Customizable units in Di- and tripeptides: Selective conversion into substituted dehydroamino acids

Article abstract of DOI:10.1021/ol301676z

The selective conversion of serine or threonine units of di- and tripeptides into substituted dehydroamino acids is reported. Thus, these common α-amino acids undergo a scission-phosphorylation process to give α-amino phosphonate residues. A Horner-Wadsworth-Emmons reaction with aldehydes or ketones follows to afford the final products with excellent Z-stereoselectivity (Z:E > 98:2). In this way, a single peptide precursor can selectively be transformed into a variety of derivatives.

Full text of DOI:10.1021/ol301676z

ORGANIC
LETTERS
2012
Vol. 14, No. 14
3788–3791
“Customizable” Units in Di- and
Tripeptides: Selective Conversion into
Substituted Dehydroamino Acids
ꢀ
Carlos J. Saavedra, Alicia Boto,* and Rosendo Hernandez*
´ ´
Instituto de Productos Naturales y Agrobiologıa CSIC, Avda. Astrofısico Fco.
ꢀ
Sanchez 3, 38206 La Laguna, Tenerife, Spain
alicia@ipna.csic.es; rhernandez@ipna.csic.es
Received June 19, 2012
ABSTRACT
The selective conversion of serine or threonine units of di- and tripeptides into substituted dehydroamino acids is reported. Thus, these common
R-amino acids undergo a scissionꢀphosphorylation process to give R-amino phosphonate residues. A HornerꢀWadsworthꢀEmmons reaction
with aldehydes or ketones follows to afford the final products with excellent Z-stereoselectivity (Z:E > 98:2). In this way, a single peptide precursor
can selectively be transformed into a variety of derivatives.
Dehydroamino acids can be found in a variety of bio-
active peptides, such as the cyclic peptide tentoxin,1 the
lantibiotics2 and thiopeptideantibiotics,3 theproteaseinhi-
bitor somamide,4a the cytotoxic kahalalide F,4 yaku’amides,5
and dolastatin,6 the fungicide pseudomycin,7 and many
others.
In addition, the introduction of dehydroamino acids
into synthetic analogues of bioactive peptides can increase
their resistance to enzymatic degradation and allow the
modulation of their biological properties.8,9 Several drug
analogues with improved properties have been developed,9
such as gramicidin analogues with potent antibiotic but
much lower hemolytic activity9a and endorphine ana-
logues for pain control9b with high μ opioid receptor
selectivity.
The rigidity provided by dehydroamino acids could also
be useful to generate folded conformations for new mate-
rials or peptide catalysts.10
In order to generate libraries of peptides with dehydro-
amino acid units, each peptide is usually prepared de novo
from the starting amino acids. Herein, we report an
alternative strategy where a single parent peptide is
(1) (a) Nandy, J. P.; Prakesch, M.; Khadem, S.; Reddy, P. T.;
Sharma, U.; Arya, P. Chem. Rev. 2009, 109, 1999–2060. (b) Jimenez,
J. C.; Chavarria, B.; Lopez-Macia, A.; Royo, M.; Giralt, E.; Albericio,
F. Org. Lett. 2003, 5, 2115. (c) Loiseau, N.; Cavelier, F.; Noel, J. P.;
Gomis, J. M. J. Peptide Sci. 2002, 8, 335–346.
(2) (a) Chatterjee, C.; Paul, M.; Xie, L.; van der Donk, W. A. Chem.
Rev. 2005, 105, 633–683. (b) van der Donk, W. A. J. Org. Chem. 2006, 71,
9561–9571. (c) Okeley, N. M.; Zhu, Y.; van der Donk, W. A. Org. Lett.
2000, 2, 3603–3609. (d) Zhang, X.; Ni, W.; van der Donk, W. A. Org.
