β-lactam-containing cyclopeptide analogs

Article abstract of DOI:10.1002/ejoc.200400295

Cyclic peptide analogs containing a β-lactam moiety were prepared. Reacting Fmoc-protected amino acid-derived diazo ketones 1, 2 with benzylidene-protected amino esters 3, 4 in a photochemically induced Staudinger-type reaction, trans-substituted β-lactams 5a/b and 6a/b were formed in 35-70% yield (dr 60:40-70:30). N-Terminal chain elongation to the respective acyclic precursors 14, 17a/b and 19a/b was achieved using conventional peptide synthesis (i.e. the pentafluorophenyl ester protocol). After saponification, activation as pentafluorophenyl esters and subsequent cleavage of the N-terminal Boc-protecting group, the pre-strained (3R,4S)-configured isomers could be cyclized without the need for high dilution or prolonged reaction times. Contrary to this, the (3S,4R)-configured isomers did not cyclize but gave polymeric material. The conformational stability of the cyclic peptidomimetics 16, 20, and 21 which were obtained in high yield, was elucidated by means of NMR spectroscopy. Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004.

Full text of DOI:10.1002/ejoc.200400295

FULL PAPER
β-Lactam-Containing Cyclopeptide Analogs
Thomas C. Maier[a] and Joachim Podlech*[a]
´
´
Dedicated to Janos Retey on the occasion of his 70th birthday
Keywords: Peptidomimetics / Lactams / Cyclization / Peptide coupling / Peptide conformation
Cyclic peptide analogs containing a β-lactam moiety were
prepared. Reacting Fmoc-protected amino acid-derived di-
the N-terminal Boc-protecting group, the pre-strained
(3R,4S)-configured isomers could be cyclized without the
azo ketones 1, 2 with benzylidene-protected amino esters 3, need for high dilution or prolonged reaction times. Contrary
4 in a photochemically induced Staudinger-type reaction,
trans-substituted β-lactams 5a/b and 6a/b were formed in
35−70% yield (dr 60:40−70:30). N-Terminal chain elongation
to this, the (3S,4R)-configured isomers did not cyclize but
gave polymeric material. The conformational stability of the
cyclic peptidomimetics 16, 20, and 21 which were obtained
to the respective acyclic precursors 14, 17a/b and 19a/b was in high yield, was elucidated by means of NMR spectroscopy.
achieved using conventional peptide synthesis (i.e. the pen-
tafluorophenyl ester protocol). After saponification, activa- ( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
tion as pentafluorophenyl esters and subsequent cleavage of
Germany, 2004)
Introduction
turn conformation might lead to an efficient cyclization of
appropriately prepared precursors. In this paper we report
on our efforts in this context.
Peptides showing a turn conformation[1Ϫ4] are of major
relevance since, in most cases, a turn region is responsible
for the biological activity of the respective peptide. Well
known representatives showing a turn conformation are, in-
ter alia, somatostatin[5Ϫ7] and oxytocin.[8] Replacing the
peptide turn by other, mostly cyclic templates[9Ϫ11] fur-
nishes substrates with a more rigid framework, that are po-
tentially capable of mimicking a biologically active confor-
mation (i.e., turn mimics). Many peptidic hormones have a
cyclic structure resulting in further advantageous features.
Not only is a possible biologically active conformation
more rigidly fixed, cyclic substrates are occasionally less po-
lar which makes separation and purification easier and they
are usually more stable against digesting enzymes what al-
lows for an oral application. Nevertheless, the cyclization
step in the synthesis of cyclic peptides is often limiting the
value of the method and low yields have to be taken into ac-
count.
Results and Discussion
For the preparation of the β-lactam-containing peptide
fragments we used a method developed in our group in
which α-amino acid-derived diazo ketones were photo-
chemically reacted with imines derived from α-amino esters
(photochemically induced Staudinger-type reaction). Al-
though this reaction is similarly possible with larger, pep-
tide-derived substrates,[13] we abolished this approach, since
yields were significantly lower therein. Instead, we used
conventional peptide coupling procedures for the elong-
ation of the corresponding amino acid-derived β-lactams
(Scheme 1).
During our investigations on peptidomimetics containing
β-lactams we found that these give very stable turn struc-
tures.[12] Crystal structure analysis showed that the angle
between incoming and leaving peptide strands (i.e. the vec-
tors at which the peptide chain enters and leaves the turn
mimetic) is about 185°. We envisioned that this pre-strained
[a]
Institut für Organische Chemie der Universität Karlsruhe (TH),
Fritz-Haber-Weg 6, 76131 Karlsruhe, Germany
Fax: (internat.) ϩ49-(0)721-608-7652
E-mail: joachim.podlech@ioc.uka.de
Supporting information for this article is available on the
WWW under http://www.eurjoc.org or from the author.
Scheme 1. General scheme for the preparation of β-lactam-contain-
ing cyclopeptide analogs
Eur. J. Org. Chem. 2004, 4379Ϫ4386
DOI: 10.1002/ejoc.200400295
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4379

T. C. Maier, J. Podlech
FULL PAPER
The diazo ketones utilized in this protocol were prepared of intermediate 11 was deprotected at the N-terminus with
according to published procedures by reacting carbamate- tris(2-aminoethyl)amine (TAEA)[18] and added without
protected amino acids (activated via the mixed anhydride further purification to the priorly synthesized active ester
method) with diazomethane.[14] Synthesis of the imine com- furnishing the linear precursor 14 in 72% yield.
pounds was achieved by liberation of amino esters from
Saponification of 14 was achieved by employing drasti-
their hydrochlorides with ammonia and subsequent con- cally prolonged reaction times (ca. 40 h) which led to a par-
densation with benzaldehyde in the presence of alum- tial cleavage of the Fmoc group. The pentafluorophenyl es-
ina.[15,16]
ter protocol was again used for activation of the carboxylic
Fmoc-protected diazo ketones 1 and 2 derived from leu- acid. However, when the Fmoc-protected amino function
cine and alanine, respectively, were treated with N-(benzyl- was deprotected under basic conditions (1,8-diazabicy-
idene)glycine and leucine methyl ester, respectively (3, 4), clo[5.4.0]undec-7-ene, DBU, or 1,5-diazabicyclo[4.3.0]non-
and mixtures of the corresponding diastereomeric trans- 5-ene, DBN, respectively) to achieve cyclization, cleavage of
substituted β-lactams were obtained (Scheme 2). Selectivit- the pentafluorophenyl ester occurred; no cyclized product
ies were in accordance with prior observations, depending could be isolated. Instead of investigating further modifi-
on the bulkiness of the diazo ketones’ side-chains. Alanine- cations of the reaction protocol we decided to use an optim-
derived substrates gave a 60:40 selectivity (5a/b), while leu- ized procedure developed for the cyclization of peptides by
cine-derived diazo ketones led to a slightly better 70:30 sel- Schmidt et al.[19,20] For this purpose we had to exchange
ectivity (6a/b). The isomers were separated by means of me- the Fmoc group for the Boc group (Ǟ 15), which was
dium pressure liquid chromatography (MPLC).
achieved by deprotection with diethylamine and subsequent
treatment of the liberated amine with di-tert-butyl dicar-
bonate (Boc2O). Saponification was now Ϫ due to the base
stability of the Boc group Ϫ possible without any loss and
activation was effected as described above. Further purifi-
cation of the obtained pentafluorophenyl ester was not
necessary since NMR spectroscopy showed that the mate-
rial was essentially pure. Cleavage of the Boc group with
trifluoroacetic acid (TFA) prior to slow addition to an
emulsion of chloroform and 1  potassium carbonate solu-
tion without the need for high dilution gave the cyclized
peptidomimetic 16 in 68% yield (from 14) within 2 h
(Scheme 3).
