Functionalized platforms based on marine cyclopeptides: Different pathways to the hexapeptide

Article abstract of DOI:10.1016/S0040-4039(00)01057-1

The synthesis of macrocyclic, roughly planar, exclusively syn-functionalized hexapeptides, related to dolastatin I and bistratamide C from enantiomerically pure oxazole precursors is reported. The platforms can either be synthesized in a stepwise procedure by final cyclization of the linear oxazole trimers or in a direct 'one-pot' reaction. The investigation on the coupling of these building blocks into linear dimers and trimers with modern peptide coupling reagents is reported. (C) 2000 Published by Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)01057-1

Tetrahedron Letters 41 (2000) 6327±6331
Functionalized platforms based on marine cyclopeptides:
di€erent pathways to the hexapeptide
Christoph Boss,*,y Palle H. Rasmussen, Alexander R. Wartini and
Siegfried R. Waldvogel*,{
The Skaggs Institute for Chemical Biology, The Scripps Research Institute, 10550 N. Torrey Pines Rd.,
MB 26, La Jolla, CA 92037, USA
Received 9 August 1999; revised 19 June 2000; accepted 20 June 2000
Abstract
The synthesis of macrocyclic, roughly planar, exclusively syn-functionalized hexapeptides, related to
dolastatin I and bistratamide C from enantiomerically pure oxazole precursors is reported. The platforms
can either be synthesized in a stepwise procedure by ®nal cyclization of the linear oxazole trimers or in a
direct `one-pot' reaction. The investigation on the coupling of these building blocks into linear dimers and
trimers with modern peptide coupling reagents is reported. # 2000 Published by Elsevier Science Ltd.
Keywords: coupling reagents; cyclisation; peptides; oxazoles; thiazoles.
Introduction. In recent years numerous biologically active cyclopeptides1 incorporating oxazole
and thiazole units2 have been identi®ed as metabolic products of algae, fungi and primitive
marine organisms and their structures have been elucidated. Compounds like dolastatin I (1)3 or
bistratamide C (2)4 (Scheme 1) containing three substituted ®ve-membered heterocycles often
exhibit biological activities, such as cytotoxicity.2b
In these compounds the three heterocycles are linked by trans amide bonds. When incorporating
only aromatic heterocycles, the cyclotrimers are roughly planar due to conjugation of the amide
bonds with the aromatic rings.5 The orientation of the three substituents depends on the absolute
con®guration of the a-carbon atoms. All syn-substituted structural analogues with functionalized
side chains are rare in the large family of marine cyclopeptides.2b,6 They may not only exhibit
di€erent biological activity than their unfunctionalized counterparts, but are also of potential
interest as template molecules in supramolecular chemistry, because of the limited conformational
y*Corresponding authors.
Present address: Actelion Ltd., Gewerbestrasse 16, CH-4123 Allschwil, Switzerland.
Present address: Organisch-Chemisches Institut, Westfalische Wilhelms-Universitat Munster, Corrensstrasse 40,
D-48149 Munster, Germany.
{
0040-4039/00/$ - see front matter # 2000 Published by Elsevier Science Ltd.
PII: S0040-4039(00)01057-1

6328
Scheme 1. Examples of molecules with a rigid platform-shaped skeleton
¯exibility within the macrocyclic backbone. A large variety of molecules have been designed that
incorporate these features; notable representatives are calixarenes, resorcinarenes and
tetraarylporphyrines.7a c In this context, a general synthetic route to 3 tolerating a variety of
functionalized side chains R1 to R3 was desirable. Furthermore, the projected synthesis should
allow the introduction of three di€erent side chains in order to provide addressability.
Based on previous pathways used for the total synthesis of natural products4,8 we investigated
the synthesis of linear precursors (Scheme 2) containing di€erently functionalized side chains
allowing for the synthesis of `addressable' platforms.
Scheme 2. Coupling of the building blocks to open-chain dimers and trimers
Results and discussion. The retrosynthetic disconnection of the three amide bonds of 3 leads to
three heterocyclic building blocks each containing an amino- and an acid-terminus. The building
blocks were synthesized according to standard procedures.5 The coupling of the mono deprotected
subunits to linear dimers and trimers was extensively studied as outlined in Tables 1 and 2. It was
®rst attempted with i-butylchloroformate and NMM.4 The reaction led to a large amount of side-
products. The desired dimers were obtained in yields of 18±26% almost independent of the dif-
ferent side chains R1 to R2. Thus, in order to improve the coupling procedure, several modern
peptide coupling reagents were investigated and a series of homo- and heterodimers was
generated. The best results were obtained with PyBOP, followed by BOP and HBTU.9 The
N-termini of the dimers 6a,c,e,f were deprotected in almost quantitative yields with TFA followed

