Efficient and regiospecific syntheses of peptides with piperazic and dehydropiperazic acids via a multicomponent reaction

Article abstract of DOI:10.1021/ol501425b

Peptides containing N2-acyl piperazic or 1,6-dehydropiperazic acids can be formed efficiently via a novel multicomponent reaction of 1,4,5,6- tetrahydropyridazines, isocyanides, and carboxylic acids. Remarkably, the reaction's induced intramolecularity can enable the regiospecific formation of products with N2-acyl piperazic acid, which counters the intrinsic and troublesome propensity for piperazic acids to react at N1 in acylations. The utility of the methodology is demonstrated in the synthesis of the bicyclic core of the interleukin-1β converting enzyme inhibitor, Pralnacasan.

Full text of DOI:10.1021/ol501425b

Letter
pubs.acs.org/OrgLett
Efficient and Regiospecific Syntheses of Peptides with Piperazic and
Dehydropiperazic Acids via a Multicomponent Reaction
Emma L. Handy, Kyle A. Totaro, Charlie P. Lin, and Jason K. Sello*
Department of Chemistry, Brown University, 324 Brook Street, Providence, Rhode Island 02912, United States
S
* Supporting Information
ABSTRACT: Peptides containing N2-acyl piperazic or 1,6-
dehydropiperazic acids can be formed efficiently via a novel
multicomponent reaction of 1,4,5,6-tetrahydropyridazines,
isocyanides, and carboxylic acids. Remarkably, the reaction’s
induced intramolecularity can enable the regiospecific
formation of products with N2-acyl piperazic acid, which
counters the intrinsic and troublesome propensity for piperazic
acids to react at N1 in acylations. The utility of the
methodology is demonstrated in the synthesis of the bicyclic
core of the interleukin-1β converting enzyme inhibitor, Pralnacasan.
umerous peptide natural products are constituted by
1
Nnonproteinogenic amino acids. The presence of these
amino acids often imparts nonribosomal peptides with proper-
ties that are critical for their biological activities and/or
stabilities.1a The peculiar structures and reactivities of noncoded
amino acids can make the chemical syntheses of peptides
containing them especially challenging. Among these unusual
constituents of peptides, a heterocyclic amino acid with a N−N
bond known as piperazic acid is particularly notable (Figure 1).
Congeners such as 1,6-dehydropiperazic acid, 5-hydroxy
piperazic acid, and 5-chloropiperazic acid have also been
Figure 2. Molecules containing the piperazic acids.
observed in peptide natural products (Figures 1 and 2).2
coupling reagents, syntheses of those containing the piperazic
acids require special considerations and efforts.2,5−8 One issue is
that these amino acids are not widely available for purchase and
must be synthesized via multistep routes.9 Examples of these
routes include a Diels−Alder cycloaddition of a diene and an
azodicarboxylate ester,10 a tandem α-aminoalkylation/olefina-
tion sequence promoted by an organocatalyst,5 or sequential
Figure 1. Piperazic acid and its congeners.
stereoselective electrophilic hydrazination and nucleophilic
cyclization.6 Importantly, the piperazic acids have been prepared
The piperazic acids can be found in many peptide antibiotics
that have antitumor, antibacterial, antifungal, and antiviral
enantioselectively and used in peptide synthesis.5−8,11a
activities.2 The conformationally constrained amino acid can
The intrinsic reactivity of the piperazic acids presents
challenges in peptide synthesis. For example, in addition to the
also be found in synthetic compounds, such as the
antihypertensive drug, Cilazapril,3 and Vertex Pharmaceuticals’
tendency of its esters to slowly oxidize and polymerize under
ambient conditions,8 piperazic acid preferentially undergoes
rationally designed inhibitor of the interleukin-1β converting
acylation at N1.11b Likewise, dehydropiperazic acid is difficult to
acylate at N2.11a These reactivity patterns are problematic
because most known piperazic acid containing peptides have
amide bonds exclusively at N2 (Figure 2). Studies by Ciufolini et
al. indicate that the carbonyl group of these amino acids exerts a
stereoelectronic effect that attenuates the nucleophilicity of
enzyme, Pralnacasan (Figure 2).4 In both of these molecules,
piperazic acid is part of a rigid, bicyclic core structure. Some
structural diversity of molecules containing the piperazic acids is
illustrated in Figure 2.
