Rapid generation of cis-constrained norstatine analogs using a TMSN3-modified Passerini MCC/N-capping strategy

Article abstract of DOI:10.1016/S0040-4039(02)01505-8

A novel application of the TMSN₃-modified Passerini three-component reaction is disclosed for the rapid solution phase synthesis of cis constrained norstatine mimetic libraries. The reaction of an N-BOC-α-amino aldehyde, an isocyanide and trimethylsilylazide in dichloromethane, followed by deprotection and N-capping with TFP esters affords cis constrained norstatine mimetics. This efficient protocol, producing products with three diversity points, can be used to generate arrays of biologically relevant small molecules for protease targeted screening.

Full text of DOI:10.1016/S0040-4039(02)01505-8

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 6833–6835
Rapid generation of cis-constrained norstatine analogs using a
TMSN3-modified Passerini MCC/N-capping strategy†
Thomas Nixey* and Christopher Hulme
Department of Small Molecule Drug Discovery, AMGEN, One AMGEN Center Drive, Thousand Oaks, CA 91320, USA
Received 18 June 2002; revised 17 July 2002; accepted 18 July 2002
Abstract—A novel application of the TMSN3-modified Passerini three-component reaction is disclosed for the rapid solution
phase synthesis of cis constrained norstatine mimetic libraries. The reaction of an N-BOC-a-amino aldehyde, an isocyanide and
trimethylsilylazide in dichloromethane, followed by deprotection and N-capping with TFP esters affords cis constrained norstatine
mimetics. This efficient protocol, producing products with three diversity points, can be used to generate arrays of biologically
relevant small molecules for protease targeted screening. © 2002 Elsevier Science Ltd. All rights reserved.
Aspartyl proteases are involved in a number of biologi-
cal processes, including the progression of a variety of
diseases,1 and are therefore important therapeutic
targets. These enzymes catalyze amide bond hydrolysis,
a process which proceeds via a well known tetrahedral
intermediate. Inhibitors of aspartyl proteases utilize a
secondary alcohol as a stable mimetic of the tetrahedral
intermediate, and common amide isosteres include
hydroxyethylamine, hydroxyethylene, and hydroxy-
methylene. Several of these classes of compounds have
proven amenable to the synthesis of libraries, leading to
enhanced preference for the cis conformation. Biologi-
cal activity of analogs with a tetrazole replacement
provide strong evidence for the role of the cis amide
conformation in receptor recognition, and hence the
constraint is valuable in assessing the nature of
enzyme–substrate interactions.
Abell has reported a combination of the above strate-
gies, producing
a tetrazole-based, cis constrained
norstatine isostere as a new class of HIV-1 protease
inhibitor.5 Although these inhibitors are quite interest-
ing for protease exploration, the linear synthesis is
lengthy and not well suited for the production of
arrays. We herein report the facile synthesis of
the discovery of
a number of potent protease
inhibitors.2
In the design of peptidomimetics it is often possible to
enhance pharmacological properties of a molecule by
replacing amides with amide isosteres.3 Common
replacements include trans olefins to mimic trans
amides, and 1,5-disubstituted tetrazoles to mimic cis
amides.4 While amides generally prefer the trans con-
formation, proline and other N-alkylated amides show
analogous cis constrained norstatine mimetics. A
TMSN3-modified Passerini6 reaction, combining N-
BOC-a-amino aldehydes, TMSN3, and isonitriles, gives
a mixture of tetrazoles 1 and 2 in moderate to high
yield (Scheme 1). The condensation products are then
deprotected and N-capped with polymer-bound
tetrafluorophenol7 (TFP) esters to produce the con-
Scheme 1.
* Corresponding author. E-mail: tnixey@amgen.com
† This article is dedicated to my daughter, Madison Lee Nixey.
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)01505-8

6834
T. Nixey, C. Hulme / Tetrahedron Letters 43 (2002) 6833–6835
strained isostere 3 with three points of diversity. This
methodology represents a significantly shortened route
to this class of peptide mimetic and further illustrates
the strength of multi-component condensation (MCC)
methodology.
reaction proved general for a variety of isocyanides and
N-BOC-a-amino aldehydes,9 tolerating a range of func-
tionalities. Following removal of the BOC group with
TFA and treatment of the resulting salt with macropo-
rous polystyrene-bound carbonate (MP-carbonate), the
deprotected Passerini product was dissolved in DMF
and added to various TFP esters. The slurry was then
heated to give the desired amides and sulfonamides in
good isolated yields.10 A selection of both the initial
condensation products and final N-capped norstatine
mimetics are shown in Fig. 1, along with their corre-
sponding yields.
The use of azides in the Passerini reaction was first
reported in 1961.8 In a slight modification of Ugi’s
original procedure, TMSN3 was employed as the azide
source since it is less toxic/explosive than several com-
mercially available alternatives and the byproduct,
methoxytrimethylsilane, is volatile. A side product of
further note was the TMS ether 1, observed as up to
40% of the crude mixture by HPLC (UV 215 nm). The
ether was readily hydrolyzed to the alcohol 2 with
TBAF, or alternatively, the BOC and TMS groups can
be removed concomitantly by treatment with TFA. The
Expanding to array synthesis, 80 resin bound TFP
esters were added to a 96 well filter plate and the plate
was assembled into a reaction frame. A solution of the
free amine in DMF was pipetted into the plate, and the
Figure 1.
Table 1.
Cmpd c
A%a
A%b
MH+
Cmpd c
A%a
A%b
MH+
14
15
16
17
18
73
71
68
69
66
83
76
77
80
80
406
519
429
474
431
19
20
21
22
23
30
36
56
67
74
69
77
84
79
83
422
434
402
508
478
a A% by LC/MS.12
b A% following PS-NCO scavenging.

