S. Behrouz, N. Kühl and C.D. Klein
Tetrahedron Letters xxx (xxxx) xxx
stable to some extent under the reduction condition (entries 9–11).
Additionally, the use of Ac2O in lower amounts lowers the yield of
O-acyl amidoxime which in turn affords the desired amidine in
lower yields (entries 13 and 14).
protection of amidine 3c using different conditions. However, we
gained no satisfactory results which can likely be attributed to
the presence of carboxylic acid group. Thus, we turned our atten-
tion to protect amidine 3h which is the methyl ester of 3c. To this
end, Boc protection of amidine 3h using di-tert-butyl dicarbonate
(Boc2O) affords the intermediate 3i as another important building
block. Then, the hydrolysis of 3i was quantitatively achieved using
lithium hydroxide [29] to prepare the respective capped amino
acid 3j.
The synthetic route to amidinophenylalanines 3k and 3l bearing
a benzoyl group at the N-terminal is outlined in Scheme 4 which is
similar to the described procedure for amidine building blocks 3c-
3g.
Thus, N-benzoyl intermediate 8 was obtained by replacing
phenylsulfonyl chloride with benzoyl chloride which was then
hydrolyzed to yield the amino acid derivative 1h (step 3a). The
N-methylation of 8 using methyl iodide and subsequent hydrolysis
of ester were achieved in a single step in the presence of sodium
hydride in DMF to afford 1i (step 3b). Subsequent reaction of inter-
mediates 1h and 1i with hydroxylamine (step 4) yielded ami-
doximes 2i and 2j. Both were reduced to amidines 3k and 3l. The
detailed synthetic procedures and characteristic data of the inter-
mediates are described in the supplementary information.
The present optimized protocol for synthesis of amidine deriva-
tives affords a general applicability and can be efficiently employed
for reduction of nitriles to amidines in the presence of hydrogena-
tion-sensitive groups such as benzyl, substituted benzyl, and Cbz
protecting groups as well as certain halogen-substituted phenyl
groups. Therefore, it avoids the complete or partial removal of
hydrogenation-sensitive protecting groups and certain halogen
on aromatic rings [26]. Additionally, the use of lower amount of
Ac2O in the present one-pot method prevents the formation of
the corresponding oxadiazole as a stable byproduct [45].
Enzyme inhibition studies for a selection of the presented com-
pounds against various viral and eukaryotic serine proteases with
preference for basic residues were performed and indicated mod-
erate activity against, in particular, the protease of West Nile Virus
(see supplementary information). We consider this result encour-
aging to further follow up on the utility of these amidinophenylala-
nine derivatives as building blocks to obtain peptidic and
peptidomimetic drug candidates. This work is currently under
investigation, with the intent to develop a novel class of protease
inhibitors.
To simplify the procedure, comparative experiments were per-
formed as shown in entries 12 and 15. In case of entry 12, O-acyl
amidoxime was first synthesized. After solvent removal and
work-up, it was dissolved again in AcOH to prepare for the reduc-
tion step. In case of entry 15, the reduction of amidoxime was car-
ried out in a one-pot procedure. Thus, after 1.5 h treatment of
amidoxime with Ac2O in AcOH, zinc powder was added to the reac-
tion mixture to reduce the in situ generated O-acyl amidoxime
intermediate. However, no considerable difference was observed
between the obtained results. In another experiment (not shown
in Table 1), after completion of the reaction for synthesis of ami-
doxime 2a, the solvent was evaporated and the concentrated crude
product was directly used in the reduction step without further
work-up, eventually yielding the desired amidine 3a in 80% yield.
Hence, the conditions shown in entry 15 were selected as the opti-
mized reaction conditions for the reduction of amidoximes to
amidines.
Having the optimized reaction conditions in hand, we were
interested to investigate its generality and applicability for reduc-
tion of other amidoximo-phenylalanine derivatives to the respec-
tive amidines. Hence, we studied the synthesis of amidines 3a-3l
(Table 2) as interesting building blocks for the synthesis of diverse
biologically active compounds. To our delight, different sub-
stituents at the N-terminal side of phenylalanine derivatives were
tolerated well. Additionally, no difference was observed for the
reduction of amidoxime in the presence of carboxylic acid or car-
boxylic ester at the C-terminal side of phenylalanine derivatives
(products 3c and 3h). Fmoc-Phe(3-amidoxime)–OH (2b) was
obtained from Fmoc-Phe(3-CN)–OH (1b) and was converted to
the corresponding amidine 3b in quantitative yield under the opti-
mized reaction conditions (Table 2, product 3b).
It is worth mentioning that the conversion of free Phe(4-CN)–
OH and Phe(3-CN)–OH into their corresponding amidines is trou-
blesome due to side reactions such as N-acetylation of the free
a-amine group of the amino acid and nucleophilic attack of this
amino to the nitrile moiety. This can lead to lower yields of the
desired amidines and tedious work-up procedures. If the aim is
the preparation of free Phe(4-amidine)–OH and/or free Phe(3-ami-
dine)–OH, then it is suggested to perform the conversion of the
Fmoc-protected amino acid into the corresponding amidine in a
final deprotection step.
Conclusion
Several amidinophenylalanine derivatives 3c-3g bearing phenyl
sulfonyl groups at the N-terminal side were synthesized according
to the pathway shown in Scheme 2. Initially, amino acid 4 was
reacted with thionyl chloride and methanol [29] to afford the
respective amino acid methyl ester 5 which was converted to the
sulfonamide derivatives 6a-6c through reaction with arene sul-
fonyl chloride in the presence of N,N-diisopropylethylamine
(DIPEA) at room temperature. Next, the sulfonamide nitrogen
was alkylated with the desired alkyl halides using potassium car-
bonate in DMF to afford intermediates 7a-7e which were then
hydrolyzed using cesium carbonate to provide amino acids 1c-1g
[29]. Subsequent reaction of 1c-1g with hydroxylamine led to ami-
doximes 2c-2g which were reduced with zinc powder under the
optimized reaction conditions to gain amidines 3c-3g as important
and promising building blocks for drug design and discovery.
Amidinophenylalanine derivative 3h was synthesized in the
procedure which is described in Scheme 2 excluding the hydrolysis
step (step 4). Thus, cyanophenylalanine derivative 7a was con-
verted to amidoxime 2h which in turn was reduced to the respec-
tive amidine 3h (Scheme 3). Since the Boc-protected amidine 3j
was essential for our further research aims, we achieved the Boc-
In summary, we have described a facile and highly efficient pro-
cedure for the synthesis of amidinophenylalanine derivatives. In
the present protocol, treatment of amidoximes with Ac2O in AcOH
followed by reduction of the in situ generated O-acyl amidoxime
with zinc at room temperature furnishes the respective amidines
in excellent yields. The mild reaction conditions, high yields, ease
of operation, avoidance of H2 gas and expensive Pd catalyst, safety,
and affordability of this approach considerably simplify the syn-
thesis of variously substituted amidines as attractive and impor-
tant building blocks for non-natural peptides and peptide
hybrids. In addition, amidoximes bearing hydrogenation-sensitive
groups can be efficiently converted to the corresponding amidines
using the present one-pot protocol.
General procedure for the synthesis of amidoxime 2
A mixture of NH2OHÁHCl (0.75 mmol, 1.5 equiv) and DIPEA
(0.75 mmol, 1.5 equiv) was dissolved in EtOH (3 mL) at r.t. for
1 h. The obtained solution was added to a solution of an appropri-
3
Behrouz, Somayeh
