Short and novel stereospecific synthesis of trisubstituted 2,5-diketopiperazines

Article abstract of DOI:10.1021/jo0501137

Novel syntheses of chiral trisubstituted 2,5-diketopiperazines using multicomponent Ugi reactions were developed.

Full text of DOI:10.1021/jo0501137

Short and Novel Stereospecific Synthesis of Trisubstituted
2,5-Diketopiperazines
Steven L. Sollis
Department of Medicinal Chemistry UK, Cardiovascular and Urogenital CEDD, GlaxoSmithKline
Research and Development, Medicines Research Centre, Gunnels Wood Road, Stevenage,
Herts SG1 2NY, United Kingdom
Steve.L.Sollis@gsk.com
Received January 20, 2005
Novel syntheses of chiral trisubstituted 2,5-diketopiperazines using multicomponent Ugi reactions
were developed.
CHART 1
Introduction
2,5-Diketopiperazines (DKPs) are an interesting class
of heterocycles, as they show varied pharmacological
activity (Chart 1). For example, compound 1 is reported
to be an inhibitor of a platelet-activating factor.1 The
conformationally restricted analogue 2 has been studied
as â-turn mimetics.2 Tricylic analogues of 2,5-diketopip-
erazine 3 are CCK2 receptor antagonists.3 The 2,5-
diketopiperazine motif has been found in the structures
of alkaloid natural products such as demethoxyfumi-
tremorgin C 4.4
We are concerned with the design and synthesis of
these topologically attractive and functionally diverse
scaffolds in the context of a selective orally bioavailable
oxytocin antagonist. We report here the facile stereose-
lective synthesis of enantiometrically pure trisubstituted
2,5-diketopiperazines with three chiral centers repre-
sented by the general structures 5a, 5b, and 5c (Chart
2).
Results and Discussion
The trisubstituted 2,5-diketopiperazines are assembled
by the cyclization of dipeptide esters via intramolecular
amide bond formation.5 The intermediate dipeptide esters
11 are readily obtained utilizing a four component Ugi
coupling reaction (Scheme 1), previously carried out on
the solid phase.6 The hydrochloride of R-leucine methyl
(1) Shimazaki, N.; Hemmi, K.; Nakaguti, O.; Miyazaki, Y.; Hash-
imoto, M. European Patent Application EP 243,122; Oct., 28, 1987, p
53.
(2) Golebiowski, A.; Klopfenstein, S. R.; Chen, J. J.; Shao, X.
Tetrahedron Lett. 2000, 41, 4841-4844.
(3) McDonald, I. M.; Dunstone, D. J.; Kalindjian, S. B.; Linney, I.
D.; Low, C. M. R.; Pether, M. J.; Steel, K. I. M.; Tozer, M. J.; Vinter,
J. G. J. Med. Chem. 2000, 43, 3518-3529.
(5) Fischer, P. M. J. Peptide Sci. 2003, 9, 9-35.
(6) Szardenings, A. K.; Burkoth, T. S.; Lu, H. H.; Tien, D. W.;
Campbell, D. A. Tetrahedron 1997, 53, 6573-6593.
(4) Wang, H.; Ganesan, A. Org. Lett. 1999, 1, 1647-1649.
10.1021/jo0501137 CCC: $30.25 © 2005 American Chemical Society
Published on Web 05/03/2005
J. Org. Chem. 2005, 70, 4735-4740
4735

Sollis
CHART 2
SCHEME 2
The partial stereocontrol comes from the addition of the
isonitrile 9 to the less hindered face of the imine 8.
Attempts to alter this ratio in favor of the RRR isomer
13 by changing the temperature, solvent, or time course
of the Ugi reaction were unsuccessful.
SCHEME 1
Several alternative syntheses of DKPs9 were consid-
ered to selectively prepare the required RRR isomer 13.
However, none were efficient or sufficiently selective. It
was envisaged that fixing the stereochemistry of the
exocyclic center prior to the cyclization to form the DKP
would be a potential advantage in our efforts to prepare
the RRR isomer 13. The most attractive of the possibili-
ties was to synthesize the secondary amine 15 and then
to acylate using R-Boc-indanyl glycine 10. A survey of
the literature found an efficient synthesis of analogues
of the secondary amine 1510 via a four center, three-
component Ugi reaction; however, the stereochemistry
of the major diastereoisomer was not reported. Thus, the
three components, R-leucine 14, benzaldehyde 7, and
t-butylisonitrile 9 (Scheme 2), were dissolved/suspended
in methanol at -30 °C for 3 h, and then the reaction
mixture was stirred at room temperature for 3 days to
produce predominately one isomer (8:1 mixture of dias-
tereoisomers).
Unfortunately, the amino ester 15 failed to undergo
intramolecular acylation with R-Boc-indanyl glycine 10,
probably due to steric bulk of both components. Further-
more, formylation using formyl acetic anhydride took
overnight to produce 16 in a 70% yield (Scheme 3).
