N-Fmoc-dehydroalanine: A versatile molecular scaffold for the rapid solid-phase synthesis of cycloaliphatic amino acids

Article abstract of DOI:10.1016/S0040-4039(00)01113-8

The synthesis of polymer-supported N-Fmoc-dehydroalanine starting from S-protected cysteine via an oxidation/elimination strategy is described. Cycloaddition with a range of dienes afforded a range of conformationally constrained amino acids in moderate yields. The potential applications of this methodology to combinatorial libraries is discussed. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)01113-8

Tetrahedron Letters 41 (2000) 6661±6664
N-Fmoc-dehydroalanine: a versatile molecular sca€old for the
rapid solid-phase synthesis of cycloaliphatic amino acids
Brendan A. Burketta and Christina L. L. Chaib,*
aResearch School of Chemistry, Australian National University, Canberra, ACT 0200, Australia
bDepartment of Chemistry, Australian National University, Canberra, ACT 0200, Australia
Received 9 June 2000; accepted 29 June 2000
Abstract
The synthesis of polymer-supported N-Fmoc-dehydroalanine starting from S-protected cysteine via an
oxidation/elimination strategy is described. Cycloaddition with a range of dienes a€orded a range of con-
formationally constrained amino acids in moderate yields. The potential applications of this methodology
to combinatorial libraries is discussed. # 2000 Elsevier Science Ltd. All rights reserved.
Keywords: solid-phase synthesis; dehydroalanine; Diels±Alder reactions.
Conformationally constrained a-amino acid derivatives such as carbocyclic a-amino acids have
applications in biological chemistry as enzyme inhibitors and peptidomimetics.1 3 Historically,
these amino acids are synthesised via the Strecker or Bucherer±Lieb synthesis,4,5 but recent strategies
employ cycloaddition reactions of dehydroalanine derivatives.6 9 In view of the potential therapeutic
uses of conformationally constrained amino acids and mindful of the possible combinatorial
applications, we recently reported the solid-phase synthesis of the bicyclic norbornene a-amino
acids from polymer-supported dehydroalanine derivatives.10 Whilst useful for the synthesis of
cycloaliphatic amino acids and peptides, the reported methodology lacked versatility in that the
N-protecting groups utilised were not orthogonal to the acid-labile polymer support. In this
report, the synthesis of polymer-supported N-Fmoc-dehydroalanine is described and applications
to the rapid synthesis of carbocyclic amino acids are outlined.
Although synthetic routes to polymer-bound dehydroalanine derivatives have been reported,
these methods are generally not applicable to the synthesis of polymer-bound N-Fmoc-
dehydroalanine.10 12 Thus, in order to circumvent this problem the procedure as outlined in
Scheme 1 was implemented. The key step in this synthesis employs thermal elimination of the
b-sulfoxide derived from N-Fmoc-S-benzylcysteine (1) for the installation of the a,b-unsaturated
system.13
* Corresponding author. Fax: +61-2-6249-0760; e-mail: christina.chai@anu.edu.au
0040-4039/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)01113-8

