Use of p-nitrobenzyloxycarbonyl (pNZ) as a permanent protecting group in the synthesis of Kahalalide F analogs

Article abstract of DOI:10.1016/j.tetlet.2005.09.044

p-Nitrobenzyloxycarbonyl (pNZ) is used for the permanent protection of ornithine in the synthesis of derivatives of the anti-tumor cyclodepsipeptide Kahalalide F that contain acid labile residues.

Full text of DOI:10.1016/j.tetlet.2005.09.044

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 7737–7741
Use of p-nitrobenzyloxycarbonyl (pNZ) as a permanent
protecting group in the synthesis of Kahalalide F analogs
a,b
Pilar E. Lopez, Albert Isidro-Llobet,a Carolina Gracia,a Luis J. Cruz,a
´
b
b
a,c,
*
´
´
´
´
Andres Garcıa-Granados, Andres Parra, Mercedes Alvarez
and
Fernando Albericioa,d,
*
aBarcelona Biomedical Research Institute, Barcelona Science Park, Josep Samitier 1, 08028 Barcelona, Spain
bDepartment of Organic Chemistry, University of Granada, 18071 Granada, Spain
cLaboratory of Organic Chemistry, Faculty of Pharmacy, University of Barcelona, 08028 Barcelona, Spain
dDepartment of Organic Chemistry, University of Barcelona, 08028 Barcelona, Spain
Received 2 August 2005; revised 2 September 2005; accepted 7 September 2005
Available online 26 September 2005
Abstract—p-Nitrobenzyloxycarbonyl (pNZ) is used for the permanent protection of ornithine in the synthesis of derivatives of the
anti-tumor cyclodepsipeptide Kahalalide F that contain acid labile residues.
ꢀ 2005 Elsevier Ltd. All rights reserved.
1. Introduction
nitrobenzyloxycarbonyl (pNZ)3 as a permanent protect-
ing group for the side chain of Orn is reported (Fig. 1).
Solid-phase fluorenylmethoxycarbonyl (Fmoc)-tert-bu-
tyl (tBu) chemistry is the strategy of choice for the prep-
aration of peptides in basic research and commercial
applications.1,2 In this strategy, the Fmoc group is nor-
mally removed with piperidine in DMF, while permanent
protecting groups are removed by trifluoroacetic acid
(TFA) in the presence of scavengers during the cleavage
of the peptide from the resin.1,2 However, certain pep-
tides can be degraded by the treatment with high concen-
2. Results and discussion
Kahalalide F (1a), the only member of the kahalalide
peptide family to be isolated from the sacoglossan mol-
lusc Elysya rufescens and the green alga Bryopsis sp.,4 is
in clinical trials for several types of cancers.4,5 Structur-
ally, Kahalalide F is a head to side-chain cyclic depsi-
peptide that terminates in an aliphatic acid. The
establishment of an efficient solid-phase synthesis of
Kahalalide F6 by our group spawned an in-house pro-
gram dedicated to creating analogs for SAR.7
t
trations of TFA. For these cases, Bu-type protecting
groups must be substituted with protecting groups that
can be removed in neutral or nearly neutral conditions.
Herein, the synthesis of analogs of the anti-tumor
cyclodepsipeptide Kahalalide F that contain acid labile
substituents (oleanolic and maslinic acids) using p-
The original solid-phase synthesis,6 similar to that out-
lined in Figure 2, is based on an orthogonal protecting
group scheme using a chlorotrityl chloride (ClTrt-Cl,
Barlos) resin together with Fmoc and allyloxycarbonyl
(Alloc) as temporary protecting groups, and Bu and
Boc for the side-chain protection of Thr and Orn,
respectively.
O
t
O
O2N
Figure 1. Structure of the pNZ protecting group.
We envisioned Kahalalide F analogs in which the N-ter-
minal aliphatic acid of the parent compound is substi-
tuted with oleanolic acid (3b-hydroxy-12-oleanen-28-
oic) or maslinic acid (2a,3b-dihydroxy-12-oleanen-28-
oic), pentacyclic triterpenoids widely found in nature.8
Keywords: Combinatorial chemistry; Cyclic pepides; Orthogonal
protecting group.