Formylation was also attempted on the corresponding
acid 17. Hydrolysis of the ester 15 to the amino acid 17
was achieved using lithium hydroxide in aqueous metha-
nol. Formylation of the amine 17 using formyl acetic
anhydride occurred very rapidly (2 h at room temp) in
quantitative yield to give 19. One possibility of the
difference in rates of the reactions could be explained by
the formation of the mixed anhydride 18, followed by an
intramolecular acylation of the amine. This suggested
that making the corresponding mixed anhydride with
R-indanyl glycine 10 would enable the preparation of the
required intermediate dipeptide ester 21. Methylation of
acid 19 gave ester 16, which showed that the stereo-
chemistry of both centers was conserved.
ester 6 and benzaldehyde 7 were mixed together in
methanol with an equivalent of triethylamine to form the
imine 8, which was not isolated. To this solution was
added t-butylisonitrile 9 and the R-indanyl glycine 10.
The crude intermediate dipeptide ester 11 was depro-
tected with 4 N hydrogen chloride in dioxane for 4 h. This
not only removed the t-Boc group of the glycine but also
gave some spontaneous cyclization to the 2,5-diketopip-
erazine. The cyclization was driven to completion by the
addition of triethylamine. This gave two trisubstituted
2,5-diketopiperazines in a ratio of 3:1. Proton NMR
experiments showed a medium to strong nOe from the
CH of the indanyl to one of the hydrogens of the CH2 of
the isobutyl group, indicating that the indanyl moiety and
the isobutyl group are cis orientated to each other across
the diketopiperazine as expected. The two isomers were
the RRS trisubstituted 2,5-diketopiperazine 12 and the
RRR trisubstituted 2,5-diketopiperazine 13, but it was
not known which was the major product. The minor
product 13 was found to have a greater potency as an
oxytocin antagonist,7 and X-ray crystallography8 showed
it to be the RRR trisubstituted 2,5-diketopiperazine 13.
R-Boc-indanyl glycine 10 was treated with isopropyl
chloroformate in the presence of N-methylmorpholine at
-20 °C to form the mixed anhydride 20 (Scheme 4). To
(7) Borthwick, A. D.; Hatley, R. J.; Hickey, D. M. B.; Liddle, J.;
Livermore, D. G. H.; Mason, A. M.; Miller, N. D.; Nerozzi, F.; Sollis, S.
L.; Szardenings, A. K.; Wyatt, P. G. PCT Int. Appl. 2003, p 90;
CODEN: WO 2003053443 A1 20030703.
(8) X-ray to be published later.
(9) Dinsmore, C. J.; Beshore, D. C. Tetrehedron 2002, 58, 3297-
3312.
4736 J. Org. Chem., Vol. 70, No. 12, 2005

Trisubstituted 2,5-Diketopiperazines
SCHEME 3
SCHEME 5
CHART 3
identical to that of the RRR isomer 13, but the retention
times by chiral HPLC were different. In addition, the
circular dichroism spectra of DKP 13 and DKP 24 were
mirror images.
SCHEME 4
Hence, the leucyl amide is inverted in the preparation
of intermediate dipeptide ester 21, and because the
isomer 24 is chirally pure with the SSS configuration,
this means that the stereochemistry of the phenyl glycine
moiety is also S and is assumed to be retained in this
reaction. Also, the chirality of the indanyl glycine is
retained during the reaction.
Therefore, in the original secondary amine derivative
15, the chirality of the phenyl glycine moiety is S, and
the major isomer is RS. Also, the chirality of DKP 22 is
RSS (Chart 3).
Further investigation of the reaction of the anhydride
20 of the R-Boc-indanyl glycine 10 with the amine 17
showed that if the reaction temperature of the acylation
is kept at -20 °C, then two major isomers were isolated,
the cis isomer 12 and the trans isomer 22 (Scheme 6).
The epimerization of the leucyl group could occur at
the mixed anhydride 25 stage or at a possible cyclic
intermediate. Molecular modeling11 indicates that the
possible intermediate 26 is of lower energy than inter-
mediate 27 by 40 kJ/mol. It is predicted, at room
temperature, that the intermediate 27 racemizes at the
leucine center to the more favored intermediate 26,
driving the reaction to form DKP 22. In contrast, at -20
°C, racemization of 27 is reduced, and a mixture of DKP
22 and DKP 12 is formed (2:1).
this was added amino acid 17, and the reaction was
warmed to room temperature to give 21. The acyclic
intermediate 21 was taken crude through to the final
DKP 22. The major isomer 22 isolated was obtained in
45% overall yield from R-Boc-indanyl glycine. Surpris-
ingly, the stereochemistry of the indanyl and isobutyl
substituents in 22 was shown to be trans by rotational
nuclear Overhauser effect spectroscopy (ROSEY) NMR.
It was reasoned that the inversion of stereochemistry
in the formation of 22 had occurred during the prepara-
tion of its precursor 21. To investigate this, R-Boc-indanyl
glycine 10 was replaced with the S-Boc-indanyl glycine
23, and the reaction was repeated, keeping the other
components the same. The major isomer isolated was the
SSS isomer 24 (Scheme 5). The structure of 24 was
confirmed by 1H and 13C NMR spectroscopy, which were
It has been shown that using R-leucine and S-Boc-
indanyl glycine, the SSS DKP 24 can be synthesized.
(10) Ugi, I.; Ho¨rl, W.; Hanusch-Kompa, C.; Schmid, T.; Herdtweck,
E. Heterocycles 1998, 47, 965-975.
(11) Both isomers were investigated using Macromodel; full details
are in the Supporting Information.
