Proline isosteres in a series of 2,4-disubstituted pyrrolo[1,2-f][1,2,4] triazine inhibitors of IGF-1R kinase and IR kinase

Article abstract of DOI:10.1016/j.bmcl.2010.07.045

Pyrrolidine, pyrrolidinone, carbocyclic, and acyclic groups were used as isosteric proline replacements in a series of insulin-like growth factor I receptor kinase/insulin receptor kinase inhibitors. Examples that were similar in potency to proline-containing reference compounds were shown to project a key fluoropyridine amide into a common space, while less potent compounds were not able to do so for reasons of stereochemistry or structural rigidity.

Full text of DOI:10.1016/j.bmcl.2010.07.045

Bioorganic & Medicinal Chemistry Letters 20 (2010) 5027–5030
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Proline isosteres in a series of 2,4-disubstituted pyrrolo[1,2-f][1,2,4]triazine
inhibitors of IGF-1R kinase and IR kinase
a
b
c
b
Anthony J. Sampognaro a, , Mark D. Wittman , Joan M. Carboni , Chiehying Chang , Ann F. Greer ,
*
Warren W. Hurlburt b, John S. Sack c, Dolatrai M. Vyas a
a Department of Discovery Chemistry, Bristol-Myers Squibb Research and Development, P.O. Box 5400, Princeton, NJ 08543-5400, USA
b Department of Discovery Biology, Bristol-Myers Squibb Research and Development, P.O. Box 5400, Princeton, NJ 08543-5400, USA
c Department of Molecular Biosciences, Bristol-Myers Squibb Research and Development, P.O. Box 5400, Princeton, NJ 08543-5400, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Pyrrolidine, pyrrolidinone, carbocyclic, and acyclic groups were used as isosteric proline replacements in
a series of insulin-like growth factor I receptor kinase/insulin receptor kinase inhibitors. Examples that
were similar in potency to proline-containing reference compounds were shown to project a key fluoro-
pyridine amide into a common space, while less potent compounds were not able to do so for reasons of
stereochemistry or structural rigidity.
Received 24 May 2010
Revised 9 July 2010
Accepted 12 July 2010
Available online 15 July 2010
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
IGF-1R
IR
Proline
Isosteres
Pyrrolotriazine
We have previously described the discovery of the clinical
development candidate 1a,1 a potent and selective inhibitor of
insulin-like growth factor I receptor kinase (IGF-1R), an important
emerging oncology target,2 and insulin receptor kinase. Both 1a
and related compound 1b feature a carbon–nitrogen bond connect-
ing the ring nitrogen of a proline to the pyrrolo[1,2-f][1,2,4]triazine
core. The early SAR exploration that led to the discovery of these
compounds focused on the synthesis of heterocyclic proline
amides. Retaining the fluoropyridine amide of 1a and 1b as an opti-
mal substituent, recent efforts shifted toward a series of analogs
(2) that replaced the nitrogen-linked proline moiety with an isoste-
re that featured a carbon–carbon bond to the pyrrolotriazine core
(Fig. 1). This carbon–carbon linkage was unprecedented in our re-
search program at the time, and provided an opportunity to intro-
duce diversity while monitoring for changes in the SAR compared
to the carbon–nitrogen-linked analogs. Specifically, we focused on
four categories of carbon-linked proline isosteres; pyrrolidine, pyr-
rolidinone, carbocyclic, and acyclic. Herein, we describe the IGF-1R
potency of these analogs.
hot aqueous KOH (Scheme 1). The protecting group was then
switched from Boc to Cbz in a one-pot procedure to give 6 because
a single protecting group compatible with all synthetic steps was
not found. Although the Cbz-protected analog of 4 underwent
cyclization with hot aqueous KOH, the Cbz was partially removed4
and provided a lower overall yield. The Boc group was found to be
unstable to later steps, necessitating the switch to the Cbz group.
Activation of 6 with BOP5 followed by exposure to 5-cyclopropyl-
1H-pyrazol-3-amine produced mostly the undesired ring-nitrogen
addition product 7 with only a minor amount of the desired isomer
8.6,7 Alternatively, 6 was converted to the heteroaryl chloride with
POCl3. The addition of the 5-cyclopropyl-1H-pyrazol-3-amine to a
preheated (>70 °C) solution of this intermediate gave a ꢀ3:1 ratio
of 8 to 7.
