748
A. R. Ekkati et al.
O
R
varied depending on the particular amino acid. The tert-
butylamide was chosen because it prevents oxidation
adjacent to the nitrogen atom of the C-terminal amide. This
necessity presented an additional challenge in forming
amide bonds with some substrates, due to the steric hin-
drance of tert-butylamine. To overcome this challenge,
four general methods were used to access substrates 1–20,
Methods A, B, C, and D. All methods involve the con-
densation of tert-butylamine with some form of the amino
acid. Method A involves the condensation of an N-acetyl
amino acid with a p-nitrophenol ester. Method B employs
EDAC or DCC/HOBt to achieve condensation. Method C
starts with tert-butoxycarbonyl protected N-terminal amino
acid, instead of an N-acetyl, and utilizes benzyl and tosyl
protecting groups for the side chains. Substrates 19 and 20
required unique syntheses and are discussed separately in
Method D.
O
R
H
N
H
N
N
N
H
H
O
O
Polypeptide
Residue Mimic
1 Ala
2 Val
3 Phe 10 Trp
4 Leu
5 Pro
6 Gly
7 Cys
15 Ser
16 Thr
17 His
18 Arg
19 Asn
20 Ile
8 Met
9 Tyr
11 Gln
12 Lys
13 Glu
14 Asp
Fig. 1 Structure of Ac-AA-NHtBu substrates that mimic amino acid
residues of polypeptides
designed substrates of the general formula Ac-AA-NHtBu
(Fig. 1, AA = amino acid) that mimic amino acid residues
in a polypeptide chain. These substrates mimic residues by
virtue of having their N- and C-termini blocked as amides,
specifically, acetyl and tert-butyl, respectively. In this
paper, we describe the synthesis of the aforementioned
amino acid substrates derived from each of the 20, natu-
rally occurring, amino acids (1–20). The synthetic routes to
access substrates 1–20 rely on solution-phase methodol-
ogy, and can be used to furnish substrates on multi-gram
scale, leading to their ready availability for reactivity
studies with metal-based oxidants.
Method A
N-Acetylated amino acids of Ala, Val, Phe, Leu, Pro, and
Gly were condensed with p-nitrophenol using DCC as the
coupling reagent to form activated p-nitrophenol esters
(Scheme 1). Each activated ester was isolated and purified
by recrystallization before treatment with excess tert-
butylamine, which furnished the corresponding C-terminal
amides. This procedure was advantageous because the salt
(tBuNH3)(OC6H4NO2) was removed easily by filtration
after the condensation was complete, allowing access to the
final substrates in good to moderate yields (Table 1).
Materials and methods
All reagents were purchased from commercial sources and
used as received unless otherwise noted. N-acetyl amino
acids and some protected amino acids were prepared
according to previously reported literature (Chenault et al.
1989; Reddy and Ravindranath 1992; Ray et al. 2006). Other
protected amino acids were purchased: Boc-Asp(OBn)-OH,
Fmoc-Asn(Trt)-OH, Boc-His(Tos)-OH, and Boc-Arg(Tos)-
OH. All experimental procedures can be found in the Sup-
porting Information. Due to the fact that amino acids can
racemize during N-acetylation (Reddy and Ravindranath
1992) and enantiopure substrates were not required for
oxidation studies, optical rotation data, and measurements of
enantiopurity were not collected. NMR spectra were recor-
ded on a Varian FT-NMR Unity-300, Mercury-400 or
500 MHz Spectrometer. Mass spectra were recorded on a
Waters ZQ2000 single quadrupole mass spectrometer using
an electrospray ionization source. IR spectra were recorded
on a Nicolet FT-IR spectrophotometer.
1) DCC, PNP
Ac-AA-OH
Ac-AA-NHtBu
2) H2NtBu
1-6
AA = Ala, Val, Phe, Leu, Pro, Gly
Scheme 1 Syntheses of substrates 1–6 via an isolated p-nitrophenol
ester
Table 1 Conditions and yields for formation of p-nitrophenol esters
and condensation products for 1–6
Ac-AA-NHtBu
AA=
Ac-AA-OPNP
yielda (%)
Condensation
yielda
Ala (1)
Val (2)
Phe (3)
Leu (4)
Pro (5)
Gly (6)
42
71
40
62b
61
66
61
79b
63b
60b
73b
44b
Results and discussion
a
rt unless otherwise stated
Although the synthetic target for substrates 1–20 are
identical, the methods used to access these substrates
b
50°C to rt
123