*
Corresponding authors. Tel.: +34 93 403 70 88; fax: +34 93 403 71
26; e-mail: albericio@pcb.ub.es
0040-4039/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.09.044

7738
P. E. Lo´pez et al. / Tetrahedron Letters 46 (2005) 7737–7741
D-allo-Ile
D-allo-Ile
D-allo-Thr
D-allo-Ile
D-allo-Ile
D-allo-Thr
O
O
D-Val
D-Val
H
H
N
H
H
N
N
N
Cl
Cl
N
H
N
H
N
H
N
H
Fmoc
Fmoc
Cl
O
O
O
O
OH
O
O
O
O
O
O
2
3
L-Val
NH
Alloc
D-allo-Ile
D-allo-Thr
D-allo-Ile
D-allo-Ile
D-allo-Ile
L-Orn
D-Pro
D-allo-Thr
D-Pro L-Orn
D-Val
D-Val
O
O
O
O
O
O
D-Val
D-Val
H
H
N
H
H
N
H
H
H
H
N
O
O
N
N
N
N
N
N
H
N
N
H
N
H
N
N
L-Val
L-Val
L-Thr
H
O
O
O
O
O
O
O
O
O
O
O
O
NH
O
NH
O
O
O
L-Thr
HN
HN
L-Val
NH
L-Val
NH
Alloc
OH
OH
HN
HN
pNZ
pNZ
Alloc
D-Val
O
O
4
5
D-Val
NH2
NH
R
D-allo-Ile
D-allo-Ile
D-allo-Ile
D-allo-Ile
D-allo-Thr
D-allo-Thr
L-Orn
O
L-Orn
D-Val
D-Val
D-Pro
D-Pro
O
O
O
O
O
D-Val
D-Val
L-Val
L-Val
H
H
H
H
N
H
N
H
N
H
H
N
O
O
N
N
N
N
N
N
H
N
H
N
N
H
N
H
O
O
O
O
O
O
O
O
O
OH
NH
O
O
NH
O
O
O
O
L-Thr
L-Thr
HN
pNZ
HN
OH
NH
OH
NH
Alloc
HN
NH2
HN
H
HN
H
pNZ
N
N
O
O
L-Val
L-Val
O
O
D-Val
D-Val
O
O
NH
R
NH
6
7
ZDhB
ZDhB
R
L-Phe
L-Phe
D-allo-Ile
D-allo-Ile
a, R = aliphatic acid (KF)
D-allo-Thr
L-Orn
D-Val
D-Pro
OH
O
OH
OH
O
O
O
D-Val
L-Val
H
H
H
H
N
O
N
N
N
O
N
N
H
N
H
O
O
O
O
O
HN
NH
O
O
L-Thr
H2N
HN
O
NH
OH
HN
L-Val
O
O
L-Phe
b, R = oleanolic acid
c, R = maslinic acid
D-Val
1
NH
ZDhB
R
Figure 2. Synthetic strategy developed for the synthesis of KF analogs.
These compounds are isolated in high yields from olive-
pressing residues following an extraction process.9 Both
acids, as well as structural analogues, have interesting
pharmacological profiles,10 including in vitro anti-HIV
activity11 and antioxidant properties.12
oleanolic and maslinic acids are not stable to high con-
centrations of TFA.
We thus sought a new protecting group for the d-amino
function of Orn that would be orthogonal to the Fmoc,
Alloc, and Cl-Trt protecting groups.13 The Thr near the
N-terminal can be incorporated with its secondary alco-
hol function unprotected due to its low nucleophilicity
and the few coupling cycles implied in the synthesis.
