Molecular diversity via amino acid derived α-amino nitriles: Synthesis of spirocyclic 2,6-dioxopiperazine derivatives

Article abstract of DOI:10.1021/jo050146m

(Chemical Equation Presented) Chiral spirocyclic 2,6-dioxopiperazines were synthesized from amino acid derived α-quaternary α-amino nitriles via H₂SO₄-promoted cyano hydration, followed by base-mediated cyclization and N-alkylation.

Full text of DOI:10.1021/jo050146m

Molecular Diversity via Amino Acid Derived r-Amino Nitriles:
Synthesis of Spirocyclic 2,6-Dioxopiperazine Derivatives
Juan A. Gonza´lez-Vera, M. Teresa Garc´ıa-Lo´pez, and Rosario Herranz*
Instituto de Quı´mica Me´dica (CSIC), Juan de la Cierva 3, E-28006 Madrid, Spain
rosario@iqm.csic.es
Received January 24, 2005
Chiral spirocyclic 2,6-dioxopiperazines were synthesized from amino acid derived R-quaternary
R-amino nitriles via H2SO4-promoted cyano hydration, followed by base-mediated cyclization and
N-alkylation. This methodology, requiring the previous preparation of the amino nitrile by a modified
Strecker reaction, was applied to Phe, Trp, Pro, Asp, Glu, and Ser derivatives. In the case of the
Trp-derived amino nitrile the major product of the treatment with H2SO4 was not the expected
carboxamide, but a new tetracyclic indoline derivative containing the novel heterocyclic system
hexahydropyrrolo[1′,2′,3′:1,9a,9]imidazo[1,2-a]indole, as a result of a domino tautomerization. The
treatment of this indoline derivative with refluxing 1 N HCl led to a Trp-derived 2,6-dioxopiperazine.
The 2,6-dioxopiperazine ring opened under the reaction conditions of methyl ester saponification,
giving N-(carboxyalkyl)amino acid derivatives. Therefore, the synthesis of 2,6-dioxopiperazines
containing free carboxylic acids from the respective methyl esters required transesterification to
benzyl esters, followed by hydrogenolysis.
Introduction
oriented synthesis.2 Multicomponent-coupling reactions,
complexity-generating reactions (domino and tandem),
and branching pathways are the most valuable in this
process. Stereoisomerism also contributes to diversity;
therefore, chirality is an additional factor to have in
mind.2d
As part of a wide program aimed to develop method-
ologies for generating peptidomimetics, we have focused
our attention on the potential of amino acid derived
R-amino nitriles as a source of diversity of privileged
scaffolds,3 such as piperazine,4 1,4-benzodiazepine,5 and
Within the past few years, the hit generation step in
the drug discovery process has been broadly approached
by means of high-throughput screening of diverse collec-
tions of small molecules. The most commonly used
synthetic strategy to access to these collections involves
the appendage of different sets of building blocks to a
common molecular skeleton.1 However, this strategy has
led to a limited structural diversity, as the diversity of
building blocks upon a common scaffold produces com-
pounds looking rather similar. Now, it is generally
accepted that gaining efficiency in the search of the
chemical space of small molecules requires access to
complex and diverse molecular skeletons via diversity-
(2) (a) Schreiber, S. L. Science 2000, 287, 1964-1968. (b) Burke,
M. D.; Berger, E. M.; Schreiber, S. L. Science 2003, 302, 613-618. (c)
Spring, D. R. Org. Biomol. Chem. 2003, 1, 3867-3870. (d) Burke, M.
D.; Schreiber, S. L. Angew. Chem., Int. Ed. 2004, 43, 46-58.
(3) (a) Patchett, A. A.; Nargund, R. P. In Annual Reports in
Medicinal Chemistry; Doherty, A. M., Ed.; Academic Press: San Diego,
2000; Vol. 35, pp 289-298. (b) Klabunde, T.; Hessler, G. ChemBioChem
2002, 3, 928-944. (c) Horton, D. A.; Bourne, G. T.; Smythe, M. L. Chem.
Rev. 2003, 103, 893-930.
* Corresponding author. Tel.: (+34) 91-5622900. Fax: (+34) 91-
5644853.
(1) (a) Obrecht, D.; Villalgordo, J. M. Solid-Supported Combinatorial
and Parallel Synthesis of Small-Molecular-Weight Compound Librar-
ies; Pergamon: Oxford, 1998. (b) Terrett, N. K. Combinatorial Chem-
istry; Oxford University Press: New York, 1998.
(4) Herrero, S.; Garc´ıa-Lo´pez, M. T.; Latorre, M.; Cenarruzabeitia,
E.; Del R´ıo, J.; Herranz, R. J. Org. Chem. 2002, 67, 3866-3873.
10.1021/jo050146m CCC: $30.25 © 2005 American Chemical Society
Published on Web 03/25/2005
3660
J. Org. Chem. 2005, 70, 3660-3666

Spirocyclic 2,6-Dioxopiperazine Derivatives
TABLE 1. Optimization of Reaction Conditions for the
SCHEME 1. Retrosynthesis of
Spiro-2,6-dioxopiperazine Derivatives
Synthesis of the Phe-Derived r-Amino Nitrile 4a
formation of imine 3a
(step a, Scheme 2)a
step bb
drying
4a
entry solvent catalyst agentc T (°C) catalystd yielde (%)
1
2
3
4
5
6
7
8
9
MeOH
MeOH
MeOH
MeOH
CH2Cl2
CH2Cl2
THF
ZnCl2
ZnCl2
-20 ZnCl2
20 ZnCl2
20 ZnCl2
65 ZnCl2
20 ZnCl2
20 ZnCl2
20 ZnCl2
20 Yb(OTf)3
20 Yb(OTf)3
0
12
73
77
77
61
10
97
98
MgSO4
MgSO4
MgSO4
CH2Cl2
CH2Cl2
a Reaction time 3 h. b First 1 h at 0 °C, then 20 h at room
temperature, except for entries 8 and 9, where this step was
carried out stirring 1 h at -20 °C and 20 h at room temperature.
