Palladium-Catalyzed Generation of ortho-Quinone Methides. A Three-Component Synthesis of L-Shaped Dimeric Steroidal Scaffolds

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.

A Crystalline Dimeric Steroidal Diboronate with Electronically Impeded Rotation

The dimeric steroid SMR-3, featuring a 1,4-phenyldiboronic ester flanked by two pregnan-triol frameworks, was synthesized to explore the intramolecular dynamics of its central component. The structural data from single-crystal X-ray diffraction studies and the Hirshfeld analyses indicate small steric effects around the aromatic ring that should favor the intended motion. However, solid-state NMR data obtained through VT ¹³C{¹H} CPMAS and ²H spin-echo experiments, using the deuterated analogue SMR-3D₄, revealed that this component is rigid even at temperatures where other reported steroidal molecular rotors experience fast rotation (85 °C). A combination of classical molecular dynamics, molecular mechanics, and correlated ab initio calculations allowed us to distinguish the steric and electronic factors that restrict the potential motion in this compound. The experimental and computational data reveal that electronic components dominate the behavior and are responsible for the high rotational barrier in the SMR-3 crystal.

Synthesis and characterization of a fluorescent steroid dimer linked through C-19 by a 1,4-Bis(phenylethynyl)phenylene fragment

The synthesis and characterization of a dimer in which two nuclei of 3β-acetoxy-19-hydroxyandrost-5-en-17-one are linked by the fluorescent 1,4-bis(phenylethynyl)phenylene bridge attached to the oxygenated functions at positions C-19 of each steroid fragment is described. The compound was obtained in five steps and 23 % overall yield and showed a strong blue emission with a quantum yield of 0.66.

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.