Synthesis of chiral piperazinones as versatile scaffolds for peptidomimetics

Article abstract of DOI:10.1016/S0040-4020(00)00787-0

Chiral piperazinones were synthesized as conformationally restricted peptidomimetics starting from inexpensive and readily available D-glucosamine hydrochloride and amino acid methyl esters. Different synthetic strategies are devised to allow attachment of side chains imitating the parent peptide as shown for the RGD motif. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4020(00)00787-0

TETRAHEDRON
Pergamon
Tetrahedron 56 (2000) 8481±8487
Synthesis of Chiral Piperazinones as Versatile Scaffolds
for Peptidomimetics
*
Frank RuÈbsam, Ralph Mazitschek and Athanassios Giannis
È È
Institut fur Organische Chemie der Universitat Karlsruhe, Richard-Willstaetter-Allee 2, D-76128 Karlsruhe, Germany
Received 17 July 2000; accepted 30 August 2000
AbstractÐChiral piperazinones were synthesized as conformationally restricted peptidomimetics starting from inexpensive and readily
available d-glucosamine hydrochloride and amino acid methyl esters. Different synthetic strategies are devised to allow attachment of side
chains imitating the parent peptide as shown for the RGD motif. q 2000 Elsevier Science Ltd. All rights reserved.
A successful method for the development of peptidomi-
metics1,2 involves synthesis of conformationally restricted
compounds, i.e. locally or globally constrained peptide
analogs that imitate the receptor-bound conformation of
the endogenous ligands as closely as possible.3±5 Bridging
between two consecutive amino acids in a peptide leads to a
dipeptide mimetic, the ¯exibility of which is limited
compared to regular dipeptides. Piperazin-2-ones are
obtained formally through Ni$Cai11 cyclization of a di-
peptide and can be used as an element for the generation
of a local constriction. Furthermore piperazin-2-ones can
serve as a rigid template exposing substituents in a con-
formation resembling the side chains of a parent peptide
in its bioactive conformation.6,7 Substituted piperazinones
are components of a wide variety of bioactive compounds
e.g. Leu-Enkephalin analogs,8 cholecystokinin receptor
antagonists,9,10 RGD (Arg-Gly-Asp) mimetics,11,12 neuro-
kinin-2 receptor ligand,13 substance P analogues,14 inhibi-
tors of farnesyltransferase,15 growth hormone secretagogue,16
and inhibitors of glycosidases.17
Here we report a simple synthetic approach to chiral piper-
azinones which starts from cheap starting materials,
proceeds in good yields, and allows easy scale-up to multi-
gram scale.
As outlined in Fig. 1, the piperazinone scaffold 1 can be
functionalized either on the ring nitrogens, through trans-
formations of the hydroxymethyl side chain, or through
derivatization of the amino acid residues introduced in
C-3 position.
Starting from Cbz-d-glucosamine (2), which is easily
accessible by N-protection of glucosamine hydrochloride
with Cbz-Cl,19 compound 4 is obtained by reductive
amination with glycine methyl ester hydrochloride (3) in
re¯uxing methanol in 80% yield (Scheme 1). Protection of
the newly generated secondary amine with Boc2O proceeds
smoothly and also appears to be crucial for the ®nal
cyclization step (vide supra). Oxidative cleavage of the
polyol 5 with NaIO4 and subsequent rapid reduction of the
aldehyde with sodium borohydride yields the amino alcohol
6 in 87% (2 steps). The piperazinone 7 is obtained through
removal of the Cbz group and spontaneous cyclization in
93% yield.
Chiral syntheses that enable an entry to all diastereomers of
several 3,6-disubstituted piperazinones have recently been
published by our group.18
Figure 1.
Keywords: piperazinones; peptide mimetics; biologically active compounds; amino sugars.
* Corresponding author. Tel.: 149-721-608-3209, fax: 149-721-608-7652; e-mail: giannis@ochhades.chemie.uni-karlsruhe.de
0040±4020/00/$ - see front matter q 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020(00)00787-0

È
F. Rubsam et al. / Tetrahedron 56 (2000) 8481±8487
8482
Scheme 1.
The piperazinone 7 is further elaborated into functionalized
target molecules 12±14 which represent mimetics of the
RGD peptide motif. This sequence is found in several
proteins, such as ®brinogen, ®bronectin, vitronectin,
laminin, osteopontin, and von Willebrand factor and serves
as an endogenous ligand for integrins, which in turn play a
critical role in cell-to-cell adhesion, cell-to-extracellular
matrix adhesion and interaction.20,21 These interactions
determine important biological phenomena: cell morph-
ology, differentiation and viability, cellular traf®c,
organogenesis, angiogenesis, and blood clotting, making
the RGD motif a promising target in modern drug design
efforts.22,23
As shown in Scheme 2, we chose to derivatize the scaffold
along the N-1±N-4 axis. A strategy to provide compounds
bearing substituents along the C-2±C-6 axis is described in
Scheme 3.
