Parallel solution- and solid-phase synthesis of spirohydantoin derivatives as neurokinin-1 receptor ligands

Article abstract of DOI:10.1016/S0960-894X(02)00488-2

The combination of the 3,5-bis(trifluoromethyl)phenyl group with a spirohydantoin motive as a central scaffold was the basis for the design of a combinatorial library targeted towards the neurokinin-1 receptor. A solution- and solid-phase procedure is described and binding affinities of representative compounds presented.

Full text of DOI:10.1016/S0960-894X(02)00488-2

Bioorganic & Medicinal Chemistry Letters 12 (2002) 2519–2522
Parallel Solution- and Solid-Phase Synthesis of Spirohydantoin
Derivatives as Neurokinin-1 Receptor Ligands
Konrad H. Bleicher,* Yves Wuthrich, Maxime De Boni, Sabine Kolczewski,
Torsten Hoffmann and Andrew J. Sleight
F. Hoffmann-La Roche AG, Pharma Research, CH-4070 Basel, Switzerland
Received 9 April 2002; revised 3 June 2002; accepted 28 June 2002
Abstract—The combination of the 3,5-bis(trifluoromethyl)phenyl group with a spirohydantoin motive as a central scaffold was the
basis for the design of a combinatorial library targeted towards the neurokinin-1 receptor. A solution- and solid-phase procedure is
described and binding affinities of representative compounds presented. # 2002 Elsevier Science Ltd. All rights reserved.
The neurokinin receptors (NK-1, NK-2 and NK-3) all
belong to the target family of 7-transmembrane G-pro-
tein coupled receptors.1,2 They are both expressed in the
periphery (mainly NK-2) as well as in the central ner-
vous system (NK-1 and NK-3). Hence, their therapeutic
utility ranges from CNS indications to the potential
treatment of respiratory and gastric diseases.
Spiropiperidines belong to a well-known class of phar-
macologically relevant molecules showing biological
activity for various target families ranging from enzyme
inhibitors7 to ion channel blockers.8 Such molecular
frameworks are of special interest in the area of G-pro-
tein coupled receptors. Therefore this class of chemical
motives are often referred to as ‘privileged structures’.
Elements of this type are recognized not only by a par-
ticular receptor or enzyme but by various protein family
members. One such representative class of spiropiper-
idines is the spiropiperidino-hydantoin which is a well-
known motive in the serotonin receptor area with Spi-
perone as a very close analogue.9 In addition we
recently reported this scaffold as novel NK-1 antago-
nists.10 This molecular architecture is of particular
interest as a scaffold for combinatorial design due to its
rigid three-dimensional organization and its low mol-
ecular weight. Additionally it can be modified in a
straightforward manner at three sites of the core moiety
which allows the rapid generation of a diverse set of
drug-like compounds.
The endogenous ligands for the neurokinin receptors
are the tachykinins, a group of peptides that all share a
common C-terminal amino acid sequence Phe-X-Gly-
Leu-Met-NH2 where X is either Phe or Val. The most
prominent member of this peptide family is the undeca-
peptide ‘Substance P’ (X=Phe) which shows highest
affinity for the NK-1 receptor, whereas NKA and NKB
(X=Val) are both decapeptides that bind preferentially
to the NK-2 and NK-3 receptor, respectively.3
In the search for novel small molecule ligands targeting
the NK-1 receptor, we initiated a focused library syn-
thesis based on a design strategy that combines the
concepts of both ‘privileged structure’ motives4,5 as well
as the ‘needle’ approach.6 Although both terms have
been widely discussed in public, their differentiation is
somewhat unclear. Therefore, we would like to outline
our understanding of these two terms in some more
detail here.
In contrast to the ‘privileged structure’ motive, the ter-
minus ‘needle’ was described in the literature as a frag-
ment of an active molecule showing very specific
interactions with one particular biological target. One
well known example for such a needle within the GPCR
area is the ortho-substituted biphenyl tetrazole (or bioi-
sosteres), an element present in most angiotensin-1
antagonists currently on the market.11 Within the
neurokinin area, one example of such a needle is the 3,5-
bis(trifluoromethyl)phenyl moiety which has been
described for several NK-1 receptor ligands.12
*Corresponding author. Tel.: +41-6168-70387; fax: +41-6168-81745;
e-mail: konrad.bleicher@roche.com
0960-894X/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(02)00488-2

