Synthesis of 2-amino-8-oxodecanoic acids (Aodas) present in natural hystone deacetylase inhibitors

Article abstract of DOI:10.1021/jo0518250

Differently substituted 2-amino-8-oxodecanoic acids (Aodas), present in naturally occurring inhibitors of hystone deacetylase (HDAC), have been prepared using a convergent approach. The configuration in position 2 was derived from enantiomerically pure allylglycine or glutamic acid, whereas the stereochemistry of the substituent in position 9 derived from lactic acid or glyceraldehyde derivatives. Starting from allylglycine, (S)-Aodas, protected at the nitrogen as Boc or Fmoc, were obtained in four steps in about 30% overall yield. These products have been used to prepare a simplified analogue of a natural cyclic tetrapeptide HDAC inhibitor by SPPS.

Full text of DOI:10.1021/jo0518250

Synthesis of 2-Amino-8-oxodecanoic Acids (Aodas) Present in
Natural Hystone Deacetylase Inhibitors
Manuela Rodriquez,‡ Ines Bruno,‡ Elena Cini, Mauro Marchetti,† Maurizio Taddei,* and
Luigi Gomez-Paloma‡
CNR, Istituto di Chimica Biomolecolare, Sez. Sassari, traV. La Crucca 3, 07040 Sassari, Italy,
Dipartimento di Scienze Farmaceutiche, UniVersita` degli Studi di Salerno, Via Ponte don Melillo,
84084 Fisciano, Salerno, Italy, and Dipartimento Farmaco Chimico Tecnologico, UniVersita` degli Studi di
Siena, Via A. Moro, 53100 Siena
taddei.m@unisi.it
ReceiVed August 30, 2005
Differently substituted 2-amino-8-oxodecanoic acids (Aodas), present in naturally occurring inhibitors
of hystone deacetylase (HDAC), have been prepared using a convergent approach. The configuration in
position 2 was derived from enantiomerically pure allylglycine or glutamic acid, whereas the
stereochemistry of the substituent in position 9 derived from lactic acid or glyceraldehyde derivatives.
Starting from allylglycine, (S)-Aodas, protected at the nitrogen as Boc or Fmoc, were obtained in four
steps in about 30% overall yield. These products have been used to prepare a simplified analogue of a
natural cyclic tetrapeptide HDAC inhibitor by SPPS.
Introduction
important class of HDAC inhibitors is constituted by cyclic
tetrapeptides of natural origin.8 With the exception of a few
examples, they are essentially constituted by a large cap group
containing hydrophobic amino acids and a lateral side chain
The organization of DNA into chromatin has important
implications in gene expression. DNA methylation and hystone
protein modification influence the effective activity of a gene
that, although present in the genome, may be silenced or
expressed by this epigenetic mechanism.1 Hystone acetylation
results in activation of genes increasing accessibility of nucleo-
some DNA, whereas hystone deacetylation may silence key
genes during differentiation.2 Several reports associate epigenetic
alterations with cancer development and, consequently, many
molecules have been evaluated to restore the initial equilibrium
in the so-called “epigenetic therapy”.3 As hystone deacetylase
(HDAC) is a class of enzymes implicated in epigenetic disorders,
HDAC inhibitors have been shown to induce growth arrest,4
terminal differentiation,5 apoptosis,6 and antiangiogenesis.7 An
(4) (a) De Ruijter, A. J.; Van Gennip, A. H.; Caron, H. N.; Kemp, S.;
Van Kuilenburg, A. B. Biochem. J. 2003, 370, 737. (b) Sowa, Y.; Orita,
T.; Minamikawa, S.; Nakano, K.; Mizuno, T.; Nomura, H.; Sakai, T.
Biochem. Biophys. Res. Commun. 1997, 241, 142. (c) Huang, L.; Sowa,
Y.; Sakai, T.; Pardee, A. B. Oncogene 2000, 19, 5712.
