Solid-Phase Synthesis of β-Lactams via the Miller Hydroxamate Approach

Article abstract of DOI:10.1021/ol006779z

(Matrix Presented) β-Lactams were prepared on solid phase starting from serine, threonine, or other β-hydroxyacids derived from naturally occurring amino acids and a resin bound hydroxylamine. The ring closure was carried out under Mitsunobu conditions. The amino group present on the β-lactam was used to assemble a short peptide. After a reductive cleavage with Sml₂, β-lactam-containing peptides were obtained.

Full text of DOI:10.1021/ol006779z

ORGANIC
LETTERS
2001
Vol. 3, No. 3
337-340
Solid-Phase Synthesis of â-Lactams via
the Miller Hydroxamate Approach
Marco Massimiliano Meloni and Maurizio Taddei*
Dipartimento di Chimica, UniVersita` degli Studi di Sassari, Via Vienna 2,
I-07100 Sassari, Italy
mtad@ssmain.uniss.it
Received October 26, 2000
ABSTRACT
â-Lactams were prepared on solid phase starting from serine, threonine, or other â-hydroxyacids derived from naturally occurring amino
acids and a resin bound hydroxylamine. The ring closure was carried out under Mitsunobu conditions. The amino group present on the
â-lactam was used to assemble a short peptide. After a reductive cleavage with SmI2, â-lactam-containing peptides were obtained
The â-lactam ring is the key component of commonly used
antibiotics such as penicillins, chephalosporins, carbapenems,
and monobactams.1 Moreover, several examples of peptides
and peptidomimetics containing the â-lactam ring have been
recently described as effective proteases inhibitors and,
consequently, as potential drugs for a wide range of diseases
implicating proteases.2
approach to â-lactams on support have been the [2 + 2]
cycloaddition3a-c,j and the enolate-imine condensation.3d,g-i
Following our interest in the synthesis of heterocyclic-
containing peptidomimetics,4 we became interested in the
solid-phase synthesis of peptides containing a â-lactam as
potential protease inhibitors. In this case the cyclization of
a â-hydroxyhydroxamate derived from an amino acid could
be a real straightforward approach.5
We now report, to the best of our knowledge, the first
example of Miller hydroxamate synthesis of â-lactams6
carried out on solid phase.
The strategy chosen was to link the amino acid derivative
to a polystyrene-supported hydroxylamine, then carry out
the cylization under Mitsunobu conditions, and finally
assemble a short peptide on the NH2 present on the ring.
This approach could be particularly suitable for solid-phase
synthesis as the supported â-lactam could be easy separated
from the byproducts of the Mitsunobu reaction.
Despite their importance, the solid-phase synthesis of
â-lactams has been barely reported.3 The favorite synthetic
(1) Mandell, G. L.; Petri, W. A., Jr. In Goodman & Gilman’s The
Pharmacological Basis of Therapeutics; Hardman J. G., Limbird, L. E.,
Molinoff, P. B., Ruddon, R. W., Goodman Gilman, A., Eds.; McGraw-
Hill: New York, 1996; p 1073.
(2) Yoakim, C.; Ogilvie, W. W.; Cameron, D. R.; Chabot, C.; Guse, I;
Hache´, B.; Naud, J.; O’Meara, J. A.; Plante, R.; De´ziel, R. J. Med. Chem.
1998, 41, 2882. Linder, M. R.; Podlech, J. Org. Lett. 1999, 1, 869. Abell,
A. D.; Gardiner, J. J. Org. Chem. 1999, 64, 9668. Palomo, C.; Aizpurua,
J. M.; Benito, A.; Galarza, R.; Khamrai, U. K.; Vasquez, J.; de Pascual-
Teresa, B.; Nieto, P. M.; Linden, A. Angew. Chem., Int. Ed. 1999, 38, 3056.
(3) See: (a) Ruhland, B.; Bhandari, A.; Gordon, E. M.; Gallop, M. A.
J. Am. Chem. Soc. 1996, 118, 253. (b) Pei, Y.; Houghten, R. A.; Kiely, J.
S. Tetrahedron Lett. 1997, 38, 3349. (c) Singh, R.; Nuss, J. M. Tetrahedron
Lett. 1999, 40, 1249. (d) Molteni, V.; Annunziata, R.; Cinquini, M.; Cozzi,
F.; Benaglia, M. Tetrahedron Lett. 1998, 39, 1257. (e) Furman, B.; Thurmer,
R.; Kaluza, Z.; Lysek, R.; Voelter, W.; Chmielewski, M. Angew. Chem.,
Int. Ed. 1999, 38, 1121. (f) Delpiccolo, M. L.; Mata, E. G. Tetrahedron:
Asymmetry 1999, 10, 3893. (g) Benaglia, M.; Cinquini, M.; Cozzi, F.
Tetrahedron Lett. 1999, 40, 2019. (h) Annunziata, R.; Benaglia, M.;
Cinquini, M.; Cozzi, F. Chem. Eur. J. 2000, 6, 133. (i) Schunk, S.; Enders,
D. Org. Lett. 2000, 2, 907 (j) Hafez, A. M.; Taggi, A. E.; Wack, H.; Drury,
W. J., III; Lectka, T. Org. Lett. 2000, 2, 3963.
The linker employed was a polystyrene resin carrying a
O-trityl-hydroxylamine linker prepared as described in the
(4) Filigheddu, S. N.; Taddei, M. Tetrahedron Lett. 1998, 39, 3853.
Filigheddu, S. N.; Masala, S.; Taddei, M. Tetrahedron Lett. 1999, 40, 6503.
(5) For a comprehensive review on the possible synthetic approaches to
azetidinones see: Perrone, E.; Franceschi, G. In Recent Progress in the
Chemical Synthesis of Antibiotics; Lukacs, G., Ohno, M., Eds.; Springer-
Verlag: Berlin, 1990; p 613.
(6) Miller, M. J.; Mattingly, P. G.; Morrison, M. A.; Kerwin, J. F., Jr. J.
Am. Chem. Soc. 1980, 102, 7026.
10.1021/ol006779z CCC: $20.00 © 2001 American Chemical Society
Published on Web 01/13/2001

