A pincer auxiliary to force difficult lactamisations.

Article abstract of DOI:10.1039/b303836j

A new method to cyclise peptides is reported based on insertion of a salicylaldehyde derived pincer auxiliary in the linear precursor sequence. H-betaAla-Phe-OH and H-Phe-betaAla-OH were chosen as representative model peptides.

Full text of DOI:10.1039/b303836j

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A pincer auxiliary to force difficult lactamisations†
Hans Bieräugel,a Hans E. Schoemaker,b Henk Hiemstraa and Jan H. van Maarseveen*a
a Institute of Molecular Chemistry, University of Amsterdam, Nieuwe Achtergracht 129,
1018 WS Amsterdam, The Netherlands. E-mail: jvm@science.uva.nl
b DSM Research, Life Science Products, PO Box 18, 6160 MD Geleen, The Netherlands
Received 7th April 2003, Accepted 7th May 2003
First published as an Advance Article on the web 13th May 2003
A new method to cyclise peptides is reported based on
insertion of a salicylaldehyde derived pincer auxiliary in
the linear precursor sequence. H-ꢀAla-Phe-OH and H-Phe-
ꢀAla-OH were chosen as representative model peptides.
combined tethered–templated strategy to cyclise medium-sized
lactams.10 In our search for new sequence independent methods
for small peptide cyclisations we report an improved auxiliary,
giving access to lactams containing multiple endocyclic amide
bonds. Our approach is based on the use of a pincer auxiliary
that facilitates two subsequent lactamisations by playing a dual
tethering and templating role (strategy B, Scheme 1).
Cyclic peptides constitute
a
versatile compound class.1
Especially small cyclic peptides are promising in drug,2
materials3 and catalysis applications.4 However, in general
head-to-tail ring-closure of small cyclopeptides (strategy A,
Scheme 1) has proven to be difficult.5 In addition and in vast
contrast to the cyclodipeptides (diketopiperazines), the
majority of the (7-membered) monocyclic homodiketopiper-
azines, made up of an α- and a linear β-amino acid, are difficult
to cyclise. Consequently, only a limited number of cyclotetra-
peptides6 and homodiketopiperazines7 have been described.
These rings have been closed successfully only due to the
presence of turn-inducing residues and the application of high-
dilution conditions. Also, the correct choice of the coupling site
has shown to be crucial. To say it concisely, to date, no generally
applicable and thus sequence independent methods exist to
effect small peptide cyclisations. The main reason for the cyclis-
ation failure is the predominant trans arrangement of the amide
bond(s) in the linear precursor, preventing a correct spatial
positioning of the terminal amine and the activated carboxylic
group for cyclisation.5,8 Unexpectedly, the inclusion of second-
ary amino acids (e.g. proline or sarcosine) in the linear pre-
cursor sequence, resulting in a conformationally unbiased
tertiary amide bond, does not necessarily lead to efficient
cyclisations.9
In strategy B, in the first macrolactamisation step (i.e. 3 4)
the auxiliary serves as an internal tether (or hinge) allowing an
unconstrained approach of the mutually reactive amino and
activated ester groups, followed by the final templated lactamis-
ation by a transannular ring-contracting O N acyl transfer
reaction (i.e. 4 1).
Another strategy (strategy C, Scheme 1), which has been
applied successfully in the cyclisation of pentapeptides in a
pioneering study by Meutermans and coworkers,11 relies on
functionalisation of the N-terminus of the peptide with a
similar auxiliary now serving as an external tether. Activation
of the carboxylic acid leads to a macrolactonisation reaction
(i.e. 5 4), still hampered by the predominant trans amide
bonds in the linear precursor, followed by the same templated
transannular ring-contraction to give the final lactam.
In the present communication the power of our novel
internal auxiliary based method (strategy B, Scheme 1) will be
demonstrated as compared to the other strategies (strategies A
and C, Scheme 1). In our study the monocyclic homodiketo-
piperazine 3-benzyl[1,4]diazepane-2,5-dione (or cyclo(-Phe-
βAla-)) 1a, composed of phenylalanine and β-alanine, was used
as a difficult to lactamise model cyclopeptide (Scheme 2).
