インドールアルカロイド類の転換反応(第3報¹⁾), Protopine型Indole Alkaloid "Burnamicine"の部分合成

抄録

Partial synthesis of dihydroburnamicine (IV) and naturally occurring 2-acylindole alkaloid, burnamicine (I), from hirsutine (IIIb) and geissoschizine methyl ether (II), respectively, is described. By the first route, 7-acetoxy-7H-hirsutine methiodide (VIb) was converted to 2-acylindole derivative (VII), whose conversion into IV has already been reported, ⁴ but the conversion of II into the corresponding 2-acylindole derivative (IX) was unsuccessful by this route. In the other route, ethyl chloroformate as a reagent for C/D ring cleavage of indole alkaloids was used. Dihydrocorynantheine (IIIa) and IIIb reacted with ethyl chloroformate to give the urethane derivatives (XIIa, b), which were converted to IV by several steps but the conversion of methane derivative (XX) prepared from II into the desired 2-acylindole derivative (XXI) was not successful. Reaction of hirsutinol (XXIV), derived from IIIb with ethyl chloroformate, gave the urethane derivative (XXV), which was also converted to IV. By the same route, geissoschizol (XXVIII), derived from II, was converted to I. This partial synthesis of I revealed its absolute configuration and geometry of the ethylidene group. It was very interesting that I and IV having the same absolute configuration at C-15 exhibited nearly opposite circular dichroism curves.