Synthesis of a novel thyrotropin releasing hormone (TRH) analog incorporating a piperazin-2-one ring

Article abstract of DOI:10.1002/(SICI)1522-2675(20000412)83:4<722::AID-HLCA722>3.0.CO;2-Z

The synthesis of a Thyrotropin Releasing Hormone (TRH) analog incorporating a piperazin-2-one ring is described. This conformationally restricted peptidomimetic attempts to retain the key recognition elements of the interaction between TRH and its receptor. The synthesis started from the protected dipeptide 2 and proceeded via the 4-nitrobenzenesulfonyl-activated intermediate 3, which was then cyclized, N-acylated, and deprotected to yield the target compound 1.

Full text of DOI:10.1002/(SICI)1522-2675(20000412)83:4<722::AID-HLCA722>3.0.CO;2-Z

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Helvetica Chimica Acta ± Vol. 83 (2000)
Synthesis of a Novel Thyrotropin Releasing Hormone (TRH) Analog
Incorporating a Piperazin-2-one Ring
1
by Ulhas Bhatt ) and George Just*
Department of Chemistry, McGill University, 801 Sherbrooke St.(W), Montreal, Quebec, Canada H3A 2K6
The synthesis of a Thyrotropin Releasing Hormone (TRH) analog incorporating a piperazin-2-one ring is
described. This conformationally restricted peptidomimetic attempts to retain the key recognition elements of
the interaction between TRH and its receptor. The synthesis started from the protected dipeptide 2 and
proceeded via the 4-nitrobenzenesulfonyl-activated intermediate 3, which was then cyclized, N-acylated, and
deprotected to yield the target compound 1.
Introduction. ± Thyrotropin Releasing Hormone (TRH) was the first hypothalamic
hormone isolated and characterized [1]. This tripeptide (GlpHisProNH2) displays dual
functions, acting both as a hormone and as a neuropeptide. TRH was initially classified
as a hormone that releases prolactin and thyrotropin from the pituitary, but it has also
been shown to act as a neurotransmitter. This characterization is based primarily on its
analeptic properties and its ability to reverse the sedation and hypothermia induced by
pentobarbital, ethanol, and diazepam. TRH augments many of the neurotransmitter
systems implicated in memory storage and retrieval independently of its hormonal
activity, and it is effective in ameliorating many forms of memory disruption. TRH-
Related alterations are associated with various diseases, including Alzheimerꢀs disease,
depression, schizophrenia, epilepsy, and metabolic disorders [2].
The biological activities of TRH are brought about by the interaction of TRH with
its receptor (TRH-R). This receptor is a member of a large family of transmembrane
proteins (TM) that couple to G-proteins. To understand TRH signaling at the
molecular level, it is important to determine the structure of the complex between TRH
and its receptor. The transmembrane nature of these G-protein-coupled receptors
makes it nearly impossible to determine the three-dimensional structure of the TRH
receptor. Moreover, all attempts to crystallize this receptor with a bound ligand have
been unsuccessful. Consequently, the only attempts to delineate the biologically active
conformation of TRH have been syntheses of various conformationally restricted
analogs of TRH and investigations of their activities2). Through these studies, it is
known that there are at least three primary sites of interaction between TRH and its
receptor. These include a) the C-terminal amide group of proline with Arg-306 in
transmembrane region 7 (TM-7); b) the imidazole ring of histidine with Tyr-282 in TM-
1
)
Current address: Institut für Organische Chemie, Universität Hannover, Schneiderberg 1B, D-30167
Hannover.
For the synthesis of various TRH analogs, which have some or all of these functional groups, see [3].
2
)

Helvetica Chimica Acta ± Vol. 83 (2000)
723
6; c) the carboxamide functionality of pyroglutamic acid (ˆ 5-oxoproline, Glp) with
Tyr-106 and Asn-110 in TM-3. TRH is rapidly metabolized by the body, and, hence, an
analog that retains these interactions while imparting rigidity to the peptide backbone
would be an ideal TRH analog.
