Structural Modifications of Deoxycholic Acid to Obtain Three Known Brassinosteroid Analogues and Full NMR Spectroscopic Characterization

An improved synthesis route for obtaining known brassinosteroid analogues, i.e., methyl 2,3-dihydroxy-6-oxo-5-cholan-24-oate (11), methyl 3-hydroxy-6-oxo-7-oxa-5-cholan-24-oate (15) and methyl 3-hydroxy-6-oxa-7-oxo-5-cholan-24-oate (16), from hyodeoxycholic acid (4) maintaining the native side chain is described. In the alternative procedure, the di-oxidized product 6, obtained in the oxidation of methyl hyodeoxycholate 5, was converted almost quantitatively into the target monoketone 7 by stereoselective reduction with NaBH4, increasing the overall yield of this synthetic route to 96.8%. The complete H-1- and C-13-NMR assignments for all compounds synthesized in this work have been made by 1D and 2D heteronuclear correlation gs-HSQC and gs-HMBC techniques. Thus, it was possible to update the spectroscopic information of H-1-NMR and to accomplish a complete assignment of all C-13-NMR signals for analogues 5-16, which were previously reported only in partial form.