Synthesis of proline derivatives of bile acids and their evaluation as organocatalysts in the asymmetric direct aldol reaction

C-12 vs C-3 substituted bile salts: An example of the effects of substituent position and orientation on the self-assembly of steroid surfactant isomers

Biomolecule derivatives are transversally used in nanotechnology. Deciphering their aggregation behavior is a crucial issue for the rational design of functional materials. To this end, it is necessary to build libraries of selectively functionalized analogues and infer general rules. In this work we enrich the highly applicative oriented collection of steroid derivatives, by reporting a rare example of C-12 selectively modified bile salt. While nature often exploits such position to encode functions, it is unusual and not trivial to prepare similar analogues in the laboratory. The introduction of a tert-butyl phenyl residue at C-12 provided a molecule with a self-assembly that remarkably switched from rigid pole-like structures to twisted ribbons at a biologically relevant critical temperature (∼25 °C). The system was characterized by microscopy and spectroscopy techniques and compared with the C-3 functionalized analogue. The twisted ribbons generate samples with a gel texture and a viscoelastic response. The parallel analysis of the two systems suggested that the observed thermoresponsive self-assemblies occur at similar critical temperatures and are probably dictated by the nature of the substituent, but involve aggregates with different structures depending on position and orientation of the substituent. This study highlights the self-assembly properties of two appealing thermoresponsive systems. Moreover, it adds fundamental insights hereto missing in the investigations of the relation between self-assembly and structure of synthetic steroids, which are valuable for the rational design of steroidal amphiphiles.

Mimicking Enzymes: Asymmetric Induction inside a Carbamate-Based Steroidal Cleft

Cholic acid has been elaborated into a carbamate-based tripodal architecture, which is able to promote an asymmetric organic transformation inside its chiral cavity. The nature of this steroidal catalyst has been disclosed by quantum-chemical calculations. It comprises the preorganization and confinement of the reagents within the cavity of the steroid to form a supramolecular complex held together by means of cooperative H-bond contacts. This operational mode resembles that of some enzymes.

Synthesis of B- and C-ring-modified lithocholic acid analogues as potential sialyltransferase inhibitors

In order to identify structural features of lithocholic acid (LCA) critical for inhibition of the enzyme sialyltransferase (ST) novel analogues with modifications of the skeleton (7-9, 16-18 and 20) were designed and synthesized. Methyl 3α-acetoxy-7-oxo-cholanate (1), methyl 3α-acetoxy-12-oxo-cholanate (2) and methyl 3α,7α-diacetoxy-12-oxo-cholanate (3) were subjected to Baeyer-Villiger oxidation to provide homolactones (7-9) or to the Beckmann rearrangement of the corresponding oximes to give homolactams (16-18). Both reactions proceed regio- and stereoselectively. Ring B homolog of lithocholic acid (20) was efficiently synthesized. Among these compounds, 7, 9 and 16 were found to have the significant activity, with IC50 values ⩽3μM against α-2,6-(N)-ST selectively, which are 5-fold lower than that of Lith-O-Asp. Given the reality that LCA and its analogue, Lith-O-Asp, have been revealed to improve inhibitory efficacy of ST and to have a wide range of antimetastatic activities in different human cancer cells, the up-to-date findings have noteworthy pharmacological significance as they open a promising path to the improvement of a prospective molecular targeted application of modified LCA analogues as agents for the treatment of cancer metastasis.

Enamine catalysis

The reversible reaction of primary or secondary amines with enolizable aldehydes or ketones affords nucleophilic intermediates, enamines. With chiral amines, catalytic enantioselective reactions via enamine intermediates become possible. In this review, structure-activity relationships and the scope as well as current limitations of enamine catalysis are discussed.

The direct catalytic asymmetric aldol reaction

Asymmetric aldol reactions are a powerful method for the construction of carbon-carbon bonds in an enantioselective fashion. Historically this reaction has been performed in a stoichiometric fashion to control the various aspects of chemo-, diastereo-, regio- and enantioselectivity, however, a more atom economical approach would unite high selectivity with the use of only a catalytic amount of a chiral promoter. This critical review documents the development of direct catalytic asymmetric aldol methodologies, including organocatalytic and metal-based strategies. New methods have improved the reactivity, selectivity and substrate scope of the direct aldol reaction and enabled the synthesis of complex molecular targets (357 references).