NH
R
NH
N
N
N
N
HN
HN
F
The syntheses of two enantiomeric carbon-linked pyrrolidine
analogs were conducted separately. The EDC-mediated condensa-
tion of 1-amino-1H-pyrrole-2-carboxamide3 3 with (S)- or (R)-N-
Boc proline, respectively, provided 4, which was cyclized to 5 in
F
O
N
N
N
proline
isostere
N
N
N
N
H
N
1a, R = Me
1b, R = H
2
* Corresponding author. Tel.: +1 203 677 5986.
E-mail address: anthony.sampognaro@bms.com (A.J. Sampognaro).
Figure 1.
0960-894X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2010.07.045

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A. J. Sampognaro et al. / Bioorg. Med. Chem. Lett. 20 (2010) 5027–5030
O
O
N
a
b
O
NH
O
NH
a
b
O
O
3
N
N
N
N
OR2
N
*
N
NH2
NH2
NH2
O
NH
NHR1
NH2
O
NH
N
R
*
NHBoc
Boc
N
O
13; R1 = Boc, R2 = H
14; R1 = H, R2 = Me
5; R = Boc
6; R = Cbz
3
4
12
c
c
Ot-Bu
NH2
NH
NH
N
N
N
N
N
HN
HN
1H NOE
observed
N
O
d
e
N
O
N
N
N
N
OMe
N
O
OR
N
*
NH2
NH
N
d
e
Cbz
7
N
15
6
F
NH
NH
N
N
N
16; R = Me
17; R = H
f
HN
HN
NH
N
g
N
HN
N
N
*
*
N
O
N
CO2H
g
N
N
N
N
Cbz
N
O
F
N
N
F
8; R = Cbz
9; R = H
f
O
N
10
F
N
11a; (S)-enantiomer
11b; (R)-enantiomer
18
Scheme 1. Reagents and conditions: (a) (R)- or (S)-N-Boc-Pro, EDC, THF, 81%; (b) aq
KOH, reflux, 3 days, 62%; (c) i—TFA, DCM; ii—Cbz-Cl, aq NaHCO3, 89%; (d) i—BOP,
MeCN, DBU, rt, 10 min; ii—5-cyclopropyl-1H-pyrazol-3-amine, DIPEA, NMP, 70 °C,
50%; (e) i—POCl3, 80 °C, 1 h; ii—5-cyclopropyl-1H-pyrazol-3-amine, DIPEA, NMP,
70 °C, 45%; (f) 10% Pd/C, MeOH, HCO2H, rt, 20 min, 89%; (g) HATU, DIPEA, NMP, 39%.
Scheme 2. Reagents and conditions: (a) Boc-Asp-Ot-Bu, EDC, THF, 85%; (b) aq KOH,
EtOH, 90%; (c) MeOH, acetyl chloride, 80%; (d) i—POCl3, 80 °C; ii—5-cyclopropyl-1H-
pyrazol-3-amine, NMP, 80 °C, 55%; (e) 10, HATU, DIPEA, NMP, 40%; (f) aq LiOH, THF,
89%; (g) i—pentafluorophenol trifluoroacetate, pyridine, THF; ii—0.1 M NaHCO3,
THF, MeCN, 67%.
Addition at lower temperatures, even with subsequent heating,
still favored the formation of 7. This pattern of reactivity was ob-
served with subsequent analogs, which typically afforded similar
ratios of isomers under the same conditions. With 8 in hand, re-
moval of the Cbz protecting group via transfer hydrogenation
cleanly afforded 9, which was then acylated with 10 to give either
11a or 11b.
Using a modified approach, a pyrrolidinone analog was con-
structed beginning with the condensation of 3 and Boc-Asp-Ot-
Bu to give 12 (Scheme 2). Cyclization was effected to afford 13,
which was then exposed to in situ generated HCl in methanol. This
simultaneously removed the Boc protecting group while protecting
the carboxylic acid as its methyl ester (14). Conversion to the het-
eroaryl chloride intermediate followed by high temperature addi-
tion of 5-cyclopropyl-1H-pyrazol-3-amine led to mostly 16, with
a minor amount of an undesired ring-nitrogen addition isomer.
Hydrolysis gave the free carboxylic acid 17, which was converted
to pyrrolidinone 18 via conversion to its pentafluorophenol ester
followed by cyclization with mild aqueous base.
