Copper-catalyzed N -arylation of semicarbazones for the synthesis of aza-arylglycine-containing aza-peptides

Article abstract of DOI:10.1021/ol100932m

Parallel synthesis of 13 aza-arylglycine peptides, based on the hexapeptide sequence of Growth Hormone Releasing Peptide-6 (GHRP-6), was accomplished via selective N-arylation of a semicarbazone peptide building block anchored on Rink amide resin. Aza-peptides possessing aza-indolylglycine and aza-imidazoylglycine residues were obtained through use of the corresponding heteroaryl iodides, yielding, respectively, aza-Trp and aza-His peptidomimics. CD spectroscopy indicated the propensity for aza-peptides, containing aza-arylglycines at the Trp⁴ position of the GHRP-6 sequence, to adopt β-turns.

Full text of DOI:10.1021/ol100932m

ORGANIC
LETTERS
2010
Vol. 12, No. 13
2916-2919
Copper-Catalyzed N-Arylation of
Semicarbazones for the Synthesis of
Aza-Arylglycine-Containing Aza-Peptides
Caroline Proulx and William D. Lubell*
De´partement de Chimie, UniVersite´ de Montre´al, C.P. 6128, Succursale Centre-Ville,
Montre´al, Que´bec H3C 3J7
william.lubell@umontreal.ca
Received April 22, 2010
ABSTRACT
Parallel synthesis of 13 aza-arylglycine peptides, based on the hexapeptide sequence of Growth Hormone Releasing Peptide-6 (GHRP-6), was
accomplished via selective N-arylation of a semicarbazone peptide building block anchored on Rink amide resin. Aza-peptides possessing
aza-indolylglycine and aza-imidazoylglycine residues were obtained through use of the corresponding heteroaryl iodides, yielding, respectively,
aza-Trp and aza-His peptidomimics. CD spectroscopy indicated the propensity for aza-peptides, containing aza-arylglycines at the Trp4 position
of the GHRP-6 sequence, to adopt ꢀ-turns.
Substituted hydrazines constitute important synthetic building
blocks employed, namely, as precursors of a wide variety
of medicinally relevant heterocycles such as pyrazolidines,1
pyrazoles,2 1,3,4-triazoles,3 as well as 1,3,4-oxa- and thia-
zoles.4 Furthermore, they are key components of aza-
peptides, a type of peptidomimetic in which the R-carbon
of one or more amino acid residues has been substituted by
a nitrogen atom.5 In aza-peptides, the replacement of CR by
nitrogen may confer enhanced stability,6 resistance to pro-
teases,7 and conformational rigidity, such that ꢀ-turn con-
formers have been observed by X-ray crystallography, NMR
spectroscopy, and computational analysis.8
(1) For a selected example, see: Ahn, J. H.; Kim, J. A.; Kim, H.-M.;
Kwon, H.-M.; Huh, H.-C.; Rhee, S. D.; Kim, K. R.; Yang, S.-D.; Park,
S.-D.; Lee, J. M.; Kim, S. S.; Cheon, H. G. Bioorg. Med. Chem. Lett. 2005,
15, 1337.
(4) (a) Taylor, E. C.; Wipf, P. Oxazoles: Synthesis, Reactions, and
Spectroscopy; John Wiley & Sons: Hoboken, 2003. (b) Lewis, J. R. Nat.
Prod. Rep. 1999, 16, 389.
(5) (a) Gante, J. Synthesis 1989, 405. (b) Huang, Y.; Malcom, B. A.;
Vederas, J. C. Bioorg. Med. Chem. 1999, 7, 607.
(2) Deng, X.; Mani, N. S. J. Org. Chem. 2008, 73, 2412. references
within.
(6) (a) Gassman, J. M. Bioorg. Med. Chem. Lett. 1996, 6, 1771. (b)
Wipf, P.; Adeyeye, C. M.; Rusnak, J. M.; Lazo, J. S. Bioorg. Med. Chem.
1996, 4, 1585.
(3) Review of synthesis and pharmacological importance: Al-Masoudi,
I. A.; Al-Soud, Y. A.; Al-Salihi, N. J.; Al-Masoudi, N. A. Chem. Heterocycl.
Comp. 2006, 42, 1377.
(7) Dugave, C.; Demange, L. Lett. Pept. Sci. 2003, 10, 1.
10.1021/ol100932m  2010 American Chemical Society
Published on Web 06/10/2010