(5) (a) Munster, P. N.; Troso-Sandoval, T.; Rosen, N.; Rifkind, R.; Marks,
P. A.; Richon, V. M. Cancer Res. 2001, 61, 8492. (b) Gore, S. D.; Weng,
L. J.; Figg, W. D.; Zhai, S.; Donehower, R. C.; Dover: G.; Grever, M. R.;
Griffin, C.; Grochow, L. B.; Hawkins, A.; Burks, K.; Zabelena, Y.; Miller,
C. B. Clin. Cancer Res. 2002, 8, 963. (c) Ferrara, F. F.; Fazi, F.; Bianchini,
A.; Padula, F.; Gelmetti, V.; Minucci, S.; Mancini, M.; Pelicci, P. G.; Lo
Coco, F.; Nervi, C. Cancer Res. 2001, 61, 2. (d) Mielnicki, L. M.; Ying,
A. M.; Head, K. L.; Asch, H. L.; Asch, B. B. Exp. Cell Res. 1999, 249,
161.
(6) (a) Guo, F.; Sigua, C.; Tao, J.; Bali, P.; George, P.; Li, Y.; Wittmann,
S.; Moscinski, L.; Atadja, P.; Bhalla, K. Cancer Res. 2004, 64, 2580. (b)
Takahashi, I.; Miyaji, H.; Yoshida, T.; Sato, S.; Mizukami, T. J. Antibiot.
1996, 49, 453. (c) Koyama, Y.; Adachi, M.; Sekiya, M.; Takekawa, M.;
Imai, K. Blood 2000, 96, 1490. (d) Huang, N.; Katz, J. P.; Martin, D. R.;
Wu, G. D. Cytokine 1997, 9, 27.
† CNR, Sassari.
‡ Universita` degli Studi di Salerno.
(1) Biel, M.; Wascholowski, V.; Giannis, A. Angew. Chem., Int. Ed. 2005,
44, 3186 and references therein.
(2) Lachner, M.; Jenuwein, T. Curr. Opin. Cell. Biol. 2002, 14, 286.
(3) Marks, P. A., Miller, T.; Richon, V. M. Curr. Opin. Pharmacol. 2003,
3, 344.
(7) Miller, T. A.; Witter, D. J.; Belvedere, S J. Med. Chem. 2003, 46,
5097.
10.1021/jo0518250 CCC: $33.50 © 2006 American Chemical Society
Published on Web 12/02/2005
J. Org. Chem. 2006, 71, 103-107
103

Rodriquez et al.
SCHEME 1
SCHEME 2a
FIGURE 1. Natural HDAC inhibitor tetrapeptides.
formed by 2-amino-8-oxa-decanoic acid (Aoda) and its deriva-
tives, ending with a chelating group for the zinc present in the
active site of the enzyme.
a Reagents and conditions: (a) BnBr, Na2CO3, NaOH, H2O, reflux, 1 h;
(b) HRh(CO)(PPh3)3 Xantphos (10% of a 1:2 mixture) H2/CO 1:1, 20 atm,
40 °C for 96 h.
All members of the Aoda family bear a carbonyl in position
8 and additional moieties relevant for protein binding in position
9 or 10 (such as OH or an epoxide). The nature of this additional
binding site influences the reversibility of the inhibition.
Epoxide-containing molecules (such as trapoxins) are irrevers-
ible inhibitors,9 whereas alkyl- or OH-containing Aoda tet-
rapeptides behave as reversible inhibitors (see Figure 1).10
Despite the importance of these noncoded amino acids, relatively
few syntheses of these molecules have been reported11 and are
mainly of the parent compound with no substituents on the last
two carbons.12
synthesis allows the introduction of different moieties (en-
antiomerically pure, if required) at the position adjacent to the
carbonyl in order to modulate their inhibitory activity. The most
straightforward approach toward aldehyde A seemed to be the
hydroformylation of (S)-allylglycine.14 As the last step of the
proposed synthetic scheme was hydrogenation, we decided to
protect allylglycine as the tribenzyl derivative, to get their
simultaneous deprotection during the reduction of the double
bond between C6-C7. Thus, (S)-allylglycine was protected with
benzyl bromide, following the procedure developed by Reetz,15
giving compound 2 in 90% yield (Scheme 2). Hydroformylation
of 2 was carried out using the catalytic system HRh(CO)(PPh3)3
Xantphos16 under 20 atm of H2/CO 1:1 at 40 °C for 96 h, and
compound 3 was isolated in 75% yield as a single regioisomer
(1HNMR 400 MHz analysis). The combined use of the hindered
protection at the nitrogen and the use of the ligand Xantphos
prevented the formation of the branched isomer.