The first attempt at synthesizing the aforementioned
analogs progressed smoothly until the last step. Final
treatment of the peptidyl-resin with TFA, which should
have removed the tBu and Boc groups, led to a complex
mixture in which the target compound could not be
identified by HPLC-MS. It was later established that
Specifically, the group had to be stable to piperidine
(used for Fmoc removal), Pd(0) in the presence of scav-

P. E. Lo´pez et al. / Tetrahedron Letters 46 (2005) 7737–7741
7739
engers (used for Alloc removal), and 1% TFA–CH2Cl2
(used to cleave the protected peptide from the Cl-Trt
resin before cyclization). The pNZ group, recently de-
scribed for the temporary protection of a-functionalities
in solid-phase peptide synthesis meets the aforemen-
tioned conditions.3f The pNZ group, removed with
SnCl2 in the presence of catalytic amounts of acid, also
does not induce side reactions that commonly result
from other temporary protecting groups.3f
(0.797 g, 3.7 mmol) was dissolved in dioxane (1.7 mL)
and a solution of sodium azide (0.289 g, 4.44 mmol) in
H2O (1.1 mL) was added. The resulting emulsion was
stirred for 2 h and the formation of the azide was fol-
lowed by TLC (CH2Cl2). This solution was added drop-
wise to a suspension of Fmoc-L-Orn-OH (1.31 g,
3.7 mmol) in 6 mL of dioxane–2% aqueous Na2CO3
(1:1) and the resulting white suspension was stirred for
24 h with the pH kept between 9 and 10 by adding
10% aqueous Na2CO3. At this point, TLC (CH2Cl2) re-
sults indicated that there was no azide left, H2O (50 mL)
was then added and the suspension was washed with
tert-butyl methyl ether (MTBE) (3 · 40 mL). The aque-
ous portion was acidified to pH 1 with 9 N HCl, afford-
ing a white precipitate that was filtered off and dried to
yield 1.67 g (85% of yield) of the title compound. The
white solid was characterized by HPLC (>98% purity),
ESMS (calcd for C28H27N3O8: 533.2, found m/z 534.4
A new synthesis was thus initiated with limited incorpo-
ration of Fmoc-D-Val-OH to ClTrt-resin,14 followed by
the stepwise coupling of the next three Na-protected
amino acids, with the side chain of D-allo-Thr unpro-
tected. Subsequent incorporation of Alloc-Val-OH
using DIPCDI/DMAP allowed formation of the ester
bond.15 At this point, the peptide chain was completed
by first introducing Fmoc-Orn(pNZ)-OH, followed by
the rest of protected amino acids and terminated by
either oleanolic or maslinic acid. The incorporation of
these two acids required strong coupling conditions,
such as benzotriazol-1-yl-N-oxy-tris(pyrrolidino)phos-
phonium hexafluoro-phosphate (PyBOP)/1-hydroxy-7-
azabenzotriazole (HOAt)/N,N-diisopropylethylamine
(DIEA). The dipeptide Alloc-L-Phe-ZDhb-OH was
introduced and the completed linear peptide was cleaved
from the resin with TFA–CH2Cl2 (1:99), cyclized
with N,N0-diisopropylcarbodiimide (DIPCDI)/hydroxy-
benzotriazole (HOBt), and deprotected using the pNZ
removal conditions described above. The target KF ana-
log was obtained with excellent purity and was charac-
terized by MS.16
1
13
(M+1)+), H and C NMR.18
4.3. KF analogs
To chlorotrityl resin (500 mg, f = 1.27 mmol/g) placed
in a 10 mL polypropylene syringe fitted with a polyeth-
ylene filter disk, was added Fmoc-D-Val-OH
(0.7 equiv), DIEA (7 equiv) and dichloromethane, and
the resulting slurry was stirred for 1 h. The reaction
was then terminated by adding piperidine–DMF (1:4)
and
the
loading
was
calculated
by
UV
(f = 0.56 mmol/g). The chain was elongated by sequen-
tially coupling Fmoc-D-allo-Ile-OH, Fmoc-D-allo-Thr-
OH and Fmoc-D-allo-Ile-OH (4 equiv), using DIPCDI
(4 equiv) and HOBt (4 equiv) in DMF for 90 min.
The completion of all couplings was confirmed with a
negative ninhydrin test result. All Fmoc groups were
removed as indicated above except the last group,
which was not removed. Alloc-Val-OH (5 equiv) was
then coupled to the peptide with DIPCDI (5 equiv)
and DIEA (1 equiv) in the presence of DMAP
(0.5 equiv). This coupling was repeated twice using
the same conditions. An aliquot of the resin was then
treated with TFA–H2O (1:99) for 1 min and, after
evaporation, the crude product corresponding to the
peptide resin 3 was characterized by HPLC (>95% pur-
ity) and ESMS (calcd for C30H53N5O9, 627.4 Found:
m/z 628.2 [M+H]+). Then, the Fmoc group of Fmoc-
D-allo-Ile-OH was removed and Fmoc-Orn(pNZ)-OH
(4 eq), Fmoc-D-Pro-OH (5 equiv), Fmoc-D-Val-OH
(4 equiv, a recoupling was needed), Fmoc-L-Val-OH
(4 equiv), Fmoc-Thr-OH (5 equiv) and Fmoc-D-Val-
OH (4 equiv) were sequentially added to the peptide re-
sin and stirred for 90 min in the presence of equimolar
amounts of DIPCDI and HOBt in DMF.