6329
by the coupling with an acid monomer activated by PyBOP to give the `open-chain' trimers
7a,c,e,f in reasonable to good yields (Table 2).
Table 1
Coupling experiments to the dimers 6a±f
Table 2
Coupling experiments to linear trimers 7
Deprotection of the acid terminus of the linear trimers 7a±c by saponi®cation of the methyl
ester followed by cleavage of the Boc-group with TFA or HCl gave the cyclization precursors
8a±c in 71 to 75%. The macrocyclization was tried under various conditions. High dilution and
activation by PyBOP, BOP or HBTU gave only low yields of the cyclopeptides 9a±c (12±15%).
However, there are several methods available for peptide cyclizations.10 In many examples the
ring closure via the penta¯uorophenyl ester method11 shows exceptional results. The methyl ester
7a was transformed into a penta¯uorophenyl ester,12,13 the Boc-group was removed by HCl in
dioxane, and the cyclization was subsequently performed in a two-phase chloroform-aqueous
base system. This procedure gave 9a in 58% yield. Removal of the protective groups on the side
chains was achieved under standard conditions.
For the synthesis of platforms with three identically functionalized substituents a `one-pot'-
procedure seems to be suitable and has been investigated. Starting from the appropriate
deprotected oxazole building blocks (Scheme 3) 10a±b, the cyclic trimers 9a±b could be synthe-
sized under high dilution conditions by using DPPA as the coupling agent in moderate yields.14
The overall yields of both pathways are in the same range (about 10±20%) starting from the

6330
monomeric building block. Therefore, the `one-pot'-procedure, with respect to yields, has no
advantage over the stepwise synthesis, but gives much faster access to molecular platforms such
as 9a±b, bearing three identical side-chain substituents. The cyclization of the open-chain
fragments 8a±c with the penta¯uorophenyl ester method highly improves the yields of cyclic
products 9a±c compared to the formerly used cyclization protocol.15
Scheme 3. Macrocyclization to the cyclopeptides 9a±c
Acknowledgements
The authors thank the Skaggs Research Foundation for ®nancial support and Julius Rebek, Jr.
for scienti®c advice. C.B. thanks the Swiss National Science Foundation for a Postdoctoral
Fellowship. S.R.W. thanks the DAAD for a NATO fellowship. P.H.R. thanks the Danish
Research Academy for supporting his research program.
References
1. (a) Pettit, G. R. Pure and Appl. Chem. 1994, 66, 2271; (b) Garson, M. J. Chem. Rev. 1993, 93, 1699; (c) Davidson,
B. S. Chem. Rev. 1993, 93, 1771.
2. (a) Juttner, F.; Linden, A.; Pluss, T.; v. Philipsborn, W. J. Org. Chem. 1995, 60, 7891; (b) Ogino, J.; Moore, R. E.;
Patterson, G. M. L.; Smith, C. D. J. Nat. Prod. 1996, 39, 581.
3. (a) Sone, H.; Kighoshi, H.; Yamanda, K. Tetrahedron 1997, 53, 8149; (b) Wipf, P. Chem. Rev. 1995, 95, 2115.
4. (a) Aguilar, E.; Myers, A. I. Tetrahedron Lett. 1994, 35, 2473 and 2477; (b) Downing, S. V.; Aguilar, E.; Meyers,
A. I. J. Org. Chem. 1999, 64, 826.
5. Mink, D.; Mecozzi, S.; Rebek Jr., J. Tetrahedron Lett. 1998, 39, 5709, and references cited therein.
6. Holzapfel, C. W.; van Zyl, W. J. Tetrahedron 1990, 46, 649.
7. (a) Bohmer, V. Angew. Chem. Int. Ed. Engl. 1995, 34, 713; (b) Timmerman, P.; Verboom, W.; Reinhoudt, D. N.
Tetrahedron 1996, 52, 2663; (c) Bonar-Law, R. P.; Sanders, J. K. M. J. Chem. Soc., Perkin Trans. 1 1995, 3085.
8. (a) Moody, C. J.; Bagley, M. C. J. Chem. Soc., Perkin Trans. 1 1998, 601; (b) Wipf, P.; Miller, C. P. J. Am. Chem.
Soc. 1992, 114, 10975.
9. Procedure for the PyBOP coupling: The amine building block (1 equiv.), the acid building block (1 equiv.) and
PyBOP (1equiv.) were dissolved in anhydrous DMF (15ml/1mmol PyBOP) at rt, followed by slow addition of
Hunigs Base (2.3 equiv.). Stirring was continued for 12 h. The reaction mixture was diluted with EtOAc, washed
with 10% HCl, sat. NaHCO3, brine and dried over MgSO4. The solvent was evaporated and the products isolated
by chromatography on silica with EtOAc:hexanes=2:1. Procedure for the BOP coupling: The amine building

