Mechanistic Insights on Pd-Catalyzed Three-Component Reactions of Alkynols, Methyl Orthoformate, and Salicylaldehyde Derivatives. Application to the Synthesis of Steroid Chroman Ketals and Spiroketals with Antioxidant Activity.

Contrary to our previous report in which a Pd-catalyzed three-component reaction of a steroid alkynol, trimethyl orthoformate, and salicylaldehyde exclusively produced chroman ketals, the same reaction employing 2,5-dihydroxysalicylaldehyde led to a mixture of a chroman ketal and a spiroketal. Provided that both courses of the reaction imply a 4 + 2 inverse demand cycloaddition between an o-quinone methide and an enol ether, density functional theory calculations revealed that the chroman ketal/spiroketal selectivity is governed by both, the rate of the formation of the o-quinone methide and the isomerization of the initially produced exocyclic enol ether─that led to the spiroketal─to its endocyclic partner that produces the chroman ketal. Remarkably, Lewis catalysis is central to the observed reactivity, and the availability of plausible catalytic species controls the overall chemoselectivity. The methodology herein applied and scrutinized enriches the palette of reactions, leading to increased molecular complexity, as demonstrated in the obtained products, whose antioxidant activity and detailed NMR characterization are presented.

Synthesis, NMR characterization and cytotoxic activity of hybrid spirostanic sapogenins-estradiol dimers.

Four hybrid steroid dimers were obtained by BF3·Et2O-catalyzed aldol condensation of acetylated steroid sapogenins with 2-formyl-estradiol diacetate. The structures of the obtained dimers were unambiguously established by NMR. The hybrid dimers 9a (IC50 18.37 µM) and 9c (IC50 9.4 µM) with the 5α configuration at the A/B rings junction showed the higher cytotoxicity against HeLa, with selectivity index of 4.36 and 11.8 respectively. The presence of a carbonyl function at position C-12 produced the highest cytotoxic effect, which is in line with our previous reports.

Methyl esters of 23,24-Dinor-5α-cholan-22-oic acids as brassinosteroid Analogues. Synthesis, evaluation of plant growth promoting activity and Molecular docking.

Five new brassinosteroid analogues were synthetized from 3ß-acetoxy-23,24-dinorchol-4-en-22-oic acid. All the obtained compound showed significant activity in the Rice Lamina Inclination Test. Interestingly the effects of the methyl ester of 3ß-hydroxy-6-oxo-23,24-dinorcholan-22-oic acid (14) at concentrations of 1 × 10-7 and 1 × 10-6 M proved to be higher than those produced by brassinolide. In silico Molecular Docking and Induced fit docking (IFD) simulations for the compounds with the highest biological activity data were carried out to investigate the binding mode interactions into the brassinolide-binding groove which revealed that the compound 14 had high binding energy values and a good affinity.

Self-Assembly of an Amphiphilic Bile Acid Dimer: A Combined Experimental and Theoretical Study of Its Medium-Responsive Fluorescence.

This work describes the synthesis and aggregation behavior of a dimeric bile acid derivative in which two steroid cores are bridged by a p-di(phenylethynyl)phenylene fluorophore. The studied compound contains three key characteristics: (a) restricted conformational equilibrium in solution, (b) efficient fluorescence conferred by the bridge, and (c) medium responsiveness encoded in the steroid fragments. The incorporation of the three components afforded a compound that generates nano- and micrometric spherical particles with aggregation-responsive fluorescence emission. The observed self-assembly process of the featured molecule was induced by the gradual addition of water to the tetrahydrofuran (THF) solution. This aggregation led to significant changes in fluorescence that went from two bands at λem values of 370 and 390 nm in pure THF to a new spectrum with two maxima at λem values of 395 and 418 nm at high water contents, without a decrease in emission. The observed changes can be ascribed to weakly coupled aggregation, a hypothesis supported by multiscale molecular modeling, which sheds light on the mechanism of the self-assembly of this unconventional amphiphile.

Reductive desymmetrization of steroid dimers.

NaBH3CN reduction of symmetric dimers in which two steroid units are linked by a 1,4-dimethylidenebenzene moiety followed two different courses: (a) hydrogenation of the benzylic double bond and (b) reductive F ring opening of the side chain. While courses a and b led to symmetrical dimers, the combination of both pathways produced an unsymmetrical dimer that bears different side chains in each half. The exhaustive NMR characterization of all obtained compounds is presented.