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Chem. Soc. 2010, 132, 7519–7527. (b) Zhang, C.; Occi, J.; Masurekar, P.;
Barrett, J. F.; Zink, D. L.; Smith, S.; Onishi, R.; Ha, S. H.; Salazar, O.;
Genilloud, O.; Basilio, A.; Vicente, F.; Gill, C.; Hickey, E. J.; Dorso, K.;
Motyl, M.; Singh, S. B. J. Am. Chem. Soc. 2008, 130, 12102–12110. (c)
Bagley, M. C.; Bashford, K. E.; Hesketh, C. L.; Moody, C. J. J. Am.
Chem. Soc. 2000, 122, 3301–3313. (d) Liao, R.; Liu, W. J. Am. Chem.
Soc. 2011, 133, 2852–2855.
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Cruz, L. J.; Luque-Ortega, J. R.; Rivas, L.; Albericio, F. Mol. Pharmac.
2009, 6, 813–824.
(5) Ueoka, R.; Ise, Y.; Ohtsuka, S.; Okada, S.; Yamori, T.; Matsunaga,
S. J. Am. Chem. Soc. 2010, 132, 17692–17694.
(6) Freidinger, R. M. J. Med. Chem. 2003, 46, 5554–5566.
(7) Sun, X.; Rodriguez, M.; Zeckner, D.; Sachs, B.; Current, W.;
Boyer, R.; Paschal, J.; McMillian, C.; Chen, S.-H. J. Med. Chem. 2001,
44, 2671–2674.
(8) For reviews on the subject, see: (a) Gupta, M.; Chauhan, V. S.
Biopolymers 2010, 95, 161–173. (b) Jain, R.; Chauhan, V. S. Biopolymers
(Pept. Sci.) 1996, 40, 105–119.
(9) (a) Yamada, K.; Shinoda, S.; Oku, H.; Komagoe, K.; Katsu, T.;
Katakai, R. J. Med. Chem. 2006, 49, 7592–7595. (b) Torino, D.; Mollica,
A.; Pinnen, F.; Feliciani, F.; Lucente, G.; Fabrizi, G.; Portalone, G.;
Davis, P.; Lai, J.; Ma, S. W.; Porreca, F.; Hruby, V. J. J. Med. Chem.
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r
10.1021/ol301676z
Published on Web 07/11/2012
2012 American Chemical Society

transformed into a variety of derivatives by selective
conversion of certain R-amino acid units (serine or
threonine) into β-substituted dehydroamino acids.
Recently, the use of “customizable” (or “tunable”)
amino acids in the site-selective modification of peptides
has elicited much interest.11,12 This selective approach
requires less time and materials than the conventional de
novo synthesis. For instance, Seebach has reported the
selective alkylation of enolates from N-alkylglycines,12a
while Kazmaier has achieved the stereoselective allylation
and alkylation of glycine residues in dipeptides.12b,c Klok
has described the addition of S-radicals to allylglycines in
peptides with 4ꢀ16 residues,12d and Skrydstrup has gen-
erated enolates in “tunable” residues of di- to tetrapep-
tides, which were trapped by electrophiles.11a,12e
Scheme 1. Site-Selective Scission of Serine Residues and Addi-
tion of P-Nucleophiles
Despite these advances, the site-selective modification of
peptides remains difficult,11 even for small peptides, be-
cause of the similar reactivity of the amino acid units. The
task is particularly difficult when several units of the
“tunable” amino acid (glycine, dehydroamino acids, etc)
are present in the peptide. The use of serine (or threonine)
residues as customizable units solves this problem, since
the lateral chains of different serine units can be protected
with orthogonal groups. Thus, free serine residues would
be selectively transformed, while the protected ones would
remain unchanged.
To determine the feasibility of this approach to obtain a
variety of peptides with dehydroamino acid units, we used
the strategy shown in Scheme 1 (conversion 1f2). Thus,
peptide 1 would undergo the radical scission of serine (or
threonine) to give a glycyl radical, which would be oxidized
in situ to a cation, and the latter would be trapped by
phosphorus nucleophiles to give the aminophosphonate 3.
