Synthesis of protected derivatives and short peptides of antAib, a novel Cα-tetrasubstituted α-amino acid of the Ac5c type possessing a fused anthracene fluorophore

Article abstract of DOI:10.1016/j.tet.2006.04.055

The N^α-Boc and N^α-Fmoc protected derivatives of 2-amino-2,3-dihydro-1H-cyclopenta[b]anthracene-2-carboxylic acid (antAib), a novel fluorescent, achiral, α-amino acid, rigid analogue of the known 9-antAla and 2-antAla residues, and be

Full text of DOI:10.1016/j.tet.2006.04.055

Tetrahedron 62 (2006) 6203–6213
Synthesis of protected derivatives and short peptides of antAib,
a novel Ca-tetrasubstituted a-amino acid of the Ac5c type
possessing a fused anthracene fluorophore
Jean-Franc¸ois Lohier,a Karen Wright,a Cristina Peggion,b Fernando Formaggio,b
a
a,
Claudio Toniolo,b, Michel Wakselman and Jean-Paul Mazaleyrat
*
*
aInstitut Lavoisier, UMR CNRS 8180, University of Versailles, F-78035 Versailles, France
bDepartment of Chemistry, University of Padova, I-35131 Padova, Italy
Received 28 February 2006; revised 18 April 2006; accepted 20 April 2006
Available online 19 May 2006
Abstract—The Na-Boc and Na-Fmoc protected derivatives of 2-amino-2,3-dihydro-1H-cyclopenta[b]anthracene-2-carboxylic acid (antAib),
a novel fluorescent, achiral, a-amino acid, rigid analogue of the known 9-antAla and 2-antAla residues, and belonging to the class of Cai /Cia
cyclized, Ca,a-disubstituted glycines (strong b-turn and helix inducers in peptides), were synthesized in seven steps from 1,2,4-trimethyl-
benzene. The UV absorption and fluorescence properties of Boc–antAib–OEt and Boc–antAib–OH are also described. Solution syntheses of the
short peptides Boc–antAib–L-Ala–OMe, Fmoc–L-Ala–antAib–L-Ala–OMe, as well as Boc–Aib–antAib–L-Ala–OMe and the side product
2,5-dioxopiperazine cyclo-[antAib–L-Ala], are presented as examples of the coupling ability at both C- and N-termini of the antAib residue.
Ó 2006 Elsevier Ltd. All rights reserved.
1. Introduction
a Ca,a-disubstituted glycine derived from 1,10-binaphthyl,
to carry out photophysical studies involving intramolecular
energy transfer (fluorescence quenching) and intramolecular
spin polarization (CIDEP) effects in conformationally con-
strained peptide-based systems.3 However, interpretation of
the data was complicated by the nonplanar structure of the
Bin 1,10-binaphthyl core. To circumvent this problem, we
have now designed 2-amino-2,3-dihydro-1H-cyclopenta-
[b]anthracene-2-carboxylic acid (antAib), a new fluorescent
a-amino acid residue which is based on a planar anthracene
core and, like Bin, belongs to the class of Cai /Cia cyclized,
Fluorescence spectroscopy has become a highly valuable
technique for conformational studies of biopolymers, the
development of peptide-based chemosensors, and biochem-
ical research in general.1 Incorporation of a fluorescent
probe into a peptide chain may be achieved by reaction
with side-chain functional groups or the direct use of fluoro-
phore-bearing amino acids. In this connection, synthetic
fluorescent amino acids may exhibit significant advantages
over the related protein (Trp, Tyr) residues in terms of poten-
tially different and improved properties. In previous studies,
we took advantage of the fluorescence, the increased rigidity,
and the axial chirality of 20,10:1,2;100,200:3,4-dinaphthcyclo-
hepta-1,3-diene-6-amino-6-carboxylic acid (Bin)2 (Fig. 1),
C
a,a-disubstituted glycines (effective b-turn and helix in-
ducers in peptides4). The achiral antAib residue may be re-
garded either as a rigid analogue of the known 9-antAla5
(or its cyano derivative Flu1a) and 2-antAla5b residues,
CO-
NH-
CO-
NH-
CO-
NH-
H
H
-HN
CO-
(
S
)-Bin
(S)-2-antAla
antAib
(S)-9-antAla
Figure 1. Chemical structure of the antAib residue compared with Bin,2 9-antAla,5 and 2-antAla.5b
Keywords: Anthracene; Aromatic amino acid; Ca,a-Disubstituted glycine; Fluorescent amino acid.
* Corresponding authors. Fax: +33 01 39 25 44 52 (J.-P.M.); +39 049 827 5239 (C.T.); e-mail addresses: claudio.toniolo@unipd.it; jean-paul.mazaleyrat@
chimie.uvsq.fr
0040–4020/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2006.04.055

6204
J.-F. Lohier et al. / Tetrahedron 62 (2006) 6203–6213
with the spatial disposition of the anthracene side-chain
fluorophore relative to the a-carbon atom being completely
defined, or as an anthracene-fused 1-aminocyclopentanecar-
boxylic acid (Ac5c). Only quinonic derivatives of antAib
have been synthesized previously by different methods.6
Reduction of the diester 5 was accomplished under various
experimental conditions. The expected diol was not isolated,
but treated directly with acetic anhydride (Ac2O) in pyri-
dine8,9 to afford the corresponding 2,3-(bis)-acetoxy-
methyl-anthracene 6 (Fig. 3). The use of lithium aluminum
hydride in THF (tetrahydrofuran) as the reducing agent
always resulted in partial over-reduction of the central ring
with formation of 2,3-(bis)-acetoxymethyl-9,10-dihydro-
anthracene 7 along with 6, in a ratio depending on reaction
time and temperature (Table 1). Large-scale separation of
the desired compound 6 from the side product 7 by chroma-
tography was not possible because of their close Rf values (as
observed by analytical TLC). However, their separation by
fractional crystallization from acetone allowed the recovery
of pure 6 in 46% yield from the first crop.
2. Results and discussion
For the synthesis of antAib, the known dimethyl anthracene-
2,3-dicarboxylate 5 was used as a key intermediate. This
compound was readily prepared by using an easily reproduc-
ible, published, procedure (Fig. 2),7 in which Friedel–Crafts
acylation of 1,2,4-trimethylbenzene 1 with benzoyl chloride
gave the resulting benzophenone derivative 2 in 67% yield.
In the second step, the two-stage oxidation of all three methyl
groups present in 2 afforded the tricarboxylic acid 3 (61%),
which upon cyclization in concentrated sulfuric acid formed
the anthraquinone dicarboxylic acid 4 (88%). Reduction of
the quinone part of 4 with activated zinc dust and ammonium
hydroxide gave 2,3-anthracene dicarboxylic acid as a soft
solid precipitate which could not be collected easily by filtra-
tion (see Section 3). Therefore, it was not purified, but
collected by centrifugation and then directly esterified in
refluxing methanol (MeOH) containing 98% H2SO4 to af-
ford the dimethyl ester 5 in 71% overall yield after extraction
followed by crystallization from acetone/methanol.
To transform 5 into the corresponding diol without over-
reduction of the central ring, the best method we found was
the use of sodium bis(2-methoxyethoxy)aluminum hydride
(RedAlÒ) at low temperature, as proposed by Sun and
Desper in a similar case.8 Here, another side product was
formed, the mono-reduced 2-acetoxymethyl-3-methyl-
anthracene 8, but in a very minor proportion relative to 6
(3% vs 86% isolated yields) when the reaction was conducted
at low temperature (ꢀ15 ꢁC to +3 ꢁC) for a short period of
time (1 h 15 min) (Table 1). Furthermore, the separation of
8 from 6 by chromatography was easy. Subsequent crystalli-
zation from boiling MeOH/CH2Cl2 afforded analytically
pure diacetate 6 in 81% yield.
CH3
Bromuration of the diacetate 6 with HBr in acetic acid9 read-
ily furnished 2,3-(bis)-bromomethyl-anthracene 9 (Fig. 4) in
98% yield after crystallization from boiling MeOH/CH2Cl2.
The dibromide 9 was then used as an electrophile for the bis-
alkylation of ethyl isocyanoacetate under the phase transfer
conditions developed by Kotha and Brahmachary,6b with po-
tassium carbonate as a base and tetra-n-butylammonium
(nBu4N+) hydrogen sulfate as the catalyst, in acetonitrile at
80 ꢁC. The resulting 2-isocyano-2,3-dihydro-1H-cyclo-
penta[b]anthracene-2-carboxylic acid ethyl ester 10a was not
isolated, but rather the crude reaction mixture was submitted
to acid hydrolysis to afford directly the desired a-amino
H3C
CH3
1
2
3
(i)
O
CH3
H3C
O
CH3
(ii)
CO2H
CO2H
HO2C
O
(iii)
CO2H
CO2H
Table 1. Product distribution (%) after reduction of the diester 5 followed by
acetylation
O
4
(iv)
Reduction conditions
6
7
8
CO2Me
CO2Me
LiAlH4 (THF) 0 ꢁC to rt (2 h)
30a
70a
LiAlH4 (THF) ꢀ14 ꢁC to ꢀ2 ꢁC (1 h)
LiAlH4 (THF) ꢀ60 ꢁC to ꢀ15 ꢁC (2 h)
RedAlÒ (THF) 0 ꢁC (2 h 30 min)
RedAlÒ (THF) 0 ꢁC (1 h 15 min)
82a 36b 18a
85a 46b 15a
54b
5
10b
5b
73b
Figure 2. Synthesis of dimethyl anthracene-2,3-dicarboxylate 5. (i)
C6H5COCl; AlCl3; 0 ꢁC to rt; 67%. (ii) (1) 20% aq HNO3; reflux; (2)
10% aq NaOH; KMnO4; reflux; (3) H+; 62%. (iii) 98% H2SO4; 120–
130 ꢁC; 87%. (iv) (1) Activated Zn powder; 20% aq NH4OH; reflux; (2)
98% H2SO4; MeOH; reflux (71%).
