A concise synthesis of chiral indanes as α1A adrenoceptor partial agonists

Article abstract of DOI:10.1016/j.tetlet.2015.10.004

The synthesis of a series of chiral indanes with reported α_1A partial agonist activity is outlined, applying a rhodium catalysed cyclisation for template construction. This method was extended to the asymmetric synthesis of lead compound PF-037

Full text of DOI:10.1016/j.tetlet.2015.10.004

Tetrahedron Letters 56 (2015) 6546–6550
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
A concise synthesis of chiral indanes as a1A adrenoceptor partial
agonists
Lee R. Roberts a, ,y, Matthew S. Corbett c, Steven J. Fussell b, Laure Hitzel a, Alan S. Jessiman a,
⇑
Helen J. Mason a, Rachel Osborne a, Michael J. Ralph a, Adam S. D. Stennett a, Simon Wheeler a,
R. Ian Storer a, ,à
⇑
a Worldwide Medicinal Chemistry, Pfizer, Ramsgate Road, Sandwich, Kent CT13 9NJ, UK
b Chemical Research & Development, Pfizer, Ramsgate Road, Sandwich, Kent CT13 9NJ, UK
c Worldwide Medicinal Chemistry, Pfizer Global Research and Development, Groton Laboratories, Eastern Point Road, Groton, CT 06340, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis of a series of chiral indanes with reported a1A partial agonist activity is outlined, applying a
Received 16 September 2015
Revised 25 September 2015
Accepted 2 October 2015
Available online 9 October 2015
rhodium catalysed cyclisation for template construction. This method was extended to the asymmetric
synthesis of lead compound PF-03774076 (1) which was prepared on >20 g scale in seven steps.
Highlights include Rh-mediated cyclocarbonylation and an asymmetric alkene hydrogenation.
Ó 2015 Elsevier Ltd. All rights reserved.
Keywords:
Adrenoceptor agonist
Asymmetric hydrogenation
Indane
Indanone
PF-03774076
Introduction
polyphosphoric acid (PPA) or Friedel Crafts cyclisation of the corre-
sponding acid chloride under treatment with aluminium trichlo-
A series of chiral substituted indanes were previously disclosed
as CNS penetrant selective partial a1A adrenoreceptor agonists.
This work culminated in the identification of lead candidate com-
pound PF-03774076 (1) (Fig. 1).1–5
Previous synthetic methods to access similar indane templates
had focused on either heating
ride to form the indanone (Scheme 1).1–3,6
O
a or b
R
R
X
X
CO2H
X = CH, N
a phenylpropionic acid with
2
3
MeO
MeO
Scheme 1. Reagents and conditions: (a) PPA, reflux; or (b)(i) (COCl)2, CH2Cl2, cat.
DMF, 0 °C to rt; (ii) AlCl3, rt.
N
NH
PF-03774076
α1A EC50 31nM (Emax 60%)
MDCK-MDR1 ER 1.1
logD 2.6
However these methods did not work well for analogues con-
taining a pendant 4-methoxymethylene substituent 2 as this was
unstable under the strongly acidic conditions. As a late stage intro-
duction of the methoxymethyl group would add numerous further
synthetic steps, an alternative synthesis of the indane ring 3 was
sought.7
Cl
MeO
1
Figure 1. Indane series a1A partial agonist lead.
⇑
Corresponding authors. Tel.: +1 6176747114 (L.R.R.), +44 1304641854 (R.I.S.).
E-mail addresses: lee.roberts@pfizer.com (L.R. Roberts), ian.storer@pfizer.com
Results and discussion
(R.I. Storer).
y
Present address: Pfizer Inc., 610 Main St. Kendall Square, Cambridge, MA 02139,
Herein, we report a concise and scalable synthesis of these
indane compounds based around a rhodium catalysed carbonyla-
tive cyclisation with aromatic C–H insertion. This key step
USA.
à
Present address: Pfizer Ltd, The Portway Building, Granta Park, Cambridge CB21
6GS, UK.
http://dx.doi.org/10.1016/j.tetlet.2015.10.004
0040-4039/Ó 2015 Elsevier Ltd. All rights reserved.