Results and Discussion
Following our interest in the synthesis of biologically relevant
R-amino acids,13 we wanted to develop a general stereocon-
trolled synthesis of all possible 2-amino-8-oxodecanoic acids
present in nature and eventually of other synthetic analogues
for the discovery of new and more selective HDAC inhibitors.
A possible retrosynthesis is reported in Scheme 1, where a
disconnection at the C6-C7 bond is proposed.
An alternate (longer) synthesis of aldehyde 3, designed for
laboratories not equipped with autoclaves and security systems
for handling CO, started from L-glutamic acid, which was fully
benzylated using the same procedure applied to allylglycine.
The tetrabenzyl derivative was reduced with DIBAL in order
to get the alcohol 5 (3 equiv of DIBAL at 0 °C), which was
then oxidized to aldehyde 6 under standard Swern conditions.17
Homologation with methoxymethyltriphenyl phosphonium chlo-
The corresponding aldehyde A could be prepared starting
from readily available natural amino acids and the phosphonates
B could be obtained from the corresponding esters. This
(8) Singh, S. B.; Zink, D. L.; Liesch, J. M.; Mosley, R. T.; Dombrowski,
A. W.; Bills, G. F.; Darkin-Rattray, S. J.; Schmatz, D. M.; Goetz, M. A. J.
Org. Chem. 2002, 67, 815.
(9) Kijima, M.; Yoshida, M.; Sugita, K.; Horinouchi, S.; Beppu, T. J.
Biol. Chem. 1993, 268, 22429.
(10) (a) Mori, H., Mori H.; Fumie, A.; Seiji, Y.; Shigehiro, T.; Motohiro,
H. PCT Application WO 00/08048, 2000. (b) Mori, H.; Urano, Y.; Abe,
F.; Furukawa, S.; Furukawa, S.; Tsurumi, Y.; Sakamoto, K.; Hashimoto,
M.; Takase, S.; Hino, M.; Fujii, T. J. Antibiot. 2003, 56, 72
(11) (a) Bajgrowicz, J. A.; El Hallaoui, A.; Jacquier, R.; Pigiere, C.;
Viallefont, P. Tetrahedron 1985, 41, 1833. (b) Kuriyama, W.; Kitahara, T.
Heterocycles 2001, 55, 1. (c) Mou, L.; Singh, G. Tetrahedron Lett. 2001,
42, 6603. (d) Kim, S.; Kim, E.-Y.; Ko, H.; Jung, Y. H. Synthesis 2003,
2194. (e) Cooper, T. S.; Laurent, P.; Moody, C. J.; Takle, A. K. Org. Biomol.
Chem. 2004, 2, 265. (f) For the total synthesis of Trapoxin B, see: Taunton,
J.; Collins, J. L.; Schreiber, S. L. J. Am. Chem. Soc. 1996, 118, 10412
(12) The total synthesis of FR235222 has been recently described,
including a different aproach to Ahoda: Xie, W.; Zou, B.; Pei, D.; Ma, D.
Org. Lett. 2005, 7, 2775.
(14) Enantiomerically pure allylglicine is commercialy available and can
be obtained following different procedures as, for example: Wolf, L.; Sonke,
T.; Tjen, K. C. M. F.; Kaptein, B.; Broxterman, Q. B.; Schoemaker, H. E.;
Rutjes, F. P. J. T. AdV. Synth. Catal. 2001, 343, 662 and references therein.