Several peptidomimetics incorporating a piperazin-2-one ring have shown interest-
ing biological properties [4], and therefore it was intriguing to investigate whether the
target TRH analog 1 would bind to the TRH receptor. In this TRH analog, the three
major sites of interaction with the TRH receptor are retained. The N-terminal
pyroglutamic acid is kept intact, histidine is made a part of the piperazin-2-one ring, and
valine has replaced proline. The synthesis of this analog 1 thus involves the generation
of the piperazin-2-one ring with His and Val, installation of Glp on the free N-terminus
of this piperazin-2-one, and converting the C-terminus of valine to amide function. We
have recently described syntheses of piperazin-2-ones from amino acids [5], and our
goal was to adapt these methods to the synthesis of the TRH analog 1.
Results and Discussion. ± The protection of the imidazole moiety of histidine is of
critical importance in this synthesis. The use of tosyl, 2,4-dinitrophenyl, and benzyl
protecting groups all led to premature deprotection during the cyclization step (vide
infra). The synthesis was eventually carried out with trityl (triphenylmethyl, Tr)
protection for the imidazole moiety of histidine. In the first step, N-(Fmoc)His(Tr)OH
(Fmoc ˆ (9H-fluoren-9-yl)methoxycarbonyl) was coupled with valine methyl ester
hydrochloride in presence of BOP and Et3N to give the protected dipeptide 2 in nearly
quantitative yield (Scheme)3). 4-(Aminomethyl)piperidine was used to deprotect the
Fmoc group [6] from the dipeptide, the reaction of which with 4-nitrobenzenesulfonyl
chloride and Et3N in CHCl3 gave the 4-nitrobenzenesulfonyl-protected dipeptide 3. On
heating 3 at 608 in DMF with excess 1,2-dibromoethane and K2CO3, 2-oxopiperazine
derivative 4 was obtained. Use of tosyl, 2,4-dinitrophenyl, or benzyl groups for
protection of the imidazole ring resulted in premature cleavage of these groups to
varying degrees. Trityl protection for the imidazole ring was thus suitable for our
purposes. The (4-nitrophenyl)sulfonyl group on 4 was removed by PhSH/K2CO3 in
DMF, and the piperazin-2-one 5 was coupled with pyroglutamic acid with BOP as the
3
)
Commercially available, optically pure amino-acid derivatives were used, but no effort was made to
1
ascertain the optical purity of the compounds prepared. H- and 13C-NMR spectra of the compounds did
not show any evidence that racemization had occurred at any stage. So, for convenience, the formulas given
represent a single possible diastereoisomer, although the optical purity has not rigorously been established.

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Helvetica Chimica Acta ± Vol. 83 (2000)
Scheme
a) 4-(Aminomethyl)piperidine; 99%. b) (4-Nitrophenyl)sulfonyl chloride, CH2Cl2; 94%. c) K2CO3, 1,2-
dibromoethane, DMF; 40%. d) K2CO3, PhSH, DMF; 88%. e) GlpOH, BOP, CH2Cl2; 79%. f) 1. LiOH, MeOH,
H2O; 2. C6F5OH, DCC, DMAP, CH2Cl2; 3. NH3, EtOH; 52% (3 steps). g) 5% AcOH in MeOH, 81%. BOP ˆ
(Benzotriazol-1-yloxy)tris(dimethylamino)phosphonium hexafluorophosphate, DCC ˆ N,N'-dicyclohexylcar-
bodiimide, DMAP ˆ 4-(dimethylamino)pyridine.