RedAlÒ (THF) ꢀ15 ꢁC to +3 ꢁC (1 h 15 min) 86b
3b
Ratio determined by 1H NMR.
Isolated yield.
a
b
(i)
CO2Me
CO2Me
CH2OAc
CH2OAc
CH2OAc
CH2OAc
CH2OAc
CH3
or (ii)
(iii)
5
6
7
8
Figure 3. Reduction of the diester 5 followed by acetylation. (i) LiAlH4; THF; low temperature (see Table 1). (ii) RedAlÒ 3.5 M in toluene; THF; low
temperature (see Table 1). (iii) Ac2O; pyridine.

J.-F. Lohier et al. / Tetrahedron 62 (2006) 6203–6213
6205
CH2OAc
CH2OAc
CH2Br
CH2Br
COOR
NC
(i)
(ii)
6
9
10a (R = Et)
10b (R = tBu)
(iii)
COOH
NH-X
COOR
NH-X
COOR
NH2
(iv)
(v)
13a (X = Boc)
12a (R = Et ; X = Boc)
11a (R = Et)
13b (X = Fmoc)
12b (R = tBu ; X = Fmoc)
11b (R = tBu)
Figure 4. Synthesis of Boc–antAib–OH 13a and Fmoc–antAib–OH 13b from the diacetate 6. (i) 33% HBr in AcOH; CH2Cl2; rt. (ii) CN–CH2–COOEt (a series)
n
or CN–CH2–COOtBu (b series); K2CO3 or Cs2CO3; Bu4N+, HSOꢀ4 (cat); CH3CN; 80 ꢁC. (iii) 10 N aq HCl; abs EtOH; 0 ꢁC to rt. (iv) Boc2O; CH3CN; rt
(a series) or Fmoc–OSu; CH3CN/CH2Cl2; rt (b series). (v) 1 N NaOH; MeOH/THF; rt (a series) or TFA/CH2Cl2 1:2; 0 ꢁC to rt (b series).
ester H–antAib–OEt 11a in 28% yield after chromatography.
It may be pointed out that this relatively moderate yield, due
to unidentified side reactions of the dibromide 9, is in agree-
ment with the 29.7% yield10 previously obtained by Kotha
et al.6b in a similar synthesis of the parent anthraquinone-
fused (instead of anthracene-fused) amino ester. Treatment
of 11a with di-tert-butyl dicarbonate in acetonitrile gave
the fully protected derivative Boc–antAib–OEt (Boc, tert-
butyloxycarbonyl; OEt, ethoxy) 12a in only 37% yield after
chromatography. In later runs we preferred not to isolate the
a-amino ester 11a but to perform N-Boc-protection directly
on the crude product obtained after acid hydrolysis, thus re-
ducing purification steps, which resulted in a slight increase
in the overall yield in 12a from 9 (18–25%). In a similar
manner, the bis-alkylation of tert-butyl isocyanoacetate
was conducted in acetonitrile at 80 ꢁC, using cesium carbon-
ate as a base and tetrabutylammonium hydrogen sulfate as
the phase transfer catalyst, to afford 2-isocyano-2,3-di-
hydro-1H-cyclopenta[b]anthracene-2-carboxylic acid tert-
butyl ester 10b in 29% yield after chromatography.
Remarkably, treatment of 10b with a few drops of concen-
trated hydrochloric acid (ca. 10 N) in ethanol (EtOH)/di-
chloromethane, from 0 ꢁC to room temperature for a few
hours (monitored by TLC) allowed the selective acidolysis
of the isonitrile function without cleavage of the tert-butyl
ester, and furnished H–antAib–OtBu (OtBu, tert-butoxy)
11b in 90% yield after chromatography. Protection of the
amino function of 11b by the Fmoc (9-fluorenylmethyloxy-
carbonyl) group was performed using Fmoc–OSu (OSu,
1-oxysuccinimide) in dichloromethane at room temperature
to afford Fmoc–antAib–OtBu 12b in 86% yield. Finally, C-
deprotection of 12a by saponification of the ethyl ester func-
tion with 1 N NaOH in MeOH/THF gave Boc–antAib–OH
13a (89–96% yield), and C-deprotection of 12b by acidoly-
sis of the tert-butyl ester function with TFA (trifluoroacetic
acid)/CH2Cl2 gave 13b (92% yield), both suitable for use
in peptide synthesis.
Solution synthesis of di- and tripeptides based on antAib
was carried out in order to investigate the coupling ability
at both C- and N-termini of such a structurally constrained
residue. Coupling of Boc–antAib–OH and HCl$L-H–Ala–
OMe was performed by EDC [N-ethyl, N0-(3-dimethyl-
aminopropyl)-carbodiimide]/HOAt (7-aza-1-hydroxy-1,2,
3-benzotriazole)11 C-activation to furnish Boc–antAib–L-
Ala–OMe 14a (Fig. 5) in 50–60% yield. This dipeptide
was Na-deprotected with TFA/CH2Cl2 1:3 and the resulting
TFA$H–antAib–L-Ala–OMe (not isolated) was coupled
with the urethane-protected N-carboxyanhydride (UNCA)12
Fmoc–L-Ala–NCA, to afford the tripeptide Fmoc–L-Ala–
antAib–L-Ala–OMe 15a in 83% overall yield. These two
methods are known to be efficient in difficult cases involving
sterically demanding Ca-tetrasubstituted a-amino acids.13 It
is also interesting to note that, as previously observed in
a similar case,14 coupling of Na-deprotected 14a with the
more hindered Boc–Aib–NCA (Aib, a-aminoisobutyric
acid) afforded the tripeptide Boc–Aib–antAib–L-Ala–OMe
16a in only 14% overall yield, because of a competitive cy-
clization to the 2,5-dioxopiperazine cyclo-[antAib–L-Ala]
17a, isolated in 48% yield. One can expect therefore that
in general cases where antAib is at the N-terminal position
of a peptide, but not of a dipeptide methyl ester (where
a fast cyclization reaction is favored), coupling at its amino
function with an additional Ca-tetrasubstituted a-amino
acid, should occur efficiently.
We have recorded the UV absorption spectra of
Boc–antAib–OEt 12a and Boc–antAib–OH 13a and their
fluorescence spectral signatures in ethanol solution. The
symmetry-allowed, intense, S0/S1 transition of the
CO-L-Ala-OMe
COOH
CO-L-Ala-OMe
NH-L-Ala-Fmoc
(ii)(iii)
(i)
NH-Boc
NH-Boc
13a
14a
(ii)(iv)
15a
O
H
H
N
CO-L-Ala-OMe
NH-Aib-Boc
+
Me
N
O
16a
17a
Figure 5. Solution synthesis of di- and tripeptides based on antAib. (i) HCl$H–L-Ala–OMe; NMM (N-methylmorpholine); EDC; HOAt; THF/CH2Cl2; rt.
(ii) TFA/CH2Cl2 (1:3); 0 ꢁC. (iii) Fmoc–L-Ala–NCA; DIEA (N,N,N-diisopropylethylamine); THF; 0 ꢁC to rt. (iv) Boc–Aib–NCA; DIEA; THF; 60 ꢁC.

6206
J.-F. Lohier et al. / Tetrahedron 62 (2006) 6203–6213
anthracene chromophore is evident as a Frank–Condon
vibronic progression with an origin (0,0) at 378 nm and
at much longer wavelengths than those typical of coded
aromatic a-amino acids. To determine antAib preferred con-
formation, peptides based on this residue in combination
with L-Ala to the hexamer level are currently being syn-
thesized in our groups. Secondary structures involving
b-turns/310-helices are expected to be efficiently induced
by antAib as it is a member of the family of Cai /Cia cy-
clized, Ca,a-disubstituted glycines,4 thus allowing its exploi-
tation as a fluorophore in photophysical studies of rigid,
folded, peptide architectures.
additional peaks (0,1–0,4) separated by ca. 16 nm (lmax
¼
359 nm)15 (Fig. 6). The emission spectra (lexc¼359 nm)
have an origin at 384–385 nm, a maximum intensity at
407–408 nm, and two other peaks of the vibronic progres-
sion clearly observed at longer wavelengths (Fig. 7).
In conclusion, the syntheses of the protected derivatives
Boc–antAib–OEt and Fmoc–antAib–OtBu were achieved
in seven steps from 1,2,4-trimethylbenzene. Saponification
of the ester function of Boc–antAib–OEt and acidolysis of
Fmoc–antAib–OtBu afforded the corresponding N-protected
amino acids, suitable for peptide elongation using either Boc
or Fmoc strategies. Solution peptide syntheses of the tri-
peptides Fmoc–L-Ala–antAib–L-Ala–OMe and Boc–Aib–
antAib–L-Ala–OMe demonstrated the coupling efficiency
at both C- and N-termini of the structurally constrained
antAib residue. The fluorescence spectra of Boc–antAib–
OEt and Boc–antAib–OH suggest that the antAib residue
may represent a novel useful spectroscopic probe in studies
of peptide molecules by virtue of the appearance of its bands
Altogether, we believe that the antAib residue will expand
the scope of fluorescence analysis of peptide conformations
and interactions in solution, as it represents a unique label
with frozen main-chain and side-chain rotational freedoms,
in contrast with all fluorescent probes proposed so far.
That may be of main interest for the design of conformation-
ally constrained bioactive peptide systems.16
3. Experimental
Melting points were measured on a Mettler apparatus with
a final temperature raise of 3 ꢁC/min or by means of a capil-
lary tube immersed in an oil bath (Tottoli apparatus, Buchi)
¨
1
and are uncorrected. H NMR and 13C NMR spectra were
recorded on a Bruker WM300 spectrometer operating at
300 MHz and 77 MHz, respectively, the solvent being
used as the internal standard: CDCl3 (1H: d¼7.27 ppm;
13C: d¼77.00 ppm), CD3COCD3 (1H: d¼2.05 ppm; 13C:
d¼29.80 ppm), CD3SOCD3 (1H: d¼2.50 ppm; 13C: d¼
39.50 ppm). Splitting patterns are abbreviated as follows:
(s) singlet, (d) doublet, (t) triplet, (q) quartet, (m) multiplet.