literature.7 Following Fmoc deprotection with 20% piperidine
in DMF, L-Cbz-serine (or L-Cbz-threonine) was coupled
using DMTMM8 in N-methylpyrrolidone (NMP) in the
presence of N,N-diisopropylethylamine (DIPEA).
6 with high conversion. The TCT-AliR test was negative
(all beads observed at the microscope were gray), and the
FT-IR showed a strong band at 1780 cm-1 typical of CdO
stretching for N-alkoxy â-lactams. The same procedure was
followed with compound 5 giving product 7.
At this point two alternatives were possible for removal
of the products from the resin: cleave the N-O bond to
give â-lactams 8 and 9 or cleave the N-trityl bond to give
1-hydroxy-â-lactams 10 and 11. The first approach was
efficiently accomplished using a reductive cleavage with
SmI2 recently described by Abell and co-workers.12 The resin
was treated with a commercially available solution of SmI2
in THF, and the products were recovered from the solution
after hydrolytic workup and passage through a short silica
gel column. Products 8 and 9 were obtained in acceptable
yields (45% and 52% calculated from the loading of the
original trityl-hydroxylamine resin 2). Alternatively, acidic
cleavage with 5% TFA in CH2Cl2 for 3 h followed by quench
with Et3N and aqueous workup gave compounds 10 and 11
in modest yields (about 35% in both cases, calculated as
above).13
Scheme 1a
After determining that Miller’s synthesis could be carried
out successfully on solid phase, we tried the construction of
a simple peptide in position 3 of the â-lactam ring. Thus
N-Fmoc-Ser-OH was coupled on resin 3 following standard
conditions. The ring closure was carried out with DEAD/
PPh3 in THF to give product 12 (IR, 1765 cm-1).
a (a) FmocNHOH, 2 eq. DIPEA, rt, 48 h.b) Piperidine, DMF, rt,
20 min. c) (L)-Cbz-Ser-OH or (L)-Cbz-Thr-OH, 4 eq. DMTMM,
4 eq. DIPEA, NMP, rt, 12 h. d) 5 eq. DEAD, 10 eq. PPh3, THF, rt,
24 h. e) SmI2 0.1 M in THF, rt, 4 h, work up as in ref 12. f) 5%
TFA in CH2Cl2, rt, 1 h followed by aqueous workup.
Scheme 2a
The choice of this coupling agent was due to its low
reactivity with alcohol, which allows the use of a free OH
group in the serine and threonine components. The coupling
was achieved efficiently,9 and the presence of the free OH
was assayed with our TCT-AliR test.10
Attempted cyclization of compound 4 following the
conditions originally reported by Miller6 failed to produce
appreciable â-lactam products (color test positive, which is
indicative of the presence of a free OH). Then the amounts
of DEAD and PPh3 were increased, and the reaction was
attempted in different solvents at room temperature or with
heating. In each case the TCT-AliR test was always positive.
On the other hand, FT-IR of the beads showed the presence
of a weak signal around 1770 cm-1 showing that the â-lactam
was partially formed. Finally we decided to use freshly
distilled DEAD,11 and the cyclization occurred in THF giving
a (a) N-Fmoc-Ser-OH, DMTMM, NMM, NMP, rt, 4 h followed
by 5 equiv of DEAD, 10 equiv of PPh3, THF, rt, 24 h. (b) Series
of Fmoc deprotections with 25% piperidine in DMF followed by
couplings using DMTMM in NMP with N-Fmoc-Phe-OH; N-Fmoc-
Ala-OH, and N-Boc-Val-OH. (c) SmI2 0.1 M in THF, rt, 4 h.
(7) Mellor, S. L.; McGuire, C.; Chan, W. C. Tetrahedron Lett. 1997,
38, 3311. Different linkers, such as, for example, a Wang-type resin carrying
an hydroxylamine group, were also tried with unsatisfactory results.
(8) (4,6-Dimethoxy-[1,3,5]-triazin-2-yl)-4-methyl-morpholinium chloride.
See: Falchi, A.; Giacomelli, G.; Porcheddu, A.; Taddei, M. Synlett 2000,
277. DMTMM is commercially available from Acros Organics.
(9) The terminal NH2 of hydroxylamine does not give a fully positive
ninhydrine test. Following Kaiser’s conditions we osberved, for compound
3, a pale yellow solution and red beads (microscope, 10X). After the
coupling, the test was completely negative (uncolored beads). For Kaiser’s
test see: Kaiser, E.; Colescott, R. L.: Bossinger, C. D.; Cook, P. I. Anal.
Biochem. 1970, 34, 595
The nitrogen was deprotected following standard condi-
tions, and a tripetide was assembled using DMTMM as the
coupling agent. The efficiency of all steps was controlled
using the classical Kaiser’s test, and the integrity of the
â-lactam was verified with FT-IR of the beads. Cleavage
was carried out using SmI2. In this case the workup procedure
(10) Attardi, M. E.; Falchi, A.; Taddei, M. Tetrahedron Lett. 2000, 41,
7395.
(11) Distillation should be conducted using a temperature-controlled bath
because of the danger of explosion on overheating DEAD. See: Pansare,
S. V.; Huyer, G.; Arnold, L. D.; Vederas, J. C. Org. Synth. 1991, 70, 1.
(12) Myers, R. M.; Langston, S. P.; Conway, S. P.; Abell, C. Org. Lett.
2000, 2, 1349. See also: Yang, H. W.; Romo, D. J. Org. Chem. 1999, 64,
7657.
338
Org. Lett., Vol. 3, No. 3, 2001