The unique structures and medicinal potential of molecules
containing the piperazic acids have made them popular targets in
target-oriented synthesis.2 Whereas peptides are typically
synthesized in a straightforward fashion by condensation of
commercially available amino acids using a familiar set of
Received: May 18, 2014
Published: June 17, 2014
© 2014 American Chemical Society
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N2.12 This problem can be solved by reacting N1-carbamate
protected piperazic acid with highly reactive acyl chlorides, but
the yields of those reactions are modest at best.2 Alternatively,
the stereoelectronic effect can be circumvented via the use of a
reduced form of piperazic acid as the acylation substrate.2 N2-
acylations of the amino alcohols can be effected with activated
carboxylic acids other than acyl chlorides and have higher yields,
but the resulting products must be oxidized to generate the
desired piperazic acid moieties. The piperazic acids’ reactivity
issues can be avoided altogether by effecting heterocycle
formation after regiospecific acylation of α-hydrazino esters.2,13
As is necessary, the dehydropiperazic acid formed in the
cyclization can be reduced to the saturated amino acid.9a
a
Table 1. Optimization of IMCR with THP
In contemplating a new strategy for achieving regioselective
syntheses of peptides with N2-acyl piperazic acids, we envisaged
an intramolecular acyl-transfer via a mixed anhydride inter-
mediate. Indeed, leveraging intramolecularity is a well-known
strategy in chemical synthesis.14 Further consideration of this
approach led to recognition of an analogy to the intramolecular
acyl transfer in the Joullie−
́
Ugi reaction.15 In this multi-
component reaction, peptides containing N-acylated proline,
or pipecolate residues are formed via the one-pot condensation
of a cyclic imine with an isocyanide and a carboxylic acid. Its
mechanism involves a series of reversible steps terminated by an
irreversible Mumm rearrangement of an O-acylimidate, which is
the intramolecular acyl transfer of interest. Beyond its favorable
intramolecularity, we deemed the multicomponent reaction as a
viable means to effect the desired transformation because its O-
acylimidate intermediate has comparable reactivity with the acyl
chlorides that are required for N2 acylation of piperazic acid. In
a
Reactions were performed at rt for 48 h with equimolar substrates.
Isolated yields. NR = No reaction.
b
c
presence of catalytic quantities (1, 10, or 25 mol %) of scandium
triflate, but were disappointed to find that its inclusion did not
improve the yields. Next, we assessed the effect of different
solvents on reactivity. The rates and yields of IMCRs are known
to be influenced by the solvent.16a The yield of the reaction in
dichloromethane was equivalent to that observed in methanol.
However, for reasons that are not obvious, no product formation
could be detected by thin layer chromatography (TLC) when the
reaction was performed in tetrahydrofuran. We then investigated
the effect of temperature on yields, as heating is known to
enhance some IMCRs.18 Disappointingly, both conventional and
microwave heating reduced product formation. These observa-
tions could be explained by reports that tetrahydropyridazines
exhibit some chemical lability.19 Since we suspected that THP
could decompose during the reaction, we performed experi-
ments wherein the reactant stoichiometry was varied. Interest-
ingly, neither reactions with THP in a 4-fold excess over the other
two reactants nor those in which the isocyanide and carboxylic
acids were used in a 3-fold excess over the THP had higher yields
than a reaction with equimolar reactants.
́
analogy to the Joullie−Ugi reaction, we predicted that peptides
with N2-acyl piperazic acid moieties could be formed
regioselectively by condensation of an isocyanide, a carboxylic
acid, and the cyclic hydrazone, 1,4,5,6-tetrahydropyridazine
(THP) (Figure 3). A series of experiments were carried out to
assess the viability of this new, hypothetical isocyanide
multicomponent reaction (IMCR).16
Figure 3. Proposed mechanism of the IMCR with THP.