Protection of the hydroxy function as TBDMS ether 8
proceeds smoothly in 98% yield. Subsequent coupling 9
with ethyl bromoacetate (96%) followed by removal of
Scheme 2.

È
F. Rubsam et al. / Tetrahedron 56 (2000) 8481±8487
8483
Scheme 3.
the silyl protecting group with TBAF (98%) yields 10.
Removal of the Boc group was achieved with gaseous
hydrochloric acid in diethyl ether since standard conditions
(TFA) gave rise to side products. Furthermore 11 precipi-
tates as the hydrochloride in cold diethyl ether and can
easily be puri®ed by simple ®ltration. Finally amino func-
tionalized side chains are introduced via carbodiimide
(EDC) mediated coupling with 5-(Bis-Boc-guanidino)
valeric acid (27%) Cbz protected g-amino butyric acid
(GABA) (89%), or glycine (43%), respectively, to afford
the protected target compounds 12±14.
with bromine in an aqueous methanol/hydrogen carbonate
solution lead to the formation of methyl ester 18 (47%),
whereas the vinylogous derivative 19 was obtained by reac-
tion with triethyl phosphonoacetate (56%, 2 steps).
In summary, we have synthesized chiral piperazinones as
scaffolds for the design of peptidomimetics from inexpen-
sive and readily available starting materials. We have
further shown their versatility by attaching different func-
tionalized groups along different axis of the rigid template
which are presented as to mimic the structure and the func-
tion of RGD peptides. The described strategy should enable
the design and synthesis of various other small molecule
peptidomimetics based on the piperazinone scaffold.
A slightly different synthetic approach sets the stage for a
functionalization along the C-2±C-6 axis of the piper-
azinone scaffold (Scheme 3). Reductive amination with
l-e-Boc-lysine methyl ester hydrochloride in re¯uxing
methanol in the presence of sodium borohydride afforded
precursor 15 (97%). Protection of the newly generated
secondary amine with Boc2O proceeds smoothly in 96%
yield to afford the polyol 16. Removal of the Cbz group
was achieved by hydrogenation in the presence of Pd-C as
catalyst to yield piperazinone 17 in 54%. Oxidative
cleavage of the polyol side chain with sodium periodate
afforded the corresponding aldehydes, which were used
without further puri®cation, although isolation is feasible
for characterization. However, to avoid racemization the
aldehydes were rapidly further elaborated as shown. Oxida-
tion according to a procedure devised by Lichtenthaler24
Biological properties of selected compounds obtained by
this strategy will be published elsewhere.
Experimental
Solvents were puri®ed in the usual way. Water sensitive
reactions were carried out in ¯ame dried glassware under
argon. Thin layer chromatography: Merck precoated
tlc-plates, silica gel 60; column chromatography: silica
gel 60 (Merck, 40±63 mm). Melting points: BuÈchi SMP
20. Melting points are uncorrected. 1H and 13C NMR:
Bruker AC-200, Bruker AC-250, Bruker AM-400. Mass

È
F. Rubsam et al. / Tetrahedron 56 (2000) 8481±8487
8484
spectrometry: A. E. I. MS-30, MS-50, ion source 1808C and
Finnigan MAT MS 70, FAB: Kratos Concept 1H,
matrixˆm-nitrobenzylic alcohol. Elemental analyses were
performed at the Institute of Organic Chemistry and
Biochemistry, Bonn, microanalytical department. Optical
rotations were determined on a Perkin±Elmer 241 polari-
meter.
extracted three times with CH2Cl2. The combined organic
solutions are dried over MgSO4 and the solvent evaporated
to afford 1.75 g (87%) of 6. Colorless sticky oil; RF (alde-
hyde): 0.71 (CH2Cl2±MeOHˆ10:1); RF (alcohol): 0.65
(CH2Cl2±MeOHˆ10:1); [a]2D0ˆ214.1 (cˆ1.00, MeOH);
1H NMR (250 MHz, CDCl3): d (ppm)ˆ1.38 (s, 9H,
C(CH3)3), 3.16 (dd, Jˆ3.7, 13.7 Hz, 0.75H, OH), 3.30±
4.00 (m, 7H, 3£CH2, CH), 3.68 (s, 3H, CO2CH3), 5.03 (s,
2H, benzyl-CH2), 5.59 (d, Jˆ7 Hz, 0.8H, NH), 7.15±7.40
(m, 5H, Ph); FAB-MS: C19H28N2O7 calcd m/zˆ396.1896
found m/zˆ397.2 (M1H1).