In spite of the importance of substituted hydrazines, the
inherent difficulty in distinguishing their two available
nitrogen atoms has often led to the extensive use of protective
groups for their regioselective assembly.9 In exploring
the combinatorial synthesis of aza-peptides on solid support,
we have recently described a three-step procedure exploiting
the use of a semicarbazone moiety to achieve selective
alkylation and incorporation of hydrazine moieties in pep-
tides.10 We now report a method for selective N-arylation
of the semicarbazone moiety in the presence of the multiple
amide and carbamate nitrogens within the aza-peptide by
employing copper catalysis. This novel chemistry constitutes
the gateway for the rapid synthesis of aza-arylglycine
moieties, whose arylglycine counterparts have long sparked
interest due to their nature as nonproteogenic R-amino acids
found in ꢀ-lactam11 and peptide antibiotics,12 such as
ampicillin and vancomycin. Furthermore, inherent issues of
racemization that plague the use of arylglycines13 are
alleviated by employing their aza-amino acid counterparts.
Our interest in aza-arylglycine peptide synthesis stems
from the desire to replace tryptophan (Trp) with aza-residues
bearing indolyl moieties in peptidomimetics of the hexapep-
tide Growth Hormone Releasing Peptide-6 (GHRP-6, His-
D-Trp-Ala-Trp-D-Phe-Lys-NH2), a lead for developing novel
antiangiogenic compounds.10 Although incorporation of an
aza-Phe residue at the Trp4 position confered selectivity for
the CD36 receptor as compared to the native sequence,10
attempts to prepare the aza-Trp peptide met with loss of the
indolylmethyl moiety giving the aza-Gly counterpart after
acidic cleavage with TFA (Figure 1).14 Pursuing aza-
bond formation gave access to unprecedented aza-Trp and
aza-His mimics. Conditions for N-arylation were initially
screened using semicarbazone 3 anchored on Rink amide
resin (Scheme 1). Among the different combinations of bases,
copper salts, ligands, and solvents tested (see Supporting
Information), the best results were achieved using a 5-fold
excess of CuI, KOtBu, EDA, and ArI in dioxane in the
presence of 4 Å powdered molecular sieves for 24 h. Under
these conditions, N-arylation of the semicarbazone at the Trp4
position of the GHRP-6 sequence proceeded with conver-
sions between 52 and 94% (Table 1). In all cases, LCMS
Table 1. N-Arylation at Residue Positions 2 and 4 of GHRP-6
analysis indicated complete consumption of starting material
with formation of byproduct. It is noteworthy that arylation
using the same conditions, yet further along the peptide
sequence, afforded Trp2 analogues in lower conversions,
suggesting that arylation may be impeded by the peptide
adopting secondary structure on solid support. In spite of
lower conversions in the arylation at the 2-position, six
GHRP-6 analogues (10a-f, R ) H) could be obtained in
suitable yield and purity after completion of the peptide
sequence, cleavage, and HPLC purification (Table 1).
To gain additional insight into the arylation reaction,
benzylidene aza-glycinyl-phenylalaninyl isopropylamide 11
(Scheme 2) was prepared and examined as a model in the
Figure 1. Synthesis of aza-peptide bearing indolyl moieties.
indolylglycine as a stable aza-Trp surrogate, we have
conceived a general method for making aza-arylglycine
peptide analogues, featuring N-arylation of a resin-bound aza-
glycine peptide with a variety of aryl and heteroaryl iodides
using CuI, ethylene diamine (EDA), and potassium tert-
butoxide in dioxane.15 To the best of our knowledge, this
method constitutes the first example of selective semicar-
bazone monoarylation. Moreover, use of N-Boc-3-iodoin-
dole16 and N-trityl-4-iodoimidazole17 in Cu-catalyzed C-N
(8) (a) Reynolds, C. H.; Hormann, R. E. J. Am. Chem. Soc. 1996, 118,
9395–9401. (b) Lee, H.-J.; Song, J.-W.; Choi, Y.-S.; Park, H.-M.; Lee, K.-
B. J. Am. Chem. Soc. 2002, 124, 11881–11893. (c) Andre´, F.; Boussard,
G.; Bayeul, D.; Didierjean, C.; Aubry, A.; Marraud, M. J. Pept. Res. 1997,
49, 556–562. (d) Andre´, F.; Vicherat, A.; Boussard, G.; Aubry, A.; Marraud,
M. J. Pept. Res. 1997, 50, 372–381.
(9) For a review on the synthesis of alkyl-substituted hydrazines, see:
Ragnarsson, U. Chem. Soc. ReV. 2001, 30, 205.
(10) Sabatino, D.; Proulx, C.; Klocek, S.; Bourguet, C. B.; Boeglin, D.;
Ong, H.; Lubell, W. D. Org. Lett. 2009, 11, 3650.
Org. Lett., Vol. 12, No. 13, 2010
2917