coupling agent to give the coupled product 6. The methyl ester was hydrolyzed with
LiOH in aqueous MeOH, and the corresponding acid reacted with C6F5OH in the
presence of DCC and DMAP to give the activated pentafluorophenyl ester. Its
treatment with 2m ethanolic NH3 solution gave the amide 7 in 52% yield with respect to
6. The amide 7 was reacted with 5% AcOH solution in MeOH at reflux for 6 h to
deprotect the Tr group. Aqueous workup resulted in the desired product partitioning
into the aqueous layer, from which it was difficult to purify. Evidently, the compound
was extremely polar and, therefore, could not be isolated in this manner. The solubility
pattern of the peptide and the trityl acetate are very different, as was evident
from previous unsuccessful attempts, and this property was then exploited to isolate
the desired tripeptide derivative. The polar peptide is expected to migrate into a

Helvetica Chimica Acta ± Vol. 83 (2000)
725
MeOH/H2O layer, whereas the non-polar trityl acetate will preferentially dissolve in a
non-polar hexanes layer. Therefore, deprotection was carried out on 7 again, and, this
time, the crude product was partitioned into a biphasic hexanes/aqueous MeOH
system. As anticipated, the peptide 1 went into the aqueous MeOH layer, and the trityl
acetate into the hexanes layer. Separation of the two phases and evaporation of the
aqueous MeOH phase under vacuum gave the desired product 1.
This piperazin-2-one derivative was tested for its affinity towards the TRH
receptor. Binding studies were carried out on the TRH receptor from rat forebrain
membrane with 3-[3H3]MeHis2) TRH. Reactions were carried out in 20 mm Na2HPO4
(pH 7.4) at 48 for 3 ± 4 h. The reactions were terminated by rapid vacuum filtration onto
glass fibers. The radioactivity trapped onto the fibers was determined and compared to
control values in order to evaluate any interactions of the analog with the TRH
receptor. The binding studies showed that the Ki value for this TRH analog was >
10 6 m, thus indicating weak binding to the receptor.
Conclusion. ± This report describes the first synthesis of a novel TRH analog
incorporating a piperazin-2-one structure. Altough this particular analog did not show a
high affinity for the TRH receptor, there is a possibility of synthesis of better analogs by
varying the amino-acid side chains or by increasing the size of the ring. These results
can perhaps pave the way for analogs with greater flexibility for enhanced interaction
with the TRH receptor.
Financial support from McGill University, NSEARC, and Astra Research Centre, Montreal, is gratefully
acknowledged. The authors thank Dr. Mamer, McGill Unversity, for the measurement of all high-resolution
mass spectra.
Experimental Part
General. Amino acids were purchased from Chem-Impex International or Aldrich Chemical Company Inc.
TLC: on silica gel thin-layer sheets 60 F 254 from Merck. Column chromatography (CC): with silica gel (40 ±
63 mm). 1H- and 13C-NMR spectra: Varian UNITY 500 spectrometer at 500 and 125 MHz, resp. 3-NBA: 3-
Nitrobenzyl alcohol. TRH Binding studies were carried out at Novascreen (www.novascreen.com).
Methyl 2-(2-{[(9H-Fluoren-9-yl)methoxycarbonyl]amino}-3-[1-(triphenylmethyl)-1H-imidazol-4-yl]pro-
panamido]-3-methylbutanoate (2). A soln. of N-(Fmoc)His(Tr)OH (1.510 g, 2.434 mmol) in 2.5 ml of dry
CH2Cl2 was treated with valine methyl ester hydrochloride (0.467 g, 2.783 mmol), BOP (1.181 g, 2.670 mmol),
and Et3N (0.40 ml, 2.70 mmol). The soln. was stirred for 12 h, then washed with 1n HCl, 10% NaHCO3, and
H2O, dried (MgSO4), and filtered. Solvent evaporation gave the crude, which was chromatographed (hexanes/
AcOEt 1:1) to yield 2 (1.348 g, 99%). White foam. Rf (hexanes/AcOEt 1:1) 0.15. 1H-NMR (500 MHz, CDCl3):
0.85 ± 0.90 (m, 2 Me); 2.10 ± 2.20 (m, Me2CH); 3.00 ± 3.10 (m, CH2 imidazole); 3.66 (s, MeO); 4.22 ± 4.26 (m,
CH); 4.30 ± 4.40 (m, CH2); 4.49 (dd, J ˆ 8.0, 4.5, CHCOOMe); 4.58 (m, CH); 6.69 (s, CˆCH); 6.89 (d, J ˆ 6.5,
1 NH); 7.10 ± 7.75 (m, 23 arom. H); 7.83 (d, J ˆ 7.5, NHCO). 13C-NMR (125 MHz, CDCl3): 17.67; 18.89; 30.30;
30.92; 46.95; 51.82; 54.71; 57.29; 67.06; 75.18; 119.25; 119.76; 125.14; 125.08; 126.89; 126.90; 127.47; 127.88;
129.58; 138.21; 141.06; 142.15; 143.74; 143.81; 171.22; 171.84. HR-FAB-MS (3-NBA): 733.3393 ([C46H44N4O5 ‡
‡
H] ; calc. 733.3390).