J. Org. Chem, Vol. 70, No. 12, 2005 4737

Sollis
SCHEME 6
ester 29 as the major product (yield 87%). Hydrolysis of
the ester 29 with lithium hydroxide in aqueous methanol
gave a 55% yield of the acid 30. Reaction of 30 via its
mixed anhydride with R-Boc-indanyl glycine 10 gave 31,
which on cyclization gave the required RRR isomer 13
in a 47% yield, identical to that prepared via Scheme 1.
Conclusion
In summary, we have developed a short and novel
stereospecific route to chirally pure RRR 13, SSS 24, and
RSS 22 DKP’s (the SRR could also be made by this route).
We have established that the new chiral center produced
in the major product from the 4-component-5-centered
Ugi is opposite to that of the leucine component and that
R-leucine gives RS 15 and S-leucine gives SR 29. This
new chiral center then induces the stereochemistry of the
leucine in the mixed anhydride reaction.
Experimental Procedures
(2R)-N-(t-Butyl)-2-[(3R,6R)-3-(2,3-dihydro-1H-inden-2-
yl)-6-isobutyl-2,5-dioxopiperazin-1-yl]-2-phenylethana-
mide (13). To a solution of R-leucine methyl ester hydrochlo-
ride 6 (600 mg) in methanol (8 mL) was added triethylamine
(0.46 mL) and benzaldehyde 7 (0.22 mL). The mixture was
stirred for 2.5 h before (2R)-[(t-butoxycarbonyl)amino](2,3-
dihydro-1H-inden-2-yl)ethanoic acid 10 (962 mg) and t-butyl-
isonitrile 9 (0.56 mL) were sequentially added. After being
stirred for 18 h, the solvent was removed in vacuo, and the
residue was dissolved in dichloromethane (4 mL) and trifluo-
roacetic acid (10 mL) and stirred for 3 h at ambient temper-
ature. After this time, the solvent was removed in vacuo. The
residue was treated with triethylamine in dioxane (2% solu-
tion, 20 mL) and was left to stir overnight. After this time,
the dioxane was removed in vacuo, and the residue was
dissolved in dichloromethane. The solution was washed with
0.1 M hydrochloric acid solution, and the organic phase was
separated using a hydrophobic frit and evaporated in vacuo.
This crude material was purified by flash chromatography
eluting with ethyl acetate/cyclohexane (50-100% ethyl acetate)
to give the less polar diastereomer, (2R)-N-(t-butyl)-2-[(3R,6R)-
3-(2,3-dihydro-1H-inden-2-yl)-6-isobutyl-2,5-dioxopiperazin-1-
yl]-2-phenylethanamide (13) as a white solid (120 mg, 19%).
1H NMR (CDCl3) 7.45-7.38 (m, 5H), 7.24-7.13 (m, 4H), 6.72
(d, 1H, J ) 4 Hz), 5.67 (s, 1H), 5.21 (s, 1H), 4.01-3.94 (m,
2H), 3.15 (m, 1H), 3.07 (m, 2H), 2.89 (m, 1H), 2.77 (m, 1H),
1.78 (m, 1H), 1.68 (m, 1H), 1.34 (m, 1H), 1.32 (s, 9H), 0.80 (d,
3H, J ) 6.5 Hz), 0.71 (d, 3H, J ) 6.5 Hz); 13C NMR (CDCl3)
21.4, 23.9, 25.2, 28.9, 36.6, 37.2, 44.8, 46.3, 52.1, 59.4, 60.4,
65.8, 124.7, 125.0, 126.9, 127.1, 129.5, 129.6, 134.6, 141.6,
142.9, 167.1, 168.0, 169.3; LCMS m/z 476 (MH+) (tr 3.54 min);
HRMS calcd for C29H37N3O3 (MH+) 476.2913, found 476.2906;
HPLC >99.5% (tr ) 14.3 min); circular dichroism λmax 204.4,
dE 21.49, λmax 228.0, dE -15.60.
SCHEME 7
This also gave the more polar diastereomer, (2S)-N-(t-butyl)-
2-[(3R,6R)-3-(2,3-dihydro-1H-inden-2-yl)-6-isobutyl-2,5-diox-
opiperazin-1-yl]-2-phenylethanamide (12) as a white solid (622
mg, 40%) 1H NMR (CDCl3) 7.45-7.38 (m, 5H), 7.24-7.14 (m,
4H), 6.67 (d, 1H, J ) 4 Hz), 5.80 (s, 1H), 5.42 (s, 1H), 3.98,
(dd, 1H, J ) 4,9.5 Hz), 3.75 (m, 1H), 3.19-3.02 (m, 3H), 2.96
(m, 1H), 2.77 (m, 1H), 1.91-1.65 (m, 3H), 1.36 (s, 9H), 0.80
(d, 3H, J ) 6.5 Hz), 0.64 (d, 3H, J ) 6.5 Hz);13C NMR (CDCl3)
21.3, 24.1, 25.2, 28.9, 36.7, 37.1, 44.2, 46.4, 52.0, 58.2, 60.3,
66.3, 124.7, 125.0, 126.9, 127.1, 129.5, 129.6, 130.2, 133.8,
141.6, 142.9, 167.3, 168.3, 169.3; LCMS m/z 476 (MH+) (tr 3.59
min); HRMS calcd for C29H37N3O3 (MH+) 476.2913, found
476.2915; HPLC >99.5% (tr )14.5 min)
Therefore, by starting with S-leucine and R-Boc-indanyl
glycine, it should be possible to synthesize the required
RRR DKP 13. Reacting L-leucine 28 with benzaldehyde
7 and t-butylisonitrile 9 (Scheme 7) gave the SR dipeptide
Methyl (2R)-2-{[2-(t-Butylamino)-2-oxo-1-phenylethyl]amino}-
4-methylpentanoate (15).