c It was filtered off before adding TMSCN. d 0.1 equiv of ZnCl2 and
0.02 equiv of Yb(OTf)3. e Determined by RP-HPLC analysis of the
reaction crude.
pyrazino[1,2-c]pyrimidine6 derivatives, via (cyanometh-
ylene)amino pseudopeptides.7 The construction of these
scaffolds takes advantage of the structural and stereo-
chemical diversity of readily accessible amino acid de-
rivatives and of the high reactivity profile of the cyano
group combined with that of the functional groups of the
starting amino acid derivatives. Now, we have explored
the extension of some of these methodologies to the
preparation of spirocyclic compounds, particularly to the
synthesis of 2,6-dioxopiperazine derivatives (A), as shown
in the retrosynthetic Scheme 1. This scheme involves the
synthesis of amino acid derived R-quaternary R-amino
nitriles B via a three-component Strecker reaction,8
followed by cyano hydration, cyclization, and N-alkyla-
tion. Piperazines and their spiro derivatives are included
among the most frequently occurring privileged substruc-
tures found in compounds of therapeutic interest.3b,c
Among them, 1,4-disubstituted piperazine derivatives
and 2,5-dioxopiperazines (cyclic dipeptides)9 have been
the main focus of interest, while 2,6-dioxopiperazine
derivatives, apart from certain antitumoral bis(2,6-di-
oxopiperazine) derivatives,9a,10 have received scarce at-
tention. Bearing in mind that the retrosynthetic Scheme
1 would allow access to diverse highly functionalized
chiral spirocyclic 2,6-dioxopiperazine derivatives, we have
studied and report herein its application to the synthesis
of spirocyclohexane derivatives, using cyclohexanone as
a model of cyclic ketone. With respect to the amino acid
component, the performance of reactions was first studied
with phenylalanine derivatives (a), and then the studies
were extended to the preparation of Pro (b), Trp (c), Asp
(d-f), Glu (g-i), and Ser (j) derivatives. Due to our
recent interest in the search for new glutamate receptor
ligands, special attention was paid to the glutamic and
aspartic acid derivatives.
Results and Discussion
Initially, the synthesis of R-amino nitriles B was
attempted following the reaction conditions previously
developed in our laboratory for the preparation of R-ami-
no aldehyde-derived amino nitriles,7 which involved the
ZnCl2-catalyzed reaction of R-amino aldehydes with a
N-unprotected amino acids at -20 °C, followed by in situ
addition of trimethylsilylcyanide (TMSCN) to the inter-
mediate imine (Table 1, entry 1). However, when these
conditions were applied to the reaction of cyclohexanone
(1) with the methyl ester of phenylalanine (2a), the
formation of the corresponding amino nitrile derivative
4a was not observed, and we obtained the cyanohydrin
5 as the only reaction product (Scheme 2). Therefore, it
was necessary to ascertain the most favorable reaction
conditions for the formation of the intermediate imine
3a and the subsequent preferential addition of TMSCN
to this intermediate, instead of to cyclohexanone. Inter-
estingly, imines 3 were unstable, and their formation
could not be detected by TLC nor HPLC. However, the
formation of imine 3a, in equilibrium with cyclohexanone
and H-Phe-OMe, was observed by 13C NMR when the
reaction was performed in CDCl3 in an NMR tube.
As shown in Table 1, increasing the temperature in
the formation of the imine 3a (step a) from -20 to +20
°C led to the amino nitrile 4a, although in very low yield
(12%). The most significant improvement was obtained
by adding the catalyst (ZnCl2) after formation of the
imine, at the same time as TMSCN (step b, entry 3).
(5) (a) Herrero, S.; Garc´ıa-Lo´pez, M. T.; Cenarruzabeitia, E.; Del
R´ıo, J.; Herranz, R. Tetrahedron 2003, 59, 4491-4499. (b) Herrero,
S.; Garc´ıa-Lo´pez, M. T.; Herranz, R. J. Org. Chem. 2003, 68, 4582-
4585.
(6) Herrero, S.; Salgado, A.; Garc´ıa-Lo´pez, M. T.; Herranz, R.
Tetrahedron Lett. 2002, 43, 4899-4902.
(7) (a) Herranz, R.; Sua´rez-Gea, M. L.; Vinuesa, S.; Garc´ıa-Lo´pez,
M. T. J. Org. Chem. 1993, 58, 5186-5191. (b) Sua´rez-Gea, M. L.;
Garc´ıa-Lo´pez, M. T.; Pe´rez, C.; Herranz, R. Bioorg. Med. Chem. Lett.
1994, 4, 1491-1496. (c) Gonza´lez-Mun˜iz, R.; Garc´ıa-Lo´pez, M. T.;
Go´mez-Monterrey, I.; Herranz, R.; Jimeno, M. L.; Sua´rez-Gea, M. L.;
Johansen, N. L.; Madsen, K.; Thøgersen, H.; Suzdak, P. J. Med. Chem.
1995, 38, 1015-1021. (d) Herrero, S.; Sua´rez-Gea, M. L.; Gonza´lez-
Mun˜iz, R.; Garc´ıa-Lo´pez, M. T.; Herranz, R.; Ballaz, S.; Barber, A.;
Fortun˜o, A.; Del R´ıo, J. Bioorg. Med. Chem. Lett. 1997, 7, 855-860.
(8) Some recent reviews on the Strecker reaction: (a) Williams, R.
