Anomeric sugar boronic acid analogues as potential agents for boron neutron capture therapy

Article abstract of DOI:10.3762/bjoc.15.135

After the development of accelerators as neutron source, the access to new suitable agents for boron neutron capture therapy (BNCT) became a major need. Among many others, sugar boronic acids have recently attracted attention as boron carriers. Herein we

Full text of DOI:10.3762/bjoc.15.135

Anomeric sugar boronic acid analogues as potential agents
for boron neutron capture therapy
Daniela Imperio*, Erika Del Grosso, Silvia Fallarini, Grazia Lombardi and Luigi Panza*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2019, 15, 1355–1359.
Department of Pharmaceutical Sciences, Università del Piemonte
Orientale, Largo Donegani 2, 28100 Novara, Italy
doi:10.3762/bjoc.15.135
Received: 18 April 2019
Accepted: 11 June 2019
Published: 19 June 2019
Email:
Daniela Imperio* - daniela.imperio@uniupo.it; Luigi Panza* -
luigi.panza@uniupo.it
Associate Editor: S. Flitsch
* Corresponding author
© 2019 Imperio et al.; licensee Beilstein-Institut.
License and terms: see end of document.
Keywords:
antitumor agents; boron neutron capture therapy; boronic acid;
hydroboration; sugar analogue
Abstract
After the development of accelerators as neutron source, the access to new suitable agents for boron neutron capture therapy
(BNCT) became a major need. Among many others, sugar boronic acids have recently attracted attention as boron carriers. Herein
we report the synthesis and preliminary biological studies of two new sugar analogues containing a boronic acid at the anomeric po-
sition. The analogues were obtained by hydroboration of proper open-chain terminal alkenes that, after quenching with water, spon-
taneously afforded cyclic boronic acids with hemiacetal-like structures.
Introduction
Boron neutron capture therapy (BNCT) belongs to the so-called clinical trials are sodium mercaptoundecahydrododecaborate
binary therapies for cancer treatment. It is based on the fission (Na2B12H11SH, BSH) and ʟ-boronophenylalanine (BPA). How-
reaction after a low-energy neutron capture by a 10B atom. The ever, these two compounds are not fully satisfactory owing to
neutron capture reaction gives rise to two high linear energy their rapid clearance from blood [2], their only moderate selec-
transfer (LET) particles (an α-particle and a lithium ion) that tivity for cancer cells and short retention times in tumors.
dissipate all their energy in a path of the order of a cell diame- Therefore, research efforts are ongoing to synthesize new
ter (5–9 μm) in biological tissues, which gives them the poten- BNCT candidates with improved tumor selectivity and cell
tial for precise cell damage. Highly selective delivery, accumu- retention.
lation of boron in tumor tissues and proper subcellular distribu-
tion are required to achieve a successful neutron capture therapy The development of boron-containing carbohydrate derivatives
[1].
for BNCT has received special attention because of the sugar-
preferential uptake by tumor cells [3]. Tumor cells perform
10B atoms to be administered should be incorporated into non- glycolysis at a faster rate than their noncancerous tissue coun-
toxic structures. So far, the two compounds currently used for terparts, so increasing the glucose consumption with respect to
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Beilstein J. Org. Chem. 2019, 15, 1355–1359.
healthy cells. Galactose and fructose also allow tumor growth in The compatibility of boronic acids/esters in the presence of
the absence of glucose.
other functional groups has been reviewed [8]. Usually,
β-alkoxyboronates are reasonably stable with some exceptions
Boronic acid derivatives have gained interest in the last years in [9], while β-hydroxyboronic acids are prone to elimination.
different fields such as the development of enzyme inhibitors, However, β-hydroxyboronic acids are described, such as in the
drug delivery polymers, saccharide sensors and as boron case of the previously mentioned glucopyranos-2-ylboronic
carriers for BNCT, e.g., amino acid derivatives or DNA ligands acid [7] as well as in other reports where they are usually isolat-
[4,5].
ed after conversion into trifluoroborates [10].
In the frame of a more general research program, we have In principle, the best way to obtain anomeric boron analogues
recently published the synthesis of boronic acids derivatives of mimicking hexoses would be the addition of a nucleophilic
monosaccharides in which one of the sugar hydroxy groups is boron reagent on a properly protected pentose. Actually, boryl-
substituted by a boronic acid moiety [6]. A related approach has lithium and magnesium reagents [11-14] have been described
been described [7] recently, where, among other derivatives, but, after a careful evaluation, we discarded this approach
1,2-dideoxy-ᴅ-glucopyranos-2-ylboronic acid has been synthe- considering the scarce information on the general applicability
sized through a regio- and stereoselective hydroboration of of these reagents and for the manipulations required after the
persilylated glucal.
addition to get the desired boronic acid.