A stirred suspension of R-leucine 14 (1.31 g) in methanol
(10 mL) under a nitrogen atmosphere was cooled to -30 °C.
4738 J. Org. Chem., Vol. 70, No. 12, 2005

Trisubstituted 2,5-Diketopiperazines
To this was added a solution of benzaldehyde 7 (1.06 g) in
methanol (0.5 mL) and t-butylisocyanide 9 (0.831 g). After 3 h
at -30 °C, the reaction was allowed to warm to room
temperature and was stirred for a further 72 h. The solvent
was removed in vacuo, and the residue purified using a column
(40 g, silica) eluting with cyclohexane/ethyl acetate (gradient
from 8:1 to 1:1). The required fractions were combined and
concentrated in vacuo to give methyl (2R)-2-{[2-(t-Butylamino)-
2-oxo-1-phenylethyl]amino}-4-methylpentanoate (15) as a col-
orless oil (2.27, 68%). The ratio of isomers was 8:1, and the
material was used without further purification.
(t, 2H, J ) 7 Hz), 1.26 (s, 9H), 0.90 (d, 3H, J ) 6.5 Hz), 0.89
(d, 3H, J ) 6.5 Hz); 13C NMR (d6-DMSO) 22.1, 23.1, 24.4, 28.3,
42.5, 49.9, 58.9, 65.6, 127.3, 128.2, 140.2, 170.6, 176.2; LCMS
m/z 321 (MH+) (tr 2.52 min); chiral HPLC 100% (tr 7.1 min);
Anal. calcd. for C18H28N2O3‚0.33 H2O: C, 66.24; H, 8.85; N,
8.58, found: C, 66.15; H, 8.62; N, 8.45.
(2R)-2-[[(1S)-2-(t-Butylamino)-2-oxo-1-phenylethyl]-
(formyl)amino]-4-methylpentanoic Acid (19). To a solu-
tion of acetic anhydride (0.102 g) in dichloromethane (3 mL)
was added formic acid (0.046 g) and pyridine (0.079 g), and
the reaction mixture was stirred under nitrogen at ambient
temperature for 20 min. To this was added (2R)-2-{[2-(t-
butylamino)-2-oxo-1-phenylethyl]amino}-4-methylpentanoic acid
(17) (0.10 g) and triethylamine (0.031 g). After 3 h, the reaction
mixture was separated between dichloromethane and 1 N
hydrochloric acid solution. The organic phase was dried using
a hydrophobic frit and evaporated in vacuo to give (2R)-2-
[[(1S)-2-(t-butylamino)-2-oxo-1-phenylethyl](formyl)amino]-4-
methylpentanoic acid (19) as an off-white solid (0.108 g, 100%).
1H NMR (d6-DMS0) 13.10 (s, 1H), 8.29 (s, 1H), 7.44 (m, 1H),
7.38 (m, 1H), 7.32 (m, 1H), 5.34 (s, 1H), 5.15 (dd, 1H, J ) 5,9
Hz), 1.70-1.51 (m, 3H), 1.31 (s, 9H), 0.87 (d, 3H, J ) 6 Hz),
0.86 (d, 3H, J ) 6 Hz); 13C NMR 20.2, 21.6, 22.5, 26.8, 36.9,
49.3, 50.6, 58.7, 126.4, 126.5, 127.2, 136.7, 162.6, 167.6, 171.6;
LCMS m/z 349 (MH+) (tr 3.13 min); HRMS calcd for C19H29N2O4
(MH+) 349.2127, found 349.2127; HPLC 99% (tr ) 12.0 min).
Methyl (2R)-2-[[(1S)-2-(t-Butylamino)-2-oxo-1-phenyl-
ethyl](formyl)amino]-4-methylpentanoate (16). To a solu-
tion of (2R)-2-[[(1S)-2-(t-butylamino)-2-oxo-1-phenylethyl]-
(formyl)amino]-4-methylpentanoic acid (19) (36 mg) in tetra-
hydrofuran (2 mL) was added a 2.0 M solution of trimethyl-
silyldiazomethane in hexanes (0.25 mL), and the reaction
mixture was stirred under nitrogen at ambient temperature
for 2 h. Methanol (2 mL) was added, and the reaction mixture
was stirred for a further 2 h. The solvent was removed in
vacuo, and the residue was purified by mass directed auto-
prep. This gave methyl (2R)-2-[[(1S)-2-(t-butylamino)-2-oxo-
1-phenylethyl](formyl)amino]-4-methylpentanoate (16) as an
off-white solid (30 mg, 80%).