was modified regarding to the literature.12 After cleavage,
the resin was washed with CH2Cl2 and MeOH. After
evaporation, the dark residue was treated several times with
water. The residual solid obtained was crude 14, which
required purification by chromatography to obtain a product
with a purity higher than 95%. The final yield of 14
(calculated with respect to the loading of resin 2) was 36%.
To extend the potential of the method we loaded on the
resin the γ-amino â-hydroxyacid 18, which was obtained
starting from N-Boc-Phe-OH 15. Claisen-type condensation
of 15, after carbonyl diimidazole (CDI) activation, gave
â-keto ester 16 in 85% yield. The carbonyl was selectively
reduced under chelation control using TiCl4 and BH3-
pyridine complex at -78 °C.14 Product 17 was obtained in
product 21 in 52% yield.16 Unfortunately the Boc protection
on 20 could not be removed without cleaving the product
from the resin. The only protection compatible with the trityl
resin is Fmoc, which is, on the other hand, unstable under
the reaction conditions used for carbonyl reduction. Conse-
quently, compound 17 was deprotected with LiOH in THF/
H2O followed by formic acid. The crude material obtained
after evaporation of the acid was treated with Fmoc-Cl and
Na2CO3 in dioxane/water to give the corresponding Fmoc
derivative 22 in 75% yield (Scheme 4). Compound 22 was
Scheme 4a
1
high diastereomeric excess (approximately 90% from H
NMR analysis of the crude). After column chromatography,
17 was isolated in 75% yield as a single diastereoisomer.
Finally, hydrolysis with LiOH in THF/H2O gave 18 in 95%
yield.15 Compound 18 was loaded on the resin with DMTMM
in NMP and NMM, and the cyclization was carried out under
standard conditions.
Scheme 3a
a (a) LiOH, THF/H2O, 24 h. (b) HCOOH (as solvent), rt, 6 h.
(c) Fmoc-Cl, Na2CO3, dioxane/water, rt, 12 h. (d) 3, NMP,
DMTMM, NMM, 2 h. (e) 5 equiv of DEAD, 10 equiv of PPh3,
THF, rt, 24 h. (f) Series of Fmoc deprotections with 25% piperidine
in DMF followed by couplings using DMTMM in NMP with
N-Fmoc-Val-OH and N-Boc-Ala-OH. (g) SmI2 0.1 M in THF, rt,
4 h.
loaded on resin 3 and cyclized to 24 following standard
conditions. The deprotection of the Fmoc (positive Kaiser’s
test) and the further assembling of amino acids was carried
out as described for compound 14. Final cleavage with SmI2
gave product 25 in 36% overall yield.17
Unfortunately attempts to cleave the O-trityl bond of
compounds 13 and 24 with TFA 5% in CH2Cl2 were
unsuccessful.
a (a) CDI, THF, rt, 24 h followed by EtAc/LDA in THF, -78
°C, 1 h. (b) TiCl4 in CH2Cl2, 30 min, followed by BH3‚Py, -78
°C, 1 h. (c) LiOH, THF/H2O, 24 h, followed by aqueous citric acid.
(d) 3, NMP, DMTMM, NMM, 2 h. (e) 5 equiv of DEAD, 10 equiv