While we found that THP was a viable substrate in a
regiospecific IMCR, the low yields led us to predict that THP
could be weakly reactive and/or prone to side reactions. We
proposed that both potential problems could be addressed by
using N1-substituted THPs as substrates in IMCRs. In principle,
substituents at N1 would both attenuate the atom’s nucleophil-
icity as protecting groups and influence the reactivity of the
THPs’ hydrazone moiety via inductive effects.
In a preliminary examination of the substituent effects, we
prepared a N1-acetyl THP (1b). We found that it did not react
with benzoic acid and tert-butyl isocyanide, even after a 96 h
reaction time. By comparing the outcome of this reaction (with
an electron-withdrawing substituent on the THP) to one
wherein unsubstituted THP was the substrate (Table 1, entries
In initial experiments, we reacted THP with benzoic acid and
tert-butyl isocyanide in methanol. The expected N2-benzoyl
piperazamide product was isolated as a racemate in 17% yield
after a reaction time of 48 h (Table 1, entry 1). Notably, the
apparent absence of N1-benzoyl piperazamide in the reaction
indicated that the desired regiospecificity was achieved. Reaction
parameters such as time, temperature, solvent, and stoichiometry
were varied systematically to gain insights into the reaction and
to improve its yield. We first investigated the parameter of
reaction time and found that the yield improved with longer
reaction times (30% after 5 days). The slow rate of the reaction
led us to revisit our findings that Lewis acidic metal catalysts can
promote IMCRs.17 Thus, we carried out the reaction in the
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a
1 and 2), one could argue that the deactivating acetyl group on
N1 further diminishes the nucleophilicity of N2 in the
acylimidate intermediate12 and thus precludes the key O-to-N
acyl transfer (Figure 3). These observations are especially
interesting in light of reports that acyclic N-acyl hydrazones are
viable substrates in IMCRs.20 In any case, we prepared THPs
functionalized at N1 with an allyl moiety (1c) or benzyl moieties
with either electron-donating or -withdrawing groups (1d−g) to
further explore the influence of substituent effects.20 They were
then used as substrates in IMCRs with benzoic acid and tert-butyl
isocyanide (Table 1, entries 3−7). The reactions with N1-
substituted THPs had much higher yields and generated fewer
side products than the reaction with unsubstituted THP (as
assessed by TLC). The lowest yield was observed in the reaction
wherein the THP substrate had an electron-deficient 4-
bromobenzyl substituent. In contrast, reactions with electron-
donating methoxybenzyl substituents on the THPs had nearly 3-
fold higher yields compared to the reaction with unsubstituted
THP (Table 1, entries 3−5). The most promising of these
activating/protecting groups was the p-methoxybenzyl (PMB),
as a THP substituted with it (1e) was the substrate in the highest
yielding IMCR (45% after 48 h). Importantly, the PMB group
can be removed from amines easily under conditions in which the
peptide product is likely to be stable.21 Unfortunately, we found
that PMB-THPs with hydroxy substituents on the heterocycle
were poor substrates in IMCRs (data not shown).
Table 3. Carboxylic Acid Scope of the Optimized IMCR
a
Reactions were performed at rt for 48 h with equimolar substrates.
b
c
Isolated yields. ∼1:1 mixture of diastereomers.
biotics.2 To demonstrate the capacity of this IMCR to form a
formal tripeptide, we also performed a reaction using PMB-THP,
N-Boc-glycine, and methyl, 2-isobutyl-α-isocyanoacetate (de-
rived from N-formyl leucine methyl ester). The expected
tripeptide product was isolated in 45% yield (2r, see Supporting
Information). These results indicate that the IMCR reaction is
useful in the efficient syntheses of peptides. Despite the ease with
which they are set up and their efficiencies, a drawback is that the
reactions with chiral substrates are not stereoselective and yield
diastereomeric products in ∼1:1 ratios. However, the diaster-
eomers can be separated by silica gel chromatography.