6331
block (1 equiv.), the acid building block (1 equiv.) and BOP (1 equiv.) were dissolved in anhydrous CH3CN or
DMF (10 ml/1 mmol BOP) at rt, followed by slow addition of Hunigs Base (2.5 equiv.). Stirring was continued for
12 h. The reaction mixture was diluted with EtOAc and the organic layer was washed with 3 M HCl, sat. NaHCO3
and brine, dried over MgSO4 and the solvent was evaporated. The products were isolated by chromatography on
1
silica with EtOAc:hexanes=2:1. All compounds were characterized by H and 13C NMR and MS/HR-MS.
10. Menz, Q.; Hesse, M. Top. Curr. Chem. 1991, 161, 109.
11. Schmidt, U.; Utz, R.; Liebknecht, A.; Griesser, H.; Potzolli, B.; Bahr, J.; Wagner, K.; Fischer, P. Synthesis 1987,
236.
12. Bagley, M. C.; Bashford, K. E.; Hesketh, C. L.; Moody, C. J. J. Am. Chem. Soc. 2000, 122, 3301.
13. General procedure for the macrocyclization (penta¯uorophenyl ester method): EDCÂHCl (1.2 equiv.) was added to
a stirred solution of compounds 7a±c (1 equiv.) in CH2Cl2, followed by the addition of penta¯uorophenol (1.3
equiv.) at ^20ꢀC. The mixture was slowly warmed to rt and stirred for 16 h, evaporated, dissolved in EtOAc,
washed with brine, dried over Na2SO4 and evaporated. The crude penta¯uorophenylester was dissolved in dioxane
and HCl in dioxane was added at 0ꢀC. Stirring was continued for 30 min at 0ꢀC then 2 h at rt followed by
evaporation of the solvent. The crude product was dissolved in CHCl3 followed by the addition of a 1 M solution
of NaHCO3. The mixture was vigorously shaken for 10 min, followed by separation of the layers. The organic
layer was dried and evaporated. The products were isolated by ¯ash chromatography (silica gel, EtOAc).
14. General procedure for the `one±pot' macrocyclization: To a 40 mM solution of 10a±b in DMF, DPPA (2 equiv.) was
slowly added at 0ꢀC, followed by slow addition of Hunigs Base. The mixture was stirred at 0ꢀC for 10 h then at rt
for 48 h. The solvent was evaporated and the cyclic products were isolated by chromatography (silica gel,
1
EtOAc:hexanes=3:1). All compounds were characterized by H and 13C NMR and MS/HR-MS.
15. Selected analytical data: compound 9a: 1H NMR (600 MHz, acetone-d6) ꢀ (ppm) 1.15±1.25(m, 6H), 1.45±1.60 (m,
3
6H), 1.90±2.05 (m, 3H), 2.05±2.20 (m, 3H), 3.05±3.20 (m, 6H), 5.02 (s, 6H), 5.21 (q, J=5.9 Hz, 3H), 6.34 (t,
3
3J=5.3 Hz, 3H), 7.20±7.35 (m, 15H), 8.42 (d, J=6.6 Hz, 3H), 8.48 (s, 3H); 13C NMR (150 MHz, acetone-d6) ꢀ
(ppm) 22.65, 30.30, 35.00, 41.29, 49.07, 66.46, 128.72, 128.84, 129.40, 136.46, 138.72, 143.18, 157.45, 160.11,
165.24; FAB-MS: [M+Cs]+ expected: 1120.3181; observed: 1120.3129. Compound 9b: After cleavage of the Cbz-
moiety in the side-chain amino functionalities by transfer-hydrogenation with Pd-black and 1,4-cyclohexadiene
the corresponding tris-amine resulted (66%) which was identical in all means of spectroscopic data with the
compound described in Ref. 5.