of PPh3, THF, rt, 24 h. (f) SmI2 0.1 M in THF, rt, 4 h, workup as
in ref 12.
See: Gordon, E. M.; Ondetti, M. A.; Pluscec, J.; Cimarusti, C. M.; Bonner,
D. P.; Sykes, R. B. J. Am. Chem. Soc. 1982, 104, 6053. 11: mp 112-117
°C; IR (KBr, cm-1) 1760. See: Woulfe, S. R.; Miller, M. J. J. Org. Chem.
1986, 51, 3133
(14) Marcantoni, E.; Alessandrini, S.; Malavolta, M.; Bartoli, G.; Bellucci,
M. C.; Sambri, L.; Dalpozzo, R. J. Org. Chem. 1999, 64, 1986.
(15) The syn relative stereochemistry of compund 18 was expected on
the basis of the chelation control. The assignment was based on the values
of melting point observed, 153-154 °C (lit. mp 153.2-153.4 °C):
Ba¨nzinger, M.; McGarrity, J. F.; Meul, T. J. Org. Chem. 1993, 58, 4010.
The melting point reported for the anti isomer is 187.5 °C: Rich, D.; Sun,
D. H.; Edgar, U. J. Med. Chem. 1980, 23, 27.
All of the steps were controlled with colorimetric tests on
the beads. The ninhydrin test was negative, and TCT-AliR
was positive for the loading. Finally, the TCT-AliR test was
negative after cyclization. Reductive cleavage with SmI2 gave
(16) Compound 21: mp 102-104 °C; IR (KBr, cm-1) 3200, 3010, 2950,
1735, 1680, 1600; 1H NMR (300 MHz, CDCl3) δ 7.20-7.00 (m, 5H), 5.48
(bs, 1H), 5.10 (bs, 1H), 4.60 (m, 1H), 4.12 (m, 1H), 3.36 (A part of an
ABX system, 1H), 3.06 (B part of an ABX system, 1H), 2.50-2.30 (m,
2H), 1.25 (s, 9H); 13C NMR (75 MHz, CDCl3) δ 178.7, 156.4, 141.3, 128.7,
127.4, 125.8, 71.7, 59.8, 48.6, 39.8, 31.7, 24.1.
(13) Products 8-11 showed melting points, spectroscopical data, and
optical rotations comparable with those reported in the literature. 8: mp
46-48 °C; [R]D ) -13.8 (c 1.4 in MeOH); IR (KBr, cm-1) 1735. See:
Cainelli, G.; Giacomini, D.; Galletti, P.; DaCol, M. Tetrahedron Asymmetry
1997, 8, 3231. 9: mp 92-94 °C; [R]D ) -17.68 (c 4.0 in CDCl3); IR (KBr,
cm-1) 1745. See: Andreoli, P.; Billi, L.; Cainelli, G.; Panunzio, M.; Bandini,
E. Tetrahedron 1991, 47, 9061. 10: mp 130-131 °C; IR (KBr, cm-1) 1765.
(17) Compounds 14 and 25 were characterized by 1H and 13C NMR and
mass spectrometry (ES/MS).
Org. Lett., Vol. 3, No. 3, 2001
339

In conclusion, we have demonstrated that peptides con-
taining a â-lactam ring can be prepared on solid phase using
the Miller approach. The reaction can be successfully carried
out on cross-linked polystyrene using a trityl linker and the
reductive cleavage of the N-O bond with SmI2 in THF. We
are currently employing this approach to prepare small
libraries of â-lactam peptidomimetics.
Acknowledgment. The work was financially supported
by the University of Sassari (Fondi 60%).
OL006779Z
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Org. Lett., Vol. 3, No. 3, 2001