Letter
solvent exposure region shaped by TM2/4c and HP2 (Figure 4e,
g). In summary, the modeling study substantiated our design
4
and motivated us to continue with the synthesis and
pharmacological evaluation of 15a−d.
Asymmetric synthesis of C-3 substituted Asp analogs is
challenging due to the required L-threo configuration at vicinal
chiral centers. This is evidenced by the published asymmetric
syntheses of L-TBOA and TFB-TBOA which are both
27,35
lengthy.
Alternatively, the Lochner group has reported an
asymmetric aminohydroxlyation strategy to access the L-threo
3
6
isomer, however, with low enantiomeric selectivity. An
efficient chemoenzymatic methodology by use of MAL-L384A
as biocatalyst was reported by Poelarends group. However, the
drawback is the narrow substrate specificity of enzyme together
31,37,38
with its limited commercial availability.
We thus turned to develop a new strategy. Our retrosynthetic
analysis suggests that 15a−d could be derived from O-alkylation
of dimethyl (L-threo)-N-Boc-3-hydroxyaspartate (7, Asp frag-
ment) with C′-3 appropriately substituted (R group) 6-
(
bromomethyl)-1-tosyl-1H-indole (13a−d, indole part) fol-
lowed by deprotection (Scheme 1). The key precursor 7 is
readily obtained from commercially available D-tartaric acid (2)
through the reported synthesis route for dimethyl (3S)-2-azido-
3
-hydroxysuccinate (5a/5b), followed by azide reduction and
39
protection. We predicted the indole part 13a−d is obtained
from commercially available methyl 1H-indole-6-carboxylate
(
8) via bromination, N′-1 position substitution, Suzuki−
Miyaura reaction, reduction, and nucleophilic substitution.
The synthesis of Asp fragment 7 (Scheme 2) commenced
with esterification of commercially available D-tartaric acid (2)
under standard conditions to give dimethyl ester 3. After
treatment with HBr in AcOH and acetate hydrolysis in acidic
methanol, the corresponding bromo alcohol 4 was obtained in
40
7
4% yield over two steps. Nucleophilic substitution with sodium
azide gave 5 in 68% yield as a mixture of L-threo 5a and D-erythro
b diastereomers (threo/erythro = 5.6:1). Hereafter the
5
diastereomeric azides 5a/5b were reduced to amine 6a/6b by
H2 and 10% Pd/C, and the diastereomeric mixture was
separated by column chromatography to give the L-threo isomer
6
a isolated in 36%. The absolute configuration of 6a was
1
Figure 4. Modeling of ligands into EAAT1cryst (green stick) or into
hmEAAT1 (white stick), and outline of the key residues (pink line)
involved in binding the ligands and hydrogen bonds (yellow dash). (a)
Overlay of binding pose of ligands (green) into hmEAAT1 and TFB-
TBOA (PDB: 5mju, white) in EAAT1cryst. (b) Binding mode of TFB-
TBOA with EAAT1cryst (PDB: 5mju). (c−g) Binding mode of
compound 15a−d from induced-fit docking into hmEAAT1.
Compound numbers are labeled on the respective figures.
confirmed by comparison with reported H NMR data for the
41
analogous diethyl ester analog. After Boc-protection, optically
pure Asp fragment 7 was ready for the following O-alkylation.
Overall, key intermediate 7 was prepared in only five steps with
an 11% overall yield.
For the synthesis of indole part 13a−d (Scheme 3), the route
began with selective bromination at the C′-3 position of 6-
methoxycarbonylindole with N-bromosuccinimide (NBS) at
low temperature (−78 °C) to give bromine 9 in 80% yield. The
strong base NaH was used to deprotonate the indole ring of
compound 9 (or 8), which was then reacted with tosyl chloride
to give N′-1 tosylamides 10 (or 11a) in over 90% yield. The
appropriate boronic acid was used in the subsequent palladium
coupling with 10 to give the series of analogs 11b−d in 69%−
79% yield. To introduce the alkyl bromide functionality, esters
11a−d were first reduced to their corresponding alcohols 12a−
According to the induced-fit docking results (Figure 4c−g), the
key interactions with the Asp fragment were conserved among
analogs 15a−d, and the residues at equivalent positions in
hmEAAT1 were Ser363, Ser365, Thr402, Asp476, Arg479, and
Thr480. No hydrogen bonds could be identified for the β-
indolyloxy part. However, for the binding modes identified for
1
5b−d in hmEAAT1, residues of differentiation to EAAT2,3
d by LiAlH . After some experimentation, we established that
4
could lead to subtype selectivity (Supporting Information).
excess equivalents of MsCl (3.0 equiv), LiBr (8.0 equiv), and
Furthermore, the incorporated groups on the C′-3 position of
Et N (4.0 equiv) in THF provided the desired alkyl bromides
3
42
1
5c and one binding mode of 15b oriented to the unfilled back
13a−d in 68%−79% yield.
subpocket constructed by TM7a/4c and HP2 (Figure 4d, 4f).
For 15d and one different binding mode of 15b, the C′-3
substituents are directed into the hitherto unexplored front
The synthesis of target molecules 15a−d (Scheme 3) was
finalized by treatment of Asp fragment 7 with NaH and
appropriate bromines 13a−d at low temperature (−15 °C) for 4
D
ACS Med. Chem. Lett. XXXX, XXX, XXX−XXX