Methyl [2-(S)-benzyloxycarbonylamino-3(R), 4(S), 5(R),
6-tetrahydroxy-hexyl]-amino-acetate (4). Cbz-glucos-
amine (2) (12.56 g, 40 mmol), glycine methyl ester hydro-
chloride (3) (5.52 g, 44 mmol), and sodium cyanoborohydride
(5.52 g, 88 mmol) were suspended in methanol (1 L) and
stirred for 24 h at re¯ux. Evaporation and separation of
the reaction mixture by column chromatography (CH2Cl2±
MeOH±NH3ˆ40:10:1) afforded 12.5 g 4 (81%) as an off-
white solid. RF: 0.48 (CH2Cl2±MeOH±NH3ˆ40:10:1), mp
117±1198C (decomposition). [a]2D0ˆ26.2 (cˆ2.25,
4-tert.-Butoxycarbonyl-6(S)-hydroxymethyl-piperazin-
2-one (7). A solution of 6 (1.74 g, 4.39 mmol) in methanol
(40 mL) was hydrogenated for 24 h at 258C and normal
pressure in the presence of 10% Pd±C. After ®ltration of
the catalyst and evaporation to dryness the crude product
was further puri®ed by column chromatography (CH2Cl2±
MeOHˆ10:1) to afford 754 mg (75%) 7 as a white solid.
Mp 104±1058C; RF: 0.36 (CH2Cl2±MeOHˆ10:1);
[a]2D0ˆ131.3 (cˆ1.10, MeOH); 1H NMR (250 MHz,
CDCl3): d (ppm)ˆ1.44 (s, 9H, C(CH3)3), 2.35 (br, 0.3H,
OH), 3.30±3.90 (m, 4H), 3.90±4.15 (m, 3H), 7.30 (br, 1H,
NH); FAB-MS: C10H18N2O4 calcd m/zˆ230.1266 found
m/zˆ230.1 (M1); C10H18N2O4 calcd C 52.16, H 7.88, N
12.17, found (%) C 52.36H 7.88 N 12.11
1
MeOH); H NMR (250 MHz, CD3OD): d (ppm)ˆ2.66±
2.90 (m, 2H, CH2), 3.34 (s, 2H, CH2), 3.42 (s, 2H, CH2),
3.53±4.00 (m, 4H), 3.71 (s, 3H, CO2CH3), 5.09 (m, 2H,
Benzyl-CH2), 7.20±7.43 (m, 5H, Ph); 13C NMR and
DEPT 135 (100.63 MHz, DMSO-d6): d (ppm)ˆ48.86
(CH2), 49.32 (CH2), 51.48 (CO2CH3), 53.22 (CH), 63.38
(CH2), 65.13 (CH2), 68.97 (CH), 70.43 (CH), 71.27 (CH),
127.26 (CH), 127.50 (CH), 128.24 (CH), 137.11 (Cq),
156.17 (CO-carbamate), 171.28 (CO2CH3); FAB-MS:
calcd for C17H26N2O8: m/zˆ386.1689, found m/zˆ387.1
(M1H1); C17H26N2O8´1H2O: calcd C 50.47, H 6.98, N
6.93; found C 50.12, H 6.58, N 6.83.
4-tert.-Butoxycarbonyl-6-(S)-(tert.-butyldimethylsiloxy-
methyl)-piperazin-2-one (8). A solution of 7 (4.00 g,
17.4 mmol) in DMF (50 mL) was treated with TBDMSCl
(6.55 g, 43.5 mmol) and imidazole (1.42 g, 21.0 mmol) at
258C. The reaction mixture was stirred for 5 h before water
(50 mL) was added. The aqueous layer was separated and
extracted with Et2O (3£20 mL), and the combined extracts
were dried (MgSO4), and concentrated in vacuo. Finally
azeotropic removal of water with toluene (3£20 mL)
afforded 5.87 g (98%) 8 as a white waxy solid. Mp 94±
958C; RF: 0.89 (CH2Cl2±MeOHˆ10:1); [a]2D0ˆ214.1
Methyl [2-(S)-benzyloxycarbonylamino-3(R), 4(S), 5(R),
6-tetrahydroxy-hexyl]-tert.-butoxy-carbonylamino-ace-
tate (5). A solution of 4 (12.49 g, 32.3 mmol) and Boc2O
(12.54 g, 57.4 mmol, 1.5 equiv.) in methanol (100 mL) was
stirred at 258C until tlc analysis indicated completion of the
reaction. After evaporation of the solvent the resulting white
foamy solid was washed several times with warm petroleum
ether to remove excess Boc2O. Evaporation yielded 15.15 g
(96%) 5 as a white solid. RF: 0.95 (CHCl3±MeOHˆ4:1), mp
92±948C (decomposition); [a]2D0ˆ216.0 (cˆ1.05, MeOH);
1H NMR (250 MHz, CDCl3): d (ppm)ˆ1.32 (s, 9H,
C(CH3)3), 2.76 (br, 4H), 3.20±4.15 (m, 6H), 3.64 (s, 3H,
CO2CH3), 4.95 (d, Jˆ12.2 Hz, 1H, benzyl-HA), 5.13 (d,
Jˆ12.2 Hz, 1H, benzyl-HB), 6.08 (br, 0.4H, NH), 7.13±
7.37 (m, 5H, Ph); FAB-MS: C22H32N2O10 calcd
m/zˆ486.2213, found m/zˆ509.2 (M1Na1).