The pre-formed turn structure of the β-lactam-containing
precursors should also facilitate the often poor yielding
cyclization of smaller cyclopeptide analogs built from four
and five amino acid residues, respectively. Attempts to syn-
thesize such compounds in a similar way as outlined above
were actually successful: The acyclic precursors were pre-
pared from dipetide analogs 5a/b by successive attachment
Scheme 2. Synthesis of β-lactam-containing dipeptide analogs
As a first cyclic target molecule we chose a symmetric of two amino acid residues at the N-terminus and from di-
analog containing two β-lactam moieties. In this case we petide analogs 6a/b by coupling with a Boc-protected tri-
expected a cyclization to be especially favored. In addition, peptide. All couplings were accomplished using the penta-
spectroscopic analysis should be simplified due to the C2 fluorophenyl ester method (Scheme 4). Yields were rather
symmetry of the substrate and Ϫ if the region containing poor for the segment coupling furnishing 19a/b since signifi-
the β-lactam is responsible for a possible biological activity cant amounts of the epimerized product (at C-1 of Phe) had
Ϫ a doubling of this region might lead to an increased ef- to be separated. This epimerization possibly occurred prior
fect.
to the activation of BocϪAlaϪAlaϪPheϪOH during sap-
Synthesis of the precursor for the cyclization step was onification of the corresponding methyl ester 18,[4] or dur-
straightforward. (3R,4S)-Configured azetidinone 5a was de- ing activation of BocϪAlaϪAlaϪPheϪOH, though we
protected at the N-terminus with diethylamine, a secondary never did encounter an epimerization during pentafluoro-
base which was easily removed in vacuo when the reaction phenyl ester activation in other cases. Given yields refer to
was complete. The thus liberated amino compound was im- analytically and diastereomerically pure material.
mediately reacted with FmocϪAlaϪOPfp (8) resulting in
While saponification of the tetrapeptide analog 17a was
the formation of tripeptide analog 11. Half the material was achieved with aqueous lithium hydroxide within two hours,
saponified with lithium hydroxide with no cleavage of the the cleavage of the other methyl esters was complete only
base-labile Fmoc group[17] and was subsequently activated after drastically prolonged reaction times (Ͼ 20 h for 17b
as pentafluorophenyl ester [prepared from N-ethyl-NЈ-(3-di- and 19b) or when sodium hydroxide in dimethylformamide
methylaminopropyl)carbodiimide hydrochloride (EDC) was used (18 h for 19a). However, the stereochemical integ-
and pentafluorophenol (PfpOH)]. The remaining fraction rity of the material was maintained even under these con-
4380
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Eur. J. Org. Chem. 2004, 4379Ϫ4386

β-Lactam-Containing Cyclopeptide Analogs
FULL PAPER
Scheme 4. Synthesis of acylic precursors 17a/b and 19a/b. a)
Et2NH, THF, room temp.; b) FmocϪPheϪOPfp (7), THF, room
temp.; c) BocϪAlaϪOPfp (9), THF, room temp.; d) Et2NH,
MeCN, room temp.; e) BocϪAlaϪAlaϪPheϪOPfp, THF, room
temp.
Scheme 3. Synthesis of cyclic peptidomimetic 16. a) 0.2  LiOHaq.
,
THF, 0 °C; b) PfpOH, EDC, CH2Cl2, 0 °C to room temp.; c)
TAEA, CH2Cl2, room temp.; d) CH2Cl2, room temp., 72%; e)
Et2NH, CH2Cl2, room temp.; f) Boc2O, CH2Cl2, room temp., 91%;
g) TFA, 0 °C; h) 1  K2CO3 aq., CHCl3/CH2Cl2, room temp., 75%;
i) DBU or DBN, THF or CH2Cl2, room temp.
ditions. Cyclization of the (3R,4S)-configured carboxylic
acids obtained from 17a and 19a, respectively, was achieved
under the above described conditions, yielding peptide anal-
ogs 20 and 21, respectively, in notably high yields (93 and
60%, Scheme 5).
On the other hand, the (3S,4R)-configured precursors
17b and 19b could not be cyclized, even under high dilution
conditions (ഠ 2·10Ϫ3 ) and with prolonged reaction times
(up to 10 h). In lieu thereof oligomeric material was ob-
tained which Ϫ due to its poor solubility Ϫ was not further
Scheme 5. Synthesis of cylic peptidomimetics 20 and 21 containing
one β-lactam moiety. a) 0.2  LiOHaq., THF, 0 °C; b) PfpOH,
EDC, CH2Cl2, 0 °C to room temp.; c) TFA, 0 °C; d) 1  K2CO3 aq.
CHCl3/CH2Cl2, room temp.; e) 1  NaOHaq., DMF, room temp.
,
purified. The reason for this differing observation is not the neighboring leucine that forms a six-membered ring at-
fully understood. Some evidence comes from simple model- tached to the azetidinone moiety. Evidence for this struc-
ling of the respective cycles. All considered conformations tural motif, which has already been recognized in the corre-
(with different starting geometries) proved to be signifi- sponding acyclic peptidomimetics,[12] comes from a down-
cantly more stable with a (3R,4S) configuration. Though field shift of the amide proton and the coupling constants
the cause for this is not clear (steric reasons should be re- (allowing for an estimation of the respective angles) of the
sponsible), it might similarly count for the deviating tend- protons in this region (see Supporting Information for
encies for cyclization. However, a complete survey of the further information, see also the footnote on the first page
conformational space, which would give more evidence, was of this article). The geminal coupling of the glycine α-pro-
not performed.
tons is rater high (16 Hz) suggesting a bisectal confor-
Due to the inherent symmetry of analog 16, its NMR mation. Chemical shifts of most protons are hardly depen-
spectra are very concise. The main structural feature of this dent on the concentration. However, slight changes of
compound is Ϫ apart from the β-lactam Ϫ a hydrogen bond chemical shifts for the amide protons and a significant in-
between the β-lactam carbonyl and the amide hydrogen of crease of the distance between the signals of the glycin α-
Eur. J. Org. Chem. 2004, 4379Ϫ4386
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T. C. Maier, J. Podlech
FULL PAPER
Table 1. Chemical shifts of selected protons in cyclic peptidomimetic 16 and their concentration dependancy
Concentration
NHleucine
NHalanine
α-Halanine
H-4β-lactam
α-Hleucine
αa-Hglycine
αb-Hglycine
H-3β-lactam
0.001 
0.050 
0.180 
7.68
7.64
7.58
6.67
6.77
6.99
4.53
4.53
4.53
4.50
4.51
4.53
4.49
4.50
4.49
3.87
3.89
3.96
3.77
3.74
3.66
3.17
3.16
3.15
THF used for reactions were distilled from sodium/benzophenone.
