STUDIES OF THE TOTAL SYNTHESIS OF (-)-RANUNCULIN

The synthesis of (—)-ranunculin from mannitol is described. The oxidation of 1,2,5,6-diacetone d-rnannitol (3) with NaIO₄ or lead tetraacetate gave 2,3-O-isopropylidene-D-glyeeraldehyde (4) with the desired α-carbon-(R)configuration. The Wittig reaction of isopropylidene-D-glyceraldehyde (4) with (C₆H₅)₃P=CHCOOCH₃ gave a mixture of Z and E methyl-3-(2,2-dimethyl-1,3-diexo-lan-4-yl)-2-propenoate (7,6) with a Z : E ratio of 85:15. The dependence of Z : E ratio upon the solvent present was studied. On careful treatment of the cis isomer with 0.1 mol/L HCl, hydrolysis of the ketal took place along with eyclization to give the optically active aglycone (8) with the expected stereochemistry, which was followed by Koenigs-Knorr reaction to give (—)-ranunculin tetraacetate. Since the glycoside is sensitive to acid and base, hydrolysis was carried out with strongly acidic cation exchange resin which yielded (—)-ranunueulin smoothly. The overall, yield from mannitol with all the intermediates purified amounted to 15%.