M. Synthesis of Optically Active R-Amino Acids; Pergamon Press:
Oxford, 1989; pp 208-229. (b) Duthaler, R. O. Tetrahedron 1994, 50,
1539-1650. (c) Enders, D.; Shilvock, J. P. Chem. Soc. Rev. 2000, 29,
359-373. (d) Ager, D. J.; Fotheringham, I. G. Curr. Opin. Drug. Discov.
Devel. 2001, 4, 800-807. (e) Gro¨ger, H. Chem. Rev. 2003, 103, 2795-
2827.
(9) Recent reviews on dioxopiperazine derivatives: (a) Witiak, D.
T.; Wei, Y. In Progress in Drug Research; Jucker, E., Ed.; Birkha´user
Verlag: Basel, 1990; Vol. 35, pp 249-363. (b) Prasad, C. Peptides 1995,
16, 151-164. (c) Horton, D. A.; Bourne, G. T.; Smythe, M. L. J. Comput.
Aid. Mol. Des. 2002, 16, 415-430. (d) Dinsmore, C. J.; Beshore, D. C.
Tetrahedron 2002, 58, 3297-3312. (e) Fischer, P. M. J. Peptide Sci.
2003, 9, 9-35.
(10) Andoh, T.; Ishida, R. Biochim. Biophys. Acta 1998, 1400, 155-
171.
J. Org. Chem, Vol. 70, No. 9, 2005 3661

Gonza´lez-Vera et al.
SCHEME 2. Synthesis of Cyclohexanone-Derived
SCHEME 3. Acid-Mediated Hydration of Amino
Nitriles 4
r-Amino Nitrilesa
TABLE 2. Hydration of Amino Nitriles 4
yield (%)
R-amino
nitrile
2,6-dioxo-
carboxamide 6 piperazine 7
Xaa-OP
4a
4b
4c
4d
4e
4g
4j
L-Phe-OMe
93
25
15
24
0
45
0
0
5
0
L-Pro-OMe
L-Trp-OMe
0
L-Asp(OMe)-OMe
L-Asp(OBn)-OBn
L-Glu(OMe)-OMe
L-Ser-OMe
68
0
55
20
20a
4k
L-Ser(OTMS)-OMe
a Reagents: (a) H-Xaa-OP‚HCl, TEA; (b) ZnCl2 or Yb(OTf)3,
TMSCN.
a Yield of 7j because the H2SO4 treatment removes the TMS
group.
Under these conditions, the increase of temperature to
65 °C (entry 4), or the equivalents of cyclohexanone (from
1 to 2) or TMSCN (from 1.2 to 2), did not produce a
significant improvement in the yield of 4a. Concerning
the solvent, MeOH (entry 3) and CH2Cl2 (entries 5 and
6) led to very similar results, while in THF (entry 7) the
yield was drastically lower. The use of MgSO4 as drying
agent, to favor the formation of the imine,11 did not lead
to any improvement either. However, as indicated in
entries 8 and 9, the substitution of ZnCl2 by ytterbium
triflate, which catalyzes the preferential addition of
nucleophiles to imines versus keto compounds,12 led to a
significant improvement and the amino nitrile 4a was
obtained in a yield higher than 90%. Applying the
conditions of entry 9, amino nitriles 4a-e,g were isolated
in good yields (75-98%). In the case of the serine
derivative 2j, a mixture of the corresponding amino
nitrile 4j (23%) and its O-trimethylsilyl derivative 4k
(22%) was obtained, which was chromatographically
resolved. It is interesting to note that, although amino
nitriles 4 were isolated as analytically pure compounds,
they reverted to the starting amino acid derivative and
cyclohexanone, particularly in the acid media of the silica
gel used for chromatography (TLC and column). A higher
instability could account for the lower yield obtained in
the serine (4j,k) and proline (4b) derivatives. The insta-
bility of R-quaternary amino nitriles had not been previ-
ously observed for R-amino aldehyde-derived amino
nitriles.
as phase transfer catalyst,14 but after 2 days under both
conditions, 4a was recovered unchanged along with
traces of decomposition products. Eventually, the cyano
hydration was achieved by treating a CH2Cl2 solution of
the amino nitrile with concentrated H2SO4 (Scheme 3),
which led to the corresponding R-amino carboxamide 6
together with the respective spirocyclic 2,6-dioxopipera-
zine derivative 7 in a variable ratio depending on the
starting amino nitrile (Table 2). The treatment of the
R-amino carboxamides 6 with NaH in dry THF at room
temperature provided the corresponding 2,6-dioxopip-
erazine derivative 7 in more than 85% yield, except for
the proline derivative 7b, which was obtained in 60%
yield.
The overall yield of the hydration was higher than 90%
for the Phe (4a), Asp(OMe) (4d), and Glu(OMe) (4g)
derivatives. Probably due to the hydrolysis of the benzyl
ester groups in the hydration reaction medium, the
O-benzyl-protected aspartic acid derivative 4e did not
provide the corresponding derivatives 6e or 7e. In the
case of the proline and serine derivatives 4b and 4j,k,
the low yield was due to their previously mentioned
instability in acid media, observing the formation of
cyclohexanone and their respective amino esters (2b and
2j) in the reaction mixture. On the other hand, the TMS
group of the serine derivative 4k did not survive after
treatment with H2SO4, which led to the corresponding
deprotected 2,6-dioxopiperazine derivative 7j.
Initially, the cyano hydration of amino nitrile 4a was
attempted in a neutral medium by treatment with MnO2
in CH2Cl213 or via oxidative hydration by treatment with
H2O2 and NaOH, using tetrabutylammonium bisulfate
With respect to the tryptophan-derived amino nitrile
2c, the low yield of hydration to give the amino carboxa-
mide 6c (15%) was due to the unfavorable competition
of this reaction with a tautomerization, as we have
recently reported,15 which involves the formation of an
(11) Basile, T.; Bocoum, A.; Savoia, D.; Umani-Ronchi, A. J. Org.
