Angewandte
Chemie
[25,26]
(
BTC), established by Jung et al.,
as a superior coupling
Finally, fully protected albicidin 17 was assembled from
agent. Other methods to generate acid chlorides with reagents
such as thionyl chloride or cyanuric chloride resulted in lower
yields or extended reaction times (> 7 d).
the three fragments (2, 3 and 4). Coupling of central
tripeptide 3 and C-terminal fragment 4 was achieved by
applying the BTC-mediated peptide coupling conditions,
[25]
With these general considerations, we set out to assemble
albicidin: The coumaric acid building block A (2) was
synthesized in two steps with 89% yield, mainly according
which resulted in pentapeptide 15 in 91% yield (Scheme 2C).
The reduction of the N-terminal nitro group of compound 15
to obtain pentapeptide 16 was followed by a quantitative
BTC-mediated coupling with coumaric acid derivative 2.
Global allyl deprotection of fully protected albicidin 17 with
Pd(PPh3)4 and phenylsilane as a scavenger resulted in
albicidin, which was purified by preparative reversed-phase
HPLC. Comparison of the analytical data with those obtained
for the natural product were in excellent accordance, thus
confirming the structure of albicidin (see the Supporting
[
27]
to literature procedures.
Initially, for assembly of the
central tripeptide 3 (Scheme 2A), the required Boc-b-Cya-
pABA-OtBu dipeptide (7) was synthesized through conden-
sation of Boc-protected b-cyanoalanine with H-pABA-OtBu
[28]
in a two-step sequence. However, the resulting dipeptide 7
was obtained in low yields and we therefore sought to shorten
this reaction sequence by making use of a known side reaction
of unprotected Asn in peptide chemistry, namely, nitrile
[
16]
Information).
[
29]
formation under amide bond coupling conditions.
cyclic intermediates resulting from this dehydration are
sensitive to racemization,
Since
Since albicidin has only one stereocenter (l-Cya), we
intended to briefly assess the influence of the configuration of
Cya on its antibacterial activity. Therefore, ent-albicidin (1b)
was synthesized. The circular dichroism (CD) spectrum of the
[30,31]
we had to study this key
reaction in more detail. Various coupling conditions were
examined and HATU/DIPEA were found to produce a high
degree of racemization, whereas carbodiimides (EDC, DIC
[
36,37]
natural product shows the same Cotton effect
as synthetic
(S)-configured albicidin (see the Supporting Information). By
contrast, ent-albicidin reveals virtually the inverted CD
spectrum, as expected for the (R) configuration of Cya.
Subsequently we performed activity studies on the
molecular target DNA gyrase from E. coli with both albicidin
enantiomers. Gyrase is also the major target of quinolone
[
32]
and DCC) without the addition of bases
enantiomeric excess (ee = 96%), albeit with low yields
13%). Through variation of the reaction conditions, we
gave excellent
(
improved the yield of this one pot reaction to 64% with
almost negligible racemization (< 4%).
[
38]
Having established a smooth access to dipeptide 7, the
acid-labile Boc and tBu-ester protecting groups were
removed with HCl/dioxane (4m) to quantitatively obtain
dipeptide 8. Most importantly, this deprotection approach
antibiotics, for example, ciprofloxacin, which is still con-
[
39]
sidered the gold standard for gyrase inhibition. A previ-
ously established protocol was employed to estimate the half
[
15,40]
maximal inhibitory concentration (IC )
and adequately
5
0
[29]
prevents rehydration of the nitrile of b-cyanoalanine.
compare it with published values. As shown in Figure 1A, the
IC50 value for synthetic albicidin is approximately 40 nm,
which is in excellent accordance the value reported for the
Subsequently, dipeptide 8 was converted into the central
tripeptide 3 by using an activated succinimide ester of 4-
nitrobenzoic acid and triethylamine as a base in yields of
[15]
natural product. Intriguingly, the IC50 for ent-albicidin was
determined to be approximately 40 nm as well (Figure 1B).
[33]
8
9%.
The synthesis of the C-terminal fragment E-F (4) is
illustrated in Scheme 2B. An important prerequisite for
successful assembly, building block 10, is available from
[
34,35]
ortho-vanillin.
Further derivatization into key building
blocks 12 and 13 required an allyl protecting group, which was
introduced in yields of 88% with K CO /allyl bromide. For
2
3
oxidation of the aldehyde functionality of compound 11 to
carboxylic acid 12, we chose mild conditions to keep the
potentially vulnerable double bond of the allyl protection
group intact. Compound 12 was obtained in a yield of 92%.
After protection of the C-terminus of 12, free amine 13 was
obtained after reduction of the nitro group under mild
conditions with SnCl ·2H O (EtOH/608C) in yields of
2
2
[18]
8
6%.
With the aromatic building blocks 12 and 13 in
hand, fragment E–F (4) was assembled through BTC-
mediated peptide coupling. As mentioned above, commonly
used peptide coupling reagents, for example T3P, failed in our
studies with substrates that have a high steric demand and
[25,26]
Figure 1. Inhibition assay of DNA gyrase supercoiling activity (E. coli)
bear aromatic amines. The method by Jung et al.
resulted
for determination of the half maximal inhibitory concentration (IC )
5
0
in satisfying yields and often shorter reaction times. The
dipeptide 14 was synthesized in a yield of 91% (BTC, 2,4,6-
collidine, DIPEA, THF) and after reduction of the nitro
values of albicidin (A) and ent-albicidin (B). The control experiment
without enzyme and drug (lane c) shows relaxed DNA (re), and the
addition of enzyme (lane 0) results in supercoiled DNA (sc). The other
lanes represent the reaction with enzyme and increasing concentra-
tions of the corresponding drug (30–200 nm).
functionality with SnCl ·2H O, we obtained the last building
2
2
block 4 for the total synthesis of albicidin in a yield of 79%.
Angew. Chem. Int. Ed. 2014, 53, 1 – 6
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3
These are not the final page numbers!
Kretz, Julian