T. Nixey, C. Hulme / Tetrahedron Letters 43 (2002) 6833–6835
6835
plate was capped and heated in a shaker oven. The
slurries were then filtered into a collection plate and
evaporated. Final compound purities were improved
(13% on average) by scavenging the unreacted amine
with PS-NCO.11 Several examples, obtained from reac-
tion of TFP esters with the deprotected analog of
compound 13, are shown below. Area % (A%) data for
crude and scavenged mixtures are listed in Table 1.
8. (a) Ugi, I.; Meyr, R. Chem. Ber. 1961, 94, 2229; (b)
TMSN3 has also been employed in the Ugi reaction: (i)
Bienayme, H. Tetrahedron Lett. 1998, 39, 2735; (ii)
Nixey, T.; Kelly, M.; Hulme, C. Tetrahedron Lett. 2000,
41, 8729; (iii) Nixey, T.; Kelly, M.; Semin, D.; Hulme, C.
Tetrahedron Lett. 2002, 43, 3681.
9. Preparation of compound 4: Solutions of N-(tert-butoxy-
carbonyl)glycinal (0.1 M, 10 mL in DCM), 2,6-
dimethylphenylisocyanide (0.1 M, 10 mL in DCM) and
TMSN3 (0.1 M, 10 mL in DCM) were added to a round
bottom flask and stirred at rt for 18 h. The solution was
concentrated and the resulting oil was fractionated by
flash-column chromatography (1% MeOH/chloroform)
In summary, a novel solution phase procedure for the
preparation of cis constrained norstatine mimetics has
been reported. With final products containing three
points of diversity and a facile and rapid production
protocol, access to thousands of diverse analogues with
the aforementioned core structure is now feasible.
1
to yield 4 as a white solid (93 mg, 28% yield). H NMR
(400 MHz, CDCl3): l 7.37 (1H, dd, J=7.5, 7.5 Hz), 7.22
(1H, d, J=7.5 Hz), 7.20 (1H, d, J=7.5 Hz), 5.33 (1H, m),
4.78 (1H, m), 3.75 (2H, m), 2.00 (3H, s), 1.91 (3H, s), 1.39
(9H, s). 13C NMR (100 MHz, CDCl3): l 157.5, 155.5,
136.4, 135.1, 132.2, 128.8, 128.6, 80.6, 64.4, 44.6, 28.2,
17.5, 17.3. HRMS: MH+ theoretical value 334.1874;
actual value 334.1876. dM/M=0.60 ppm.
Acknowledgements
We would like to thank Sam Thomas for LC/MS
determinations and Randall Hungate for proofreading.
10. Preparation of compound 5: Compound 4 (12 mg, 0.036
mmol) was deprotected by reaction with 50% TFA/DCM
(1 mL) for 5 min. The solution was concentrated and the
resulting oil was dissolved in DCM, MP-carbonate (3.15
mmol/g, 50 mg) was added, and the mixture was shaken
for 16 h, filtered, and concentrated. The oil was dissolved
in DMF (1.2 mL) and was added to polymer bound
quinoline-3-carboxylate TFP ester (54 mg, 0.83 mmol/g,
0.045 mmol). The slurry was heated at 60°C for 16 h,
filtered, and the DMF was removed in vacuo. The result-
ing oil was purified by preparative HPLC to give 5 as an
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1
off-white solid (12 mg, 86% yield). H NMR (400 MHz,
CDCl3): l 9.85 (1H, s), 9.35 (1H, s), 9.12 (1H, m), 8.40
(1H, d, J=8.5 Hz), 8.18 (1H, d, J=8.5 Hz), 8.08 (1H, dd,
J=8.5, 8.5 Hz), 7.89 (1H, dd, J=8.5, 8.5 Hz), 7.35 (1H,
dd, J=7.5, 7.5 Hz), 7.21 (1H, d, J=7.5 Hz), 7.19 (1H, d,
J=7.5 Hz), 4.98 (1H, m), 4.00 (2H, m), 1.99 (3H, s), 1.90
(3H, s). 13C NMR (100 MHz, CDCl3): l 155.7, 144.7,
144.3, 140.0, 136.0, 135.8, 135.6, 131.5, 131.2, 130.4,
129.8, 129.0, 128.9, 128.0, 122.3, 62.8, 44.6, 17.3. HRMS:
MH+ theoretical value 389.1726; actual value 389.1735.
dM/M=2.3 ppm.
11. Production of an 80-member array was successfully com-
pleted using a 96 well filter plate encapsulated in a
reaction frame assembly. A slurry of TFP resins (20
mmol), DMF (350 mL) and the free amine (18 mmol) was
heated at 60°C for 18 h, cooled, and the mixture was then
filtered into a collection plate and the solvent was evapo-
rated in vacuo at 65°C. Scavenging with PS-NCO (1
equiv., 24 h) was performed to remove any remaining free
amine. The secondary alcohol present in the products
showed little reactivity towards the polymer bound isocy-
anate under the reaction conditions.
12. LC/MS analysis was performed using a C18 Hypersil
BDS 3m 2.1×50 mm column with a mobile phase of 0.1%
TFA in CH3CN/H2O, gradient from 10% CH3CN to
100% over 15 min. The HPLC was interfaced with an
APCI probe, and detection was performed at UV 215
nm.