Solid-phase synthesis of conformationally constrained peptidomimetics based on a 3,6-disubstituted-1,4-diazepan-2,5-dione core

Article abstract of DOI:10.1021/jo034785d

Starting from a Cl-trytyl linked hydroxylamine, a hydroxamic dipeptide having serine in the second position was prepared by using DMTMM as the coupling agent. Mitsunobu cyclization carried out under microwave heating gave very good yields of a 3,6-disubstituted-perhydro-diazepin-2,5-dione. This heterocycle can be used as a new platform for combinatorial chemistry or as a constraint to rigidify a small peptide.

Full text of DOI:10.1021/jo034785d

concerning the seven-member diazepane ring.6 Although
this heterocycle offers an attractive structure for the
development of new biologically active compounds,7 all
the syntheses reported until now describe the preparation
of 1,4-diazepan-2,5-diones. The syntheses proceeded
through the lactonization of the terminal NH2 and the
â-COOH of a dipeptide containing aspartic acid6 or the
head-to-tail cyclization of a dipeptide containing phenyl-
alanine.8
Looking for a new general and versatile synthesis of
this class of heterocycles, we decided to follow an alterna-
tive route based on the construction of a serine-containing
dipeptide linked to a hydroxylamine resin followed by
cyclization of the hydroxyhydroxamate under Mitsunobu
conditions.9 This approach provides the new 3,6-disub-
stituted-diazepan-2,5-dione skeleton having an amino
group in position 6 that can be employed for the elonga-
tion of the peptide chain or to increase the molecular
diversity. Moreover, if the substituent in position 3
carries a protected carboxylic group, it is possible to have
two sites for peptide elongation.
To optimize the reaction conditions for each step we
decided to intially explore the cyclization of a simple
model compound in the homogeneous phase. Thus,
O-benzylhydroxylamine was reacted with N-Boc-PheOH
in the presence of DMTMM as the coupling agent.10
The hydroxamate 1 was deprotected (TFA/CH2Cl2/Et3SiH
1/1/0.1) and further coupled with Boc-Ser-OH. Product
2, isolated in 72% yield after a simple aqueous workup,
was ready to undergo the cyclization.
The first attempt was made with use of classical
Mitsunobu conditions, DEAD/PPh3 in THF at room tem-
perature.11 Cyclization occurred after an overnight reac-
tion but compound 3 was isolated, after column chroma-
tography, in 31% yield. The other compounds recovered
in the reaction mixture were unreacted starting material
and the alkylated hydrazine dicarboxylate 4 typical of a
tricky Mitsunobu reaction.12 Attempts to change the
Solid -P h a se Syn th esis of Con for m a tion a lly
Con str a in ed P ep tid om im etics Ba sed on a
3,6-Disu bstitu ted -1,4-d ia zep a n -2,5-d ion e
Cor e
Lucia Raffaella Lampariello,‡ Daniela Piras,§
Manuela Rodriquez,‡ and Maurizio Taddei*,‡
Dipartimento di Chimica, Universita` degli Studi
di Sassari, Via Vienna 2, I-07100 Sassari, Italy, and
Dipartimento di Chimica and Dipartimento Farmaco
Chimico Tecnologico, Universita` degli Studi di Siena,
Via A. Moro, 53100 Siena, Italy
taddei.m@unisi.it
Received J une 9, 2003
Abstr a ct: Starting from a Cl-trytyl linked hydroxylamine,
a hydroxamic dipeptide having serine in the second position
was prepared by using DMTMM as the coupling agent.
Mitsunobu cyclization carried out under microwave heating
gave very good yields of a 3,6-disubstituted-perhydro-diaze-
pin-2,5-dione. This heterocycle can be used as a new plat-
form for combinatorial chemistry or as a constraint to
rigidify a small peptide.
In the search for new and more effective peptidomi-
metics, the synthesis of peptides containing a ring system
that acts as a conformationally restricted core has been
extensively employed to prepare new leads for the drug
discovery process.1 When inserted into peptide sequences,
such constraint can enhance the biological properties as,
for example, the binding with the active site or the
stability of the modified peptide to endopeptidases.
Different approaches have been pursued to introduce a
(macro)cyclic structure inside a peptide chain. Head-to-
tail,2 residue-to-residue,3 and residue-to-backbone cy-
clization4 are the most common strategies employing
natural R-amino acids that also can be carried out on
solid phase.