Some of the mixture was purified to give the following
1
isomers: major isomer H NMR (CDCl3) 7.37-7.27 (m, 5H),
6.51 (s, 1H), 4.11 (s, 1H), 3.71 (s, 3H), 3.11 (t, 1H, J ) 7 Hz),
2.19 (broad signal, 1H), 1.70 (m, 1H), 1.48 (m, 2H), 1.31 (s,
9H), 0.87 (d, 3H, J ) 6.5 Hz), 0.77 (d, 3H, J ) 6.5 Hz); 13C
NMR (CDCl3) 22.0, 22.8, 24.8, 28.6, 42.5, 50.9, 51.8, 57.5, 66.3,
127.8, 128.3, 128.9, 138.9, 170.8, 175.4; LCMS m/z 335 (MH+)
(tr 3.38 min); HRMS calcd for C19H31N2O3 (MH+) 335.233468,
found 335.232462; chiral HPLC 99.5% (tr 14.1 min).
1
Minor isomer H NMR (CDCl3) 7.38-7.27 (m, 6H), 3.96 (s,
1H), 3.72 (s, 3H), 3.32 (dd, 1H, J ) 5, 8.5 Hz), 2.06 (broad
signal, 1H), 1.87 (m, 1H), 1.50-1.42 (m, 2H), 1.39 (s, 9H), 0.98
(d, 3H, J ) 6.5 Hz), 0.96 (d, 3H, J ) 6.5 Hz); 13C NMR (CDCl3)
22.2, 23.2, 24.9, 28.7, 43.3, 50.7, 52.0, 59.0, 66.9, 127.2, 128.1,
128.8, 139.5, 170.9, 175.9; LCMS m/z 335 (MH+) (tr 3.42 min);
HRMS calcd for C19H31N2O3 (MH+) 335.233468, found
335.232511; chiral HPLC 100% (tr 7.7 min)
Methyl (2R)-2-[[2-(t-Butylamino)-2-oxo-1-phenylethyl]-
(formyl)amino]-4-methylpentanoate (16). To a solution of
acetic anhydride (0.51 g) in dichloromethane (10 mL) was
added formic acid (0.23 g) and pyridine (0.39 g), and this was
stirred under nitrogen at ambient temperature for 1 h. To this
was added a solution of methyl (2R)-2-{[2-(t-butylamino)-2-
oxo-1-phenylethyl]amino}-4-methylpentanoate (15) (0.334 g)
in dichloromethane (2 mL) and triethylamine (0.101 g), and
the reaction mixture was left to stand under nitrogen at
ambient temperature for 20 h. The reaction mixture was
separated between dichloromethane and saturated sodium
bicarbonate solution. The organic phase was washed with 2
N hydrochloric acid solution and brine. The organic phase was
evaporated in vacuo to give methyl (2R)-2-[[2-(t-butylamino)-
2-oxo-1-phenylethyl](formyl)amino]-4-methylpentanoate (16)
(2S)-N-(t-Butyl)-2-[(3R,6S)-3-(2,3-dihydro-1H-inden-2-
yl)-6-isobutyl-2,5-dioxopiperazin-1-yl]-2-phenylethana-
mide (22). To a solution of (2R)-[(t-butoxycarbonyl)amino](2,3-
dihydro-1H-inden-2-yl)ethanoic acid (10) (58 mg) in dry
tetrahydrofuran (3 mL) under a nitrogen atmosphere at -25
°C was added N-methylmorpholine (20 mg) and a solution of
isopropylchloroformate in toluene (1.0 M, 0.2 mL). After 10
min, (2R)-2-{[2-(t-butylamino)-2-oxo-1-phenylethyl]amino}-4-
methylpentanoic acid (17) (70 mg) was added, and the result-
ant mixture was stirred at room temperature for 3 h. The
solvent was then removed in vacuo, and the residue was
treated with 4 N hydrochloric acid in dioxane (8 mL). After 4
h, methanol (15 mL) was added to the reaction mixture, and
this was left to stand for 18 h. The solvent was then removed
in vacuo, and the residue was purified on an SPE cartridge
(50 g, silica) eluting with cyclohexane/ethyl acetate (gradient
from 4:1 to neat ethyl acetate), which furnished (2S)-N-(t-
butyl)-2-[(3R,6S)-3-(2,3-dihydro-1H-inden-2-yl)-6-isobutyl-2,5-
dioxopiperazin-1-yl]-2-phenylethanamide (22) as a white solid
1
as a white solid (0.274 g, 75%). H NMR (d6-DMSO) 8.24 (s,
1H), 7.99 (s, 1H), 7.49-7.27 (m, 6H), 5.24 (dd, 1H, J ) 5.6,8.8
Hz), 3.70 (s, 3H), 1.83-1.46 (m, 3H), 1.31-1.20 (m, 9H), 0.90-
0.75 (m, 6H); 13C NMR (d6-DMSO) 21.5, 22.9, 23.8, 28.1, 38.0,
50.7, 51.8, 52.1, 60.0, 127.8, 127.9, 128.7, 137.8, 163.9, 168.6,
171.7; LCMS m/z 363 (MH+) (tr 3.24 min); HRMS calcd for
C20H30N2O4 (MH+) 363.2284, found 363.2277; HPLC 93% (tr
) 13.0 min).