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K. H. Bleicher et al. / Bioorg. Med. Chem. Lett. 12 (2002) 2519–2522
Following the idea of combining a ‘privileged structure’
motive with the ‘needle’ concept, a library design was
generated as depicted in Scheme 1. The intention was to
couple the 3,5-bis(trifluoromethyl)phenyl needle to all
positions available in the core structure and decorate
one of the remaining nitrogens with various residues.
The three sets of compounds were prepared in a
parallel fashion using solution- and solid-phase
methodologies.
intermediate was generated starting from commercially
available piperidone monohydrate 4. The resulting
spirohydantoin 5 was further modified to give the cor-
responding N-Boc-protected intermediate 6 using stan-
dard conditions. Regioselective N-methylation of the
imide nitrogen of the hydantoin moiety yielded in com-
pound 7. Subsequent 3,5-bis(trifluoromethyl)benzylation
led to the desired Boc-protected spirohydantoin 8.
Finally, deprotection of the piperidine nitrogen by
treatment with trifluoroacetic acid gave the TFA salt of
spirohydantoin 9 ready for further modification.
First, a collection of compounds for library A was syn-
thesized in solution by modification of the piperidine
nitrogen of template 9 as outlined in Scheme 2. This
In addition, compound 9 was further coupled with bro-
moacetic acid to yield intermediate 10. Both derivatives
were used as precursors for additional modification to
result in products of type 11a–e (Table 1). For the
synthesis of the first set of compounds a collection of
aliphatic and aromatic carboxylic acids were coupled
under standard conditions to compound 9 resulting in
spiropiperidine amides of type 11a–c. For the genera-
tion of a second subset of compounds intermediate 10
was treated with secondary amines resulting in the cor-
responding chemotype 11d,e. For the library generation
in-house developed parallel synthesis equipment was
applied. The compounds were worked up via liquid/
liquid extraction and further purified by preparative
HPLC. All products were analyzed and characterized by
LC–MS.
Scheme 1. Targeted library design for NK-1 receptor ligands by com-
bination of a GPCR ‘privileged structure’ motive with the 3,5-bis(tri-
fluoromethyl)phenyl ‘needle’ [Ph(CF3)2].
Table 1 summarizes five representative compounds 11a–e
generated for this particular series showing moderate
NK-1 receptor binding affinities.
For the generation of libraries B and C, a solid-phase
protocol was envisaged since the synthesis of hydan-
toins from the corresponding a-amino acids is well
Table 1. NK-1 receptor affinities of representatives from library A
Compd
R
pKi (hNK-1)
11a
6.34
11b
11c
11d
11e
6.19
5.83
5.77
5.43
Scheme 2. (a) 1.5 equiv NaCN, 5 equiv (NH4)2CO3, EtOH/water
(4:1), 80 ꢀC, 3.5 atm, 18 h; (b) 1.1 equiv Boc2O, 1.1 equiv Et3N, diox-
ane/water (1:1), rt, 2 h; (c) 1.1 equiv NaH, 1.1 equiv MeI, DMA, rt, 72
h; (d) 1.1 equiv NaH, 1.1 equiv 3,5-bis(trifluoromethyl)benzyl bro-
mide, DMA, rt, 48 h; (e) TFA/DCM (1:1), rt, 2 h; (f) 2.2 equiv
RCOOH, 1.1 equiv DIC, rt, 16 h; (g) 2.2 equiv bromoacetic acid, 1.1
equiv DIC, rt, 16 h; (h) 2 equiv amine, rt, 16 h.

