Chemically-enabled synthesis of 1,2,3,4-tetrahydroisoquinolin-1-ones

Article abstract of DOI:10.1016/j.tetlet.2009.02.157

We have developed a number of efficient protocols for the facile synthesis of 1,2,3,4-tetrahydroisoquinolin-1-ones. This synthetic methodology allowed concise and efficient exploration of the SAR in all areas of the molecule. A number of these methods pro

Full text of DOI:10.1016/j.tetlet.2009.02.157

Tetrahedron Letters 50 (2009) 2140–2143
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Chemically-enabled synthesis of 1,2,3,4-tetrahydroisoquinolin-1-ones
*
Paul S. Humphries , Gayatri Balan, Bruce M. Bechle, Edward L. Conn, Kenneth J. Dirico, Yu Hui,
Robert M. Oliver, James A. Southers, Xiaojing Yang
Pfizer Global R&D, CVMED Chemistry, 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:
We have developed a number of efficient protocols for the facile synthesis of 1,2,3,4-tetrahydroisoquin-
olin-1-ones. This synthetic methodology allowed concise and efficient exploration of the SAR in all areas
of the molecule. A number of these methods proved to be versatile, efficient and amenable to parallel
synthesis.
Received 5 February 2009
Accepted 20 February 2009
Available online 25 February 2009
Ó 2009 Elsevier Ltd. All rights reserved.
G protein-coupled receptor 40 (GPR40) belongs to a family of
fatty acid (FA) binding GPCRs, which include GPR40, GPR41,
GPR43, and GPR120.1 GPR41 and GPR43 are activated by short-
chain FAs, GPR40 is activated by medium- and long-chain FAs,
and GPR120 is activated by long-chain FAs.2 GPR40 is highly ex-
pressed in pancreatic b-cells and insulin-secreting cell lines.
GPR40-deficient b-cells secrete less insulin in response to FAs,
and loss of GPR40 protects mice from obesity-induced hyper-insu-
linemia, hyperglycemia, and glucose intolerance. Conversely, over-
expression of GPR40 in b-cells of mice leads to impaired b-cell
function, hyperinsulinemia, and diabetes.3 These results suggest
that GPR40 plays an important role in linking obesity and type 2
diabetes.
Figure 1. Initial hits 1 and 2 from high-throughput screening.
Our first parallel chemistry effort required a Mitsunobu reaction
between phenol 7 and a variety of alkyl alcohols (Scheme 2).8 This
reaction was attempted utilizing polymer-supported triphenyl-
phosphine, but with no success.9 It was also found that DMF was
required as a co-solvent, due to the poor solubility of template 7
in THF alone. Diisopropyl azodicarboxylate (DIAD) was found to
be optimal when compared with DEAD, DMAD and PPh3CN. The
resulting products 8 were taken on crude into the ensuing sapon-
ification reaction to afford the required acids 9. In singleton exper-
iments, the crude acids 9 were subjected to an SPE ‘catch and
release’ work-up with SAX cartridges.10 This work-up allowed for
the products to be isolated in pure form, free from any residual
DMF solvent or triphenylphosphine oxide by-products. Utilizing
this procedure, hit compound 1 was obtained in a 75% yield over
the two steps. A small array (ꢀ185) of diverse alkyl alcohols were
subjected to this synthetic sequence (exchanging a base-acid wash
for the SPE work-up) with a 53% success rate.
Our next parallel array required a method for obtaining diaryl
ethers 11 from phenol 7 (Scheme 3). Utilizing elegant methodology
developed in the Evans laboratories, copper-promoted arylation of
phenol 7 was carried out with a diverse set of arylboronic acids to
yield required products 10.11 Once again, the resulting products 10
were taken on crude into the saponification reaction to afford the
required acids 11. Utilizing this methodology, hit compound 2
was obtained in a 71% yield over the two steps. A small array
As part of an ongoing GPR40 drug discovery program in our lab-
oratories, compounds 1 and 2 were identified from high-through-
put screening (HTS) and possessed attractive pharmacological
properties as well as structural features amenable to optimization
by rapid parallel synthesis (Fig. 1).
We initially wanted to explore the SAR of the terminal cyclo-
hexyl and phenyl moieties of 1 and 2. In order to do this in an effi-
cient manner, we required
a concise synthesis of phenol
intermediate 7. This intermediate was accessed in a straightfor-
ward fashion following the five step protocol below (Scheme 1).4
4-Benxyloxybenzaldehyde 3 was condensed with propylamine to
afford imine 4 in excellent yield. Treatment of imine 4 with hom-
ophthalic anhydride resulted in the formation of cis-lactam 5 in
moderate yield along with minor amounts of trans-lactam 6.5
The thermodynamically less stable isomer 5 could be epimerized
under acidic conditions, resulting in the exclusive formation of
trans-lactam 6 in excellent yield.6 Esterification of acid 6 with iodo-
methane afforded the intermediate methyl ester, which was subse-
quently debenzylated utilizing boron tribromide to yield the
required phenol intermediate 7.7
* Corresponding author. Tel.: +1 860 705 0559.
E-mail address: phumphri@gmail.com (P.S. Humphries).
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.02.157