Diazo ketones 1 and 2[14] were prepared as described elsewhere.
Fmoc- and Boc-protected amino acids and amino ester hydrochlo-
rides were purchased from Bachem, BocϪAlaϪAlaϪPheϪOMe
(18) and pentafluorphenyl esters 7Ϫ9 were purchased from Bachem
or prepared according to standard protocols.[4] Common amino
acid abbreviations are used.[21] EDC: N-ethyl-NЈ-[3-(dimethylamin-
o)propyl]carbodiimide hydrochloride, TAEA: tris(2-aminoethyl)a-
mine. Flash column chromatography: Merck silica gel 60 (230Ϫ400
mesh). TLC: precoated sheets from MachereyϪNagel, Alugram
protons with increasing concentration show a change in the
molecule’s geometry, most likely due to some sort of inter-
molecular interaction. The signal of the leucine amide pro-
ton forming the hydrogen bond shows almost no solvent or
concentration dependency [∆δ(leucine-NH) ϭ ϩ0.05 ppm
for a change from CDCl3 to CDCl3/CD3OD (5:1) as com-
pared with ∆δ(alanine-NH) ϭ ϩ0.47 ppm for CDCl3 to
CDCl3/CD3OD (5:1)] (Table 1). However, these findings are
in disagreement with a fast H/D exchange of this proton
[leucine-NH: about 5 min when changing from CDCl3 to SIL G/UV254; detection by UV extinction, by cerium molybdato-
phosphate solution [phosphomolybdic acid (25 g), Ce(SO4)2·H2O
(10 g), concd. H2SO4 (60 mL), H2O (940 mL)], by anisaldehyde
solution [anisaldehyde (9.2 mL), AcOH (3.75 mL), H2SO4 (12.5
mL), EtOH (335 mL)] or by ninhydrin solution (2% in EtOH).
Medium pressure liquid chromatography (MPLC): Detection by
UV absorption (Latek UVIS 200). HPLC: Analyses of dia-
stereoisomer distribution were carried out with a Pharmacia LKB,
RSD 2140 apparatus with a Pharmacia LKB, RSD 2249 mixer
and diode-array detection (Pharmacia RSD 2140) with a Merck
LiChrosorb Si 60 chromatographic column (hexane/EtOAc, flow:
CDCl3/CD3OD (5:1) as compared with alanine-NH: about
30 min when the solvent was changed from CDCl3 to
CDCl3/CD3OD (5:1)].
Signals of the cyclic tetrapeptide analog 20 are signifi-
cantly broadened at room temperature giving evidence for
a more floppy structure. Sharper signals revealing the coup-
ling pattern were obtained only at 330 K. Although the sig-
nal of the amide proton next to the β-lactam carbonyl in
this peptide analog also appears at significantly lower field
compared to the other amide protons, the hydrogen bond 2.0 mL/min). 1H and 13C NMR spectra were recorded with a
Bruker ARX 500 spectrometer at room temp. in CDCl3 unless
otherwise indicated. Chemical shifts (δ in ppm) relative to internal
TMS (0 ppm) or to resonances of the solvent (1H: CHCl3,
7.24 ppm; 13C: CDCl3, 77.0 ppm), coupling constants J in Hz. The
numbering used for the assignment of the NMR spectroscopic data
for the cyclic peptidomimetics 16, 20, and 21; detailed spectro-
scopic data for all compounds and NMR spectra for key sub-
stances are included in the Supporting Information. Mass spectra
were recorded using a Finnigan MAT 95 [FAB or CI (CH4 or NH3)
technique] or a Varian MAT 711 instrument (EI). IR spectra were
(with a participation of the β-lactam carbonyl) seems Ϫ
most probably due to steric reasons Ϫ to be not present or
at least rather weak in this substrate. The conformation of
cyclopeptide analog 21 seems to be very sensitive towards
the conditions under which the spectra are recorded. While
the NMR spectra in CD2Cl2 show sharp signals even at
room temperature, only broadened signals could be ob-
served when CDCl3 was used as a solvent. The ambiguous
data obtained for this substrate may be explained by coordi-
nation of different amounts of water to the cyclopeptide recorded with a Bruker IFS 28 or a PerkinϪElmer 283 instrument.
Elemental analyses were performed by the service of the Institut
für Organische Chemie, Stuttgart. Melting points are not corrected.
mimetic since traces of water were unavoidable in this case.
General Procedure for the Preparation of Imines 3 and 4:[15,16] The
amino acid methyl ester hydrochloride (20.0 mmol) was suspended
at 0 °C in a ca. 0.5  solution of NH3 in CHCl3 (100 mL), stirred
for 30 min, left without stirring for further 20 min, filtered (Celite)
and carefully concentrated in vacuo (300 mbar, 35 °C). Al2O3 (neu-
tral, type 90, mesh 63Ϫ200 µm, activity grade I, 10.0 g), benzal-
dehyde (18.0 mmol) and CH2Cl2 (2 mL) were added, and the mix-
ture was stirred at room temp. for 3 h. The suspension was filtered
(Celite) and washed with CH2Cl2. The solvent was evaporated
(300 mbar, 35 °C) and the residue purified by a bulb-to-bulb distil-
lation.
Conclusion
We were able to show that a (3R,4S)-configured β-lactam
moiety incorporated into small peptides enforces a turn
conformation leading to a facilitated cyclization of suitable
precursors. This is not only valid for larger rings but as well
for small cyclopetide analogs consisting of only four or five
amino acid residues, respectively. Contrary to this, the
(3S,4R)-configured isomers could not be cyclized but gave
polymeric material.
Dipeptide Analogs 5a,b: A quartz photo reactor was charged with
FmocϪLeuϪCHN2 (1, 2.72 g, 7.20 mmol), BnϭGlyϪOMe (3,
2.04 g, 11.5 mmol), and Et2O (280 mL), then flushed with N2,
cooled to Ϫ30 °C, and irradiated with a UV lamp for 1.5 h (moni-
toring by TLC) with vigorous stirring. The solution was warmed
to room temp. and the solvent was removed (300 mbar, 35 °C).
Excess imine was removed by flash chromatography (SiO2). The
Experimental Section
General Remarks: Solvents for chromatography and for workup,
e.g. ethyl acetate and light petroleum (PE) were distilled prior to
use, diethyl ether was distilled from KOH/FeSO4. Diethyl ether and
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β-Lactam-Containing Cyclopeptide Analogs
FULL PAPER
diastereomeric ratio was determined by HPLC (hexane/iPrOH,
95:5) and 1H NMR spectroscopy (5a/b ϭ 70:30). Separation of side
products and impurities by MPLC (PE/EtOAc, 80:20) and separ-
ation of the isomers by MPLC (PE/iPrOH, 98:2) afforded the azeti-
Fmoc-Protected Acyclic Precursor 14: To a solution of the tripep-
tide analog 11 (550 mg, 920 µmol) in THF (20 mL), 0.2  aqueous
LiOH (28 mL) was added in 3 portions at 0 °C within 1.5 h. Stir-
ring was continued for 2 h, and the solution was added to a mixture
dinone 5a (1.59 g, 3.02 mmol, 42%) and the azetidinone 5b (0.76 g, of 0.2  HCl (300 mL) and EtOAc (100 mL). The aqueous phase
1.44 mmol, 20%) as colorless solids. A mixture of isomers was ob- was extracted with EtOAc (2ϫ) and the combined organic layers
tained in an additional fraction (303 mg, 576 µmol, 8%). Major
Isomer 5a: M.p. 64Ϫ65 °C. [α]2D0 ϭ Ϫ17.0 (c ϭ 1.00, CHCl3).