Considering the different substitution positions for the boronic Therefore, we decided to explore the synthesis of two simpli-
acid in the sugar skeleton, we were curious to observe the fied compounds, namely the 2-deoxy and the 2,3-dideoxy deriv-
chemical and biological behavior of novel derivatives contain- atives, whose structures are reported in Figure 2, in order to
ing this functional group at the anomeric position, giving rise to have a reliable synthesis and to get preliminary information on
an isostere analogue able to mimic the monosaccharide (see their stability.
Figure 1).
Figure 2: Structures of boron analogues.
Figure 1: Structure of boronic acid analogues (for clarity, sugar
numbering has been conserved into the analogues).
We planned a strategy based on a hydroboration reaction of ter-
minal, suitably functionalized alkenes, which can be obtained
from properly protected sugar precursors (Figure 3).
Results and Discussion
In the present paper the synthesis of two analogues bearing a The synthesis of the first analogue 8, shown in Scheme 1,
boron atom at the anomeric position is reported. These model started from known compound 1, that is easily obtained from
compounds, never described before in literature, allowed to 2,3,5-tri-O-benzyl-ᴅ-arabinose according to a literature proce-
perform a preliminary biological evaluation as well as an evalu- dure [15]. Benzoylation of the free hydroxy group of 1, fol-
ation of their stability.
lowed by removal of the silyl protecting group gave the inter-
Figure 3: Synthetic strategy.
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Beilstein J. Org. Chem. 2019, 15, 1355–1359.
Scheme 1: Synthesis of 2-deoxy analogue 8.
mediate 3 in good yield. Standard substitution of the hydroxy pound 7 [17]. Moreover, the coupling constants of 4.6 and
group gave the 1-deoxy-1-iododerivative 4. Base-promoted 5.7 Hz of H-3 with the protons at C-2 further confirm the
dehydroiodination, followed by Zémplen removal of the assigned C-3 configuration. We were unable to isolate the
benzoate gave the desired enol ether 6, which was selected as arabino-configurated epimer from the reaction mixture, which,
precursor for the preparation of the first anomeric boron ana- if present, may therefore be formed in a very small amount. As
logue.
expected, no regioisomer was detected, due to the contribution
of both steric and electronic effects on the hydroboration at C-1
Different conditions and reagents were tested for the hydro- [7].
boration of the enol ethers 5 and 6 (including the use of cate-
cholborane or dibromoborane at low temperature), but complex In order to explain the stereoselectivity in the hydroboration
mixtures containing elimination byproducts were obtained in reaction we hypothesized the transition states bearing to the two
any condition. We were able to obtain the desired product 7 by epimers. We assumed that borane would react firstly with the
the simple treatment of 6 with an excess of borane-THF com- free hydroxy group generating an intermediate alkoxyborane,
plex (5 molar equiv) followed by quenching with water, while and that the hydroboration reaction occurs intramolecularly on
in the same conditions compound 5 again gave extensive de- such intermediate. Based on these premises, two transition
composition. These conditions allowed the isolation of the
cyclic anomeric boron analogue 7 in moderate yield. Unfortu-
nately, we isolated as main side product the known elimination
compound 7a [16], which further confirms the strong tendency
of β-alkoxyboronic acids to give rise to elimination.
The stereochemistry of the newly formed stereocenter was
inferred from the coupling constant between H-3 and H-4
(sugar numbering). In fact, the measured J3,4 value was 1.8 Hz,
Figure 4: Postulated transition states for the hydroboration reaction.
which is only compatible with a ribo configuration of the com-
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Beilstein J. Org. Chem. 2019, 15, 1355–1359.
states can be identified (Figure 4) for the hydroboration reac- shows two isochronal protons at C-2 with a J2,3 = 4.9 Hz, and
tion on each of the two diastereotopic faces of the double bond. superposition of protons H-3 and H-5 not allowing to hypothe-
The transition state deriving from the attack on the si face, size a clear picture of the 6-membered ring conformation. Con-
which leads to the arabino-configurated product, contains two cerning compound 10, the molecule is rigid around the benzyl-
destabilizing pseudoaxial substituents. The alternative, a more idene ring, while a broadening of the H-2, H-3 and H-4 signals
stable transition state formed from the re-face attack would bear suggest a slow equilibrium between different conformations.
to the actually obtained 3S configurated boronic acid analogue After deprotection of the benzylidene group in addition to line
7.
broadening, signal superposition is observed, hampering any
conclusion on the conformation of compound 11.