L. R. Roberts et al. / Tetrahedron Letters 56 (2015) 6546–6550
6547
comprises the reaction of a trimethylsilyl phenylacetylene 4 with
R1
R1
R1
rhodium cyclooctadiene as catalyst (1 mol %) and triphenylphos-
phine (20 mol %) under a 1000 psi CO atmosphere at 160 °C for
4 h to form the corresponding indanone 5. An amine base promoter
(Et3N) and water co-solvent were both required for successful
reaction (Scheme 2).
b
a
I
I
I
Me
10
Br
MeO
11
14
12
e
d
N
N
I
c
Cl
Cl
SiMe3
13
O
CO, H2O
O
[Rh(COD)Cl]2
R
R
R1
f
R1
PPh3, Et3N, THF
160 °C, 1000 psi
4
5
SiMe3
X
X
R2
SiMe3
R2
16
15
Scheme 2. Rhodium catalysed carbonylative cyclisation.
Scheme 5. Reagents and conditions: (a) NBS, CCl4, (PhCO2)2, reflux; (b) Na, MeOH,
reflux, 30 min; (c) TMS-acetylene, Pd(PPh3)2Cl2 (1 mol %), CuI (2.5 mol %), Et3N,
DMF, rt, 16 h; (d) TMS-acetylene, Pd(PPh3)2Cl2 (1 mol %), CuI (2.5 mol %), Et3N, DMF,
rt, 16 h; (e) Fe(acac)2 (0.1 equiv), nPrMgCl (2.4 equiv), NMP, THF, rt; (f) [Rh(COD)Cl]2
(1 mol %), PPh3 (20 mol %), Et3N, water (10 equiv), THF, 160 °C, >1000 psi CO, 12 h.
See Table 1 for yields.
This rhodium catalysed reaction was first reported by Takeuchi,
building on earlier work by Takahashi.8–14 The mechanism
postulated by Takeuchi in which co-ordination of rhodium to the
acetylene 4 induces oxidative addition to the neighbouring aro-
matic C–H bond was also corroborated by experimental observa-
tions. This results in the formation of complex 6 after initial
intramolecular insertion of the coordinated acetylene into the
Rh–H bond followed by isomerisation. Insertion of CO results in
the assembly of the indenone skeleton 7. Association of hydrogen
derived from water and CO via the water gas shift reaction gives
complex 8 leading in turn to reduction of the indenone to yield
9. After 1,3-rearrangement of the silyl group, hydrolysis affords
the desired indanone 5, releasing the catalytic Rh (Scheme 3).
Iodotoluenes 10 were converted to the corresponding benzyl-
bromides 11 via reaction with NBS in refluxing carbon tetrachlo-
ride. Freshly prepared sodium methoxide was then used to
install the requisite methoxymethyl substituent 12. Sonogashira
reaction of the aryl iodide with TMS-acetylene subsequently
provided the desired alkyne precursors 15a–e for cyclisation.
Alternatively, an aza analogue could be prepared from iodopy-
ridine 13 via Sonogashira coupling, followed by an iron catalysed
coupling of the resulting chloropyridine 14 with n-propyl Grignard
to provide the corresponding alkyne 15f.16 Considering the cyclisa-
tion to the indanone, there was concern that the heteroatoms of
the methoxy or pyridine groups in the substrates may competi-
tively chelate with the catalyst to either reduce reactivity or direct
the regioselectivity of the formylation to an undesired position on
the aromatic ring. However, when the acetylenes were reacted
under carbonylative conditions, all the desired indanones 16 were
formed cleanly in moderate to good yields with some unreacted
starting material remaining. Increasing the reaction time and tem-
perature had little effect on the reaction conversion and yield.
However, it was noted that if the CO pressure dropped below
1000 psi then the yields dropped considerably. A number of related
indanones 16, including challenging aza-indanone variants
(Table 1), were synthesised via this general procedure. Interest-
ingly, comparing yields for fluorinated examples (Table 1, entries
2–4) it was apparent that the lowest yield resulted for the cyclisa-
tion of 15d despite all three substrates being electronically similar.
In theory this could be accounted for by the fluorine substituent in
15d being ortho to the C–H bond undergoing insertion, thereby
providing an opportunity for fluorine to coordinate to the interme-
diate rhodacycle, impairing carbonylation. Similarly, the lower
yields observed for the pyridine examples (Table 1, entries 6 and
7) could potentially be due to coordination of rhodium to the pyr-
idine nitrogen, thereby impairing reactivity.
O
R
R
4
5
[Rh]
SiMe3
Rh
H2O
O
SiMe3
R
SiMe3
R
9
6
[Rh]
CO
O
O
CO + H2O
R
R
SiMe3
SiMe3
[Rh]
[Rh]
8
7
H
CO2
Scheme 3. Proposed catalytic cycle for the Rh-catalysed cyclisation.