Elemental analyses were performed by the C.N.R.S.
Service of Microanalyses in Gif-sur-Yvette (France). Mass
spectra (electrospray mode) were recorded by Vincent
Steinmetz (Institut Lavoisier), and high-resolution mass
spectra by Nicole Morin (Service of mass spectrometry,
ENS, Paris). Analytical TLC and preparative column chro-
matography were performed on Kieselgel F 254 and
Kieselgel 60 (0.040–0.063 mm) (Merck), respectively, with
the following eluant systems: CH2Cl2 (I); 2.5% MeOH–
97.5% CH2Cl2 (II); 2.5% EtOAc–97.5% CH2Cl2 (III); 5%
EtOAc–95% CH2Cl2 (IV); 20% EtOAc–80% CH2Cl2 (V);
50% EtOAc–50% CH2Cl2 (VI); 5% MeOH–95% CH2Cl2
(VII); 10% MeOH–90% CH2Cl2 (VIII). UV light
(l¼254 nm) allowed visualization of the spots after TLC
runs for all compounds. Except when noted, all starting ma-
terials and solvents were obtained from commercial suppliers
and were used as received. Fmoc–L-Ala–NCA and Boc–Aib–
NCA were purchased from Fluka and Isochem, respectively.
359
378
0.5
342
326
312
0.0
300
350
Wavelength (nm)
400
Figure 6. UV absorption spectrum (>300 nm region) of Boc–antAib–OEt
12a (solid line) and Boc–antAib–OH 13a (dotted line) in ethanol solution.
Amino acid derivative concentration: 1ꢂ10ꢀ4 M.
1.0
0.5
0.0
3.1. 2,4,5-Trimethylbenzophenone (2)
A mixture of 1,2,4-trimethylbenzene 1 (20 g; 166.6 mmol)
and AlCl3 (23.4 g; 175 mmol) in CH2Cl2 (20 mL) was
treated with benzoyl chloride (23.4 g; 166.5 mmol) as de-
scribed in the literature,7 to yield 24.9 g (67%) of pure 2
as a colorless liquid after vacuum distillation. Bp 152–
155 ꢁC/ca. 0.3 Torr (lit.7 bp 130 ꢁC/0.15 Torr). 1H NMR
(CDCl3): d 7.81 [m, 2H, ArH], 7.58 [m, 1H, ArH], 7.46
[m, 2H, ArH], 7.12 [s, 1H, ArH], 7.07 [s, 1H, ArH], 2.30
[s, 3H, ArCH3], 2.29 [s, 3H, ArCH3], 2.25 [s, 3H, ArCH3].
400
450
Wavelength (nm)
500
Figure 7. Fluorescence spectra in the 370–500 nm range of Boc–antAib–
OEt 12a (solid line) and Boc–antAib–OH 13a (dotted line) in ethanol solu-
tion: lexc¼359 nm. Amino acid derivative concentration: 1ꢂ10ꢀ6 M.

J.-F. Lohier et al. / Tetrahedron 62 (2006) 6203–6213
6207
13C NMR (CDCl3): d 198.2 [C]O], 138.9, 138.0, 135.8,
134.2, 133.0, 132.5, 132.2, 129.9, 129.7, 129.1, 128.5,
128.1 [CAr], 19.35, 19.29, 18.8 [ArCH3].
(MgSO4), filtered, and evaporated in vacuo to afford 5.76 g
of crude diester 5 as a solid, which was pure by H NMR
1
and TLC. The original mother liquor (MeOH solution)
was concentrated in vacuo at 40 ꢁC to ca. 150 mL, H2O
(700 mL) added, and the mixture was extracted with
CH2Cl2 (250 mL). The decanted CH2Cl2 solution was
washed with 5% NaHCO3 (100 mL) and H2O (2ꢂ400 mL)
as above, dried (MgSO4), filtered, and evaporated in vacuo
to afford 1.01 g of impure diester 5. A second similar run
starting from the remaining mixture of diacid 4 and NH4Cl
(11.70 g) gave 7.39 g of pure diester 5 after extraction of
the filtered precipitate and 1.39 g of impure compound
from the mother liquor. The combined pure samples of di-
ester (13.14 g) were dissolved in boiling acetone (400 mL)
and the solution concentrated to ca. 60 mL. Crystallization
occurred from the boiling solution to which methanol (ca.
60 mL) was added by portions in order to increase the quan-
tity of crystals. The mixture was concentrated again to ca.
60 mL, and then cooled to room temperature. The crystals
were filtered, abundantly washed with methanol, and air
dried (weight 11.96 g). More crystals (1.60 g) were obtained
from the mother liquor and by repeated crystallization of the
combined impure samples, to give a total of 13.56 g (71%)
of analytically pure diester 5, obtained as yellow-orange
3.2. Benzophenone-2,4,5-tricarboxylic acid (3)
A mixture of 2,4,5-trimethylbenzophenone 2 (19.7 g;
88 mmol) and 20% HNO3 (150 mL) was refluxed with stir-
ring for 5 days. The resulting thick semisolid was decanted,
rinsed with cold water (2ꢂ75 mL), dissolved in boiling
10% NaOH (200 mL), and treated with KMnO4 (55.3 g;
350 mmol) as described in the literature,7 to yield 17.0 g
(61%) of pure 3 obtained as white crystals. Mp 224 ꢁC (lit.7
1
mp 281–283 ꢁC). H NMR (CD3COCD3): d 8.47 [s, 1H,
ArH], 7.76 [m, 2H, ArH], 7.75 [s, 1H, ArH], 7.67–7.46 [m,
3H, ArH]. 13C NMR (CD3COCD3): d 195.7 [C]O], 167.9,
167.4, 165.8 [COOH]; 145.5, 137.7, 137.6, 134.1, 133.8,
132.2, 131.7, 130.0, 129.4, 128.6 [CAr].
3.3. Anthraquinone-2,3-dicarboxylic acid (4)
The triacid 3 (4.20 g; 13.4 mmol) was treated with 98%
H2SO4 (42 g) at 120–130 ꢁC for 3 h as described in the liter-
ature,7 to yield 3.49 g (88%) of pure 4 as a pale brown solid.
Mp >300 ꢁC (lit.7 mp >310 ꢁC). 1H NMR (CD3SOCD3): d
8.36 [s, 2H, ArH], 8.26–8.19 [m, 2H, ArH], 8.00–7.94 [m,
2H, ArH]. 13C NMR (CD3SOCD3): d 181.4 [C]O], 167.6
[COOH]; 137.5, 134.9, 134.2, 132.9, 127.0, 126.9 [CAr].
1
crystals. Mp 152 ꢁC (lit.7 mp 149–151 ꢁC). Rf 0.18 (I). H
NMR (CDCl3): d 8.52 [s, 2H, ArH], 8.45 [s, 2H, ArH],
8.09–8.04 [m, 2H, ArH], 7.60–7.55 [m, 2H, ArH]. 13C
NMR (CDCl3): d 168.0 [C]O], 132.9, 131.1, 130.2,
128.2, 127.8, 127.4, 126.7 [CAr], 52.5 [OCH3].
3.4. Dimethyl anthracene-2,3-dicarboxylate (5)
3.5. Reduction of dimethyl anthracene-2,3-dicarboxylate
Following the literature procedure,7 the diacid 4 (2.00 g;
6.76 mmol) was added by portions to a solution of 20%
NH4OH (100 mL) with stirring at room temperature. A clear
red solution was obtained after ca. 15 min, to which was
added by portions activated zinc dust (7.50 g). The resulting
blood-red mixture was refluxed until the color was gone (ca.
2 h) and then filtered while hot. The solid residue was re-
fluxed with 20% NH4OH (50 mL) for 2 h and filtered while
hot. The combined filtrates were cooled on an ice bath and
then acidified to pH <1 with 10 N HCl, which resulted in
the precipitation of a yellow colloidal solid which slowly set-
tled overnight at room temperature. Attempted filtration on
(a) With lithium aluminum hydride: To a suspension of
LiAlH4 (0.190 g; 5 mmol) in THF (15 mL) magnetically
stirred at 0 ꢁC was added the diester 5 (0.294 g; 1 mmol)
by portions under an argon atmosphere. The reaction mixture
was stirred at room temperature for 2 h, cooled to 0 ꢁC,
and quenched by dropwise addition of saturated aqueous
Na2SO4 (ca. 2 mL). After stirring at room temperature for
15 min, the mixture was filtered on glass wool and the filtrate
was evaporated to dryness in vacuo at 40 ꢁC. The residue was
repeatedly evaporated again invacuo after addition of metha-
nol in order to remove water. The obtained crude product
was dissolved in pyridine (10 mL) and acetic anhydride
(1 mL) was added. The resulting solution was stirred at
room temperature overnight and then evaporated to dryness
in vacuo at 40 ꢁC. The residue was taken up in ethyl acetate
(100 mL) and 0.5 N aq HCl (100 mL) with stirring. The
decanted organic phase was washed with 0.5 N aq HCl
(100 mL), then with H2O (2ꢂ100 mL), dried (MgSO4),
filtered, and evaporated in vacuo. The crude product
(0.175 g) presented a single spot on TLC with either eluant
(I), (II), (III) or (IV), but contained compounds 6 and 7 in
the ratio 30:70, determined by integration of their respective
a Buchner was very slow and difficult. Therefore, the solid
¨
was collected by centrifugation, and then repeatedly dried
by evaporation in vacuo at 50 ꢁC after addition of methanol.