C32H34N2O5 (526.62): calcd. C 72.98, H 6.51, N 5.32; found C
were dried (MgSO4) and concentrated in vacuo (50 mbar, 40 °C).
The residue was dissolved in CH2Cl2 (15 mL), the solution stirred
and cooled to 0 °C, then pentafluorophenol (170 mg, 920 µmol)
and EDC (194 mg, 1.01 mmol) were added. Stirring was continued
at 0 °C for 2 h and at room temp. for 2 h. The solvents were re-
moved (300 mbar, 35 °C), EtOAc and brine were added to the resi-
due. The organic layer was extracted with saturated NaHCO3 (2ϫ)
and with brine, dried (MgSO4), filtered through a pad of Celite,
and the solvents evaporated (50 mbar, 40 °C) to yield the intermedi-
ate pentafluorophenyl ester 12.
72.66, H 6.75, N 5.11. Minor Isomer 5b: M.p. 85Ϫ88 °C. [α]2D0
ϭ
ϩ20.5 (c ϭ 1.00, CHCl3). C32H34N2O5 (526.62): calcd. C 72.98, H
6.51, N 5.32; found C 72.90, H 6.63, N 5.22.
Dipeptide Analogs 6a,b: FmocϪAlaϪCHN2 (2, 2.08 g, 6.20 mmol)
and BnϭLeuϪOMe (4, 1.88 g, 8.06 mmol) in Et2O (280 mL) were
reacted as described for compounds 5a/b. The diastereomeric ratio
was determined by HPLC (hexane/EtOAc, 75:25) and 1H NMR
spectroscopy (6a/b ϭ 60:40). Separation of side products and im-
purities by MPLC (PE/EtOAc, 88:12) and separation of the isomers
by MPLC (PE/iPrOH, 98:2) afforded azetidinone 6a (403 mg, 930
µmol, 15%) and azetidinone 6b (369 mg, 682 µmol, 11%) as color-
less foams. A mixture of isomers was obtained in an additional
fraction (302 mg, 558 µmol, 9%). Major Isomer 6a: M.p. 63Ϫ71 °C
(softening range). [α]D 20 ϭ Ϫ23.1 (c ϭ 1.00, CHCl3). C33H36N2O5
(540.65): calcd. C 73.31, H 6.71, N 5.18; found C 73.22, H 6.96, N
To a second portion of 11 (530 mg, 890 µmol) in CH2Cl2 (20 mL)
TAEA (6.48 g, 6.63 mol, 44.3 mmol) was added, and the mixture
was stirred for 10 min (monitoring by TLC), diluted with ad-
ditional CH2Cl2, extracted with brine (2ϫ), phosphate buffer (3ϫ,
pH 5.5: 6.54 g Na2HPO4 and 20.3 g NaH2PO4·2 H2O in 100 mL
H2O), brine (1ϫ), dried (MgSO4), and concentrated in vacuo
(100 mbar, 40 °C). The thus liberated amine 13 was diluted in
CH2Cl2 (25 mL) and the pentafluorphenyl ester 12 was added.
After stirring at room temp. for 2.5 h the solvents were removed
(100 mbar, 40 °C), the residue was dissolved in EtOAc, extracted
with brine (3ϫ), dried (MgSO4), filtered, concentrated in vacuo,
and purified by column chromatography (SiO2, PE/EtOAc, 1:2) to
yield the Fmoc-protected acyclic precursor 14 (623 mg, 660 µmol,
74%) as a colorless foam. M.p. 120Ϫ123 °C. [α]2D0 ϭ Ϫ52.6 (c ϭ
1.01, CHCl3). C54H64N6O9 (941.12): calcd. C 68.92, H 6.85, N 8.93;
5.12. Minor Isomer 6b: M.p. 66Ϫ70 °C (softening range). [α]2D0
ϭ
ϩ8.8 (c ϭ 1.01, CHCl3). C33H36N2O5 (540.65): calcd. C 73.31, H
6.71, N 5.18; found C 73.01, H 6.73, N 5.05.
Tripeptide Anolog 10a: A mixture of the azetidinone 5a (300 mg,
570 µmol) and Et2NH (753 mg, 1.07 mL, 10.3 mmol) in THF (3
mL) was stirred for ca. 5 h (monitoring by TLC). The volatile com-
ponents were removed (20 mbar, 40 °C) and the residue was repeat-
edly dissolved in THF and the solvents were evaporated in order
to remove traces of Et2NH. The residue was dissolved in THF (6
mL) and FmocϪPheϪOPfp (7, 376 mg, 680 µmol) was added.
After stirring for 2.5 h the solvent was removed (100 mbar, 40 °C),
the residue was dissolved in EtOAc, extracted with brine (3ϫ),
dried (MgSO4), filtered, and concentrated in vacuo. Column chro-
matography (SiO2, PE/EtOAc, 4:1 to 3:1) yielded the tripeptide an-
alog 10a (353 mg, 520 µmol, 92%) as a colorless foam. M.p. 89Ϫ91
°C (softening range). [α]2D0 ϭ Ϫ25.6 (c ϭ 0.99, CHCl3). C41H43N3O6
found
C
68.58,
H
6.87,
N
8.85. HR-MS: calcd. for
12
C
54
1H6514N616O9 941.4813; found m/z ϭ 941.4807.
Boc-Protected Acyclic Precursor 15: Et2NH (1.28 g, 1.82 mL,
17.5 mmol) was added to a solution of the Fmoc-protected acyclic
precursor 14 (551 mg, 580 µmol) in CH2Cl2 (10 mL) and the mix-
ture was stirred at room temp. for 5 h (monitoring by TLC). The
volatile components were removed (20 mbar, 40 °C) and the residue
was repeatedly dissolved in THF and the solvents were evaporated
in order to remove traces of Et2NH. The residue was dissolved in
CH2Cl2 (15 mL), Boc2O (317 mg, 1.45 mmol) was added, and the
solution was stirred at room temp. for 14 h. EtOAc was added and
the mixture was extracted with saturated NH4Cl solution (2ϫ),
saturated NaHCO3 solution, and with brine, dried (MgSO4), fil-
tered, and concentrated in vacuo (50 mbar, 40 °C). Column chro-
matography (SiO2, PE/EtOAc, 2:3) afforded the Boc-protected
acyclic precursor 15 (431 mg, 530 µmol, 91%) as a colorless foam.