Then, a HornerꢀWadsworthꢀEmmons reaction with dif-
ferent aldehydes or ketones would afford peptides with
dehydroamino acids 2.
Since the decarboxylation is much more favored than the
radical scission of alcohols (in particular, primary alcohols
such as serine), there were concerns that the scissionꢀ
phosphorylation process would not work as desired or that
side reactions (H-abstraction, oxidation of the alcohol,
cleavage of the peptide chain) would take place.14,15
The selective radical scissionꢀoxidation was studied
with peptides 4 and 5 (Scheme 2), which present two serine
residues or a serine/threonine pair. Using the reported
procedure [(diacetoxyiodo)benzene (DIB)/I2, hν, 26 °C,
2ꢀ4 h, then 0 °C, Lewis acid, nucleophile],13,15 a complex
mixture of compounds was formed, due either to side
reactions or to the formation of unstable scission products,
such as a peptide with an R-acetoxyglycine unit.
In order to determine whether the low yields were due to
the generation of unstable N,O-intermediates or to other
causes, the scissionꢀoxidation was followed by addition of
methanol, since this nucleophile usually adds in good to
excellent yields, providing stable methoxy acetals.16 There-
fore, peptide 4 was treated with PhI(OAc)2 (DIB) and
iodine under irradiation with visible light, affording the
methoxy derivative 6 in improved but still moderate yield
(<40%).
For the first step, we used a variation of our reported
amino acid decarboxylationꢀphosphorylation process.13
(10) For different applications of dehydroamino acid derivatives, see:
(a) Chen, H.; Luzy, J. P.; Gresh, N.; Garbay, C. Eur. J. Org. Chem. 2006,
2329–2335. (b) Baldisserotto, A.; Marastoni, M.; Lazzari, I.; Trapella,
C.; Gavioli, R.; Tomatis, R. Eur. J. Med. Chem. 2008, 43, 1403–1411. (c)
Jones, M. C.; Marsden, S. P. Org. Lett. 2008, 10, 4125–4128. (d) van den
Broek, S. M. A. W.; Rensen, P. G. W.; van Delft, F. L.; Rutjes, F. P. J. T.
Eur. J. Org. Chem. 2010, 5906–5912. (e) Wang, C. J.; Xu, Z. P.; Wang,
X.; Teng, H. L. Tetrahedron 2010, 66, 3702–3706. (f) Ramapanicker, R.;
Mishra, R.; Chandrasekaran, S. J. Pept. Sci. 2010, 16, 123–125.
(11) (a) Ebran, J. P.; Jensen, C. M.; Johannesen, S. A.; Karaffa, J.;
Lindsay, K. B.; Taaning, R.; Skrydstrup, T. Org. Biomol. Chem. 2006, 4,
3553–3564. (b) Antos, J. M.; Francis, M. B. Curr. Opin. Chem. Biol.
2006, 10, 253–262. (c) Qi, D.; Tann, C. M.; Distefano, M. D. Chem. Rev.
2001, 101, 3081–3112.
(12) (a) Seebach, D.; Bech, A. K.; Studer, A. Modern Synthetic
Methods; Ernst, B., Leumann, C., Eds.; VCH: Weinheim, 1995; Vol. 7. (b)
Datta, S.; Kazmaier, U. Org. Biomol. Chem. 2011, 9, 872–880. (c) Deska,
J.; Kazmaier, U. Chem.;Eur. J. 2007, 13, 6204–6211. (d) Franz, N.;
Menin, L.; Klok, H. A. Org. Biomol. Chem. 2009, 7, 5207–5218. (e) Ricci,
M.; Madariaga, L.; Skrydstrup, T. Angew. Chem., Int. Ed. 2000, 39, 242–
245. (f) For other interesting approaches, see: Chapman, C. J.; Hargrave,
J. D.; Bish, G.; Frost, C. G. Tetrahedron 2008, 64, 9528–9539. (g) Wan,
Q.; Danishefsky, S. J. Angew. Chem., Int. Ed. 2007, 46, 9248–9252.