O
a
b
NH
O
3
N
N
N
HO2C
CO2Me
NH2
O
NH
RO2C
CO2Me
( )20
( )21; R = H
( )22; R = Me
( )19
c
NH
N
NH
N
N
HN
HN
N
e
d
NH2
N
N
*
N
O
N
*
MeO2C
N
NH
F
N
Two cyclopentane-based analogs were synthesized starting
with trans-1,2-cyclopentanedicarboxylic acid monomethyl ester
[( )19] (Scheme 3).8b Condensation with 3 afforded ( )20, which
was cyclized to ( )21 in high yield. These conditions also removed
the methyl ester, which was reintroduced under Fischer conditions
to give ( )22. Activation with POCl3 and addition of 5-cyclopropyl-
1H-pyrazol-3-amine produced ( )23, again with only a minor
F
(+)-25
+
(-)-25
( )23
24
(trans)
Scheme 3. Reagents and conditions: (a) 3, EDC, THF, 72%; (b) aq KOH, EtOH, reflux,
90%; (c) MeOH, acetyl chloride, 84%; (d) i—POCl3, 85 °C; ii—5-cyclopropyl-1H-
pyrazol-3-amine, NMP, 85 °C, 50%; (e) i—MeMgBr, 24, Et2O, THF, 69%; ii—chiral
separation.

A. J. Sampognaro et al. / Bioorg. Med. Chem. Lett. 20 (2010) 5027–5030
5029
Table 1
amount of an undesired ring-nitrogen addition isomer. The addi-
SAR of proline-replacement analogs
tion of the magnesium salt of 6-fluoropyridin-3-amine (24) to
( )23 followed by chiral separation gave (+)-25 and (À)-25.9 The
achiral cyclopentene analog 27 was constructed in a similar man-
ner, starting with the commercially available anhydride 26
(Scheme 4).
The synthesis of a simple acyclic analog began with the conden-
sation of N-Boc glycine with 3 to give 28, which was smoothly cy-
clized to 29 (Scheme 5). A switch of the protecting group from Boc
to phthalate gave 30 in high yield. This was necessary because the
phthalate-protected analog of 28 failed to cyclize under a variety of
conditions, while the Boc-protected compound 29 was not stable
to the POCl3 used in the following step to convert 30 to the hetero-
aryl chloride intermediate 31. High-temperature addition of 5-
cyclopropyl-1H-pyrazol-3-amine to 31 gave mostly 32 with a min-
or amount of an undesired ring-nitrogen addition isomer. Removal
of the phthalate protecting group with hydrazine afforded 33,
which was then acylated with 10 to produce 34.
Compound
IGF-1R IC50 (nM)
IGF-Sal IC50 (nM)
PAMPA (nm/s)
1a
1b
8.4
2.7
7.0
8400
11
9.7
190
32
2.1
1.0
634
770
721
935
600
509
854
822
525
11a
11b
18
(+)-25
(À)-25
27
162
n/a
508
206
389
237
40
34
5.3
and PAMPA permeability. All analogs exhibited no selectivity ver-
sus IR.
Structural differences between 11a and 11b compared to 1a in-
clude the number of connecting atoms between the fluoropyridine
and the pyrrolotriazine core, the secondary versus tertiary nature
of the amide, and the direction of the amide connectivity. Despite
these differences, IGF-1R potency of 11a was comparable to that of
1a and 1b. The (R)-enantiomer (11b) showed a 1200-fold decrease
in kinase potency versus 11a, which led us to suspect a suboptimal
placement of the fluoropyridine amide.
Table 1 shows the IGF-1R kinase potencies for the new analogs
in comparison to 1a and 1b as well as IGF-Sal cellular potencies
NH
N
Pyrrolidinone analog 18 showed only a small reduction of IGF-
1R potency. Because of the dramatic loss of potency demonstrated
by 11b compared to 11a, the (R)-enantiomer of 18 was not made.
Carbocycles have previously been used as proline isosteres.8
One of the cyclopentane analogs, (+)-25, showed comparable po-
tency to 1a, while (À)-25 was approximately 20-fold less potent.
Cyclopentene analog 27 suffered a moderate loss of IGF-1R
potency.
O
HN
O
O
N
3
+
N
N
O
NH
N
F
The acyclic analog 34 was considered experimentally equipo-
tent to 11a,10 demonstrating that a cyclic proline isostere was
not necessary for kinase potency.
26
27
Individual X-ray structures of 1a, 11a, 11b, and 34 co-crystal-
lized with IGF-1R were obtained.11 All four compounds exhibit
the hydrogen bond donor–acceptor–donor triad across the amino-
pyrazole substituent to the hinge binding region of IGF-1R
Scheme 4.