A straightforward one-pot two-step conversion of bile acids into dehomologated alcohols.

Irradiation of dichloroethane solutions of different bile acids with diacetoxy(iodobenzene) and iodine followed by treatment of the resulting raw mixture with MCPBA led to the 41-50% yields of the corresponding dehomologated alcohols in an uncomplicated one-pot protocol that can be completed in less than one day of work.

Application of palladium-catalyzed cross-coupling between bile acids and 2-furanylboronic acid to the synthesis of 24-(2'-furanyl)-24-oxo steroids.

Palladium-catalyzed cross-coupling of bile acids with 2-furanylboronic acid produced steroid furanyl ketones in low yields. Unambiguous assignments of the NMR signals were made with the aid of combined 1D and 2D NMR techniques.

Palladium catalyzed synthesis of benzannulated steroid spiroketals.

Nine cytotoxic [5/7] and [6/6] benzannulated steroid spiroketals were synthesized by palladium catalyzed spiroketalization of 5α and 5ß-alkynediols derived from testosterone, diosgenin and cholesterol. The regioselectivity of the spiroketalization reaction is decisively influenced by the α or ß-orientation of the hydroxyl group at C-5. NMR and single crystal X-ray diffraction corroborated the structure of the obtained compounds. Compounds bearing a cholestane skeleton exhibited higher cytotoxicity against U251 and T47D cells, and the BrdU incorporation assay suggests that the synthesized spiroketals inhibit cell proliferation.

Studies on the reactivity of 23E-benzylidene spirostanes.

A study of the reactivity of 25R and 25S 23E-benzylidene spirostanes that includes epoxidation, catalytic hydrogenation as well as Lewis or Brønsted acid-catalyzed rearrangements is described. Exhaustive NMR characterization of the obtained compounds is presented. Additionally the structures of some of the obtained compounds were confirmed by single crystal X-Ray Diffraction studies.

Synthesis of dimeric spirostanols linked through a 1,4-dimethylidenebenzene moiety by double BF3·Et2O-catalyzed aldol condensation of steroid sapogenins and terephtalaldehyde.

BF3·Et2O-catalyzed double aldol condensation between acetylated steroid sapogenins and terephtalaldehyde led to acetylated dimeric spirostanols linked through a 1,4-dimethylidenebenzene moiety in moderate to good yields. The E configurations of the introduced double bonds were corroborated by NOE experiments. Saponification of the dimeric steroids led to the corresponding dimeric spirostanols.

Synthesis of novel hybrid steroid dimers by BF3·Et2O-catalyzed aldol condensation of 2-formyl-estradiol diacetate and steroid sapogenins.

BF3·Et2O-catalyzed aldol condensation of steroid sapogenins with 2-formyl-estradiol diacetate afforded two novel classes of steroid dimers in which an estrogenic core is attached to the spirostanic side chain of an steroid sapogenin through an exocyclic double bond in position C-23, or through a spiro centre in C-22.

Baeyer-Villiger reaction of steroid sapogenins by CF3COOH-H2O2. A short cut to pregnan-3ß,16ß,20-triol 3-monoacetates.

Treatment of steroid sapogenins with H2O2 in CF3COOH for 15min followed by reflux in CH3OH/H2O afforded good yields of pregnan-3ß,16ß,20-triol 3-monoacetates. When the hydrolysis step was carried out with KOH in refluxing methanol excellent yields pregnantriols were obtained. The resulting compounds were characterized by their melting points and NMR spectral data. An X-ray diffraction analysis of compound 3a confirmed the proposed structure and provided detailed information about the bond lengths, bond angles and conformation.

Synthesis and in vitro anticancer activity of 23(23')E-benzylidenespirostanols derived from steroid sapogenins.

Benzylidenespirostanols were prepared by two-step synthesis including BF3·Et2O-catalyzed aldol condensation of several acetylated steroid sapogenins with benzaldehyde followed by saponification. The obtained compounds showed moderate cytotoxicity against three cancer cell lines (T-lymphoblastic leukemia cell line CEM, breast carcinoma cell line MCF7 and cervical carcinoma cell line HeLa) and normal human fibroblasts (BJ). The most active of the five tested substances was 3c (lowest IC50 for MCF7 cells 19.9±0.1µM) without any selectivity towards human cancer and normal cells, respectively.