(15) Reetz, M. T.; Drewes, M. W.; Schwickardi, R. Org. Synth. 1998,
76, 110.
(16) Xantphos: 9,9-dimethyl-4,5-bis(diphenylphosphine)-xanthene. Kranen-
burg, M.; van der Burgt, Y. E. M.; Kamer, P. C. J.; van Leeuwen, P. W. N.
M Organometallics 1995, 14, 3081.
(17) (a) Rodriquez, M.; Taddei, M. Synthesis 2005, 493. For the synthesis
of the γ-aldehyde of glutamic acid with different protective groups, see:
(b) Kokotos, G.; Padron, J. M.; Martin, T.; Gibbons, W. A.; Martin, V. S.
J. Org. Chem. 1998, 63, 3741 (c) Padron, J. M.; Kokotos, G.; Martin, T.,
Markidis, T.; Gibbons, W. A.; Martin, V. S. Tetrahedron: Asymmetry 1998,
3381.
(13) For the last paper of the series, see: Esposito, A.; Piras, P. P.;
Ramazzotti, D.; Taddei, M. Org. Lett. 2001, 3, 3273.
104 J. Org. Chem., Vol. 71, No. 1, 2006

Synthesis of 2-Amino-8-oxodecanoic Acids
SCHEME 3a
reaction times gave the N-monobenzyl amino acid. The reaction
was carried out by hydrogenation using Pd(OH)2/C in dry
MeOH at 6 atm for 12 h (Parr apparatus). After catalyst removal
by filtration on Celite and purification through a small silica
plug, compounds 16-19 were obtained in 70-80% yields.
Using these conditions, the other protections on the side chain
remained untouched. In the case of compounds 13 and 14, when
CHCl3 was used as the solvent, the simultaneous deprotection
of the OTBDMS group was observed. When the hydrogenation
was carried out in the presence of Boc2O (1.5 equiv), the
corresponding N-Boc-protected amino acids 20-23 were ob-
tained in a single step from the tribenzyl derivatives. On the
other hand, the introduction of the Fmoc protection was possible
by reaction of the free amino acid with FmocOSu in aqueous
Na2CO3. Compounds 16 and 17 were transformed into the
Fmoc-protected derivatives 24 and 25 in high yields. These
products, like 20-23, are suitable for use in SPPS.
a Reagents and conditions: (a) BnBr, Na2CO3, NaOH, H2O, reflux, 1 h;
(b) DIBAL, THF, 0 °C; (c) (COCl)2, DMSO, Et3N in CH2Cl2, -78 °C, 30
min; (d) MeOCH2PPh3Cl, LiN(SiMe3)2, THF, 0 °C then 12 h rt; (e) HCl
6N EtOAc, 20 min, rt.
SCHEME 4a
HPLC analysis and high field 1H NMR spectra of compounds
21, 22, and 23 showed the presence of a single diastereoisomer,
confirming that the stereochemical integrity was maintained.
As expected, (R)- and (S)-Aodas gave rise to subtle differences
1
in their pattern of NMR signals (in both H and 13C NMR
spectra), very likely owing to the remote character of their two
stereogenic centers separated by a flexible six-carbon chain.
However, a more sensible spectroscopic difference stemmed
from the characteristic pattern of doubled signals contained in
the NMR spectra of (R)-Aoda, originating from the slow
interconversion (on the NMR time scale) of rotamers with
different Boc spatial orientation.