O
O
a
b
d
NH
N
3
N
NR1R2
NH2
O
NH
N
HN
Boc
29; R1 = H, R2 = Boc
28
c
30; R1, R2 = phthalate
NH
N
Cl
HN
N
O
e
N
N
N
N
NR1R2
N
N
O
31
32; R1, R2 = phthalate
33; R1 = H, R2 = H
f
NH
N
HN
N
F
g
N
H
N
N
N
O
34
Scheme 5. (a) N-Boc Gly, EDC, THF, 82%; (b) EtOH, aq KOH, 100 °C, 86%; (c) i—HCl,
dioxane, 100%; ii—phthalic anhydride, DMF, 98%; (d) POCl3, 100 °C, 64%; (e) 5-
cyclopropyl-1H-pyrazol-3-amine, DIPEA, NMP, 80 °C; (f) hydrazine hydrate, EtOH,
40 °C, 55%; (g) 10, HATU, DIPEA, NMP.
Figure 2. Overlay of the X-ray co-crystal structures of IGF-1R with 1a (blue), 11a
(green), and 34 (magenta).

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A. J. Sampognaro et al. / Bioorg. Med. Chem. Lett. 20 (2010) 5027–5030
In summary, this series of analogs demonstrate that a variety of
groups could replace the proline of 1a or 1b and retain IGF-1R ki-
nase potency provided the key fluoropyridine amide in each was
able to be positioned into a similar space. As evidenced by X-ray
co-crystal structures with IGF-1R, both cyclic and acyclic proline
isosteres demonstrated this ability. Analogs that had reduced
IGF-1R potency were shown or presumed to place the fluoropyri-
dine into a suboptimal space, due to stereochemical factors or
structural rigidity.
Acknowledgments
The authors thank the following for their contributions: Gordon
Todderud and the Lead Evaluation Department for generating the
IGF-1R in vitro kinase data; the Lead Profiling Department of
BMS; Yingzi Wang for the NMR characterization of 7, Christopher
Poronsky for the chiral separation of (+)-25 and (À)-25; Yaqun
Zhang, Susan E. Kiefer, and John A. Newitt of the Molecular Biosci-
ences Department for IGF-1R kinase domain expression, purifica-
tion, and characterization; and David Langley for helpful
discussions about the X-ray co-crystal structures.
References and notes
1. Wittman, M. D.; Carboni, J. M.; Yang, Z.; Lee, F. Y.; Antman, M.; Attar, R.;
Balimane, P.; Chang, C.; Chen, C.; Discenza, L.; Frennesson, D.; Gottardis, M. M.;
Greer, A.; Hurlburt, W.; Johnson, W.; Langley, D. R.; Li, A.; Li, J.; Liu, P.;
Mastalerz, H.; Mathur, A.; Menard, K.; Patel, K.; Sack, J.; Sang, X.; Saulnier, M.;
Smith, D.; Stefanski, K.; Trainor, G.; Velaparthi, U.; Zhang, G.; Zimmermann, K.;
Vyas, D. M. J. Med. Chem. 2009, 52, 7360.
2. (a) Macaulay, V. M.; Riedemann, J. Endocr. Relat. Cancer 2006, 13, S33; (b)
Mitsiades, C. S.; Mitsiades, N. S.; McMullan, C. J.; Poulaki, V.; Shringapure, R.;
Akiyama, M.; Hideshima, T.; Chauhan, D.; Joseph, M.; Libermann, T. A.; García-
Echeverría, C.; Pearson, M. A.; Hofmann, F.; Anderson, K. C.; Kung, A. L. Cancer
Cell 2004, 3, 221; (c) Baserga, R. Expert Opin. Ther. Targets 2005, 9, 753; (d)
Hofmann, F.; García-Echeverría, C. Drug Discovery Today 2005, 10, 1041; (e)
Werner, H.; Le Roith, D. Crit. Rev. Oncog. 1997, 8, 71.
3. Mastalerz, H.; Wittman, M. D.; Zimmermann, K.; Saulnier, M. G.; Velaparthi, U.;
Vyas, D. M.; Zhang, G.; Johnson, W. L.; Frennesson, D. B.; Sang, X.; Liu, P.;
Langley, D. R. World Patent WO 2008005956 A2, 2008.