Methyl 3-Methyl-2-{2-(4-nitrobenzenesulfonamido)-3-[1-(triphenylmethyl)-1H-imidazol-4-yl]propanami-
do}butanoate (3). A soln. of dipeptide 2 (0.450 g, 0.614 mmol) was taken in 5 ml of CHCl3 and 5 ml 4-
(aminomethyl)piperidine was added. The soln. was stirred for 2 h, diluted with CHCl3, and washed twice with
sat. NaCl soln. It was then washed with phosphate buffer (prepared by adding 90 g of NaH2PO4 ´ H2O and 32.7 g
of Na2HPO4 in 500 ml of dist. H2O; pH 5.5), H2O, dried (MgSO4), and filtered. Partial solvent evaporation i.v.
gave the free dipeptide as a soln. in CHCl3. It was taken with 4-nitrobenzenesulfonyl chloride (0.163 g,
0.735 mmol) and Et3N (0.10 ml, 0.717 mmol), and the soln. was stirred for 5 h. It was washed with 1n HCl, 10%

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Helvetica Chimica Acta ± Vol. 83 (2000)
NaHCO3, and H2O, dried (MgSO4), and filtered. Solvent evaporation gave 3 (0.400 g, 93.6%). M.p. 192 ± 1948.
1
Rf (hexanes/AcOEt 1:2) 0.32. H-NMR (500 MHz, CDCl3): 0.72 (d, J ˆ 7.0, Me); 0.76 (d, J ˆ 6.5, Me); 2.00 ±
2.10 (m, Me2CH); 2.70 ± 3.10 (m, CH2); 3.69 (s, MeO); 4.08 ± 4.10 (m, CH); 4.36 (dd, J ˆ 9.0, 5.0, CHCOOMe);
6.60 (s, CˆCH); 7.00 ± 7.36 (m, CHˆN, 3 Ph); 7.59 (d, J ˆ 9.0, CONH); 8.09 (d, J ˆ 8.5, 2 H, C6H4); 8.30 (d, J ˆ
8.5, 2 H, C6H4). 13C-NMR (125 MHz, CDCl3): 17.50; 18.73; 29.81; 30.86; 51.92; 56.73; 57.28; 73.45; 119.68;
124.10; 127.96; 128.08; 128.46; 129.48; 135.55; 138.18; 141.80; 145.63; 149.87; 169.71; 171.30. HR-FAB-MS (3-
‡
NBA): 696.2490 ([C37H37N5O7S ‡ H] ; calc. 696.2492).
Methyl
3-Methyl-2-(4-(4-nitrobenzenesulfonamido)-2-oxo-3-{[1-(triphenylmethyl)-1H-imidazol-4-yl]-
methyl}piperazin-1-yl)butanoate (4). A soln. of 3 (0.093 g, 0.134 mmol) in 1 ml of dry DMF was treated with
K2CO3 (0.143 g, 1.032 mmol), and the soln. was heated at 608 for 30 min. Then, 1,2-dibromoethane (0.11 ml,
1.277 mmol) was added. The soln. was stirred for 14 h, and then solvent was evaporated under high vacuum.