(2R)-2-{[2-(t-Butylamino)-2-oxo-1-phenylethyl]amino}-
4-methylpentanoic Acid (17). Methyl (2R)-2-[[2-(t-buty-
lamino)-2-oxo-1-phenylethyl]amino]-4-methylpentanoate (15)
(0.59 g) was dissolved in methanol (15 mL), and to this was
added lithium hydroxide (0.12 g) and water (2.5 mL). After
18 h, the methanol was removed in vacuo, and the residue
was dissolved in water. The pH was adjusted to 7 using 2 N
hydrochloric acid solution. This was applied to an OASIS
cartridge (6 g) and eluted twice with water and twice with
methanol. The methanol fractions were evaporated in vacuo
to give (2R)-2-{[2-(t-butylamino)-2-oxo-1-phenylethyl]amino}-
4-methylpentanoic acid (17) as a white solid (0.36 g, 64%).
Some of the mixture was purified to give the following
isomers: major isomer 1H NMR (d6-DMSO) 7.63 (s, 1H), 7.37-
7.21 (m, 5H), 4.12 (s, 1H), 2.80 (t, 1H, J ) 7 Hz), 1.73 (m, 1H),
1.36 (m, 2H), 1.21 (s, 9H), 0.82 (d, 3H, J ) 6.5 Hz), 0.70 (d,
3H, J ) 6.5 Hz); 13C NMR (d6-DMSO) 21.9, 22.9, 24.3, 28.3,
42.2, 50.0, 57.6, 64.7, 127.0, 127.4, 127.9, 140.4, 171.0, 176.5;
LCMS m/z 321 (MH+) (tr 2.46 min); chiral HPLC 100% (tr 9.2
min); Anal. calcd. for C18H28N2O3‚0.66 H2O: C, 65.06; H, 8.89;
N, 8.43, found: C, 65.16; H, 8.83; N, 8.36.
1
(54 mg, 57%) H NMR (CDCl3) d 7.42 (m, 5H), 7.20 (m, 2H),
7.17 (m, 2H), 5.64 (s, 1H), 5.59 (s, 1H), 5.48 (s, 1H), 4.23 (d,
1H, J ) 4 Hz), 4.00 (m, 1H), 3.39 (m, 2H), 3.13 (m, 1H), 2.91-
2.79 (m, 2H), 1.63-1.51 (m, 2H), 1.33 (s, 9H), 0.87 (m, 1H),
0.71 (d, 3H, J ) 6 Hz), 0.58 (d, 3H, J ) 6 Hz); 13C NMR (CDCl3)
20.9, 23.4, 24.2, 28.6, 34.1, 36.3, 38.6, 40.8, 51.8, 57.6, 58.7,
63.3, 124.5, 124.8, 126.7, 126.9, 129.17, 129.19, 129.9, 134.2,
141.4, 142.0, 167.5, 167.6, 169.5; LCMS m/z 476 (MH+) (tr 3.55
min); HRMS calcd for C29H37N3O3 (MH+) 476.2913, found
476.2908; HPLC 96% (tr ) 14.4).
(2S)-N-(t-Butyl)-2-[(3S,6S)-3-(2,3-dihydro-1H-inden-2-
yl)-6-isobutyl-2,5-dioxopiperazin-1-yl]-2-phenylethana-
mide (24). To a solution of (2S)-[(t-butoxycarbonyl)amino](2,3-
dihydro-1H-inden-2-yl)ethanoic acid (23) (58 mg) in dry
tetrahydrofuran (3 mL) under a nitrogen atmosphere at -25
°C was added N-methylmorpholine (20 mg) and a solution of
1
Minor isomer H NMR (d6-DMSO) 7.66 (s, 1H), 7.38-7.23
(m, 5H), 3.95 (s, 1H), 2.97 (t, 1H, J ) 7 Hz), 1.84 (m, 1H), 1.39
J. Org. Chem, Vol. 70, No. 12, 2005 4739

Sollis
1
isopropylchloroformate in toluene (1.0 M, 0.2 mL). After 10
min, (2R)-2-{[2-(t-butylamino)-2-oxo-1-phenylethyl]amino}-4-
methylpentanoic acid (17) (64 mg) was added, and the result-
ant mixture was stirred at room temperature for 3 h. The
solvent was then removed in vacuo, and the residue was
treated with 4 N hydrochloric acid in dioxane (8 mL). After 3
h, the solvent was removed in vacuo, and the residue was
dissolved in dichloromethane (2 mL). To this was added
diisopropylethylamine (78 mg) and WSCDI (58 mg), and the
reaction mixture was left to stand for 18 h. The solvent was
then removed in vacuo, and the residue was purified on an
SPE cartridge (50 g, silica) eluting with cyclohexane/ethyl
acetate (gradient from 4:1 to neat ethyl acetate), which
furnished (2S)-N-(t-butyl)-2-[(3S,6S)-3-(2,3-dihydro-1H-inden-
2-yl)-6-isobutyl-2,5-dioxo piperazin-1-yl]-2-phenylethanamide
(24) as a white solid (34 mg, 36%). 1H NMR (CDCl3) 7.47-
7.37 (m, 6H), 7.24-7.13 (m, 4H), 5.78 (s, 1H), 5.23 (s, 1H),
4.01-3.94 (m, 2H), 3.17-2.98 (m, 3H), 2.94-2.77 (m, 2H), 1.78
(m, 1H), 1.67 (m, 1H), 1.38-1.29 (m, 10H), 0.81 (d, 3H, J )
6.5 Hz), 0.72 (d, 3H, J ) 6.5 Hz); 13C NMR (CDCl3) 21.4, 23.9,
25.2, 28.9, 36.6, 37.2, 44.8, 46.3, 52.1, 59.4, 60.4, 65.8, 124.7,
125.0, 126.9, 127.1, 129.5, 129.6, 134.6, 141.6, 142.9, 167.1,
168.0, 169.3; LCMS m/z 476 (MH+) (tr 3.38 min); HRMS calcd
for C29H37N3O3 (MH+) 476.2913, found 476.2903; HPLC >99.5%
(tr )14.3 min); circular dichroism λmax 204.4, dE -21.61, λmax
228.0, dE 15.30.