Scheme 1. Synthesis of [Aza-Arylglycine] GHRP-6 Analogues
deprotection of the hydrazone was achieved using
NH2OH·HCl in pyridine.10 The remaining sequences were
completed according to general methods for peptide synthe-
sis19 to afford [aza-arylglycine] GHRP-6 analogues with
crude purities ranging from 7 to 65% (Table 2). Material of
Scheme 2. Solution-Phase N-Arylation of
Aza-Glycinyl-phenylalanine Isopropylamide
solution-phase N-arylation, which occurred with concurrent
hydrazone deprotection to afford aza-toluylglycine dipeptide
amide 12 as a single product, albeit in a 20% yield.
Hydrazone removal may occur as a result of a nucleophilic
attack by ethylene diamine. By comparison, deprotection of
arylated material was also observed as a minor product peak
in the LCMS analysis of material cleaved from the solid
support.
On solid support, completion of the peptide sequence
necessitated scavenging of copper by shaking the resin with
a 3:1 DMF:0.1 N HCl mixture overnight.18 Chemoselective
(11) Townsend, C. A.; Brown, A. M. J. Am. Chem. Soc. 1983, 105,
913.
(12) Williams, D. H.; Waltho, J. P. Biochem. Pharmacol. 1988, 37, 133.
(13) Smith, G. G.; Sivakua, T. J. Org. Chem. 1983, 48, 627.
(14) Boeglin, D.; Lubell, W. D. J. Comb. Chem. 2005, 7, 864.
(15) Similar conditions used for arylation of indoles on solid support
in: Wu, T. Y. H.; Schultz, P. Org. Lett. 2002, 4, 4033.
(16) Wiltulski, B.; Buschmann, N.; Bergstra¨ber, U. Tetrahedron 2000,
56, 8473.
Figure 2. Circular dichroism spectra of GHRP-6 with [aza-arylgly2]-
(upper) and with [aza-arylgly4]-GHRP-6 analogues (lower) in water.
(17) Cliff, M. D.; Pyne, S. G. Synthesis 1994, 681.
(18) Xu, W.-M.; Huang, X.; Tang, E. J. Comb. Chem. 2005, 7, 726.
2918
Org. Lett., Vol. 12, No. 13, 2010