AcOEt was added, and the soln. was washed with 1n HCl, H2O, dried (MgSO4), and filtered. Solvent
evaporation gave the crude, which was chromatographed (AcOEt/hexanes 1:1) to give 4 (0.038 g, 40%). Rf
(hexanes/AcOEt 1:2) 0.18. 1H-NMR (500 MHz, CDCl3): 0.74 (d, J ˆ 6.8, Me); 0.90 (d, J ˆ 6.3, Me); 1.90 ± 2.10
(m, Me2CH); 3.05 ± 3.10 (m, 1 H, CH2N); 3.18 (d, J ˆ 6.5, CH2 imidazole); 3.25 ± 3.50 (m, 1 H, CH2N); 3.53 (s,
MeO); 3.60 ± 3.65 (m, 1 H, CH2N); 4.69 (t, J ˆ 6.5, CH); 4.75 (d, J ˆ 10.5, CHCOOMe); 6.67 (s, CˆCHN);
7.10 ± 7.40 (m, 16 H, CHNCPh3); 7.95 (d, J ˆ 9.0, 2 H, C6H4); 8.26 (d, J ˆ 9.0, 2 H, C6H4). 13C-NMR (125 MHz,
CDCl3): 18.70; 19.36; 27.12; 32.03; 40.42; 41.02; 51.58; 58.81; 60.43; 75.08; 120.11; 124.31; 127.84; 127.88; 128.28;
129.59; 135.49; 138.13; 142.22; 145.11; 149.95; 166.88; 170.61. HR-FAB-MS (3-NBA): 722.2645
‡
([C39H39N5O7S ‡ H] ; calc. 722.2648).
Methyl 3-Methyl-2-(2-oxo-3-{[1-(triphenylmethyl)-1H-imidazol-4-yl]methyl}piperazin-1-yl)butyrate (5). A
soln. of K2CO3 (0.166 g, 1.201 mmol) in 0.2 ml of dry DMF at r.t. was treated with PhSH (0.085 ml, 0.828 mmol).
The soln. was stirred at r.t. for 30 min, and 4 (0.144 g, 0.200 mmol) was added as a 0.5-ml soln. in DMF. The soln.
was stirred at r.t. for 12 h. The solvent was evaporated i.v., and AcOEt was added. The soln. was then washed
with 10% NaHCO3, H2O, dried (MgSO4), and filtered. Evaporation gave the crude, which was chromato-
1
graphed (AcOEt/MeOH 10 :1) to yield 5 (88.3%). Rf (AcOEt) 0.05. H-NMR (500 MHz, CDCl3): 0.80, 0.95
(2d, J ˆ 7.0 each, Me2CH); 2.10 ± 2.20 (m, Me2CH); 2.80 ± 2.85 (br. s, NH); 2.89 (m, 1 H, CH2 imidazole); 2.92 ±
3.20 (m, CH2N); 3.21 (m, 1 H, CH2 imidazole); 3.25 ± 3.50 (m, NCH2); 3.68 (s, MeO); 3.78 (m, CH); 4.92 (d,
J ˆ 11.0, CHCOOMe); 6.66 (s, CH); 7.10 ± 7.40 (m, CHNCPh3). 13C-NMR (125 MHz, CDCl3): 18.74; 19.36;
26.63; 30.35; 41.95; 44.26; 51.60; 59.75; 60.58; 75.02; 119.25; 127.83; 129.57; 138.10; 138.42; 142.27; 170.07; 171.42.
‡
ES-MS: 537 ([C33H36N4O3 ‡ H] ; calc. 537).
