Synthesis and Structural Determination of New Brassinosteroid 24-Nor-5α-Cholane Type Analogs

Abstract

Natural brassinosteroids possess a 22R, 23R configuration that appears essential for biological activity. It is, therefore, interesting to elucidate if the activity of brassinosteroids with a short side chain depends on the C22 configuration. Herein, we describe the synthesis of new brassinosteroids analogs with 24-norcholane type of side chain and R configuration at C22. The initial reaction is the dihydroxylation of a terminal olefin that leads to S/R epimers. Three different methods were tested in order to evaluate the obtained S/R ratio and the reaction yields. The results indicate that Upjohn dihydroxylation is the most selective reaction giving a 1.0:0.24 S/R ratio, whereas a Sharpless reaction leads to a mixture of 1.0:0.90 S/R with 95% yield. Using the latter mixture and following a previous reported method, benzoylated derivatives and both S and R brassinosteroids analogs were synthesized. All synthesized compounds were completely characterized by NMR spectroscopy, and HRMS of new compounds are also given. In conclusion, a synthetic route for preparation of new analogs of brassinosteroids of 24-norcholane type and R configuration at C22 were described. It is expected that this will help to elucidate if a configuration at C22 is a structural requirement for hormonal growth activity in plants.

Biological Activities and Molecular Docking of Brassinosteroids 24-Norcholane Type Analogs

“…However, this difference disappears when a benzoyl group is attached to the side chain, i.e., compounds 6 and 7. The calculated bending values for mixtures 3/4 can be obtained by using the individual activities obtained for analogs 3 and 4 and the epimeric composition determined by NMR, i.e., 1.0:0.24 and 1.0:0.9 [44]. The calculated bending angles at 1 × 10 −7 M are 54 ± 4 and 46 ± 4, respectively, which compare quite well with the experimental values.…”

“…These analogs have been previously synthesized by the dihydroxylation of a terminal olefin (3α-acetoxy-24-nor-5α-cholan-22-en-6-one) obtained from hyodeoxycholic acid in a six-step route [43]. Briefly, the formation of vicinal diol function in the alkyl chain was performed by direct Upjohn dihydroxylation or direct Sharpless asymmetric dihydroxylation [44]. Application of these transformations led to mixtures of 22S/22R epimers of 22,23-dihydroxy-6-oxo-24-nor-5α-cholan-3α-yl acetate, with different diastereomeric ratios, i.e., 1.0:0.24 and 1.0:0.9, respectively [43,44].…”

“…Briefly, the formation of vicinal diol function in the alkyl chain was performed by direct Upjohn dihydroxylation or direct Sharpless asymmetric dihydroxylation [44]. Application of these transformations led to mixtures of 22S/22R epimers of 22,23-dihydroxy-6-oxo-24-nor-5α-cholan-3α-yl acetate, with different diastereomeric ratios, i.e., 1.0:0.24 and 1.0:0.9, respectively [43,44]. Compound 5 was obtained by recrystallization from the former mixture [43].…”

“…Saponification reaction (K 2 CO 3 /CH 3 OH, reflux) of these mixtures leads to mixtures of 3/4 with the same epimer proportion, and the benzoylation reaction produced a mixture of compounds 6-8. Finally, analogs 3 and 4 were obtained by the saponification of monobenzoylated derivatives 6 and 7 [44].…”

“…To determine the structural effect of the side alkyl chain on growth-promotion activity, we have previously reported the synthesis of BRs 24-nor-5α-cholan type analogs (compounds 3-9, Figure 1) [43,44]. These compounds were obtained by the dihydroxylation of a terminal olefin synthesized from hyodeoxycholic acid [43,44]. Thus, the aim of this study is to evaluate the growth-promoting activity of these analogs by using the RLIT and to explain the observed SAR by applying molecular docking.…”