Table 2. Yields and Purities of Aza-Arylglycine-Containing GHRP-6 Analogues 10a-f and 6a-g
purityb (%)
HRMS [M + H]+ or [M + Na]+ ions
peptide
crude puritya (%) MeOH MeCN yieldc (%)
m/z (calcd)
m/z (obsd)
10a His-Aza(phenylgly)-Ala-Trp-D-Phe-Lys-NH2
23
22
34
16
7
>99
>99
>99
>99
>99
91
>99
>99
>99
>99
>99
92
>99
>99d
>99
>99
>99d
>99
>99
2.0
1.2
1.4
1.2
1.4
0.8
1.0
1.0
1.2
3.4
2.8
1.6
1.6
821.4205
851.4311
851.4311
835.4362
839.4111
882.4134
821.4205
851.4311
851.4311
835.4362
839.4111
860.4314
811.4110
821.4204
851.4296
851.4300
835.4358
839.4105
882.4121
821.4217
851.4293
851.4307
835.4352
839.4106
860.4321
811.4080
10b His-Aza(m-methoxyphenylgly)-Ala-Trp-D-Phe-Lys-NH2
10c His-Aza(p-methoxyphenylgly)-Ala-Trp-D-Phe-Lys-NH2
10d His-Aza(p-toluylgly)-Ala-Trp-D-Phe-Lys-NH2
10e His-Aza(p-fluorophenylgly)-Ala-Trp-D-Phe-Lys-NH2
10f His-Aza(3-indolylgly)-Ala-Trp-D-Phe-Lys-NH2
6a His-D-Trp-Ala-Aza(phenylgly)-D-Phe-Lys-NH2
9
65
22
23
45
28
27
65
>99
>99
>99
92
6b His-D-Trp-Ala-Aza(m-methoxyphenylgly)-D-Phe-Lys-NH2
6c His-D-Trp-Ala-Aza(p-methoxyphenylgly)-D-Phe-Lys-NH2
6d His-D-Trp-Ala-Aza(p-toluylgly)-D-Phe-Lys-NH2
6e His-D-Trp-Ala-Aza(p-fluorophenylgly)-D-Phe-Lys-NH2
6f His-D-Trp-Ala-Aza(3-indolylgly)-D-Phe-Lys-NH2
6g His-D-Trp-Ala-Aza(5-imidazoylgly)-D-Phe-Lys-NH2
a RP-HPLC purity at 214 nm of the crude peptide in MeOH/H2O eluant containing 0.1% formic acid. b RP-HPLC purity at 214 nm of the purified peptide
in MeOH/H2O and MeCN/H2O eluant containing 0.1% formic acid. c Yields after purification by RP-HPLC are based on resin loading. d Aza-peptide was
isolated as a mixture of His epimers.
>99d
>99
96
g91% purity was subsequently isolated by reverse-phase
HPLC and used in circular dichroism (CD) spectroscopic
studies (Figure 2) for assessing the conformational effects
of the aza-arylglycine moiety on peptide conformation.
Affinity for the CD36 receptor is under investigation and
will be reported in due time.
In the CD spectra in water, native GHRP-6 and aza-peptide
analogues 10a-f (R ) H), in which D-Trp2 was replaced by
the aza-arylglycine moiety, all displayed CD signatures
characteristic of a random coil, exhibiting a negative
maximum around 190 nm. Conversely, CD signatures of
[aza-arylgly4]-GHRP-6 analogues 6a-g (R ) H) displayed
distinctive negative maxima at 230 and 190 nm and a positive
maximum at 215 nm, suggestive of a ꢀ-turn conformation.
A novel solid-phase methodology featuring the copper-
catalyzed N-arylation of a semicarbazone has furnished 13
aza-arylglycine GHRP-6 analogues. Employing heteroaryl
iodides, aza-peptides bearing indole and imidazole side
chains were synthesized as aza-Trp and aza-His surrogates.
Replacement of Trp4 with aza-arylglycine residues in the
GHRP-6 sequence induced a ꢀ-turn conformation in water
as indicated by circular dichroism spectroscopy. Their
effective synthesis and the conformational bias of the aza-
arylglycinyl residues make this methodology particularly
attractive for studying structure-activity relationships of
biologically active peptides.
Acknowledgment. This research was supported by the
Natural Sciences and Engineering Research Council of
Canada (NSERC) and the Fonds Que´becois de la Recherche
sur la Nature et les Technologies (FQRNT). C.P. is grateful
to NSERC and Boehringer Ingelheim for graduate student
fellowships. The authors thank Dr. A. Fu¨rto¨s, K. Venne, and
M.-C. Tang (Universite´ de Montre´al) for assistance with mass
spectrometry and Dr. David Sabatino (Universite´ de Mon-
tre´al) for help with CD spectroscopy.
Supporting Information Available: Experimental pro-
cedures and characterization data. This material is available
free of charge via the Internet at http://pubs.acs.org.
(19) Lubell, W. D.; Blankenship, J. W.; Fridkin, G.; Kaul, R. Peptides.
Science of Synthesis 21.11, Chemistry of Amides; Thieme: Stuttgart,
Germany, 2005; pp 713-809.
OL100932M
Org. Lett., Vol. 12, No. 13, 2010
2919