ACS Medicinal Chemistry Letters
Letter
With these influences in mind, we set out to design a novel
sp3-rich screening library and demonstrate its utility by
identifying novel antimalarial leads. As previously reported,
we have focused some of our efforts on analogues and
derivatives of 2,4-methanoproline 1 (Figure 1).6 For this effort,
Scheme 2. Synthesis of 1-(Amino-methyl)-2-benzyl-2-aza-
bicyclo[2.1.1]hexane 2a
a
a
Reagents and conditions: (a) LiAlH4, THF (78%); (b) phthalimide,
polymer-supported PPh3, DIAD, THF (83%); (c) H2NNH2, EtOH
(72%); (d) DIPEA, HATU, DCM, R1 carboxylic acid or DIPEA,
COMU, DCM, R1 carboxylic acid (See Table 1 for yields).
A diverse 234-member array of amide derivatives of 2a was
then prepared using a parallel synthesis/purification method
employing standard amide forming reactions with HATU or
full list of the compounds synthesized).
The compounds were assessed for their antimalarial activity
using a 3D7 SYBR Green I in vitro assay.15,16 All compounds
were screened in a single point assay at a concentration of 10
μM to identify those with activity >50% inhibition. Actives and
a representative selection of less active compounds were
screened in potency mode with a top concentration of 25 μM
following a 10-point 1 in 3 dilution series (see Table 1; see
the compounds synthesized).
The 68 compounds titrated displayed IC50 values in the
range 1 to 25 μM with a reasonable correlation between the
activity in the primary assay and the titrated IC50 assay (Figure
2). Three compounds have activity <2 μM providing a hit rate
of 1.3% based on this cut off, which compares extremely
favorably with the hit rates of the two recent screens described
earlier.
Figure 1. Proline analogues.
we selected 1-(amino-methyl)-2-benzyl-2-aza-bicyclo[2.1.1]-
hexane 2a as an ideal key scaffold for the preparation of the
library. During the completion of the work in this publication,
2a was reported by Levterov et al. as a precursor to the two
differentially protected N-Boc derivatives 2b and 2c.9 Although
there have been numerous reports of the “aminoproline” 3, its
analogues, and their biological activities, there are relatively few
reports of biological activity associated with the 2,4-methano-
bridged variant. Two examples are the insect repellent/
antifeedant activity of compounds such as 2d10 and the
potential therapeutic application of compounds such as 4,
found to be potent inhibitors of the glycine transporter
GlyT1.11
The N-benzoyl ethyl ester of 2,4-methanoproline 5 was
prepared by the method of Malpass et al. in a two step process
from ethyl pyruvate (Scheme 1) via an intramolecular [2 + 2]
Scheme 1. Synthesis of Ethyl 2-Benzoyl-2-
azabicyclo[2.1.1]hexane-1-carboxylate 5
a
The unsubstituted benzamide (203) was found to be
inactive (−20% @ 10 μM); however, positioning a chloro at
the 3-position of the phenyl ring gave useful activity (51, IC50
13 μM). The bioisosteric 3-bromo (89, 35% @ 10 μM) and 3-
methyl (109, 7.6% @ 10 μM) analogues showed no advantage.
However, the 3-trifluoromethoxy (38, IC50 9.3 μM) showed a
possible improvement. The 2-chloro (143, −7.9% @ 10 μM)
and 4-chloro (81, 38% @ 10 μM) proved to be less active.
Using the 3-chloro as the starting point for further analysis,
additional substitution at the 2-position (e.g., the 2,3-dichloro
compound 147, −8.2% @ 10 μM) and the 6-position (e.g., the
3-chloro-6-methyl analogue 62, IC50 19 μM) proved
detrimental. In contrast, 3,4-disubstitution offered a modest
improvement in activity (21, IC50 5.1 μM). Additional
substitution at the 5-position proved to be the most
advantageous (e.g., the 3-chloro-5-trifluoromethyl analogue 9,
IC50 1.3 μM). A range of derivatives with lipophilic
substituents in the 3,5-positions were also active. In general,
the analogous benzylamides were less active (see 43 vs 21, 67
vs 42, 80, vs 46, and 146 vs 72); however substitution at the
benzylic position dramatically improved activity with the
spirocyclobutyl compound 26 (IC50 6.2 μM) proving to be
a
Reagents and conditions: (a) allylamine then PhCOCl, Et3N,
toluene (47%); (b) hν, acetone (40%).
olefin photocycloaddition reaction.12 The original syntheses of
the N-benzoyl methyl ester of 2,4-methanoproline were
published in back-to-back articles, the first utilizing the methyl
pyruvate13 and the second starting from serine.14
The N-benzoyl ethyl ester of 2,4-methanoproline 5 was used
to prepare 2a in a three stage process (Scheme 2); first
reduction with lithium aluminum hydride to produce the N-
benzyl alcohol 6, followed by a Mitsunobu reaction with
phthalimide to generate the phthalimide derivative 7, and
finally hydrazinolysis to yield the desired material. Levterov et
al. used an alternative approach via the tosylate of 6, followed
by conversion to the azide and then Staudinger reduction.9
B
ACS Med. Chem. Lett. XXXX, XXX, XXX−XXX