Chem. 1994, 59, 7766-7773.
(12) (a) Kobayashi, S.; Nagayama, S. J. Org. Chem. 1997, 62, 232-
233. (b) Kobayashi, S.; Nagayama, S. J. Am. Chem. Soc. 1997, 119,
10049-10053.
(14) (a) Herranz, R.; Conde, S.; Ferna´ndez-Resa, P.; Arribas, E. J.
Chem. Soc., Perkin Trans. 1 1988, 649-655. (b) Sua´rez-Gea, M. L.;
Garc´ıa-Lo´pez, M. T.; Herranz, R. J. Org. Chem. 1994, 59, 3600-3603.
(15) Gonza´lez-Vera, J. A.; Garc´ıa-Lo´pez, M. T.; Herranz, R. Org. Lett.
2004, 6, 2641-2644.
(13) (a) Gateau-Olesker, A.; Cle´ophax, J.; Ge´ro, S. D. Tetrahedron
Lett. 1986, 27, 41-44. (b) Herranz, R. An. Quim. 1987, 83, 318-322.
3662 J. Org. Chem., Vol. 70, No. 9, 2005

Spirocyclic 2,6-Dioxopiperazine Derivatives
SCHEME 4. Hydration and Tautomerization of the
Trp-Derived Amino Nitrile 4c
quantity obtained of these dioxotetrahydropyrazines, only
the phenylalanine derivative 13a was completely char-
acterized. This compound was obtained in 70% yield from
piperazine 9a after 7 days of reflux in CH3CN in the
1
presence of 1.5 equiv of Cs2CO3. The H NMR spectrum
of 13a showed the disappearance of the signal corre-
sponding to the 2-H and a higher than 0.70 ppm
deshielding for its 2-CH2 protons with respect to the
starting dioxopiperazine 9a. Correspondingly, the 13C
NMR spectrum of 13a showed the disappearance of the
aliphatic C2 and its appearance in the zone of quaternary
carbons at 157.1 ppm.
In view of the low overall yield of the proline deriva-
tives 10b and 11b (∼10%) and the tryptophan derivative
11c (∼8%) from their respective starting amino acid
derivatives 2b and 2c in the sequential Strecker reac-
tion-hydration-cyclization-alkylation, we studied their
alternative preparation via an Ugi four-component
reaction.2a,16 This reaction was carried out as previously
described for the synthesis of other 2,6-dioxopiperazine
analogues,17 which involved the reaction of cyclohexanone
with the corresponding amino acid and methyl isocy-
anoacetate in the presence of TEA. However, this method
did not provide any increase in yield over our methodol-
ogy.
SCHEME 5. Alkylation of Dioxopiperazinesa
In those dioxopiperazine derivatives containing methyl
ester groups, the synthesis of their corresponding free
carboxylic acids was initially attempted by means of
saponification. However, when this was first tried using
the Asp derivative 10d, by treatment with 1 equiv of 1
N NaOH in (1:9) H2O/MeOH, the free acid 10f was
obtained as a minor product (29%), along with 47% of
14, resulting from the simultaneous saponification and
opening of the N4-C5 bond by the nucleophilic attack of
MeOH to the C5 (Scheme 6). On the other hand, when
the saponification was attempted by reaction with 1 equiv
of 1 N NaOH in acetonitrile or dioxane containing 10%
of H2O, the only reaction product was 15, resulting from
the nucleophilic attack of H2O to the C5. To avoid the
opening of the 2,6-dioxopiperazine ring, we considered
the transesterification of methyl to benzyl esters followed
by hydrogenolysis. As shown in Scheme 7, transesteri-
fication was performed for the Pro and Trp derivatives
11b and 11c and the Asp and Glu derivatives 9-11d,g
by treatment of the methyl esters with benzyl alcohol in
the presence of 3-chloro-1-hydroxytetrabutyldistan-
noxane, freshly prepared from Bu2SnO and Bu2SnCl2,18,19
yielding 65-80% of the benzyl esters 12b,c and (9, 10,
and 12)e,h. Subsequently, catalytic hydrogenolysis of
these benzyl esters and the Phe derivative 12a led to the
corresponding carboxylic acids 9f,i, 10f,i, and 16a-c,f,i,
quantitatively.
a Meaning of a-d, g as indicated in Scheme 2. Reagents: (a)
R2-X, Cs2CO3 or K2CO3, CH3CN, ∆.
endocyclic amidine within the novel indole-based tetra-
cyclic system of compound 8 (Scheme 4). The treatment
of this compound with aqueous 1 N HCl at 100 °C for 48
h led to the 2,6-dioxopiperazine derivative 7c, which was
also prepared by NaH-mediated cyclization of the R-ami-
no carboxamide 6c.
Further functionalization of dioxopiperazine deriva-
tives 7, by alkylation with alkyl halides, was first
attempted in dry THF, using NaH as base. Under these
conditions, the products of selective alkylation at position
4 were obtained in a very low yield within a complex
mixture of reaction products. The alkylation was then
studied in dry CH3CN at 50 °C, using 1.5 equiv of alkyl
halide and Cs2CO3 or K2CO3 as base (Scheme 5). In both
cases, the alkylation products 9-12 were obtained in
higher than 85% yield, although when K2CO3 was used
the reaction was slower. Partial dialkylation at position
4 of the piperazine ring and at position 1 of the indole
(∼25%) was observed in the tryptophan derivative 7c.
