Racemic, yet diastereomerically pure azido acids as both γ-turn and inverse γ-turn mimetics for solid-phase peptide synthesis

Article abstract of DOI:10.1055/s-1998-3143

The chemoselective arylation of cyclohexane-1,3,5-tricarboxylate derivatives gave access to ketodiesters. Corey's variation of Mukayama's 2-mercaptopyridine activation was superior to acid chlorides and Weinreb amides. The obtained meso-diesters were desymmetrized by L-Selectride and further elaborated to azido acids. These azido acids (e.g. 7) are novel γ-turn templates for solid-phase synthesis of peptides such as 8a and 8b.

Full text of DOI:10.1055/s-1998-3143

1240
LETTERS
SYNLETT
Racemic, Yet Diastereomerically Pure Azido Acids as Both γ-Turn and Inverse γ-Turn
Mimetics for Solid-Phase Peptide Synthesis
Boris Schmidt,* Christian Kühn
Department of Organic Chemistry, University of Hannover, Schneiderberg 1B, D-30167 Hannover, Germany
Fax +49-511-762 3011, e-mail: boris.schmidt@mboc.oci.uni-hannover.de
Received 21 August 1998
Abstract: The chemoselective arylation of cyclohexane-1,3,5-tricar-
boxylate derivatives gave access to ketodiesters. Corey’s variation of
Mukayama’s 2-mercaptopyridine activation was superior to acid
chlorides and Weinreb amides. The obtained meso-diesters were
desymmetrized by L-Selectride™ and further elaborated to azido acids.
These azido acids (e.g. 7) are novel γ-turn templates for solid-phase
synthesis of peptides such as 8a and 8b.
We expected cuprates to be more sensitive towards the activation, but
again the yields were below 37 % for 4d. The Weinreb amides18,19
usually provide a solution to such selectivity problems, yet for the
polyfunctional 3 the overall yield from the acid to the ketone did not
exceed 50 %. We therefore turned to the recently reported activation of
acid chlorides by trialkyl phosphines20 (Scheme 1, method C; Table 1,
entry 6), but it proved to be inferior to a Friedel-Crafts acylation
(method B), yet the latter reaction is limited to few substrates such as
anisol (Table 1, entry 2, 4, 7).
Turns are an important element in protein secondary structure and are
often located at the protein surface, where they display characteristic
amino acid residues and stimulate immunoresponses. In most cases β-
turns are observed which involve 4 amino acids. The less common, and
less investigated γ-turns are defined as the reversal of the protein or
peptide chain by just 3 amino acids, if the first carbonyl interacts via a
hydrogen bond with the NH of the third amino acid. A γ-turn (Φ 70° to
85°, Ψ -60° to -70°) or inverse γ-turn (Φ -70 to -85°, Ψ 60° to 70°) are
differentiated by the dihedral angles of the second amino acid.9 Several
γ-turn mimetics1-8 have been developed for incorporation into peptides.
A
R
1. 2, TEA, CH2Cl2
2. ArMgX 1 eq,
THF, -30°C
O
CO2H
B
1. SOCl2
2. Ar-H, AlCl3
MeO2C
CO2Me
MeO2C
CO2Me
C
1. SOCl2
2. PBu3
3. ArMgX
1
4a-d
Φ
R2
Ψ
Scheme 1
H
O
N
R1
N
R3
O
H
Figure 1 formal γ-turn: Φ 70° to 85°, Ψ -60° to -70°
The γ-turn concept10 for the receptor bound conformation of angio-
tensin II stimulated the synthesis of a number of novel turn5,11
mimetics.The observed structure-affinity relationship and the individual
binding constants at the AT1 receptor indicate an open turn motif12,13 in
the binding-stimulation sequence at the receptor. We therefore turned to
the design of rigid peptide mimetics, which differ from their natural
counterparts by reduced conformational freedom,14-16 yet do not adopt
the precise γ-turn conformation. Such turns may be replaced by an
amino acid with a 1,3-syn-substituted cyclohexane framework. To
reduce unnecessary protection like N-Fmoc, these amino acids can be
introduced by solid-phase peptide synthesis17 (SPPS) as azido acids,
e.g. 7. The enantiomers of 7 have access to a similar conformational
space of the inverse γ-turn because of the free rotation of the
aminomethyl and the carboxyl groups. Therefore neither an
enantioselective synthesis, nor a resolution of the racemate are required.
This is due to the regiocontrol of the substitution and the instability of
the acid labile phenol ethers required for the final deprotection in SPPS.
Thus we decided to investigate the chemoselective reaction of the
2-mercaptopyridine21 thioester 3. The conversion of the acid 1 to the
thioester by 2-mercaptopyridyl chloroformiate22 was followed by a
reaction with 1 equivalent of the Grignard-reagent (method A). These
thioesters lack the stabilisation of the tetrahedral intermediate, which
could control the nucleophilic addition as in the Weinreb amides, yet the
enhanced activity leads to a rapid mono addition (entry 1, 3, 7).
The precursor for these derivatives is accessible by an improved
synthesis11 from the cyclohexylcarboxylic acid 1. The required key
substrate 4a was obtained from 1 by 3 different methods depending on
the nature of the substituents. Initially we planned a reaction of an
intermediate acid chloride with zinc or magnesium aryl reagents. How-
ever, the selectivity was poor for all investigated solvent and
temperature combinations. Therefore the isolated yield in 4 did not
exceed 30 % and was accompanied by 15-50 % yields of the tertiary
alcohols.
Under these conditions (60 min, -30 °C) no contamination by 3°
alcohols was observed, which resulted in 65-67 % yields for both steps.
The Corey procedure gives 4-6 % lower yields in comparison to
Mukayama’s protocol, but provided a simpler purification by acid/base
separation to remove the mercaptopyridine, whereas the Mukayama
method required chromatography to remove triphenylphosphine oxide.
The crude product was then purified by repeated crystallisation.