6662
Scheme 1. (i) DCC (1.6 equiv.), DMAP (1.5 equiv.), 4:1 dichloromethane:DMF, 5.5 h. (ii) MCPBA (1.05 equiv.),
dichloromethane, 0ꢁC, 3 h. (iii) toluene, 110ꢁC, 16 h
Coupling of N-Fmoc-S-benzylcysteine to Wang resin (1.22 mmol/g) under standard coupling
procedures (DCC) gave the polymer-bound amino acid 2 in 76% yield as determined by photo-
metric analysis.14 Subsequent treatment of polymer-bound N-Fmoc-S-benzylcysteine (2) with
careful stoichiometric amounts of MCPBA a€orded the sulfoxide 3.
As the characteristic IR sulfoxide SˆO vibration (typically ꢀ1030 cm^1) was partially obscured
1
by polystyrene backbone vibrations, the oxidation reaction was monitored by H NMR analysis
on a small portion of cleaved product. Conversion to the polymer-supported N-Fmoc-dehydro-
alanine 4 was readily achieved by heating the polymer-bound sulfoxide 3 at 100ꢁC for 16 h. The
polymer-supported dehydroalanine derivative 4 was then reacted with dienes 5a±d (Scheme 2) to
a€ord cycloadducts 6a±d following cleavage with TFA, as summarised in Table 1.
Scheme 2. (i) Diene 5a±d (10 equiv.), toluene, 110ꢁC, sealed tube, 16±72 h. (ii) 20% TFA/dichloromethane, 30 min
With the exception of furan, cycloadditions with the dienes (5a,b,d) proceeded to give moderate
to good isolated yields of the desired cycloadducts.y Deprotection and hydrogenation of 6a con-
®rmed the preferential formation of the exo cycloadduct by comparison with literature 1H NMR
data.7 The Diels±Alder reaction of polymer-supported N-Fmoc-dehydroalanine with furan was
sluggish and, during the extended period of heating, degradation of the polymer support ensued.
For ease of characterisation, the N-Fmoc group of the cycloadduct resulting from the Diels±Alder
reaction of 4 with diene 5d, was removed prior to cleavage with TFA, which in turn resulted
y
1
All new compounds gave satisfactory spectroscopic and analytical data. Representative H NMR data for 6a (300
MHz, CD3OD): (exo) ꢀ 1.43 (dd, 2J 12.5 Hz, 3J 3 Hz, 1H, cycloaliphatic CHaHb), 1.56 (dm, 2J 9 Hz, 1H, bridge
CHaHb), 1.93 (dm, 2J 9 Hz, 1H, bridge CHaHb), 2.52 (dd, 2J 12.5 Hz, 3J 3 Hz, 1H, cycloaliphatic CHaHb), 2.96 (br
m, 1H, CH), 3.43 (br m, 1H, CH), 4.20±4.50 (m, 3H, ¯uorenyl CH2 and CH), 6.11 (m, 1H, ole®nic CH), 6.42 (m, 1H,
ole®nic CH), 7.30±8.00 (8H, aromatics); (endo) ꢀ 1.30±2.30 (multiple signals, 4H, cycloaliphatic and bridge CH2), 2.80±
3.20 (2Âbr m, 2H, 2ÂCH), 4.20±4.50 (m, 3H, ¯uorenyl CH2 and CH), 5.89 (m, 1H, ole®nic CH), 6.40 (m, 1H, ole®nic
CH), 7.30±8.00 (8H, aromatics).

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Table 1
Isolated yields of cycloadducts following Diels±Alder reaction of 4 with dienes 5a±d
in the concomitant removal of the TMS protecting group. Two cycloadducts in the ratio of 3:1
1
were observed in the H NMR spectrum of the crude cleavage mixture. The major product was
1
identi®ed as the 1,2- or ortho-adduct as determined by 2D H±1H NMR spectroscopy. The
cleaved material was then hydrogenated to a€ord the corresponding cyclohexane derivatives and
comparison of the 1H NMR data with those in the literature15 identi®ed the major product from
the cycloaddition as the exo-cycloadduct.
In order to demonstrate the potential of utilising N-Fmoc-dehydroalanine derivatives for the
synthesis of novel amino acids and peptides of therapeutic interest, the N-Fmoc group of the
polymer-supported cycloadduct 7 was deprotected and then functionalised under a variety of
conditions. Thus, elaboration of the polymer-bound cycloadduct 8 into the dipeptide 9 was
achieved via standard peptide coupling conditions followed by cleavage from the Wang resin
(Scheme 3a). Alternatively, the polymer-bound cycloadduct 8 may be readily acylated, which
upon cleavage a€ords N-acyl cycloaliphatic amino acids such as 10 (Scheme 3b). N-Alkylation of the
polymer-bound cycloadducts is also possible via well established reductive alkylation protocols.16 18
Thus, when the polymer-bound cycloadduct 8 was treated with benzaldehyde under standard
conditions, the N-benzyl cycloaliphatic amino acid 11 was isolated in 63% yield following cleavage
from the resin (Scheme 3c). In view of the extensive range of commercially available aldehydes,
reductive alkylation reactions are particularly well suited to combinatorial methodologies, lead-
ing to the synthesis of diverse libraries of potentially biologically active compounds. In addition,
polymer-supported N-alkylated cycloaliphatic amino acids may be further functionalised by
simple peptide coupling techniques to a€ord the corresponding peptoids.
These studies nicely illustrate the utility of polymer-supported N-Fmoc-dehydroalanine in the solid
phase synthesis of carbocyclic amino acids. The use of the base-labile N-Fmoc functionality allows
further manipulation of the N-terminus of the amino acid, enabling a number of N-functionalised