P. S. Humphries et al. / Tetrahedron Letters 50 (2009) 2140–2143
2141
0
Scheme 1. Reagents and conditions: (a) C3H7NH2, 4 ÅA MS, CH2Cl2, rt, 16 h, 100%; (b) homophthalic anhydride, CH3CN, 60 °C, 16 h, 50%; (c) AcOH, 120 °C, 16 h, 95%; (d) MeI,
K2CO3, Me2CO, rt, 16 h, 90%; (e) BBr3, CH2Cl2, rt, 16 h, 90%.
(ꢀ110) of diverse arylboronic acids were subjected to this syn-
thetic sequence with a 85% success rate.
The final parallel library to utilize phenol 7 involved SNAr reac-
tions with 2-bromoheteroaryls to obtain the required products 13
(Scheme 4) This step was optimized under a variety of conditions,
utilizing 2-bromopyridine as our partner of choice. Ullman-type
coupling of phenol 7, in the presence of catalyst 14, afforded 12,
which was saponified to give 13 (where HetAr = 2-pyridyl) in
85% yield.12 A small array (ꢀ20) of 2-bromoheteroaryls were sub-
jected to this synthetic sequence with a 100% success rate.
The next endeavor required more diverse SAR exploration of the
chemical series (Scheme 5). To achieve this undertaking, we
needed to optimize the chemistry in Scheme 1 in order to allow
for it to be utilized in a number of parallel arrays. Two of the most
immediate challenges were the solubility of the reagents/products
in the existing solvents and the requirement for Dean–Stark condi-
tions on formation of the imine (e.g., 4) Solutions to these chal-
lenges involved utilizing conditions similar to reductive
aminations by choosing methanol as the solvent of choice for imine
formation. The poor solubility of homophthalic anhydride necessi-
tated the use of DMF as the solvent for the formation of the cis- and
Scheme 2. Reagents and conditions: (a) ROH, PPh3, DIAD, THF, DMF, 80 °C, 16 h; (b)
LiOH, THF, MeOH, H2O, rt, 16 h.
Scheme0 3. Reagents and conditions: (a) ArB(OH)2, Cu(OAc)2, Et3 N, Py, 1,2-DCE,
DMF, 4 ÅA MS, air, 50 °C, 16 h; (b) LiOH, THF, MeOH, H2O, rt, 16 h.
Scheme 4. Reagents and conditions: (a) 2-Br-HetAr, CuI (cat.), 14 (cat.), K3PO4,
MeCN, 100 °C, 16 h; (b) LiOH, THF, MeOH, H2O, rt, 16 h.

