Efficient three-step one-pot synthesis of a novel 2,3,5-substituted pyrazine library

Article abstract of DOI:10.1021/co200062n

The partnership between rational synthesis design and mass-triggered preparative LCMS is a powerful one, capable of furnishing very large libraries in a selective manner in a very short space of time. Herein, we communicate one example of possibly a perfect marriage between the synthetic chemistry and the subsequent purification method employed, affording a ～1000-member library supplying 50 mg on average of final compound in less than a month.

Full text of DOI:10.1021/co200062n

LETTER
pubs.acs.org/acscombsci
Efficient Three-Step One-Pot Synthesis of a Novel 2,3,5-Substituted
Pyrazine Library
Christian Delvare, Craig S. Harris,* Laurent Hennequin, Patrice Koza, Christine Lambert-van der Brempt,
Jacques Pelleter, and Olivier Willerval
AstraZeneca, Centre de Recherches, Z.I. la Pompelle, BP1050, 51689 Reims Cedex
S Supporting Information
b
ABSTRACT: The partnership between rational synthesis design and
mass-triggered preparative LCMS is a powerful one, capable of furnish-
ing very large libraries in a selective manner in a very short space of time.
Herein, we communicate one example of possibly a perfect marriage
between the synthetic chemistry and the subsequent purification
method employed, affording a ∼1000-member library supplying 50
mg on average of final compound in less than a month.
KEYWORDS: mass-triggered preparative LCMS, three-step one-pot
pyrazine library, Mitsunobu, SuzukiꢀMiyaura
raditional cancer chemotherapy uses cytotoxic agents target-
ting inhibition of DNA synthesis and function or interrup-
T
tion of the cell-cycle.1 Recently, the pharmaceutical industry has
been focusing on new treatments for cancer concentrating on
inhibition of cell-signaling pathways, notably on kinase inhibition.2
From this new strategy to combat this terrible disease at epidemic
proportions,3 we have already seen emerge successes on the
market in the field of epidermal-growth factor receptor (EGFR)
inhibition, such as gefinitib4 and erlontib,5 both compounds
based on mimicking ATP with 4-(anilino)quinazoline scaffolds.5
Since 2000, the industry has heavily exploited this particular
structural template with many compounds currently in devel-
opment;6 therefore, the quest for new scaffolds for kinase inhibitors
has become very important.7 Alongside their core therapeutic
programs, AstraZeneca initiates a certain number of nontargetted
library design and synthesis projects searching for novel scaffolds,
which are tested in high throughput assays in a hit-to-lead style
initiative.8 We designed our speculative library around a pyr-
azine-carboxamide hub offering a hinge binding capacity while
leaving two vectors to explore: the selectivity pocket using Suzuki
reaction9 and the solvent region using Mitsunobu chemistry
(Figure 1).10
Figure 1. Proposed retrosynthetic analysis of the library scaffold.
The synthesis of the scaffolds started from commercially avail-
able 3-aminopyrazine-2-carboxylic acid methyl ester (1). Bromi-
nation followed by diazotisation/bromination (Sandemyer) of
the amino group afforded only 32% yield of dibromide 2 but with
no chromatography. The intermediate phenol 3 was prepared in
excellent yield by SNAr using a slight excess of 4-aminophenol,
under pseudomelt conditions at 100 °C.
To meet our library goals with a 2 diversity points to explore,
we wanted an efficient synthesis preferably with a single purifica-
tion step at the end. We initially set our sights on a solid-phase
approach attaching the scaffold to polystyrene supported Rink
amide linker.11 Saponification of 3 afforded the corresponding
acid which, after significant development work, was successfully
bound to the Rink resin using mixed anhydride methodology
(i-BuOCOCl, NMM, THF, 0 °C to r.t.). The subsequent
Mitsunobu alkylation required a 3-fold excess of reagents to
ensure a complete reaction but the following SuzukiꢀMiyaura
Received: April 3, 2011
Revised:
June 6, 2011
Published: June 07, 2011
r
2011 American Chemical Society
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dx.doi.org/10.1021/co200062n ACS Comb. Sci. 2011, 13, 449–452
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LETTER
Scheme 1a
Scheme 3a
a Reagents and conditions: (a) Br2, NaOAc, AcOH, rt, overnight, 74%;
(b) NaNO2, 48% HBr (aq), Br2, AcOH, H2O, 0ꢀ5 °C, 50 min, 43%; (c)
4-aminophenol, minimum MeOH, 100 °C, 1 h, 97%.
Scheme 2a
a Reagents and conditions: (a) 3 PS-TPP (3 mmol/g), 3 R1-OH, 3 DTAD,
DCM, rt, 30 minutes, quantitative; (b) R2-B(OH)2, Pd(dppf)Cl2.CH2Cl2,
MeOH, CsF, 120°C, microwave, 20 minutes; (c) NH3 in MeOH (7N),
120°C, microwave, 30 minutes.
reaction leaving an ammonolysis “drown-out” of the methyl ester
to afford the final primary carboxamides thus allowing for a simple