6664
Scheme 3. (i) 1:1 Piperidine:DMF, 10 min. (ii) N-Fmoc-glycine (3 equiv.), HBTU (3 equiv.), DIPEA (3 equiv.), DMF,
1 h. (iii) 20% TFA/dichloromethane. (iv) Ac2O (6 equiv.), DMAP (1 equiv.), DMF, 3 h. (v) PhCHO (1 equiv.),
NaCNBH3 (0.95 equiv.), 90:5:5 THF:H2O:AcOH, 3 h. (Note: only the exo-cycloadduct is shown in each case. In each
case the functionalised cycloadducts were obtained as a 4:1 mixture of exo:endo isomers)
products to be accessed from a single precursor. In addition, a variety of carbocyclic frameworks
are accessible via Diels±Alder methodologies, enabling large libraries of potentially biologically
active amino acids to be generated from a readily accessible, inexpensive molecular sca€old.
Acknowledgements
We gratefully acknowledge the Australian Research Council for ®nancial support, and Paci®c
Dunlop for the award of a research scholarship to B.A.B.
References
1. Gibson (nee Thomas), S. E.; Guillo, N.; Tozer, M. J. Tetrahedron 1999, 55, 585±615.
2. Gante, J. Angew. Chem., Int. Ed. Engl. 1994, 33, 1699±1720.
3. Cativiela, C.; Dõaz-de-Villegas, M. D. Tetrahedron: Asymmetry 1998, 9, 3717±3599.
4. Munday, L. J. Chem. Soc. 1961, 4372±4378.
5. Connors, T. A.; Ross, W. C. J. J. Chem. Soc. 1960, 2119±2132.
6. Horikawa, H.; Nishitani, T.; Iwasaki, T.; Mushika, Y.; Inoue, I. Tetrahedron Lett. 1980, 21, 4101±4104.
7. Cativiela, C.; Lopez, P.; Mayoral, J. A. Tetrahedron: Asymmetry 1990, 1, 379±388.
8. Yamazaki, H.; Horikawa, H.; Nishitani, T.; Iwasaki, T. Tetrahedron 1996, 47, 541±548.
9. Burkett, B. A.; Chai, C. L. L.; Hockless, D. C. R. Aust. J. Chem. 1998, 51, 993±997.
10. Burkett, B. A.; Chai, C. L. L. Tetrahedron Lett. 1999, 40, 7035±7038.
11. Gosselin, F.; Di Renzo, M.; Ellis, T. H. J. Org. Chem. 61, 7980±7981.
12. Blettner, C.; Bradley, M. Tetrahedron Lett. 1994, 35, 467±470.
13. Burrage, S. A.; Raynham, T.; Bradley, M. Tetrahedron Lett. 1998, 39, 2831±2834.
14. Meienhofer, J.; Waki, M.; Heimer, E. P.; Lambros, T. J.; Makofske, R. C.; Chang, C.-D. Int. J. Pept. Prot. Res.
1979, 13, 35±42.
15. Fondekar, K. P.; Volk, F.-J.; Frahm, A. W. Tetrahedron: Asymmetry 1999, 10, 727±735.
16. Brown, E. G.; Nuss, J. M. Tetrahedron Lett. 1997, 38, 8457±8460.
17. Gordon, D. W.; Steele, J. Bioorg. Med. Chem. Lett. 1995, 5, 47±50.
18. Ede, N. J.; Ang, K. H.; James, I. W.; Bray, A. M. Tetrahedron Lett. 1996, 37, 9097±9100.