3. Conclusions
This work demonstrates the utility of the pNZ group as
a permanent protecting group in solid-phase peptide
synthesis. pNZ can be used with Cl-Trt-resin to prepare
t
peptides having acid sensitive residues, for which Bu-
type protection is not possible. The pNZ group is re-
moved in solution or on-resin with SnCl2 and a catalytic
amount of acid.
4. Experimental section
4.1. Fmoc-L-Orn-OH
Fmoc-L-Orn(Boc)-OH (2 g, 4.4 mmol) was dissolved in
TFA–DCM (3:7) and stirred for 30 min. After evaporat-
ing off the solvent, diethyl ether was added to, and evap-
orated from, the crude residue five times in order to
ensure that all of the TFA had been removed. The final
product (1.31 g, 84% yield) was characterized by analyt-
ical HPLC (<98% purity) and ES-MS (calcd for
C20H22N2O4: 354.15, found m/z 355.1 (M+1)+).
After removing the last Fmoc group, an aliquot of the
peptide resin was cleaved as indicated above and the
crude product obtained (corresponding to peptide resin
4) characterized by HPLC (>98% of purity) and MAL-
DI-TOF (calcd for C67H109N13O20: 1416.7, found: m/z
1417.1 [M+H]+, 1439.1 [M+Na]+, 1455.0 [M+K]+).
The remaining resin was suspended in DMF and divided
into two portions. Oleanolic acid (3 equiv) was coupled
to one of the portions, and maslinic acid (3 equiv) was
4.2. Fmoc-L-Orn(pNZ)-OH
Protection of the side chain of Orn was carried out using
the azide method.17 p-Nitrobenzyl chloroformate

7740
P. E. Lo´pez et al. / Tetrahedron Letters 46 (2005) 7737–7741
coupled to the other using PyBOP (3 equiv), HOAt
(3 equiv) and DIEA (9 equiv) in DMF. An aliquot of
each peptide resins was cleaved and characterized by
HPLC (5b: >78% of purity, 5c: >80% of purity) and
ESMS (5b: m/z calcd for C97H155N13O22: 1854.1, found
m/z 1855.6 [M+H]+; 5c: m/z calcd for C97H155N13O23:
1870.1, found m/z 1871.6 [M+H]+).
References and notes
1. (a) Stewart, J. M.; Young, J. D. Solid-Phase Peptide Syn-
thesis, 2nd ed.; Pierce Chemical Co.: Rockford, IL, 1984;
(b) Lloyd-Williams, P.; Albericio, F.; Giralt, E. Chemical
Approaches to the Synthesis of Peptides and Proteins; CRC:
Boca Raton, FL (USA), 1997; (c) Houben-Weyl, Synthesis
of Peptides and Peptidomimetics; Goodman, M., Felix, A.,
Moroder, L. A., Toniolo, C., Eds.; Georg Thieme:
Stuttgart (Germany), 2002; Vol. E 22a-e; (d) Bruckdorfer,
T.; Marder, O.; Albericio, F. Current Pharm. Biotech.
2004, 5, 29–43.
2. (a) Fmoc Solid Phase Peptide Synthesis; Chan, W. C.,
White, P. D., Eds.; Oxford University Press: Oxford (UK),
2000; (b) Fields, G. B.; Lauer-Fields, J. L.; Liu, R.-q.;
Barany, G. In Synthetic Peptides: A UserÕs Guide, 2nd ed.;
Grant, G. A., Ed., WH. Freeman & Co.: New York, 2001;
pp 93–219.
3. (a) Carpenter, F. H.; Gish, D. T. J. Am. Chem. Soc. 1952,
74, 3818–3821; (b) Gish, D. T.; Carpenter, F. H. J. Am.
Chem. Soc. 1953, 75, 950–952; (c) Shields, J. E.; Carpenter,
F. H. J. Am. Chem. Soc. 1961, 83, 3066–3070; (d) Hocker,
M. D.; Caldwell, C. G.; Macsata, R. W.; Lyttle, M. H.
Pept. Res. 1995, 8, 310–315; (e) Peluso, S.; Dumy, P.;
Nkubana, C.; Yokokawa, Y.; Mutter, M. J. Org. Chem.