M.p. 118Ϫ121 °C (softening range). [α]2D0 ϭ Ϫ69.6 (c ϭ 1.00,
CHCl3). C44H62N6O9 (819.00): calcd. C 64.53, H 7.63, N 10.26;
found C 64.65, H 7.77, N 9.99.
(673.80): calcd. C 73.08, H 6.43, N 6.24; found C 72.55, H 6.45, N
12
6.15. HR-MS: calcd. for
C
41
1H4414N316O6 674.3241; found
m/z ϭ 674.3230.
Tripeptide Anolog 10b: The azetidinone 5b (526 mg, 1.00 mmol)
was deprotected with Et2NH (1.83 g, 2.60 mL, 25.0 mmol) in THF
(6 mL) and subsequently reacted with FmocϪPheϪOPfp (7,
664 mg, 1.20 mmol) in THF (11 mL) as described for compound
10a. Column chromatography (SiO2, PE/EtOAc, 3:1) yielded the
tripeptide analog 10b (532 mg, 790 µmol, 79%) as a colorless foam.
M.p. 96Ϫ99 °C (softening range). [α]2D0 ϭ Ϫ8.5 (c ϭ 1.00, CHCl3).
C41H43N3O6 (673.80): calcd. C 73.08, H 6.43, N 6.24 found C
72.74, H 6.48, N 6.13.
Cyclic Peptidomimetic 16: To a solution of Boc-protected acyclic
precursor 15 (324 mg, 390 µmol) in THF (28 mL), 0.2  aqueous
LiOH (18 mL) was added in 3 portions at 0 °C within 1.5 h. Stir-
ring was continued for 2 h and the solution was added to a mixture
Tripeptide Anolog 11: The azetidinone 5a (1.07 g, 2.03 mmol) was
deprotected with Et2NH (2.68 g, 3.81 mL, 36.6 mmol) in THF (17 of 0.2  HCl (300 mL) and EtOAc (100 mL). The aqueous phase
mL) and subsequently reacted with FmocϪAlaϪOPfp (8, 1.16 g,
2.44 mmol) in THF (17 mL) as described for compound 10a. Col-
was extracted with EtOAc (2ϫ) and the combined organic layers
were dried (MgSO4) and concentrated in vacuo (50 mbar, 40 °C).
umn chromatography (SiO2, PE/EtOAc, 3:1) yielded the tripeptide The residue was dissolved in CH2Cl2 (13 mL), the solution stirred
analog 11 (1.14 g, 1.91 mmol, 94%) as a colorless foam. M.p. and cooled to 0 °C, then pentafluorophenol (72 mg, 0.39 mmol)
94Ϫ96 °C (softening range). [α]2D0 ϭ Ϫ49.4 (c ϭ 1.01, CHCl3).
C35H39N3O6 (597.70): calcd. C 70.33, H 6.58, N 7.03; found C
70.36, H 6.41, N 6.85.
and EDC (82 mg, 0.42 mmol) were added. Stirring was continued
at 0 °C for 2 h and at room temp for another 2 h. The solvents
were removed (300 mbar, 35 °C) and EtOAc and brine were added
Eur. J. Org. Chem. 2004, 4379Ϫ4386
www.eurjoc.org
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4383

T. C. Maier, J. Podlech
FULL PAPER
to the residue. The organic layer was extracted with saturated
NaHCO3 (2ϫ) and with brine, dried (MgSO4), filtered through a
pad of Celite, and concentrated in vacuo (50 mbar, 40 °C).
tide analog 17b (351 mg, 560 µmol, 95%) as a colorless foam. M.p.
92Ϫ104 °C (softening range). [α]2D0 ϭ Ϫ30.9 (c ϭ 1.00, CHCl3).
C34H46N4O7 (622.75): calcd. C 65.57, H 7.45, N 9.00; found C
12
65.19, H 7.49, N 8.77. HR-MS: calcd. for
C
34
1H4714N416O7
Deprotection was achieved by treatment of the residue with TFA
(4.47 g, 3.00 mL, 39.2 mmol) at 0 °C for 2.5 h. After removal of
excess TFA in vacuo (20 mbar, 40 °C) the obtained salt was dis-
solved in CH2Cl2. The cyclization reaction was started by dropwise
addition (1 h) of the CH2Cl2 solution to a vigorously stirred emul-
sion of 1  aqueous K2CO3 (20 mL) and CHCl3 (40 mL). The
aqueous phase was separated after further stirring for 1 h and ex-
tracted with CHCl3 (3ϫ). The combined organic layers were dried
(MgSO4), filtered, and the solvent was evaporated (50 mbar, 40 °C).
Column chromatography (SiO2, PE/EtOAc, 1:1 to 1:3) yielded the
cyclic peptidomimetic 16 (201 mg, 0.29 mmol, 75%) as a colorless
solid. M.p. 164Ϫ166 °C. [α]2D0 ϭ Ϫ74.8 (c ϭ 1.03, CHCl3). IR
(KBr): ν˜ ϭ 3395, 3290 (NϪH), 3041, 3011 (arom. CϪH), 2933,
2911, 2844 (aliphat. CϪH), 1740, 1652 (CϭO), 1520 (δ CϪNϪH
ϩ ν CϪN, amide II), 1443 (δ CϪH and CϭC), 728, 677 (arom.
CϪH o,o,p) cmϪ1. MS (FAB, pos., matrix: NBA): m/z ϭ 709 (23)
623.3445; found m/z ϭ 623.3445.
Acyclic Precursor 19a: To a solution of BocϪAlaϪAlaϪPheϪOMe
(18, 227 mg, 540 µmol) in THF (30 mL), 0.2  aqueous LiOH (90
mL) was added in 3 portions at 0 °C within 1.5 h. Stirring was
continued for 20 h and the solution was added to a mixture of 0.2
 HCl (300 mL) and EtOAc (100 mL). The aqueous phase was
extracted with EtOAc (2ϫ) and the combined organic layers were
dried (MgSO4) and the solvent evaporated (50 mbar, 40 °C). The
residue was dissolved in CH2Cl2 (10 mL), the solution stirred and
cooled to 0 °C, then pentafluorophenol (199 mg, 1.08 mmol) and
EDC (125 mg, 650 µmol) were added. Stirring was continued at 0
°C for 2 h and at room temp. for 2 h The solvents were removed
(300 mbar, 35 °C), EtOAc and brine were added to the residue. The
organic layer was extracted with saturated NaHCO3 (2ϫ) and with
brine, dried (MgSO4), filtered through a pad of Celite, and
concentrated in vacuo (50 mbar, 40 °C) to yield
BocϪAlaϪAlaϪPheϪOPfp. During this procedure ca. 20% epi-
merization at C-α of Phe was observed.
