Steroidal ribbons from (3α,5β,20S)-3-hydroxy-20-methyl-pregnan-21-oic acid

Background: Steroidal ribbons (or steroid containing ribbons) are being studied for several reasons, firstly, they may be included in the lipid bilayer, especially if they have suitable size (as "extended cholesterol"), and they are interesting in binding to steroidal receptors, gelators, and/or superassembly synthons. Bile acids seem to be very suitable as their components have natural tendency to form supramolecular assemblies. Methods: Synthesis of ester bonded tetramer of (3α,5β,20S)-3-hydroxy-20-methyl-pregnan-21-oic acid was achieved using 2,6-dichlorobenzoyl chloride and DMAP in toluene. To prepare shortened side chain steroid, the oxidative decarboxylation of protected lithocholic acid was used, followed by oxidation of thus formed terminal double bond. Aggregation properties of the prepared steroidal ribbon were studied alone and after binding with copper(II) ions. Results: There was synthesized 3α-{3α-{3α-[(3α,5β,20S)-3-hydroxy-pregnane-20-carbonyl)oxy]-[(5β,20S)-pregnane-20-carbonyl)oxy]}-[(5β,20S)-pregnane-20-carbonyl)oxy]}-(5β,20S)pregnane-20-carboxylate as a steroidal ribbon derived from bile acid, however, with the shortened side chain. The aggregation of the ribbon prepared in chloroform after binding of Cu(II) ions was monitored by absorption and circular dichroism spectroscopy at different ratios [tetramer]/[Cu(II)]. A positive bisignate band centered at 280 nm emerges; whose intensity increases with the [tetramer]/[Cu(II)] ratio. This result suggests that chiral aggregates are obtained as a consequence of Cu(II) complexation. Time dependent experiments did not show any variation in both UV-Vis and CD spectra. Conclusion: Prepared and documented new type of tetrasteroidal ribbon is unique one for the length of the connecting steroidal side chain. Its construction was done aiming to have more rigid ribbon than the one of lithocholic acid, and on the other hand, less rigid than the one derived from etienic acid. Contrary to the fact that no (chiral) aggregation was found by studying the ribbon itself, absorption and CD spectra showed defined aggregates formed after binding of Cu(II) on tetramer.