Synthesis of Cyclic Peptide Mimetics by the Successive Ring Expansion of Lactams

Article abstract of DOI:10.1002/chem.201703316

A successive ring-expansion protocol is reported that enables the controlled insertion of natural and non-natural amino acid fragments into lactams. Amino acids can be installed into macrocycles via an operationally simple and scalable iterative procedure, without the need for high dilution. This method is expected to be of broad utility, especially for the synthesis of medicinally important cyclic peptide mimetics.

Full text of DOI:10.1002/chem.201703316

A Journal of
Accepted Article
Title: Synthesis of cyclic peptide mimetics via the successive ring
expansion of lactams
Authors: Thomas C Stephens, Mahendar Lodi, Andrew M Steer, Yun
Lin, Matthew T Gill, and William Paul Unsworth
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To be cited as: Chem. Eur. J. 10.1002/chem.201703316
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Synthesis of cyclic peptide mimetics via the successive ring
expansion of lactams
Thomas C. Stephens, Mahendar Lodi, Andrew M. Steer, Yun Lin, Matthew T. Gill and William P.
Unsworth*
Abstract: A successive ring expansion protocol is reported that
enables the controlled insertion of natural and non-natural amino acid
fragments into lactams. Amino acids can be installed into macrocycles
via an operationally simple and scalable iterative procedure, without
ester.[12] Average yields of ≈50–60% and the fact that this method
only works consistently well with β-amino acid derivatives were
also identified as areas in need of improvement.
Herein we describe a new SuRE reaction mode, based on an
the need for high dilution. This method is expected to be of broad utility, operationally simple, versatile method for the ring expansion of
especially for the synthesis of medicinally important cyclic peptide
mimetics.
lactams (9 → 11, Scheme 1C). As in our earlier work, successive
ring expansion reactions can be performed, but in this case, a
secondary lactam group (highlighted) is the repeating unit that
enables subsequent iterations. Higher yields (typically ≈80–90%)
and much improved substrate scope have been demonstrated,
and furthermore, the only functional groups formed in the products
are stable amides, auguring well for its use in applied fields that
rely on the design and synthesis of macrocycles, especially the
synthesis of cyclic peptide mimetics.[13]
Macrocycles have important roles across several scientific fields,
including medicinal chemistry,[1] catalysis,[2] self-assembly,[3]
molecular sensing[4] and supramolecular chemistry.[5] However,
functionalised macrocycles are often difficult to make,[6] and in
many cases, this can limit their utility. Macrocycles are typically
prepared via the end-to-end cyclisation of linear precursors (1 →
2, Scheme 1A), which can be difficult and unpredictable
processes;[7] the main problem is ensuring that the ‘ends’ of the
linear molecule react whilst avoiding unwanted side reactions,
especially intermolecular coupling. Macrocyclisation reactions are
usually performed at high dilution to minimise such problems, but
they are rarely avoided entirely, often resulting in low yielding,
impractical processes, especially on large scale. While various
strategies have emerged to address these problems,[8] high
dilution methods remain the most commonly used, hence new
versatile and scalable procedures for the synthesis of
functionalised macrocycles will always be of value.
One approach is to assemble macrocycles via ring expansion
(5 → 2, Scheme 1A).[9, 10] ‘Growing’ macrocycles in this way has
a distinct advantage over most other approaches, in that rather
than seeking to make the difficult macrocyclisation step easier,
ring expansion enables it to be avoided altogether. One such
method reported by our group is summarised in Scheme 1B; thus,
cyclic β-keto esters (6) were shown to undergo a telescoped C-
acylation/deprotection sequence, resulting in the overall insertion
of β-amino acid derivatives (e.g. 7) into ring enlarged products
(8).[11] A key design feature of this method is that that the β-keto
ester group (highlighted) needed to promote the rearrangement is
regenerated upon ring expansion, meaning that controlled
‘Successive Ring Expansion’ (SuRE) can be achieved by
repeating the same acylation/deprotection sequence on the
product 8. This proof-of-concept study highlighted the potential of
SuRE-type methods, although its reliance on a relatively reactive
β-keto ester motif is a drawback, as this limits the utility of the
products; e.g. for medicinal applications, additional synthetic
steps are needed to remove the metabolically labile β-keto
Scheme 1. Macrocyclisation strategies and Successive Ring Expansion (SuRE).
The key ring expansion procedure was tested using 13-
membered lactam 9a, with the aim of performing an N-acylation
to form an imide (12), which could then rearrange to form 11a
upon protecting group cleavage as shown in Scheme 2.