November 1998
SYNLETT
1241
OH
N
S
S
N
8a
2
CO2H
O
O
Cl
Asp-Arg-NH
NH-His-Pro-Phe
TEA
CH2Cl2
0°C
MeO2C
MeO2C
O
CO2Me
CO2Me
1
3
74%
OH
OPMB
OBn
1.) H2 4×105 Pa,
Pd/C, HOAc, 50°C
2.) PMB-Cl
8b
TBAI, Na2CO3
acetone, 55°C
4-BnOC6H4MgBr
Asp-Arg-NH
NH-His-Pro-Phe
O
4a
THF , - 30°C
65%
87 %
O
5
Scheme 3
MeO2C
MeO2C
CO2Me
CO2Me
Acknowledgement. Financial support from the DAAD/SI and the
OPMB
OPMB
Deutsche Forschungsgemeinschaft is gratefully acknowledged.
1.) DEAD, HN3
PPh3, THF
2.) LiOH
dioxane/H2O
72 %
References and Notes
LiBH(secBu)3
(1) Brickmann, K., Somfai, P., Kihlberg, J. Tetrahedron Lett. 1997,
38, 3651.
THF, -78°C
80 %
(2) Burgess, K., Ho, K.-H., Pettitt, B. M. J. Am. Chem. Soc. 1995,
117, 54.
6
7
MeO2C
HO2C
OH
N3
(3) Callahan, J. F., Bates, J. W., Burgess, J. L., Eggleston, D. S.,
Hwang, S. M., Kopple, K. D., Koster, P. F., Nichols, A., Peishoff,
C. E., Samanen, J. M., Vasko, J. A., Wong, A., Huffman, W. F. J.
Med. Chem. 1992, 35, 3970.
Scheme 2
The arylketones were deoxygenated by a catalytic hydrogenation over
10 % Pd/C in glacial acetic acid, which was found to be superior to
other catalysts or reagents (e.g. Et3SiH, TFA). However, the reaction
required both elevated temperature (50 °C) and higher pressure (4.0 ×
105 Pa) to proceed. Yet the simultaneous loss of the benzyl protection
allowed introduction of the more acid labile para-methoxybenzyl group
required for a smooth final deprotection in SPPS.
(4) Geyer, A., Müller, G., Kessler, H. J. Am. Chem. Soc. 1994, 116,
7735.
(5) Schmidt, B., Lindman, S., Tong, W., Lindeberg, G., Gogoll, A.,
Lai, Z., Thörnwall, M., Synnergren, B., Nilsson, A., Welch, C. J.,
Sohtell, M., Westerlund, C., Nyberg, F., Karlén, A., Hallberg, A. J.
Med. Chem. 1997, 40, 903.
The crucial desymmetrization of the meso-diester 5 was first achieved
by a charge controlled mono saponification of the diester by KOH in
aqueous methanol. The reduction of the crude acid by BH3·Me2S gave
the alcohol 6 in 60 % yield. A significant improvement was established
by treatment of 5 with L-Selectride™ in THF at low temperature, which
provided racemic 6 in 80 % yield23 in a single step, contaminated by
less than 8 % of the doubly reduced product. The following Mitsunobu
reaction required high purity DEAD and controlled reaction conditions
(0 °C, 5 min activation, then addition of 10 eq of HN3 in C6H6, 90 min)
to convert the alcohol to the azide without sidereactions and in good
yield. The final saponification of the methyl ester by LiOH in aqueous
dioxane provided the desired acid 7 quantitatively.
(6) Graf von Roedern, E., Lohof, E., Hessler, G., Hofmann, M.,
Kessler, H. J. Am. Chem. Soc. 1996, 118, 10156.
(7) Curran, T. P., Chandler, N. M., Kennedy, R. J., Keaney, M. T.
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Sepinwall, J., Vincent, G. P., Winokur, A., Gary, K. A. J. Med.
Chem. 1995, 38, 2866.
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Academic Press, Orlando, 1985, p. 1.
(10) Printz, M. P., Nemethy, G., Bleich, H. Nature 1972, 237, 135.
The introduction of such azido acids into peptides by solid-phase
synthesis was recently pioneered by Meldal.17 His method elegantly
combines introduction of the amino substituent and simultaneous
protection in a single step as it is demonstrated in the Somfai/Kihlberg
mimetic.1 The key step is the reduction of the azide to the amine by
dithiols after condensation of the acid to the resin linked peptide.
Utilizing this protocol G. Lindeberg (Uppsala University) introduced the
turn motif into one of our peptidic targets (8a + 8b) which is currently in
evaluation in AT1 receptor assays.
(11) Kühn, C., Lindeberg, G., Gogoll, A., Hallberg, A., Schmidt, B.
Tetrahedron 1997, 53, 12497.
(12) Nikiforovich, G. V., Marshall, G. R. Biochem. Biophys. Res.
Commun. 1993, 195, 222.
(13) Nikiforovich, G. V., Kao, J. L.-F., Plucinska, K., Zhang, W. J.,
Marshall, G. R. Biochemistry 1994, 33, 3591.
(14) Schiller, P. W. in Medicinal Chemistry for the 21st Century (Eds.:
C. G. Wermuth, N. Koga, H. König, B. W. Metcalf), Blackwell
Scientific Publications, Oxford, 1994, p. 215.