Although the presence of the PMB group on the THP
precludes the advantageous regiospecificity of the IMCR, its
positive effect on the reaction yield and the possibilities of its
removal from the reaction product warranted further studies.
Accordingly, we set out to determine the compatibility of the
PMB-THP in reactions with various carboxylic acids and
isocyanides. We were gratified to find that these reactions
yielded the expected products in moderate to good yields
(Tables 2 and 3). Conveniently, reactions of PMB-THP with
a
Table 2. Isocyanide Scope of the Optimized IMCR
The aforementioned IMCRs yield peptides containing a
piperazic acid moiety with a PMB-group on N1. We predicted
that its removal from the peptide product under reductive
conditions would yield a piperazic acid moiety and under
oxidative conditions would yield a dehydropiperazic acid
moiety.21 We found that the PMB group can be removed from
N1 of the IMCR product oxidatively by utilizing either ceric
ammonium nitrate (CAN) or 2,3-dichloro-5,6-dicyano-1,4-
benzoquinone (DDQ) to produce dehydropiperazic acid
(Scheme 1). If desired, the unsaturated heterocycle can be
transformed to piperazic acid using NaCNBH3 as a reductant.
Otherwise, the piperazic acid residue can be formed directly from
the IMCR product via reductive cleavage of the PMB group using
Scheme 1. Formation of Piperazic Acids via PMB Removal
a
Reactions were performed at rt for 48 h with equimolar substrates.
Isolated yields. ∼1:1 mixture of diastereomers.
b
c
either an isocyanoacetate (Table 2, entry 5) or N-Boc-protected
amino acids (Table 3, entries 4−5) yielded dipeptide products. It
is particularly noteworthy that the reaction of racemic Cbz-N1-
protected piperazic acid (Table 3, entry 6), PMB-THP, and tert-
butyl isocyanide yielded a diastereomeric mixture of dipeptide
products with contiguous piperazic acid residues in 34% yield
because this connectivity is observed in many peptide anti-
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catalytic hydrogenation (Scheme 1). Through judicious selection
of conditions for removal of the PMB group, it is possible to
prepare peptides with the piperazic acid moiety in the desired
oxidation state.
To demonstrate this novel IMCR’s utility, we used it to
efficiently synthesize the bicyclic core structure of Pralnacasan
(Figure 1). This interleukin 1β-converting enzyme inhibitor was
rationally designed and synthesized by Vertex Pharmaceuticals
for use as a potential anti-inflammatory drug.4 The first and key
transformation in the synthetic route was condensation of N-
Cbz-Glu(OMe)-OH, PMB-THP, and tert-butyl isocyanide
(Scheme 2). Following removal of the IMCR product’s PMB
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In summary, we have developed a novel IMCR that can be
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residue or its oxidized congener in only two steps. We have
shown the IMCR can tolerate a variety of carboxylic acid and
isocyanide substrates, including those that can be used for the
direct formation of peptides with tandem piperazic acid
constituents. Although the reaction is not stereoselective, we
found that the diastereomeric products can be separated easily.
This work is a practical and conceptual advance in peptide
synthesis because it capitalizes on induced intramolecularity to
circumvent challenges associated with the peculiar reactivities of
piperazic and dehydropiperazic acids. Application of the
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ASSOCIATED CONTENT
* Supporting Information
Procedures and spectroscopic data for all compounds. This
material is available free of charge via the Internet at http://pubs.
acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: jason_sello@brown.edu.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
J.K.S. is the recipient of a NSF CAREER Award. E.L.H. was
supported by the NSF Graduate Fellowship Program. C.P.L. was
supported by an Undergraduate Teaching and Research
Assistantship Award from Brown University. We are grateful to
T. Shen and R. Hopson at Brown for expert assistance in MS and
NMR analyses, respectively.
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