Precedent for related transformations meant that these studies
15]
were undertaken with relatively high confidence;[14,
similar
transamidation-type ring expansion processes have been
reported by Hesse and coworkers[14a-b] and others,[14c-g] while
recent research by Szostak et al. demonstrates the surprising
ease with which N-acyl-Boc-carbamates undergo intermolecular
transamidation.[15] Thus, the N-acylation of lactam 9a was first
attempted using Fmoc-protected amino acid chloride 10a, and
was successfully achieved upon reaction with pyridine and DMAP
at reflux in CH2Cl2. The resulting imide 12 was then treated with
[a]
T. C. Stephens, Dr. M. Lodi, A. M. Steer, Y. Lin, M. T. Gill, Dr. W. P.
Unsworth*
University of York
York, YO10 5DD (UK)
E-mail: william.unsworth@york.ac.uk
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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DBU in CH2Cl2 at RT, which promoted concomitant protecting
group cleavage (12 → 13) and ring expansion (13 → 14 → 11a),
furnishing 17-membered ring lactam 11a in 91% yield over the
telescoped two-step sequence. Next, having confirmed the
viability of this ring expansion method, it was important to
establish whether it could be applied successively; to test this,
lactam 11a was reacted under the same conditions, and with no
additional optimisation, the 21-membered ring product 15a was
formed in 81% isolated yield. As the second ring expansion was
performed in the same way as the first, the product also contains
a secondary lactam group (highlighted), and hence is another
potential starting substrate for further SuRE iterations (vide infra).
rings,[9, 10] the formation of 11e and 11f using the standard method
is particularly pleasing.
Cyclic peptides and their mimics are of significant medicinal
interest,[13] hence we were keen to demonstrate that α-amino acid
derivatives could also be used in this method. α-Amino acid
derivatives proved to be much more challenging than their β-
homologues in our published β-keto ester SuRE reactions (which
only work consistently well with glycine derivatives),[11, 12] but
pleasingly, the current lactam ring expansions are far more robust.
Thus, N-acylation and ring expansion can be achieved with a
range of α-amino acid chlorides, including examples with
hydrocarbon side chains, tryptophan, methionine and proline
derivatives (11l–u, Scheme 3C).[18] Some variation in yield was
observed across this series, but note that these reactions were all
performed using the same conditions and are unoptimised. Cyclic
peptoids are also of interest for their medicinal properties,[19] and
we have demonstrated that 4 peptoid building blocks are well
tolerated, furnishing expanded lactams 11v–y in high yields
(Scheme 3D).
Selected special cases (Scheme 3E) illustrate that the
scope of this method is likely to extend beyond cyclic peptides.
For example, the same rearrangement has been demonstrated
on a linear amide (N-ethyl propanamide), to form diamide 11z. 2-
Azetidinone can also undergo N-acylation and ring expansion,
with a Cbz-protection strategy favoured in this case, as this
helped with the purification of the polar 8-membered ring product
11aa.[20] Lactone groups can also be installed into ring-expanded
products, by replacing the protected amine with a benzyl-
protected alcohol; following N-acylation, the alcohol is revealed
by hydrogenolysis to initiate ring expansion (11ab, 11ac).[21] We
anticipate that this will be useful for the synthesis of medicinally
important lactones, e.g. macrolide-class natural products and
their derivatives, which are well known for their anti-microbial
properties.[22] It is also worth noting that every ring expansion
reaction reported in this manuscript was performed using a 0.1 M
reaction concentration, and hence should be easier to scale than
typical high-dilution macrocycle syntheses.
Scheme 2. Successive ring expansion of lactam 9a.
One of the most important features of this research is the
fact that all the products shown in Scheme 3 are potential starting
materials for further ring expansion, as the secondary amide
group (highlighted) produced in the reaction provides a synthetic
handle for further iterations. To showcase this feature, 6
macrocycles formed via a second ring expansion reaction (15a–
f) are illustrated in Scheme 4. These reactions, which were
performed using the standard conditions, include macrocycles
made with α- and β-amino acid derivatives and peptoid building
blocks. Of course, these products represent a small fraction of
those potentially accessible; by using the full array of linear
fragments shown in Scheme 3, the synthesis of a much larger
array of functionalised macrocycles should be possible. Finally,
we have confirmed that ring expansion for a third time to form a
tripeptide system is also possible (16a). Additional acid chloride
10a was needed in this case to ensure that the N-acylation to
proceeded to completion, but the isolated yield (77%) remained
comparable with other examples.