A high loading of triphenylphosphine proved essential for suc-
cessful reaction. It is believed that this likely pushes the catalyst
equilibrium to the more reactive rhodium species, which is highly
coordinated to triphenylphosphine, as previously demonstrated by
Wilkinson’s work on hydroformylations (Scheme 4).15
Indanones 16a–f were subsequently converted into the corre-
sponding chiral imidazole-products 22a–f in order to enable the
structure activity relationship for indane ring substitution to be
investigated for a1A agonist potency (Scheme 6).
+ CO
+ CO
RhH(CO)(PPh3)3
RhH(CO)2(PPh3)2
RhH(CO)3(PPh3)
+ PPh3
+ PPh3
In order to achieve this, a six-step route was developed
whereby the indanones 16 were reduced to racemic alcohols 17
using sodium borohydride, converted to the corresponding chlo-
rides 18 via treatment with thionyl chloride before reacting with
sodium cyanide to provide the respective nitriles 19. Conversion
of the nitriles to carbimidates 20 then amidines 21 enabled the
requisite imidazole rings to be constructed. The racemic products
Scheme 4. Catalyst equilibrium with triphenylphosphine.
In order to apply this cyclisation to the desired 4-methoxy-
methylene substituted indanes, the necessary acetylene systems
were prepared from commercially available iodotoluenes 10
(Scheme 5).

6548
L. R. Roberts et al. / Tetrahedron Letters 56 (2015) 6546–6550
Table 1
Table 2
Entry Acetylene intermediate Yielda (%) Indanone product Yield (%)
Product
Yielda a1A
Product
Yielda a1A EC50
b
(%)
EC50
(%)
(Emax)
O
O
O
b
(Emax
)
1
2
3
4
94
60
52
46
57
70
52
34
N
N
SiMe3
SiMe3
NH
NH
NH
NH
NH
MeO
15a
15b
37 nM
(69%)
62 nM
(59%)
16a
16b
16c
MeO
8
17
[n = 15]
[n = 8]
F
F
22a
22b
MeO
F
MeO
F
MeO
N
N
MeO
F
59 nM
(87%)
[n = 4]
658 nM
(49%)
[n = 7]
F
F
23
14
5
8
SiMe3
22c
22d
15c
MeO
MeO
MeO
MeO
N
N
F
F
O
O
NH
31 nM
(60%)
2930 nM
(43%)
N
[n = 8]
[n = 4]
Cl
SiMe3
SiMe3
nPr
22f
16d
15d
15e
MeO
rac-1
MeO
MeO
a
Yield over 6-steps to racemic product.
EC50 and Emax determined in vitro by measuring calcium mobilisation in a FLIPR
b
Cl
5
6
63
60
28
20
Cl
assay configuration; data shown for most potent enantiomer following chiral HPLC
MeO
separation.1,17
16e
O
MeO
N
at human a1A receptors (Table 2).1 On balance of the in vitro
EC50 functional potency and partial Emax, compound 1 was selected
as the optimal lead.4
30b
81b
N
nPr
SiMe3
15f
nPr
16f
O
Once it had been established that compound 1 had a suitable
profile for further project studies, it was deemed necessary to syn-
thesise multi-gram quantities of this compound.4 However, despite
the route outlined (Scheme 6) being suitable for small scale
analogue synthesis, this route was less attractive for larger scale
compound provision. Overall this route to synthesise compound
1 was low yielding (10-steps, 5% yield) and required preparative
chromatography after each step. As a result, a shorter improved
sequence was developed to support larger scale provision of lead
compound 1 by introduction of the imidazole via a Negishi cross-
coupling as opposed to ring construction (Scheme 7).
N
7
N
SiMe3
MeO
15g
16g
MeO
a
Yield over 3-steps.
Yield over 2-steps (see Scheme 5 for details).
b
O
OH
Cl
R1
R1
R1
a
b
X
X
X
R2
R2
R2
16
17
HN
18
19
a
b
c
Cl
I
Cl
I
Cl
MeO
I
H
N
HN
OMe
OEt
CN
NH
Me
10e
Br
.HCl
OMe
e
11e
12e
.HCl
R1
R1
R1
d
X
X
X
Me2NSO2
N
N
c
N
O
R2
f
R2
R2
21
20
d
e
Cl
MeO
Cl
MeO
Cl
MeO
N
N
N
NH
NH
TMS
NH
23
16e
15e
g
R1
R1
+
R1
X
X
X
f
N
N
NH
NH
R2
R2
R2
(R)-
22
22
22
rac-
(S)-
g
h
*
R
Scheme 6. Reagents and conditions: (a) NaBH4, EtOH, rt, 3 h; (b) SOCl2, CH2Cl2, rt,
1 h; (c) NaCN, DMSO, rt, 12 h; (d) HCl, EtOH, 0 °C, 12 h; (e) 2,2-dimethoxyethan-1-
amine, MeOH, rt, 2 h; (f) 4 M HCl, dioxane, reflux, 1 h; (g) chiral HPLC.