[M
ϩ ϩ
Na]ϩ, 687 (100) [M H]ϩ, 189 (29) [PhCHϭ
NHCH2C(O)NC2H4]ϩ, 131 (25) [PhC3H2O]ϩ, 86 (26) [H2Nϭ
CHCH2CHMe2]ϩ. 1H NMR (500 MHz, CDCl3): δ ϭ 0.87, 0.92 (2
ϫ d, 3J ϭ 6.6 Hz, 2 ϫ 6 H, HLeu-δ), 1.40 (ddd, 2J ϭ 13.9, 3J ϭ
3
3
To the dipeptide analog 6a (292 mg, 540 µmol) dissolved in aceto-
nitrile (15 mL), Et2NH (4.74 g, 6.73 mL, 64.8 mmol) was added,
and the mixture was stirred for 0.5 h at room temp. (monited by
TLC). The volatile components were removed (20 mbar, 40 °C) and
the residue was repeatedly dissolved in acetonitrile and the solvents
were evaporated in order to remove traces of Et2NH. The residue
was diluted in CH2Cl2 (15 mL) and BocϪAlaϪAlaϪPheϪOPfp
(as described above) was added. After stirring at room temp. for
2.5 h, the solvents were removed (100 mbar, 40 °C), the residue was
dissolved in EtOAc, extracted with brine (3ϫ), dried (MgSO4), fil-
tered, and concentrated in vacuo. The major isomer was partly
separated by recrystallization (EtOAc), the mother liquor was then
resolved by MPLC (PE/iPrOH, 91:9) affording the acyclic precur-
sor 19a (177 mg, 250 µmol, 46%), its epimer epi-19a (57 mg, 81
µmol, 15%) and an additional fraction containing a mixture of the
isomers (35 mg, 49 µmol, 9%) as colorless solids. Major Isomer 19a:
M.p. 214Ϫ215 °C. [α]2D0 ϭ Ϫ72.0 (c ϭ 0.50, CHCl3). C38H53N5O8
(707.86): calcd. C 64.48, H 7.55, N 9.89; found C 62.08, H 7.29, N
8.9, J ϭ 5.1 Hz, 2 H, HLeu-βa), 1.53 (d, J ϭ 7.3 Hz, 6 H, HAla
-
β), 1.66 (dseptd, 3J ϭ 8.9, 3J ϭ 6.6, 3J ϭ 5.2 Hz, 2 H, HLeu-γ),
2
3
3
2.00 (ddd, J ϭ 13.9, J ϭ 10.5, J ϭ 5.2 Hz, 2 H, HLeu-βb), 3.16
3
3
2
(dd, J ϭ 3.5, J ϭ 2.5 Hz, 2 H, H-3), 3.74 (d, J ϭ 15.8 Hz, 2 H,
HGly-αa), 3.89 (d, 2J ϭ 15.8 Hz, 2 H, HGly-αb), 4.48Ϫ4.54 (m, 2 H,
3
3
3
HLeu-α), 4.51 (d, J ϭ 2.5 Hz, 2 H, H-4), 4.52 (dq, J ϭ 7.3, J ϭ
7.1 Hz, 2 H, HAla-α), 6.77 (d, 3J ϭ 7.1 Hz, 2 H, NHAla), 7.32Ϫ7.42
(2 ϫ m, 10 H, Ph), 7.65 (d, 3J ϭ 9.2 Hz, 2 H, NHLeu) ppm. 13C
NMR (125 MHz, CDCl3): δ ϭ 17.79 (q, CAla-β), 21.62 (q, CLeu
-
δa), 23.09 (q, CLeu-δb), 24.94 (d, CLeu-γ), 41.19 (t, CLeu-β), 45.05
(d, CLeu-α), 48.17 (t, CGly-α), 51.14 (d, CAla-α), 59.37 (d, C-4), 64.32
(d, C-3), 126.52, 128.79, 129.15 (3 ϫ d, aryl-C Ph), 136.52 (s, aryl-
C ipso Ph), 166.07 (s, COGly), 169.27 (s, C-2), 172.91 (s, COAla
)
ppm. C38H50N6O6 (686.84): calcd. C 66.45, H 7.34, N 12.24; found
12
C 65.03, H 7.28, N 11.87. HR-MS: calcd. for
C
1H5114N616O6
38
687.3870; found m/z ϭ 687.3873.
Tetrapeptide Analog 17a: Et2NH (627 mg, 890 µL, 8.60 mmol) was
added to a solution of the tripeptide analog 10a (321 mg, 470 µmol)
in CH2Cl2 (7 mL) and the mixture was stirred at room temp. for
5 h (monitored by TLC). The volatile components were removed
(20 mbar, 40 °C) and the residue was repeatedly dissolved in THF
and the solvents were evaporated to remove traces of Et2NH. The
residue was dissolved in THF (8 mL) and BocϪAlaϪOPfp (9,
176 mg, 520 µmol) was added. After stirring for 2.5 h the solvent
was removed (100 mbar, 40 °C), the residue was dissolved in
EtOAc, extracted with brine (3ϫ), dried (MgSO4), filtered, and
concentrated in vacuo (50 mbar, 40 °C). Column chromatography
(SiO2, PE/EtOAc, 2:1) yielded the tetrapeptide analog 17a (281 mg,
9.55. HR-MS: calcd. for 12C38 H5414N516O8 708.3972; found m/z ϭ
1
708.3984. Minor Isomecr epi-19a: M.p. 188Ϫ189 °C. [α]2D0 ϭ Ϫ35.4
(c ϭ 0.50, CHCl3). C38H53N5O8 (707.86): calcd. C 64.48, H 7.55,
N 9.89; found C 64.34, H 7.47, N 9.91.
Acyclic Precursor 19b: Dipeptide analog 6b (292 mg, 540 µmol) was
deprotected and reacted with BocϪAlaϪAlaϪPheϪOPfp as de-
scribed for compound 19a (reaction of intermediate 6a). Separation
of side products and impurities by column chromatography (PE/
EtOAc, 3:1 to 1:1) and separation of the isomers by MPLC (PE/
iPrOH, 9:1) afforded the acyclic precursor 19b (195 mg, 275 µmol,
51%) and its epimer epi-19b (65 mg, 92 µmol, 17%) as colorless
solids. Major Isomer 19b: M.p. 180Ϫ182 °C. [α]2D0 ϭ Ϫ39.5 (c ϭ
1.00, CHCl3). C38H53N5O8 (707.86): calcd. C 64.48, H 7.55, N 9.89;
450 µmol, 96%) as a colorless foam. M.p. 135Ϫ137 °C. [α]D20
ϭ
Ϫ53.7 (c ϭ 1.01, CHCl3). C34H46N4O7 (622.75): calcd. C 65.57, H
7.45, N 9.00; found C 65.12, H 7.24, N 8.85. HR-MS: calcd. for
12
1H4714N416O7 623.3445; found m/z ϭ 623.3448.
found
C
63.99,
H
7.57,
N
9.70. HR-MS: calcd. for
C
34
12
C
38
1H5414N516O8 708.3972; found m/z ϭ 708.3971. Minor Isomer
Tetrapeptide Analog 17b: Et2NH (4.29 g, 6.10 mL, 58.7 mmol) and
the tripeptide analog 10b (400 mg, 590 µmol) dissolved in aceto-
nitrile (12 mL) were allowed to react as described for compound
17a. Traces of Et2NH were removed by evaporation with aceto-
nitrile. The residue was dissolved in THF (12 mL) and reacted with
BocϪAlaϪOPfp (9, 218 mg, 650 µmol) as described above. Col-
umn chromatography (SiO2, PE/EtOAc, 7:3) yielded the tetrapep-
epi-19b: M.p. 186Ϫ187 °C. [α]2D0 ϭ Ϫ12.2 (c ϭ 0.50, CHCl3). HR-
MS: calcd. for 12C38 H5414N516O8 708.3972; found m/z ϭ 708.3976.