Methyl 3-Methyl-2-(2-oxo-4-[(5-oxopyrrolidin-2-yl)carbonyl]-3-{[1-(triphenylmethyl)-1H-imidazol-4-yl]-
methyl}piperazin-1-yl)butanoate (6). A soln. of 5 (0.096 g, 0.178 mmol) in 0.2 ml of dry CH2Cl2 was treated
with pyroglutamic acid (0.028 g, 0.214 mmol), BOP (0.100 g, 0.226 mmol), and Et3N (0.03 ml, 0.215 mmol). The
soln. was stirred at r.t. for 48 h. The soln. was diluted with 20 ml of CH2Cl2 and washed with 10% NaHCO3, 1n
HCl, H2O, dried (MgSO4), and evaporation i.v. gave the crude, which was chromatographed (hexanes/AcOEt
1:4) to yield 6 (0.091 g, 79%). White solid. M.p. 186 ± 1888. Rf (AcOEt/MeOH 10 :1) 0.07. 1H-NMR (500 MHz,
CDCl3): 0.67, 0.91 (2d, J ˆ 6.6 each, 2 Me); 2.00 ± 2.30 (m, CH, CH2CH2); 3.00 ± 3.15 (m, 2 CH2 imidazole);
3.15 ± 3.50 (m, NCH2CH2N); 3.65 (s, MeO); 4.36 ± 4.46 (m, CH); 4.90 (d, J ˆ 10.5, CHCOOMe); 5.07 (dd, J ˆ
5.9, 11.8, CH); 6.58 ± 6.62 (m, NH); 6.65 (s, CˆCH); 7.00 ± 7.30 (m, CˆCHNPh3). 13C-NMR (125 MHz, CDCl3):
18.53; 19.32; 24.26; 26.80; 28.97; 29.29; 40.64; 42.46; 51.79; 53.35; 56.56; 60.32; 75.08; 119.71; 127.86; 129.53;
‡
135.87; 138.41; 142.16; 167.13; 169.13; 170.72; 177.62. HR-FAB-MS (3-NBA): 648.3188 ([C38H41N5O5 ‡ H] ;
calc. 648.3186).
3-Methyl-2-(2-oxo-4-[(5-oxopyrrolidine-2-yl)-carbonyl]-3-{[1-(triphenylmethyl)-1H-imidazol-4-yl]methyl}-
piperazin-1-yl)butanamide (7). A soln. of 6 (12 mg, 0.018 mmol) was dissolved in 0.2 ml of MeOH/H2O 1:1 at
r.t. and treated with 2.4 mg of LiOH. The soln. was stirred for 6 h, and then dil. HCl and AcOEt were added. The
org. layer was collected, dried (MgSO4), filtered, and the solvent was evaporated. The solid obtained was then
treated with 5 mg of C6F5OH, 5 mg of DCC, 1 mg of DMAP, and 3 ml of Et3N in 0.3 ml of dry CH2Cl2 at 08. The
soln. was stirred at 08 for 15 min and then at r.t. overnight. After the reaction was complete (TLC), the soln. was
diluted with CH2Cl2 and filtered to remove DCU. The solvent was evaporated from the filtrate, and then the
filtrate was treated with 1.5 ml of 2m NH3 soln. in EtOH at r.t. for 8 h. 1n HCl was then added to the soln., which
was extracted with AcOEt. The org. layer was dried (MgSO4) and then filtered. Evaporation gave the crude,
which was chromatographed (5% MeOH in AcOEt) to yield 7 (6.2 mg, 52.2%). Yellow oil. Rf (MeOH/AcOEt
1:10) 0.31. 1H-NMR (500 MHz, CDCl3): 0.62 (d, J ˆ 7.0, Me); 0.70 (d, J ˆ 6.0, Me); 2.00 ± 2.40 (m, CH,
CH2CH2); 2.80 ± 2.95 (m, CH2 imidazole); 3.00 ± 3.40 (m, NCH2CH2N); 4.24 ± 4.34 (m, CH); 4.58 (d, J ˆ 6.5,
CHCOOMe); 4.98 ± 5.10 (m, CH); 5.60, 6.20 (2 br. s, CONH2); 6.56 (br. s, NHCO); 6.56 (s, CˆCH); 7.00 ± 7.40

Helvetica Chimica Acta ± Vol. 83 (2000)
727
(m, NˆCHNCPh3). 13C-NMR (125 MHz, CDCl3): 18.15; 19.33; 25.25; 29.16; 29.67; 31.54; 40.42; 41.56; 51.90;
56.48; 61.42; 75.18; 119.79; 127.95; 129.53; 135.73; 138.83; 141.95; 168.04; 169.32; 170.91; 177.79. HR-FAB-MS
‡
(3-NBA): 633.3188 ([C37H40N6O4 ‡ H] ; calc. 633.3189).
