Journal of the American Chemical Society, Vol.116, No.20, 9009-9018, 1994
Symmetry-Driven Synthesis of Indole Alkaloids - Asymmetric Total Syntheses of (+)-Yohimbine, (-)-Yohimbone, (-)-Yohimbane, and (+)-Alloyohimbane
Total asymmetric syntheses of the target alkaloids are reported. The syntheses involve the preparation of enantiomerically pure (S,S)-1,3,3a,4,7,7a-hexahydro-2(H)-inden-2-one 7 and its meso isomer 5. Each ketone is then converted into a ring-expanded lactam using an oxaziridine synthesis/rearrangement protocol. The applications of Bischler-Napieralski ring constructions along with appropriate functional group transformations afford enantiomerically enriched alloyohimbane or yohimbane from the meso- or C-2-symmetric ketones, respectively. A cis-5,6-diacetoxy compound (18) derived from the (S,S)-ketone served as the starting material for the total syntheses of the more highly functionalized alkaloids. Accordingly, a site-specific insertion of the indole-containing side chain was accomplished via stereoselective formation of an oxaziridine followed by its stereospecific rearrangement. The selectivity of this sequence allowed for the differentiation of alcohols at C-17 and C-18 (yohimbine numbering) and the synthesis of Delta 18,19-yohimbone. This alpha,beta-unsaturated ketone was converted into either (-)-yohimbone or (+)-yohimbine using standard chemistry.
Keywords:SULFINYL KETIMINE ANIONS;ENANTIOSELECTIVE SYNTHESIS;THERMAL REARRANGEMENT;YOHIMBAN ALKALOIDS;ENE ESTERS;(-)-ALLOYOHIMBANE;INDOLO(2;3-A)QUINOLIZIDINE;DERIVATIVES;ROUTE