TOTAL SYNTHESES OF YOHIMBE ALKALOIDS, WITH STEREOSELECTION FOR THE NORMAL, ALLO, AND 3-EPIALLO SERIES, BASED ON ANNELATIONS OF 4-METHOXY-1,2-DIHYDROPYRIDONES

N-[2-(1H-3-indolyl)ethyl]-2,3-dihydro-4-pyridone (31) was generated in two steps (77% yield) from tryptamine and N-methyl-4-piperidone methiodide. Its cyclization (90% yield) and oxidation (91% yield) provided the tetracyclic analogue 32. O-Methylation and Robinson-type annelation of these vinylogous lactams (the latter in form of its N(a)-carbamate) furnished the dienones 38 (64%) and 43 (90%). Further elaboration by cyclization and/or reduction reactions selectively provided the 15,16-didehydroyohimbinones 7 or 44. Their reductions then led to yohimbinone (52, 20% overall yield from tryptamine), alloyohimbinone (11, 19% overall yield), and 3-epi-alloyohimbinone (10, 23% overall yield), which led to yohimbine (3), beta-yohimbine (9), 3-epi-alloyohimbine (53), and 3-epi-17-epi-alloyohimbine (54).