To a solution of (2R)-[(t-butoxycarbonyl)amino](2,3-dihydro-
1H-inden-2-yl)ethanoic acid (10) (58 mg) in dry tetrahydrofu-
ran (3 mL) under a nitrogen atmosphere at -25 °C was added
N-methylmorpholine (20 mg) and a solution of isopropylchlo-
roformate in toluene (1.0 M, 0.2 mL). After 10 min, (2R)-2-{-
[2-(t-butylamino)-2-oxo-1-phenylethyl]amino}-4-methylpen-
tanoic acid (17) (64 mg) was added, and the resultant mixture
was stirred at between -20 and -15 °C for 2 h. The solvent
was then removed in vacuo, and the residue was treated with
4 N hydrochloric acid in dioxane (8 mL). After 3 h, the solvent
was removed in vacuo, and the residue was dissolved in
dichloromethane (2 mL). To this was added diisopropylethy-
lamine (78 mg) and WSCDI (58 mg), and the reaction mixture
was left to stand for 18 h. The solvent was then removed in
vacuo, and the residue was purified on an SPE cartridge (50
g, silica) eluting with cyclohexane/ethyl acetate (gradient from
4:1 to neat ethyl acetate), which furnished (2S)-N-(t-butyl)-2-
[(3R,6S)-3-(2,3-dihydro-1H-inden-2-yl)-6-isobutyl-2,5-dioxopip-
erazin-1-yl]-2-phenylethanamide (22) as a white solid (26 mg,
27%) and (2S)-N-(t-butyl)-2-[(3R,6R)-3-(2,3-dihydro-1H-inden-
2-yl)-6-isobutyl-2,5-dioxopiperazin-1-yl]-2-phenylethanamide
(12) as a white solid (14 mg, 15%).
Minor isomer H NMR (CDCl3) 7.38-7.27 (m, 6H), 3.96 (s,
1H), 3.72 (s, 3H), 3.32 (dd, 1H, J ) 5,8.5 Hz), 2.06 (broad
signal, 1H), 1.87 (m, 1H), 1.50-1.42 (m, 2H), 1.39 (s, 9H), 0.98
(d, 3H, J ) 6.5 Hz), 0.96 (d, 3H, J ) 6.5 Hz); 13C NMR (CDCl3)
22.2, 23.2, 24.9, 28.7, 43.3, 50.7, 52.0, 59.0, 66.9, 127.2, 128.1,
128.8, 139.5, 170.9, 175.9; LCMS m/z 335 (MH+) (tr 3.46 min);
HRMS calcd for C19H31N2O3 (MH+) 335.233468, found
335.234355; chiral HPLC 100% (tr 13.7 min).
(2S)-2-{[2-(t-Butylamino)-2-oxo-1-phenylethyl]amino}-
4-methylpentanoic acid (30). Methyl (2S)-2-[[2-(t-buty-
lamino)-2-oxo-1-phenylethyl]amino]-4-methylpentanoate (29)
(1.32 g) was dissolved in methanol (15 mL), and to this was
added lithium hydroxide (0.29 g) and water (10 mL). After 4
h, the methanol was removed in vacuo, and the residue was
dissolved in water. The pH was adjusted to 7 using a 2 N
hydrochloric acid solution. This was applied to an OASIS
cartridge (6 g) and eluted twice with water and twice with
methanol. The methanol fractions were evaporated in vacuo
to give (2S)-2-{[2-(t-butylamino)-2-oxo-1-phenylethyl]amino}-
4-methylpentanoic acid (30) as a white solid (0.36 g, 55%).
Some of the mixture was purified to give the following
isomers: major isomer 1H NMR (d6-DMSO) 7.65 (s, 1H), 7.37-
7.20 (m, 5H), 4.11 (s, 1H), 2.79 (dd, 1H, J ) 6,7 Hz), 1.74 (m,
1H), 1.36 (m, 2H), 1.21 (s, 9H), 0.82 (d, 3H, J ) 6.5 Hz), 0.70
(d, 3H, J ) 6.5 Hz); 13C NMR (d6-DMSO) 22.0, 23.0, 24.3, 28.4,
42.3, 50.0, 57.8, 64.8, 127.1, 127.5, 128.0, 140.6, 171.5, 176.5;
LCMS m/z 321 (MH+) (tr 2.46 min); chiral HPLC 100% (tr 7.8
min); Anal. calcd. for C18H28N2O3‚0.5 H2O: C, 65.63; H, 8.87;
N, 8.50, found: C, 65.58; H, 8.56; N, 8.40.