To minimize the alkylation at the indole ring, the amount
of alkyl halide was lowered to 0.95 equiv. Thus, the N4-
monoalkylated product 11c was obtained in 83% yield
along with a 5% yield of the N,4Ni-dialkylation product,
which were separated by column chromatography. In the
alkylation of all dioxopiperazines 7, traces (1-4%) of
dioxotetrahydropyrazines were obtained, resulting from
the slow oxidation of the dioxopiperazine ring in the basic
media of the alkylation reaction. Due to the small
Although in previous studies on the chemical manipu-
lation of amino acid derived R-amino nitriles3-7 we had
(16) Recent reviews on the Ugi four component reaction: (a) Ugi,
I.; Lohberger, S.; Karl, R. In Comprehensive Organic Synthesis; Trost,
B. M., Fleming, I., Heathcock, C. H., Eds.; Pergamon Press: Oxford,
1991; Vol. 2, pp 1083-1109. (b) Domling, A. Comb. Chem. High
Throughput Screen 1998, 1, 1-22. (c) Do¨mling, A.; Ugi, I. Angew.
Chem., Int. Ed. 2000, 39, 3169-3210. (d) Hulme, C.; Gore, V. Curr.
Med. Chem. 2003, 10, 51-80.
(17) Ugi, I.; Ho¨rl, W.; Hanusch-Kompa, C.; Schmid, T.; Herdtweck,
E. Heterocycles 1998, 47, 965-975.
(18) Okawara, R.; Wada, M. J. Organomet. Chem. 1963, 1, 81-88.
(19) Otera, J.; Dan-oh, N.; Nozaki, H. J. Org. Chem. 1991, 56, 5307-
5311.
J. Org. Chem, Vol. 70, No. 9, 2005 3663

Gonza´lez-Vera et al.
SCHEME 8. Study of the Optical Purity
SCHEME 6. Reactivity of the Asp-Derived
Dioxopiperazine 10d with NaOH
second case, as shown in Scheme 8, the coupling of the
free carboxylic acid 16a with the methyl ester of L-Ala,
using BOP and TEA as coupling reagents, led to the
corresponding pseudotripeptide analogue 17a. The RP
HPLC and NMR analyses of the crude reaction products
did not show the existence of duplicity of signals. This
result indicated the absence of diastereoisomers and,
therefore, the absence of racemization.
In conclusion, the studies herein reported show that
amino acid derived R-quaternary R-amino nitriles are
good starting materials for the synthesis of diverse
spirocyclic 2,6-dioxopiperazine derivatives with retention
of configuration, by means of H2SO4-promoted cyano
hydration, followed by base-mediated cyclization and
N-alkylation. In the case of tryptophan-derived R-amino
nitriles a domino tautomerization to the novel heterocy-
clic ring system hexahydropyrrolo[1′,2′,3′:1,9a,9]imidazo-
[1,2-a]indole competes favorably with the cyano hydration
to the corresponding carboxamide derivative. The treat-
ment of the new tetracyclic indoline derivative with
refluxing aqueous 1 N HCl solution leads to the corre-
sponding 2,6-dioxopiperazine derivative. To avoid the
reactivity of the 2,6-dioxopiperazine ring under the basic
conditions of saponification, the synthesis of 2,6-diox-
opiperazines containing free carboxylic acids requires the
transesterification of the methyl to benzyl esters and
subsequent hydrogenolysis. Finally, the reactivity of the
2,6-dioxopiperazine derivatives herein observed toward
nucleophiles in basic media could be used to synthesize
N-(carboxyalkyl)amino acid derivatives of interest in the
field of peptidase inhibitors.20 Studies in this direction
are in progress.
SCHEME 7. Synthesis of Free Carboxylic Acidsa
Experimental Section
General Methods. All reagents were of commercial quality.
Solvents were dried and purified by standard methods.
Analytical TLC were performed on aluminum sheets coated
with a 0.2 mm layer of silica gel 60 F254. Silica gel 60 (230-
400 mesh) was used for flash chromatography. Preparative
radial chromatography was performed on 20 cm diameter glass
plates coated with a 1-mm layer of silica gel PF254. Analytical
RP-HPLC was performed on a Novapak C18 (3.9 × 150 mm, 4
µm) column, with a flow rate of 1 mL/min, and using a tunable
UV detector set at 214 nm. Mixtures of CH3CN (solvent A)
and 0.05% TFA in H2O (solvent B) were used as mobile phases.
Melting points were taken on a micro hot stage apparatus and
a Reagents: (a) BnOH, (nBu4Sn2ClOH)2, PhCH3; (b) H2, Pd(C),
MeOH.
not observed racemization, taking into account the strong
basic media used in the cyclization of R-aminocarbox-
amides and in the alkylation of the 2,6-dioxopiperazine
derivatives, as well as the low values of optical rotation
measured for most of these derivatives, a priori, the
existence of racemization could not be discarded. This
aspect was studied in the phenylalanine derivatives 7a
and 16a. In the first case, we tried the acylation at
positions 1 or 4 of the dioxopiperazine ring with an excess
of the chloride of the Mosher acid [(R)-R-methoxy-R-
(trifluoromethyl)phenylacetic acid chloride, (R)-MTPA].
However, this acylation did not take place, and the
starting compound was recovered unaltered. In the
(20) (a) Neustadt, B. R.; Smith, E. M.; Tulshian, D. PCT Int. Appl.
WO 9403481, 1994; Chem. Abstr. 1994, 122, 56581. (b) Robinson, R.
P.; Cronin, B. J.; Donahue, K. M.; Jones, B. P.; Lopresti-Morrow, L.
L.; Mitchell, P. G.; Rizzi, J. P.; Reeves, L. M.; Yocum, S. A. Bioorg.
Med. Chem. Lett. 1996, 6, 1725-1730. (c) Quibell, M.; Johnson, T.;
Hart, T. PCT Int. Appl. WO 9742216, 1997; Chem. Abstr. 1997, 128,
30375. (d) Quibell, M.; Johnson, T.; Hart, T. PCT Int. Appl. WO
9740065, 1997; Chem. Abstr. 1997, 128, 1461. (e) Robinson, R. P. PCT
Int. Appl. US 5854275, 1998; Chem. Abstr. 1999, 130, 95848.