The so-obtained crude anthracene-2,3-dicarboxylic acid
1
(2.14 g) was pure by H NMR (CD3SOCD3): d 8.77 [s,
2H, ArH], 8.46 [s, 2H, ArH], 8.17–8.12 [m, 2H, ArH],
7.64–7.59 [m, 2H, ArH] with only the presence of NH4Cl
(d 7.18 [t, J¼51.0 Hz]) as contaminant. Ten similar runs
starting with in total 19.25 g (65.0 mmol) of diacid 4 gave
21.80 g of such a mixture of anthracene-2,3-dicarboxylic
acid and NH4Cl. To this mixture (10.10 g) was added
MeOH (400 mL) and 98% H2SO4 (10 mL). The resulting
yellow suspension was stirred and refluxed. After ca. 1 h,
a clear orange-brown solution was obtained, which was re-
fluxed for 5 days to give a suspension again. The mixture
was cooled to 0 ꢁC, the solid was filtered, abundantly
washed with MeOH, air dried (weight 6.27 g), and then dis-
solved in a mixture of H2O (400 mL) and CH2Cl2 (600 mL)
with stirring. The decanted CH2Cl2 solution was washed
with 5% NaHCO3 (100 mL), H2O (2ꢂ400 mL), dried
1
ArCH2O and OCOCH3 singlets in H NMR (vide infra).
Preparative TLC on silica gel with eluant (III) gave 0.105 g
of the same mixture 6 (30%) and 7 (70%). Fractional crystal-
lization from a concentrated acetone solution (ca. 1 mL) gave
0.012 g (4%) of pure 6 as pale yellow crystals.
In another run, the diester 5 (0.294 g; 1 mmol) was treated by
LiAlH4 (0.190 g; 5 mmol) in THF (15 mL) in the same
experimental conditions and workup as above, except that

6208
J.-F. Lohier et al. / Tetrahedron 62 (2006) 6203–6213
the reaction was carried out at ꢀ14 ꢁC to ꢀ2 ꢁC for 1 h. The
crude product obtained after acetylation (0.213 g) contained
6 and 7 in the ratio 82:18 by 1H NMR. Fractional crystalliza-
tion of this crude product from concentrated acetone (ca.
1 mL) gave 0.116 g (36%) of pure 6 as pale yellow crystals.
at ꢀ13 ꢁC to +3 ꢁC for a further 1 h. Column chromato-
graphy of the crude product obtained after acetylation
(3.917 g) gave 6 (3.437 g; 86%) and 8 (0.114 g; 3%). The
pure compound 6 (3.437 g) was dissolved in boiling
CH2Cl2 (100 mL) (water bath at 80 ꢁC) and MeOH
(50 mL) was added. The solution was concentrated to ca.
50 mL upon boiling (crystallization started to occur from
the boiling solution). Methanol (50 mL) was added, the mix-
ture was concentrated again to ca. 30 mL upon boiling, and
then cooled to room temperature. The resulting crystals were
filtered, abundantly washed with MeOH, and air dried to
give 3.203 g (81%) of analytically pure 6. Similar crystalli-
zation of the side product 8 (0.553 g; combined samples
from several runs) from boiling MeOH (CH2Cl2) gave
0.384 g of yellow crystals.
In another run, the diester 5 (1.470 g; 5 mmol) was treated
by LiAlH4 (0.950 g; 25 mmol) in THF (75 mL) in the
same experimental conditions and workup as above, except
that the reaction was carried out at ꢀ60 ꢁC to ꢀ15 ꢁC for
2 h. The crude product obtained after acetylation (1.486 g)
1
contained compounds 6 and 7 in the ratio 85:15 by H
NMR. Fractional crystallization of this crude product from
concentrated acetone (ca. 5 mL) gave 0.746 g (46%) of
pure 6 as pale yellow crystals.
(b) With sodium bis(2-methoxyethoxy)aluminum hydride: To
a solution of diester 5 (2.940 g; 10 mmol) in THF (100 mL)
cooled to 0 ꢁC and stirred under an argon atmosphere was
added dropwise by syringe 16 mL (56 mmol) of ca. 3.5 M
solution of RedAlÒ in toluene (Acros) over a period of
30 min. The resulting red-brown solution was stirred at
0 ꢁC for 2 h, and then quenched by dropwise addition of
aq 5% H2SO4 (100 mL). The mixture was poured into a sep-
arating funnel containing 5% H2SO4 (200 mL) and EtOAc
(500 mL). After shaking and decantation, a large amount
of foam at the interface between the milky yellow organic
phase and the clear colorless aqueous acidic phase was pres-
ent, due to the low solubility of the expected 2,3-bis(hydroxy-
methyl)-anthracene. The separated organic phase and the
foam at the interface were washed with H2O (500 mL), dec-
anted, and then directly (without addition of drying agent)
evaporated to dryness in vacuo. The residue was repeatedly
evaporated again in vacuo after addition of methanol in order
to remove water. The obtained crude product (2.454 g) was
dissolved in pyridine (100 mL) and acetic anhydride
(20 mL) was added. The resulting clear orange solution
was magnetically stirred at room temperature for 20 h and
then evaporated to dryness in vacuo at 40 ꢁC. The resi-
due was taken up in ethyl acetate (400 mL) and 0.5 N aq
HCl (300 mL) with stirring. The decanted organic phase
was washed with 0.5 N aq HCl (100 mL), then with
brine (2ꢂ400 mL), dried (MgSO4), filtered, and evaporated
in vacuo. The crude product (3.161 g) was chromatographed
on a 2.3ꢂ54 cm column of silica gel with eluant (IV) to
afford the desired pure 2,3-bis(acetoxymethyl)-anthracene
6 (1.752 g; 54%) and pure 2-acetoxymethyl-3-methyl-
anthracene 8 (0.272 g; 10%) as a side product.
3.6. 2,3-Bis(acetoxymethyl)-anthracene (6)
Yellow crystals. Mp 185 ꢁC. Rf 0.2–0.3 (I), 0.75 (II), 0.35
(III), 0.50 (IV). H NMR (CDCl3): d 8.43 [s, 2H, ArH],
1
8.04 [s, 2H, ArH], 8.04–8.00 [m, 2H, ArH], 7.53–7.48 [m,
2H, ArH], 5.39 [s, 4H, ArCH2O], 2.17 [s, 6H, OCOCH3].
13C NMR (CDCl3): d 170.6 [C]O], 132.2, 131.3, 130.8,
129.7, 128.2, 126.4, 125.8 [CAr], 64.6 [ArCH2O], 20.9
[OCOCH3]. Anal. Calcd for C20H18O4 (322.344): C,
74.52; H, 5.63. Found: C, 74.92; H, 5.65.
3.7. 2,3-Bis(acetoxymethyl)-9,10-dihydroanthracene (7)
Not isolated. Characterized by 1H and 13C NMR in mixtures
with 6. Rf 0.2–0.3 (I), 0.75 (II), 0.35 (III), 0.50 (IV). 1H NMR
(CDCl3): d 7.36 [s, 2H, ArH], 7.32–7.27 [m, 2H, ArH], 7.25–
7.20 [m, 2H, ArH], 5.22 [s, 4H, ArCH2O], 3.95 [s, 4H,
ArCH2Ar], 2.11 [s, 6H, OCOCH3]. 13C NMR (CDCl3):
d 170.6 [C]O], 137.4–125.7 [CAr], 63.7 [ArCH2O], 35.6
[ArCH2Ar], 20.9 [OCOCH3].
3.8. 2-Acetoxymethyl-3-methyl-anthracene (8)
Yellow crystals. Mp 194 ꢁC. Rf 0.70 (IV). 1H NMR (CDCl3):
d 8.43 [s, 1H, ArH], 8.40 [s, 1H, ArH], 8.02–7.97 [m, 2H,
ArH], 7.98 [s, 1H, ArH], 7.80 [s, 1H, ArH], 7.49–7.44 [m,
2H, ArH], 5.32 [s, 2H, ArCH2O], 2.55 [s, 3H, ArCH3],
2.18 [s, 3H, OCOCH3]. 13C NMR (CDCl3): d 170.9
[C]O], 133.9, 132.5, 132.1, 131.7, 131.5, 130.3, 128.5,
128.2, 128.1, 126.2, 125.5, 125.1, 125.0 [CAr], 65.2
[ArCH2O], 21.0 [OCOCH3], 19.5 [ArCH3].
3.9. 2,3-Bis(bromomethyl)-anthracene (9)
In another run, the diester 5 (2.940 g; 5 mmol) in THF
(100 mL) was treated by a 3.5 M solution of RedAlÒ in tol-
uene (16 mL; 56 mmol) in the same experimental conditions
and workup as above, except that the reaction was conducted
at 0 ꢁC for 15 min (addition) and then for a further 1 h.
Column chromatography of the crude product obtained after
acetylation (3.198 g) gave 6 (2.354 g; 73%) and 8 (0.140 g;
5%).
To a solution of diacetate 6 (3.222 g; 10 mmol) in CH2Cl2
(200 mL) was added a 33 wt % solution of hydrogen bro-
mide in acetic acid (Aldrich) (20 mL; ca. 110 mmol). The
solution, which became more and more turbid, was stirred
at room temperature for 24 h. Water (250 mL) was added,
the reaction mixture was transferred to a separating funnel,
and extracted with CH2Cl2 (800 mL necessary for complete
solubilization). The separated CH2Cl2 solution was washed
with 5% NaHCO3 (150 mL) and then H2O (2ꢂ150 mL),
dried (MgSO4), filtered, and evaporated in vacuo. The
obtained crystalline crude product (3.680 g) was stirred in
boiling CH2Cl2 (200 mL) but could not be totally solubi-
lized. Methanol (50 mL) was added and the mixture was
In another run, the diester 5 (3.633 g; 12.4 mmol) in THF
(125 mL) was treated by a 3.5 M solution of RedAlÒ in
toluene (19 mL; 66 mmol) in the same experimental condi-
tions and workup as above, except that the reaction was con-
ducted at ꢀ15 ꢁC to ꢀ13 ꢁC for 15 min (addition) and then

J.-F. Lohier et al. / Tetrahedron 62 (2006) 6203–6213
6209
concentrated to ca. 30 mL with almost complete crystalliza-
tion occurring from the boiling solution. After cooling to
room temperature, the crystals were filtered, abundantly
washed with MeOH, and air dried to afford 3.573 g (98%)
of analytically pure dibromide 9 as yellow crystals. Mp
afford 0.043 g (28%) of a-amino ester 11a. Rf 0.05 (IV);
0.20 (V). H NMR (CDCl3): d 8.33 [s, 2H, ArH], 8.00–
1
7.95 [m, 2H, ArH], 7.82 [s, 2H, ArH], 7.46–7.41 [m, 2H,
ArH], 4.27 [q, J¼7.1 Hz, 2H, CH2 OEt], 3.71 and 3.08 [d,
J¼16.3 Hz, 2H, ArCH2 and d, J¼16.3 Hz, 2H, ArCH2],
2.02 [br s, 2H, NH2], 1.33 [t, J¼7.1 Hz, 3H, CH3 OEt].