Strategy A: That indeed ring-closure to give cyclo(-Phe-
βAla-) 1a by direct head-to-tail lactamisation of H-βAla-
Phe-OH 2a or H-Phe-βAla-OH 2b (Scheme 3) is difficult is
underscored by the finding that no cyclisation could be
Recently, we have developed a novel auxiliary mediated
† Electronic supplementary information (ESI) available: experimental
details. See http://www.rsc.org/suppdata/ob/b3/b303836j/
Scheme 1 Cyclisation strategies towards bislactams.
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 1 8 3 0 – 1 8 3 2
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 3
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liberation of the carboxylic acid and amine functional groups
by catalytic hydrogenolysis under neutral conditions, the
tethered lactamisation precursors 3a and 3b were isolated as
stable compounds.
The introduction of the sterically demanding tert-butoxy
group at the ortho position with respect to the ester group has
shown to be crucial because preliminary experiments, starting
from salicylaldehyde, suffered from extensive side-reactions due
to premature intermolecular aminolysis.13
Scheme 2 Target homodiketopiperazine 1a.
Activation of the carboxylic acids of 3a and 3b by treatment
with EDC–HOBt effected the tethered cyclisation reactions to
give the macrocyclic lactams 4a and 4b in yields of 84% and
71%, respectively. Removal of the Boc and tert-butyl protective
groups by stirring in TFA–dichloromethane was followed by
the addition of solid NaHCO3 to induce the final templated
transannular O N acyl transfer reaction furnishing the final
homodiketopiperazines 1g and 1h, both in yields of 79%.
These results show that our approach, featuring an auxiliary
which is inserted in the peptide bond, and serves as a tethering
hinge or molecular pincer facilitating the mutually reactive
terminal functional groups to approach one and another,
is sequence independent. Also, due to the connection of the
auxiliary onto the C-terminal amino acid as a tertiary amide
(i.e. compounds 3), this residue is shielded from racemisation,
which often occurs with difficult peptide cyclisations.5
Scheme 3 Strategy A; direct cyclisation. Reagents and conditions:
EDC (5 equiv), HOBt (4 equiv), DMF–CH2Cl2 (1:4), RT.
accomplished using the most powerful coupling reagents
(i.e. EDC–HOBt, BOP, TBTU and PfPyU) at high dilution.
Lactamisation of the amide N-benzylated precursors H-βAla-
BnPhe-OH 2c and H-Phe-BnβAla-OH 2d, employing a tertiary
amide bond under high-dilution conditions, indeed gave the
corresponding homodiketopiperazines cyclo(-βAla-BnPhe-) 1c
and cyclo(-Phe-BnβAla-) 1d, albeit in yields of only 11% and
30%, respectively.
Strategy B: The recent results published by Meutermans and
coworkers11 and our preliminary study,10 prompted us to use
salicylaldehyde-derived auxiliaries for the combined tethered–
templated approaches towards cyclo(-Phe-βAla-) 1a. Salicyl-
aldehyde derived auxiliaries combine the high reactivity of
an aryl ester towards aminolysis (enthalpic activation) with
a correct spatial positioning of the secondary amine and
carbonyl functional groups for the transannular ring-
contraction reaction (entropic activation).
Removal of the remaining 1-(2,3-dihydroxybenzyl) group
was accomplished by reduction with sodium in liquid ammonia
after methylation of the phenolic-OH groups to give cyclo-
(-Phe-βAla-) 1a in overall yields of 57% (from 1g) and 65%
(from 1h) (Scheme 4).
Strategy C: For the synthesis of 1a via strategy C, the
N-termini of H-βAla-Phe-OBn and H-Phe-βAla-OBn were
reductively alkylated with salicylaldehyde 7, followed by
liberation of the carboxyl group by catalytic hydrogenolysis
to furnish 5a and 5b, respectively.