Compound 21 was employed for the synthesis of a new
synthetic tetrapeptide by SPSS starting with a 2-chlorotrityl
resin.18 D-Pro was loaded on the resin followed by HATU/
HOBT promoted couplings with L-Phe and L-Aib. Product 21
(N-Boc-OTBDMS-Ahoda) was introduced as the last amino acid
before cleavage from the resin with AcOH/TFE/TIS that gave
the linear precursor 27 in 70% yield.
a Reagents and conditions: (a) CH3PO(OMe)2, BuLi, THF, -78 °C, 1
h; (b) 3, LiCl, DIPEA, MeCN, rt, 72 h; (c) H2 (6 atm), Pd(OH)2 on C
(10%) MeOH, rt, 12 h; (d) H2 (6 atm.), Pd(OH)2 on C (10%) Boc2O (1.5
equiv), MeOH, rt, 12 h; (e) FmocOSu, Na2CO3, acetone/H2O 1/1, rt, 24 h.
ride and LiN(SiMe3)2 in THF, followed by acidic workup, gave
the required aldehyde 3 (Scheme 3). All steps of the process
were optimized, and aldehyde 3 was obtained in 75% overall
yield on a multigram scale.
After cleavage and removal of TBDMS and Boc groups from
Ahoda by treatment with TFA-H2O-TIS (95% yield),19 the
unprotected linear tetrapeptide 28 was cyclized by exposure to
HATU (2 equiv) and DIEA (2 equiv) in a very diluted solution
(7.7 × 10-5 M in DCM) to avoid the side formation of the
cyclodimeric sequence of 29 as byproduct (Scheme 5). The
cyclized product 29 was obtained in 68% yield after HPLC
At the same time, the â-ketophosphonates 8-11 were
obtained from the corresponding commercially available esters
and dimethyl methylphoshonate in the presence of BuLi in THF
at -78 °C. For the synthesis of the two epimers of 2-amino-
9-hydroxy-8-oxadecanoic acids (17 and 18), commercially
available (S)-methyl lactate and (R)-isobutyl lactate were
employed as the starting esters after protection of the OH as
OTBDMS (Scheme 4). For the synthesis of 19 (precursor of
epoxide-containing tetrapeptides), commercially available (R)-
methyl-2,2-dimethyl-1,3-dioxolane-4-carboxylate was employed.
Phosphonates 9 and 10 were sufficiently stable to be isolated,
purified by column chromatography, and fully characterized,
whereas compounds 8 and 11 showed low stability and had to
be used in the next step without purification. Wittig-Horner-
Emmons reaction of phosphonates 8-11 with aldehyde 3 gave
the unsaturated ketones 12-15 (Scheme 4). The reaction was
carried out under strictly anhydrous conditions in MeCN using
LiCl and DIPEA as base. After stirring at room temperature
for 72 h, the required compounds were obtained in yields higher
than 80% after column chromatography (see Experimental
Section). The last step (hydrogenation and hydrogenolysis of
the protective groups) was crucial for the success of the
synthesis. The use of Pd/C did not succeed in deprotection of
the benzyl groups (exclusively reduction of the double bond
occurred). Moreover, working at low H2 pressure or reducing
1
purification and was characterized by ES-MS and H and 13C
NMR spectroscopic data. Compound 29 has been tested gainst
class III HDACs (experiments carried out on rat smooth muscle
cell line A7r5), showing an IC50 of 10 mM.20 Athough the
activity of this product was not high as some of the natural
tetrapeptides, we have demonstrated that it is possible to prepare
simplified HDAC inhibitor tetrapeptides with the incorporation
of the Aoda moiety.
In conclusion, we have developed a general efficient method
for the synthesis of all members of the Aoda family present in
nature and their synthetic analogues. The final products are
(18) Terracciano, S.; Bruno, I.; Bifulco, G.; Copper, J. E.; Smith, C. D.;
Gomez-Paloma, L.; Riccio, R. J. Nat. Prod. 2004, 67, 1325-1331.
Napolitano, A.; Rodriquez, M.; Bruno, I.; Marzocco, S.; Autore, G.; Riccio,
R.; Gomez-Paloma, L. Tetrahedron 2003, 59, 10203-10211.
(19) The amount of TIS used as carbocation scavenger had to be carefully
adjusted because of its ability to promote a reductive amination (as side
reaction) between the free amino terminus and the Ahoda carbonyl group
in position 8, giving rise to an eight-membered ring.