13C NMR (CDCl3): d 172.5 [C]O], 139.8, 131.7, 131.4,
128.0, 125.5, 125.0, 122.8 [CAr], 65.4 [Ca], 61.5 [CH2
OEt], 45.4 [ArCH2], 14.2 [CH3 OEt]. HRMS (FAB+) of
11a$CF3CO2H. Calcd [M+H]+ for C20H20NO2: 306.1494.
Found: 306.1487.
1
225 ꢁC. Rf 0.85 (III). H NMR (CDCl3): d 8.39 [s, 2H,
ArH], 8.04 [s, 2H, ArH], 8.03–7.99 [m, 2H, ArH], 7.53–
7.49 [m, 2H, ArH], 4.95 [s, 4H, ArCH2Br]. 13C NMR
(CDCl3): d 133.1, 132.4, 131.2, 131.0, 128.2, 126.6, 126.1
[CAr], 31.6 [ArCH2Br]. HRMS (DCI+). Calcd [M+H]+ for
C16H13 79Br2: 362.9384. Found: 362.9388. Calcd [M+H]+
for C16H13 79Br81Br: 364.9364. Found: 364.9373. Calcd
[M+H]+ for C16H13 81Br2: 366.9346. Found: 366.9350.
Anal. Calcd for C16H12Br2 (364.088): C, 52.78; H, 3.32.
Found: C, 52.93; H, 3.29.
3.12. 2-Amino-2,3-dihydro-1H-cyclopenta[b]anthra-
cene-2-carboxylic acid tert-butyl ester (H–antAib–OtBu
11b)
3.10. 2-Isocyano-2,3-dihydro-1H-cyclopenta[b]anthra-
cene-2-carboxylic acid tert-butyl ester (C]antAib–
OtBu 10b)
The isonitrile 10b (0.103 g; 0.300 mmol) was dissolved in
CH2Cl2 (20 mL) and ethanol (20 mL) was added. The solu-
tion was cooled to 0 ꢁC and 10 N HCl (0.5 mL) was added.
The reaction mixture was allowed to warm to room temper-
ature and stirred for ca. 2 h, until total hydrolysis had
occurred (TLC monitoring). Water (150 mL) and CH2Cl2
(150 mL) were added, and the mixture was made alkaline
by slow addition of a large excess of NaHCO3 (0.840 g;
10 mmol) with stirring. The separated CH2Cl2 solution
was washed with water (2ꢂ100 mL), dried (MgSO4), fil-
tered, and evaporated in vacuo. The crude product was chro-
matographed on a column of silica gel with eluant (V) to
afford 0.090 g (90%) of pure 11b as a pale yellow solid.
To a suspension of 9 (1.00 g; 2.75 mmol), tetrabutylammo-
nium hydrogen sulfate (0.42 g; 1.24 mmol) and Cs2CO3
(5.38 g; 16.5 mmol) in CH3CN (150 mL) was added tert-
butyl isocyanoacetate (0.60 mL, 4.12 mmol). The mixture
was stirred under argon at 75–80 ꢁC for 48 h and then filtered
through sintered glass. The filtrate was evaporated in vacuo
and the residue dissolved in CH2Cl2 (150 mL). The organic
layer was washed with water, dried over magnesium sulfate,
and evaporated to dryness. The crude product was purified
by column chromatography on silica gel using CH2Cl2 as el-
uant to give 0.275 g (29%) of pure 10b as a pale yellow solid.
Crystallization of an aliquot from cyclohexane/CH2Cl2 af-
forded analytically pure crystals (needles). Mp 203 ꢁC. Rf
0.55 (I). 1H NMR (CDCl3): d 8.37 [s, 2H, ArH], 8.00–7.97
[m, 2H, ArH], 7.85 [s, 2H, ArH], 7.48–7.45 [m, 2H, ArH],
3.81 and 3.61 [d, J¼16.4 Hz, 2H and d, J¼16.4 Hz, 2H,
ArCH2], 1.54 [s, 9H, CH3 OtBu]. 13C NMR (CDCl3):
d 167.3 [C]O], 137.1, 131.9, 131.7, 128.3, 126.2, 125.6,
123.3 [CAr], 111.9 [C]N–], 84.3 [O–C OtBu], 77.4 [Ca],
45.5 [ArCH2], 28.0 [CH3 OtBu]. Anal. Calcd for
C23H21NO2 (343.406): C, 80.44; H, 6.16; N, 4.08. Found:
C, 80.43; H, 6.13; N, 3.88.
1
Mp 227 ꢁC. Rf 0.31 (V). H NMR (CDCl3): d 8.33 [s, 2H,
ArH], 8.00–7.96 [m, 2H, ArH], 7.81 [s, 2H, ArH], 7.47–
7.42 [m, 2H, ArH], 3.66 and 3.05 [d, J¼16.3 Hz, 2H,
ArCH2 and d, J¼16.3 Hz, 2H, ArCH2], 1.86 [br s, 2H,
NH2], 1.51 [s, 9H, CH3 OtBu]. 13C NMR (CDCl3): d 175.7
[C]O], 140.4, 131.9, 131.5, 128.2, 125.7, 125.1, 122.9
[CAr], 81.6 [O–C OtBu], 66.1 [Ca], 45.6 [ArCH2],
28.2 [CH3 OtBu]. Anal. Calcd for C22H23NO2 (333.412):
C, 79.25; H, 6.95; N, 4.20. Found: C, 79.07; H, 6.98;
N, 4.05.
3.13. 2-tert-Butyloxycarbonylamino-2,3-dihydro-1H-
cyclopenta[b]anthracene-2-carboxylic acid ethyl ester
(Boc–antAib–OEt 12a)
3.11. 2-Amino-2,3-dihydro-1H-cyclopenta[b]anthra-
cene-2-carboxylic acid ethyl ester (H–antAib–OEt 11a)
(a) A solution of the a-amino ester 11a (0.034 g; 0.11 mmol)
and Boc2O (0.050 g; 0.22 mmol) in CH3CN (2 mL) was
stirred at room temperature for 3 days and then evaporated
in vacuo. The crude product was chromatographed on a pre-
parative TLC plate of silica gel with eluant (III) to afford
0.0167 g (37%) of pure 12a.
To a suspension of 9 (0.182 g; 0.5 mmol), tetrabutylammo-
nium hydrogen sulfate (0.068 g; 0.2 mmol) and K2CO3
(1.38 g; 10 mmol) in CH3CN (25 mL) was added ethyl iso-
cyanoacetate (0.55 mL; 5 mmol). The mixture was stirred
under argon at 75–80 ꢁC for 20 h, then cooled to room tem-
perature, and filtered through sintered glass. The solid was
abundantly washed with CH2Cl2 and the filtrate was evapo-
rated in vacuo. To the residue (containing C]antAib–OEt
10a, not isolated) was added CH2Cl2 (10 mL), abs EtOH
(25 mL) and 10 N HCl (1 mL). The mixture was magneti-
cally stirred at room temperature for 2 h and then diluted
with CH2Cl2 (100 mL). Water (150 mL) was added, and
the mixture was made alkaline by slow addition of
NaHCO3 with stirring. The separated CH2Cl2 solution was
washed with H2O (2ꢂ100 mL), dried (MgSO4), filtered,
and evaporated in vacuo. The crude product was purified
on a preparative TLC plate of silica gel with eluant (VI) to
(b) In other experiments on a larger scale, without isolation/
characterization of the isonitrile and a-amino ester inter-
mediates, a mixture of dibromide 9 (0.364 g; 1 mmol), tetra-
butylammonium hydrogen sulfate(0.136 g; 0.4 mmol), K2CO3
(2.76 g; 10 mmol), and ethyl isocyanoacetate (0.30 mL;
2.75 mmol) in CH3CN (50 mL) was reacted in the same ex-
perimental conditions and workup as above (in Section 3.10).
The crude product was submitted to direct acidic hydrolysis
with 10 N HCl (1 mL) in CH2Cl2 (10 mL) and abs EtOH
(25 mL), also as above. The crude product obtained after ex-
traction (containing the a-amino ester 11a) was reacted with
Boc2O (0.347 g; 1.59 mmol) in CH3CN (5 mL) and CH2Cl2

6210
J.-F. Lohier et al. / Tetrahedron 62 (2006) 6203–6213
(10 mL) at room temperature for 5 days and then evaporated
in vacuo. The crude product was chromatographed on
a 2.3ꢂ27 cm column of silica gel with eluant (III), to afford
0.102 g (25%) of pure N-Boc-protected a-amino ester 12a. In
another run on a larger scale, a mixture of dibromide 9
(1.820 g; 5 mmol), tetrabutylammonium hydrogen sulfate
(0.678 g; 2 mmol), K2CO3 (13.8 g; 100 mmol), and ethyl iso-
cyanoacetate (1.5 mL; 2.75 mmol) in CH3CN (250 mL) was
reacted in the same experimental conditions and workup as
above. The crude product was submitted to direct acidic hy-
drolysis with 10 N HCl (10 mL) in CH2Cl2 (75 mL) and abs
EtOH (175 mL), also as above. The crude product obtained
after extraction was reacted with Boc2O (1.816 g;
8.33 mmol) in CH3CN (25 mL) and CH2Cl2 (50 mL) at
room temperature for 4 days, then diluted with CH2Cl2 (ca.