The final deprotection of compound 7 by conventional catalytic
hydrogenolysis gave the final compound 8 in almost quantita- In order to get more information about the solution behavior of
tive yield. Again, the final product proved to be unstable these compounds we performed MS analysis. The mass spec-
towards elimination giving rise slowly to the corresponding trum of compound 8 in aqueous solution showed the presence
alkene 8a [18]. In fact, when compound 8 was kept in water or of the cyclic boronic ester both in the positive ionization mode,
methanol solution at room temperature, the formation of 8a was as adduct at m/z 180 ([M + NH4]+) and m/z 185 ([M + Na]+),
observed in about 20% within a few hours. Moreover, the same and in the negative ionization mode, as m/z 161 ([M − H]−). In
elimination occurs even when 8 was stored at −20 °C.
the same positive spectrum the presence of a pseudomolecular
ion at m/z 119 revealed the presence of the elimination product
To avoid β-elimination we planned the synthesis of a new final 8a as [M + H]+ (C5H10O3) confirming the instability of the
sugar devoid of the hydroxy group at position 3. Therefore, we β-hydroxyboronic ester. On the other hand, the positive mass
selected the alkene 9 as substrate for the hydroboration reaction spectrum of compound 11 did not reveal the presence of any
(Scheme 2).
elimination product, while the negative mass spectrum showed
only the pseudomolecular ion at m/z 145 corresponding to the
The hydroboration reaction of the known compound 9 [19], [M − H]−.
again with 5 equiv of reagent, afforded the cyclic boronic acid
10 in a fair yield. Benzylidene group hydrogenolysis gave the The toxicity of compounds 8 and 11, compared to commercial
deprotected pure cyclic boronic acid 11 in quantitative yield and BPA, on cell viability, was evaluated on human primary fibro-
compound 11 proved to be stable for months. In this case the blasts. The cells were treated (24–72 h) with increasing concen-
elimination reaction was not observed as expected for a 3-deoxy trations (1–100 μM) of each compound, and cell viability
derivative. Moreover, compound 11 also proved to be stable measured by MTT assay. No toxicity was measured at all con-
towards oxidation of the C–B bond which could occur on centrations and times considered (see Supporting Information
boronic acids.
File 1), indicating that all compounds under evaluation are
biocompatible and suitable for future biological tests.
Inspection of the NMR spectra can give some information on
the conformation of cyclic boronic acids. Considering that the Conclusion
boron atom should be sp2 hybridized it is not possible to assign In conclusion, we have reported the synthesis, stability evalua-
any α/β configuration to the position 1 of the sugar analogue. tion, and preliminary biological testing of two sugar analogues
Compound 7 showed the following coupling constants: bearing a boron atom at the anomeric position. Compound 8
J2a,3 = 4.6 Hz, J2b,3 = 5.7 Hz, J3,4 = 1.8 Hz and J4,5 = 7.2 Hz. showed a tendency towards elimination reaction, probably
From these values, a chair-like conformation could be inferred, through boronic ester ring opening, while compound 11 demon-
with some flattening around the boron atom. Compound 8 strated to be stable. The two compounds showed no toxicity on
Scheme 2: Synthesis of 2,3-dideoxy analogue 11.
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Beilstein J. Org. Chem. 2019, 15, 1355–1359.
15.van Straten, N. C. R.; van der Marel, G. A.; van Boom, J. H.
Tetrahedron 1997, 53, 6523–6538.
human primary fibroblasts. Further studies will be conducted to
determine the cellular uptake of compounds 8 and 11.
doi:10.1016/s0040-4020(97)00308-6
16.Bravo, F.; Castillón, S. Eur. J. Org. Chem. 2001, 507–516.
doi:10.1002/1099-0690(200102)2001:3<507::aid-ejoc507>3.0.co;2-r
17.Schmidt, R. R.; Frische, K. Liebigs Ann. Chem. 1988, 209–214.
doi:10.1002/jlac.198819880305
18.Jäger, V.; Schröter, D.; Koppenhoefer, B. Tetrahedron 1991, 47,
2195–2210. doi:10.1016/s0040-4020(01)96130-7
Supporting Information
Supporting Information File 1
Experimental procedures and analytical data. Copies of
NMR spectra of all new compounds, mass analysis of
compounds 8 and 8a and toxicity data.
19.Paquette, L. A.; Zeng, Q.; Tsui, H.-C.; Johnston, J. N. J. Org. Chem.
1998, 63, 8491–8509. doi:10.1021/jo981749j
[https://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-15-135-S1.pdf]
License and Terms
This is an Open Access article under the terms of the
Creative Commons Attribution License
(http://creativecommons.org/licenses/by/4.0). Please note
that the reuse, redistribution and reproduction in particular
requires that the authors and source are credited.
Acknowledgements
We gratefully thank Compagnia di San Paolo (GLYCOBNCT
project - CUP: C61J12000280007) for financial support.
ORCID® iDs
Daniela Imperio - https://orcid.org/0000-0001-6810-1439
Erika Del Grosso - https://orcid.org/0000-0002-2398-5944
Silvia Fallarini - https://orcid.org/0000-0001-8871-7417
Luigi Panza - https://orcid.org/0000-0002-0785-0409
The license is subject to the Beilstein Journal of Organic
Chemistry terms and conditions:
(https://www.beilstein-journals.org/bjoc)
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