Total synthesis of dl-dendrobine has now been completed. The ketol (17) was derived to the tosylate (18) which was treated with KCN in anhydr. DMSO to give the keto-nitrile (19). Hydrogenation of (19) over 5% Pd-SrCO_3 catalyst afforded the compound (21) in high yield. Treatment of (21) with one equivalent of Br_2, followed by dehydrobromination provided the enone (19) and (34) in a 3-4: 1 ratio. The compound (19) is circulated as the starting material for the compound (34) by hydrogenation of (19) to the compound (21). The compound (35) obtained by ketalization of (34) was refluxed in KOH-ethyleneglycol-H_2O (2:4:1) to give the keto-lactone (5). The compound (5) or (46) was heated with aq. CH_3NH_2 in the presence of CH_3NH_2・HCl to provide the lactam (6). The lactam (6) was successively treated with isopropyl-MgBr, KHSO_4 (dehydration), I_2-AgOAc-AcOH-H_2O, aq. MeOH-KOH, and CrO_3-pyridine to give the enone (7) and (49) in 10 and 20% yield, respectively. Hydrocyanation of (7) with Et_2AlCN gave the cyano-ketones (50), (51), and (8) in 18, 20, and 29% yield, respectively. Reduction of (50) with NaBH_4, followed by hydrolysis with aq. KOH and acidification with dil. HCl provided dl-oxodendrobine (53) and its isomer (52) in 25 and 50%, respectively. dl-Oxodendrobine (53) was also synthesized from the compound (8) through the hydroxy-nitrile (54), imino-lactone (55), and tosylate (56). Another route to dl-oxodendrobine from the compound (8) through the compound (57) and (58) was also carried out. Reduction of dl-oxodendrobine by the method of Borch gave dl-dendrobine.