(5) For a recent review on diketopiperazine synthesis see: Dinsmore,
C. J .; Beshore, D. C. Tetrahedron 2002, 58, 3297. For diketopyperazine
as peptidomimetics see: Creighton, C. J .; Zapf, C. W.; Goodman, M.
Org. Lett. 1999, 1, 1407. Baures, P. W.; Ojala, W. H.; Costain, W. J .;
Ott, M. C.; Pradhan, A.; Gleason, W. B.; Mishra, R. K.; J ohnson, R. L.
J . Med. Chem. 1997, 40, 3594.
(6) Curran, T. P.; McEnaney Tetrahedron Lett 1995, 36, 191.
Krchnak, V.; Weichesel, A. S. Tetrahedron Lett. 1997, 38, 7299. Nefzi,
A.; Ostresh, J . M.; Houghten, R. A. Tetrahedron Lett. 1997, 38, 4943.
Hulme, C.; Morrissette, M. M.; Volz, F. A.; Burns, C. J . Tetrahedron
Lett. 1998, 39, 1113.
(7) The 1,4-diazepan-2,5-dione is present, for example, in the anti
methicillin-resitant becteria antibiotic TAN-1057C: Yuan, C.; Wil-
liams, R. M. J . Am. Chem. Soc. 1997, 119, 11777. Nefzi, A.; Dooley,
C.; Ostrech, J . M.; Houghten, R. A. Bioorg. Med. Chem. Lett. 1998, 8,
2273.
To get an effective constraint, the introduction of rela-
tively small or medium size rings is preferred. Whereas
the six-member diketopiperazine ring has been largely
employed for this goal,5 there are few literature reports
‡ Dipartimento di Chimica, Universita` degli Studi di Siena.
§ Dipartimento di Chimica, Universita` degli Studi di Sassari.
(1) Giannis, A.; Kolter, T. Angew. Chem., Int. Ed. Engl. 1993, 32,
1244. Goodman, M.; Zhang, J . Chemtracts-Org. Chem. 1997, 10, 629.
Kleber-Emmons, T.; Murali, R.; Greene, T. Curr. Op. Biotech. 1997,
8, 435. Kazmierski, W. M., Walker, J . M.; Eds. Methods in Molecular
Medicine; Vol 30, Peptidomimetics Protocols; Human Press: Totowa,
NJ , 1999.
(2) Hruby, V. J .; Al-Obeidi, F.; Kazmierski, W. Biochem. J . 1990,
268, 249. See also: Cavelier-Frontin, F.; Achmad, S.; Verducci, J .;
J acquier, R.; Pepe, G. J . Mol. Struct (THEOCHEM) 1993, 286, 125.
(3) Loyd-Williams, P.; Albericio, F.; Giralt, E. Tetrahedron 1993, 49,
11065.
(4) Miller, S. J .; Grubbs, R. H. J . Am. Chem. Soc. 1995, 117, 5855.
Miller, S. J .; Blackwell, H. E.; Grubbs, R. H. J . Am. Chem. Soc. 1996,
118, 9606. Reichwein, J . F.; Liskamp, R. M. J . Eur. J . Org. Chem. 2000,
2335. Blackwell, H. E.; Sadowsky, J . D.; Howard, R. J .; Sampson, J .
N.; Chao, J . A.; Steinmetz, W. E.; O’Leary, D. J .; Grubbs, R. H. J . Org.
Chem. 2001, 66, 5291. Creighton, C. J .; Reitz, A. B. Org. Lett. 2001, 3,
895. Hanessian, S.; Angiolini, M. Chem. Eur. J . 2002, 8, 111. Saha,
B.; Das, D.; Banertji, B.; Iqbal, J . Tetrahedron Lett. 2002, 43, 6467.
(8) El Mahdi, O.; Lavergne, J .-P.; Martinez, J .; Viallefont, P.;
Essassi, E. M.; Riche, C. Eur. J . Org. Chem. 2000, 251. Giovannoni,
J .; Subra, G.; Amblard, M.; Martinez, J . Tetrahedron Lett 2001, 42,
5389.
(9) Miller, M. J .; Mattingly, P. G.; Morrison, M. A.; Kerwin, J . F. J .
Am. Chem. Soc. 1980, 102, 7026.
(10) DMTMM: 4-(4,6-dimethoxy-1,3,5-triazin-2-yl)-4-methyl-mor-
pholinium chloride. Kunishima, M.; Kawachi, G.; Morita, J .; Terao,
K.; Iwasaki, F.; Tani, S. Tetrahedron 1999, 55, 13159. Falchi, A.;
Giacomelli, G.; Porcheddu, A.; Taddei, M. Synlett 2000, 277. DMTMM
is commercially available from Acros Organics.