(cˆ1.83, MeOH); 1H NMR (400 MHz, CDCl3):
d
(ppm)ˆ0.00 (s, 6H, Si(CH3)2), 0.83 (s, 9H, SiC(CH3)3),
1.38 (s, 9H, C(CH3)3), 3.00±4.30 (m, 7H, 3-CH2, 5-CH2,
6-CH, CH2±O), 6.20 (br, 1H, NH); HR-MS:
C16H32N2O4Si calcd m/zˆ344.2131 found m/zˆ344.2138
(M1).
Ethyl (4-tert.-butoxycarbonyl-6(S)-tert.-butyldimethyl-
siloxymethyl-2-oxo-piperazin-1-yl)-acetate (9). A solu-
tion of 8 (4.00 g, 11.6 mmol) in dry DMF was treated
with sodium hydride (60% oil dispersion in mineral oil,
480 mg, 23.2 mmol) at 08C under argon. After addition of
18-crown-6 (5 mg), ethyl bromoacetate (2.9 g, 17.4 mmol)
was added dropwise under argon at 08C. The resulting
reaction mixture was stirred at 258C for 4 h before being
treated with brine (20 mL) at 08C. The aqueous layer was
separated and extracted with Et2O (3£20 mL), and the
combined extracts were dried (MgSO4), and concentrated
in vacuo. Column chromatography (petroleum ether±ethyl
acetateˆ1:1) afforded 4.8 g (96%) 9 as a pale yellow oil. RF:
0.72 (petroleum ether±ethyl acetateˆ1:1); [a]2D0ˆ156.1
Methyl [2(S)-benzyloxycarbonylamino-3-hydroxy-propyl-
(tert.-butoxycarbonyl)-amino]-acetate (6). To an ice cold
suspension of 5 (2.48 g, 5.1 mmol) in water (50 mL) a solu-
tion of sodium metaperiodate (3.28 g, 19.8 mmol) in water
(30 mL) was added dropwise. Since the mixture turns sticky
after a few minutes, CH2Cl2 (20 mL) is added and the
mixture is stirred for 1 h at 08C. The aqueous layer is sepa-
rated and extracted with CH2Cl2 (3£20 mL). The combined
organic solutions are dried over MgSO4 and the solvent
evaporated. To avoid racemization of the a-amino aldehyde
the white solid is rapidly dissolved in methanol (50 mL) and
sodium borohydride (820 mg, 21.68 mmol) is added at 08C.
After stirring for 1 h at 08C the solvent is removed in vacuo,
and the residue is dissolved in water and neutralized with a
sat. aqueous solution of NH4Cl. The aqueous solution is
(cˆ1.50, MeOH); 1H NMR (400 MHz, CDCl3):
d
(ppm)ˆ0.00 (s, 6H, Si(CH3)2), 0.82 (s, 9H, SiC(CH3)3),
1.23 (t, Jˆ7.6 Hz, 3H, CO2CH2CH3), 1.40 (s, 9H, (CH3)3),
3.15±3.48 (m, 2H), 3.53±3.68 (m, 2H), 3.72±3.90 (d,

È
F. Rubsam et al. / Tetrahedron 56 (2000) 8481±8487
8485
Jˆ17 Hz, 2H), 4.05±4.40 (m, 2H), 4.15 (q, Jˆ7.6 Hz, 2H,
CO2CH2CH3), 4.45±4.60 (d, Jˆ17 Hz, 1H); HR-MS:
C20H38N2O6Si calcd m/zˆ430.2499 found m/zˆ430.2503
(M1).
mixture was stirred for another hour at 08C. To drive the
reaction to completion, the mixture was stirred for addi-
tional 12 h at 258C. After evaporation to dryness the residue
was dissolved in ethyl acetate. The organic phase was
extracted with water, 5% aqueous citric acid, water, satu-
rated aqueous NaHCO3 and water, dried (MgSO4), and
concentrated in vacuo. Column chromatography (CH2Cl2±
MeOHˆ10:1) afforded 183 mg (89%) 13 as a colorless oil.
RF: 0.80 (CH2Cl2±MeOH±NH3ˆ40:10:1); [a]2D0ˆ123.4
Ethyl (4-tert.-butoxycarbonyl-6(S)-hydroxymethyl-2-oxo-
piperazin-1-yl)-acetate (10). A solution of 9 (921 mg,
2.1 mmol) in THF (20 mL) at 08C was treated dropwise
with a solution of TBAF (1.01 g, 3.21 mmol) in THF
(5 mL) under argon. Saturated aqueous NH4Cl (5 mL) and
ethyl acetate (5 mL) were added, and the aqueous phase was
extracted with ethyl acetate (3£10 mL). The combined
organic phases were washed with water (3£10 mL) and
brine (3£10 mL), dried (MgSO4), and concentrated in
vacuo. Column chromatography (petroleum ether±ethyl
acetateˆ1:1) afforded 662 mg (98%) 10 as a colorless oil.