Cl
MeO
Cl
MeO
Cl
MeO
24
rac-1
(R)-1
Scheme 7. Reagents and conditions: (a) NBS, 1,10-azobiscyclohexanecarbonitrile
(VAZO-88), MeCN, 80 °C, 16 h, 79%; (b) Na, MeOH, 65 °C, 30 min, 100%; (c) TMS-
acetylene, Pd(PPh3)2Cl2 (1 mol %), CuI (2.5 mol %), Et3N, DMF, rt, 16 h, 100%; (d) [Rh
(cod)Cl]2 (1 mol %), PPh3 (40 mol %), Et3N, water (10 equiv), THF, 160 °C, >1000 psi
CO, 12 h, 42%; (e) (i) Tf2O, 2,6-di-t-butyl-4-methylpyridine, CH2Cl2, 0 °C, 30 min, (ii)
sulfamoyl protected imidazole, nBuLi, ZnCl2 (3 equiv), THF, ꢀ40 °C to rt, 2 h, (iii) Pd
(PPh3)4 (8 mol %), 60 °C, 2 h, 77%; (f) HCl (2 equiv), EtOH, 55 °C, 1 h, 81%; (g) Pd/C,
100 psi H2, rt, 18 h; (h) HPLC separation, Chiracel OD-H, heptane/EtOH/Et2NH,
90:10:0.1.
could then be separated into individual enantiomers by chiral
preparative HPLC.
Although this route enabled the delivery of the desired target
compounds, this procedure involved many steps and generally
resulted in low overall yields of products (Table 2). All final com-
pounds were assessed in vitro for their functional agonist activity

L. R. Roberts et al. / Tetrahedron Letters 56 (2015) 6546–6550
6549
Table 3
Commercially available 1-chloro-6-iodotoluene 10e was bromi-
nated with NBS in acetonitrile using 1,10-azobiscyclohexanecar-
bonitrile (VAZOÒ-88) as
radical initiator to afford benzyl
Asymmetric hydrogenation of alkene 24
a
N
N
bromide 11e. The yield was found to be critically dependent on
the purity of toluene 10e, the use of recrystallised NBS and a reac-
tion concentration of 20 mL/g.18 The product could be isolated in
good purity simply by precipitation of the product upon the addi-
tion of water followed by filtration. The crude material was further
purified by slurry trituration with methanol followed by filtration
to give product 11e in high purity and 79% yield without the
requirement for chromatography.
NH
NH
30 mol% cat.
150 psi H2
80 °C, 20 h
Cl
MeO
Cl
MeO
24
(R)-1
Catalyst
% conv.
% ee
RuCl2[(S)-BINAP]
RhCl2[(S)-BINAP]
IrCl2[(S)-BINAP]
100
100
100
100
100
100
100
100
100
100
92
47
37
23
29
24
23
44
27
0
Subsequent displacement of benzyl bromide 11e to provide
methyl ether 12e was achieved in quantitative yield and also with-
out the need for chromatography using freshly prepared sodium
methoxide. Aryl iodide 12e was then coupled via a Sonogashira
reaction in degassed solvent to give alkyne 15e in an excellent
quantitative yield after chromatography. Interestingly, the
corresponding aromatic bromide gave almost no reaction under
identical conditions. With bulk quantities of silyl protected alkyne
15e available, the critical cyclocarbonylation to form indanone 16e
was explored. Due to the sensitive nature of this reaction, initial
attempts to scale the reaction led to diminished yields. However,
further optimisation highlighted that strict control of initial CO
pressure to >1000 psi, an internal temperature of at least 160 °C,
and a reaction time of 12 h were required. Concentration was also
found to be important with 50 mL/g providing consistent results.
Careful maintenance of these parameters ensured the reaction
could be scaled to provide indanone 16e in a moderate 42% yield
on a 40 g scale after chromatographic purification. The moderate
yield was the result of incomplete conversion of starting material
15e which could be recovered and recycled.
Treatment of indanone 16e with triflic anhydride and 2,6-di-t-
butyl-4-methylpyridine gave the corresponding enol triflate.