(20) Govin, J.; Caron, C.; Rousseaux, S.; Khochbin, S. Trends Biochem.
Sci. 2005, 30, 357.
J. Org. Chem, Vol. 71, No. 1, 2006 105

Rodriquez et al.
SCHEME 5
EtOAc (8 mL) followed by a solution of 6 N HCl (4 mL).22 The
mixture was stirred at room temperature for 20 min, and then a
saturated solution of Na2CO3 added. The organic phase was
separated, and the aqueous layer was washed several times with
EtOAc. The organic fractions were collected and dried, and the
solvent was evaporated to give compound 3 (0.436 g. 94% yield).
The spectra of the product were identical to the product obtained
by hydroformilation.
(2S,9S,E)-2-Dibenzylamino-9-(tert-butyldimethylsilyloxy)-8-
oxo-6-decenoic Acid Benzyl Ester 14, General Procedure. To a
solution of phosphonate 10 (0.293 g, 0.94 mmol) in dry MeCN (5
mL) was added dry LiCl (42.7 mg, 0.94 mmol), followed by freshly
distilled DIPEA (98.0 mg, 0.78 mmol). After stirring for 2 h at
room temperature, aldehyde 3 (0.300 g, 0.72 mmol) in MeCN was
added and the mixture was stirred at room temperature for 72 h. A
saturated solution of NaCl was used for the quench and the organic
layer was separed. The aqueous phase was extracted several times
with EtOAc, and all the organic fractions were collected, dried,
and purified using a Sepacore system (silica gel column, petroleum
ether 60-80/ EtOAc 6:1) to give compound 14 (0.37 g, 87% yield).
1H NMR (200 MHz, CDCl3): δ 0.04 (s, 3H), 0.06 (s, 3H), 0.89 (s,
9H), 1.28 (d, J ) 7 Hz, 3H), 1.35-1.82 (m, 4H), 1.87-2.01 (m,
2H), 3.24 (X part of an ABX system, 1H), 3.49 (d, J ) 8 Hz, 2H),
3.88 (d, J ) 8 Hz, 2H), 4.21 (q, J ) 7 Hz, 1H), 5.18 (AB system,
obtained in four steps, starting from (S)-allylgycine or in six
steps from L-Glu. The synthesis is general and may be used to
introduce different functional groups at the R position, just
starting from different esters. Moreover, these compounds are
ready for the introduction in a tetrapeptide for further cyclization
and synthesis of new compounds related to apicidin and trapoxin
families.21
B
2H), 6.58 (d, J ) 13 Hz, 1H), 6.89 (dt, AJ ) 13 Hz, J ) 8 Hz,
1H), 7.20-7.40 (m, 15H). 13C NMR (50 MHz, CDCl3): δ -4.9,
-4.8, 18.1, 21.1, 24.5, 25.7(3C), 28.9, 32.1, 54.4(2C), 60.2, 66.0,
74.4, 124.2, 127.0(2C), 128.2(4C), 128.3(2C), 128.4(4C), 128.5-
(2C), 128.8(2C), 136.1, 139.4, 148.2, 172.0, 201.7. IR (neat, cm-1
)
ν 3070, 1740, 1705. MS (ES) m/z 600 (M + H)+. Anal. Calcd for
C37H 49NO4Si: C, 74.08; H, 8.23; N, 2.23. Found: C, 73.87; H,
8.20; N, 2.22.