1
Cyclic Peptidomimetic 20: Saponification of the Boc-protected
acyclic precursor 17a (205 mg, 329 µmol) in THF (25 mL) with
0.2  aqueous LiOH (15 mL), activation with pentafluorophenol
(63 mg, 0.34 mmol after addition of 15 mL CH2Cl2) and EDC
4384
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
Eur. J. Org. Chem. 2004, 4379Ϫ4386

β-Lactam-Containing Cyclopeptide Analogs
FULL PAPER
7.2, J ϭ 7.0 Hz, 1 H, HAla-III-α or HAla-IV-α), 3.91 (dq, J ϭ 7.3,
3
3
(79 mg, 0.41 mmol), deprotection with TFA (4.10 g, 2.75 mL,
36.0 mmol) and cyclization in vigorously stirred 1  aqueous 3J ϭ 7.2 Hz, 1 H, HAla-III-α or HAla-IV-α), 4.10 (ddq, 3J ϭ 7.0, 3J ϭ
3
3
K2CO3 (18 mL)/CHCl3 (36 mL) was achieved as described for com-
pound 16. Recrystallization afforded the cyclic peptidomimetic 20
(150 mg, 307 µmol, 93%) as a colorless solid. M.p. 225Ϫ227 °C.
[α]2D0 ϭ Ϫ12.7 (c ϭ 1.00, CHCl3). IR (KBr): ν˜ ϭ 3310 (NϪH),
3038, 3005 (arom. CϪH), 2929, 2903, 2842 (aliphat. CϪH), 1749,
6.8, J ϭ 3.8 Hz, 1 H, HAla-I-α), 4.11 (d, J ϭ 2.6 Hz, 1 H, H-4),
3
3
3
4.46 (ddd, J ϭ 9.0, J ϭ 8.2, J ϭ 7.8 Hz, 1 H, HPhe-α), 7.03 (d,
3J ϭ 7.3 Hz, 1 H, NHAla-III or NHAla-IV), 7.17Ϫ7.23, 7.27Ϫ7.35 (2
ϫ m, 11 H, 2 Ph, NHAla-I), 7.57 (d, 3J ϭ 9.0 Hz, 1 H, NHPhe), 7.87
(d, 3J ϭ 7.1 Hz, NHAla-III or NHAla-IV) ppm. 13C NMR (125 MHz,
1672, 1652 (CϭO), 1520 (δ CϪNϪH and ν CϪN, amide II), 1443 CD2Cl2): δ ϭ 16.32 (2 ϫ q, CAla-III-β, C-IV-β), 18.62 (q, CAla-I-β),
(δ-CϪH and CϭC), 724, 677 (arom. CϪHo,o,p) cmϪ1. MS (FAB,
21.41, 22.96 (2 ϫ q, CLeu-δ), 25.69 (d, CLeu-γ), 38.20 (t, CPhe-β),
pos., matrix: NBA): m/z ϭ 513 (43) [M ϩ Na]ϩ, 491 (100) [M ϩ 40.07 (t, CLeu-β), 43.91 (d, CAla-I-α), 50.70, 50.77 (2 ϫ d, CAla-III
-
H]ϩ, 189 (9) [PhCHϭNHCH2C(O)NC2H4]ϩ, 131 (13) α, CAla-IV-α), 57.08 (d, CPhe-α), 58.27 (d, C-4), 59.65 (d, CLeu-α),
[PhC3H2O]ϩ, 120 (27) [H2NϭCHCH2Ph]ϩ, 91 (15) [C7H7]ϩ. 1H
63.25 (d, C-3), 127.18, 127.31, 128.80, 129.07, 129.23, 129.62 (6 ϫ
3
NMR (300 MHz, CDCl3, 330 K): δ ϭ 0.89 (d, J ϭ 6.5 Hz, 3 H, d, aryl-C Ph), 137.52, 137.72 (2 ϫ s, aryl-C ipso Ph), 169.65, 171.59,
HLeu-δa), 0.95 (br. d, 3J ϭ 6.6 Hz, 3 H, HLeu-δb), 1.33 (d, 3J ϭ
171.75, 171.82, 172.91 (5 ϫ s, 5 CϭO) ppm. HR-MS: calcd. for
2
3
3
12
6.9 Hz, 3 H, HAla-β), 1.37 (ddd, J ϭ 13.9, J ϭ 8.8, J ϭ 4.8 Hz,
C
32
1H4214N516O5 576.3186; found m/z ϭ 576.3201.
2
3
1 H, HLeu-βa), 1.67 (mc, 1 H, HLeu-γ), 1.89 (ddd, J ϭ 13.9, J ϭ
10.5, 3J ϭ 5.4 Hz, 1 H, HLeu-βb), 3.06 (d, 2J ϭ 14.3 Hz, 1 H,
HGly-αa), 3.14 (dd, 3J ϭ 2.6, 3J ϭ 2.5 Hz, 1 H, H-3), 3.26 (dd, 2J ϭ
13.7, 3J ϭ 5.3 Hz, 1 H, HPhe-βa), 3.77 (mc, 1 H, HPhe-βb), 3.96 (mc,
Polymerized Material 22: To a solution of the Boc-protected acyclic
precursor 17b (100 mg, 160 µmol) in THF (2.5 mL), 0.2  aqueous
LiOH (7.2 mL) was added in 3 portions at 0 °C within 1.5 h. Stir-
ring was continued for 2 h and the solution was added to a mixture
of 0.2  HCl (300 mL) and EtOAc (100 mL). The aqueous phase
was extracted with EtOAc (2ϫ) and the combined organic layers
were dried (MgSO4), filtered, and concentrated in vacuo (50 mbar,
40 °C). The residue was dissolved in CH2Cl2 (5 mL), pentafluoro-
phenol (33 mg, 0.18 mmol) and EDC (36 mg, 0.19 mmol) were ad-
ded at 0 °C. Stirring was continued for 2 h at 0 °C and for 2 h at
room temp. The solvents were removed (300 mbar, 35 °C) and
EtOAc and brine were added to the residue. The organic layer was
extracted with saturated NaHCO3 (2ϫ) and with brine, dried
(MgSO4), filtered through a pad of Celite, and the solvent was
evaporated (50 mbar, 40 °C).