1999, 64, 7114–7120; (f) Isidro-Llobet, A.; Guasch-Cam-
The Alloc group of both resins was removed using
Pd(PPh3)4 (0.1 equiv) and PhSiH3 (10 equiv) in DMF
(three treatments of 15 min). The resin was then
washed with DCM and DMF, followed by 0.02 M
sodium diethyldithiocarbamate in DMF (3 · 15 min).
Alloc-Phe-ZDhb-OH (2.5 equiv), HOAt (2.5 equiv)
and DIPCDI (2.5 equiv) in DMF were added to each
resin, and the mixture was stirred overnight to yield
the peptidyl resins 6b and 6c. The removal of Alloc
group and subsequent washings were carried out as de-
scribed above.
Both protected peptides were cleaved from the resin with
TFA–DCM (1:99) (5 · 30 s) and successive washings
with DCM. Filtrates were collected in H2O and the
solvent was partially removed in vacuo. Acetonitrile
(MeCN) was added to dissolve the solid that appeared
during H2O removal, the solutions were lyophilized
and the products characterized by HPLC (7b: >91% of
purity, 7c: >73% of purity) and ESMS (7b: calcd for
C106H165N15O22, 2001.5, found m/z 2002.0 [M+H]+;
7c: calcd for C106H165N15O23, 2017.5, found m/z
2018.5 [M+H]+).
´
ell, J.; Alvarez, M.; Albericio, F. Eur. J. Org. Chem. 2005,
3031–3039.
4. Hamann, M. T.; Scheuer, P. J. J. Am. Chem. Soc. 1993,
115, 5825–5826.
5. (a) Jimeno, J.; Lopez-Martin, J. A.; Ruiz-Casado, A.;
Izquierdo, M. A.; Scheuer, P. J.; Rinehart, K. Anti-Cancer
Drugs 2004, 15, 321–329; (b) Hamann, M. T. Curr. Op.
Molecular. Therap. 2004, 6, 657–665; (c) Rademaker-
Lakhai, J. M.; Horenblas, S.; Meinhardt, W.; Stokvis, E.;
de Reijke, T. M.; Jimeno, J. M.; Lopez-Lazaro, L.; Lopez
Martin, J. A.; Beijnen, J. H.; Schellens, J. H. M. Clin.
Cancer Res. 2005, 11, 1854–1862.
6. Lopez-Macia, A.; Jimenez, J. C.; Royo, M.; Giralt, E.;
Albericio, F. J. Am. Chem. Soc. 2001, 123, 11398–11401.
7. Albericio, F.; Fernandez-Donis, A.; Giralt, E.; Gracia, C.;
Lopez-Rodriguez, P.; Varon, S.; Cuevas, C.; Lopez-
The resulting crudes were dissolved in DCM and cyc-
lized in the presence of DIPCDI (3 equiv), HOBt
(4 equiv), and DIEA (3 equiv). After 1 h of stirring, no
starting materials remained as indicated by HPLC, so
the solvent was removed.
Removal of the pNZ groups was carried out by stirring
the peptides with 1 M SnCl2 and 1.6 mM HCl–dioxane
in DMF for 1 h. The solvent was removed in vacuo,
and the crudes were purified by HPLC (Kromasil C8
5 lm, 205 · 50 mm) then characterized by HPLC (1b:
32 mg, >98% of purity, 1c: 19 mg, >97% of purity)
and MALDI-TOF-MS (1b: calcd for C98H157N14O17,
1803.8. Found: m/z 1804.8 [M+H]+, 1825.8 [M+Na]+,
1841.8 [M+K]+; 1c: calcd for C98H158N14O18, 1819.2.
Found: m/z 1820.0 [M+H]+, 1842.9 [M+Na]+, 1857.9
[M+K]+).
´
Macia, A.; Francesch, A.; Jimenez-Garcia, J. C.; Royo,
M. PCT Int. Appl., 2005, WO 2005023846.
8. (a) Dictionary of Natural Products on CD-ROM, ISSN
0966-2146 ver. 5:1 Chapman & Hall: New York, 1996;
oleanolic acid, CAS [508-02-1]; maslinic acid, CAS [4373-
41-5]; (b) Phytochemical and Ethnobotanical DB. http://
www.ars-grin.gov/duke/plants.html.
9. Garcıa-Granados, A.; Martınez, A.; Parra, A.; Rivas F.
PCT Int. Appl. W0 98 04331, 1998. Chem. Abstr. 1998,
128, 179706.