2142
P. S. Humphries et al. / Tetrahedron Letters 50 (2009) 2140–2143
Scheme 5. Reagents and conditions: (a) MeOH, rt, 16 h then homophthalic
anhydride, DMF, rt, 16 h then 1 N aq NaOH, rt, 16 h.
trans-lactams (e.g., 5 and 6). The resulting mixture was converted
to all trans-lactam (e.g., 6) by subjection to strong base, rather than
the original acidic conditions. Two libraries were executed utilizing
this synthetic sequence with a success rate of 67% (varying R2) and
71% (varying R1).
Diversification of the left-hand phenyl ring required an efficient
synthesis of diverse homophthalic anhydrides. A representative
synthetic scheme from this exercise is shown below (Scheme 6).
Acid 18 was subjected to copper-catalyzed coupling with the anion
of dimethyl malonate to afford diester 19, which was taken on
crude. Decarboxylation of 19 was achieved under acidic conditions
to yield required bis-acid 20 in moderate yield over two steps.13
Dehydrative cyclization of 20, utilizing 22,14 resulting in the effi-
cient formation of anhydride 21 in excellent yield.15 Attempts to
achieve this transformation utilizing acetyl chloride,16 acetic anhy-
dride,17 or thionyl chloride18 resulted in lower yields. A number of
other substituted anhydrides were also synthesized in a similar
fashion.
Scheme 7. Reagents and conditions: (a) LDA, THF, ꢁ78 °C, MeI, 20 min, 92%; (b)
LiOH, THF, MeOH, H2O, rt, 16 h, 95%.
SAR exploration also required us to synthesize the
a-methyl-
carboxylic acid 25 and it’s diastereomer 30. Intermediate ester 23
(obtained via Scheme 5) was deprotonated and the resulting eno-
late was subjected to iodomethane to afford the single diastereo-
mer 24 in 92% yield. Relative stereochemistry was confirmed by
the observed NOE (2.5%) between the C-3 methine and the C-4
methyl group. This facial selectivity is also in line with that re-
ported by others.19 Saponification of 24 yielded required acid 25
in a straightforward fashion (Scheme 7).
Due to the complete facial selectivity in the formation of 24, ac-
cess to diastereomer 30 required the synthesis of anyhdride 29
(Scheme 8). Esterification of bis-acid 26 afforded bis-ester 27.
Deprotonation of 27, treatment of the resulting enolate with iodo-
methane, followed by saponification afforded 28 in good yield.20
Dehydrative cyclization, utilizing 22, occurred in moderate yield
to form required anyhdride 29.
Scheme 8. Reagents and conditions: (a) Me2SO4, K2CO3, 1,4-dioxane, 70 °C, 16 h,
87%; (b) LDA, THF, ꢁ78 °C, MeI, HMPA, 30 min, 75%; (c) LiOH, 1,4-dioxane, MeOH,
H2O, 50 °C, 16 h, 62%; (d) 22, CH2Cl2, rt, 1 h, 52%.
ture of two diastereomers 30 and 25. 1H NMR analysis of the crude
reaction mixture revealed a ratio of 1.4–1.7:1 favoring diastereomer
30, which was isolated in 25% yield. Diastereomer25 was found to be
Formation of the imine from 4-phenoxybenzaldehyde and pro-
pylamine, followed by treatment with anyhdride 29 afforded a mix-
Scheme 6. Reagents and conditions: (a) NaH, CuBr2 (cat.), CH2(CO2Me)2, 70 °C, 2 h;
(b) HCl, 110 °C C, 48 h, 50% (two steps); (c) 22, CH2Cl2, rt, 1 h, 93%.
Scheme 9. Reagents and conditions: (a) 4-phenoxybenzaldehyde, propylamine,
MeOH, rt, 16 h then 29, DMF, rt, 16 h, 30 = 25% and 25 = 17%.

P. S. Humphries et al. / Tetrahedron Letters 50 (2009) 2140–2143
2143
3. Steneberg, P.; Rubins, N.; Bartoov-Shifman, R.; Walker, M. D.; Edlund, H. Cell
Metab. 2005, 1, 245.
4. All new compounds were characterized by full spectroscopic data, yields refer
to chromatographed material with purity >95%.
identical to that obtained by the method in Scheme 7. Unlike 25, dia-
stereomer 30 showed no observable NOE between the C-3 methine
and the C-4 methyl group (Scheme 9).
5. (a) Ryckebusch, A.; Garcin, D.; Lansiaux, A.; Goossens, J.-F.; Baldeyrou, B.;
Houssin, R.; Bailly, C.; Henichart, J.-P. J. Med. Chem. 2008, 51, 3617; (b) Morrell,
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A.; Antony, S.; Kohlhagen, G.; Pommier, Y.; Cushman, M. J. Med. Chem. 2006, 49,
7740.
In summary, we have developed a number of efficient protocols
for the facile synthesis of 1,2,3,4-tetrahydroisoquinolin-1-ones.
This synthetic methodology allowed concise and efficient explora-
tion of the SAR in all areas of the molecule. A number of these
methods proved to be versatile, efficient and amenable to parallel
synthesis.
6. (a) Cushman, M.; Gentry, J.; Dekow, F. J. Org. Chem. 1977, 42, 1111; (b)
Cushman, M.; Mohan, P. Tetrahedron Lett. 1985, 26, 4563.
7. Paliakov, E.; Strekowski, L. Tetrahedron Lett. 2004, 45, 4093.
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Mitsunobu, O. Synthesis 1981, 1; (c) Hughes, D. L. Org. React. 1992, 42, 335; (d)
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Acknowledgements
The authors would like to thank Chris Limberakis for stimulat-
ing discussions and feedback on this manuscript.
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