4. Greene, T. W.; Wuts, P. G. M. Protective Groups in Organic Synthesis; Wiley &
Sons: New York, 1999. p 534.
5. (a) Wan, Z.-K.; Wacharasindhu, S.; Binnun, E.; Mansour, T. Org. Lett. 2006, 11,
2425; (b) Wan, Z.-K.; Wacharasindhu, S.; Levins, C. G.; Lin, M.; Tabei, K.;
Mansour, T. S. J. Org. Chem. 2007, 72, 10194; (c) Wan, Z.-K.; Binnun, E.; Wilson,
D. P.; Lee, J. Org. Lett. 2005, 7, 5877.
Figure 3. Overlay of the X-ray co-crystal structures of IGF-1R with 11a (green) and
11b (red).
(Glu1050, Leu1051, and Met1052). Despite the structural differ-
ences in the groups linking the fluoropyridine amide to the pyrrol-
otriazine core, 1a, 11a, and 34 occupy much the same space in the
regions critical for IGF-1R potency (Fig. 2). In contrast, 11b was
shown to display the fluoropyridine into a space that brings it into
steric conflict with Asp1056, explaining its sharp loss of potency
versus its enantiomer 11a (Fig. 3). The additional carbonyl present
in 18 did not substantially affect IGF-1R potency, implying that the
placement of the fluoropyridine was not significantly changed
compared to 11a. Although the difference in potency between
(+)-25 and (À)-25 is not as dramatic as that of 11a and 11b, a sim-
ilar situation could reasonably be envisioned whereby one enantio-
mer has a clear stereochemical advantage over the other in the
context of IGF-1R binding. Finally, the moderate decrease in po-
tency of 27 could be explained by the rigid double bond of the
cyclopentene ring at least partially preventing what appears to
be the need for a slight turn or bend necessary for optimal place-
ment of the fluoropyridine moiety.
For reasons which were not clear, none of the new analogs dem-
onstrated potency in the IGF-Sal cellular assay.12 Passive perme-
ability as measured in the PAMPA assay indicated the
compounds were permeable, and the IGF-Sal cell line does not
have a functional efflux mechanism. Nevertheless, in this case
PAMPA was not predictive of cellular potency despite the compa-
rable kinase potencies of some of the new analogs compared to
1a and 1b.
6. The other ring-nitrogen addition isomer was not observed.
7. The structure of the (R)-enantiomer of 7 was established by NOE 1H NMR (see
Scheme 1).
8. (a) Turbanti, L.; Cerbai, G.; Di Bungo, C.; Giorgi, R.; Garzelli, G.; Criscuoli, M.;
Renzetti, A. R.; Subissi, A.; Bramanti, G. J. Med. Chem. 1993, 36, 699; (b)
Nöteberg, D.; Brånalt, J.; Kvarnström, I.; Linschoten, M.; Musil, D.; Nyström, J.-
A.; Zuccarello, G.; Samuelsson, B. J. Med. Chem. 2000, 43, 1705; (c) Jarho, E. M.;
Venäläinen, J. I.; Huuskonen, J.; Christiaans, J. A. M.; Garcia-Horsman, J. A.;
Forsberg, M. M.; Järvinen, T.; Gynther, J.; Männistö, P. T.; Wallén, E. A. A. J. Med.
Chem. 2004, 47, 5605.
9. (+)-25 and (À)-25 were separated from each other using supercritical fluid
chromatography: ChiralCel OD-H, 30 Â 250 mm, 5
lm; 30% i-PrOH, 70% CO2,
150 bar, 70 mL/min, monitored at 230 nm.
10. Experimental variability is typically 2Â. See Ref. 1 and citations therein for a
more complete description of both the IGF-1R kinase and IGF-Sal cellular
assays.
11. The atomic coordinates have been deposited in the RSCB Protein Data Bank as
entries 3I81 (1a), 3NW5 (11b), 3NW6 (11a), and 3NW7 (34).
12. Carboni, J. M.; Lee, A. V.; Hadsell, D. L.; Rowley, B. R.; Lee, F. Y.; Bol, D. K.;
Camuso, A. E.; Gottardis, M.; Greer, A. F.; Ho, C. P.; Hurlburt, W.; Li, A.; Saulnier,
M.; Velaparthi, U.; Wang, C.; Wen, M.-L.; Westhouse, R. A.; Wittman, M.;
Zimmermann, K.; Rupnow, B. A.; Wong, T. W. Can. Res. 2005, 65, 3781.