The consecutive macrolactonisation and ring-contractive
lactamisation reactions were conducted using the optimised
conditions as developed by Meutermans and coworkers.11
Activation of 5a with BOP (1.1 equiv) using DiPEA (2 equiv)
as the base in DMF under high dilution (1 mM) only gave
complex reaction mixtures. On the other hand, 5b cyclised
smoothly to give homodiketopiperazine 1f in a yield of 65%
(Scheme 5). Although the reason for the cyclisation failure
of 5a is unclear to us,14 it is obvious that strategy B, featuring
an external tethering auxiliary, is strongly sequence dependent.
In conclusion, we have developed a new and powerful
auxiliary mediated lactamisation strategy towards small
cyclopeptides. This was demonstrated by the successful
Synthesis of the linear cyclisation precursors 3a and 3b to test
our internal tethered strategy was accomplished by a reductive
amination reaction of H-βAla-OBn and H-Phe-OBn with
3-tert-butoxy-2-hydroxybenzaldehyde 6,12 followed by Boc-
protection of the resulting amine and esterification with
Cbz-Phe-OH or Cbz-βAla-OH, respectively (Scheme 4). After
Scheme 4 Strategy B; internal tether–template. Reagents and conditions: i, H-βAla-OBn or H-Phe-OBn, Na(OAc)3BH, THF; ii, (Boc)2O, CH2Cl2;
iii, Cbz-Phe-OH or Cbz-βAla-OH, DCC, DMAP, CH2Cl2; iv, H2, Pd/C, EtOAc–iPrOH = 4 : 1; v, EDC–HOBt (5 equiv); vi, TFA–CH2Cl2 = 1 : 1;
vii, NaHCO3, EtOAc; viii, MeI, K2CO3, followed by Na, NH3(l).
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Scheme
5
Strategy C; external tether–template. Reagents and
conditions: i, H-βAla-Phe-OBn or H-Phe-βAla-OBn, Na(OAc)3BH,
CH2Cl2; ii, H2, Pd/C, EtOAc–iPrOH = 4 : 1; iii, BOP (1.1 equiv), DiPEA
(2 equiv), DMF (1 mM).
sequence independent ring-closure to a homodiketopiperazine,
which could not be prepared efficiently using the currently
known lactamisation methods. Currently, our novel internal
auxiliary mediated combined tethered–templated approach
is expanded towards the synthesis, of to date, inaccessible
homodetic and heterodetic cyclotetrapeptides.
12 The synthesis of the novel auxiliary 6 was easily accomplished in one
step from commercially available 2,3-dihydroxybenzaldehyde (see
electronic supplementary material).†
Acknowledgements
Dr. Olivier David is acknowledged for the helpful discussions.
We thank Mr. A. van den Hoogenband and Mr. K. Stegman
(Solvay Pharmaceuticals, Weesp, The Netherlands) for measur-
ing the ESI-MS spectra.
13 The shielding effect of ortho-alkoxy groups for aminolysis reactions
of aryl esters has been described before. After removal of the
tert-butyl group, the reactivity of the phenolic ester is enhanced
due to the formation of a hydrogen bond between the ortho-phenolic
OH and the ester carbonyl group. See: (a) G. T. Bourne, S. W.
Golding, R. P. McGeary, W. D. F. Meutermans, A. Jones,
G. R. Marshall, P. F. Alewood and M. L. Smythe, J. Org. Chem.,
2001, 66, 7706–7713; (b) C. L. Beech, J. F. Coope, G. Fairley,
P. S. Gilbert, B. G. Main and K. Plé, J. Org. Chem., 2001, 66,
2240–2245.
14 In pentapeptide cyclisation studies it has been shown that steric
effects at the C-terminus play an important role, although not so
pronounced as found by us. See: X.-M. Gao, Y.-H. Ye, M. Bernd and
B. Kutscher, J. Peptide Sci., 2002, 8, 418–430.
Notes and references
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