Commercially available dimethylsulfamoyl imidazole was lithiated
at the 2-position then transmetallated to the zincate. Negishi
coupling with the enol triflate using catalytic Pd(PPh3)4 yielded
imidazole-indene 23, applying an excess of zinc chloride to inhibit
aggregation of the zincate.19 The product 23 was isolated in 77%
yield with the application of disodium EDTA during reaction work
up proving critical to break down residual metal complexes.
Cleavage of the dimethylsulfamoyl group was carried out in
ethanol using 2 M HCl. The resulting salt was collected, purified
via ethanol trituration, and converted to the free base with
aqueous sodium bicarbonate. The addition of base gave rise to a
precipitate that was filtered and dried to afford indene 24 as a
white solid in 81% yield and high purity.
RuCl2[(S)-tolBINAP]23
RuCl2[(S)-segPhos]24
RuCl2[(S)-dmsegPhos]24
RuCl2[(S)-dtbmsegPhos]24
RuCl2[(S)-PPhos]25
RuCl2[(S)-Cl, OMe-BIPHEP]26
RuCl2[(S)-TunePhos]27
RhBF4[SL-J011-1 JosiPhos]28
RhBF4[W-009 WalPhos]29
19
20
29
100
Based on this precedent, the selection of chiral metal–ligand
catalysts was investigated for the asymmetric hydrogenation of
alkene 24. All reactions were carried out using 30 mol % catalyst,
40 mL methanol per g of substrate, 80 °C, 150 psi H2, 20 h. All reac-
tions proceeded to good conversion but the enantioselectivities
were only moderate (Table 3).
Interestingly, RuCl2[(S)-BINAP] as catalyst proved optimal for
this substrate, providing the product in 85% yield, 47% ee
(3:1 er).30 It can be post-rationalised that this lower enantioselec-
tivity relative to that achieved for reduction of the Ramelteon
example 26 may be due to lack of suitably positioned coordinating
groups leading to a less constrained transition state.
This reaction was not optimised further but did translate well
on scale up and could be used to process multi-gram quantities
of material in 85% yield using 5 mol % of catalyst. The stereochem-
ical mixture obtained was further enriched using preparative HPLC
resulting in chemically and stereochemically pure compound 1.
Conclusion
In conclusion, a concise route to synthesise a series of indane
core a1A partial agonists has been described. The route was further
modified for the large scale preparation of lead compound
PF-03774076 (1).4 The optimised route provided >20 g of material
in 7 steps with 18% overall yield and with only four chromato-
graphic purifications required. Significant features of the work
were the harnessing of Rh-mediated cyclocarbonylation to
construct the indane core of the molecule, a Negishi coupling to
introduce a pendant imidazole and the asymmetric hydrogenation
of a challenging electron poor tri-substituted alkene. Notably all
new C–C bonds were formed via metal-catalysed processes.
Compound 1 (PF-03774076, Catalogue No. PZ0263) is now avail-
able from Sigma Aldrich.
Initial routes had relied on achiral hydrogenation to provide
racemic final compound material 1 that was subsequently sepa-
rated into single enantiomers via chiral HPLC. However, in order
to maximise the yield of the desired (R)-enantiomer, as had been
determined by an X-ray structure, an asymmetric synthesis was
sought. Catalytic enantioselective alkene reduction remains a field
of great interest and is the subject of several recent reviews.20,21
The reductions of trisubstituted cyclic alkenes are particularly
challenging and not commonly exemplified in the published
literature. However, some precedent exists for their asymmetric
reaction. Notably, chemists at Takeda were able to reduce alkene
25 with Ru(OAc)2[(R)-BINAP] in 86% yield and 96% ee en route to
the drug molecule Ramelteon 26 (Scheme 8).22
Acknowledgements
We would like to acknowledge David Wallace, John Deering and
Trevor Newbury for help with hydrogenation reactions.
O
NH2
O
Supplementary data
N
H
O
O
Me
Supplementary data (full experimental details, compound char-
acterisation and selected spectra for key compounds) associated
with this article can be found, in the online version, at http://
dx.doi.org/10.1016/j.tetlet.2015.10.004.
96%ee
25
26
Ramelteon,
Scheme 8. Asymmetric hydrogenation of trisubstituted cyclic alkene 25.

6550
L. R. Roberts et al. / Tetrahedron Letters 56 (2015) 6546–6550
17. Human a1A was expressed in Chinese Hamster Ovary (CHO) cells. Receptor
References and notes
activation was determined via calcium mobilisation through the Gq pathway
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measured by a Fluorescent Light Imaging Plate Reader (FLIPR). Concentration–
response curves (11-point) were calculated, with Emax calculated as a percent
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