2-{3-[(1H-Imidazol-4-yl)methyl]-2-oxo-4-[(5-oxopyrrolidine-2-yl)carbonyl]piperazin-1-yl}-3-methylbutyr-
amide (1). A soln. of 7 (9 mg) in 2 ml MeOH was treated with 0.1 ml of AcOH and refluxed for 6 h. The solvent
was evaporated i.v. to remove excess AcOH, and then MeOH and hexanes were added in equal amounts by
volume. A few drops of H2O were added to create two layers. The two layers were thoroughly mixed, and then
the top hexane layer was removed. The MeOH/H2O mixture was evaporated i.v. to give 1 (4.5 mg, 81%).
1H-NMR (500 MHz, CDCl3): 0.90 ± 1.00 (m, 2 Me); 1.80 ± 2.00 (m, 1 H, CH2); 2.05 ± 2.10 (m, Me2CH); 2.20 ±
2.40 (m, 3 H, CH2CH2); 3.20 ± 3.85 (m, CH2 imidazole, NCH2CH2N); 4.60 ± 4.65 (m, CH); 4.90 ± 4.95 (m,
CHCOOMe); 5.15 ± 5.20 (m, CH); 6.95 (br. s, 1 H, C3H2N2); 8.10 (br. s, 1 H, C3H2N2). 13C-NMR (125 MHz,
CDCl3): 18.73; 19.29; 21.83; 24.39; 27.17; 29.23; 40.90; 42.43; 53.68; 56.58; 60.87; 118.06; 129.90; 134.31; 167.14;
‡
170.22; 176.34; 179.36. HR-FAB-MS (3-NBA): 391.2095 ([C18H26N6O4 ‡ H] ; calc. 391.2094).
REFERENCES
[1] a) J. Bler, F. Enzmann, K. Folkers, C. Y. Bowers, A. V. Schally, Biochem. Biophys. Res. Commun. 1969, 37,
705; b) R. Burgus, T. F. Dunn, D. Desiderio, W. Vale, R. C. Guillemin, R. Acad. Sci. (Paris) 1969, 269, 1870.
[2] A. A. Mazurov, S. A. Andronati, T. I. Korotenko, N. I. Sokolenko, A. I. Dyadenko, Y. E. Shapiro, V. Y.
Gorbatyuk, T. A. Voronina, Bioorg. Med. Chem. Lett. 1997, 5, 2029.
[3] a) G. L. Olson, H.-C. Cheung, E. Chiang, V. S. Madison, J. Sepinwall, G. P. Vincent, A. Winkour, K. A. Gary,
J. Med. Chem. 1995, 38, 2866; b) M. Bos, G. L. Olson, G. P. Vincent, Helv. Chim. Acta 1994, 77, 463; c) K.
Ikeda, Y. Iwasaki, M. Kinoshita, Neurosci. Lett. 1998, 250, 9; d) K. Toide, M. Shinoda, M. Takase, K. Iwata,
H. Yoshida, Eur. J. Pharmacol. 1993, 233, 21.
[4] a) Y. Tong, Y. M. Fobian, M. Wu, N. D. Boyd, K. D. Moeller, Bioorg. Med. Chem. Lett. 1998, 8, 1679; b) H.
Sugihara, H. Fukushi, T. Miyaki, Y. Imai, Z. Terashita, M. Kawamura, Y. Fujisawa, S. Kita, J. Med. Chem.
1998, 41, 489; c) I. Torrini, G. Mastropietro, G. P. Zecchini, M. P. Paradishi, G. Lucente, S. Spisani, Arch.
Pharm. Med. Chem. 1998, 331, 170.
[5] a) U. Bhatt, N. Mohammed, G. Just, E. Roberts, Tetrahedron Lett. 1997, 38, 3679; b) N. Mohamed, U. Bhatt,
G. Just, Tetrahedron Lett. 1998, 39, 8213.
[6] L. A. Carpino, D. Sadat-Aalaee, M. Beyermann, J. Org. Chem. 1990, 55, 1673.
Received November 18, 1999