(cˆ1.22, MeOH); 1H NMR (400 MHz, CDCl3):
d
(ppm)ˆ1.05±1.32 (m, 3H, CH3), 1.65±2.00 (m, 2H),
2.18±2.60 (m, 2H), 3.10±3.30 (m, 2H), 3.31±4.30 (m,
11H), 4.48±5.20 (m, 1H), 5.00±5.28 (m, 3H), 7.20±7.43
(m, 5H, Ph); FAB-MS: C21H29N3O7 calcd m/zˆ435.2
found m/zˆ436.2 (M1H1).
RF:
0.10
(petroleum
ether±ethyl
acetateˆ1:1);
Ethyl
[6(S)-hydroxymethyl-2-oxo-4-(2-benzyloxycar-
[a]2D0ˆ173.0 (cˆ1.00, MeOH); 1H NMR (400 MHz,
CDCl3): d (ppm)ˆ1.23 (t, Jˆ7.2 Hz, 3H, CH3), 1.40 (s,
9H, C(CH3)3), 3.20±4.40 (m, 11H, 5£CH2, CH); FAB-
MS: C14H24N2O6 calcd m/zˆ316.1 found m/zˆ316.1 (M1).
bonylamino-acetyl)-piperazin-1-yl]-acetate (14). A solu-
tion of 11 (100 mg, 0.47 mmol) and Cbz-glycine (109 mg,
0.52 mmol) in CH2Cl2 was treated with triethylamine
(90 mg, 0.9 mmol) at 08C. After stirring for 10 min, EDC
(96 mg, 0.5 mmol) was added and the reaction mixture was
stirred for another hour at 08C. To drive the reaction to
completion, the mixture was stirred for additional 12 h at
258C. After evaporation to dryness the residue was
dissolved in ethyl acetate. The organic phase was extracted
with water, 5% aqueous citric acid, water, saturated aqueous
NaHCO3 and water, dried (MgSO4), and concentrated in
vacuo. Column chromatography (CH2Cl2±MeOHˆ10:1)
afforded 82 mg (43%) 12 as a colorless oil. RF: 0.81
(CH2Cl2±MeOH±NH3ˆ40:10:1)); [a]2D0ˆ116.4 (cˆ1.10,
MeOH); 1H NMR (400 MHz, CDCl3): d (ppm)ˆ0.70±
1.00 (m, 2H), 1.00±1.70 (m, 3H, CH3), 3.14±4.85 (m,
12H), 5.00±5.15 (m, 2H, benzylic-CH2), 5.60±5.82 (m,
1H), 7.20±7.50 (m, 5H, Ph); HR-MS: C19H25N3O7 calcd
m/zˆ407.1693 found m/zˆ407.1696 (M1).
Ethyl (6(S)-hydroxymethyl-2-oxo-piperazin-1-yl)-acetate
A
hydrochloride (11).
solution of 10 (300 mg,
0.95 mmol) in diethyl ether was cooled to 08C and treated
with gaseous HCl for 15 min. The precipitate was ®ltered,
washed several times with cold diethyl ether, and dried in
vacuo to afford 141 mg (59%) 11 as an off-white solid. Mp
115±1188C (decomposition); RF: 0.59 (CH2Cl2±MeOH±
1
NH3ˆ40:10:1); [a]2D0ˆ112.2 (cˆ2.90, DMSO); H NMR
(200 MHz, DMSO-d6): d (ppm)ˆ1.18 (t, Jˆ6.6 Hz, 3H,
CH3), 2.95±4.60 (m, 12H), 9.35 (br, 3H, NH); FAB-MS:
C9H17N2O4Cl calcd m/zˆ252.0877 found m/zˆ217.1
(M2Cl1).
Ethyl [6(S)-hydroxymethyl-2-oxo-4-(5-(bis-tert.-butoxy-
carbonyl-guanidino)-valeriyl)-piperazin-1-yl]-acetate (12).