200 mL), filtered through Celite, and evaporated in vacuo.
The crude product was dissolved in CH2Cl2 (75 mL) and
chromatographed on a 3ꢂ42 cm column of silica gel with el-
uant (III) to afford 0.373 g (18%) of pure 12a. Several other
runs in the same experimental conditions and workup gave
a similar yield. The pure compound 12a from several runs
(0.514 g) was dissolved in CH2Cl2 (25 mL) and EtOAc
(5 mL) was added. The clear yellow solution was concen-
trated under a slight vacuum at 60 ꢁC to ca. 4 mL where crys-
tallization started to occur. Methanol (10 mL) was added and
the mixture was concentrated again to ca. 2 mL, then cooled
at +4 ꢁC for 3 h. More MeOH (10 mL) was added, the crys-
tals were filtered, abundantly washed with MeOH, and air
dried to give 0.418 g (81%) of analytically pure 12a as a yel-
low solid. Another similar crystallization process starting
from pure 12a (0.722 g) gave 0.598 g of analytically pure
crystals. Mp 221 ꢁC. Rf 0.30 (III); 0.90 (V). 1H NMR
(CDCl3): d 8.33 [s, 2H, ArH], 7.99–7.96 [m, 2H, ArH],
7.79 [s, 2H, ArH], 7.47–7.43 [m, 2H, ArH], 5.11 [br s, 1H,
NH], 4.27 [q, J¼7.1 Hz, 2H, CH2 OEt], 3.77 and 3.42 [d,
J¼16.9 Hz, 2H, ArCH2 and br d, J¼16.7 Hz, 2H, ArCH2],
1.45 [s, 9H, CH3 Boc], 1.29 [t, J¼7.1 Hz, 3H, CH3 OEt].
13C NMR (CDCl3): d 173.2 [C]O], 155.0 [C]O Boc],
139.0, 131.6, 131.4, 128.0, 125.6, 125.1, 122.6 [CAr], 80.1
[C–O Boc], 66.4 [Ca], 61.6 [CH2 OEt], 43.1 [ArCH2], 28.2
[CH3 Boc], 14.1 [CH3 OEt]. ESI+ MS m/z (relative intensity):
428.3 (100) [M+Na]+, 833.5 (58) [2M+Na]+. HRMS (FAB+).
Calcd [M]+ for C25H27NO4: 405.1940. Found: 405.1953.
Calcd [M+H]+ for C25H28NO4: 406.2018. Found:
406.2014. Anal. Calcd for C25H27NO4 (405.474): C, 74.05;
H, 6.71; N, 3.45. Found: C, 73.87; H, 6.95; N, 3.71.
intermediates, a mixture of dibromide 9 (3.64 g; 10 mmol),
tetrabutylammonium hydrogen sulfate (1.53 g; 4.5 mmol),
Cs2CO3 (3.26 g; 50 mmol), and tert-butyl isocyanoacetate
(1.75 mL; 12 mmol) in CH3CN (250 mL) was reacted in
the same experimental conditions and workup as in Section
3.9. The crude product (containing the isonitrile 10b) was
submitted to direct acidic hydrolysis with 10 N HCl (25
drops) in CH2Cl2 (150 mL) and abs EtOH (75 mL), in the
same experimental conditions and workup as in Section
3.11. At this stage, the crude product (containing the a-amino
ester 11b) was combined with the crude products obtained
from two other similar runs in which 2.27 g (6.24 mmol)
and 1.14 g (3.12 mmol) of dibromide 9 were engaged, and
the mixturewas purified by column chromatography on silica
gel with eluant (V) to afford 1.81 g (28%) of a-amino ester
11b containing minor impurities. This sample was treated
with Fmoc–OSu (3.10 g; 9.20 mmol) in CH2Cl2 (400 mL)
in the same experimental conditions and workup as above
in Section 3.13 (a). The crude product was chromatographed
on a column of silica gel with eluant (I) to afford 1.84 g
(17% overall from 9) of pure 12b as a pale yellow solid.
1
Mp 243 ꢁC. Rf 0.63 (III). H NMR (CDCl3): d 8.35 [s, 2H,
ArH], 8.01–7.96 [m, 2H, ArH], 7.79 [br s, 2H, ArH], 7.73
[br d, Jw7.5 Hz, 2H, ArH Fmoc], 7.58 [d, J¼7.0 Hz, 2H,
ArH Fmoc], 7.48–7.43 [m, 2H, ArH], 7.38–7.24 [m, 4H,
ArH Fmoc], 5.40 [br s, 1H, NH], 4.41 [br m, 2H, CH2
Fmoc], 4.22 [m (t-like), Jw6.7 Hz, 1H, CH Fmoc], 3.72
and 3.52 [br d, Jw17.3 Hz, 2H, ArCH2 and br d,
Jw17.3 Hz, 2H, ArCH2], 1.41 [s, 9H, CH3 OtBu]. 13C
NMR (CDCl3): d 172.3 [C]O], 155.7 [C]O Fmoc],
144.1, 141.5, 139.6, 131.8, 131.6, 128.2, 127.9, 127.3,
125.8, 125.3, 125.2, 122.6, 120.2 [CAr], 82.2 [O–C OtBu],
67.4 [CH2 Fmoc], 66.9 [Ca], 47.4 [CH Fmoc], 43.1
[ArCH2], 28.0 [CH3 OtBu]. HRMS (FAB+). Calcd
[M+Na]+ for C37H33NO4Na: 578.2307. Found: 578.2313.
Anal. Calcd for C37H33NO4 (555.642): C, 79.97; H, 5.99;
N, 2.52. Found: C, 80.45; H, 6.12; N, 2.42.
3.15. 2-tert-Butyloxycarbonylamino-2,3-dihydro-1H-
cyclopenta[b]anthracene-2-carboxylic acid (Boc–
antAib–OH 13a)
To a solution of 12a (0.203 g; 0.5 mmol) in THF (20 mL)
and MeOH (40 mL) was added a solution of 1 N NaOH
(20 mL). The reaction mixture was magnetically stirred at
room temperature for 24 h, cooled by addition of ice, acidi-
fied by addition of 0.5 N HCl (50 mL), and extracted with
CH2Cl2 (2ꢂ250 mL). The CH2Cl2 solution was washed
with H2O (250 mL), filtered, and evaporated in vacuo. The
residue was dried by repeated evaporation in vacuo at
40 ꢁC after addition of methanol to afford 0.183 g (96%)
of pure 13a as a pale yellow crystalline powder. Several sim-
ilar runs gave 89–96% yields. Mp >300 ꢁC. Rf 0.03 (V); 0.10
3.14. 2-(9-Fluorenylmethoxycarbonylamino)-2,3-di-
hydro-1H-cyclopenta[b]anthracene-2-carboxylic acid
tert-butyl ester (Fmoc–antAib–OtBu 12b)
(a) A solution of the a-amino ester 11b (0.120 g; 0.36 mmol)
and Fmoc–OSu (0.146 g; 0.43 mmol) in CH2Cl2 (10 mL)
was magnetically stirred at room temperature for 48 h, and
then diluted with CH2Cl2 (100 mL). The CH2Cl2 solution
was washed with 0.5 N HCl (100 mL), 5% NaHCO3
(100 mL) and then H2O (2ꢂ100 mL), dried (MgSO4), fil-
tered, and evaporated in vacuo. The crude product was chro-
matographed on a column of silica gel with eluant (I) to
afford 0.172 g (86%) of pure 12b as a pale yellow solid.
1
(VI). H NMR (CDCl3/CD3OD 4:1): d 8.22 [s, 2H, ArH],
7.87 [m, 2H, ArH], 7.68 [s, 2H, ArH], 7.34 [m, 2H, ArH],
3.66 and 3.32 [d, Jw17.0 Hz, 2H and d, Jw17.0 Hz, 2H,
Ar–CH2], 1.32 [s, 9H, CH3 Boc]. 13C NMR (CDCl3/
CD3OD 4:1): d 175.5 [C]O], 155.6 [C]O Boc], 139.0,
131.2, 131.0, 127.5, 125.0, 124.5, 121.9 [CAr], 79.5 [C–O
Boc], 65.7 [Ca], 42.4 [Ar–CH2], 27.5 [CH3 Boc]. ESI+ MS
m/z (relative intensity): 400.2 (100) [M+Na]+, 416.2 (18)
[M+K]+, 777.4 (97) [2M+Na]+. HRMS (FAB+). Calcd
[M]+ for C23H23NO4: 377.1627. Found: 377.1629. Calcd
(b) In other experiments on a larger scale, without isolation/
characterization of the isonitrile and a-amino ester

J.-F. Lohier et al. / Tetrahedron 62 (2006) 6203–6213
6211
[M+H]+ for C23H24NO4: 378.1705. Found: 378.1709. Anal.
Calcd for C23H23NO4 (377.422): C, 73.19; H, 6.14; N, 3.71.
Found: C, 72.72; H, 6.55; N, 3.74.
Jw7.2 Hz and 7.2 Hz, 1H, CHa Ala], 3.85 [d, Jw16.7 Hz,
1H, ArCHA antAib], 3.76 [d (partly masked), Jw16.7 Hz,
1H, ArC0HA antAib], 3.52–3.36 [br m, 2H, ArCHB and
ArC0HB antAib], 3.74 [s, 3H, OCH3], 1.44 [s, 9H, CH3
Boc], 1.43 [d (partly masked), Jw7.1 Hz, 3H, CH3 Ala].