3664 J. Org. Chem., Vol. 70, No. 9, 2005

Spirocyclic 2,6-Dioxopiperazine Derivatives
1
are uncorrected. H NMR spectra were recorded at 200, 300,
corresponding dioxopiperazine derivative 7a-d,g (0.25 mmol)
in dry CH3CN (2.5 mL). After 2 h of stirring at 50 °C, the
mixture was evaporated, the residue was dissolved in CH2Cl2
(20 mL), and the resulting solution was successively washed
with H2O (5 mL) and brine (5 mL), dried over Na2SO4, and
evaporated to dryness. The residue was purified by circular
chromatography, using 15-25% gradient of EtOAc in hexane
as eluant. Significant analytical and spectroscopic data of the
resulting dioxopiperazine derivatives 9a,d,g, 10a,b,d,g, 11a-
d,g, and 12a,g are summarized in Tables 4-7 of the Support-
ing Information.
or 400 MHz, using TMS as reference, and 13C NMR spectra
were recorded at 50, 75, or 100 MHz. The NMR spectra
assignment was based on COSY and HSQC spectra. ESI-MS
spectra were performed, in positive mode, using MeOH as
solvent.
General Procedure for the Synthesis of the r-Amino
Nitriles 4a-e,g,j-k. TEA (139.2 µL, 1 mmol) was added to a
solution of the corresponding amino acid derivative [H-Phe-
OMe‚HCl (2a), H-Pro-OMe‚HCl (2b), H-Trp-OMe‚HCl (2c),
H-Asp(OMe)-OMe‚HCl (2d), H-Asp(OBz)-OBz‚Tos (2e), H-Glu-
(OMe)-OMe‚HCl (2g), H-Ser(OMe)-OH‚HCl (2j)] (1 mmol) in
dry CH2Cl2 (20 mL). After 30 min of stirring at room temper-
ature under argon, cyclohexanone (103.6 µL, 1 mmol) was
added, and the stirring was maintained for 3 h. Then, the
solution was cooled to -23 °C, and ytterbium triflate (13.6 mg,
0.02 mmol) and trimethylsilyl cyanide (TMSCN, 187.7 µL, 1.5
mmol) were added. After being successively 1 h at -23 °C and
20 h at room temperature, the mixture was evaporated to
dryness. The residue was dissolved in CH2Cl2 (20 mL), and
the solution was washed with H2O (5 mL) and brine (5 mL),
dried over Na2SO4, and evaporated. The residue was purified
by flash chromatography, using 20-50% gradient of EtOAc
in hexane as eluant, to yield the corresponding R-amino nitrile
4a-e,g,j-k. Significant analytical and spectroscopic data of
these R-amino nitriles are summarized in Table 1 of the
Supporting Information.
General Procedure for the Hydration of the r-Amino
Nitriles 4(a-e,g,j-k). Synthesis of the r-Amino Car-
boxamides 6 and the (2S)-3,5-Dioxo-1,4-diazaspiro[5.5]-
undecane Derivatives 7. Concentrated H2SO4 (5 mL) was
added to a solution of the corresponding amino nitrile (4a-
e,g,j-k) (3 mmol) in CH2Cl2 (12 mL), and this mixture was
stirred at room temperature for 1 h. Afterward, the reaction
mixture was sequentially poured into ice, neutralized with
NH4OH, and extracted with CH2Cl2 (20 mL). The organic
extracts were successively washed with H2O (5 mL) and brine
(5 mL), dried over Na2SO4, and evaporated to dryness. The
residue was purified by flash chromatography, using 75-100%
gradient of EtOAc in hexane as eluant, to give the spiricyclic
2,6-dioxopiperazines 7a,d,g,j of higher Rf and the R-amino
carboxamides derivatives 6a-d,g. In the case of the tryp-
tophan-derived amino nitrile 4c, the hexahydropyrrolo[1′,2′,3′:
1,9a,9]imidazo[1,2-a]indole derivative 8 was the major product
of this reaction.15 Significant analytical and spectroscopic data
of these R-amino carboxamides 6a-d,g and of the (2S)-3,5-
dioxo-1,4-diazaspiro[5.5]undecane derivatives 7a,d,g,j are sum-
marized in Tables 2 and 3, respectively, and those of the
proline derivative 7b in Table 7 of the Supporting Information.
General Procedure for the Cyclization of the r-Amino
Carboxamides 6. Synthesis of the (2S)-3,5-Dioxo-1,4-
diazaspiro[5.5]undecane Derivatives 7. Method B. NaH
(60% dispersion in mineral oil; 6.6 mg, 0.28 mmol) was added
to a solution of the corresponding R-amino carboxamide (6a-
d,g) (0.25 mmol) in dry THF (7.5 mL), and this mixture was
stirred at room temperature for 1 h. Then, the solvent was
evaporated, the residue was dissolved in CH2Cl2 (20 mL), and
the resulting solution was successively washed with H2O (5
mL) and brine (5 mL), dried over Na2SO4, and evaporated to
dryness. The residue was purified by circular chromatography,
using 25-40% gradient of EtOAc in hexane as eluant. Sig-
nificant analytical and spectroscopic data of the resulting
dioxopiperazine derivatives 7a-d,g are summarized in Table
3 of the Supporting Information, except for those of the proline
derivative 7b which are summarized in Table 7.