A solution of 11 (100 mg, 0.40 mmol) and 5-(Bis-tert.-
butoxycarbonyl-guanidino)-valeric acid25 (150 mg, 0.42
mmol) in 5 mL CH2Cl2 was treated with triethylamine
(90 mg, 0.9 mmol) at 08C. After stirring for 10 min, EDC
(96 mg, 0.5 mmol) was added and the reaction mixture was
stirred for another hour at 08C. To drive the reaction to
completion, the mixture was stirred over night at room
temperature. After evaporation to dryness the residue was
dissolved in ethyl acetate. The organic phase was extracted
with water, 5% aqueous citric acid, water saturated aqueous
NaHCO3 and water, dried (MgSO4), and concentrated in
vacuo. Column chromatography (CH2Cl2±MeOHˆ10:1)
afforded 60 mg (27%) 12 as a colorless oil. RF: 0.80
(CH2Cl2±MeOH±NH3ˆ40:10:1); [a]2D0ˆ156.8 (cˆ1.64,
Methyl 2-(S)-[(2-(S)-benzyloxycarbonylamino-3(R), 4(S),
5(R), 6-tetrahydroxy-hexyl)-amino]-6-tert.-butoxycar-
bonylamino-hexanoate (15). A suspension of Cbz-gluco-
samine 2 (1.66 g, 5.29 mmol) and e-Boc-LysOMe´HCl
(1.73 g, 5.79 mmol) in methanol (130 mL) was treated
with sodium cyanoborohydride (0.73 g, 11.64 mmol). The
mixture was stirred for 8 h under re¯ux before the volatiles
were removed in vacuo. Flash chromatography (CH2Cl2±
MeOH±NH3ˆ40:10:1) of the resulting white solid afforded
2.85 g (97%) 15 as a white solid. Mp 102±1048C; RFˆ0.67
(CH2Cl2±MeOH±NH3ˆ40:10:1); [a]2D0ˆ25 (cˆ1.00,
1
MeOH); H NMR (200 MHz, CDCl3): d (ppm)ˆ1.40 (s,
9H, (CH3)3), 1.20±1.70 (m, 6H, 3£CH2), 1.80±2.13 (m,
2H, CH2), 2.75±4.16 (m, 12H), 3.69 (s, 3H, CO2CH3),
4.65±4.90 (br, 2H), 4.97±5.20 (m, 2H, Benzyl-CH2),
7.20±7.45 (m, 5H, Ph); FAB-MS: C26H43N3O10 calcd
1
MeOH); H NMR (250 MHz, CDCl3): d (ppm)ˆ1.42 (t,
Jˆ7.0 Hz, 3H, CH3), 1.62 (s, 9H, C(CH3)3), 1.67 (s, 9H,
C(CH3)3), 1.78±1.88 (br, 4H), 2.55±2.73 (br, 2H), 3.35±
4.80 (m, 11H), 4.35 (q, Jˆ7.0 Hz, 2H); FAB-MS:
C25H43N5O9 calcd m/zˆ557,3 found m/zˆ558,2 (M1H1).
m/zˆ557.2948
found
m/zˆ558.3
(M1H1);
C26H43N3O10´1/2H2O, calcd C 55.09, H 7.83, N 7.42;
found C 55.29, H 7.79, N 7.74.
Ethyl
bonylamino-butyryl)-piperazin-1-yl]-acetate (13).
solution of 11 (100 mg, 0.47 mmol) and Cbz-g-butyric
acid (112 mg, 0.52 mmol) in CH2Cl2 was treated with
triethylamine (90 mg, 0.9 mmol) at 08C. After stirring for
10 min, EDC (96 mg, 0.5 mmol) was added and the reaction
[6(S)-hydroxymethyl-2-oxo-4-(4-benzyloxycar-
A
Methyl 2-(S)-[(2-(S)-benzyloxycarbonylamino-3(R), 4(S),
5(R), 6-tetrahydroxy-hexyl)-tert.-butoxycarbonylamino]-
6-tert.-butoxycarbonylamino-hexanoate (16). A solution
of 15 (1.00 g, 1.79 mmol) in methanol (20 mL) was treated
with Boc2O (5.86 g, 2.69 mmol). After stirring for 12 h at
258C the solvent was removed in vacuo, and the residue was

È
F. Rubsam et al. / Tetrahedron 56 (2000) 8481±8487
8486
washed several times with warm petroleum ether to remove
residual Boc2O. Removal of the volatiles in vacuo afforded
1.13 g (96%) 16 as a white solid. Mp .2408C; RFˆ0.77
(CH2Cl2±MeOH±NH3ˆ40:10:1); [a]2D0ˆ236.9 (cˆ0.98,
cooled to 08C. After addition of DBU (155 mg, 1.02 mmol),
the mixture was stirred for another 30 min before the alde-
hyde component (dissolved in 10 mL THF) was added drop-
wise through a septum. After 0.5±1 h the reaction was
complete according to tlc and was quenched with aqueous
saturated NH4Cl and then extracted with diethyl ether
(3£50 mL). The combined organic extracts were dried
over MgSO4, ®ltered and evaporated in vacuo. Column
chromatography (CH2Cl2±MeOHˆ10:1) afforded 119 mg
(56% for 2 steps) 19 as a white solid. Mp 135±1388C;
RFˆ0.46 (CH2Cl2±MeOHˆ10:1); [a]2D0ˆ184.1 (cˆ1.55,
1
MeOH); H NMR (250 MHz, CDCl3): d (ppm)ˆ1.40 (s,
9H, (CH3)3), 1.46 (s, 9H, (CH3)3), 1.20±1.70 (m, 6H,
3£CH2), 1.80±2.13 (m, 2H, CH2), 2.75±4.16 (m, 12H),
3.69 (s, 3H, CO2CH3), 4.65±4.90 (br, 2H), 4.97±5.20 (m,
2H, Benzyl-CH2), 5.58±5.97 (br, 1H, NH), 7.20±7.45 (m,
5H, Ph); FAB-MS: C31H51N3O12 calcd m/zˆ657.3473 found
m/zˆ658.3 (M1H1).
