Angewandte
Communications
Chemie
[
23]
Recent reports have revealed that these molecules function as
telomerase inhibitors, with the [6,5]-spiroketal ring system in
this case playing an essential role in the rubromycinꢀs
[6,6]-bisbenzannulated spiroketals
and is described in
Scheme 1. We reacted aldehyde 3 with the lithium acetylide
of 2 to obtain alkynol 4 in a reasonable 58% yield. After
subsequent catalytic hydrogenation of 4, we performed
[
22]
pharmacophore.
Despite the abundance of spiroketals
and their highlighted potential as medicinal chemistry scaf-
folds, the structure-based design and structural validation of
spiroketals as medicinal chemistry scaffolds has not been
clearly demonstrated.
A reported crystal structure of a [6,6]-bisbenzannulated
[23]
spiroketal
inspired us to consider these structures as
nuclear receptor (NR) ligands, which we speculated would
possess the correct size, shape, and hydrophobicity to target
the L-shaped ligand binding pocket (LBP) of the retinoid X
[
24]
receptor (RXR),
a member of the superfamily of gene
transcription factors. RXR plays a central role in hormone-
driven cell-signaling events through its ability to heterodi-
[
25]
merize with other type II nuclear receptors. RXR is a drug
target for the treatment of cutaneous T-cell lymphoma and is
under investigation as a potential treatment for Alzheimerꢀs
[26]
Disease.
thoroughly investigated,
Despite the fact that RXR ligands have been
[
27]
only a very few RXR partial
agonists with limited structural diversity have been charac-
terized, and rules guiding the design of heterodimer-specific
RXR ligands are essentially non-existent. In part as a response
to these challenges, and in continuation of our groupꢀs recent
Scheme 1. Reagents and conditions: a) n-BuLi, THF, À788C to RT;
b) 10% Pd/C, KHCO , EtOAc, RT; c) Dess–Martin periodinane,
3
CH
Cl
, RT; d) TMSBr, CH
Cl
, À308C to RT; e) NaOH, dioxane/
2
2
2
2
1
13
[
28]
MeOH, 408C. Characteristic H and C resonance peaks are summar-
efforts to identify selective RXR and other NR modula-
[
23]
[
29]
ized for 7 and (Æ)-1.
tors,
we report the first designed spiroketal protein
modulator, as exemplified by RXR modulation.
To assess the potential of bisbenzannulated spiroketals
to target RXR, we adopt a classical scaffold-hopping
a Dess–Martin oxidation on the resulting secondary alcohol
5 to generate the spirocyclization precursor, 6, which we
treated with trimethylsilyl bromide to effect a one-pot
deprotection/cyclization, which produced the [6,6]-bisben-
zannulated spiroketal 7 in a yield of 64%. The synthesis of
[
30]
approach
commencing with the co-crystal structure of
commercial nanomolar-potent RXR full agonist BMS 649
[
31]
(
otherwise known as SR11237) bound to hRXRa (PDB
[
16]
code: 1MVC).
We then replaced the rigid acetal linker
(Æ)-1 concluded with
a straightforward base-mediated
hydrolysis of the methyl ester group. The resonance peak at
present in BMS 649 with a [6-6]-bisbenzannulated spiroketal
1
3
linker while retaining the key tetramethyl-tetrahydro-
dc 97.0 ppm in the C-NMR spectra of 7 and (Æ)-1 is
[
32]
1
naphthyl- and carboxylic acid groups, to generate 1. We
then modelled and docked both enantiomers of 1 into the
space occupied by BMS 649 in the LBD of the hRXRa-BMS
diagnostic for the spirocarbon, while the H resonances
3
corresponding to the two sets of diastereotopic protons H ,
3
’
4
4’
H , H and H suggest that the spiroketal adopts a diaxial ring
conformation (Supporting Information) similar to analogous
[
16]
6
49 co-crystal structure using the FlexX docking module in
[
23]
the LeadIT suite followed by evaluation using the scoring
function HYDE in SEESAR (Figure 1). While all attempts at
docking the S-enantiomer in this PDB structure failed to
generate poses, we succeeded in docking the R-enantiomer,
with the best pose shown in Figure 1B. Although our docking
studies did not take into account the thermodynamic prefer-
ences of the spiroketal ring system, interestingly, R-1 adopts
a diaxial ring conformation, which would be favored owing to
bis-anomeric stabilization. In this ring conformation favor-
able polar interactions are maintained between the carboxylic
acid of R-1, Arg316, and the backbone of Ala327.
structures.
We profiled the RXRa-activity of (Æ)-1 alongside full
agonist LG100268 (Figure 1A) in a fluorescence-based
cofactor recruitment assay (Figure 2, left and Table 1). As
[
36]
[
28]
expected,
D22 peptide
LG100268 induced potent recruitment of the
[
37]
with an EC = 0.10 Æ 0.01 mm. Intriguingly,
5
0
(Æ)-1 was also active, with an EC = 0.73 Æ 0.06 mm, and
5
0
additionally exhibited a partial agonist behavior, as judged by
the levelling off of polarization at 53% of the maximum
response induced by LG100268. We tested (Æ)-1 against two
other LXXLL-derived peptides (Table 1), ribosome display
[
35]
[38]
To enable an expedient testing of our binding hypothesis,
we elected for a racemic synthesis of (Æ)-1, with a view to
separating the enantiomeric spiroketals by chiral HPLC at
peptide Pro22 and the naturally occurring peptide TIF2,
and observed similar EC50 values but different % efficacies.
One of the separated enantiomers, 1-ent1, was found to
approach the potency of LG100268 in the same FP assay
(Figure 2, left and Table 1). Furthermore, 1-ent1 displayed
seven-fold higher potency and two-fold higher % efficacy
than the other enantiomer, 1-ent2. The isolated enantiomers
did not evidently epimerize under the acidic separation
[
33,34]
a later stage. In reported syntheses of [6,5]-
and [6,6]-
[3]
bisbenzannulated spiroketals, the spiroketal core is fre-
quently formed under thermodynamically driven conditions
through a dehydrative ring cyclisation. Our synthesis of (Æ)-
1
was based on work by Brimble and co-workers on analogous
2
ꢀ 2017 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2017, 56, 1 – 6
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