Experimental Section
(2S,9S)-2-(tert-Butoxycarbonylamino)-9-(tert-butyldimethyl-
silyloxy)-8-oxodecanoic Acid 21, General Procedure. Pd(OH)2
on C (20 mg) was dissolved in dry MeOH (4 mL) and inserted
into the bottle connected with a Parr apparatus. After 2 cycles of
vacuum-nitrogen, ketone 14 (0.200 g, 0.334 mmol), dissolved in
dry MeOH (2 mL), was added, followed by Boc2O (0.145 g, 0.66
mmol). The bottle was filled with H2 at 6 atm and shaken at room
temperature for 12 h. The bottle was degassed, and the catalyst
was filtered (attention: the Pd residue may be pyrophoric) and
washed several times with MeOH. The solvent was evaporated and
product 21 was purified using the Sepacore system (silica gel
(S)-2-Dibenzylamino-6-oxo-hexanoic Acid Benzyl Ester (3)
from Tribenzylallylglycine. A solution containing compound 2
(1.0 g, 2.6 mmol), HRh(CO)(PPh3)3 (47.6 mg, 0.05 mmol) and
Xanthphos (0.168 g, 0.306 mmol) in dry toluene (20 mL) was
inserted into a 150 mL stainless steel autoclave, closed under
nitrogen, and pressurized with 20 atm of CO/H2 1:1. The mixture
was stirred for 96 h at 40 °C. After cooling, the autoclave was
degassed and the solvent was evaporated under vacuum. The crude
was dissolved in diethyl ether and passed through a short path of
silica gel to recover 3 (0.78 g, 75% yield) sufficiently pure for the
1
column, CHCl3/MeOH 98:2). Obtained 0.108 g, 75% yield. H
1
NMR (200 MHz, CDCl3): δ 0.08 (s, 6H), 0.87 (s, 9H), 1.21 (d, J
) 7 Hz, 3H), 1.45 (s, 9H), 1.25-1.87 (m, 8H), 2.49-2.59 (m,
2H), 4.12 (q, J ) 7 Hz, 1H), 4.20-4.31 (m, 1H), 4.95-5,05 (bs,
1H). 13C NMR (50 MHz, CDCl3): δ -3.7, -3.5, 18.1, 21.1(3C),
24.3(3C), 25.0, 25.7, 28.9, 29.2, 33.4, 38.4, 57.6, 79.6, 80.6, 158.2,
172.5, 215.2. IR (neat, cm-1) ν 3250, 3100-2660 (broad), 1740,
1730, 1725. MS (ES) C21H41NO6 m/z 454 (M + Na)+. Anal. Calcd
for : C, 58.43; H, 9.57; N, 3.25. Found: C, 58.37; H, 9.54; N,
3.22.
next step. H NMR (CDCl3, 200 MHz): δ 1.60-1.80 (m, 4H),
2.11-2.19 (m, 2H), 3.23 (X part of an ABX system, 1H), 3.41 (d,
J ) 8 Hz, 2H), 3.80 (d, J ) 8 Hz, 2H), 5.12 (AB system, 2H),
7.15-7.40 (m, 15H), 9.57 (s, 1H). 13C NMR (50 MHz, CDCl3): δ
18.6, 28.9, 42.2, 54.4 (2C), 60.2, 66.1, 125.2 (2C), 125.3, 126.3
(4C), 126.5 (2C), 128.6 (4C), 128.9 (2C), 136.2 (2C), 139.2, 172.1,
202.1. IR (neat, cm-1) ν 3090, 2870, 2800, 1740, 1730, 1550. MS-
(ES) C27H29NO3 m/z 416 (M + H)+.
(S)-2-Dibenzylamino-6-oxo-hexanoic Acid Benzyl Ester (3)
from Aldehyde 6. Methoxymethyltriphenylphosphomium chloride
(0.771 g, 2.25 mmol) was dissolved in dry THF (8 mL), and to
this solution, cooled to 0 °C, was slowly added LiN(SiMe3)2 (2.4
mL of a 1 M solution in THF). After 1 h at 0 °C, aldehyde 617a
(0.597 g, 1.5 mmol) in dry THF (8 mL) was added. The mixture
was stirred overnight at room temperature. Water was added and
the two phases separated. The aqueous layer was washed several
times with Et2O, and the organic fractions were collected, dried,
and evaporated. The crude was purified using the Sepacore system
(silica gel column, petroleum ether 60-80/EtOAc 3:1) to give crude
compound 22 (0.48 g, 75% yield). This product was dissolved in
Cyclo-(2S,9R)-Ahoda-Aib-L-Phe-D-Pro (29). The tetrapeptide
wa prepared using classical procedures for solid-phase synthesis
and cleaved from the resin as reported in Supported Information.