1
CH2Cl2); H NMR (400 MHz, CDCl3): d (ppm)ˆ1.20 (t,
4-tert.-Butoxycarbonyl-3(S)-(4-tert.-butoxycarbonylamino-
butyl)-6(S)-[10(R), 20(S), 30(R), 40-tetrahydroxybutyl]-
Jˆ7 Hz, 3H, CH3), 1.32 (s, 9H, (CH3)3), 1.37 (s, 9H,
(CH3)3), 1.25±1.55 (m, 4H, CH2), 1.65±1.78 (m, 1H),
1.80±1.96 (m, 1H), 3.05 (br, 2H), 3.12±3.40 (m, 1H),
3.95±4.20 (m, 1H), 4.10 (q, Jˆ7 Hz, 2H, CO2CH2CH3),
4.30±4.70 (m, 2H), 5.90 (d, Jˆ15.3 Hz, 1H, CO2CHvCH),
6.15 (br, 1H, amide-NH), 6.85 (dd, Jˆ15.5, 4.8 Hz, 1H,
CHvCH±CH); HR-MS: C23H39N3O7 calcd m/zˆ469.2799
found m/zˆ469.2794 (M1); C23H39N3O7´1/3H2O, calcd C
58.09, H 8.41, N 8.84, found C 58.05, H 8.17, N 8.78.
piperazin-2-one (17).
A solution of 16 (800 mg,
1.22 mmol) in dry methanol (20 mL) was hydrogenated at
258C and 3 bar pressure in the presence of 10% Pd-C. Upon
completion the catalyst is ®ltered off over celite and the
solvent was removed in vacuo. Column chromatography
(CH2Cl2±MeOHˆ2:1) afforded 324.8 mg (54%) 17 as a
white solid. Mp 1488C; RFˆ0.72 (CH2Cl2±MeOHˆ2:1
11% NH3); [a]2D0ˆ176.1 (cˆ2.39, MeOH); 1H NMR
(250 MHz, CDCl3±D2O): d (ppm)ˆ1.10±2.00 (m, 24H,
2£(CH3)3 and 3£CH2), 2.92±3.12 (m, 2H), 3.14±3.30 (d,
Jˆ12 Hz, 1H), 3.40±3.90 (m, 5H), 3.95±4.10 (d,
Jˆ13.6 Hz, 1H), 4.30±4.50 (br, 1H), 4.90±5.10 (br, 1H);
FAB-MS: C22H41N3O9 calcd m/zˆ491.2843 found m/
zˆ492.3 (M1H1) and 514.3 (M1Na1).
Acknowledgements
R. M. is grateful to the Land Baden-WuÈrttemberg for a
È
scholarship from the Landesgraduiertenforderung.
Methyl 3-[4-tert.-butoxycarbonyl-3(S)-(4-tert.-butoxycar-
bonyl-amino-butyl)-2-oxo-piperazin-6(S)-yl]-carboxy-
late (18). A solution of 17 (150 mg, 0.31 mmol) in H2O±
MeOHˆ2:1 (20 mL) was cooled to 08C and treated with
sodium metaperiodate (196 mg, 0.92 mmol). After stirring
for 30 min the reaction mixture was treated with solid
NaHCO3 (126 mg, 1.5 mmol) and bromine (160 mg,
1 mmol) was added dropwise over 30 min. After stirring
for additional 45 min the reaction was quenched with
sodium thiosulfate, ®ltered, extracted with chloroform
(3£10 mL), dried (MgSO4), and concentrated in vacuo.
Column chromatography (CH2Cl2±MeOHˆ10:1) afforded
62 mg (47%) 18 as a white solid. Mp 76±788C; RFˆ0.7
(CH2Cl2±MeOHˆ10:1); [a]2D0ˆ172.3 (cˆ1.56, MeOH);
1H NMR (400 MHz, CDCl3): d (ppm)ˆ1.10±2.00 (m,
24H, 2£(CH3)3 and 3£CH2), 3.03±3.17 (m, 2H), 3.29±
3.52 (m, 1H), 3.65 (s, 3H, CO2CH3), 3.98±4.06 (m, 1H),
4.30±4.70 (m, 3H), 6.20 (br, 1H, NH); FAB-MS:
C20H35N3O7 calcd m/zˆ429.2475 found m/zˆ430.2
(M1H1).
References
1. Giannis, A.; Kolter, T. Angew. Chem. 1993, 105, 1303±1326;
Angew. Chem., Int. Ed. Engl. 1993, 32, 1244±1267.
2. Giannis, A.; RuÈbsam, F. In Advances in Drug Research, Testa,
B. Ed.; Academic: San Diego, 1997; 29, pp 1±78.
3. Veber, D. F.; Freidinger, R. M. Trends Neurosci. 1985, 8, 392±
396.
4. Burt, S. K.; Greer, J. Annu. Rep. Med. Chem. 1988, 23, 285±
294.