(h) Dialer, H.; Steglich, W.; Beck, W. Tetrahedron 2001, 57, 4855–4861.
(14) In other complex substrates, such as carbohydrates, the
ꢀ
H-abstraction often predominates over the scission: Boto, A.; Hernandez,
ꢀ
ꢀ
D.; Hernandez, R.; Suarez, E. J. Org. Chem. 2006, 71, 1938–1948.
(15) (a) For other work on scissionꢀaddition processes, see: Saave-
dra, C.; Boto, A.; Hernandez, R. Org. Biomol. Chem. 2012, 10, 4448–
4461. (b) Boto, A.; Romero-Estudillo, I. Org. Lett. 2011, 13, 3426–3429.
(c) Saavedra, C.; Hernandez, R.; Boto, A.; Alvarez, E. J. Org. Chem.
€
ꢀ
2009, 74, 4655–4665. (d) Boto, A.; Gallardo, J. A.; Hernandez, D.;
ꢀ
Hernandez, R. J. Org. Chem. 2007, 72, 7260–7269.
(i) Schuemann, S.; Zeitler, K.; Jager, M.; Polborn, K.; Steglich, W.
Tetrahedron 2000, 56, 4187–4195.
(13) Boto, A.; Gallardo, J. A.; Hernandez, R.; Saavedra, C. J.
ꢀ
ꢀ
ꢀ
(16) Boto, A; Hernandez, D.; Hernandez, R. Eur. J. Org. Chem. 2010,
3847–3857 and references cited therein.
Tetrahedron Lett. 2005, 46, 7807–7811.
Org. Lett., Vol. 14, No. 14, 2012
3789

Scheme 2. Formation of a Set of Compounds from a Peptide Precursor by Selective Conversion of Ser and Thr Units into
Dehydroamino Acids
Fortunately, when the system DIBꢀiodine was replaced
byleadtetraacetate(LTA) and iodine, a singleproduct was
formed. After purification by chromatography, the dipep-
tide 6 was isolated in 63% yield. Interestingly, the scission
of the threonine analogue 5 resulted in increased yield
(74%). To account for this result, both the scission and the
addition of methanol must have proceeded in good to
excellent yields.
The conversion of dipeptide 6 into the aminophospho-
nate 7 was studied under several conditions, using different
phosphorus nucleophiles and Lewis acids. The best results
were obtained with P(OEt)3 and TMSOTf, affording
compound 7 in 73% yield.
The aminophosphonate 7 underwent the Hornerꢀ
WadsworthꢀEmmons reaction with dihydrocinnamal-
dehyde to give the dehydro(phenyl)norvaline 8 in good
yield and excellent Z-stereoselectivity. The reaction
also proceeded with ketones (acetone) to give the
dehydrovaline derivative 9. It should be noticed that
in both cases the protected N-terminal serine unit
remained unaffected.
In a similar way, the Cbz-protected peptide 10
(Scheme 2) underwent the scissionꢀaddition of methanol
process, affording the methoxyglycine derivative 11 in
78% yield. Then compound 11 was converted into the
aminophosphonate 12 in good yield.
Product 12 was treated with different aryl and akyl
aldehydes17 to provide compounds 13 and 14, which
present units of dehydrophenyl alanine and dehydroleu-
cine, respectively. The reaction also proceeded with acet-
one to give the dehydrovaline derivative 15 in good yield.
In the case of compounds 13 and 14, the process took
place with complete stereoselectivity to give the Z-isomers.
