Rui M. A. Pinto

Bismuth compounds in medicinal chemistry

In recent years, the chemical potential of bismuth and bismuth compounds has been actively exploited. Bismuth salts are known for their low toxicity, making them potential valuable reagents for large-scale synthesis, which becomes more obvious when dealing with products such as active pharmaceutical ingredients or synthetic intermediates. Conversely, bismuth compounds have been widely used in medicine. After extensive use in the treatments of syphilis and other bacterial infections before the advent of modern antibiotics, bismuth compounds remain important for the treatment of several gastrointestinal disorders and also exhibit antimicrobial properties and cytotoxic activity, among others. This review updates relevant advances in the past few years, concerning the application of bismuth reagents and catalysts in innovative synthetic processes for the preparation of compounds of medicinal interest, as well as the preparation, biological evaluation and potential medicinal uses of bismuth compounds.

Bismuth(III) Triflate-Catalyzed Direct Conversion of Corticosteroids into Highly Functionalized 17-Ketosteroids by Cleavage of the C17-Dihydroxyacetone Side Chain

The use of bismuth(III) triflate as catalyst for the direct conversion of corticosteroids into highly functionalized 17-ketosteroids by cleavage of the C17-dihydroxyacetone side chain is reported. This catalytic process is very chemoselective, since functionalities of the starting corticosteroids, such as Delta(4)-3-keto, Delta(1,4)-3-keto, 11beta-hydroxyl, and 9beta,11beta-epoxide, remained intact.

Efficient oxidation of oleanolic acid derivatives using magnesium bis(monoperoxyphthalate) hexahydrate (MMPP): A convenient 2-step procedure towards 12-oxo-28-carboxylic acid derivatives

Summary A new, straightforward and high yielding procedure to convert oleanolic acid derivatives into the corresponding δ-hydroxy-γ-lactones, by using the convenient oxidizing agent magnesium bis(monoperoxyphthalate) hexahydrate (MMPP) in refluxing acetonitrile, is reported. In addition, a two-step procedure for the preparation of oleanolic 12-oxo-28-carboxylic acid derivatives directly from Δ12-oleananes, without the need for an intermediary work-up, and keeping the same reaction solvent in both steps, is described as applied to the synthesis of 3,12-dioxoolean-28-oic acid.

5β,6β-Epoxy-17-oxoandrostan-3β-yl acetate and 5β,6β-epoxy-20-oxopregnan-3β-yl acetate

In the title compounds, C(21)H(30)O(4), (I), and C(23)H(34)O(4), (II), respectively, which are valuable intermediates in the synthesis of important steroid derivatives, rings A and B are cis-(5beta,10beta)-fused. The two molecules have similar conformations of rings A, B and C. The presence of the 5beta,6beta-epoxide group induces a significant twist of the steroid nucleus and a strong flattening of the B ring. The different C17 substituents result in different conformations for ring D. Cohesion of the molecular packing is achieved in both compounds only by weak intermolecular interactions. The geometries of the molecules in the crystalline environment are compared with those of the free molecules as given by ab initio Roothan Hartree-Fock calculations. We show in this work that quantum mechanical ab initio methods reproduce well the details of the conformation of these molecules, including a large twist of the steroid nucleus. The calculated twist values are comparable, but are larger than the observed values, indicating a possible small effect of the crystal packing on the twist angles.

19β,28-Ep-oxy-18α-olean-3β-ol.

The title triterpene, C30H50O2, is an 18α-oleanane derivative prepared by the Wagner–Meerwein rearrangement of betulin with Bi(OTf)3.xH2O (OTF is trifluoromethanesulfonate). There are two symmetry-independent molecules in the asymmetric unit that show no significant differences concerning bond lengths and angles. The conformation of the six-membered rings is close to a chair form, while the five-membered epoxide rings adopt envelope conformations. All rings are trans-fused. In the crystal, molecules are held together by O—H⋯O hydrogen bonds. A quantum-mechanical ab initio Roothan Hartree–Fock calculation on the isolated molecule gives values for bond lengths and valency angles close to the experimental values. The calculations also reproduce well the molecular conformation with calculated puckering parameters that match well the observed values.

6β-Hydroxy-5β-methyl-20-oxo-19-norpregn-9(10)-en-3β-yl acetate

In the title compound, C(23)H(34)O(4), which is an intermediate in the synthesis of pregnane derivatives with a modified skeleton that show potent abortion-inducing activity, the conformation of ring B is close to half-chair due to the presence of both the C=C double bond and the axial 5beta-methyl group. Rings A and C have conformations close to chair, while ring D has a twisted conformation around the bridgehead C-C bond. Molecules are hydrogen bonded via the hydroxyl and acetoxy groups into infinite chains. Quantum-mechanical ab initio Roothan Hartree-Fock calculations show that crystal packing might be responsible for the low values of the angles between rings A and B, and between ring A and rings C and D, as well as for a different steric position of the methyl ketone side chain compared to the geometry of the free molecule.

6β-Chloro-5α-hydr­oxy-20-oxopregnan-3β-yl acetate

The title steroid, C23H35ClO4, is a pregnane derivative prepared by ring opening of the corresponding 5α,6α-epoxy steroid with BiCl3. There are two symmetry-independent molecules in the asymmetric unit that show no significant differences concerning bond lengths and angles. The conformation of the six-membered rings in both molecules is close to a chair form, while the five-membered ring adopts an envelope conformation. All rings in both molecules are trans-fused. The molecules are held together by an extensive O—H⋯O hydrogen-bonding network.

16α,17α-Ep-oxy-5α-hydr-oxy-6β-nitrooxy-20-oxopregnan-3β-yl acetate.

The title steroid, C23H33NO8, is a pregnane derivative obtained regio-, stereo- and chemoselectively from the ring opening of the corresponding 5α,6α;16α,17α-diepoxide with bismuth(III) nitrate. There are two symmetry-independent molecules in the asymmetric unit that show no significant differences concerning bond lengths and angles. All rings are trans-fused. The conformations of the six-membered rings are close to chair forms, while the five-membered ring adopts an envelope conformation. The molecules are held together by an extensive O—H⋯O hydrogen-bonding network of chains runnning along the a axis.

6β-Acetamido-5α-hydroxy­cholestan-3β-yl acetate

The title steroid, C31H53NO4, was prepared from the corresponding 5α,6α-epoxycholestane. The conformation of the six-membered rings is close to a chair form, while the five-membered ring adopts a twist conformation. The hydroxyl and acetamide groups are in axial positions. The nucleophilic species bound to the steroid nucleus at position 6 by the β-face, whereas the hydroxyl group at position 5 has α-orientation. All rings are trans-fused. The crystal packing shows that the molecules related by twofold symmetry exist as O—H⋯O hydrogen-bonded dimers.

3-Oxo-18α-olean-28,13β-olide

The title terpene, C30H46O3, is a 28,13β-lactone of oleanolic acid prepared with bismuth trifluoromethanesulfonate (OTf), Bi(OTf)3·xH2O. All rings are trans-fused. The X-ray study shows the inversion of the orientation of 18-H in the lactonization reaction. A quantum chemical ab-initio Roothaan Hartree–Fock calculation of the equilibrium geometry of the isolated molecule gives values for bond lengths and valency angles in close agreement with experimental values. The calculation also reproduces the observed molecular conformation, with puckering parameters that agree well with those determined from the crystallographic study.