one-pot process with concentrations between steps (Scheme 3).
While the alkylation of 3 can be adequately achieved using a
3-fold excess of Mitsunobu reagents using free triphenylpho-
sphine, we used polymer-supported triphenylphosphine in order
to reduce the mass of crude product to purify at the end of the
sequence and avoid contamination of final compounds with
triphenylphosphine oxide.12 The presence of large quantities of
triphenylphosphine oxide (Ph3PO) also led to difficulty in
collecting the final compounds due to the strong mass response
(ESþ) of Ph3PO which tended to drag over a wide area of the
spectrum. These problems were eliminated using polymer-
supported triphenylphosphine. Therefore, after the alkylation
step was complete, the resulting suspension was filtered to
remove the polymer and the filtrate concentrated, dissolved in
MeOH and the resulting solution of 7 was exposed to standard
Suzuki reaction conditions under microwave conditions. The
subsequent solutions of 8 were concentrated and treated with
methanolic ammonia under microwave conditions to afford the final
compounds 9 (Scheme 3).
An important aspect of this approach was the quality of the
crude reaction mixtures after 3 quick operations with excess
reagents. A typical crude LCMS analysis is shown in the
Supporting Information. One can quickly deduce from the
spectra, that after each stage, the crude profile remains almost
100% pure by u.v. allowing for easy final stage purification. In
addition, this class of compounds had a strong mass response
allowing for collection by mass-triggered LCMS.
Library Synthesis. Encouraged by the excellent LCMS crude
profile after each operation during the validation work, we
embarked on a simple type of 10-tube multiparallel experiment.13
The reaction mixture obtained after the Mitsunobu alkylation
was evaporated to dryness taken up in methanol and the solution
was divided into 10 separate microwave pressure tubes for the
a Reagents and conditions: (a) LiOH, MeOH, 50 °C, 68%; (b) iBuOCOCl,
NMM, THF, PS-Rink Amide, quant.; (c) 1-(2-hydroxyethyl)pyrrolidine,
TPP, DTAD, DCM 0 °Ctort, quant.;(d) (Het)ArylB(OH)2, Pd(dppf)Cl2.
CH2Cl2, 1,4-dioxane, THFꢀMeOH, CsF, 80 °C; (e) TFA, TES, DCM,
50%.
cross-coupling reaction could only be progressed to around 50%
completion. Despite numerous attempts to improve on this con-
version by changing solvent, catalytic system, we had difficulty
pushing the SuzukiꢀMiyaura to completion on the resin so we
abandonned the solid-phase approach (Scheme 2). Moreover,
with the basic chain already successfully installed, the small
difference in retention time between the 5-Br, the 5-H impurity
and the desired compound posed certain purification issues using
our standard preparative LCMS conditions.
Preferring something other than a solid-phase approach, we
turned our attention to the possibility of using a straightfoward
3-step one-pot solution phase approach, cascading the 3 reaction
steps together in one-pot starting with the most sensitive step
and finishing with the least sensitive step. In this particular case,
we postulated that the byproduct generated by the Mitsunobu
reaction should be tolerated by the subsequent SuzukiꢀMiyaura
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Table 1. Selected Results from This “One-Pot” 3-Step
Pyrazine Library Preparation
Yield and purity data for selected compounds can be seen in
Table 1.
To summarize, we have developed an efficient 3-step one-
pot reaction by rational reaction design, relying on cascading
reactions types of increasing tolerance with simple concentra-
tions between steps. In this particular case, we started the sequence
with the most sensitive reaction (Mitsunobu alkylation) and
finished with the least sensitive reaction (ammonolysis “drown
out”) and proved that, with a well designed route and a rapid
preparative mass-triggered LCMS purification method, one can
access large libraries in a matter of weeks on pharmacologically
interesting and diverse skeletons. We also hope that this paper
does illustrate nicely that, chemistry permitting of course, one
can achieve acceptable to excellent final purity profiles and yields
without the constraints of developing a solid-phase supported
synthesis.
’ ASSOCIATED CONTENT
1
S
Supporting Information. Experimental procedures, H
b
NMR and LCMS data of a selection of compounds highlighted in
the manuscript. This material is available free of charge via the
Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: craig.harris2@astrazeneca.com.
’ ACKNOWLEDGMENT
The authors would like to thank the Reims support team,
particularly Pascal Boutron and Nadine Gondouin, for their help
during the preparation of this large library.
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microwave, the reaction mixtures were concentrated to dryness
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