus, a mixture of the acids 7 (cis:trans ratio of 3:1) resulted
after stirring ester (−)-3 in a methanolic 1 M NaOH solution at room temperature for 30 h (Scheme 2).
The cis configuration of the major stereoisomer was confirmed by NOE experiments, allowing significant
enhancements of the protons at the two stereogenic centers to be observed when, in separate experiments,
each proton was selectively irradiated to presaturation.
Scheme 2.
The amine protection was removed by Pd/C catalyzed hydrogenation of 7 to furnish the free amino
acid 9 as a hygroscopic solid, [α]D −9.0, in 40% overall yield from 4. The diastereomeric homogeneity
of 9 was established by 13C NMR.12 Alternatively, catalytic hydrogenation of (−)-3 yielded the amino
ester 8. Both compounds 7 and 8 are suitable for incorporation into peptides.
2. HPLC determinations
In order to develop an appropriate analytical method to assess the enantiomeric content of 3, several
chiral stationary phases based on immobilized polysaccharide derivatives14 were tested. Heptane:2-
propanol mixtures were used as the mobile phase. The low wavelength of maximal absorption (210
nm) and the low absorptivity of 3 prevented the use of other solvent systems. Refractive index detec-
tion was not possible because of the low response of the sample. Under these conditions,15 a slight
splitting of peaks was observed when using columns based on amylose 3,5-dimethylphenylcarbamate
(α=1.08)16 or chitosan 3,5-dichlorophenylcarbamate (α=1.17). Cellulose 3,5-dimethylphenylcarbamate
and amylose 4-chlorophenylcarbamate were unable to distinguish between the two enantiomers of
3. Resolved peaks were obtained with cellulose 3,5-dichlorophenylcarbamate (α=1.32), chitosan 3,5-
dimethylphenylcarbamate (α=2.17) and amylose 3,5-dichlorophenylcarbamate (α=1.47). However,
completely resolved peaks were only obtained with the latter two (Rs=3.07 and Rs=2.55, respectively)
0
0
when using a heptane:2-propanol (95:5) mixture as the mobile phase (k1 =1.90 and k1 =5.33, respecti-
vely). When (−)-(1R,2S)-3 was chromatographed on both columns, an enantiomeric excess of 91% was
determined.
Acknowledgements
Financial support from DGICYT (Project PB94-0694) and from DGES (Projects PB 96-0382 and
PB97-0214) is gratefully acknowledged. M. M.-V. thanks the MEC for a predoctoral fellowship. The
authors are indebted to Prof. Ernest Giralt, Universitat de Barcelona, for his interesting comments at the
beginning of this work.

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References
1. Boge, T. C.; Georg, G. I.; Tamariz, J. In Enantioselective Synthesis of β-Amino Acids; Juaristi, E., Ed.; Wiley-VCH, Inc.:
New York, 1997; pp. 1 and 45.
2. Seebach, D.; Overhand, M.; Kühnle, F. N. M.; Martinoni, B.; Oberer, L.; Hommel, U.; Widmer, H. Helv. Chim. Acta 1996,
79, 913.
3. Appella, D.; Christianson, L. A.; Karle, I. L.; Powell, D. R.; Gellman, S. H. J. Am. Chem. Soc. 1996, 118, 13071.
4. Iverson, B. L. Nature 1997, 385, 113.
5. For a recent example on a biologically active peptide including highly substituted β-amino acid residues to restrict the
conformational flexibility, see: Hayashi, Y.; Katade, J.; Harada, T.; Tachiki, A.; Iijima, K.; Takiguchi, Y.; Muramatsu, M.;
Miyazaki, H.; Asari, T.; Okazaki, T.; Dato, Y.; Yasuda, E.; Yano, M.; Uno, I.; Ojima, I. J. Med. Chem. 1998, 41, 2345.
6. (a) Jiménez, J. M.; Rifé, J.; Ortuño, R. M. Tetrahedron: Asymmetry 1996, 7, 537. (b) Díaz, M.; Ortuño, R. M. Ibid, 1996,
7, 3465. (c) Díaz, M.; Jiménez, J. M.; Ortuño, R. M. Ibid, 1997, 8, 2465. (d) Martín-Vilà, M.; Hanafi, N.; Jiménez, J. M.;
Alvarez-Larena, A.; Piniella, J. F.; Branchadell, V.; Oliva, A.; Ortuño, R. M. J. Org. Chem. 1998, 63, 3581.
7. For the synthesis of optically active cyclobutane dehydro α-amino acids from (+)-α-pinene, see: Moglioni, A. G.; García-
Expósito, E.; Moltrasio, G. Y.; Ortuño, R. M. Tetrahedron Lett. 1998, 39, 3593.
8. Avotins, F. Russ. Chem. Rev. 1993, 62, 897.
9. Racemic cis-9 had previously been synthesized: Kennewell, P. D.; Mathrau, S. S.; Taylor, J. B.; Westwood, R.; Sammes,
P. G. J. Chem. Soc., Perkin Trans. 1, 1982, 2563.
10. Owsley, D.; Bloomfield, J. J. J. Org. Chem. 1971, 36, 3768.
11. Sabbioni, G.; Jones, J. B. J. Org. Chem. 1987, 52, 4565.
12. All new products were fully characterized. Some selected data follow. Compound (−)-2, [α]D −3.6 (c 2.36, CHCl3) (Lit11
[α]D −3.0 (c 2.11, CHCl3)). Compound (−)-3, [α]D −83.2 (c 2.05, CHCl3). Compound 7, [α]D −96.0 (c 1.06, methanol).
Compound 8, [α]D −27.4 (c 3.57, methanol). Amino acid 9, [α]D −9.0 (c 1.55, H2O); 62.5 MHz 13C NMR (D2O) δ 25.1,
28.0, 38.9, 41.8, 184.8 (only one set of signals was observed).
13. Isomerization during the Curtius rearrangement has been reported to occur in related systems: (a) Csuk, R.; von Scholz,
Y. Tetrahedron, 1994, 50, 10431; (b) Ibid 1995, 51, 7193.
14. (a) Franco, P.; Minguillón, C.; Oliveros, L. J. Chromatogr. A, 1998, 793, 239. (b) Franco, P.; Senso, A.; Minguillón, C.;
Oliveros, L. J. Chromatogr. A, 1998, 796, 265.
15. Columns: 150×4.6 mm, flow rate: 1 ml/min, UV detection: 210 nm.
16. Selectivity factor, α=t2−t0/t1−t0; where ti is the retention time for each enantiomer and t0 the dead time of the column.
Resolution factor, Rs, calculated as Rs=2 (t2−t1)/w2+w1; where wi is the peak width at the 0baseline. An Rs value over 1.5
implies the complete resolution of peaks. Capacity factor for the first eluted enantiomer, k1 =t1−t0/t0