10. (a) Liu, J. Ethnopharmacology 1995, 49, 57; (b) Assefa, H.;
Nimrod, A.; Walker, L.; Sindelar, R. Bioorg. Med. Chem.
Lett. 2001, 11, 619; (c) Honda, T.; Rounds, B. V.; Bore, L.;
Finlay, H. J.; Favaloro, F. G., Jr.; Suh, N.; Wang, Y.;
Sporn, M. B.; Gribble, G. W. J. Med. Chem. 2000, 43,
4233.
´
´
Acknowledgements
11. (a) Kashiwada, Y.; Ikeshiro, Y.; Nagao, T.; Okabe, H.;
Cosentino, L. M.; Lee, K. H. J. Nat. Prod. 2000, 63, 1619;
(b) Mengoni, F.; Lichtner, M.; Battinelli, L.; Marzi, M.;
Mastronianni, C. M.; Vullo, V.; Mazzanti, G. Planta Med.
2002, 68, 111.
12. Montilla, M. P.; Agil, A.; Navarro, C.; Jimenez, M. I.;
Garcia-Granados, A.; Parra, A.; Cabo, M. M. Planta
Med. 2003, 69, 472–474.
This work was partially supported by PharmaMar
S.A.U. (Madrid), CICYT (BQU 2003-00089, PETRI
95-0658OP), the Generalitat de Catalunya (Grup Con-
`
solidat and Centre de Referencia en Biotecnologia),
and the Barcelona Science Park. A.I. thanks the Gener-
alitat de Catalunya for a predoctoral fellowship. The
authors thank Dr. Carmen Cuevas (PharmaMar
S.A.U.) for her encouragement to perform the present
work.
13. (a) Barany, G.; Albericio, F. J. Am. Chem. Soc. 1985, 107,
4936–4942; (b) Albericio, F. Biopolymers (Pept. Sci.)
2000, 55, 123–139.

P. E. Lo´pez et al. / Tetrahedron Letters 46 (2005) 7737–7741
7741
14. The use of high loading (F >0.7 mmol/g) ClTrt-Cl resins
for the preparation of hydrophobic peptides such as
Kahalalide F usually leads to impurities Chiva, C.;
Vilaseca, M.; Giralt, E.; Albericio, F. J. Pept. Sci. 1999,
5, 131–140.
(2H, d, J = 7.2 Hz, H4 and H5); 7.6 (2H, d, J = 8.8 Hz,
H20 and H60); 7.4 (2H, dd, J1 = J2 = 7.2 Hz, H3 and H6);
7.3 (2H, dd, J1 = J2 = 7.2 Hz, H2 and H7); 5.1 (2H, s,
–O–CH2–4-nitrophenyl); 4.2 (3H, m, H9 and fluorene–
~
~
CH2–O–); 3.8 (1H, m, aCH Orn); 3.0 (2H, m, aCH2 Orn);
~
~
15. When this Val was incorporated after the peptide chain
had been completed, the yields were low. This is presum-
ably due to the hydrophobicity of the peptide chain, which
favors interchain aggregation. See Ref. 6.
16. Tin metal content in the purified KF analogue was showed
to be nil by atomic absorption.
1.7 (1H, m, aCH2 Orn); and 1.6 (1H, m, aCH2 Orn); 1.4
(2H, m, aCH2 Orn). 13C NMR (100 MHz, DMSO-d6):
~
174.5 (–CO–OH); 156.6 and 156.5 (–N–CO–O–) 147.6
(C40); 146.1 ( C10); 144.6 and 144.5 (C12 and C13); 141.4
(C10 and C11); 128.7 (C20 and C60); 128.3 (C2 and
C7); 127.8 (C3 and C6); 126.0 (C4 and C5); 124.2 (C30 and
C50); 120.8 (C1 and C8); 66.2 (fluorene–CH2O–);
64.6 (–O–CH2–4-nitrophenyl); 54.8 (aCH Orn); 47.36
(C9); 40.8 (aCH2 Orn); 29.4 (aCH2 Orn); 26.7
17. Cruz, L. J.; Beteta, N. G.; Ewenson, A.; Albericio, F. Org.
Proc. Res. Develop. 2004, 8, 920.
~
18. 1H NMR (400 MHz, DMSO-d6): 8.2 (2H, d, J = 8.4
Hz, H30 and H50); 7.9 (2H, d, J = 7.2 Hz, H1 and H8); 7.7
~
~
~
(aCH2 Orn).