(11) Mitsunobu, O. Synthesis 1981, 1.
(12) Kolasa, T.; Miller, M. J . J . Org. Chem. 1987, 52, 4978.
10.1021/jo034785d CCC: $25.00 © 2003 American Chemical Society
Published on Web 08/30/2003
J . Org. Chem. 2003, 68, 7893-7895
7893

TABLE 1. Cycliza tion Con d ition s
SCHEME 2. Solid -P h a se Syn th esis of
1,4-Dia zep a n -2,5-d ion ea
yieldsa
reagents
conditions
THF, rt, 12 h
THF, rt, 12 h
THF, rt, 12 h
THF, rt, 12 h
toluene, reflux, 12 h
DMF, rt, 12 h
3, 4
DEAD/PPh3 1.1.
DIAD/PPh3 1:1
DTAD/PPh3 1:1
DTAD/PPh3 3:1
DIAD/PPh3 1.1
DIAD/PPh3 1.1
DIAD/PPh3 1.1
31%, 40%
46%, 36%
42%, 26%
47%, 28%
63%, 26%
46%, 22%
75%, 4%
DMF, MW, 210 °C, 10 min
a
Yield after separation by column chromatography. Starting
material is also present.
a
Reagents and conditions: (a) N-FmocPheOH, DMTMM,
SCHEME 1. Syn th esis of 1,4-Dia zep a n -2,5-d ion e
Cor ea
DIPEA, NMP, 4 h, rt and piperidine 25% in DMF. (b) DIAD, PPh3,
DMF, MW, 60 W, 210 °C. (c) Piperidine, 25% DMF followed by
As2O, CH2Cl2, DIPEA, rt 1 h, TFA:CH2Cl2:TFE 10:40:40, rt ,6 h.
SCHEME 3. Syn th esis of P ep tid om im etic
Sca ffold a
a
Reagents and conditions: (a) TFA:CH2Cl2:Et3SiH 1:1:0.1, 6 h,
rt, DMTMM, N-BocSerOH, NMM, THF, 6 h, rt. (b) See Table 1.
nature of the azodicarboxylate and to increase the
dilution did not improve the yield of 3 (see Table 1).
The presence of product 4 obtained even with hindered
di-tert-butyl azodicarboxylate (DTAD), although in a
smaller amount than with DEAD or DIAD, suggested
that dipeptide 2 was reluctant to cyclize probably because
of the preferential transoid conformation assumed by the
peptide bond. Indeed, the 1H NMR spectrum of 2,
registered in d6-dmso at room temperature, showed two
signals for the amide NH at δ 7.70 and 7.93 integrating
for 0.8 and 0.2 H, respectively. The signals collapsed to
a singlet in a range from 60 to 90 °C. Thus we considered
that heating the reaction mixture would accelerate the
trans-cis equilibration allowing a rapid replacement of
the cis isomer consumed by the cyclization and conse-
quently decreasing the amount of byproduct 4.13 When
the cyclization was carried out in boiling toluene product
3 was obtained in 65% yield with the formation of a
considerably smaller amount of 4. When heating was
more efficiently carried out under microwave irradiation14
(DIAD/PPh3 in DMF at 100 W for 10 min in a sealed
tube)15 product 3 was obtained in 75% yield and 4 was
formed in a negligible amount.16
a
Reagents and conditions: (a) DIAD, PPh3, DMF, MW, 60 W,
210 °C. (b) 25% piperidine 25% in DMF followed by N-AcGlyOH,
DMTMM, DIPEA, NMP, rt, 4 h. (c) Pd(PPh3)4 (3 equiv), CHCl3:
AcOH:NMM 37:2:1, HProOMe, DMTMM, DIPEA, NMP, rt, 6 h.
(d) TFA:CH2Cl2:TFE, 10:40:40, rt, 6 h. (e) SmI2 0.1 M in THF, 8
h, rt and workup as in ref 23.
For the solid-phase approach we chose hydroxylamine
linked to a PS-DVB 2-chlorotrytyl resin.17 Coupling with
FmocPheOH gave compounds 6 (Scheme 2). Cyclization
was carried out with DMF as the solvent, in the presence
of 2 equiv of DIAD and 4 equiv of PPh3 in a sealed tube
under microwave irradiation (60 W, 210 °C). After 6 min,
the beads were recovered by filtration, washed several
times with DMF and CH2Cl2, and submitted to a second
round of reaction under the same conditions.