The cyclization step was performed in solution at a concentration
of 7.7 × 10-5 M with HATU (18.8 mg, 0.05 mmol) and DIEA
(10.9 µL, 0.062 mmol) in DCM. The solution was stirred at 4 °C
for 1 h and then allowed to warm to room temperature for 1 h.
The solvent was removed under reduced pressure. The crude
cyclopeptide (36.9 mg) was purified by semipreparative RP HPLC
using the following gradient: from 5% B to 100% B over 30 min
at flow rate of 4 mL/min. The binary solvent system (A/B) was as
(22) One of the referees remarked on the possibility of reaction between
AcOEt and 1 N HCl. In all cases investigated, we never experienced this
problem. However, changing the solvent from AcOEt to THF, Et2O, or
acetone gave the required aldehyde in much lower yield.
(21) For the preparation of the epoxide, the acetonide must be deprotected
and cyclized as described in: Chow, S.; Kitching W Tetrahedron:
Asymmetry 2002, 13, 779.
106 J. Org. Chem., Vol. 71, No. 1, 2006

Synthesis of 2-Amino-8-oxodecanoic Acids
follows: 0.1% TFA in water (A) and 0.1% TFA in acetonitrile
(B). The absorbance was detected at 240 nm. The HPLC analysis
showed one main peak tR ) 19.20 min that was identified as pure
Acknowledgment. The work was financially supported by
MIUR (Rome) as a project within PRIN-2002. The authors thank
Saadi Khochbin (INSERM U309, Grenoble, F) for testing
compound 29. L.G.P. and I.B. also wish to acknowledge the
use of the instrumental (NMR and MS) facilities of the Centre
of Competence in Diagnostics and Molecular Pharmaceutics
supported by Regione Campania (Italy) through POR funds.
1
29 (63% in yield). H NMR (600 MHz, CDCl3) δ: Aoh 4.16 (H-
2, m, 1H), 1.80 (H-3a, m, 1H,), 1.60 (H-3b, m, 1H,), 1.30 (H2-4,
m, 2H,), 1.31 (H2-5, m, 2H,), 1.61 (H2-6, m, 2H,), 2.50 (H-7a, m,
1H), 2.42 (H-7b, m, 1H), 4.25 (H-9, m, 1H,), 1.38 (H3-10, d, J )
7 Hz, 3H); Pro 4.65 (H-13, d, J ) 8 Hz, 1H) 2.32 (H-14, m, 1H)
2.12 (H-14, m, 1H) 2.52 (H2-15, m, 2H) 3.85 (H-16a, m, 1H) 2.47
(H-16b, m, 1H); Phe 5.15 (H-19, ddd, J ) 9.5, 9.5, 6 Hz, 1H)
3.22 (H-20a, dd, J ) 13.5, 9.5 Hz, 1H) 2.98 (H-20b, dd, J ) 13.5,
6 Hz, 1H) 7.22 (H-22/H-26, m, 2H) 7.25 (H-23/H-25, m, 2H) 7.20
(H-24, t, J ) 8.0 Hz 1H); Aib 1.8 (H3-30, s, 3H) 1.35 (H3-31, s,
3H). Exchangeable protons not detectable due to low sample
concentration. ES HRMS calcd for C28H41N4O6 (M+) 529.3026,
obsd 529.3049.
Supporting Information Available: Procedures for the prepa-
ration of compounds 2, 8-11, 16-19, 24-25, and 29 and
characterization of compounds 12, 13, 15-20, 22-25, and 29. This
material is available free of charge via the Internet at http://
pubs.acs.org.
JO0518250
J. Org. Chem, Vol. 71, No. 1, 2006 107