13C NMR (CDCl3): d 173.3, 172.3 [C]O Ala and antAib],
154.9 [C]O Boc], 139.1, 131.6, 131.4, 128.0, 125.6,
125.1, 122.9, 122.8 [CAr], 80.6 [C–O Boc], 67.5 [Ca
3.16. 2-(9-Fluorenylmethoxycarbonylamino)-2,3-di-
hydro-1H-cyclopenta[b]anthracene-2-carboxylic acid
(Fmoc–antAib–OH 13b)
To an ice-cold solution of 12b (0.290 g; 0.52 mmol) in
CH2Cl2 (10 mL) was added TFA (5 mL). The solution was
magnetically stirred from 0 ꢁC to room temperature for 4 h
and then evaporated in vacuo. The solid residue was repeat-
edly taken up in CH2Cl2 and the suspension evaporated inva-
cuo at 40 ꢁC. The crude product was dissolved in CH2Cl2
(150 mL necessary) with heating, the warm solution was fil-
tered and then concentrated to ca. 50 mL under reduced pres-
sure at 40 ꢁC resulting in crystallization. Cyclohexane (ca.
30 mL) was added and the mixture was concentrated again
to ca. 25 mL under reduced pressure. The crystals were trit-
urated at room temperature, filtered, abundantly washed
with a solution of cyclohexane/CH2Cl2 ca. 95:5 (v/v), and
air dried to afford 0.240 g (92%) of pure 13b as a pale yellow
crystalline powder. Mp 244–246 ꢁC. Rf 0.08 (VII), 0.42
antAib], 52.4 [OCH3], 48.3 [Ca Ala], 42.6, 42.1 [ArCH2
25
589
antAib], 28.2 [CH3 Boc], 18.3 [CH3 Ala]. [a]
ꢀ16.7,
25
25
25
[a]
ꢀ17.3, [a]
ꢀ19.3, [a]
ꢀ34.0 (c 0.2; EtOAc).
578
546
436
ESI+ MS m/z (relative intensity): 485.3 (100) [M+Na]+,
385.3 (40) [M+Na-Boc]+, 947.6 (44) [2M+Na]+. Anal.
Calcd for C27H30N2O5$0.5H2O (471.534): C, 68.77; H,
6.63; N, 5.94. Found: C, 69.15; H, 6.85; N, 5.93.
3.18. Fmoc–L-Ala–antAib–L-Ala–OMe (15a)
To an ice-cold solution of dipeptide 14a (0.109 g;
0.23 mmol) in CH2Cl2 (6 mL) was added TFA (2 mL). The
solution was stirred at 0 ꢁC for 3 h and evaporated in vacuo
at 25 ꢁC. The residue was repeatedly dissolved in CH2Cl2
and the solution evaporated in vacuo to yield crude
TFA$H–antAib–L-Ala–OMe (not characterized). To this
product was added THF (5 mL), the mixture was magneti-
cally stirred at 0 ꢁC for 10 min and DIEA (0.180 mL;
1.03 mmol) was added, followed by solid Fmoc–L-Ala–
NCA (0.238 g; 0.71 mmol). The solution was stirred at
room temperature for 66 h and evaporated in vacuo at
40 ꢁC. The residue was dissolved in EtOAc by portions and
the solutions combined and transferred to a separatory fun-
nel. The organic solution (ca. 150 mL of EtOAc) was washed
with 0.5 N HCl (2ꢂ75 mL), H2O (2ꢂ100 mL), dried
(MgSO4), filtered, and evaporated in vacuo. The crude prod-
uct was chromatographed on a preparativeTLC plate of silica
gel with eluant (II) (three consecutive elutions) to afford
0.128 g (83%) of pure 15a as a solid. Mp 210–212 ꢁC. Rf
1
(VIII). H NMR (CDCl3/CD3OD 9:1 v/v): d 8.29 [s, 2H,
ArH], 7.94–7.91 [m, 2H, ArH], 7.75 [br s, 2H, ArH], 7.66
[br d, Jw7.3 Hz, 2H, ArH Fmoc], 7.51 [d, J¼7.3 Hz, 2H,
ArH Fmoc], 7.41–7.37 [m, 2H, ArH], 7.31 [m (t-like),
Jw7.4 Hz, 2H, ArH Fmoc], 7.21 [m (t-like), Jw7.4 Hz, 2H,
ArH Fmoc], 4.33 [br m, 2H, CH2 Fmoc], 4.15 [m (t-like),
Jw6.7 Hz, 1H, CH Fmoc], 3.74 and 3.46 [br d, Jw16.2 Hz,
2H, ArCH2 and br d, Jw16.9 Hz, 2H, ArCH2]. 13C NMR
(CDCl3/CD3OD 9:1 v/v): d 175.2 [C]O], 156.0 [C]O
Fmoc], 143.6, 141.1, 139.0, 131.5, 131.3, 127.9, 127.5,
126.9, 125.5, 125.0, 122.4, 119.8 [CAr], 67.1 [Ca], 66.6
[CH2 Fmoc], 47.0 [CH Fmoc], 42.7 [ArCH2]. ESI+ MS m/z
(relative intensity): 522.3 (100) [M+Na]+, 538.3 (27)
[M+K]+. Anal. Calcd for C33H25NO4$0.5H2O (508.546):
C, 77.93; H, 5.15; N, 2.75. Found: C, 78.13; H, 5.39;
N, 2.55.
1
0.15 (II); 0.55 (VII). H NMR (CDCl3): d 8.22 [br s, 1H,
ArH antAib], 8.15 [s, 1H, ArH antAib], 7.92–7.85 [m, 2H,
ArH antAib], 7.73 [br m, 2H, ArH Fmoc], 7.70 [s, 1H, ArH
antAib], 7.66 [s, 1H, ArH antAib], 7.43–7.34 [m, 6H, 2
ArH antAib and 4 ArH Fmoc], 7.29 [m (partly masked),
1H, NH Ala–OMe], 7.30–7.22 [m, 2H, ArH Fmoc], 6.95
[br s, 1H, NH antAib], 5.34 [d, Jw6.4 Hz, 1H, NH Ala–
Fmoc], 4.58 [dq, Jw7.2 Hz and 7.2 Hz, 1H, CHa Ala–
OMe], 4.18 [m (d-like), Jw6.9 Hz, 2H, CH2O Fmoc], 4.06
[br dq, Jw6.8 Hz and 6.8 Hz, 1H, CHa Ala–Fmoc], 3.95 [br
m, t-like), 1H, Ar–CH Fmoc], 3.83 [d, Jw16.8 Hz, 1H,
ArCHA antAib], 3.72 [d, Jw17.1 Hz, 1H, ArC0HA antAib],
3.60 [d, Jw16.9 Hz, 1H, ArCHB antAib], 3.47 [d,
Jw16.7 Hz, 1H, ArC0HB antAib], 3.66 [s, 3H, OCH3], 1.38
[d, J¼7.1 Hz, 3H, CH3 Ala–OMe], 1.30 [d, J¼6.9 Hz, 3H,
CH3 Ala–Fmoc]. 13C NMR (CDCl3): d 173.3, 171.8 [C]O
Ala–OMe, Ala–Fmoc and antAib], 156.5 [C]O Fmoc],
143.6, 143.4, 141.1, 138.5, 131.4, 131.3, 128.7, 128.0,
127.7, 127.0, 126.9, 125.6, 125.1, 124.9, 122.8, 122.7,
119.9 [CAr], 67.8 [Ca antAib], 67.1 [CH2–O Fmoc], 52.3
[OCH3], 50.9, 48.5 [Ca Ala–OMe and Ala–Fmoc], 46.8
3.17. Boc–antAib–L-Ala–OMe (14a)
To a suspension of 13a (0.133 g; 0.35 mmol), HCl$L-H–Ala–
OMe (0.148 g; 1.06 mmol), and HOAt (0.097 g; 0.71 mmol)
in THF (3 mL) and CH2Cl2 (3 mL) was added NMM
(0.160 mL; 1.45 mmol) and then EDC (0.102 g;
0.53 mmol). The reaction mixture was stirred at room tem-
perature for 44 h and evaporated to dryness invacuo. The resi-
due was solubilized in several portions of EtOAc (total of ca.
150 mL) and 0.5 N HCl (total of ca. 100 mL) with stirring,
and the solutions combined and transferred into a separatory
funnel. The decanted organic phase was extracted again with
0.5 N HCl (2ꢂ50 mL), and then H2O (100 mL), 5% NaHCO3
(50 mL), and H2O (2ꢂ100 mL), dried (MgSO4), filtered and
evaporated in vacuo. The crude product was chromato-
graphed on a preparative TLC plate of silica gel with eluant
(II) to afford 0.082 g (50%) of pure 14a as a solid. Another
run under similar experimental conditions and starting
from 0.062 g (0.16 mmol) of 13a gave 0.047 g (62%) of
1
pure 14a. Mp 226–228 ꢁC. Rf 0.40 (II). H NMR (CDCl3):
d 8.33 [s, 2H, ArH], 7.99–7.96 [m, 2H, ArH], 7.81 [s, 1H,
ArH], 7.79 [s, 1H, ArH], 7.46–7.43 [m, 2H, ArH], 7.15 [br
m, 1H, NH Ala], 5.22 [s, 1H, NH antAib], 4.64 [dq,
[Ar–CH Fmoc], 42.7, 41.5 [ArCH2 and ArC0H2 antAib],
25
589
17.8, 17.4 [CH3 Ala–OMe and Ala–Fmoc]. [a]
ꢀ39.4,
25
578
25
546
25
436
[a] ꢀ40.4, [a] ꢀ45.9, [a] ꢀ84.9 (c 0.11, CHCl3).
ESI+ MS m/z (relative intensity): 678.3 (100) [M+Na]+.

6212
J.-F. Lohier et al. / Tetrahedron 62 (2006) 6203–6213
Anal. Calcd for C40H37N3O6$0.5H2O (664.728):C, 72.27; H,
5.76; N, 6.32. Found: C, 72.46; H, 5.83; N, 5.91.
(35) [2M+H]+. Anal. Calcd for C21H18N2O2$0.3H2O
(335.775): C, 75.11; H, 5.58; N, 8.34. Found: C, 75.14;
H, 5.58; N, 8.47.
3.19. Synthesis of Boc–Aib–antAib–L-Ala–OMe (16a)
and cyclo-[antAib–L-Ala] (17a)
3.22. Ultraviolet absorption and fluorescence
The dipeptide 14a (0.043 g; 0.09 mmol) was Na-deprotected
in CH2Cl2 (3 mL) and TFA (1 mL) as above (Section 3.18).