1
Minor isomer H NMR (d6-DMSO) 7.66 (s, 1H), 7.38-7.23
(m, 5H), 3.96 (s, 1H), 2.98 (t, 1H, J ) 7 Hz), 1.84 (m, 1H), 1.39
(t, 2H, J ) 7 Hz), 1.26 (s, 9H), 0.90 (d, 3H, J ) 6.5 Hz), 0.89
(d, 3H, J ) 6.5 Hz); 13C NMR (d6-DMSO) 22.1, 23.1, 24.4, 28.3,
42.5, 49.9, 58.8, 65.5, 127.3, 127.4, 128.2, 140.1, 170.6, 176.1;
LCMS m/z 321 (MH+) (tr 2.52 min); chiral HPLC 100% (tr 5.5
min); Anal. calcd. for C18H28N2O3‚0.33 H2O: C, 66.24; H, 8.85;
N, 8.58, found: C, 66.36; H, 8.66; N, 8.48.
(2R)-N-(t-Butyl)-2-[(3R,6R)-3-(2,3-dihydro-1H-inden-2-
yl)-6-isobutyl-2,5-dioxopiperazin-1-yl]-2-phenylethana-
mide (13). To a solution of (2R)-[(t-butoxycarbonyl)amino](2,3-
dihydro-1H-inden-2-yl)ethanoic acid (10) (58 mg) in dry
tetrahydrofuran (3 mL) under a nitrogen atmosphere at -25
°C was added N-methylmorpholine (20 mg) and a solution of
isopropylchloroformate in toluene (1.0 M, 0.2 mL). After 10
min, (2S)-2-{[2-(t-butylamino)-2-oxo-1-phenylethyl]amino}-4-
methylpentanoic acid (30) (70 mg) was added, and the result-
ant mixture was stirred at room temperature for 3 h. The
solvent was then removed in vacuo, and the residue was
treated with 4 N hydrochloric acid in dioxane (8 mL). After 4
h, methanol (15 mL) was added to the reaction mixture, and
this was left to stand for 18 h. The solvent was then removed
in vacuo, and the residue was purified on an SPE cartridge
(50 g, silica) eluting with cyclohexane/ethyl acetate (gradient
from 4:1 to neat ethyl acetate), which furnished (2R)-N-(t-
butyl)-2-[(3R,6R)-3-(2,3-dihydro-1H-inden-2-yl)-6-isobutyl-2,5-
dioxopiperazin-1-yl]-2-phenylethanamide (13) as a white solid
Methyl (2S)-2-{[2-(t-Butylamino)-2-oxo-1-phenylethyl]-
amino}-4-methylpentanoate (29). A stirred suspension of
S-leucine (28) (2.62 g) in methanol (200 mL) under a nitrogen
atmosphere was cooled to -30 °C. To this was added a solution
of benzaldehyde (7) (2.12 g) in methanol (10 mL) and a solution
of t-butylisocyanide (9) (1.66 g) in methanol (10 mL). After 4
h at -30 °C, the reaction was allowed to warm to room
temperature and was stirred for a further 72 h. The solvent
was removed in vacuo, the residue was purified using a column
(40 g, silica) eluting with cyclohexane/ethyl acetate (gradient
from 8:1 to 1:1). The required fractions were combined and
concentrated in vacuo to give methyl (2S)-2-{[2-(t-butylamino)-
2-oxo-1-phenylethyl]amino}-4-methylpentanoate (29) as a col-
orless oil (5.82 g, 87%). The ratio of isomers was 9:1, and
material used without further purification.
1
(45 mg, 47%) H NMR (CDCl3) 7.45-7.38 (m, 5H), 7.24-7.14
(m, 4H), 6.38 (m, 1H), 5.64 (s, 1H), 5.19 (s, 1H), 3.97 (m, 2H),
3.20-3.01 (m, 3H), 2.91 (m, 1H), 2.77 (m, 1H), 1.91-1.65 (m,
2H), 1.32 (m, 10H), 0.79 (d, 3H, J ) 6.5 Hz), 0.71 (d, 3H, J )
6.5 Hz); LCMS m/z 476 (MH+) (tr 3.59 min).
Acknowledgment. Dave Davies and Alan Borth-
wick are thanked for their valuable input into this
paper.
Some of the mixture was purified to give the following
1
isomers: major isomer H NMR (CDCl3) 7.37-7.27 (m, 5H),
6.52 (s, 1H), 4.12 (s, 1H), 3.71 (s, 3H), 3.12 (t, 1H, J ) 7 Hz),
1.70 (m, 1H), 1.48 (m, 2H), 1.31 (s, 9H), 0.87 (d, 3H, J ) 6.5
Hz), 0.77 (d, 3H, J ) 6.5 Hz); 13C NMR (CDCl3) 22.0, 22.8,
24.8, 28.6, 42.5, 50.1, 51.8, 57.5, 66.3, 127.8, 128.3, 128.9, 138.8,
170.8, 175.4; LCMS m/z 335 (MH+) (tr 3.40 min); HRMS calcd
for C19H31N2O3 (MH+) 335.233468, found 335.232467; chiral
HPLC 99.5% (tr 7.0 min).
Supporting Information Available: 1H NMR for isolated
compounds. Circular dichroism spectra for 13 and 24. This
material is available free of charge via the Internet at
http://pubs.acs.org.
JO0501137
4740 J. Org. Chem., Vol. 70, No. 12, 2005