K. H. Bleicher et al. / Bioorg. Med. Chem. Lett. 12 (2002) 2519–2522
2521
documented13 and the advantages of solid-phase syn-
thesis for library generation were recognized. Therefore,
the orthogonally protected amino acid 14 was generated
starting from the spirohydantoin 5 as outlined in
Scheme 3. Treatment with excess of Boc anhydride gave
the fully Boc-protected spirohydantoin 12. Ring open-
ing under basic conditions gave the corresponding Boc-
protected amino acid 13. Final protection of the a-
nitrogen resulted in orthogonally protected amino acid
14 ready for coupling to the solid support.
Therefore the cesium salt of 14 was generated and cou-
pled under standard conditions to give resin bound
compound 15. For the generation of compounds of
library B the Boc protecting group was cleaved first via
TFA treatment of resin 15 which resulted in the free
piperidine nitrogen. Standard coupling of 3,5-bis(tri-
fluoromethyl)phenyl carboxylic acid chloride resulted in
resin 16 where R represents the NK-1 needle. Pd(0)
catalyzed Alloc deprotection was achieved under mild
conditions using morpholine as a nucleophile. Subse-
quently, the resin was treated with 4-nitrophenyl
chloroformate to yield resin 17. This activated carba-
mate was converted into the corresponding urea by
treating the resin with primary amines to generate resin
bound urea derivatives 18. A bright yellow color indi-
cated the liberation of 4-nitrophenolate and was there-
fore used for real time monitoring of the reaction
progress. The advantage of this two-step procedure is
obvious due to the accessibility of far more primary
amines compared to the limited number of available
isocyanates. The final step to generate compounds of
type 19a–j was a cyclization and cleavage approach.
This can be achieved under either acidic14 or basic con-
ditions.15 We preferred the resin cleavage via isopropyl
amine treatment due to higher yielding reactions and
ease of parallel handling. Although the compounds
showed very high purity due to the cyclization/cleavage
strategy an additional HPLC purification step was
undertaken to ensure reliable biological data. All com-
pounds were analyzed and characterized by LC–MS.
The solid supported synthesis protocol for the genera-
tion of libraries B and C is schematically outlined in
Scheme 4.
As a first step, amino acid 14 was immobilized on
chloromethylated polystyrene beads (Merrifield resin).
Five representative compounds of collection B with
moderate NK-1 binding affinities are shown in Table 2.
The compounds generated for library C mainly differ at
the piperidine moiety. Once again, a compound subset
of type 3 was obtained by standard coupling of
Scheme 3. (a) 5 equiv Boc2O, 1 equiv DMAP, 1 equiv Et3N, DME, rt,
18 h; (b) 1 N NaOH/DME (2:1), rt, 20 h; (c) 1.5 equiv allyl chloro-
formate, 5% aq Na2CO3 dioxane/water (1:2), rt, 16 h.
Table 2. NK-1 receptor affinities of representatives from library B
Compd
R0
pKi (hNK-1)
19a
6.88
19b
19c
19d
19e
6.86
6.61
6.24
5.69
Scheme 4. (a) 1 equiv Cs2CO3, cat. KI, 14, 60 ꢀC, 16 h; (b) TFA/
CH2Cl2 (1:1), rt, 2 h; (c) 3 equiv RCOCl, 3 equiv Et3N, CH2Cl2, rt, 1
h; (d) 3 equiv RNCO, CH2Cl2, rt, 2 h; (e) 3 equiv RSO2Cl, 3 equiv
Et3N, rt, 4 h; (f) 10 equiv RCHO, 5 equiv NaBH3CN, DMF/MeOH/
CH3COOH (87:10:3), rt, 16 h; (g) 0.2 equiv Pd(PPh3)4, 10 equiv mor-
pholine, CH2Cl2, rt, 2 h; (h) 3 equiv RNCO, CH2Cl2, rt, 2 h; (i) 2 equiv
4-nitrophenyl chloroformate, 2 equiv Et3N, CH2Cl2, rt, 1 h; (j) 2 equiv
amine, CH2Cl2, rt, 16 h; (k) isopropyl amine, 80 ꢀC, 2 h.