To the obtained crude TFA$H–antAib–L-Ala–OMe was
added THF (3 mL), the mixture was magnetically stirred
at 0 ꢁC for 10 min and DIEA (0.065 mL; 0.37 mmol)
was added, followed by solid Boc–Aib–NCA (0.085 g;
0.37 mmol). The solution was stirred at 60 ꢁC for 21 h and
evaporated in vacuo at 40 ꢁC. The residue was stirred in the
presence of EtOAc (10 mL). The residual solid was filtered,
washed with EtOAc (2ꢂ10 mL), then 0.5 N HCl (2ꢂ10 mL),
then H2O (2ꢂ10 mL), and air dried to afford 0.015 g (48%)
of pure 17a as a solid, sparingly soluble in the usual organic
solvents. The filtrate (mixture of EtOAc and aqueous HCl)
was diluted with more EtOAc (100 mL) and transferred to
a separatory funnel. The separated organic phase was washed
with 0.5 N HCl (2ꢂ50 mL), H2O (2ꢂ100 mL), dried
(MgSO4), filtered, and evaporated in vacuo. The crude prod-
uct was chromatographed on a preparativeTLC plate of silica
gel with eluant (II) to afford 0.007 g (14%) of pure 16a as
a solid.
The electronic absorption spectra were recorded between
300 and 400 nm using a Shimadzu model UV-2501 PC
spectrophotometer. The fluorescence spectra were mea-
sured between 370 and 500 nm (upon excitation at either
359 nm or 378 nm) using a Perkin Elmer model LS-50B
spectrofluorimeter. Ethanol (99.8%) was purchased from
Fluka.
References and notes
1. (a) Torrado, A.; Imperiali, B. J. Org. Chem. 1996, 61, 8940–
´
8948; (b) Szymanska, A.; Wiczk, W.; Kankiewicz, L. Amino
Acids 2001, 21, 265–270; (c) Guzow, K.; Milewska, M.;
´
´
Wroblewski, D.; Gie1don, A.; Wiczk, W. Tetrahedron 2004,
60, 11889–11894; (d) Wang, W.; Li, H. Tetrahedron Lett.
2004, 45, 8479–8481; (e) Brun, M.-P.; Bischoff, L.; Garbay,
´
C. Angew. Chem., Int. Ed. 2004, 59, 3175–3178; (f) Vazquez,
M. E.; Blanca, J. B.; Imperiali, B. J. Am. Chem. Soc. 2005,
127, 1300–1306 and references therein.
2. Formaggio, F.; Peggion, C.; Crisma, M.; Toniolo, C.;
Tchertanov, L.; Guilhem, J.; Mazaleyrat, J. P.; Goubard, Y.;
Gaucher, A.; Wakselman, M. Helv. Chim. Acta 2001, 84,
481–501.
3. (a) Toniolo, C.; Formaggio, F.; Crisma, M.; Mazaleyrat, J. P.;
Wakselman, M.; George, C.; Deschamps, J. R.; Flippen-
Anderson, J. L.; Pispisa, B.; Venanzi, M.; Palleschi, A.
Chem.—Eur. J. 1999, 5, 2254–2264; (b) Corvaja, C.; Sartori,
E.; Toffoletti, A.; Formaggio, F.; Crisma, M.; Toniolo, C.;
Mazaleyrat, J. P.; Wakselman, M. Chem.—Eur. J. 2000, 6,
2775–2782; (c) Pispisa, B.; Mazzuca, C.; Palleschi, A.;
Stella, L.; Venanzi, M.; Wakselman, M.; Mazaleyrat, J. P.;
Rainaldi, M.; Formaggio, F.; Toniolo, C. Chem.—Eur. J.
2003, 9, 4084–4093.
4. Toniolo, C.; Crisma, M.; Formaggio, F.; Peggion, C.
Biopolymers (Pept. Sci.) 2001, 60, 396–419.
5. (a) Egusa, S.; Sisido, M.; Imanishi, Y. Bull. Chem. Soc. Jpn.
1986, 59, 3175–3178; (b) Hohsaka, T.; Kajihara, D.;
Ashizuka, Y.; Murakami, H.; Sisido, M. J. Am. Chem. Soc
1999, 121, 34–40.
6. (a) Kotha, S.; Brahmachary, E.; Sreenivasachary, N.
Tetrahedron Lett. 1998, 39, 4095–4098; (b) Kotha, S.;
Brahmachary, E. J. Org. Chem. 2000, 65, 1359–1365; (c)
Kotha, S. Acc. Chem. Res. 2003, 36, 342–351; (d) Kotha, S.;
Sreenivasachary, N.; Brahmachary, E. Eur. J. Org. Chem.
2001, 787–792; (e) Kotha, S.; Ghosh, A. K. Tetrahedron
2004, 60, 10833–10841.
7. Hallman, J. L.; Bartsch, R. A. J. Org. Chem. 1991, 56, 6243–
6245.
8. Sun, S.; Desper, J. Tetrahedron 1998, 54, 411–422.
3.20. Boc–Aib–antAib–L-Ala–OMe (16a)
1
Mp 180–185 ꢁC. Rf 0.25 (II). H NMR (CDCl3): d 8.32 [s,
2H, ArH antAib], 7.99–7.96 [m, 2H, ArH antAib], 7.78 [m
(s-like), 3H, 2 ArH antAib and NH Ala], 7.46–7.43 [m,
2H, ArH antAib], 6.65 [br s, 1H, NH antAib], 4.79 [s, 1H,
NH Aib], 4.61 [dq, Jw7.1 Hz and 7.1 Hz, 1H, CHa Ala],
4.02 [d, Jw17.0 Hz, 1H, ArCHA antAib], 3.80 [d,
Jw17.0 Hz, 1H, ArC0HA antAib], 3.54 [d, Jw17.0 Hz, 1H,
ArCHB antAib], 3.23 [d, Jw17.0 Hz, 1H, ArC0HB antAib],
3.73 [s, 3H, OCH3], 1.47 [d (partly masked), Jw6.8 Hz,
3H, CH3 Ala], 1.45 [s, 3H, CH3 Aib], 1.42 [s, 12H, CH3
Boc and CH3 Aib]. 13C NMR (CDCl3): d 173.5, 172.1
[C]O Ala, antAib and Aib], 155.2 [C]O Boc], 138.7,
131.5, 128.0, 125.6, 125.1, 122.7 [CAr], 81.2 [C–O Boc],
67.6 [Ca antAib], 57.1 [Ca Aib], 52.1 [OCH3], 48.6 [Ca
Ala], 41.2 [ArCH2 antAib], 28.2 [CH3 Boc], 26.3, 22.7
25
25
578
[CH3 Aib], 17.2 [CH3 Ala]. [a]
ꢀ35.6, [a]
ꢀ36.9,
589
[a] ꢀ41.9, [a] ꢀ73.3 (c 0.12, CHCl3). ESI+ MS m/z
25
25
546
436
(relative intensity): 570.4 (100) [M+Na]+. Anal. Calcd for
C31H37N3O6 (547.630): C, 67.99; H, 6.81. Found: C,
68.07; H, 7.46.
3.21. cyclo-[antAib–L-Ala] (17a)
1
Mp >300 ꢁC. H NMR (CD3SOCD3): d 8.61 [s, 1H, NH
antAib], 8.45 [s, 2H, ArH antAib], 8.27 [br s, 1H,
NH Ala], 8.05–8.01 [m, 2H, ArH antAib], 7.84 [s, 1H,
ArH antAib], 7.83 [s, 1H, ArH antAib], 7.48–7.44 [m,
2H, ArH antAib], 4.61 [m (q-like), Jw6.9 Hz, 1H, CHa
Ala], 3.72 [d, Jw17.0 Hz, 1H, ArCHA antAib], 3.61
[d, Jw17.0 Hz, 1H, ArC0HA antAib], 3.32 [d (partly
masked), 1H, ArCHB antAib], 3.25 [d, Jw17.0 Hz, 1H,
ArC0HB antAib], 1.34 [d, Jw6.9 Hz, 3H, CH3 Ala]. ESI+
MS m/z (relative intensity): 331.2 (100) [M+H]+, 661.3
´
9. Valdes, C.; Spitz, U. P.; Toledo, L. M.; Kubik, S. W.; Rebek, J.,
Jr.. J. Am. Chem. Soc. 1995, 117, 12733–12745.
10. Calculated by us from the reported 45% yield relative to
the coupling step and 66% yield relative to the hydrolysis
step.6b

J.-F. Lohier et al. / Tetrahedron 62 (2006) 6203–6213
6213
11. Carpino, L. A. J. Am. Chem. Soc. 1993, 115, 4397–4398.
12. (a) Fuller, W. D.; Cohen, M. P.; Shabankareh, M.; Blair, R. K.;
Goodman, M.; Naider, F. R. J. Am. Chem. Soc. 1990, 112,
7414–7416; (b) Fuller, W. D.; Goodman, M.; Naider, F. R.;
Zhu, Y.-F. Biopolymers 1996, 40, 183–205.
13. For a recent review-article on the coupling methods used for
Ca-tetrasubstituted a-amino acids, see: Formaggio, F.;
Broxterman, Q. B.; Toniolo, C. Houben-Weyl: Methods of
Organic Chemistry; Goodman, M., Felix, A., Moroder, L.,
Toniolo, C., Eds; Synthesis of Peptides and Peptidomimetics;
Thieme: Stuttgart, 2003; Vol. E, 22c, pp 292–310.
14. Wright, K.; Melandri, F.; Cannizzo, C.; Wakselman, M.;
Mazaleyrat, J.-P. Tetrahedron 2002, 58, 5811–5820.
15. Turro, N. J. Modern Molecular Photochemistry; Benjamin/
Cummings: Menlo Park, CA, 1978.
16. Hruby, V. J. Acc. Chem. Res. 2001, 34, 389–397.