An important concern was that the basic reaction condi-
tions would produce the epimerization of the adjacent
amino acid(s). We compared the optical activity of com-
pound 13 and a sample formed by coupling the leucine and
(17) (a) Buck, R. T.; Clarke, P. A.; Coe, D. M.; Drysdale, M. J.;
Ferris, L.; Haigh, D.; Moody, C. J.; Pearson, N. D.; Swann, E. Chem.;
Eur. J. 2000, 6, 2160–2167. (b) See also: Alexander, P. A.; Marsden, S. P.;
~
Munoz-Subtil, D. M.; Reade, J. C. Org. Lett. 2004, 6, 1433–1436.
3790
Org. Lett., Vol. 14, No. 14, 2012

Scheme 3. Simplified ScissionꢀPhosphorylation Procedure and Application to the Synthesis of Modified Peptides
the Z-dehydrophenylalanine units;18a to our satisfaction,
both activities matched completely.
dehydroamino acids with excellent (Z) stereoselectivity,
and no epimerization of other positions was observed.
This methodologyallows for the preparation ofa variety
of peptide derivatives from a single precursor. The process
takes place under mild conditions in good yields.
The use of the serine (or threonine) units to generate
diversity is particularly interesting. Since its hydroxy-
methylene group can be protected with different orthogo-
nal groups, thestarting peptide could contain several serine
residues, but only the unprotected one(s) would be mod-
ified. For further modifications, the orthogonal protecting
groups could be sequentially removed. The application of
this methodology to the synthesis of other peptides of
different sizes will be reported in due course.
We reasoned that a simplification of the previous pro-
cedure could allow the direct transformation of peptide
1018 into the phosphonate 12. Thus, the dipeptide 10
(Scheme 3) underwent scission with LTA/I2 followed by
aqueous workup. The intermediate was not purified but
treated with the phosphorylation reagents to give com-
pound 12 in 54% yield (the global yield for the two-step
procedure was 56%). The aminophosphonate 12 was
treated with dihydrocinnamaldehyde to give the dehydro-
(phenyl)norvaline derivative 16 in 74% yield.
Finally, we studied the process with tripeptide 17 where the
customizable unit is threonine. We are interested in polyleu-
cine-substituted peptides where one of the residues is replaced
by a dehydroamino acid since these derivatives present inter-
esting conformational and biological properties.19
Acknowledgment. This work was supported by the
Research Program CTQ2009-07109 of the Plan Nacional
ꢀ
The tripeptide 17 underwent the simplified scissionꢀ
phosphorylation process to give the aminophosphonate 18
in good yield. Then, compound 18 was treated with
dihydrocinnamaldehyde to give the dehydro(phenyl)nor-
valine derivative 19.
de IþD, Ministerio de Ciencia e Innovacion (currently
Economıa y Competitividad), Spain, and European Re-
´
gional Development Funds (FEDER). C.J.S. thanks Go-
bierno de Canarias for a predoctoral grant.
In summary, a scission of serine/threonine unitsꢀ
phosphorylation process was developed, which is suitable
for the selective modification of peptides; other alterna-
tive procedures reported in the literature do not work
with these substrates. The resulting aminophosphonates
underwent a HornerꢀWadsworthꢀEmmons reaction
with aldehydes or ketones to give the corresponding
Supporting Information Available. Procedures for the
synthesis of substrates 4, 5, and 17. Formation of the R-
methoxyglycine derivatives 4, 5, and 11, the amino phos-
phonates 7, 12, and 18, and the dehydroamino acid-
1
containing peptides 8, 9, 13ꢀ16, and 19. H and 13C
NMR spectra of products 4ꢀ9 and 11ꢀ19. NOE experi-
ments for compounds 8, 13, 14, 16, and 19. This material
is available free of charge via the Internet at http://pubs.
acs.org
(18) (a) Falorni, M.; Giacomelli, G.; Satta, M.; Cossu, S. Synthesis
1994, 391–395. (b) Falorni, M.; Satta, M.; Conti, S.; Giacomelli, G.
Tetrahedron: Asymmetry 1993, 4, 2389–2398.
(19) Nandel, F. S.; Jaswal, R. Biomacromolecules 2007, 8, 3093–3101.
The authors declare no competing financial interest.
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