General Procedure for the Alkylation of the Diox-
opiperazine Derivatives 7. Synthesis of (2S)-2,4-Disub-
tituted-3,5-dioxo-1,4-diazaspiro[5.5]undecane Deriva-
tives 9a,d,g, 10a,b,d,g, 11a-d,g, and 12a,g. Cs2CO3 (122.2
mg, 0.375 mmol) and the corresponding alkylating agent
(methyl iodide, benzyl bromide, methyl bromoacetate, or benzyl
bromoacetate, 0.375 mmol) were added to a solution of the
Synthesis of 2-Phenylmethyl-4-methyl-3,5-dioxo-1,4-
diazaspiro[5.5]undec-1-ene 13a. Cs2CO3 (25.59 mg, 0.078
mmol) was added to a solution of 9a (15 mg, 0.052 mmol) in
CH3CN (3 mL), and this mixture was stirred at 60 °C for 7
days. Afterward, the solvent was evaporated, the residue was
dissolved in CH2Cl2 (20 mL), and the resulting solution was
successively washed with H2O (5 mL) and brine (5 mL), dried
over Na2SO4, and evaporated to dryness. The residue was
purified by circular chromatography, using 10-12% gradient
of EtOAc in hexane as eluant: foam (10.3 mg, 70%); [R]20
D
-1.70 (c 0.8, MeOH); RP-HPLC [Novapak C18 (3,9 × 150 mm,
4µm), (A/B 50:50)] tR 16.51 min; 1H NMR (400 MHz, CDCl3) δ
(ppm) 1.42-1.94 [m, 10H, (7-11)-H], 2.05-2.11 (m, 2H, 7-Hax
and 11-Hax), 3.13 (s, 3H, 4-CH3), 4.01 (s, 2H, 2-CH2), 7.29 (m,
5H, Ph); 13C NMR (100 MHz, CDCl3) δ (ppm) 20.5 (C8 and C10),
25.4 (C9), 25.6 (4-CH3), 37.1 (C7 and C11), 40.5 (2-CH2), 65.5
(C6), 126.7, 128.4, 129.4 and 136.3 (Ph), 155.8 (C3), 157.1 (C2),
177.0 (C5); EM-ES m/z 285.0 (100) [M + 1]+. Anal. Calcd for
C17H20N2O2: C, 71.81; H, 7.09; N, 9.85. Found: C, 71.75; H,
7.40; N, 9.88.
Reactivity of the Asp-Derived Dioxopiperazine 10d
with NaOH. NaOH (1 N, 200 µL, 0.2 mmol) was added to a
solution of 10d (68.9 mg, 0.2 mmol) in 10:1 MeOH/H2O or 10:1
MeCN/H2O (3 mL). After 2 h of stirring at room temperature,
the mixture was evaporated, the residue was dissolved in H2O
(4 mL), and the resulting solution was washed with CH2Cl2
(12 mL). The aqueous solution was acidified with 1 N HCl to
pH 3, then the mixture was extracted with EtOAc (3 × 12 mL).
The organic extracts were dried over Na2SO4 and evaporated
to dryness. The residue was purified by radial chromatography
using 2-30% gradient of MeOH in CH2Cl2 as eluant, yielding
the dioxopiperazine derivative 10f (19.2 mg, 29%) and the
opened compound 14 (34.2 mg, 47%) in the case of the reaction
in MeOH/H2O, and 15 (64.4 mg, 89%) in the case of the
reaction in MeCN/H2O.
N-[1-(Methoxycarbonyl)cyclohexyl]aspartic acid ben-
zyl amide (14): amorphous solid (34.2 mg, 47%); [R]20D +2.75
(c 1, MeOH); RP-HPLC [Novapak C18 (3.9 × 150 mm, 4µm),
(A/B 30:70)] tR 3.98 min; 1H NMR (200 MHz, DMSO-d6) δ
(ppm) 1.15-1.84 (m, 10H, cyclohexyl), 2.22 [(dd, 1H, J ) 7.5
and 14.5 Hz, 3-H (Asp)], 2.38 [(dd, 1H, J ) 5 and 14.5 Hz,
3-H(Asp)], 3.33 [dd, 1H, 2-H(Asp)], 3.50 (s, 3H, OCH3), 4.17
(d, 1H, J ) 6 Hz, CH2-Ph), 4.34 (d, 1H, J ) 6 Hz, CH2-Ph),
7.26 (m, 5H, Ph), 8.70 (bs, 1H, NH); 13C NMR (50 MHz, DMSO-
d6) δ (ppm) 22.0 and 22.3 [C3 and C5 (cyclohexyl)], 25.2 [C4
(cyclohexyl)], 32.3 and 34.9 [C2 and C6 (cyclohexyl)], 40.1 (C3),
42.0 (CH2-Ph), 53.6 (C2), 126.7, 127.2, 128.2 and 139.5 (Ph),
171.0 (C1), 174.7 (CO2CH3); EM-ES m/z 363.3(100) [M + 1]+.
Anal. Calcd for C19H26N2O5: C, 62.97; H, 7.23; N, 7.73. Found
: C, 63.21; H, 7.44; N, 7.59.
N-[1-(Carboxyl)cyclohexyl]-Asp(OMe) benzyl amide
(15): amorphous solid (64.4 mg, 89%); [R]20D -0.84 (c 1, MeOH);
RP-HPLC [Novapak C18 (3.9 × 150 mm, 4 µm), (A/B 30:70)] tR
1.85 min; 1H NMR (200 MHz, acetone-d6) δ 1.18-2.06 (m, 10H,
cyclohexyl), 2.63 [dd, 1H, J ) 6 and 14.5 Hz, 3-H (Asp)], 2.76
[dd, 1H, J ) 5 and 14.5 Hz, 3-H (Asp)], 3.65 [dd, 1H, 2-H (Asp)],
3.51 (s, 3H, OCH3), 4.33 (d, 1H, J ) 6 Hz, CH2-Ph), 4.43 (d,
1H, J ) 6 Hz, CH2-Ph), 7.24 (m, 5H, Ph), 8.02 (bs, 1H, NH);
13C NMR (50 MHz, acetone-d6) δ 23.2 (C3 and C5 cyclohexyl),
26.6 (C4 cyclohexyl), 34.0 and 35.1 (C2 and C6 cyclohexyl), 38.7
(C3), 44.2 (CH2-Ph), 53.9 (C2), 126.8, 127.4, 128.2 and 139.6
J. Org. Chem, Vol. 70, No. 9, 2005 3665

Gonza´lez-Vera et al.