After 3 cycles, the color test for free OH18 carried out
on the beads was negative. The Fmoc protecting group
was removed, the NH2 was acetylated to simplify the
NMR spectra, and product 8 was emoved from the resin
with TFA/CH2Cl2 1/1 and isolated in 66% yield.
(13) Miller, S. J .; Kim, S.-H.; Chen, Z. R.; Grubbs, R. H. J . Am.
Chem. Soc. 1995, 117, 2108.
(14) For a discussion on the rate enhance effects of microwaves
see: Kuhnert, N. Angew. Chem., Int. Ed. 2002, 41, 1863. Lindstro¨m,
P.; Tierney, J .; Wathey, B.; Westman, J . Tetrahedron 2001, 57, 9225.
Perreux, L.; Loupy, A. Tetrahedron 2001, 57, 9199.
(15) We used a domestic oven under MORE conditions (Banik, B.
K.; Barakat, K. J .; Wagle, D. R.; Manhas, M. S.; Bose, A. K. J . Org.
Chem. 1999, 64, 5746) and a Discover Microwave Labstation from
CEM. The highest temperature reached inside the reaction tube was
210 °C.
(16) To the best of our knowledge, in the literature there is only
one example involving a microwave-promoted Mitsunobu reaction.
Steinreiber, A.; Stadler, A.; Mayer, S. F.; Faber, K.; Kappe, C. O.
Tetrahedron Lett. 2001, 42, 6283.
(17) We used the commercially available resin from Novabiochem.
(18) Attardi, M. E.; Taddei, M. Tetrahedron Lett. 2001, 42, 2927.
7894 J . Org. Chem., Vol. 68, No. 20, 2003

The versatility of this synthetic approach is outlined
in Scheme 3. FmocGlu(OAll)OH19 was linked to the
hydroxylamine resin, deprotected, and coupled with
N-FmocSerOH.20 Cyclization of compound 9 was carried
out under standard conditions under microwave activa-
tion to give product 10 (free OH test negative). Depro-
tection of the Fmoc group and coupling with N-AcGlyOH
gave product 11. Deprotection of the allyloxy group
(Pd(PPh)4 CHCl3, AcOH, NMM 37/2/1)21 was followed by
coupling with H-ProOMe until free carboxylic acid disap-
peared.22
In conclusion, with the crucial assistance of microwave
irradiation, we have expanded the use of the Miller
cyclization of hydroxy hydroxamates to the synthesis of
new 3,6-disubstituted-1,4-diazepan-2,5-diones that can be
employed as new scaffolds for combinatorial chemistry
and as conformational constrains for short peptides. The
extension of this approach to macrocylization is actually
underway in our laboratory and will be reported in due
course.
Ack n ow led gm en t. The work was financially sup-
ported by the MIUR-MURST (Rome) within the project
PRIN 2001 at the University of Sassari. The financial
support of NikemResearch srl (Milan) is gratefully
acknowledged.
Cleavage from the resin with TFA/CH2Cl2 1/1 gave the
N-hydroxy-derivative 13 in 75% yield, whereas with SmI2
in THF solution23 14 was isolated in 51% yield after
aqueous workup and a short path silica gel column.
Su p p or tin g In for m a tion Ava ila ble: Experimental pro-
cedures and characterization data for compounds 3, 8, 13, and
14. This material is available free of charge via the Internet
at http://pubs.acs.org.
(19) Booth, S.; Wallace, E. N. K.; Singhal, K.; Bartlett, P. N.;
Kilburn, J . D. J . Chem. Soc., Perkin Trans. 1 1998, 1467.
(20) When we tried to link N-FmocAsp(OAll)OH to the resin-
supported hydroxylamine, a considerable amount of the corresponding
succinimide was formed. With glutamic acid the formation of the
corresponding cyclic N-alkoxyimide was not observed.
(21) Kaes, S. A.; Sole´, N. A.; J ohnson, C. R.; Hudson, D.; Barany,
G.; Albericio, F. Tetrahedron Lett. 1993, 34, 1549.
J O034785D
(22) Attardi, M. E.; Porcu, G.; Taddei, M. Tetrahedron Lett. 2000,
41, 7391.
(23) Myers, R. M.; Langston, S. P.; Conway, S. P.; Abell, C. Org.
Lett. 2000, 2, 1349. Meloni, M. M.; Taddei, M. Org. Lett. 2001, 3, 337.
J . Org. Chem, Vol. 68, No. 20, 2003 7895