1242
LETTERS
SYNLETT
(15) Giannis, A., Kolter, T. Angew. Chem. 1993, 105, 1303; Angew.
Chem., Int. Ed. Engl. 1993, 32, 1244.
(23) 6: M+. 399; C24H30O5 requires C: 72.3, H: 7.6, found C: 71.3, H:
7.5; νmax 3520, 1728, 1512, 1240 cm-1; δH (400.1 MHz, CDCl3)
7.35 (d, J = 8.6 Hz, 2H, H-11"+H-15"), 7.04 (d, J = 8.6 Hz, 2H, H-
3"+H-7"), 6.91 (d, J = 8.6 Hz, 2H, H-12"+H-14"), 6.88 (d, J = 8.6
Hz, 2H, H-4"+H-6"), 4.95 (s, 2H, H-16"), 3.81 (s, 3H, H-9"), 3.64
(s, 3H, CO2CH3), 3.41 - 3.50 (m, 2 H, H-1´), 2.50 (d, J = 7.0 Hz,
2H, H-1"), 2.33 (tt, J =3.4 Hz, J = 12.4 Hz, 1H, H-1), 1.91 - 2.06
(m, 2H, Heq), 1.71 - 1.79 (m, 1H, Heq), 1.48 - 1.66 (m, 2H, H-3 u.
H-5), 1.08 (q, J = 12.5 Hz, 1H), 1.07 (q, J = 12.5 Hz, 1H), 0.66 (q,
J = 12.2 Hz, 1H); δC (100.6 MHz, CDCl3) 176.2 (s, CO2CH3),
159.3 (s, C-13"), 157.8 (s, C-5"), 132.5 (s, C-2"), 130.0 (d), 129.2
(d), 129.1 (s, C-10"), 114.5 (d), 113.9 (d), 69.8 (t, C-9"), 67.9 (t,
C-1´), 55.2 (q, C-16"), 51.6 (q, CO2Me), 42.7 (t, 1´), 42.6 (d, 1),
39.4 (d), 38.6 (d), 35.1 (t), 35.0 (t), 31.8 (t).
(16) Gante, J. Angew. Chem. 1994, 106, 1780; Angew. Chem., Int. Ed.
Engl. 1994, 33, 1699.
(17) Meldal, M., Juliano, M. A., Jansson, A. M. Tetrahedron Lett.
1997, 38, 2531.
(18) Mentzel, M., Hoffmann, H. M. R. J. Prakt. Chem. 1997, 339, 517.
(19) Nahm, S., Weinreb, S. M. Tetrahedron Lett. 1981, 22, 3815.
(20) Maeda, H., Okamoto, J., Ohmori, H. Tetrahedron Lett. 1996, 37,
5381.
(21) Mukaiyama, T., Araki, M., Takei, H. J. Am. Chem. Soc. 1973, 95,
4763.
(22) Corey, E. J., Clark, D. A. Tetrahedron Lett. 1979, 2875.