Having uncovered an efficient new ring expansion method,
and proved that it can be applied successively, attention turned to
exploring its scope (Scheme 3). First, other β-amino acid
chlorides were tested in place of initial test substrate 10a (Scheme
3A). N-Methylated variant 11b was prepared in high yield (from
both Fmoc- and Cbz-protected amino acid chlorides), which is
important given that N-methylation is of much interest as a tool to
improve the druggability of cyclic peptides.[16] Branched β-amino
acids are also tolerated (e.g. 11c), as are protected primary amino
acids, exemplified by the formation of lactam 11d in high yield; of
further note, this example was repeated on larger scale (10 mmol)
with no significant drop in yield, and its structure was confirmed
by X-ray crystallography.[17] Importantly, the method works
consistently well across a range of ring sizes, with 6–12-
membered lactams all undergoing ring expansion in high yield
(78–90%) using acid chloride 10a (Scheme 3B). In view of the
well-known problems associated with making medium-sized
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Scheme 3. Lactam ring expansion scope. a Conditions A: i) To lactam 9 (1 equiv.), pyridine (6 equiv.) and DMAP (0.1 equiv.) in CH2Cl2 (7 mL/mmol of 12) add 10
(1.5 equiv.) in CH2Cl2 (3 mL/mmol of 12), 18 h at 50 °C; ii) DBU or piperidine (10 equiv.), CH2Cl2 (10 mL/mmol), RT, 18 h. b Conditions B: i) Acylation step as in
conditions A; ii) H2, Pd/C, EtOAc (10 mL/mmol), RT, 18 h. c Also performed on 10 mmol scale (≈2.5 g, 94%).
In summary, a new SuRE protocol for the formation of cyclic
peptide mimetics is described, based on the expansion of simple,
readily available lactams. The method is operationally simple,
Acknowledgements
scalable, and allows
a wide range of peptide-containing
macrocycles to be generated, typically in high yield. This is
expected to be of use in the synthesis of macrocycles with
medicinal applications, such as stapled peptides.[23] Cyclic
peptide mimetics are also of much medicinal interest, and this
method appears particularly well suited for the synthesis of this
compound class; macrocycles based on short amino acid
sequences and a non-peptidic portion are important in this
area,[13] with exemplar macrocycle classes with proven
therapeutic potential shown in Scheme 4 (17 and 18).[24] In this
manuscript, we have mainly focused on the potential of SuRE to
be an enabling technology in medicinal chemistry, but by
increasing the freedom with which researchers can design and
synthesise peptide-containing macrocycles, useful applications in
other areas[2-5] are also anticipated to emerge in time.
The authors thank the Leverhulme Trust (for an Early Career
Fellowship, ECF-2015-013, for W. P. U.) and the University of
York (T. C. S., A. M. S., Y. L., M. T. G., W. P. U.) for funding, the
Science and Engineering Research Board, Department of
Science & Technology, Government of India (for an overseas
postdoctoral fellowship, M. L.) and Dr. Adrian. C. Whitwood
(University of York) for X-ray crystallography.
Keywords: Macrocycles • Medium-sized Rings • Ring
Expansion • Cyclic peptides • Lactams
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Scheme 4. Successive Ring Expansion reactions (SuRE). a Conditions A: i) To lactam 9 (1 equiv.), pyridine (6 equiv.) and DMAP (0.1 equiv.) in CH2Cl2 (7
mL/mmol of 12) add 10 (1.5 equiv.) in CH2Cl2 (3 mL/mmol of 12),18 h at 50 °C; ii) DBU (10 equiv.), CH2Cl2 (10 mL/mmol), RT, 18 h. b 4.5 equiv. of 10a used.
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A successive ring expansion protocol
is reported that enables the controlled
insertion of natural and non-natural
amino acid fragments into lactams.
Amino acids can be installed into
macrocycles via an operationally
simple and scalable iterative
procedure, without the need for high
dilution. This method is expected to be
of broad utility, especially for the
synthesis of medicinally important
cyclic peptide mimetics.
Thomas C. Stephens, Mahendar Lodi,
Andrew M. Steer, Yun Lin, Matthew T.
Gill and William P. Unsworth*
Page No. – Page No.
Synthesis of cyclic peptide mimetics
via the successive ring expansion of
lactams
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Author(s), Corresponding Author(s)*
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Title
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