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K. H. Bleicher et al. / Bioorg. Med. Chem. Lett. 12 (2002) 2519–2522
Table 3. NK-1 receptor affinities of representatives from library C
In summary, we have described the generation of a
focused compound library targeted towards the neuro-
kinin-1 receptor. The basis for the library design were
both the identification of spiropiperidino-hydantoin as a
‘privileged structure’ motive for G-protein coupled
receptors as well as an NK-1 specific 3,5-bis(tri-
fluoromethyl)phenyl ‘needle’. Follow up workon the
privileged structure/needle design concept for the iden-
tification of further NK-1 ligands will be reported in due
course.
Compd
R0
pKi (hNK-1)
19f/n=1
7.34
Acknowledgements
19g/n=1
19h/n=1
19i/n=0
7.13
6.97
6.49
The authors would like to thank Dr. Mark Rogers-
Evans for critical reading of the manuscript and Mr.
Alain Rudler for biological testing.16
References and Notes
1. Watson, S.; Arkinstall, S. The G-Protein LinkedReceptors
Facts Book; Academic: New York, 1994.
19j/n=0
6.47
2. Flower, D. R. Biochim. Biophys. Acta 1999, 1422, 207.
3. Leroy, V.; Mauser, P.; Gao, Z.; Peet, N. P. Exp. Opin.
Invest. Drugs 1999, 9, 735.
4. Mason, J. S.; Morize, I.; Menard, P. R.; Cheney, D. L.;
Hulme, C.; Labaudiniere, R. F. J. Med. Chem. 1999, 42, 3251.
5. Patchett, A. A.; Nargund, R. P. Annu. Rep. Med. Chem.
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6. Boehm, H.-J.; Boehringer, M.; Bur, D.; Gmuender, H.;
Huber, W.; Klaus, W.; Kostrewa, D.; Kuehne, H.; Luebbers,
T.; Meunier-Keller, N.; Mueller, F. J. Med. Chem. 2000, 43,
2664.
7. Anon. Expert Opion. Ther. Pat. 2000, 10, 125
8. Kolocouris, N.; Kolocouris, A.; Foscolos, G. B.; Fytas, G.;
Neyts, J.; Padalko, E.; Balzarini, J.; Snoeck, R.; Andrei, G.;
De Clercq, E. J. Med. Chem. 1996, 39, 3307.
9. Weinhardt, K. K.; Bonhaus, D. W.; De Souza, A. Bioorg.
Med. Chem. Lett. 1996, 6, 2687.
10. Galley, G.; Godel, T.; Goergler, A.; Hoffmann, T.; Kolc-
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11. Burnier, M.; Brunner, H. R. Lancet 2000, 355, 637.
12. Seward, E. M.; Swain, C. Exp. Opin. Ther. Pat. 1999, 9,
571.
13. Chong, P. Y.; Petillo, P. A. Tetrahedron Lett. 1999, 40,
2493.
14. DeWitt, S. H.; Kiley, J. S.; Stankovic, C. H.; Schroeder,
M. C.; Cody, D. M. R.; Pavia, M. R. Proc. Natl. Acad. Sci.
U.S.A. 1993, 90, 6900.
15. Kim, S. W.; Anh, S. Y.; Koh, J. S.; Lee, J. H.; Ro, S.;
Cho, H. Y. Tetrahedron Lett. 1997, 38, 4603.
16. Affinities for the human NK1 receptor were evaluated in
CHO cells infected with the human NK1 receptor (using the
Semliki virus expression system) and radiolabelled with [3H]
substance P. Displacement curves were determined with at
least seven concentrations.
carboxylic acid chlorides onto the deprotected piperi-
dine nitrogen of resin bound amino acid 16 (R=H) for
tertiary amid formation. Treatment of resin 16 with
sulfonyl chlorides resulted in the corresponding sulfon-
amides as a second subset. In addition, isocyanates were
coupled to the piperidine nitrogen to form the corre-
sponding piperidine urea derivatives. For the introduc-
tion of a basic nitrogen to the molecular frameworkof
compounds 3, a reductive alkylation step was applied
using aldehydes as building blocks resulting in resin
bound tertiary amines of type 16. Subsequent urea for-
mation after Alloc cleavage at the a-nitrogen was either
achieved via coupling of commercially available 3,5-
bis(trifluoromethyl)phenyl isocyanate directly to the a-
nitrogen of the immobilized amino acid or via activa-
tion of resin 16 and subsequent treatment of activated
carbamate resin 17 with 3,5-bis(trifluoromethyl)benzyl
amine to yield resin 18. The generated urea derivatives
were again cleaved under basic conditions to result in
compounds of type 19f–j (Table 3) where n equals either
0 or 1.
A subset of NK-1 active compounds from library C is
depicted in Table 3 showing moderate to high binding
affinities.