5. Rizo, J.; Gierasch, L. M. Annu. Rev. Biochem. 1992, 61, 387±
418.
6. Toniolo, C. Int. J. Pept. Protein Res. 1990, 35, 287±300.
7. We have previously shown by X-ray analysis of a valine
derived piperazinone, that in the crystal the six-membered hetero-
cycle adopts a distorted halfchair conformation with C5 lying out
of plane: Kolter, T.; RuÈbsam, F.; Giannis, A. Acta Cryst. 1996,
C52, 978±980.
8. DiMaio, J.; Belleau, B. J. Chem. Soc., Perkin Trans. 1 1989,
1687±1689, cited Lit.
9. Kendrick, D. A.; Ryder, H.; Semple, G.; Szelke, M. BioMed.
Chem. Lett. 1992, 2, 9±12.
Ethyl 3-[4-tert.-butoxycarbonyl-3(S)-(4-tert.-butoxycar-
bonylamino-butyl)-2-oxo-piperazin-6(S)-yl]-acrylate
(19). A solution of 17 (250 mg, 0.51 mmol) in MeOH±
H2Oˆ2:1 (25 mL) was cooled to 08C and treated with
sodium metaperiodate (327 mg, 1.53 mmol). After stirring
for 30 min the reaction mixture was extracted with CH2Cl2
(3£10 mL). The combined organic phases were dried
(MgSO4), and concentrated in vacuo. The crude aldehyde
was kept at 08C and used without further puri®cation. Anhy-
drous lithium chloride (45 mg, 1.018 mmol) was suspended
under argon atmosphere in absolute THF (25 mL) and
triethyl phosphonoacetate (225 mg, 1.02 mmol) was
added. The mixture was stirred for 15 min at r.t. and then
10. Batt, A. R.; Kendrick, D. A.; Mathews, E.; Rooker, D. P.;
Ryder, H.; Semple, G.; Szelke, M. Bioorg. Med. Chem. Lett.
1994, 4, 867±872.
11. Askew, B. C.; McIntyre, C. J.; Hunt, C. A.; Claremon, D. A.;
Gould, R. J.; Lynch, R. J.; Armstrong, D. J. BioMed. Chem. Lett.
1995, 5, 475±480.
12. Askew, B. C.; McIntyre, C. J.; Hunt, C. A.; Claremon, D. A.;
Baldwin, J. J.; Anderson, P. S.; Gould, R. J.; Lynch, R. J.; Chang,
C. C.-T.; Cook, J. J.; Lynch, J. J.; Holahan, M. A.; Sitko, G. R.;
Stranieri, M. T. Bioorg. Med. Chem. Lett. 1997, 7, 1531±1536.
13. Logan, M. E.; Goswami, R.; Tomczuk, B. E.; Venepalli, B. R.
Annu. Rep. Med. Chem. 1989, 26, 43±51.

È
F. Rubsam et al. / Tetrahedron 56 (2000) 8481±8487
8487
14. Tong, Y.; Fobian, Y. M.; Wu, M.; Boyd, N. D.; Moeller, K. D.
J. Org. Chem. 2000, 65, 2484±2493.
21. Pfaff, M. Integrin±Ligand Interaction; Eble, J. A., KuÈhn, K.,
Eds.; Springer: Heidelberg, 1997, pp 101±121.
22. Kessler, H.; Haubner, R.; Finsinger, D. Angew. Chem. 1997,
109, 1140±1156; Angew. Chem., Int. Ed. Engl. 1997, 36, 1374±
1389.
15. Theresa, M. W. et al., J. Med. Chem. 1999, 42, 3779±
3784.
16. Hansen, T. K.; Schlienger, N.; Hansen, B. S.; Andersen, P. H.;
Bryce, M. R. Tetrahedron Lett. 1999, 740, 3651±3654.
17. Lohse, A.; Ernholt, B. V.; Bols, M. Acta Chem. Scand. 1998,
52, 499±502.
23. Keenan, R. M.; Miller, W. H.; Kwon, C.; Ali, F. E.; Callahan,
J. F.; Calvo, R. R.; Hwang, S. M.; Kopple, K. D.; Peishoff, C. E.;
Samanen, J. M.; Wong, A. S.; Yuan, C. K.; Huffman, W. F. J. Med.
Chem. 1997, 40, 2289±2292.
18. Kolter, T.; Dahl, C.; Giannis, A. Liebigs Ann. Chem. 1995,
625±629.
24. Lichtenthaler, F. W.; Jarglis, P.; Lorenz, K. Synthesis 1988,
10, 790±792.
19. Bergmann, M.; Zervas, L. Ber. Dtsch. Chem. Ges. 1932, 65,
1192±1201.
25. 5-(Bis-tert.-butoxycarbonyl-guanidino)-valeric acid was
a
20. Ruoslahti, E.; Pierschbacher, M. D. Science 1987, 238, 491±
497.
generous gift of E. Addicks, Institute of Organic Chemistry,
University of Karlsruhe.