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Page 1 of 16 Organic & Biomolecular Chemistry
DOI: 10.1039/C7OB00774D
Journal Name
ARTICLE
Fluorous-Tag Assisted Synthesis of Bile Acid-Bisphosphonate
Conjugates via Orthogonal Click Reactions: An Access to Potential
Anti-Resorption Bone Drugs
Received 00th January 20xx,
Accepted 00th January 20xx
Chiara Massarenti,a Olga Bortolini,a Giancarlo Fantin,a Dario Cristofaro,a Daniele Ragno,a Daniela
Perrone,*a Elena Marchesi,a Gianluca Toniolob and Alessandro Massi*a
DOI: 10.1039/x0xx00000x
The synthesis of a small collection of novel bile acid-bisphosphonate (BA-BP) conjugates as potential drug candidates is
reported. The disclosed methodology relied on the installation of azide and thiol functionalities at the head and tail positions,
respectively, of the BA scaffold and its subsequent decoration by orthogonal click reactions (copper-catalyzed azide-alkyne
cycloaddition, thiol-ene or thiol-yne coupling) to introduce BP units and a fluorophore. Because of the troublesome isolation
of the target conjugates by standard procedures, the methodology culminated with the functionalization of the BA scaffold
with a light fluorous tag to rapidly and efficiently purify intermediates and final products by fluorous solid-phase extraction.
ibandronate and zoledronic acid through the generation of
molecular complexes with lysine-linked deoxycholic acid.6 Our
group also contributed to this area of research reporting on the
synthesis and biological evaluation of the chenodeoxycholic-
derived bisphosphonate 1
(Figure 1).7 This BA-BP conjugate was
Introduction
prepared by a stepwise synthesis strategy and exhibited high
affinity toward hydroxyapatite. Significantly, together with the
lower cytotoxicity compared to neridronate in L929 murine
Geminal bisphosphonates (BPs) are stable analogues of
pyrophosphate and represent an important class of bioactive
compounds, which are currently employed for the treatment
and prevention of several bone disorders such as bone
metastasis, myeloma, rheumatoid arthritis, osteoporosis, and
Paget’s disease.1 BPs mainly act by decreasing osteoclast
activity and inducing osteoclast apoptosis.2 Structure-activity
relationship (SAR) studies highlighted that bioactivity of BPs is
strictly dependent on the nature of substituents installed at the
geminal position of the bisphosphonic moiety, as confirmed by
the evolution of this class of drugs across three generations of
active molecules.3 Despite their successful use, bioavailability
remains a critical feature of BPs since these highly hydrophilic
derivatives are poorly absorbed from the gastrointestinal tract
after oral administration.4 A number of delivery systems have
been investigated to overcome bioavailability limitations of BPs
and improve patient compliance including liposome
encapsulation, use of nanoparticles and co-administration with
adsorption enhancing agents such as surfactants, salicylates,
and bile acids (BAs).5 Indeed, the unique features of facial
amphiphilic BAs have been successfully exploited by Park and
co-workers for improving the intestinal permeability of
fibroblast culture cells, the conjugate
1 displayed a higher
activity in inhibition of osteoclastogenesis.7 These promising
results encouraged us to set-up a modular and general synthetic
strategy to rapidly explore the chemical space around the BA
scaffold through the efficient generation of a small collection of
BA-BP conjugates eventually functionalized with additional
molecular portions such as fluorophores. Therefore, herein we
report on the design and synthesis of the multi-functional BA
scaffold I suitably equipped with azide and thiol groups at the
‘head’ and ‘tail’ positions of the BA unit, respectively, for further
elaborations via orthogonal click reactions (Figure 1). The
copper-catalyzed azide-alkyne cycloaddition (CuAAC),8 thiol-
ene coupling (TEC),9 and the thiol-yne coupling (TYC)9c,10 were
selected for the scope. On the other hand, the high potential
and versatility of the above click reactions performed
sequentially or simultaneously have been amply documented in
materials science and bioconjugation studies.11 Although not
initially planned, the post-reaction phase of BA-BP conjugates
synthesis was addressed in this study by taking advantage of the
incorporation of a light fluorous tag onto the BA moiety
(scaffold II) to facilitate the purification of click intermediates
and the target conjugates. In fact, tagged substrates can be
efficiently separated from nonfluorous-tagged side-products by
fluorous solid-phase extraction (F-SPE).12 Noteworthy, while
fluorous technology has been widely utilized for the synthesis
of biomolecules such as peptides and carbohydrates,13 its
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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