(Ph), 175.5 (C1), 176.2 (CO2CH3); EM-ES m/z 363.3(100) [M +
1]+. Anal. Calcd for C19H26N2O5: C, 62.97; H, 7.23; N, 7.73.
Found : C, 63.23; H, 7.38; N, 7.63.
were successively added. After stirring successively for 1 h at
0 °C and 24 h at room temperature, the mixture was
evaporated to dryness. The residue was dissolved in CH2Cl2
(20 mL), and the solution was washed successively with
saturated solution of Na2CO3 (2 × 5 mL), H2O (2 × 5 mL),
citric acid 10% (2 × 5 mL), and brine (5 mL), dried over Na2-
SO4, and evaporated. The residue was purified by circular
chromatography using 50-70% gradient of EtOAc in hexane
as eluant, yielding 17a as a foam (38.2 mg; 94%): [R]20D -47.87
(c 1, MeOH); RP-HPLC [Novapak C18 (3.9 × 150 mm, 4 µm),
General Procedure for the Transesterification of
Methyl to Benzyl Esters in the Dioxopiperazine Deriva-
tives 9-11(a-d,g). Synthesis of the Benzyl Esters 9-10-
(e,h) and 12a-c,e,h. Benzyl alcohol (259 µL, 2.5 mmol) and
3-chloro-1-hydroxytetrabutyldistannoxane (13 mg, 0.025 mmol),
freshly prepared from Bu2SnO and Bu2SnCl2,18,19 were added
to a solution of the corresponding methyl ester 9-11(a-d,g)
(0.25 mmol) in toluene (2 mL), and this mixture was refluxed
for 2 h. Afterward, the solvent was evaporated to dryness, and
the residue was purified by circular chromatography using 12-
15% gradient of EtOAc in hexane as eluant. Significant
analytical and spectroscopic data of the benzyl esters 9-10(e,h)
and 12a-c,e,h are summarized in Tables 4-7 of the Support-
ing Information.
General Procedure for the Hydrogenolysis of the
Benzyl Esters 9-10(e,h) and 12a-c,e,h. Synthesis of the
Free Carboxylic Acids 9-10(f,i) and 16a-c,f,i. Pd(C) (10%
w/w) was added to a solution of the corresponding benzyl ester
[9-10(e,h) and 12a-c,e,h] (0.09 mmol) in MeOH (20 mL), and
the resulting suspension was hydrogenated at room temper-
ature and 15 psi of H2 pressure for 1 h. After filtration of the
catalyst, the solvent was evaporated yielding the corresponding
free carboxylic acid 9-10(f,i) and 16a-c,f,i quantitatively.
Significant analytical and spectroscopic data of these free
carboxylic acids are summarized in Tables 4-7 of the Sup-
porting Information.
1
(A/B 50:50)] tR 4.25 min; H NMR (300 MHz, CDCl3) δ (ppm)
1.11-1.63 [m, 8H, (7-11)-H], 1.41 [d, 3H, J ) 7 Hz, CH3 (Ala)],
1.98 (m, 1H, 7-Hec), 1.83 (bs, 1H, 1-H), 2.09 (dt, 1H, J ) 4, 13
Hz, 7-Hax), 2.93 (dd, 1H, J ) 9 and 14 Hz, 2-CH2), 3.38 (dd,
1H, J ) 4 and 14 Hz, 2-CH2), 3.68 (s, 3H, OCH3), 3.80 (bs, 1H,
2-H), 4.28 [d, 1H, J ) 15 Hz, 2-H (Gly)], 4.42 [d, 1H, J ) 15
Hz, 2-H (Gly)], 4.51 [q, 1H, J ) 7 Hz, 2-H (Ala)], 6.20 (d, 1H,
J ) 7 Hz, CONH), 7.24 (m, 5H, Ph); 13C NMR (75 MHz, CDCl3)
δ (ppm) 18.6 (4-CH3), 19.9 and 20.5 (C8 and C10), 25.1 (C9),
29.5 and 33.9 (C7 and C11), 37.0 (2-CH2), 41.9 [C2 (Gly)], 48.2
[C2 (Ala)], 52.5 (OCH3), 54.4 (C2), 58.1 (C6), 127.0, 128.7, 129.2
y 137.0 (Ph), 166.1 and 173.2 (CO), 172.5 (C3), 176.1 (C5); EM-
ES, m/z 416.1(100) [M + 1]+. Anal. Calcd for C22H29N3O5 : C,
63.60; H, 7.04; N, 10.11. Found : C, 63.82; H, 7.24; N, 10.03.
Acknowledgment. This work was supported by
CICYT (SAF2000-0147).
Supporting Information Available: Tables of significant
analytical and spectroscopic data of R-quaternary R-amino
nitriles 4, R-amino carboxamides 6, and 2,6-dioxopiperazine
derivatives 7, 9-12, and 16. This material is available free of
charge via the Internet at http://pubs.acs.org.
Synthesis of the Pseudotripeptide Derivative 17a.
TEA (31 µL, 0.196 mmol) was added to a solution of H-Ala-
OMe‚HCl (27.4 mg, 0.196 mmol) in dry CH2Cl2 (20 mL). After
30 min of stirring at room temperature under argon, the
solution was cooled to 0 °C, and 16a (32.4 mg, 0.098 mmol),
BOP (86.8 mg, 0.196 mmol), and TEA (31 µL, 0.196 mmol)
JO050146M
3666 J. Org. Chem., Vol. 70, No. 9, 2005