Crystalline arrays of side chain modified bile acids derivatives. Two novel self-assemblies based on π-π and belly-to-belly interactions.

Crystalline derivatives of side chain modified bile acids were efficiently prepared from the naturally occurring steroids by palladium-catalyzed cross coupling reaction as a key step. The solvent-free crystalline bile acids derivatives 2b-e are readily accessed by slow evaporation from selected solvents. A variety of steroidal scaffolds were found and elucidated by SXRD studies. The crystal packing of the title compounds are dominated by hydrogen-bonding interactions established between differently positioned acetyl protecting groups, which in the case of 2b and 2e take advantage of the facial amphiphilicity producing two novel steroidal supramolecular self-assemblies combining π-π and strong facial interactions. Thus, these crystalline arrays of side chain modified bile acids represent promising scaffolds for research and implementation in biomolecular materials or inclusion phenomena.

Regioselective hypervalent iodine-induced Favorskii rearrangement of 3-oxo-5ß-steroids.

Treatment of 3-oxo-5ß-steroids with diacetoxyiodobenzene/KOH triggered a fast and regioselective Favorskii rearrangement that exclusively led to 3ß-methoxycarbonyl-5ß-4-norsteroids in good yields. The outcome of the reaction indicates that although both Cyclopropanone and Semi-benzylic pathways are possible, in the case of 3-oxo-5ß-steroids, only the last participates. Unambiguous characterization of the products was achieved by NMR and X-ray Diffraction studies.

Synthesis, NMR and crystal characterization of dimeric terephthalates derived from epimeric 4,5-seco-cholest-3-yn-5-ols.

Two dimeric steroidal terephthalates derived from epimeric 4,5-seco-cholest-3-yn-5-ols were prepared starting from cholesterol in a five-step synthetic sequence. X-ray crystallography shows that the obtained compounds display novel supramolecular networks in the solid state in which the facial hydrophobicity of the steroidal skeletons plays an important role. Unambiguous NMR characterization of the obtained dimers is also provided.

Application of palladium-catalyzed carboxyl anhydride-boronic acid cross coupling in the synthesis of novel bile acids analogs with modified side chains.

Palladium-catalyzed cross coupling of 4-methoxycarbonyl phenyboronic acid with acetylated bile acids in which the carboxyl functions was activated by formation of a mixed anhydride with pivalic anhydride afforded the cross coupled compounds, which were converted in novel side chain modified bile acids by one pot carbonyl reduction/removal of the protecting acetyl groups by Wolff-Kishner reduction. Unambiguous assignments of the NMR signals and crystal characterization of the heretofore unknown compounds are provided.

Synthesis and radical scavenger properties of novel spirochromenes derived from steroid sapogenins.

Tandem aldol condensation between steroid sapogenins and hydroxylated benzaldehydes afforded steroidal spirochromenes. Compounds that bear a phenolic hydroxyl group at position C-6', obtained by a reaction with 2,5-dihydroxybenzaldehyde, showed approximately 80% of maximal radical scavenging activity in the 1,1-diphenyl-2-picrylhydrazyl radical (DPPH) assay at 288 nM. In contrast, the starting steroid sapogenins and the spirochromenes without a phenolic group in the side chain proved to be inactive.

Diacetoxyiodobenzene-mediated synthesis of unnatural furospirostane sapogenins derived from diosgenin and tigogenin.

Two unnatural steroid sapogenins bearing a furospirostane side chain were prepared starting from the readily available spirostane sapogenins, tigogenin and diosgenin following a synthetic protocol that included: (i) introduction of a carbonyl group at position C-23, (ii) diacetoxyiodobenzene-induced F-ring contraction and (iii) LiAlH4 reduction of the newly emerged methoxycarbonyl moiety. The structures of the new compounds were corroborated by NMR and X-ray studies.

Mechanistic insights on the reactivity of furospirostanes with the 16ß,22:22,25-diepoxy-23-acetoxymethyl-24-methyl side chain.

F-ring opening in spirostanes with the 16ß,22:22,25-diepoxy-23-acetoxymethyl-24-methyl side chain produces a Δ(22)-intermediate with an allylic acetoxy group. For this reason the reactivity profile of these compounds deviates from that observed in other naturally occurring or synthetic spirostanes and furospirostanes.