2
3
1 H, HPhe-α), 4.22 (d, J ϭ 14.3 Hz, 1 H, HGly-αb), 4.35 (dq, J ϭ
9.6, 3J ϭ 6.9 Hz, 1 H, HAla-α), 4.42 (d, 3J ϭ 2.5 Hz, 1 H, H-4),
4.49 (dddd, 3J ϭ 10.5, 3J ϭ 9.7, 3J ϭ 4.8, 3J ϭ 2.6 Hz, 1 H, HLeu-α),
6.57 (br. s, 1 H, NHPhe), 6.70 (d, 3J ϭ 9.6 Hz, 1 H, NHAla),
7.18Ϫ7.39 (m, 10 H, 2 Ph) ppm, signal of NHLeu is hidden. 13C
NMR (125 MHz, CDCl3): δ ϭ 14.37 (q, CAla-β), 21.44, 23.22 (2 ϫ
q, CLeu-δ), 25.14 (d, CLeu-γ), 35.55 (t, CPhe-β), 41.63 (t, CLeu-β),
44.30 (d, CLeu-α), 46.04 (t, CGly-α), 49.05 (d, CAla-α), 57.96 (d, C-
4), 62.93 (d, CPhe-α), 65.53 (d, C-3), 126.33, 126.90, 128.54, 128.78,
128.95, 129.06 (6 ϫ d, aryl-C Ph), 136.51, 137.44 (2 ϫ s, aryl-C
ipso Ph), 169.90, 169.43, 173.24, 173.56 (4 ϫ s, CϭO amide, β-
lactam) ppm. C28H34N4O4 (490.59): calcd. C 68.55, H 6.99, N
11.42; found C 67.92, H 6.96, N 11.19. HR-MS: calcd. for
Deprotection was achieved by treatment of the residue with TFA
(1.82 g, 1.22 mL, 16.0 mmol) at 0 °C for 2.5 h. After removal of
excess TFA in vacuo (20 mbar, 40 °C) the obtained salt was dis-
solved in CH2Cl2, and cyclization was attempted by dropwise ad-
dition (1 h) of the solution to a vigorously stirred emulsion of 1 
aqueous K2CO3 (9 mL) and CHCl3 (16 mL) as described pre-
viously. The aqueous phase was separated after further stirring for
1 h and extracted with CHCl3 (3ϫ). The combined organic layers
were dried (MgSO4), filtered through a pad of Celite, and the
solvent was evaporated (50 mbar, 40 °C) to leave a polymeric mate-
rial 22 (95 mg) which was not further purified.
1H3514N416O4 491.2658; found m/z ϭ 491.2671.
12
C
28
Cyclic Peptidomimetic 21: To a solution of the Boc-protected
acyclic precursor 19a (100 mg, 141 µmol) in DMF (6.5 mL), 1 
aqueous NaOH (420 µL) was added dropwise at room temp. within
3 h. Stirring was continued for 15 h and the solution was added to
a mixture of 0.5  HCl (25 mL) and CH2Cl2 (45 mL). The aqueous
phase was extracted with CH2Cl2 (3ϫ) and the combined organic
layers were dried (MgSO4), filtered, and concentrated in vacuo
(200 mbar, 40 °C). Activation with pentafluorophenol (28 mg,
0.15 mmol after addition of 5 mL CH2Cl2) and EDC (31 mg,
0.16 mmol), deprotection with TFA (3.19 g, 2.14 mL, 28.0 mmol)
and cyclization in vigorously stirred 1  aqueous K2CO3 (14 mL)/
CHCl3 (28 mL) was achieved as described for compound 16. Col-
umn chromatography (SiO2, PE/EtOAc, 1:3) afforded the cyclic
peptidomimetic 21 (49 mg, 85 µmol, 60%) as colorless solid. M.p.
254Ϫ256 °C. [α]2D0 ϭ Ϫ94.1 (c ϭ 0.80, CHCl3). IR (KBr): ν˜ ϭ 3400,
3307 (NϪH), 3043, 3012 (arom. CϪH), 2938, 2910, 2851 (aliphat.
CϪH), 1758Ϫ1740, 1672Ϫ1643 (CϭO), 1521 (δ CϪNϪH and ν
CϪN, amide II), 1442 (δ CϪH and CϭC), 722, 677 (arom.
CϪHo,o,p) cmϪ1. MS (FAB, pos., matrix: NBA): m/z ϭ 598 (18) [M
ϩ Na]ϩ, 576 (100) [M ϩ H]ϩ, 131 (32) [PhC3H2O]ϩ, 120 (21)
[H2NϭCHCH2Ph]ϩ. AlaI is connected to the β-lactam followed by
Phe, AlaIII, AlaIV and Leu: 1H NMR (500 MHz, CD2Cl2): δ ϭ 0.71
Polymerized Material 23: Saponification of the Boc-protected
acyclic precursor 19b (124 mg, 180 µmol) in THF (3 mL) with 0.2
 aqueous LiOH (7.8 mL), activation in CH2Cl2 (6 mL) with penta-
fluorophenol (33 mg, 0.18 mmol) and EDC (38 mg, 0.20 mmol),
deprotection with TFA (2.09 g, 1.40 mL, 18.3 mmol) and addition
to an emulsion of 1  K2CO3 solution (9 mL) and CHCl3 (18
mL) as described for compound 22 yielded a polymeric material 23
(128 mg) which was not further purified. MS (MALDI-TOF, pos.,
matrix: ATT): m/z ϭ 2306 (traces, n ϭ 4), 1730 (traces, n ϭ 3),
1153 (35, n ϭ 2), 577 (100, n ϭ 1).
Supporting Information: Spectroscopic data with peak assignments
for all compounds. 1H and 13C NMR spectra of compounds 15,
16, 17a, 19a, 20, 21. A scheme rationalizing the conformation in
the vicinity of the β-lactam in compound 16.
(d, 3J ϭ 6.5 Hz, 3 H, HLeu-δa), 0.91 (d, 3J ϭ 6.6 Hz, 3 H, HLeu
-
3
3
δb), 1.25 (d, J ϭ 6.8 Hz, 3 H, HAla-I-β), 1.26 (d, J ϭ 7.3 Hz, 3 H,
HAla-III-β or HAla-IV-β), 1.44 (d, 3J ϭ 7.0 Hz, 3 H, HAla-III-β or
3
3
HAla-IV-β), 1.61 (ddd, 2J ϭ 13.7, J ϭ 9.0, J ϭ 5.8 Hz, 1 H, HLeu
-
Acknowledgments
This work was supported by the Fonds der Chemischen Industrie,
βa), 1.81 (dseptd, 3J ϭ 9.0, 3J ϭ 6.6, 3J ϭ 4.9 Hz, 1 H, HLeu-γ),
2.38 (ddd, 2J ϭ 13.7, 3J ϭ 10.6, 3J ϭ 4.9 Hz, 1 H, HLeu-βb),
3
3
3.09Ϫ3.14 (m, 2 H, HPhe-β), 3.14 (dd, J ϭ 3.8, J ϭ 2.6 Hz, 1 H, the Deutsche Forschungsgemeinschaft, and the Landesgraduierten-
H-3), 3.49 (dd, 3J ϭ 10.6, 3J ϭ 5.8 Hz, 1 H, HLeu-α), 3.90 (dq, 3J ϭ förderung Baden-Württemberg (stipend to T. C. M.).
Eur. J. Org. Chem. 2004, 4379Ϫ4386
www.eurjoc.org
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4385

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Received June 2, 2004
4386
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www.eurjoc.org
Eur. J. Org. Chem. 2004, 4379Ϫ4386